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1. Introduction
This section discusses the sensors, signal processors and data presentation elements commonly used in engineering. The term sensor is used for an element which produces a signal relating to the quantity being measured. The term signal processor is used for the element that takes the output from the sensor and converts it into a form which is suitable for data presentation. Data presentation is where the data is displayed, recorded or transmitted to some control system.
2. Displacement sensors
A displacement sensor is here considered to be one that can be used to:
1. Measure a linear displacement, i.e. a change in linear position. This might, for example, be the change in linear displacement of a sensor as a result of a change in the thickness of sheet metal emerging from rollers.
2. Measure an angular displacement, i.e. a change in angular position.
This might, for example, be the change in angular displacement of a drive shaft.
3. Detect motion, e.g. this might be as part of an alarm or automatic light system, whereby an alarm is sounded or a light switched on when there is some movement of an object within the 'view' of the sensor.
4. Detect the presence of some object, i.e. a proximity sensor. This might be in an automatic machining system where a tool is activated when the presence of a work piece is sensed as being in position.
Displacement sensors fall into two groups: those that make direct contact with the object being monitored, by spring loading or mechanical connection with the object, and those which are non-contacting. For those linear displacement methods involving contact, there is usually a sensing shaft which is in direct contact with the object being monitored, the displacement of this shaft is then being monitored by a sensor. This shaft movement may be used to cause changes in electrical voltage, resistance, capacitance, or mutual inductance. For angular displacement methods involving mechanical connection, the rotation of a shaft might directly drive, through gears, the rotation of the sensor element, this perhaps generating an e.m.f. Non-contacting proximity sensors might consist of a beam of infrared light being broken by the presence of the object being monitored, the sensor then giving a voltage signal indicating the breaking of the beam, or perhaps the beam being reflected from the object being monitored, the sensor giving a voltage indicating that the reflected beam has been detected. Contacting proximity sensors might be just mechanical switches which are tripped by the presence of the object. The following are examples of displacement sensors.
FIG. 1 Potentiometer
Application---The following is an example of part of the specification of a commercially available displacement sensor using a plastic conducting potentiometer track:
Ranges from 0 to 10 mm to 0 to 2 m
Non-linearity error ±0.1 % of full range
Resolution ±0.02% of full range
Temperature sensitivity ±120 parts per million / C
Resolution ±0.02% of full range
FIG. 2 Strain gauges
2.1 Potentiometer
A potentiometer consists of a resistance element with a sliding contact which can be moved over the length of the element and connected as shown in FIG. 1. With a constant supply voltage Vs, the output voltage Vo between terminals 1 and 2 is a fraction of the input voltage, the fraction depending on the ratio of the resistance Rn between terminals 1 and 2 compared with the total resistance R of the entire length of the track across which the supply voltage is connected. Thus Vo/V12 = R12/R. If the track has a constant resistance per unit length, the output is proportional to the displacement of the slider from position 1. A rotary potentiometer consists of a coil of wire wrapped round into a circular track, or a circular film of conductive plastic or a ceramic-metal mix termed a cermet, over which a rotatable sliding contact can be rotated. Hence an angular displacement can be converted into a potential difference. Linear tracks can be used for linear displacements.
With a wire wound track the output voltage does not continuously vary as the slider is moved over the track but goes in small jumps as the slider moves from one turn of wire to the next. This problem does not occur with a conductive plastic or the cermet track. Thus, the smallest change in displacement which will give rise to a change in output, i.e. the resolution, tends to be much smaller for plastic tracks than wire-wound tracks. Errors due to non-linearity of the track for wire tracks tend to range from less than 0.1% to about 1% of the full range output and for conductive plastics can be as low as about 0.05%. The track resistance for wire-wound potentiometers tends to range from about 20 ohm to 200 k-ohm and for conductive plastic from about 500 ohm to 80 kOhm.
Conductive plastic has a higher temperature coefficient of resistance than wire and so temperature changes have a greater effect on accuracy.
2.2 Strain-gauged element
Strain gauges consist of a metal foil strip (FIG. 2(a)), flat length of metal wire (FIG. 2(b)) or a strip of semiconductor material which can be stuck onto surfaces like a postage stamp. When the wire, foil, strip or semiconductor is stretched, its resistance R changes. The fractional change in resistance delta_R/R is proportional to the strain e, i.e.:
delta_R/R = Ge
where G, the constant of proportionality, is termed the gauge factor.
Metal strain gauges typically have gauge factors of the order of 2.0. When such a strain gauge is stretched its resistance increases, when compressed its resistance decreases. Strain is (change in length/original length) and so the resistance change of a strain gauge is a measurement of the change in length of the gauge and hence the surface to which the strain gauge is attached. Thus a displacement sensor might be constructed by attaching strain gauges to a cantilever (FIG. 3), the free end of the cantilever being moved as a result of the linear displacement being monitored. When the cantilever is bent, the electrical resistance strain gauges mounted on the element are strained and so give a resistance change which can be monitored and which is a measure of the displacement. With strain gauges mounted as shown in FIG. 3, when the cantilever is deflected downwards the gauge on the upper surface is stretched and the gauge on the lower surface compressed. Thus the gauge on the upper surface increases in resistance while that on the lower surface decreases. Typically, this type of sensor is used for linear displacements of the order of 1 mm to 30 mm, having a non-linearity error of about ± 1% of full range.
A problem that has to be overcome with strain gauges is that the resistance of the gauge changes when the temperature changes and so methods have to be used to compensate for such changes in order that the effects of temperature can be eliminated. This is discussed later in this section when the circuits used for signal processing are discussed.
FIG. 3 Strain-gauged cantilever
Application -- A commercially available displacement sensor, based on the arrangement shown in FIG. 3, has the following in its specification: Range 0 to 100 mm Non-linearity error ±0.1 % of full range Temperature sensitivity ±0.01 % of full range/ degr. C
FIG. 4 Parallel plate capacitor
FIG. 5 Capacitor sensors
Application---A commercially available capacitor displacement sensor based on the use of the sliding capacitor plate (FIG. 5 (b)) includes in its specification: Ranges available from 0 to 5 mm to 0 to 250 mm Non-linearity and hysteresis en-or ±0.01% of full range
2.3 Capacitive element
The capacitance C of a parallel plate capacitor (FIG. 4) is given by:
C=eT eO A/d
where Cr is the relative permittivity of the dielectric between the plates, 6 b a constant called the permittivity of free space, A the area of overlap between the two plates and d the plate separation. The capacitance will change if the plate separation d changes, the area A of overlap of the plates changes, or a slab of dielectric is moved into or out of the plates, so varying the effective value of et (FIG. 5). All these methods can be used to give linear displacement sensors.
One form that is often used is shown in FIG. 6 and is referred to as a push-pull displacement sensor. It consists of two capacitors, one between the movable central plate and the upper plate and one between the central movable plate and the lower plate. The displacement x moves the central plate between the two other plates. Thus when the central plate moves upwards it decreases the plate separation of the upper capacitor and increases the separation of the lower capacitor. Thus the capacitance of the upper capacitor is increased and that of the lower capacitor decreased. When the two capacitors are incorporated in opposite arms of an alternating current bridge, the output voltage from the bridge is proportional to the displacement. Such a sensor has good long-term stability and is typically used for monitoring displacements from a few millimeters to hundreds of millimeters. Non-linearity and hysteresis errors are about ± 0.01% of full range.
FIG. 6 Push-pull displacement sensor
FIG. 7 LVDT
Application---A commercially available displacement sensor using a LVDT has the following in Its specification: Ranges ±0.125 mm to ±470 mm Non-linearity error ±0.25% of full range Temperature sensitivity ±0.01% of full range Signal conditioning incorporated within the housing of the LVDT
FIG. 8 Optical incremental encoder
2.4 Linear variable differential transformer
The linear variable differential transformer, generally referred to by the abbreviation LVDT, is a transformer with a primary coil and two secondary coils. FIG. 7 shows the arrangement, there being three coils symmetrically spaced along an insulated tube. The central coil is the primary coil and the other two are identical secondary coils which are connected in series in such a way that their outputs oppose each other. A magnetic core is moved through the central tube as a result of the displacement being monitored. When there is an alternating voltage input to the primary coil, alternating e.m.f.s are induced in the secondary coils. With the magnetic core in a central position, the amount of magnetic material in each of the secondary coils is the same and so the e.m.f.s induced in each coil are the same. Since they are so connected that their outputs oppose each other, the net result is zero output.
However, when the core is displaced from the central position there is a greater amount of magnetic core in one coil than the other. The result is that a greater e.m.f. is induced in one coil than the other and then there is a net output from the two coils. The bigger the displacement the more of the core there is in one coil than the other, thus the difference between the two e.m.f.s increases the greater the displacement of the core.
Typically, LVDTs have operating ranges from about ±2 mm to ±400 mm. Non-linearity errors are typically about ±0.25%. LVDTs are very widely used for monitoring displacements.
2.5 Optical encoders
An encoder is a device that provides a digital output as a result of an angular or linear displacement. Position encoders can be grouped into two categories: incremental encoders, which detect changes in displacement from some datum position, and absolute encoders, which give the actual position. FIG. 8 shows the basic form of an incremental encoder for the measurement of angular displacement of a shaft. It consists of a disc which rotates along with the shaft. In the form shown, the rotatable disc has a number of windows through which a beam of light can pass and be detected by a suitable light sensor. When the shaft rotates and disc rotates, a pulsed output is produced by the sensor with the number of pulses being proportional to the angle through which the disc rotates. The angular displacement of the disc, and hence the shaft rotating it, can thus be determined by the number of pulses produced in the angular displacement from some datum position.
Typically the number of windows on the disc varies from 60 to over a thousand with multi-tracks having slightly offset slots in each track.
With 60 slots occurring with 1 revolution then, since 1 revolution is a rotation of 360 degrees, the minimum angular displacement, i.e. the resolution, that can be detected is 360/60 = 6 degree
The resolution thus typically varies from about 6° to 0.3° or better.
With the incremental encoder, the number of pulses counted gives the angular displacement, a displacement of, say, 50 degree giving the same number of pulses whatever angular position the shaft starts its rotation from. However, the absolute encoder gives an output in the form of a binary number of several digits, each such number representing a particular angular position. FIG. 9 shows the basic form of an absolute encoder for the measurement of angular position.
FIG. 9 The rotating wheel of the absolute encoder. Note that though the normal form of binary code is shown in the figure, in practice a modified form of binary code called the Gray code is generally used. This code, unlike normal binary, has only one bit changing in moving from one number to the next Thus we have the sequence 0000, 0001, 0011, 0010, 0011, 0111, 0101, 0100, 1100, 1101, 1111.
With the form shown in the figure, the rotating disc has four concentric circles of slots and four sensors to detect the light pulses. The slots are arranged in such a way that the sequential output from the sensors is a number in the binary code, each such number corresponding to a particular angular position. A number of forms of binary code are used. Typical encoders tend to have up to 10 or 12 tracks. The number of bits in the binary number will be equal to the number of tracks. Thus with 10 tracks there will be 10 bits and so the number of positions that can be detected is V\ i.e. 1024, a resolution of 360/1024 = 0.35 degree.
The incremental encoder and the absolute encoder can be used with linear displacements if the linear displacement is first converted to a rotary motion by means of a tracking wheel (FIG. 10).
FIG. 10 Tracking wheel
FIG. 11 (a) Moire fringes, (b) transmission and (c) reflection forms of
instruments
2.6 Moire fringes
Moire fringes are produced when light passes through two gratings which have rulings inclined at a slight angle to each other. Movement of one grating relative to the other causes the fringes to move. FIG. 11(a) illustrates this. FIG. 11(b) shows a transmission form of instrument using Moire fringes and FIG. 11(c) a reflection form.
With both, a long grating is fixed to the object being displaced. With the transmission form, light passes through.
The long grating and then a smaller fixed grating. The transmitted light being detected by a photocell.
With the reflection form, light is reflected from the long grating through a smaller fixed grating and onto a photocell.
Coarse grating instruments might have 10 to 40 lines per millimeter, fine gratings as many as 400 per millimeter. Movement of the long grating relative to the fixed short grating results in fringes moving across the view of the photocell and thus the output of the cell is a sequence of pulses which can be counted. The displacement is thus proportional to the number of pulses counted. Displacements as small as 1 mm can be detected by this means. Such methods have high reliability and are often used for the control of machine tools.
FIG. 12 Photoelectric proximity sensors
FIG. 13 Proximity sensor
2.7 Optical proximity sensors
There are a variety of optical sensors that can be used to determine whether an object is present or not. Photoelectric switch devices can either operate as transmissive types where the object being detected breaks a beam of light, usually infrared radiation, and stops it reaching the detector (FIG. 12(a)) or reflective types where the object being detected reflects a beam of light onto the detector (FIG. 12(b)). In both types the radiation emitter is generally a light-emitting diode (LED). The radiation detector might be a phototransistor, often a pair of transistors, known as a Darlington pair, using the pair increases the sensitivity. Depending on the circuit used, the output can be made to switch to either high or low when light strikes the transistor. Such sensors are supplied as packages for sensing the presence of objects at close range, typically at less than about 5 mm. FIG. 12(c) shows a U-shaped form where the object breaks the light beam.
Another possibility is 2i photodiode. Depending on the circuit used, the output can be made to switch to either high or low when light strikes the diode. Yet another possibility is a photoconductive cell. The resistance of the photoconductive cell, often cadmium sulphide, depends on the intensity of the light falling on it.
FIG. 13 illustrates a proximity sensor based on reflection. A LED emits infrared radiation which is reflected by the object. The reflected radiation is then detected by a phototransistor. In the absence of the object there is no detected reflected radiation; when the object is in the proximity, there is.
Another form of optical sensor is the pyroelectric sensor. Such sensors give a voltage signal when the infrared radiation falling on them changes, no signal being given for constant radiation levels. Lithium tantulate is a widely used pyroelectric material. FIG. 14 shows an example of such a sensor. Such sensors can be used with burglar alarms or for the automatic switching on of a light when someone walks up the drive to a house. A special lens is put in front of the detector. When a object which emits infra-red radiation is in front of the detector, the radiation is focused by the lens onto the detector. But only for beams of radiation in certain directions will a focused beam fall on the detector and give a signal. Thus when the object moves then the focused beam of radiation is effectively switched on and off as the object cuts across the lines at which its radiation will be detected. Thus the pyroelectric detector gives a voltage output related to the changes in the signal.
FIG. 14 Pyroelectric sensor
FIG. 15 Limit switches: (a) Lever, (b) roller, (c) cam
2.8 Mechanical switches
There are many situations where a sensor is required to detect the presence of some object. The sensor used in such situations can be a mechanical switch, giving an on-off output when the switch contacts are opened or closed by the presence of an object. FIG. 15 illustrates the forms of a number of such switches. Switches are used for such applications as where a work piece closes the switch by pushing against it when it reaches the correct position on a work table, such a switch being referred to as a limit switch. The switch might then be used to switch on a machine tool to carry out some operation on the work piece.
Another example is a light being required to come on when a door is opened, as in a refrigerator. The action of opening the door can be made to close the contacts in a switch and trigger an electrical circuit to switch on the lamp.
FIG. 16 shows another form of a non-contact switch sensor, a reed switch. This consists of two overlapping, but not touching, strips of a spring magnetic material sealed in a glass or plastic envelope. When a magnet or current carrying coil is brought close to the switch, the strips become magnetized and attract each other. The contacts then close.
Typically a magnet closes the contacts when it is about 1 mm from the switch.
2.9 Capacitive proximity switch
A proximity switch that can be used with metallic and non-metallic objects is the capacitive proximity switch. The capacitance of a pair of plates separated by some distance depends on the separation, the smaller the separation the higher the capacitance. The sensor of the capacitive proximity switch is just one of the plates of the capacitor, the other plate being the metal object whose proximity is to be detected (FIG. 17). Thus the proximity of the object is detected by a change in capacitance.
The sensor can also be used to detect non-metallic objects since the capacitance of a capacitor depends on the dielectric between its plates. In this case the plates are the sensor and the earth and the non-metallic object is the dielectric. The change in capacitance can be used to activate an electronic switch circuit and so give an on-off device. Capacitive proximity switches can be used to detect objects when they are typically between 4 and 60 mm from the sensor head.
3. Speed sensors
The following are examples of sensors that can be used to monitor linear and angular speeds.
FIG. 16 Reed switch
FIG. 17 Capacitive proximity switch
FIG. 18 Measurement of linear speed
FIG. 19 The tachogenerator
3.1 Optical methods
Linear speeds can be measured by determining the time between when the moving object breaks one beam of radiation and when it breaks a second beam some measured distance away (FIG. 18). Breaking the first beam can be used to start an electronic clock and breaking the second beam to stop the clock.
3.2 Incremental encoder
The incremental encoder can be used for a measurement of angular speed or a rotating shaft, the number of pulses produced per second being counted.
3.3 Tachogenerator
The basic tachogenerator consists of a coil mounted in a magnetic field (FIG. 19). When the coil rotates electromagnetic induction results in an alternating e.m.f being induced in the coil. The faster the coil rotates the greater the size of the alternating e.m.f. Thus the size of the alternating e.m.f is a measure of the angular speed. Typically such a sensor can be used up to 10 000 revs per minute and has a non-linearity error of about ±1% of the full range.
4. Fluid pressure sensors
Many of the devices used to monitor fluid pressure in industrial processes involve the monitoring of the elastic deformation of diaphragms, bellows and tubes. The following are some common examples of such sensors.
The term absolute pressure is used for a pressure measured relative to a vacuum, differential pressure when the difference between two pressures is measured and gauge pressure for the pressure measured relative to some fixed pressure, usually the atmospheric pressure.
FIG. 20 Diaphragm sensors
FIG. 21 Diaphragm pressure gauge using strain gauges
FIG. 22 MPXIOOAP
Application---The specification of a MPX pressure sensor typically includes: Pressure range 0 to 100 kPa
Supply voltage 10 V
Sensitivity 0.4 mV/kPa
Linearity ±0.25% of full scale
Pressure hysteresis ±0.1 % of full scale
Response time (10% to 90%) 1.0 ms
4.1 Diaphragm sensor
The movement of the center of a circular diaphragm as a result of a pressure difference between its two sides is the basis of a pressure gauge (FIG. 20(a)). For the measurement of the absolute pressure, the opposite side of the diaphragm is a vacuum, for the measurement of pressure difference the pressures are connected to each side of the diaphragm, for the gauge pressure, i.e. the pressure relative to the atmospheric pressure, the opposite side of the diaphragm is open to the atmosphere. The amount of movement with a plane diaphragm is fairly limited; greater movement can, however, be produced with a diaphragm with corrugations (FIG. 20(b)). The movement of the center of a diaphragm can be monitored by some form of displacement sensor. FIG. 21 shows the form that might be taken when strain gauges are used to monitor the displacement, the strain gauges being stuck to the diaphragm and changing resistance as a result of the diaphragm movement. Typically such sensors are used for pressures over the range 100 kPa to 100 MPa, with an accuracy up to about ±0.1%. One form of diaphragm pressure gauge uses strain gauge elements integrated within a silicon diaphragm and supplied, together with a resistive network for signal processing, on a single silicon chip as the Motorola MPX pressure sensor (FIG. 22). With a voltage supply connected to the sensor, it gives an output voltage directly proportional to the pressure. In one form it has a built-in vacuum on one side of the diaphragm and so the deflection of the diaphragm gives a measure of the absolute pressure applied to the other side of the diaphragm. The output is a voltage which is proportional to the applied pressure with a sensitivity of 0.6 mV/kPa. Other versions are available which have one side of the diaphragm open to the atmosphere and so can be used to measure gauge pressure, others allow pressures to be applied to both sides of the diaphragm and so can be used to measure differential pressures. Such sensors are available for use for the measurement of absolute pressure, differential pressure or gauge pressure, e.g. MPX2100 has a pressure range of 100 kPa and with a supply voltage of 16 V d.c. gives a voltage output over the full range of 40 mV. FIG. 23 shows the form that might be taken by a capacitance diaphragm pressure gauge. The diaphragm forms one plate of a capacitor, the other plate being fixed. Displacement of the diaphragm results in changes in capacitance. The range of such pressure gauges is about 1 kPa to 200 kPa with an accuracy of about ±0.1%. Another form of diaphragm sensor uses a LVDT to monitor the displacement of the diaphragm. Such an arrangement is typically used for low pressure measures where high stability is required. The total error due to non-linearity, hysteresis and repeatability can be of the order of±0.5% of full scale.
FIG. 23 Diaphragm gauges using capacitance
FIG. 24 Basic form of a piezoelectric sensor
Application --- A commercially available piezo electric diaphragm pressure gauge has in its specification:
Ranges 0 to 20 MPa, 0 to 200 MPa, 0 to 500 MPa, 0 to 1000 MPa
Non-linearity error ±0.5%
Sensitivity -0.1 pC/kPa
Temperature sensitivity ±0.5% of full scale for use +20 C tp +100°C
4.2 Piezoelectric sensor
When certain crystals are stretched or compressed, charges appear on their surfaces. This effect is called piezo-electricity. Examples of such crystals are quartz, tourmaline, and zirconate-titanate.
A piezoelectric pressure gauge consists essentially of a diaphragm which presses against a piezoelectric crystal (FIG. 24). Movement of the diaphragm causes the crystal to be compressed and so charges produced on its surface. The crystal can be considered to be a capacitor which becomes charged as a result of the diaphragm movement and so a potential difference appears across it. The amount of charge produced and hence the potential difference depends on the extent to which the crystal is compressed and hence is a measure of the displacement of the diaphragm and so the pressure difference between the two sides of the diaphragm. If the pressure keeps the diaphragm at a particular displacement, the resulting electrical charge is not maintained but leaks away. Thus the sensor is not suitable for static pressure measurements.
Typically such a sensor can be used for pressures up to about 1000 MPa with a non-linearity error of about ±1.0% of the full range value.
4.3 Bourdon tube
The Bourdon tube is an almost rectangular or elliptical cross-section tube made from materials such as stainless steel or phosphor bronze.
With a C-shaped tube (FIG. 25(a)), when the pressure inside the tube increases the closed end of the C opens out, thus the displacement of the closed end becomes a measure of the pressure. A C-shaped Bourdon tube can be used to rotate, via gearing, a shaft and cause a pointer to move across a scale. Such instruments are robust and typically used for pressures in the range 10 kPa to 100 MPa with an accuracy of about ±1% of full scale.
Another form of Bourdon instrument uses a helical-shaped tube (FIG. 25(b)). When the pressure inside the tube increases, the closed end of the tube rotates and thus the rotation becomes a measure of the pressure. A helical-shaped Bourdon tube can be used to move the slider of a potentiometer and so give an electrical output related to the pressure.
Helical tubes are more expensive but have greater sensitivity. Typically they are used for pressures up to about 50 MPa with an accuracy of about ±1% of full range.
FIG. 25 Bourdon tube instruments: (a) geared form, (b) potentiometer
form.
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