Chapter 13: Sensors Flashcards
What is the function of sensors?
They serve to inform the mechatronic system about their environment, enabling them to make decisions and respond appropriately to stimulus. they provide info about the physical world to microcontrollers and other circuit elements.
What is the primary purpose of a sensor?
To produce a measurable electrical signal correlated to the quantity of the measurand
Sensors are a category of transducer, what does this mean?
Transducers are devices that transforms energy from one form into another, eg. a force transducer translates a mechanical force to an electrical signal
Actuators are also transducers, for example a solenoid, what is its purpose?
Many actuators serve to translate electrical energy into mechanical energy
What must first be known before connecting a sensor to a microcontroller?
What microcontrollers accept as inputs, making sure the sensor translates the quantity of interest into one of the appropriate input types. microcontrollers are very limited on what inputs they accept
Fundamentally, what are the only two things a microcontroller can measure?
Voltage and time
Do microcontrollers accept digital or analogue inputs?
Practically all accept digital inputs, most have an analogue-digital converter on the chip, so in theory most can do both.
How do microcontrollers read voltage?
They read voltages that are above and below threshold levels that indicate logic levels on (1) and off (0)
How do microcontrollers time external events?
They make use of a clock source, most have a clock source within them, or can use an external source
Fundamentally, the job of a sensor is to translate a single physical parameter into what?
A digital voltage, analogue voltage, or a voltage signal whose variations in time encode its output
Why is it important that the sensor takes a single physical parameter?
A key characteristic of a good sensor is that they are insensitive to physical parameters, other than the one of interest
How must the sensor be connected electrically?
In such a way that the final result is a change in the sensor’s interface electronics output voltage
A popular sensor is a thermistor, what do they do?
Their resistance changes as a function of temperature
When a thermistor is in equilibrium with an environment, what will its resistance reflect?
The temperature
What do you need to do to measure the resistance directly from the thermistor on the microcontroller?
Need to translate the signal from the sensor into a voltage or time-varying signal in order for the microcontroller to accept it.
What is it that you need to connect to the thermistor in order to translate the signal to a voltage or time-varying signal?
Interface electronics
What is interface electronics?
This is a circuit that will perform the necessary translation between the thermistor and microcontroller
What is one of the simplest ways of making this translation?
Creating a voltage divider circuit, thermistor on the high side (connected to a 5 volt power supply), the output of the voltage divider is then a analogue voltage that varies as the resistance of the thermistor varies. this is easy for a micro-controller with adc to read and interpret
What is adc?
analogue to digital converter
Measuring acceleration is typically more challenging than measuring temp with a thermistor, what is a more simple way of measuring the acceleration?
Simple mass attached to a spring
How can we determine the acceleration from a mass on a spring?
A force can be applied to a mass by subjecting it to an acceleration, we can determine the acceleration by measuring the displacement of the mass.
What equations are required to calculate this?
f=ma and f=kx
Name a simple, inexpensive method of sensing the displacement of the mass?
Attach the mass directly to the wiper of a potentiometer, this would result in a device whose resistance changes with acceleration
What is a potentiometer?
A potentiometer is a three terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider
What are the drawbacks of using a potentiometer as a displacement sensor, and what would need to be considered to achieve higher performance?
- poor accuracy
- poor repeatability
- friction creates a challenge at each phase of the movement
in order to achieve higher performance, it would be advantageous to seek an other means of measuring the displacement, something that doesn’t involve direct contact or friction
Name some other methods for measuring displacement without direct contact?
- reflective infrared emitter-detector pairs
- capacitive sensors
- force-balance accelerometer/servo accelerometer
What is a force-balance accelerometer (or servo accelerometer)?
It’s essentially the mass on a spring approach flipped on its head. build a device that actively works to hold a moveable mass in a fixed position. we can then measure the amount of control effort is required to maintain the position of the mass, and this can be corelated to an external acceleration applied to the mass.
What is the definition of transfer function?
The functional relationship between the physical input signal (the measurand) and amp; the electrical output signal. this may be expressed in a formula, or plotted on a graph
What is the definition of sensitivity?
The relationship between input physical signal and output electrical signal.
How is sensitivity often expressed?
The sensitivity is often expressed as the ratio between a change in output electrical signal to a given change in the input physical signal.
How may sensitivity be expressed?
Sensitivity may be expressed as the derivative of the transfer function with respect to the input physical signal
What units is sensitivity stated as for a thermistor?
Ω/°C
What is the definition of span or dynamic range?
This is the range of input physical signals which may be converted to electrical signals by the sensor
What happens to input signals outside of the span/dynamic range?
Input signals out of this range are not guaranteed to produce output signals that meet the device’s specifications
What can happen to the sensor if signals outside of the span/dynamic range are used?
The sensor may be permanently damaged
A temperature sensor may state the span/dynamic range in °C, but if the range is large, what may the span/dynamic range be stated as, and what formula?
It may be stated in decibels (db) and the formula:
dB=20log_10 (max-measurable-signal)/(min-measurable-signal)
What types of sensors would use decibels?
Microphones, photo-sensors, where the dynamic range is large
What is the definition of accuracy?
The largest expected error between the actual and ideal output signal. This maybe stated as a value, or a percentage of full-scale output
How can you substantially improve a sensor’s accuracy?
By calibrating it against a known standard
What different techniques are required for calibration of a sensor and when should you apply them?
- single-point calibration. this is where the value of the measurand reported by the sensor is compared with the known value of the measurand at a known point somewhere within the sensor’s dynamic range. This is applied when a sensor’s offset error is significant but not its gain error
- two-point calibration. this involves taking two measurements from the sensor, one at the low end of the range, and one at the high end, comparing these with known values. this is used when both the offset error and gain error are significant.
- multi-point calibration may be required if the output of a sensor is highly non-linear
There are multiple ways non-linearity is commonly described, what does it mean?
Non-linearity is the maximum deviation from a linear transfer function over the specified dynamic range. Most commonly, it compares the actual transfer function with the ‘best straight line’ which lies between the two parallel lines which encompasses the entire transfer function over the specified dynamic range of the device.
Why is the comparison method for non-linearity popular?
It produces the smallest value for non-linearity and makes sensors look their best
What is another possible definition of non-linearity?
This compares the possible range of errors with the least squares fit line
What units may non-linearity have?
It is likely to have the units of the measured signal, or be expressed as a percentage of the full-scale output.
What is the definition of hysteresis?
For a given value of the input signal, this is the difference between a sensor’s output reading when approached from a previous reading below the new value and when approached from a previous reading above
What units is the hysteresis expressed as?
The hysteresis is expressed as units of the measurement quantity
If a temperature sensor has a hysteresis of 2°C, what does this mean?
This indicates that the output will differ by up to 2°C, depending on whether the sensor was heated to the new conditions or cooled to the new conditions
What is the definition of noise?
A constituent of a sensor’s output that does not contain information about the input physical signal being measured. noise may be thought of ‘corrupting’ the signal of interest, and is generally undesirable
Do all sensors produce some output noise?
Yes, in some cases this noise is less than the noise of the other elements in the electronics, or less than the fluctuations in the physical signal, in this case it is not significant. However in many cases the noise of the sensor limits the performance of the system.
How is the sensor noise usually quoted in most sensor data?
It is often quoted as the rms (root mean square) noise amplitude. it will some- times include plots of typical noise distributions with respect to frequency
What is common for noise sources to produce?
A white noise distribution
What is a white noise distribution?
A white noise distribution is where the spectral noise density is the same at all frequencies
Give an example of a white noise distribution, and where is it found?
Johnson noise, this is present in conductive materials such as wires & resistors
What is johnson noise caused by?
Johnson noise is caused by thermal agitation of the charge carriers, & typicallyappears as white noise
For white noise, what units is the noise spectral density characterised in?
volts/√Hz
A white noise distribution adds noise to a measurement with what amplitude?
A distribution of this nature adds noise to a measurement with amplitude inversely proportional to the square root of the measurement bandwidth
What does the inverse relationship between bandwidth and measurement time mean for way the noise decreases?
The noise decreases with the square root of the measurement time
What is the signal-to-noise ratio?
A measure of how big the desired output signal is relative to the noise present
How can the signal-to-noise ration be improved?
Signal averaging
What is signal averaging?
Signal averaging is effectively a low-pass filter that reduces the bandwidth of the signal by increasing the average time of the measurement
What is the definition of resolution?
The minimum detectable change in the measured quantity
Fluctuations are a temporal phenomena, what does this mean for the resolution?
This means there must be a relationship between the timescale for the fluctuation and the minimum detectable amplitude. therefore a definition of resolution must include some information about the nature of the measurement being carried out
What units would the resolution be stated as for sensors limited by noise with a white spectral distribution?
(physical signal)/√Hz
What units do sensor data sheets usually quote resolution as, or how else may it be stated?
signal/√Hz
they may otherwise give a minimum detectable signal for a specific measurement
How is the resolution stated for sensors that are limited by other error sources (other than noise with a white spectral distribution)?
The resolution is often not even mention in data sheets
What is the definition of bandwidth?
The range of frequencies of an input physical signal that a sensor can detect
Just read this pls-
Sensors have finite response time to an instantaneous change in the physical signal. Many sensors exhibit a behaviour in which they follow a step change reasonably quickly, but have an output that then decays in the absence of further changes in the input.
What do the reciprocal of the time constants for the step response correspond to?
The upper cutoff frequency
What does the reciprocal of the decay time correspond to?
The lower cutoff frequency
How is the bandwidth defined with the upper and lower cutoff frequencies?
The bandwidth of a sensor is defined as the frequency range between the upper and lower cutoff frequencies
What units define bandwidth?
Hz
What assumption is made for the lower cutoff frequency if a single frequency is specified?
It is assumed that the lower cutoff frequency is 0 Hz
How is the bandwidth stated in the case that sensors do not produce signals when their input signals are at or near DC, or 0hz?
The bandwidth will be stated as a range
Give an example of a sensor that does not produce signals when their input signals are at or close to DC or 0hz?
Microphones
What is the ‘freescale mma1250’?
This is a commercially available ‘mems’ accelerometer, fitting in a 16 pin ‘soic’ package
What does mems mean?
Micro Electro-Mechanical System
What does soic mean?
Small Outline Integrated Circuit
The Freescale mma1250 incorporates a proof mass, a spring element, a displacement sensor, as well as all the circuitry necessary to translate the output to a varying voltage. but, how does it work?
A very small mass is created from the base silicon wafer and capacitive displacement sensor on either side of the proof mass determine how far from the equilibrium it has moved due to externally applied accelerations
How does the Freescale mma1250 perform signal conditioning and optimize the sensor’s final output?
Filter circuits
What factors can affect the accuracy?
Offset and gain errors as well as non-linearity
What happens when a sensors output doesn’t match the transfer function’s linear equation?
Where the sensors output doesn’t match the transfer function’s linear equation, additional inaccuracies are introduced
How can we reduce or eliminate the effects of non-linearity?
Using a multipoint calibration
How can inaccuracies be greatly reduced or eliminated in sensors?
Calibration of individual sensors
What is one of the most common and simplest sensor?
The switch
What is a switch?
A device with two or more electrical terminals that form an open circuit in one configuration (i.e. the terminals are not in electrical contact), and a short circuit in another configuration (i.e. they make electrical contact)
What types of switches are there?
-push button
-slide
-toggle
-rotary
-knife
-Hall
-tactile
-micro
-lever
-membrane
-momentary
-keyed
-mercury
and reed
What is the common purpose of all switches?
To sense and indicate when an object or force has interacted with them in such a way that it causes them to take on a given state or to change states
Give some common example of applications of a switch
- Enabling or disabling power to a device
- Providing a human interface
- Sensing limits of travel
- Detecting proximity
- Sensing angles
How are switches categorised?
Switches are categorised according to the number of poles and throws it has.
What does the number of poles a switch has mean?
The number of poles a switch has is the number of independent circuits it can switch
How many poles do most switches have?
Single, double or triple, but some have higher numbers
What does the number of throws a switch has mean?
The number of throws of a switch is the number of discrete active positions the switch contacts may assume
What is the general format for describing a switch, and what do they stand for?
xPyT
x- number of poles (S=single, D=double etc, numbers are used for cases with more than three)
y- number of throws (again S=single, D=double etc)
Name a switch with a very high number of throws?
Rotary switches
What label would a simple ON/OFF switch have?
Single pole, single throw, so SPST
How is a switch interfaced?
Ideally the circuits output voltage is a digital logic level low in one state, and in the opposite state, the circuits output voltage is a digital logic level high
In general, what voltages are used to interface a switch?
In general, a logic level low is very closer to 0V and a logic level high approaches the supply voltage, often 5V.
What forms when a switch incorporates moving electrical contacts forced into physical contact by spring elements?
Switches that incorporate moving electrical contacts which are forced into physical contact by spring elements form a classic friction-damped mass-spring dynamic system
What is switch bounce (or contact bounce)?
When the switches have moving electrical contacts, it forms a friction-damped mass-spring dynamic system. When the switch changes state, the electrical contact literally bounce against each other, making and breaking the circuit several times (~10 typically) within a few milliseconds even though the switch was only actuated once.
Is switch bounce a good or bad thing?
In many applications, switch bounce is undesirable
What is it called to eliminate switch bounce?
Switch debouncing
There are a number of ways of effectively debouncing a switch, what is the simplest hardware approach?
The simplest hardware approach involves adding a RC low pass filter between the switch and its pull-up resistor and the rest of the circuit.
What is the affect of adding a RC low pass filter between the switch and its pull-up resistor and the rest of the circuit when trying to debounce a switch?
The effect of the low pass filter is to slow down the switch’s transmission from low to high. If the component values are selected so that the rise times are longer than the period between the switch bounces, then only a single switch transition will occur at the output of the circuit
What is a Schmitt trigger inverter used for?
A Schmitt trigger inverter is used to translate the relatively slowly changing voltage at the output of the low-pass filter into a sharp clean transition that is appropriate for the digital logic input that we will connect at Vout
How should the low-pass filter’s resistor and capacitor be chosen for the Schmitt trigger?
The low-pass filter’s resistor and capacitor should be chosen so that the resulting rise time does not reach the Schmitt trigger’s upper voltage threshold when the switch is bouncing. Rather it should reach the threshold only when the switch has finished bouncing and has a stable output.
How do switch manufacturers usually specific bounce time?
Switch manufacturers usually specify the bounce time as the amount of time it takes for the contacts to stop bouncing after the switch changes state, rather than the interval between bounces.
There are several other hardware techniques other than the Schmitt trigger, for dealing with switch bounce, name another very common method.
Several IC manufacturers sell components specifically designed for the sole task of debouncing switches. Examples of switch interface chips include the MAX6816 and the ON14490
Switch bounce can also be dealt with effectively in software, this is a standard approach in systems where a switch is interfaced directly to a microcontroller. Why can this be done?
It is impossible for a human to actuate a switch multiple times within 10ms, its straightforward to distinguish between a legitimate switch transition and bouncing switch contacts in software.
What do potentiometers, thermistors, photo-cells, strain gages and flex sensors have in common?
Their resistance is a function of the measurand
We can’t measure resistance directly with a microcontroller, what must we do?
First we must translate the varying resistance into a varying voltage, which may then be connected to an analogue-to-digital converter and measured
What is one of the simplest ways to create a varying voltage from a varying resistor?
One of the simplest ways to create a varying voltage from a varying resistor is to incorporate it into a voltage divider
Diagram 1: What could you do with the resistor if it would be useful for the lower resistances to correspond to lower voltages?
For cases where it is useful for lower resistances to correspond to lower voltages, connecting the sensor between Vout and ground will give that result.
Diagram 1: What could you do with the resistor if it would be useful for the lower resistances to correspond to higher voltages?
In the case you prefer to have low values of resistance correspond to higher Vout, swap the position of R1 and the sensor R2.
Diagram 2: The diagram shows a simple and effective voltage divider circuit. But, there are a few characteristics that may make them a poor choice in some applications. What are they?
- One issue is that the output is not linear with changes in the sensors resistance. For many circuits, this is fine. In cases where high accuracy is need, the sensor and interface circuit can be calibrated and a curve can be fit to the results to determine the transfer function.
- The other issue with the voltage divider approach is its high output impedance. In the circuit shown in diagram 1, its 10kohms in parallel with Rs. Connecting Vout to any circuit element will effectively ‘load’ the sensor interface circuit and affect the voltage at Vout. Regardless of what we connect to Vout, some current Iout, will flow. Our goal is to minimize this current flow, and thus minimize its effect on Vout, since that’s what we hope to accurately measure.
If we want to cause a known and constant current to flow across the resistor, and measure the resulting voltage drop across the resistor, calculating the resistance, what is required?
This means we must provide a current source. This is a circuit that causes a constant current to flow regardless of what other elements in the circuit are doing.
Why don’t ideal general-purpose current sources exist?
This is because the range of operating voltages would have to be enormous (essentially -infinity to +infinity) to cause a constant flow of current under all conditions, including open circuits and short circuits.
How is resistance measured in modern ohms meters?
Real current sources can be constructed to perform well over limited ranges.
What is a common method for implementing a real current source?
One common method for implementing this approach in practice is the constant current circuit shown in diagram 3.
What is used in diagram 3 to implement a real circuit source?
An op-amp fixes the current flow through R1 as a constant. In the ideal case, the Golden Rules of Op-Amps allows us to assume that the inputs draw no current, and that the output does whatever necessary to hold the voltages at the inputs equal (when connected in a negative feedback configuration, as is case here). With these assumptions, the voltage at the noninverting input will be very close to 1V and thus the voltage at the inverting input will also be held at 1V.
Diagram 3, how will the voltage at the noninverting input be close to 1V and the voltage at the inverting input be held at 1V?
To achieve this, the op-amp’s output will source whatever current is necessary through R1 and Rs to hold the node between them at 1V. The current flowing through the R1 is fixed since the resistance is constant, and the voltage drop is fixed (1V-0V=1V). A current of 1mA flows through R1 (1V/1kohm=1mA) and since the (ideal) inverting input draws no current, all the current must also flow through the sensor Rs.
Diagram 3, Depending on the resistance Rs, the op-amp’s output will range from 1 to 5V, and source 1mA. For values of Rs above 3.9kohm, what will happen?
For values of Rs above 3.9kohm, the op-amp’s output will saturate at 5V, since that’s as high as Vout can go.
Diagram 4 shows what type of relationship between Vout and Rs?
There is a linear relationship between Rs and Vout. This makes it very easy to deduce the circuit’s transfer function and determine the value of the measurand we’re trying to sense.
Give an example of a sensor who’s resistance has some nominal, nonzero baseline value, with a range that deviates from this value by a very small amount.
There are several sensors like this, such as strain gages
What approach can be used if the resistance has some nominal nonzero baseline value, with a range that deviates only a very small amount from this value?
a Wheatstone bridge circuit
Diagram 5 shows a whetstone bridge circuit, what is it comprised of?
It is comprised of two voltage dividers, one formed by R1 and R3 (on the left side) and the other by R2 and Rs (on the right side)
What is the output voltage of the wheatstone bridge circuit?
The output voltage of the wheatstone bridge circuit is the difference between the mid-point of the two voltage dividers
What is the equation for the output voltage of the wheatstone bridge circuit shown in diagram 5?
Vout=V+-V-=+V[Rs/(Rs+R2) - R3/(R3+R1)]
If a wheatstone bridge is said to be ‘balanced’ what does this mean?
It is common (but not required) to choose all resistors so that they have the same nominal value, in this case it is said to be ‘balanced’ when Rs has its nominal value and Vout = 0V. This is also true any time the resistors are chosen so that the output of each side of the bridge is equal under nominal conditions, that is, when R1/R3=R2/Rs. Choosing other resistor value has the effect of altering the effective offset and gain of the bridges differential output.
As Rs deviates from its nominal value in the wheatstone bridge configuration, what shows how much it has changed?
As Rs deviated from its nominal value in the wheatstone bridge configuration, the difference between V+ and V- indicates how much it has changed.
What is a typical range of Vout for the Wheatstone bridge configuration?
The range of Vout is usually small, around +/-25mV.
What is required to do before connecting the Vout to the other circuit elements due to the range of Vout being very small?
It is common to amplify the Vout before connecting it to the other circuit elements, such as an A/D convertor pin on a microcontroller
What type of amplifier are well suited to amplifying Vout from the wheatstone bridge for connection with the rest of the circuit elements?
Instrumentation amplifiers are particularly well suited for amplifying differential signals like the output of the wheatstong bridge.
The resistance of a strain gage is a function of what?
Their resistance is a function of both strain and temperature. They have strong temperature dependencies, dealing with this can often pose serious challenges for the designer.
What is a useful aspect of the wheatstone bridge configuration to combat temperature dependencies?
A very useful feature of the wheatstone bridge configuration is that the resistors can often be arranged so that they’re all the same temperature.
How could the resistors all be arranged for the same temperature in the wheatstone bridge configuration?
You could do this by mounting them all on a single block of aluminium or in close proximity on the same printed circuit board, this will ensure that temperatures of all the elements is essentially the same at all times.
Why does making all the resistors in the Wheatstone bridge configuration the same or similar temperature coefficient and the same temperature, greatly reduce or eliminate the effects of temperature on the output of the circuit?
If all resistors have the same or similar temperature coefficients and they’re all the same temperature, then this greatly reduces or eliminates the effects of temperature on the output of the circuit, since the values for the resistors will drift together with temperature, in lock step.
How can the effective gain of a Wheatstone bridge configuration be increased?
The effective gain of a Wheatstone bridge configuration can be increased by using sensing elements in more than one position within the circuit, and arranging them so that they result in larger differential voltages (the difference between V+ and V-).
What is another way of increasing the effective gain of a Wheatstone bridge configuration?
Replace all elements of the bridge with sensors, and arranging them so that the output voltages are driven further in opposite directions.
What are three cases you may want to use a Wheatstone bridge configuration for?
- The ability to initially balance (or zero) the bridge output enables you to use high gains with a differential measurement
- You are using resistive sensors that have a nonzero nominal resistance and a range that deviates only a small amount from the nominal value
- You can mitigate the impact of temperature effects by ensuring that the temperature of all bridge elements is held uniform.
What is a capacitive sensor?
It is a sensor whose capacitance varies as a function of a physical input signal.
Give six examples of capacitive sensors?
- Proximity sensor
- Displacement sensor
- Computer touch pad
- Wall stud sensors
- Liquid level sensors
- Chemical composition sensors
How do we need to measure capacitance?
Measuring capacitance requires us to take dynamic measurements, where we need to manage signals that vary with time and amplitude.
If a step input is introduced to an RC circuit, what will happen and why?
The output voltage will achieve the value of the input voltage, this is because of the equation Vout=Vin(1-e^(-t/RC))
What is an archetypal RC circuit?
What is the circuit’s time constant τ=RC?
This is the amount of time required for the output voltage to rise to 63.2% of Vin after a step input.
How can we determine the value of Cs, if values Vin and R are known?
Determining Cs involves introducing a step response and determining how much time is required for Vout to reach a given value (τ or 2τ, or any voltage that is convenient for you to measure), and then solving equation Vout=Vin(1-e^(-t/RC)) for Cs.
What is required about the step response method needed to determine the value of Cs?
This approach requires either the use of a comparator to indicate when the voltage threshold is crossed, or else the ability to perform rapid analogue-to-digital conversions. You must be able to accurately time the interval between the step input and the crossing of the threshold voltage.
What can be used well to time and convert from a-to-d in order to determine the values of Cs using the step response method?
Microcontrollers are very good at performing the timing and the a-to-d conversions, but for some values of Cs and R, very short timing intervals may be required, making this approach potentially problematic.
What is another way of measuring capacitance?
Another common method for measuring capacitance is with an oscillator circuit that relies on the combination of resistors and capacitors to determine the frequency of oscillation.
How is the baseline frequency of an oscillator circuit determined?
The baseline frequency of the oscillator circuit (without Cs connected) is determined by the values of R2 and C1, and will be approximately f=1/(2.2R2C1) (R1 should be 10xR2)
When Cs is added in parallel with C1, what happens to the total value of capacitance, and the value of frequency, why?
When Cs is added in parallel with C1, the total capacitance value increases because the equivalent capacitance of multiple capacitance in parallel is the sum of their individual capacitance. The frequency correspondingly decreases.
How much does the frequency decrease when Cs is added in parallel with C1?
The amount of the decrease depends on the value of the added capacitance Cs, and thus provides a means of measuring Cs.
What is the advantage of the method of adding Cs in parallel with C1 to provide a means of measuring Cs?
The advantage of this method is that it is entirely frequency based, only the timing of the output square wave’s rising and falling edges is affected by changes to the capacitance, not the amplitude.
How can we take the approach of treating capacitors as frequency-dependent resistors one step further?
We can take the approach of treating capacitors as frequency-dependent resistors one step further by using the Wheatstone bridge circuits and replacing the resistors with capacitors.
For the capacitive Wheatstone bridge, the left and right side voltage dividers that make up the circuit are both what?
For the capacitive Wheatstone bridge, the left and right side voltage dividers that make up the circuit are both frequency dependent.
What is the difference between the capacitive Wheatstone bridge behaviour and the resistive Wheatstone bridge behaviour?
The behaviour of the capacitive Wheatstone bridge is similar to the behaviour of the resistive Wheatstone bridge, but the excitation voltage must be time varying (e.g. a sine wave) so that the dynamic response to the |RC networks on both sides of the bridge can be differentially compared.
The capacitive Wheatstone bridge approach shares the same advantages for measuring capacitance as for resistance, what are these?
- The bridge’s output may be initially balanced (or zeroed) removing any offset. Changes in the variable capacitance can then be measured as deviations away from the zero point, rather than having to account for the initial offset.
- Capacitive Wheatstone bridge allow for a direct measurement of a capacitor’s deviation from a nominal value.
- If all the elements comprising a capacitive Wheatstone bridge are at the same temperature and have the same characteristics with respect to temperature, this mitigates or eliminates the effects of changes in temperature.
Light sensors are among the most versatile and commonly used sensors in mechatronics. What are light sensors used for?
In addition to enabling us to detect the amount of incident light, they can be used as building blocks and configured in a number ways that allow for the determination of many other measurands.
What is the most fundamental light sensor?
Photo-diode.
What is a photo-diode?
A photo-diode is a semi-conductor device that is composed of a semiconductor junction housed in an optically clear package that allows incident light to fall directly on the junction.
How is the optically clear package of a photo-diode designed?
This is usually done by encapsulating the semiconductor in a material that is optically clear at the desired wavelength(s) of light such as polycarbonate. The semiconductor junction is sensitive to light within a relatively narrow range of wavelengths. When exposed to light within its range sensitivity, each photon liberates an electron, causing a small amount of electrical current to flow.
What is the resulting current called in a photo-diode once an photon has liberated an electron?
Photo-current.
What does the photo-current in a photo-diode correspond to?
The photo-current corresponds to the irradiance of the incident light.
What is irradiance?
The amount of radiant flux per unit area.
To obtain the desired response from the photo-diode, what must be done?
To obtain the desired response from the photo-diode, it must be either zero biased or reverse biased.
What is zero biased and reverse biased?
Zero biased is then a voltage potential of 0V is held across the anode and cathode. Reverse biased, is where the cathode is held at a higher potential than the anode, usually 1V or so.
What is a common way to achieve either zero biased or reverse biased for a photo-diode?
A common means of achieving either of these schemes is to use a trans-resistive circuit to maintain a constant voltage across a reversed photo-diode, and against resistor Rg to translate the photo-current into a voltage.
Diagram 6: shows an interface circuit for a photo-diode, what do R1 and R2 in this circuit form?
R1 and R2 form a voltage divider with an output of 1V, which is connected to the op-amp’s noninverting input.
Diagram 6: What does the trans-resistive circuit connect to?
The trans-resistive circuit connects the output to the inverting input across Rg, establishing negative feedback, so we can apply Golden Rule 2 and say that the voltage at the inverting input is held at the same potential as the non-inverting input (1V). This maintains the voltage difference of -1V across the photodiode, which is reverse-biased. Under these conditions, (and invoking Golden Rule 1) all of the photo-current flowing into the photodiode comes from the op-amp’s output, through the gain resistor.
What does the wavelength of light that a photo-diode can detect depend on?
The wavelength of light that a photo-diode can detect depends on the semiconductor used to make the manufacturing process.
What are the most common type of photo diode?
Silicon photo-diode
Which end of the spectrum are silicon photo diodes sensitive to electromagnetic radiation?
Silicon photo diodes are sensitive to electromagnetic radiation at the low end of the near IR spectrum, with a peak in the region near λ=1μm
What is the dynamic range of a photo-diode like?
Photo-diodes have a very large dynamic range.
What size is the output signal of a photo diode? What size is the range ?
The output signal of a photo-diode is very small, with a very large range.
Why is it necessary to condition the output signal of a photodiode into a voltage with a usable range of values?
With such small signal levels, considerable care is necessary to condition the output signal of a photodiode into a voltage with a usable range of values.
What is special about a photodiode in a very dark environment?
Even in a very dark environment that have essentially no IR light within the sensors range of sensitivity, a small amount of current will still flow. (Dark Current)
What is dark current?
Photodiodes still have a small amount of current flowing when the sensor is in a very dark environment with no IR light within the sensors range of sensitivity.
What is dark current caused by?
Dark current is the result of thermal excitation of the diode’s semiconductor charge carriers.
As dark current is a thermal effect, what does this mean?
This means that dark current scales with temperature.
What contributes error to a measurement from the photodiode without any compensation?
The dark current is usually small compared with the photo-current, however as the temperature of the device rises, the dark current increases substantially, contributing error to the measurement.
What is an advantage of a photodiode?
The response time is very quick, meaning they can be used in high speed applications or applications where dynamic range is needed.
Name a suitable application of a photodiode due to its quick response time.
Photodiodes are commonly used in high speed applications such as fibre optic data communications.
What sensing elements are usually used to detect IR remote control signals for most consumer electronic products?
Photodiodes.
What is essential for a photodiode to do while being used to detect IR remote control signals?
In this application, they must reliably produce usable signal levels (and not saturate) when the environment is drenched in direct sunlight during the day (which has very high levels of IR light), as well as in completely dark rooms (where there is no ambient IR light).
What else is a key requirement for a photodiode to be used as a consumer electronic product?
Because it is a consumer electronic application, extremely low cost is a strict requirement too.
What is offered as another choice when the low sensitivity characteristic of photo diodes is problematic?
Photo-transistors
How are photo-transistors similar to photo-diodes?
Photo-transistors function similar to photo-diodes in that they allow a current to flow (a collector-emitter current, in this case) related to the amount of incident light.
What is the difference between photo-transistors and photo-diodes?
Photo-transistors provide much higher output currents than photo-diodes, albeit over a much narrower dynamic range with much slower response times.
Photo-transistors are BJT’s, what does this mean?
Photo-transistors are bipolar junction transistors, and their semiconductor base region is optically exposed.
Instead of connecting a wire to the base and externally causing base current to flow and controlling the flow of collector current, what do the photo-transistors do?
The base current in a photo-transistor results from photons striking the base region with sufficient energy to liberate electrons. This base current is then multiplied by the gain of the BJT, resulting in a collector current in the milliamp range, rather than the microamp characteristics of photo-diodes.
What is the output current of a photo-transistor referred to?
The collector-current.
What is a key difference between a photo-transistor and a photo-diode?
The dynamic range. The photo-transistor dynamic range is tiny compared with the photo-diode.
What is the output of the photo-transistors like?
The output is linear for each individual part, regardless of what its gain is, there can be wide part-to-part variation. They can also have nonlinear output.
If we want to make the most of a photo-transistors linear output characteristics, what must we do?
We must maintain a constant voltage across the collector and emitter of about 2.5V or more.
For photo-transistors, there are two approaches to interface circuits that are most commonly used. In cases where linearity of the output is not a requirement, what can be used?
In cases where linearity of the output is not a requirement, a simple pull-up pull-down resistor will suffice.
What is the benefit of using a simple pull-up pull-down resistor with a photo-transistor?
This approach is quick and simple, and yields an output that varies with incident light, however it is nonlinear. It also links response time to gain, since the value of the resistor not only sets the gain but also determines the time required to charge and discharge any stray capacitance (which is often significant).
Diagram 7: For sourcing configuration, when does Vout have its minimum and when does it increase?
Vout has its minimum when value (close to 0V) when no light is incident on the photo-transistor Q1, and increasing light causes Vout to increase.
Diagram 7: For sinking configuration, when does Vout have its maximum and when does it decrease?
Vout has its maximum value when no light is present (near 5V), and increasing light levels causes Vout to decrease.
The nonlinearity of the output of the photo-transistor is a result of what?
The nonlinearity is a result of not holding a constant voltage drop across the photo-transistor.
Diagram 7; what is the amount of current flowing determined by?
For both configurations, Vout is essentially the output of a voltage divider, and is determined by the voltage drop across the gain resistor R1, which Ohms law tells us is V=IR. In this circuit, I is the collector current from the photo-transistor, and R is the resistor value. The amount of current flowing is determined by the light level incident to the sensor, which determines the voltage drop across the resistor, which in turn determines the voltage drop across the photo-transistor’s collector and emitter which affects the collector current.
Is a nonlinear output of a photo-transistor useful?
Yes, such as detecting whether light is on or off, or whether an object is in close proximity to a sensor, or whether a beam of IR light is bouncing off a white object or a black object.
For applications which involve measurements of irradiance, what is easier to work with rather than the nonlinear response of the sourcing or sinking photo-transistor?
For applications which involve measurements of irradiance, an interface circuit with a linear output characteristic is much easier to work with than the nonlinear response of a sourcing or sinking configuration.
For applications which involve measurements of irradiance, an interface circuit with a linear output characteristic is easier to work with, what is a good option of circuit for this?
A trans resistive circuit is a good option.
Diagram 8: For a trans resistive circuit, what is the non-inverting input of the op-amp held at with a voltage divider?
2.5V
Diagram 8, since the non-inverting input of the op-amp is held at 2.5 V with a voltage divider, what can be said for the voltage at the inverting input?
Golden rule 2 allows us to assume that the voltage at the inverting input will also be 2.5V, and this fixes the Vce across the photo-transistor at 5V-2.5V=2.5V.
What does Golden Rule 1 allow us to assume about the configuration diagram 8?
Golden rule 1 allows us to assume that all the photo-current flows across the Rg to the op-amp’s output, Vout.
How does the trans-resistive circuit perform a linear current to voltage transform?
The trans-resistive circuit performs a linear current to voltage transform by multiplying the collector current by the gain resistor Rg. Additionally, the trans-resistive circuit maintains a constant voltage across the photo-transistor’s collector and emitter, so it eliminates the response time effects of charging and discharging any stray capacitance.
What improves the photo-transistors dynamic response?
There are no changes in voltage, there are no contributions to the sensors output rise and fall times due to stray capacitance.
Diagram 9: The output voltage of the trans-resistive interface shown will be what when there is no light incident on the sensor?
2.5V.
Diagram 9: What causes the linearly decreasing trend as the light level increases?
This is a result of op-amp’s output sinking current from the photo-transistor.
Diagram 9: Is the photo-transistor sinking current or sourcing current from the trans-resistive circuit?
This is from the photo-transistor sinking current from the trans-resistive circuit.
Diagram 9: What would be the circuit if the photo-transistor was sourcing current from the trans-resistive circuit?
The characteristics would be flipped by connecting the photo-transistors emitter to the ground and the collector to the op-amp’s inverting input. Vce would be 2.5V in this new configuration. If the same Rg is kept from before, the resulting output voltage will be 2.5V when there’s no incident light, and the output voltage will increase with the increasing light levels.
Photo-transistors are often paired in a single package with what?
Photo-transistors are often paired in a single package with IR LEDs that emit light at wave lengths matched to the photo-transistors sensitivity, and packaged so they can be applied very conveniently for a few specific applications.
What is a very common arrangement of emitter and detector pair?
One very common arrangement of emitter and detector pairs are opto-interrupters in which an emitter shines directly towards a detector. When an object passes in front of the detector and interrupts the beam of light, the output of the detector changes.
What is an opto-interrupter?
This is an emitter and detector pair in which an emitter shines directly towards a detector. When an object passes in front of the detector and interrupts the beam of light, the output of the detector changes
Another common configuration for of an emitter and detector pair is a reflective optical sensor, what is this?
A reflective optical sensor is an emitter and detector arranged side by side. This configuration relies on light from the emitter reflecting off a nearby object and reaching the detector. If the reflectance characteristics of the object are known, the amount of reflected light can be used to measure relatively short distances accurately (a few mm to a few cm), or it can be used to simply detect the presence or absence of an object.
Some applications require detection of light at wavelengths outside the near IR spectrum, which type of sensors are and aren’t appropriate for this?
Standard photo-diodes and photo-transistors may not have the appropriate spectral response. One common and inexpensive sensor that is sensitive to light within the visible spectrum is the photo-cell.
What are photo-cells otherwise known as?
Photo-cells are also commonly known as photo-detectors and photo-resistors.
What are photo-cells?
Photo-cells are sensors whose resistance varies with the amount of incident light across a range of wavelengths similar to those detectable by the human eye.
What is the mechanism of action for photo-cells?
The mechanism of action for photo-cells is similar to that of photo-diodes in that photons with sufficient energy that strike the sensing surface liberating electrons.
For photo-cells, changes in the electron’s energy alter what?
For photo-cells, changes in the electron’s energy alter the material’s ability to conduct electricity-that is, its resistance.
What is one of the most common and inexpensive types of photo-cell sensors?
CdS photo-cell (cadmium sulphide)
What is part-to-part variation like for photo-cells?
Part-to-part variation is wide for photo-cells, and data sheets usually specify absolute resistance vs light intensity with wide tolerances.
Photo-cells; the relative response of each individual sensor is well characterised with respect to what?
The relative response of each individual sensor is well characterised with respect to its response at reference light levels.
Why is a photo-cell straightforward to interface to?
Interfacing to a photo-cell is straightforward given that its resistance varies with light levels.
What is a weakness with photo-cells?
Photo-cell’s response times are very slow; a rise time of 50ms and a fall time of 25ms is typical. This compares very badly with photo-diodes and photo-cells who have a much quicker response time.
The poor response time of a photo-cells means what?
This makes photo-cells unsuitable for medium to high-speed applications.
Give some examples of devices that use photo-cells.
Photodiodes are ideal for lower speed applications, examples include toys, street lights that automatically switch on at dusk/off at dawn, alarm systems and light meters for photography.
How is strain defined?
Strain is defined as the deformation of a material subjected to mechanical stresses from loading.
How is strain expressed?
Strain is expressed as the ratio of the change of length of a material under stress over the original length of the unstressed material.
Measurements of strain can be taken directly to determine how materials respond to loading. They can also be used as the basis of measurement of secondary quantities such as what?
They can also be used as the basis of measurement of secondary quantities such as deflection and force.
Why can measuring strain accurately be very difficult?
Accurately measuring strain can be quite challenging because in most cases it is a very small quantity.
What device can be used to measure such small dimensional changes of materials(strain)?
Strain gages
What are strain gages?
These are sensors whose resistances changes as a function of the strain they undergo.
What happens when a strain gage is rigidly attached to a base material?
The strain experienced by the base material is imparted to the strain gage.
How can a strain gage be rigidly attached to a base material?
Typically with an adhesive such as cyanoacrylate.
What needs to be true for the deformation of the base material and the gage to be the same?
The coupling between the gage and the base material must be secure.
Strain gages are conceptually simple, and consist of what?
Strain gages consist of little more than sensing elements whose resistance changes slightly when deformed along the sensing axis.
For a material with resistivity ρ, length L, area A, what is the resistance R given by?
R=ρL/A
All strain gages take advantage of two fundamental properties of materials that affect the overall resistance, what are they?
They take advantage of two fundamental properties of materials, changes to length L and area A that result from applying loads to deforming a material.
Strain gages made from semiconductors exhibit significant changes to what?
Strain gages made from semiconductors exhibit significant changes to their resistivity ρ when subjected to strain.
There are two-dimensional effects from strain that contribute to the changes in resistance, what are they?
- Subjecting a material to tension causes it to elongate, and subjecting it to compression causes it to shorten. We can see from the Resistance expression that elongating a material will increase its resistance and shortening it will decrease its resistance.
- When a material elongates, its cross-sectional area decreases. When it shortens, its cross-sectional area increases (there are exceptions to this, but these materials are very rare and aren’t used to make strain gages).
What is Poisson’s Ratio?
The amount that the cross-sectional area changes for a given change of length.
For Poisson’s ratio, what direction strain is the εtransverse ?
Normal to the direction of loading
For Poisson’s ratio, what direction strain is the εaxial?
This is the strain in the direction of loading.
What is the Poisson’s ratio equation?
v=-εtransverse/εaxial
Almost all materials have a positive Poisson’s ratio, and range from what?
They range from near 0 (cork) to 0.5 (rubber)
What can be said for all materials with a positive Poisson’s ratio?
For all materials with a positive Poisson’s ratio, the resistance R of a given length under tension increases because L increases and A decreases, and under compression, R decreases because L decreases and A increases.
How does the change of the strain gage’s resistance relate to the strain of the base material? (Equation)
GF=(dR/R)/ε
What does GF stand for?
GF is the strain gage’s gage factor.
How is the GF determined for a strain gage?
The GF is determined by the properties of the material used to make the strain gage.
What is a typical range for metal foil strain gages?
Metal foil strain gages have constant gage factors that range from 1 to 2.
What is a typical range for semiconductor strain gages?
Semiconductor strain gages have gage factors that range from 50-100, but the value can vary depending on the strain.
Diagram 10: the left side shows dark regions of a metal foil strain gage, what are these?
The dark regions are a conductive strain-sensing material.
Diagram 10: the left side shows transparent regions of a metal foil strain gage, what are these?
The transparent regions are an insulating substrate.
What else can affect a strain gage other than strain?
Temperature effects can dominate those of strain.
Why do strain gages get affected by temperature?
This is because of thermal expansion or contraction of the base material relative to the thermal expansion of the strain gage material.
How are some metal foil strain gages made to reduce the impact of temperature on the reading?
Many metal foil strain gages are made with thermal expansion coefficients that match a particular base material and mostly cancel. For example, a strain gage designed for use with aluminium will compensate for the dimensional changes that occur as the temperature of the base aluminium material changes. However, if the same gage was mounted instead to a sample of stainless steel, the temperature effects would be pronounced.
What makes a good choice of interface circuit for a strain gage?
Wheatstone bridge configuration, this is because it works best at measuring very small deviations in resistance from some relatively large nominal value, and allows for the compensation of temperature effects.
What are load cells?
Load cells are sensors that measure force or, stated another way, an applied load.
How can a load cell be made?
To make a load cell, a material that deforms under load (anything that exhibits strain) is fitted with strain gages. When the materials properties, geometry and response to loading is known, the force that cause the strain may be determined.
How are load cells seen integrated into mechatronic devices?
Load cells are rarely seen integrated into mechatronic devices outside of laboratory settings.
What sort of variation is there in load cells?
Load cells are available in a very wide variety of configurations, some are capable of measuring forces as low as a few grams, while others can measure millions of kilograms.
Load cells are often highly precise sensors, how?
The normal course of action is to calibrate each one.
How expensive are load cells?
As load cells are usually highly precise, the resulting cost for each load cell can be substantial.
What do designers of load cells strive for in their designs?
Designers of load cells strive for designs that accurately measure forces in a desired direction (or axis), whilst optimising linearity, sensitivity, temperature stability, and rejecting loads in other directions (off-axis loading).
What are critical to achieving design goals of a load cell?
The shape of the strained element, the way forces are applied to it, and the location, number, and type of strain gages used are all critical to achieving the design goals.
The most straightforward load cell designs are those that do what?
The most straightforward load cell designs are those that couple strain gages to a simple strain element such as a disk, cylinder, or tube to measure axial compressive or tensile loads. Disk, washer and canister load cells are examples of this approach.
Larger disk and washer load cells have room for, and make use of what? Whilst smaller examples sometimes use what?
Larger disk and washer load cells have room for, and make use of metal foil strain gages. Whilst smaller examples sometimes use semiconductor strain gages because of their compact size and high gage factors.
What is a more complex approach to load cells rather than disk, washer etc?
A more complex approach is to apply a bending force to the load cell’s mechanical element and measure the resulting deflection. E.g beam, cantilever and parallelogram load cells
Name examples of the more complex approach to load cells.
Beam, cantilever and parallelogram load cells are examples of the application of bending force to a load cell’s mechanical element, and measuring the resulting deflection.
What does the more complex approach to load cells allow?
The more complex approach to load cells allows for incorporation of several strain gages- some under tension and others in compression.
How can a Wheatstone bridge help a load cell?
A full Wheatstone bridge can be built into the load cell, bringing the benefits of temperature compensation as well as increasing the sensitivity.
The most complex load cells make use of what measurements?
The most complex load cells make use of measurements of shear strain to measure the applied force. For example shear beam load cells.
What are shear beam load cells?
They are load cells that make use of measurements of shear strain to measure the applied force
How are shear beam load cells (strain gages) mounted (what angle)?
The strain gages are mounted at 45 degree angles to the neutral axis of a beam, half the gage above and half below the neutral axis. In this arrangement, strain resulting from bending is rejected, and only the shear strain registers.
What is the advantage of mounting a shear beam load cell at 45 degrees, allowing bending to be rejected and only shear strain to register?
The advantage of this is that shear strain is not dependent on the location of the loading, so how the load cell is mounted becomes less critical. Load cells using this approach are capable of achieving high load capacities in compact packages with low compliance- these are all highly desirable characteristics.
What are thermocouples?
Thermocouples are temperature sensors that make use of the thermoelectric effect.
What is the thermoelectric effect otherwise known as?
the Seebeck effect
When dos a thermoelectric effect occur?
Thermoelectric effect occurs when a thermal gradient is established along a conductive material. This results in the development of a small voltage potential between any two points which are at different temperatures along the conductor.
Rather than directly measuring the voltage between a hot point and a cold point on a single conductor, what is much more common in practice?
It is much more common in practice to join to dissimilar conductors together (usually welded) to form a thermocouple. The junction is sometimes left exposed, or sheathed in a housing to create a thermocouple probe.
Selecting two conductors that have very different thermoelectric characteristics means what for a thermocouple?
Selecting two conductors that have very different thermoelectric characteristics results in more sensitive thermocouples.
How are the different types of thermocouples labelled?
There are several standard combinations of conductors that are commonly used to make thermocouples, and each has been assigned a letter to indicate its construction.
What are type K thermocouples made of?
Made with chromel and alumel conductors
What are type J thermocouples made of?
Made with iron and constantan
What do the different conductor material properties result in for thermocouples?
The conductors used have different material properties and result in different thermocouple sensitivities, so special applications and requirements may call for the use of particular type of thermocouple.
How accurate are uncalibrated thermocouples?
Uncalibrated thermocouples are typically accurate to within a relatively unimpressive +/- 2 degrees, but they can be calibrated by the manufacturer to be far more accurate if needed.
What is one method of measuring temperature with a thermocouple?
One method of measuring temperature with thermocouples involves the use of a reference temperature. Two thermocouple junctions are connected together to form a loop: one junction is held at a reference temperature, such as an ice bath at 0degrees and the other junction is exposed to an unknown temperature to be the measurand. If a voltmeter is introduced at any point in between the junctions, the voltage measurand corresponds to the difference in temperature between the two junctions.
Using the reference temperature approach is straightforward, but in many circumstances its inconvenient. What is another method that can be used for a thermocouple to measure temperature?
Another method is to connect the leads of a single thermocouple at a point whose temperature can be measured using another type of sensor, and then compensate for the difference in temperature of that point to a 0 degrees reference. This is called cold reference compensation, and is the scheme used by thermocouple meters, readouts and data loggers.
What is cold reference compensation?
A method for measuring temperature with a thermocouple is to connect the leads of a single thermocouple at a point whose temperature can be measured using another type of sensor, and then compensate for the difference in temperature of that point to a 0 degrees reference.
What is a thermistor?
A thermistor is a device whose resistance changes as a function of temperature.
Nearly all devices (including resistors) have some temperature dependency, and this is usually an undesirable characteristic. Except for when?
Except for when making a thermistor, as the temperature dependency is exploited to measure temperature itself.
What are thermistors made of?
Thermistors are made of ceramic and polymer materials.
Why are thermistors different from resistive temperature detectors?
Resistive temperature detectors (RTDs) are made from conductive metals such as platinum and they typically have lower sensitivity than thermistors, and a more linear response.
Functionally, thermistors are divided into two major categories, depending on what?
Functionally, thermistors are divided into two major categories, depending on the slope of the relationship between temperature and resistance.
What are the two categories that thermistors are divided into, based on the slope of the relationship between temperature and resistance?
The first category is negative temperature coefficient (NTC) thermistors. The second category is positive temperature coefficient (PTC) thermistors.
What are NTC thermistors?
Negative temperature coefficient (NTC) thermistors have a resistance that decreases with increasing temperature.
A high degree of nonlinearity is readily apparent for NTC thermistor relationships, but what is required for conversions between resistance and temperature?
Conversions between resistance and temperature require calculation from a transfer function or lookup table supplied by the manufacturer, or a calibration curve generated by the user.
What are PTC thermistors?
Positive temperature coefficient (PTC) thermistors. They are not commonly used to measure temperature. Their resistance is characterised by a relatively flat, stable region at lower temperatures, a transition point and then a region of high resistance at higher temperatures. This behaviour lends itself well to applications in current limiting and overload protection.
What are PTC’s very useful for?
PTC thermistors are very useful as resettable fuses.
How are PTCs used in resettable fuses?
In this type of application, they are placed in series with an electrical load, such as a power supply or a motor. When the current is flowing through the load is within the allowable limits, the internal heating from power dissipation within the thermistor is low, and the resistance of the PTC thermistor is low. However, when the current exceeds the allowable range, the PTC thermistor is internally heated to a much higher temperature, and the resistance increases significantly. When the current increases due to a fault condition, this causes increased internal heating within the PTC thermistor, which in turn causes the resistance of the device to increase greatly- usually by several orders of magnitude. The result is a large reduction in the amount of current allowed to flow.
How long does it take PTC thermistors to respond to sudden increases in current above allowable limits and why?
Since the process for PTC thermistors is governed by heat transfer, PTC thermistors take several milliseconds to respond to sudden increases in current above allowable limits.
What is the advantage of using PTC thermistors as fuses?
When used as fuses, PTCs have the tremendous advantage of automatically resetting after they cool off, their resistance returns to a much lower value and the device may be used again.
Other than fuses, what else are PTC thermistors commonly used for?
PTC thermistors are also commonly used as heating elements. Because their resistance increases as their temperature increases, the amount of current and hence the amount of heat generated, is inherently self-regulating, and requires little or no additional control effort.
What is another example (other than thermistor) of a devices temperature characteristic being exploited to create a temperature sensor?
A semiconductor temperature sensor
What is a diode thermometer otherwise known as?
A semiconductor temperature sensor.
A common example of a semiconductor temperature sensor uses changes in what, which is a function of temperature?
A common example of a semiconductor temperature sensor uses changes in Vbe (the voltage drop between a BJT’s base and emitter terminals), which is a function of temperature. This is very straightforward to use.
Give some examples of where measurements of magnetic field can be useful in applications?
Examples include the determination of position, proximity or orientation, and the use of magnetic media for data storage. Once a magnetic field has been established or identified, there are several methods for detecting and measuring it.
What is Hall Effect?
The Hall effect is a phenomenon in which a magnetic field induces a measurable voltage, (called the Hall voltage) across a conductor through which a current is flowing.
What does diagram 11 show?
The current flowing across the conductor is warped and caused to deviate from a straight path across the conductor by the magnetic field with at least a component perpendicular to the current flow, and this displacement results in the development of the Hall voltage.
The Hall effect occurs for what?
The Hall effect occurs for currents flowing across metallic conductors and semiconductors.
What does the magnitude of the Hall voltage depend on?
The magnitude of the Hall voltage depends on the magnetic flux and the properties of the conductive material.
Hall effect sensors are most appropriate for measuring strong magnetic fields, why?
This is because Hall effect sensors have a range of hundreds of thousands of gauss.
Are Hall effect sensors available with analogue or digital outputs?
Hall effect sensors are available with analogue or digital outputs.
Why is it common to integrate a Hall sensor and amplifier together into a single package?
This is because the Hall effect results in small voltages.
What is the result of a Hall sensor and an amplifier being integrated together into one single package called?
This is known as a linear output Hall effect transducer, sometimes called a LOHET
What is a LOHET?
linear output Hall effect transducer. It is a Hall effect sensor and amplifier integrated together in a single package.
Linear Hall sensors are what?
Bipolar, this means that they are capable of sensing both north and south magnetic polarities, and their output is proportional to the magnetic flux detected.
What else does the integrated amplifier in the Hall sensor help with?
The integrated amplifier also helps to minimize the effects of nearby noise sources by buffering and amplifying the small signals from the Hall element immediately adjacent to the sensing material.
Why is magnetic field strength highly non-linear with distance?
Since the magnetic field strength varies as 1/d^3, where d is distance from magnetic pole.
As magnetic field strength is highly non-linear with distance, what is an implication of this for a Hall effect sensor?
One implication of this is that it isn’t practical to use the simple arrangement between a sensor and a magnet to measure distances greater than a few millimetres, the magnetic strength very quickly decreases to immeasurable levels. For this reason there are other arrangements available.
What are Linear Hall sensors also useful for?
Linear Hall sensors are also useful for measuring angles and continuous rotation.
What are applications of Linear Hall sensors limited by?
Applications of Linear Hall sensors are limited by the magnetic field strength.
Hall sensors are also available in digital outputs, instead of coupling a Hall sensor with a linear amplifier as done in a LOHET, the sensing element in a digital Hall switch is amplified and then coupled to what to create a digital output?
The sensing element in a digital Hall switch is amplified and then couples to a Schmitt trigger to create the digital output and add hysteresis.
Hall switches are designed to take on how many digital states?
Hall switches are designed to take on one digital state (usually off) when the applied magnetic flux density is below a threshold level, and switch to the opposite digital state (usually on) when the flux density is above another threshold value.
Because the Schmitt trigger of the Hall switch implements hysteresis, the transition back to the original state will occur at what?
Because the Schmitt trigger of the Hall switch implements hysteresis, the transition back to the original state will occur at a lower threshold.
What is the purpose of hysteresis in the Hall switch?
The hysteresis serves its usual purpose in this application; it prevents multiple crossings of a single threshold level from causing multiple transitions of the output state.
Digital Hall sensors can be programmed to be unipolar, what does this mean?
Digital Hall sensors can be programmed to be unipolar, so that a single polarity and magnitude of magnetic flux density is required to trigger a change in the output state.
Digital Hall sensors can be programmed to be bipolar, what does this mean?
Digital Hall sensors can be programmed to be bipolar, where one magnetic polarity and magnitude of magnetic flux density is required to cause a change of state, and the other magnetic polarity at another magnitude of flux density is required to change it back.
What are bipolar digital Hall sensors also referred to as?
Bipolar, digital Hall sensors are also referred to as ‘latched’ since they require the opposite magnetic pole to change states. The field that caused them to take on the current state may go all the way to zero and the switch will retain the state.
What is one of the simplest magnetic sensors called?
Reed switches
What do Reed switches consist of?
Reed switches consist of a set of contacts, at least one of which is ferromagnetic.
What does ferromagnetic mean?
This means having a high susceptibility to magnetization, the strength of which depends on that of the applied magnetizing field.
How does the Reed switch work?
In the presence of a strong magnetic field (which can be supplied by either a permanent magnet or an electromagnet), the contacts are either drawn together to close a normally open switch, or pulled apart to open a switch that is normally closed.
What happens to a Reed switch when the magnetic field is removed?
When the magnetic field is removed, the contacts revert to their normal inactivated state.
As the name suggests, Reed switches have contacts that are shaped like reeds, long and thin, what does this mean?
This means that the contacts impart the spring tension that allows the contacts to assume their normal, inactive state.
A Reed switch is a simple way to detect what?
A Reed sensors is a simple way to detect the presence or absence of a strong magnetic field with minimal expense.
What is the expense of a Reed switch like?
When low cost and simplicity are strict requirements, reed switches are an excellent choice.
Name some applications that use Reed switches?
Reed switches are used in a very large number of applications, including automobiles, home applications, and toys. One familiar example is a bicycle computer, which senses wheel rotation and sometimes pedal rotation with reed switches.
How do reed switches work on a bicycle computer?
A small magnet is mounted to the bikes front wheel, and a reed switch is mounted on the front fork so that the magnet passes very near the switch as the wheel rotates. Each time the magnet passes the reed switch, the contacts close and the cycle computer calculates quantities such as speed and distance.
Proximity detection is usually performed in which domain? digital or analogue
Proximity detection is usually performed in the digital domain, with a sensor that detects when an object is within the range of the sensor.
Proximity sensors give what type of answer to the question of whether an object is within a range of a sensor?
Proximity sensors give yes or no answer to the question of whether an object is within a range of a sensor.
What is the range of proximity sensors like?
Usually the range of proximity sensors is small, rarely more than a few cm.
What is a common application of proximity sensors?
A common application of proximity sensing is when a motorised stage in a piece of automated equipment reaches the home, or zero, position.
Many other sensors use proximity sensing, name some?
Contact switches, microswitches, opto-interrupter emitter-detector pairs, reflective emitter-detector pairs, Hall switches, Magento resistive switches, and reed switches.
What do Capacitive proximity sensors do?
Capacitive proximity sensors detect changes in the capacitance between two plates of a capacitor when an object moves close enough to measurably disturb the electric field.
Where have Capacitive proximity sensors become popular?
Capacitive proximity sensors have also become popular in user interfaces. Capacitive touch sensors capable of detecting the presence of one or more fingers are now commonplace in a wide variety of applications, such as computer touch pads, mobile phones etc.
What is the difference between Inductive Proximity sensors and capacitive proximity sensors?
Inductive Proximity sensors use a similar approach to that of capacitive proximity sensors except that they use an oscillating signal in a coil to establish an electromagnetic field rather than an electric field.
What do Inductive Proximity Sensors do?
Inductive proximity sensors detect when a metallic object has moved close enough to measurably disturb a magnetic field.
Do non-metallic objects disturb the Inductive proximity sensors?
Nonmagnetic objects do not disturb the field, and are not detected by inductive sensors.
What do ultrasonic proximity detectors do?
Ultrasonic proximity detectors emit high frequency sound waves and measure the amount of time required for reflections to return.
What does a shorter return time mean for ultrasonic proximity detectors?
Shorter return time indicates that an object has moved in a close proximity.
What is the job of a position sensor?
A position sensor must determine the distance from the sensor to a location or the distance that something has moved.
What are position sensors also known as?
Displacement sensors
What is one of the most simple position sensors?
Potentiometer
What is a potentiometer?
A pot is a three terminal device with a fixed resistance between two terminals, and the third terminal is connected to a moveable wiper that contacts the resistance somewhere midspan. This creates two adjustable resistances that depend on the relative position of the wiper between the two ends of the resistance
How can the position of the wiper in the potentiometer be determined?
The position of the wiper in the potentiometer can be determined by measuring either of the variable resistances.
Are potentiometers high performing? Why?
Potentiometers are not particularly high performance position measurement sensors, as they add frictional loads to the system being measured, and are subject to temperature effects, vibration and long term drift resulting from wear and tear.
Despite their low performance abilities, what is positive about potentiometers?
They are very inexpensive, and readily available in a wide variety of packaging options and resistance values,
Name some applications for potentiometers?
Rotary or linear pots are used as user input devices on stereo equipment. Potentiometer based joysticks are used in video game console user interfaces.
What is another variation of the potentiometer?
Another interesting and useful variant is the cable-driven linear displacement transducer, more commonly known as string pot.
What is string pot comprised of?
Cable-driven linear displacement transducer (string pot) is comprised of a precision multiturn rotary potentiometer whose input shaft is attached to a constant tension torsional spring and a spool holding a long string.
What is the resistance of the string pot proportional to?
The resistance of the string pot output is proportional to the length of string that has been pulled from the spool.
When used to measure position, potentiometers are referred to as ‘absolute position sensors’, what does this mean?
Absolute position sensors; the output corresponds to the location of the wiper and does not require a separate position measurement to be used as a relative reference or serve as the origin . This contrasts with relative position sensors, which measure incremental motion away from an initial position .
Name a very popular type of position sensor that is available in both relative and absolute versions.
An Optical Encoder
What is the most common commercially available approach for optical encoders?
Light from an LED is directed towards an optically opaque material such as a thin stainless steel disc that has a series of holes in it. When the disc rotates relative to the light source, a photo-diode on the opposite side detects transitions in the levels of light that occur as the light is alternately blocked or allowed to pass through a hole. Counting the transitions enables the user to track any rotation that occurs.
Why are optical encoders sometimes called incremental encoders?
Because of the incremental nature of the measurements, the sensors often get called incremental encoders.
Why can a high resolution be achieved from the approach for optical encoders?
Very high resolutions are possible because the holes can be made to be very tiny and closely packed through the use of manufacturing techniques such as chemical etching or laser cutting. Resolutions of better than 500 counts per turn are readily available.
What type of optical encoder can be used to determine the direction of motion rotation?
Quadrature encoding can be used to determine the direction of motion or rotation.
What is quadrature encoding work?
By using two photo-diodes to detect light transitions and off-setting them by half the spacing between holes so that one sensor is blocked when the other is illuminated, the relative phase of the resulting signals indicates the direction of motion.
How much does quadrature encoding increase the resolution of the sensor by? and why?
Quadrature encoding increases the resolution of the sensor by a factor of four. This is because for each hole in the encoder disc, a total of four transitions occur for channel A and channel B.
Diagram 12 shows the quadrature encoding system. The right side of the image shows the wave trains that result from clockwise rotation and counter clockwise rotation . For clockwise rotation, which of channel A or B’s rising edge precedes each other?
For clockwise rotation, the rising edge of Channel A precedes the rising edge of Channel B.
Diagram 12 shows the quadrature encoding system. The right side of the image shows the wave trains that result from clockwise rotation and counter clockwise rotation . For counter clockwise rotation, which of channel A or B’s rising edge precedes each other?
For counter clockwise rotation, Channel B’s falling edge precedes Channel A’s falling edge.
What is a Reflective Optical Encoder?
Reflective encoders bounce the light from an LED off a surface that has alternating reflecting and non-reflecting regions, and use a photo-diode to sense the level of reflected light. Both variations of encoders are available in rotary and linear configurations.
Optical encoders are also available as absolute position sensors. How can an absolute optical encoder be made?
One method for creating an absolute optical encoder is to add more tracks of windows (holes) to the encoder wheel at separate radii that allow for more than simple incrementing or decrementing a count of transistors.
For absolute encoder wheels, what would in theory be an intuitive approach, but isn’t used very often.
Arranging the slots so that the tracks simply count up and down in binary would be an intuitive approach, but it is rarely used because two or more bits of data would transition simultaneously at several locations. Very nearly, but not quite simultaneous transitions is about the best that could be done.
What would be required if a almost (very nearly) simultaneous transitions for absolute encoders was possible?
An additional logic and error checking would be required to correctly interpret this.
What is Gray Code Encoding?
This is the approach taken for absolute encoders with very near simultaneous transitions. The code is rearranged so that only one bit changes at a time.
What can an Inductive Pickup sensor do?
Some applications call for measuring the rotation of a rotating gear or shaft, this can be done with an inductive pick up sensor, also known as a gear tooth sensor.
What type of industry are inductive pickup sensors/gear tooth sensors popular?
Inductive pickup/gear tooth sensors are popular in the automotive industry to determine the crankshaft position and rotational speed.
What makes inductive pickup/gear tooth sensors popular in many different applications?
- inexpensive
- robust
- noncontact
- they’re not optical so don’t have to stay clean to work
How do inductive pickups work?
Inductive pickups consist of a magnet whose magnetic field is directed through the centre of a coil by a flux concentrator. When placed in proximity to a ferrous material with an air gap that varies as the ferrous material moves, such a rotating gear or a pulley with a groove in it , voltage spikes occur across the leads of the coil, because of the changes induced in the magnetic field. The magnitude of the output voltage increases as the speed of the ferrous material increases, and also when the air gap is reduced.
What is a challenge of interfacing to an inductive pickup?
One of the challenges of interfacing to these sensors is that the output voltage can vary greatly, variation in the 10s of volts is typical, so care must be taken to ensure that the interface electronics function properly across a wide and often unpredictable range of input voltages and that the electronics aren’t damaged by input voltages well above and below power supply rails.
How can a reflective infrared sensor be used to determine the distance to a target?
When an object is relatively close to the emitter (within a cm or so) some of the light from the emitter is reflected back by the object and sensed by the detector. The amount of light reflected and hence the amount of collector current that flows depends on the optical properties of the object, and the distance. If the optical properties are known, and are consistent, then the output depends simply on the distance.
Capacitive displacement sensors can also be used to measure the distance to an object, what sort of distances can they determine?
They can measure the distance to an object when the distance is small. Very small capacitive sensors are capable of achieving subnanometer resolutions, with a typical maximum range of less than a centimetre.
When is a capacitive displacement sensor created?
Whenever two conductive objects are brought into close proximity, since that’s all that’s required to create a capacitor.
A simple plate capacitor is made by placing two conductive plates in close proximity. What is the equation for the resulting capacitance?
C=(εεοA)/d
If we hold the contributions from a dielectric material (the εεο term) and the shared area of the conductors (A) constant, then what does the capacitance depend on?
If we hold the contributions from a dielectric material (the εεο term) and the shared area of the conductors (A) constant, then the capacitance only depends on the distance between the conductors d.
How do can you use a capacitive displacement sensor to read a distance?
A capacitive displacement sensor consists of one half of a simple plate capacitor, the target makes up the other plate, and the resulting capacitance depends on the spacing between them. Measuring the capacitance is then all that is needed to determine the distance to the target. It is hard to get a reading of high resolution in this way.
What are capacitive sensors also capable of measuring?
Capacitive sensors are also capable of measuring the thickness of coating on conductive surfaces, such as adhesives or surface finishes, as well as thickness of conductive materials. They are, however sensitive to target size, alignment and surface irregularities.
What sensors would be more appropriate for measuring large distances?
Ultrasonic sensors
What is the operating principle of an ultrasonic sensor?
A high frequency impulse of sound is emitted by a piezo or electrostatic element, and the time between the emission of the sound and the receipt of its reflection is measured. The sensor reports the sound waves round trip time of flight, which is proportional to the distance between the sensor and the object that reflected the sound wave.
Some applications call for measuring radius of curvature rather than straight-line distance. In some cases curvature can be related to a quantity you’d like to measure, but it isn’t the primary measurand, such as arc length or spring deflection. What type of sensor would be suitable?
A flex sensor/bend sensor.
How are flex sensors made?
Flex sensors are made by printing a thin coating of a material that forms a resistive element onto a flexible substrate such as Mylar or polyimide. The entire sensor can be as little as 0.004 in. thick.
How does the flex sensor work?
When the sensor is bent or flexed, microscopic cracks form in the printed sensing material, and the bulk resistance increases as the radius of curvature decreases. That is, the more they are flexed, the higher their resistance.
Flex sensors have high dependencies to what?
Flex sensors have strong temperature and humidity dependencies and so they are not typically used for high-precision measurements or applications that require stability.
What type of measurements are flex sensors more appropriate for?
They are best for measurements that involve relatively fast changes and for those where relative changes in radius of curvature are more important than quantifying the actual radius.
Give examples of typical applications for flex sensors?
Typical applications include interface devices that measure body motion for computer games and virtual reality simulations, automotive horn buttons, and sensing if a passenger is seated in a car for an air bag activation/deactivation.
What do Tilt sensors do?
Tilt sensors compare the direction of acceleration due to earth’s gravity to the orientation of the sensor.
What can the measurement of tilt be performed on?
Tilt measurements can be performed with an accelerometer, or it can be achieved with sensors intended for this purpose.
What can a tilt direction sensor do?
This is a very simple type of tilt sensor. It only determines if the orientation has changed in large increments such as 90 degrees, it provides a go/no-go indication of tilt.
Give an example of an application for a tilt direction sensor?
A tilt direction sensor would be useful in cases where you wish to determine the orientation of a camera as a picture is being taken so the image can be stored automatically in either landscape or portrait format.
How can gross determination of tilt be achieved?
In a number of ways, including with an element that moves with gravity and blocks the light between an opto-interrupter pair.
In cases where a measurement of tilt is needed with a higher resolution, what may be used?
Inclinometers or MEMs accelerometer-based devices are often used.
What does an inclinometer consist of?
An inclinometer consists of rotating a disk that is heavier at a point away from the centre, and a protractor.
What are Incremental inclinometers?
Incremental inclinometers are a clever means of sensing tilt angles with very high resolution and linearity.
What does an Incremental inclinometer consist of ?
The weighted disk from a standard inclinometer is coupled to the input shaft of an optical encoder (either incremental or absolute encoders) and the force of gravity draws the heavy portion of the weighted disc to the position where is has the lowest potential energy. The position of the weight relative to the orientation of the encoder body gives an indication of the tilt of the device.
What is an undesirable aspect of Incremental inclinometers?
One potentially undesirable aspect of this approach is that a weight pivoting around a point creates a pendulum and oscillations may be induced if inputs to the sensor have frequency content at or near the natural frequency of the system.
What can be done to reduce the affect of oscillations in an incremental inclinometer?
To reduce these effects versions of these sensors that incorporate damping of the rotor’s motion are available. This reduces the potential for oscillations and decreases overshoot and settling time, but it also decreases the sensors response time.
What is one of the most common methods for determining force indirectly?
Measuring the strain of a known material and solving for the force that must have created it.
What is a force sensitive resistor?
An FSR has a pressure sensitive ink sandwiched between flexible conductive layers. The pressure sensitive ink is comprised of conductive and non-conductive materials, suspended in a polymer matrix. When the material is compressed the density of the conductive material increases, resulting in a reduction of the bulk resistance.
How is a force sensitive resistor made?
The sensor assembly is made by first applying conductive ink layers to two separate pieces of flexible substrate, such as polyimide or polyester, and then applying the pressure sensitive ink on the conductive ink of one side. The two pieces of substrate are then bonded together so that the printed surfaces are sandwiched together and sealed inside. The resulting sensor is less than 0.001in. thick.
What are the characteristic force vs resistance output of FSR like?
The characteristic force vs resistance output of FSRs are highly nonlinear, however their conductance 1/R is very nearly a straight line.
What are the advantages of FSRs?
- ultrathin
- robust
- low cost
- easily integrated sensors
- indicate force over a large range.
What are the disadvantages of FSRs?
- repeatability
- hysteresis
- drift specifications
What happens in a Piezoelectric Force Sensor?
In a piezoelectric force sensor, a load is applied to a piezo element, which results in the development of electrical charge proportional to the applied load.
How is the force found in a piezoelectric force sensor?
A preloaded stud clamps the mounting pads together around the piezo element so that the element is in compression when no external load is applied. Because of the preload, tensile forces can be measured since they serve to counteract the preload.
What type of measurements are piezoelectric force sensors useful for?
Piezoelectric force sensors are only useful for dynamic load measurements.
What is the sensing component of a piezoelectric force sensor?
The quartz element.
What is the process that happens when a static load is applied quickly to a piezoelectric force sensor and then held static?
When a static load is quickly applied and then held constant, the sensors output responds to the initial transient but then quickly decays back to zero. This happens because the charge that develops leaks through any adjacent material, even highly insulative materials. This can be problematic.
Give some examples for applications of piezoelectric force sensors, despite the problems with charge leak.
Automated manufacturing equipment, measurements of internal combustion engine parameters like cylinder pressure, and vibration or impact measurements.
How is pressure defined (equation)?
Pressure=force/area
How can measurements of pressure be found?
Measurements of pressure can be directly based on measurements of force, as long as the area over which the force is applied is well understood.
How do Strain-Gage-Based Pressure sensors work?
For strain-gage-based pressure sensors, sensing pressure involves establishing a pressure differential across a material and measuring the resulting deflection with strain gages.
What is the strained material of a strain-gage-based pressure sensor usually?
The strained material is typically a membrane or a diaphragm, so that even very small pressure differences result in measurable displacements. Sensors that incorporate metal foil strain gages have long been available and popular.
How might strain-gage-based pressure sensors incorporate metal foil strain gages?
These may be separate metal foil strain gages that are bonded to a diaphragm, or the strain gage may be formed in place on the surface of the diaphragm itself using a spluttering process. In either case, this type of sensor is more difficult and expensive than those that make exclusive use of semiconductor manufacturing technique.
What is a Semiconductor Pressure Transducer?
A pressure sensor constructed using standard semiconductor manufacturing techniques, resulting in highly consistent and inexpensive device.
How does the Semiconductor Pressure Transducer work?
It works by comparing two pressures by applying them to opposite sides of a silicon sensing element, called the Die.
How is the Die kept separate in a semiconductor pressure transducer?
The Die is kept sealed and physicaly separated from the environment by a silicone gel which fills the space inside the packaging above the Die and also serves to transmit the pressure from the inlet port at the centre of the Stainless steel metal cover. Refer to diagram 13.
What are the benefits of a semiconductor pressure transducer?
- very low hysteresis
- low temperature sensitivity
- lower costs
There are a number of ways to sense the deflection of a diaphragm without involving a strain gage, give an example.
Capacitive position sensors.
