Chapter 13: Sensors Flashcards

Question

What are the drawbacks of using a potentiometer as a displacement sensor, and what would need to be considered to achieve higher performance?

Answer 1

* 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

Answer 2

* reflective infrared emitter-detector pairs * capacitive sensors * force-balance accelerometer/servo accelerometer

Answer 3

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.

Answer 4

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

Answer 5

The relationship between input physical signal and output electrical signal.

Answer 6

The sensitivity is often expressed as the ratio between a change in output electrical signal to a given change in the input physical signal.

Answer 7

Sensitivity may be expressed as the derivative of the transfer function with respect to the input physical signal

Answer 8

This is the range of input physical signals which may be converted to electrical signals by the sensor

Answer 9

Input signals out of this range are not guaranteed to produce output signals that meet the device's specifications

Answer 10

The sensor may be permanently damaged

Answer 11

It may be stated in decibels (db) and the formula: dB=20log_10 (max-measurable-signal)/(min-measurable-signal)

Answer 12

Microphones, photo-sensors, where the dynamic range is large

Answer 13

The largest expected error between the actual and ideal output signal. This maybe stated as a value, or a percentage of full-scale output

Answer 14

By calibrating it against a known standard

Answer 15

* 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

Answer 16

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.

Answer 17

It produces the smallest value for non-linearity and makes sensors look their best

Answer 18

This compares the possible range of errors with the least squares fit line

Answer 19

It is likely to have the units of the measured signal, or be expressed as a percentage of the full-scale output.

Answer 20

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

Answer 21

The hysteresis is expressed as units of the measurement quantity

Answer 22

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

Answer 23

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

Answer 24

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.

Answer 25

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

Answer 26

A white noise distribution

Answer 27

A white noise distribution is where the spectral noise density is the same at all frequencies

Answer 28

Johnson noise, this is present in conductive materials such as wires & resistors

Answer 29

Johnson noise is caused by thermal agitation of the charge carriers, & typicallyappears as white noise

Answer 30

volts/√Hz

Answer 31

A distribution of this nature adds noise to a measurement with amplitude inversely proportional to the square root of the measurement bandwidth

Answer 32

The noise decreases with the square root of the measurement time

Answer 33

A measure of how big the desired output signal is relative to the noise present

Answer 34

Signal averaging

Answer 35

Signal averaging is effectively a low-pass filter that reduces the bandwidth of the signal by increasing the average time of the measurement

Answer 36

The minimum detectable change in the measured quantity

Answer 37

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

Answer 38

(physical signal)/√Hz

Answer 39

signal/√Hz | they may otherwise give a minimum detectable signal for a specific measurement

Answer 40

The resolution is often not even mention in data sheets

Answer 41

The range of frequencies of an input physical signal that a sensor can detect

Answer 42

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.

Answer 43

The upper cutoff frequency

Answer 44

The lower cutoff frequency

Answer 45

The bandwidth of a sensor is defined as the frequency range between the upper and lower cutoff frequencies

Answer 46

It is assumed that the lower cutoff frequency is 0 Hz

Answer 47

The bandwidth will be stated as a range

Answer 48

Microphones

Answer 49

This is a commercially available 'mems' accelerometer, fitting in a 16 pin 'soic' package

Answer 50

Micro Electro-Mechanical System

Answer 51

Small Outline Integrated Circuit

Answer 52

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

Answer 53

Filter circuits

Answer 54

Offset and gain errors as well as non-linearity

Answer 55

Where the sensors output doesn't match the transfer function's linear equation, additional inaccuracies are introduced

Answer 56

Using a multipoint calibration

Answer 57

Calibration of individual sensors

Answer 58

The switch

Answer 59

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)

Answer 60

-push button -slide -toggle -rotary -knife -Hall -tactile -micro -lever -membrane -momentary -keyed -mercury and reed

Answer 61

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

Answer 62

- Enabling or disabling power to a device - Providing a human interface - Sensing limits of travel - Detecting proximity - Sensing angles

Answer 63

Switches are categorised according to the number of poles and throws it has.

Answer 64

The number of poles a switch has is the number of independent circuits it can switch

Answer 65

Single, double or triple, but some have higher numbers

Answer 66

The number of throws of a switch is the number of discrete active positions the switch contacts may assume

Answer 67

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)

Answer 68

Rotary switches

Answer 69

Single pole, single throw, so SPST

Answer 70

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

Answer 71

In general, a logic level low is very closer to 0V and a logic level high approaches the supply voltage, often 5V.

Answer 72

Switches that incorporate moving electrical contacts which are forced into physical contact by spring elements form a classic friction-damped mass-spring dynamic system

Answer 73

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.

Answer 74

In many applications, switch bounce is undesirable

Answer 75

Switch debouncing

Answer 76

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.

Answer 77

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

Answer 78

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

Answer 79

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.

Answer 80

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.

Answer 81

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

Answer 82

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.

Answer 83

Their resistance is a function of the measurand

Answer 84

First we must translate the varying resistance into a varying voltage, which may then be connected to an analogue-to-digital converter and measured

Answer 85

One of the simplest ways to create a varying voltage from a varying resistor is to incorporate it into a voltage divider

Answer 86

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.

Answer 87

In the case you prefer to have low values of resistance correspond to higher Vout, swap the position of R1 and the sensor R2.

Answer 88

- 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.

Answer 89

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.

Answer 90

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.

Answer 91

Real current sources can be constructed to perform well over limited ranges.

Answer 92

One common method for implementing this approach in practice is the constant current circuit shown in diagram 3.

Answer 93

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.

Answer 94

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.

Answer 95

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.

Answer 96

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.

Answer 97

There are several sensors like this, such as strain gages

Answer 98

a Wheatstone bridge circuit

Answer 99

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)

Answer 100

The output voltage of the wheatstone bridge circuit is the difference between the mid-point of the two voltage dividers

Answer 101

Vout=V+-V-=+V[Rs/(Rs+R2) - R3/(R3+R1)]

Answer 102

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.

Answer 103

As Rs deviated from its nominal value in the wheatstone bridge configuration, the difference between V+ and V- indicates how much it has changed.

Answer 104

The range of Vout is usually small, around +/-25mV.

Answer 105

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

Answer 106

Instrumentation amplifiers are particularly well suited for amplifying differential signals like the output of the wheatstong bridge.

Answer 107

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.

Answer 108

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.

Answer 109

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.

Answer 110

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.

Answer 111

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-).

Answer 112

Replace all elements of the bridge with sensors, and arranging them so that the output voltages are driven further in opposite directions.

Answer 113

1. The ability to initially balance (or zero) the bridge output enables you to use high gains with a differential measurement 2. You are using resistive sensors that have a nonzero nominal resistance and a range that deviates only a small amount from the nominal value 3. You can mitigate the impact of temperature effects by ensuring that the temperature of all bridge elements is held uniform.

Answer 114

It is a sensor whose capacitance varies as a function of a physical input signal.

Answer 115

1. Proximity sensor 2. Displacement sensor 3. Computer touch pad 4. Wall stud sensors 5. Liquid level sensors 6. Chemical composition sensors

Answer 116

Measuring capacitance requires us to take dynamic measurements, where we need to manage signals that vary with time and amplitude.

Answer 117

The output voltage will achieve the value of the input voltage, this is because of the equation Vout=Vin(1-e^(-t/RC))

Answer 118

**************

Answer 119

This is the amount of time required for the output voltage to rise to 63.2% of Vin after a step input.

Answer 120

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.

Answer 121

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.

Answer 122

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.

Answer 123

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.

Answer 124

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)

Answer 125

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.

Answer 126

The amount of the decrease depends on the value of the added capacitance Cs, and thus provides a means of measuring Cs.

Answer 127

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.

Answer 128

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.

Answer 129

For the capacitive Wheatstone bridge, the left and right side voltage dividers that make up the circuit are both frequency dependent.

Answer 130

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.

Answer 131

1. 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. 2. Capacitive Wheatstone bridge allow for a direct measurement of a capacitor's deviation from a nominal value. 3. 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.

Answer 132

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.

Answer 133

Photo-diode.

Answer 134

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.

Answer 135

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.

Answer 136

Photo-current.

Answer 137

The photo-current corresponds to the irradiance of the incident light.

Answer 138

The amount of radiant flux per unit area.

Answer 139

To obtain the desired response from the photo-diode, it must be either zero biased or reverse biased.

Answer 140

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.

Answer 141

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.

Answer 142

R1 and R2 form a voltage divider with an output of 1V, which is connected to the op-amp's noninverting input.

Answer 143

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.

Answer 144

The wavelength of light that a photo-diode can detect depends on the semiconductor used to make the manufacturing process.

Answer 145

Silicon photo-diode

Answer 146

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

Answer 147

Photo-diodes have a very large dynamic range.

Answer 148

The output signal of a photo-diode is very small, with a very large range.

Answer 149

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.

Answer 150

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)

Answer 151

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.

Answer 152

Dark current is the result of thermal excitation of the diode's semiconductor charge carriers.

Answer 153

This means that dark current scales with temperature.

Answer 154

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.

Answer 155

The response time is very quick, meaning they can be used in high speed applications or applications where dynamic range is needed.

Answer 156

Photodiodes are commonly used in high speed applications such as fibre optic data communications.

Answer 157

Photodiodes.

Answer 158

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).

Answer 159

Because it is a consumer electronic application, extremely low cost is a strict requirement too.

Answer 160

Photo-transistors

Answer 161

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.

Answer 162

Photo-transistors provide much higher output currents than photo-diodes, albeit over a much narrower dynamic range with much slower response times.

Answer 163

Photo-transistors are bipolar junction transistors, and their semiconductor base region is optically exposed.

Answer 164

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.

Answer 165

The collector-current.

Answer 166

The dynamic range. The photo-transistor dynamic range is tiny compared with the photo-diode.

Answer 167

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.

Answer 168

We must maintain a constant voltage across the collector and emitter of about 2.5V or more.

Answer 169

In cases where linearity of the output is not a requirement, a simple pull-up pull-down resistor will suffice.

Answer 170

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).

Answer 171

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.

Answer 172

Vout has its maximum value when no light is present (near 5V), and increasing light levels causes Vout to decrease.

Answer 173

The nonlinearity is a result of not holding a constant voltage drop across the photo-transistor.

Answer 174

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.

Answer 175

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.

Answer 176

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.

Answer 177

A trans resistive circuit is a good option.

Answer 178

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.

Answer 179

Golden rule 1 allows us to assume that all the photo-current flows across the Rg to the op-amp's output, Vout.

Answer 180

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.

Answer 181

There are no changes in voltage, there are no contributions to the sensors output rise and fall times due to stray capacitance.

Answer 182

This is a result of op-amp's output sinking current from the photo-transistor.

Answer 183

This is from the photo-transistor sinking current from the trans-resistive circuit.

Answer 184

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.

Answer 185

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.

Answer 186

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.

Answer 187

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

Answer 188

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.

Answer 189

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.

Answer 190

Photo-cells are also commonly known as photo-detectors and photo-resistors.

Answer 191

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.

Answer 192

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.

Answer 193

For photo-cells, changes in the electron's energy alter the material's ability to conduct electricity-that is, its resistance.

Answer 194

CdS photo-cell (cadmium sulphide)

Answer 195

Part-to-part variation is wide for photo-cells, and data sheets usually specify absolute resistance vs light intensity with wide tolerances.

Answer 196

The relative response of each individual sensor is well characterised with respect to its response at reference light levels.

Answer 197

Interfacing to a photo-cell is straightforward given that its resistance varies with light levels.

Answer 198

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.

Answer 199

This makes photo-cells unsuitable for medium to high-speed applications.

Answer 200

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.

Answer 201

Strain is defined as the deformation of a material subjected to mechanical stresses from loading.

Answer 202

Strain is expressed as the ratio of the change of length of a material under stress over the original length of the unstressed material.

Answer 203

They can also be used as the basis of measurement of secondary quantities such as deflection and force.

Answer 204

Accurately measuring strain can be quite challenging because in most cases it is a very small quantity.

Answer 205

Strain gages

Answer 206

These are sensors whose resistances changes as a function of the strain they undergo.

Answer 207

The strain experienced by the base material is imparted to the strain gage.

Answer 208

Typically with an adhesive such as cyanoacrylate.

Answer 209

The coupling between the gage and the base material must be secure.

Answer 210

Strain gages consist of little more than sensing elements whose resistance changes slightly when deformed along the sensing axis.

Answer 211

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.

Answer 212

Strain gages made from semiconductors exhibit significant changes to their resistivity ρ when subjected to strain.

Answer 213

1. 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. 2. 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).

Answer 214

The amount that the cross-sectional area changes for a given change of length.

Answer 215

Normal to the direction of loading

Answer 216

This is the strain in the direction of loading.

Answer 217

v=-εtransverse/εaxial

Answer 218

They range from near 0 (cork) to 0.5 (rubber)

Answer 219

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.

Answer 220

GF=(dR/R)/ε

Answer 221

GF is the strain gage's gage factor.

Answer 222

The GF is determined by the properties of the material used to make the strain gage.

Answer 223

Metal foil strain gages have constant gage factors that range from 1 to 2.

Answer 224

Semiconductor strain gages have gage factors that range from 50-100, but the value can vary depending on the strain.

Answer 225

The dark regions are a conductive strain-sensing material.

Answer 226

The transparent regions are an insulating substrate.

Answer 227

Temperature effects can dominate those of strain.

Answer 228

This is because of thermal expansion or contraction of the base material relative to the thermal expansion of the strain gage material.

Answer 229

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.

Answer 230

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.

Answer 231

Load cells are sensors that measure force or, stated another way, an applied load.

Answer 232

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.

Answer 233

Load cells are rarely seen integrated into mechatronic devices outside of laboratory settings.

Answer 234

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.

Answer 235

The normal course of action is to calibrate each one.

Answer 236

As load cells are usually highly precise, the resulting cost for each load cell can be substantial.

Answer 237

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).

Answer 238

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.

Answer 239

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.

Answer 240

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.

Answer 241

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

Answer 242

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.

Answer 243

The more complex approach to load cells allows for incorporation of several strain gages- some under tension and others in compression.

Answer 244

A full Wheatstone bridge can be built into the load cell, bringing the benefits of temperature compensation as well as increasing the sensitivity.

Answer 245

The most complex load cells make use of measurements of shear strain to measure the applied force. For example shear beam load cells.

Answer 246

They are load cells that make use of measurements of shear strain to measure the applied force

Answer 247

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.

Answer 248

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.

Answer 249

Thermocouples are temperature sensors that make use of the thermoelectric effect.

Answer 250

the Seebeck effect

Answer 251

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.

Answer 252

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.

Answer 253

Selecting two conductors that have very different thermoelectric characteristics results in more sensitive thermocouples.

Answer 254

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.

Answer 255

Made with chromel and alumel conductors

Answer 256

Made with iron and constantan

Answer 257

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.

Answer 258

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.

Answer 259

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.

Answer 260

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.

Answer 261

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.

Answer 262

A thermistor is a device whose resistance changes as a function of temperature.

Answer 263

Except for when making a thermistor, as the temperature dependency is exploited to measure temperature itself.

Answer 264

Thermistors are made of ceramic and polymer materials.

Answer 265

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.

Answer 266

Functionally, thermistors are divided into two major categories, depending on the slope of the relationship between temperature and resistance.

Answer 267

The first category is negative temperature coefficient (NTC) thermistors. The second category is positive temperature coefficient (PTC) thermistors.

Answer 268

Negative temperature coefficient (NTC) thermistors have a resistance that decreases with increasing temperature.

Answer 269

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.

Answer 270

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.

Answer 271

PTC thermistors are very useful as resettable fuses.

Answer 272

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.

Answer 273

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.

Answer 274

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.

Answer 275

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.

Answer 276

A semiconductor temperature sensor

Answer 277

A semiconductor temperature sensor.

Answer 278

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.

Answer 279

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.

Answer 280

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.

Answer 281

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.

Answer 282

The Hall effect occurs for currents flowing across metallic conductors and semiconductors.

Answer 283

The magnitude of the Hall voltage depends on the magnetic flux and the properties of the conductive material.

Answer 284

This is because Hall effect sensors have a range of hundreds of thousands of gauss.

Answer 285

Hall effect sensors are available with analogue or digital outputs.

Answer 286

This is because the Hall effect results in small voltages.

Answer 287

This is known as a linear output Hall effect transducer, sometimes called a LOHET

Answer 288

linear output Hall effect transducer. It is a Hall effect sensor and amplifier integrated together in a single package.

Answer 289

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.

Answer 290

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.

Answer 291

Since the magnetic field strength varies as 1/d^3, where d is distance from magnetic pole.

Answer 292

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.

Answer 293

Linear Hall sensors are also useful for measuring angles and continuous rotation.

Answer 294

Applications of Linear Hall sensors are limited by the magnetic field strength.

Answer 295

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.

Answer 296

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.

Answer 297

Because the Schmitt trigger of the Hall switch implements hysteresis, the transition back to the original state will occur at a lower threshold.

Answer 298

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.

Answer 299

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.

Answer 300

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.

Answer 301

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.

Answer 302

Reed switches

Answer 303

Reed switches consist of a set of contacts, at least one of which is ferromagnetic.

Answer 304

This means having a high susceptibility to magnetization, the strength of which depends on that of the applied magnetizing field.

Answer 305

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.

Answer 306

When the magnetic field is removed, the contacts revert to their normal inactivated state.

Answer 307

This means that the contacts impart the spring tension that allows the contacts to assume their normal, inactive state.

Answer 308

A Reed sensors is a simple way to detect the presence or absence of a strong magnetic field with minimal expense.

Answer 309

When low cost and simplicity are strict requirements, reed switches are an excellent choice.

Answer 310

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.

Answer 311

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.

Answer 312

Proximity detection is usually performed in the digital domain, with a sensor that detects when an object is within the range of the sensor.

Answer 313

Proximity sensors give yes or no answer to the question of whether an object is within a range of a sensor.

Answer 314

Usually the range of proximity sensors is small, rarely more than a few cm.

Answer 315

A common application of proximity sensing is when a motorised stage in a piece of automated equipment reaches the home, or zero, position.

Answer 316

Contact switches, microswitches, opto-interrupter emitter-detector pairs, reflective emitter-detector pairs, Hall switches, Magento resistive switches, and reed switches.

Answer 317

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.

Answer 318

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.

Answer 319

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.

Answer 320

Inductive proximity sensors detect when a metallic object has moved close enough to measurably disturb a magnetic field.

Answer 321

Nonmagnetic objects do not disturb the field, and are not detected by inductive sensors.

Answer 322

Ultrasonic proximity detectors emit high frequency sound waves and measure the amount of time required for reflections to return.

Answer 323

Shorter return time indicates that an object has moved in a close proximity.

Answer 324

A position sensor must determine the distance from the sensor to a location or the distance that something has moved.

Answer 325

Displacement sensors

Answer 326

Potentiometer

Answer 327

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

Answer 328

The position of the wiper in the potentiometer can be determined by measuring either of the variable resistances.

Answer 329

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.

Answer 330

They are very inexpensive, and readily available in a wide variety of packaging options and resistance values,

Answer 331

Rotary or linear pots are used as user input devices on stereo equipment. Potentiometer based joysticks are used in video game console user interfaces.

Answer 332

Another interesting and useful variant is the cable-driven linear displacement transducer, more commonly known as string pot.

Answer 333

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.

Answer 334

The resistance of the string pot output is proportional to the length of string that has been pulled from the spool.

Answer 335

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 .

Answer 336

An Optical Encoder

Answer 337

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.

Answer 338

Because of the incremental nature of the measurements, the sensors often get called incremental encoders.

Answer 339

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.

Answer 340

Quadrature encoding can be used to determine the direction of motion or rotation.

Answer 341

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.

Answer 342

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.

Answer 343

For clockwise rotation, the rising edge of Channel A precedes the rising edge of Channel B.

Answer 344

For counter clockwise rotation, Channel B's falling edge precedes Channel A's falling edge.

Answer 345

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.

Answer 346

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.

Answer 347

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.

Answer 348

An additional logic and error checking would be required to correctly interpret this.

Answer 349

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.

Answer 350

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.

Answer 351

Inductive pickup/gear tooth sensors are popular in the automotive industry to determine the crankshaft position and rotational speed.

Answer 352

- inexpensive - robust - noncontact - they're not optical so don't have to stay clean to work

Answer 353

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.

Answer 354

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.

Answer 355

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.

Answer 356

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.

Answer 357

Whenever two conductive objects are brought into close proximity, since that's all that's required to create a capacitor.

Answer 358

C=(εεοA)/d

Answer 359

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.

Answer 360

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.

Answer 361

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.

Answer 362

Ultrasonic sensors

Answer 363

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.

Answer 364

A flex sensor/bend sensor.

Answer 365

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.

Answer 366

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.

Answer 367

Flex sensors have strong temperature and humidity dependencies and so they are not typically used for high-precision measurements or applications that require stability.

Answer 368

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.

Answer 369

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.

Answer 370

Tilt sensors compare the direction of acceleration due to earth's gravity to the orientation of the sensor.

Answer 371

Tilt measurements can be performed with an accelerometer, or it can be achieved with sensors intended for this purpose.

Answer 372

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.

Answer 373

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.

Answer 374

In a number of ways, including with an element that moves with gravity and blocks the light between an opto-interrupter pair.

Answer 375

Inclinometers or MEMs accelerometer-based devices are often used.

Answer 376

An inclinometer consists of rotating a disk that is heavier at a point away from the centre, and a protractor.

Answer 377

Incremental inclinometers are a clever means of sensing tilt angles with very high resolution and linearity.

Answer 378

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.

Answer 379

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.

Answer 380

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.

Answer 381

Measuring the strain of a known material and solving for the force that must have created it.

Answer 382

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.

Answer 383

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.

Answer 384

The characteristic force vs resistance output of FSRs are highly nonlinear, however their conductance 1/R is very nearly a straight line.

Answer 385

- ultrathin - robust - low cost - easily integrated sensors - indicate force over a large range.

Answer 386

- repeatability - hysteresis - drift specifications

Answer 387

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.

Answer 388

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.

Answer 389

Piezoelectric force sensors are only useful for dynamic load measurements.

Answer 390

The quartz element.

Answer 391

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.

Answer 392

Automated manufacturing equipment, measurements of internal combustion engine parameters like cylinder pressure, and vibration or impact measurements.

Answer 393

Pressure=force/area

Answer 394

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.

Answer 395

For strain-gage-based pressure sensors, sensing pressure involves establishing a pressure differential across a material and measuring the resulting deflection with strain gages.

Answer 396

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.

Answer 397

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.

Answer 398

A pressure sensor constructed using standard semiconductor manufacturing techniques, resulting in highly consistent and inexpensive device.

Answer 399

It works by comparing two pressures by applying them to opposite sides of a silicon sensing element, called the Die.

Answer 400

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.

Answer 401

- very low hysteresis - low temperature sensitivity - lower costs

Answer 402

Capacitive position sensors.