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
