The term sensor is used for an input device that provides a usable output in response to aspecified physical input. For example, a thermocouple is a sensor that converts atemperaturedifference into an electrical output. The term transducer is generally used to refer to a devicethat converts a signal from one form to a different physical form. Thus sensors are oftentransducers, but also other devices can be transducers, such as a motor that converts anelectrical input into rotation.
Sensors that give digital or discrete, that is, on/off, outputs can be easily connected to theinput ports of PLCs. An analog sensor gives an output proportional to the measured variable.
Such analog signals have to be converted to digital signals before they can be input to PLCports.
The following are some of the more common terms used to define the performance ofsensors:
As an illustration of the use of these terms in specification, the following were included in thespecification of a MX100AP pressure sensor :
Supply voltage: 3 V (6 V max)
Supply current: 6 mA
Full-scale span: 60 mV
Range: 0 to 100 kPa
Sensitivity: 0.6 mV/kPa
Nonlinearity error: Æ0.05% of full range
Temperature hysteresis: Æ0.5% of full scale
Input resistance: 400 to 550 O
Response time: 1 ms (10% to 90%)
The following are examples of some of the commonly used PLC input devices and theirsensors.
A mechanical switch generates an on/off signal or signals as a result of some mechanicalinput causing the switch to open or close. Such a switch might be used to indicate thepresence of a workpiece on a machining table, the workpiece pressing against the switchandso closing it. The absence of the workpiece is indicated by the switch being open and itspresence by it being closed. Thus, with the arrangement shown in Figure, the inputsignals to a single input channel of the PLC are thus the logic levels:
Workpiece not present: 0
Workpiece present: 1
The 1 level might correspond to a 24 V DC input, the 0 to a 0 V input.
With the arrangement shown in Figure when the switch is open the supply voltage isapplied to the PLC input; when the switch is closed the input voltage drops to a low value.
The logic levels are thus:
Workpiece not present: 1
Workpiece present: 0
Switches are available with normally open (NO) or normally closed (NC) contacts or can beconfigured as either by choice of the relevant contacts. An NO switch has its contacts open intheabsence of a mechanical input and the mechanical input is used to close the switch. An NCswitch has its contacts closed in the absence of a mechanical input and the mechanical inputisused to open the switch. Mechanical switches are specified in terms of number of poles, thatis,the number of separate circuits that can be completed by the same switching action, andnumberof throws, that is, the number of individual contacts for each pole. A problem with mechanical switches is that when a switch is closed or opened, bounce canoccur and the contacts do not make or open cleanly. Because they involve an elasticmember,they bounce back and forth like an oscillating spring. This “bounce” may produce amplitudesthat change logic levels over perhaps 20 ms, and so a single switch change may give rise toanumber of signals rather than just the required single one. There are a number of ways ofeliminating these spurious signals. One way is to include in the software program a delay ofapproximately 20 ms after the first detected signal transition before any further signals areread. A possibility for a single pole/double throw (SPDT) switch is to use two NAND logic gates, as illustrated in Figure When the switch is in position A,the output is a logic 1. When the switch moves to position B, the output becomes logic 0 andremains latched at this spot, even when the switch bounces. Figure shows how a D flipflop can be used to debounce a single pole/single throw(SPST) switch. The output of the D flip-flop does not change until a position-edged clocksignal is imposed, and if this is greater than the bounce time, the output is debounced.
The term limit switch applies to a switch that is used to detect the presence or passage ofa moving part. It can be actuated by a cam, roller, or lever. Figure shows someexamples. The cam can be rotated at a constant rate and so can turn the switchon and off for particular time intervals.
Liquid-level switches are used to control the level of liquids in tanks. Essentially, theseare vertical floats that move with the liquid level, and this movement is used to operateswitch contacts.
Proximity switches are used to detect the presence of an item without making contact withit. There are a number of forms of such switches, some being suitable only for metallic objects.
The eddy current type of proximity switch has a coil that is energized by a constantalternating current and produces a constant alternating magnetic field. When a metallicobjectis close to it, eddy currents are induced in it The magnetic field due to theseeddy currents induces an EMF back in the coil with the result that the voltage amplitudeneeded to maintain the constant coil current changes. The voltage amplitude is thus ameasure of the proximity of metallic objects. The voltage can be used to activate anelectronic switch circuit, basically a transistor that has its output switched from low to highby the voltage change, creating an on/off device. The range over which such objects can bedetected is typically about 0.5 to 20 mm.
Another switch type is the reed switch. This consists of two overlapping, but not touching,strips of a springy ferromagnetic material sealed in a glass or plastic envelope. When a magnet or current-carrying coil is brought close to the switch, the strips becomemagnetized and attract each other. The contacts then close. The magnet closes the contactswhen it is typically about 1 mm from the switch. Such a switch is widely used with burglaralarms to detect when a door is opened, with the magnet being in the door and the reedswitch in the frame of the door. When the door opens, the switch opens. A proximity switch that can be used with metallic and nonmetallic objects is the capacitiveproximity switch. The capacitance of a pair of plates separated by some distance depends onthe separation; the smaller the separation, the higher the capacitance. The sensor of thecapacitive proximity switch is just one of the plates of the capacitor, the other plate beingthemetal object for which the proximity is to be detected. Thus the proximity ofthe object is detected by a change in capacitance. The sensor can also be used to detectnonmetallic objects, since the capacitance of a capacitor depends on the dielectric betweenitsplates. In this case the plates are the sensor and the earth and the nonmetallic object is thedielectric. The change in capacitance can be used to activate an electronic switch circuitand so create an on/off device. Capacitive proximity switches can be used to detectobjects when they are typically between 4 mm and 60 mm from the sensor head. Anexample of the use of such a sensor might be to determine whether a cake is presentinside a cardboard box, when such boxes move along a conveyor belt.
Another type, the inductive proximity switch, consists of a coil wound a round a ferrousmetallic core. When one end of this core is placed near a ferrous metal object, there iseffectively a change in the amount of metallic core associated with the coil and so a changein its inductance. This change can be monitored using a resonant circuit, the presence of theferrous metal object thus changing the current in that circuit. The current can be used toactivate an electronic switch circuit and so create an on/off device. The range over whichsuch objects can be detected is typically about 2 mm to 15 mm. An example of the use ofsuch a sensor is to detect whether bottles passing along a conveyor belt have metal caps on.
Photoelectric Sensors and Switches
Photoelectric switch devices can either operate as transmissive types, in which the objectbeing detected breaks a beam of light, usually infrared radiation, and stops it reaching thedetector (Figure), as in Figure, which shows a U-shaped form in which theobject breaks the light beam; or reflective types, in which the object being detected reflectsa beam of light onto the detector (Figure). The transmissive form of sensor istypically used in applications involving the counting of parts moving along conveyor beltsand breaking the light beam; the reflective form is used to detect whether transparentcontainers contain liquids to the required level.
The radiation emitter is generally a light-emitting diode (LED). The radiation detector mightbe a phototransistor, often a pair of transistors, known as a Darlington pair, to increase thesensitivity. 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 thepresence of objects at close range, typically less than about 5 mm. Another possible detectoris a photodiode. Depending on the circuit used, the output can be made to switch to eitherhigh or low when light strikes the diode. Yet another possibility is a photoconductive cell. The resistance of the photoconductive cell, often cadmium sulfide, depends on the intensityof the light falling on it.
With these sensors, light is converted to a current, voltage, or resistance change. If theoutput is tobe used as a measure of the intensity of the light, rather than just the presence or absenceof someobject in the light path, the signal will need amplification and then conversion from analog todigital by an analog-to-digital converter. An alternative is to use a light-to-frequencyconverter,the light then being converted to a sequence of pulses, with the frequency of the pulsesbeing ameasure of the light intensity. Integrated circuit sensors, such as the Texas InstrumentTSL220,incorporate the light sensor and the voltage-to-frequency converter (Figure).
The term encoder is used for a device that provides a digital output as a result of angular orlineardisplacement. An incremental encoder detects changes in angular or linear displacementfromsome datum position; an absolute encoder gives the actual angular or linear position.
Figure shows the basic form of an incremental encoder for the measurement of angulardisplacement. A beam of light, perhaps from an LED, passes through slots in a disc and isdetected by a light sensor, such as a photodiode or phototransistor. When the disc rotates,
Thelight beam is alternately transmitted and stopped, and so a pulsed output is produced fromthelight sensor. The number of pulses is proportional to the angle through which the disc hasrotated, the resolution being proportional to the number of slots on a disc. With 60 slots,then,since one revolution is a rotation of 360 , a movement from one slot to the next is a rotationof 6 . By using offset slots it is possible to have over a thousand slots for one revolution andthus a much higher resolution.
This setup with just one track is a very basic form of incremental encoder with no way ofdetermining the direction of rotation. With a single track, the output is the same for bothdirections of rotation. Thus, generally such encoders have two or three tracks with sensors(Figure). With two tracks, one track is one-quarter of a cycle displaced from the othertrack. As a consequence, the output from one track will lead or lag that from the other track,depending on the direction of rotation. A third track of just a single aperture is also included;this gives one pulse per revolution and so can be used for counting the number of fullrevolutions. The absolute encoder differs from the incremental encoder in having a pattern of slots thatuniquely defines each angular position. With the form shown in Figure, the rotating dischas four concentric circles of slots and four sensors to detect the light pulses. The slots arearranged in such a way that the sequential output from the sensors is a number in the binarycode, each number corresponding to a particular angular position. With four tracks there willbe 4 bits, and so the number of positions that can be detected is 24 ¼ 16, that is, a resolutionof 360/16 ¼ 22.5 . Typical encoders have up to 10 or 12 tracks. The number of bits in thebinary 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 210, that is, 1024, a resolutionof 360/1024 ¼ 0.35 .
Though the normal form of binary code is shown in the figure, in practice a modified formof binary code called the Gray code is generally used. This, unlike normal binary, hasonly 1 bit that changes in moving from one number to the next. This codeprovides data with the least uncertainty, but since we are likely to need to run systemswith binary code, a circuit to convert Gray to binary code has to be used.
A simple form of temperature sensor that can be used to provide an on/off signal when aparticular temperature is reached is the bimetal element. This consists of two strips of different metals, such as brass and iron, bonded together (Figure). The two metals havedifferent coefficients of expansion. Thus, when the temperature of the bimetal stripincreases,the strip curves in order that one of the metals can expand more than the other.
The higherexpansion metal is on the outside of the curve. As the strip cools, the bending effect isreversed. This movement of the strip can be used to make or break electrical contacts andhence, at some particular temperature, give an on/off current in an electrical circuit. Thedevice is not very accurate but is commonly used in domestic central heating thermostatsbecause it is a very simple, robust device. Another form of temperature sensor is the resistive temperature detector (RTD). Theelectrical resistance of metals or semiconductors changes with temperature. In the case of ametal, the ones most commonly used are platinum, nickel, or nickel alloys. Such detectorscan be used as one arm of a Wheatstone bridge and the output of the bridge taken as ameasure of the temperature (Figure). For such a bridge, there is no output when theresistors in the bridge arms are such that P/Q ¼ R/S. Any departure of a resistance from thisbalance value results in an output. The resistance varies in a linear manner with temperatureover a wide range of temperatures, though the actual change in resistance per degree isfairlysmall. A problem with a resistance thermometer is that the leads connecting it to the bridgecan be quite long and themselves have significant resistance, which changes withtemperature. One way of overcoming this problem is to use a three-wire circuit, as shown in Figure. Then changes in lead resistance affect two arms of the bridge and balance out.
Such detectors are very stable and very accurate, though expensive. They are available in The form of wire-wound elements inside ceramic tubes or as thin film elements deposited on asuitable substrate.
Semiconductors, such as thermistors (Figure), show very large changes in resistancewith temperature. The change, however, is nonlinear. Those specified as NTC have negativetemperature coefficients, that is, the resistance decreases with increasing temperature, andthose specified as PTC have positive temperature coefficients, that is, the resistanceincreaseswith increasing temperature. They can be used with a Wheatstone bridge, but anotherpossibility that is widely used is to employ a potential divider circuit with the change inresistance of the thermistor changing the voltage drop across a resistor (Figure). Theoutput from either type of circuit is an analog signal that is a measure of the temperature.
Thermistors have the advantages of being cheap and small, giving large changes inresistance, and having fast reaction to temperature changes, though they have thedisadvantage of being nonlinear, with limited temperature ranges.
Thermodiodes and thermotransistors are used as temperature sensors since the rate atwhichelectrons and holes diffuse across semiconductor junctions is affected by the temperature. Integrated circuits can combine such a temperature-sensitive element with the relevantcircuitry to give an output voltage related to temperature. A widely used integrated packageis the LM35, which gives an output of 10 mV/ C when the supply voltage is þ5 V(Figure). A digital temperature switch can be produced with an analog sensor byfeeding the analog output into a comparator amplifier, which compares it with some setvalue, producing an output that gives a logic 1 signal when the temperature voltage input isequal to or greater than the set point and otherwise gives a logic 0 signal. Integrated circuits, such as LM3911N, are available, combining a thermotransistor temperature-sensitiveelementwith an operational amplifier. When the connections to the chip are so made that theamplifier is connected as a comparator, the output will switch as thetemperature traverses the set point and so directly give an on/off temperature controller.
Such temperature sensors have the advantages of being cheap and giving a reasonably linearresponse. However, they have the disadvantage of a limited temperature range.
Another commonly used temperature sensor is the thermocouple. The thermocouple consistsessentially of two dissimilar wires, A and B, forming a junction When thejunction is heated so that it is at a higher temperature than the other junctions in the circuit,which remain at a constant cold temperature, an EMF is produced that is related to the hotjunction temperature. The EMF values for a thermocouple are given in Table 2.2, assumingthat the cold junction is at 0 C. The thermocouple voltage is small and needs amplificationbefore it can be fed to the analog channel input of a PLC. There is also circuitry required tocompensate for the temperature of the cold junction, since often it will not be at 0 C, butroom temperature and its temperature affects the value of the EMF. The amplification andcompensation, together with filters to reduce the effect of interference from the mainssupply,are often combined in a signal processing unit. Thermocouples have the advantages of beingable to sense the temperature at almost any point, ruggedness, and being able to operateovera large temperature range. They have the disadvantages of giving a nonlinear response, giving only small changes in EMF per degree change in temperature, and requiringtemperature compensation for the cold junction.
The term position sensor is used for a sensor that gives a measure of the distance between A reference point and the current location of the target, while a displacement sensor gives ameasure of the distance between the present position of the target and the previouslyrecorded position.
Resistive linear and angular position sensors are widely used and relatively inexpensive. These are also called linear and rotary potentiometers. A DC voltage is provided across thefull length of the track and the voltage signal between a contact that slides over theresistancetrack and one end of the track is related to the position of the sliding contact between theends of the potentiometer resistance track he potentiometer thus provides ananalog linear or angular position sensor.
Another form of displacement sensor is the linear variable differential transformer (LVDT),which gives a voltage output related to the position of a ferrous rod. The LVDT consists ofthree symmetrically placed coils through which the ferrous rod moves Whenan alternating current is applied to the primary coil, alternating voltages, v1 and v2, areinduced in the two secondary coils. When the ferrous rod core is centered between the twosecondary coils, the voltages induced in them are equal. The outputs from the two secondarycoils are connected so that their combined output is the difference between the two voltages,that is, v1 – v2. With the rod central, the two alternating voltages are equal and so there is nooutput voltage. When the rod is displaced from its central position, there is more of the rod inone secondary coil than the other. As a result, the size of the alternating voltage induced inone coil is greater than that in the other. The difference between the two secondary coilvoltages, that is, the output, thus depends on the position of the ferrous rod. The output fromthe LVDT is an alternating voltage. This is usually converted to an analog DC voltage andamplified before inputting to the analog channel of a PLC.
Capacitive displacement sensors are essentially just parallel plate capacitors. Thecapacitance will change if the plate separation changes, the area of overlap of the plateschanges, or a slab of dielectric is moved into or out of the plates. All thesemethods can be used to give linear displacement sensors. The change in capacitance hasto be converted into a suitable electrical signal by signal conditioning.
When a wire or strip of semiconductor is stretched, its resistance changes. The fractionalchange in resistance is proportional to the fractional change in length, that is, strain.
where DR is the change in resistance for a wire of resistance R and G is a constant called thegauge factor. For metals, the gauge factor is about 2; for semiconductors, about 100. Metalresistance strain gauges are in the form of a flat coil so that they get a reasonable length ofmetal in a small area. Often they are etched from metal foil and attached to abacking of thin plastic film so that they can be stuck on surfaces, like postage stamps on anenvelope. The change in resistance of the strain gauge, when subject to strain, is usuallyconverted into a voltage signal by the use of a Wheatstone bridge. A problem that occurs isthatthe resistance of the strain gauge also changes with temperature, and thus some means oftemperature compensation has to be used so that the output of the bridge is only a functionofthe strain. This can be achieved by placing a dummy strain gauge in an opposite arm of thebridge, that gauge not being subject to any strain but only the temperature. Apopular alternative is to use four active gauges as the arms of the bridge and arrange themso that one pair of opposite gauges is in tension and the other pair in compression. This notonly gives temperature compensation; it also gives a much larger output change when strainis applied. The following paragraph illustrates systems employing such a form ofcompensation.
By attachingstrain gauges to other devices, changes that result in strain of those devicescan be transformed, by the strain gauges, to give voltage changes. They might, forexample, be attached to a cantilever to which forces are applied at its free end.
The voltage change, resulting from the strain gauges and the Wheatstone bridge, thenbecomes a measure of the force. Another possibility is to attach strain gauges to adiaphragm,which deforms as a result of pressure (Figure). The output from the gauges andassociated Wheatstone bridge then becomes a measure of the pressure.
Pressure sensors can be designed to give outputs that are proportional to the difference inpressure between two input ports. If one of the ports is left open to the atmosphere, thegaugemeasures pressure changes with respect to the atmosphere and the pressure measured isknown as gauge pressure. The pressure is termed the absolute pressure if it is measuredwithrespect to a vacuum. Commonly used pressure sensors that give responses related to thepressure are diaphragm and bellows types. The diaphragm type consists of a thin disc ofmetal or plastic, secured around its edges. When there is a pressure difference between thetwo sides of the diaphragm, its center deflects. The amount of deflection is related to thepressure difference. This deflection may be detected by strain gauges attached to thediaphragm, by a change in capacitance between it and a parallel fixedplate, or by using the deflection to squeeze a piezoelectric crystal.
When a piezoelectric crystal is squeezed, there is a relative displacement of positive andnegative charges within the crystal and the outer surfaces of the crystal become charged.
Hence a potential difference appears across it. An example of such a sensor is the MotorolaMPX100AP sensor (Figure). This has a built-in vacuum on one side of the diaphragmand so the deflection of the diaphragm gives a measure of the absolute pressure applied to theother side of the diaphragm. The output is a voltage that is proportional to the appliedpressure, with a sensitivity of 0.6 mV/kPa. Other versions are available that have one side ofthe diaphragm open to the atmosphere and so can be used to measure gauge pressure;othersallow pressure to be applied to both sides of the diaphragm and so can be used to measuredifferential pressures.
Pressure switches are designed to switch on or off at a particular pressure. A typical forminvolves a diaphragm or bellows that moves under the action of the pressure and operates amechanical switch. Figure shows two possible forms. Diaphragms are less sensitive thanbellows but can withstand greater pressures.
Pressure sensors may be used to monitor the depth of a liquid in a tank. The pressure due to a height of liquid h above some level is hrg, where r is the density of the liquid and g theacceleration due to gravity. Thus a commonly used method of determining the level of liquidin a tank is to measure the pressure due to the liquid above some datum level (Figure). Often a sensor is just required to give a signal when the level in some container reaches aparticular level. A float switch that is used for this purpose consists of a float containing amagnetthat moves in a housing with a reed switch. As the float rises or falls, it turns the reed switchon oroff, the reed switch being connected in a circuit that then switches a voltage on or off.
Fluid Flow Measurement
A common form of fluid flow meter is one based on measuring the difference in pressure thatresults when a fluid flows through a constriction. Figure shows a commonly used form,the orifice flow meter. As a result of the fluid flowing through the orifice, the pressure at A ishigher than that at B, the difference in pressure being a measure of the rate of flow. This pressure difference can be monitored by means of a diaphragm pressure gauge and thusbecomes a measure of the rate of flow.
To use a sensor, we generally need to add signal conditioning circuitry, such as circuits whichamplify and convert from analog to digital, to get the sensor signal in the right form, takeaccount of any nonlinearities, and calibrate it. Additionally, we need to take account of drift,that is, a gradual change in the properties of a sensor over time. Some sensors have alltheseelements taken care of in a single package; they are called smart sensors.
The term smart sensor is thus used in discussing a sensor that is integrated with the requiredbuffering and conditioning circuitry in a single element and provides functions beyond that ofjust a sensor. The circuitry with the element usually consists of data converters, a processorandfirmware, and some form of nonvolatile electrically erasable programmable read onlymemory. The term nonvolatile is used becausethe memory has to retain certain parameters when the power supply is removed. Such smartsensors can have all their elements produced on a single silicon chip. Because the elementsareprocessor-based devices, such a sensor can be programmed for specific requirements. Forexample, it can be programmed to process the raw input data, correcting for such things asnonlinearities, and then send the processed data to a base station. It can be programmed tosenda warning signal when the measured parameter reaches some The IEEE 1451.4 standard interface for smart sensors and actu actroitrisc aisl vbaalsueed. on an electronicdata sheet (TEDS) format that is aimed at allowing installed analog transducers to be easilyconnected to digital measurement systems. The standard requires the nonvolatile EEPROMembedded memory to hold and communicate data, which will allow a plug-and-playcapability. It thus would hold data for the identification and properties for the sensor andmight also contain the calibration template, thus facilitating digital interrogation.
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