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EEE205. Yrd. Doç. Dr. Mehmet Ali Aktaş. Sensors. In order to perform useful tasks electronic systems must interact with the world about them. To do this they use sensors to sense external physical quantities and actuators to affect or control them .
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EEE205 Yrd. Doç. Dr. Mehmet Ali Aktaş
Sensors • In order to perform useful tasks electronic systems must interact with theworld about them. To do this they use sensors to sense external physicalquantities and actuators to affect or control them. • Sensors and actuators are often referred to as transducers. • A transducer is a device that converts a signal in one form of energyto another form of energy.
Sensors • ForExample • Examplesinclude a mercury-in-glass thermometer, which converts variations in temperatureinto variations in the length of a mercury column, and a microphone,which converts sound into electrical signals. • Thermometers and microphones are both examples of sensors that convertone form of analogue quantity into another. Other sensors can be usedwith digital quantities, converting one digital quantity into another. • A third class of sensors take an analogue quantity and represent it in adigital form. In some instances the output is a simple binary representationof the input, as in a thermostat, which produces one of two output values
Sensors • Almost any physical property of a material that varies in response to someexcitation can be used to produce a sensor. Commonly used devices includethosewhoseoperation is: • resistive • inductive • capacitive • piezoelectric • photoelectric • elastic • thermal.
Describing Sensor Performance • When describing sensors and instrumentation systems we make use ofa range of terms to quantify their characteristics and performance. It isimportant to have a clear understanding of this terminology.
Describing Sensor Performance • Range • This defines the maximum and minimum values of the quantity that thesensor or instrument is designed to measure. • Resolutionordiscrimination • This is the smallest discernible change in the measured quantity that the sensor is abletodetect. This is usually expressed as a percentage of the range ofthedevice. • Error • This is the difference between a measured value and its true value. Errorsmay be divided into random errors and systematic errors. • Random errorsproduce scatter within repeated readings. The effects of such errors maybe quantified by comparing multiple readings and noting the amount of scatterpresent. The effects of random errors may also be reduced by taking theaverage of these repeated readings. • Systematic errors affect all readings ina similar manner and are caused by factors such as mis-calibration. Since allreadings are affected in the same way, taking multiple readings does notallow quantification or reduction of such errors.
Describing Sensor Performance • Accuracy, inaccuracyanduncertainty • The term accuracy describes the maximum expected error associated witha measurement (or a sensor) and may be expressed as an absolute value oras a percentage of the range of the system. For example, the accuracy of avehicle speed sensor might be given as ±1 mph or as ±0.5 per cent of thefull-scalereading. • Precision • This is a measure of the lack of random errors (scatter) produced by asensor or instrument. Devices with high precision will produce repeatedreadings with very little spread. It should be noted that precision is veryoften confused with accuracy, which has a very different meaning. Asensor might produce a range of readings that are very consistent but thatareallveryinaccurate.
Describing Sensor Performance • Sensitivity • This is a measure of the change produced at the output for a given change inthe quantity being measured. A sensor that has high sensitivity will producea large change in its output for a given input change.
TemperatureSensors • The measurement of temperature is a fundamental part of a large number ofcontrol and monitoring systems, ranging from simple temperature-regulatingsystems for buildings to complex industrial process-control plants. • Temperature sensors may be divided into those that give a simple binaryoutput to indicate that the temperature is above or below some thresholdvalue and those that allow temperature measurements to be made. • A large number of different techniques are used for temperature measurement,but here we will consider just three forms.
TemperatureSensors • Resistivethermometers • The electrical resistance of all conducting materials changes with temperature.The resistance of a piece of metal varies linearly with its absolutetemperature. This allows temperature to be measured by determining theresistance of a sample of the metal and comparing it with its resistance ata known temperature. Typical devices use platinum wire; such devices areknown as platinum resistance thermometers or PRTs. • PRTs can produce very accurate measurements at temperatures from lessthan −150 °C to nearly 1000 °C to an accuracy of about 0.1 °C, or 0.1 percent. • However, they have poor sensitivity. A typical PRT might have a resistance of 100 Ω at 0 °C, which increases toabout 140 Ω at 100 °C.
TemperatureSensors • Thermistors • Like PRTs, these devices also change their resistance with temperature.However, they use materials with high thermal coefficients of resistance togive much improved sensitivity. A typical device might have a resistanceof 5 kΩ at 0 °C and 100 Ω at 100 °C. • pnjunctions • A pnjunction is a semiconductor device that has the properties of a diode.That is, it conducts electricity in one direction (when the device is said to beforward-biased) but opposes the flow of electricity in the other direction(when the device is said to be reverse-biased).At a fixed current, the voltage across a typical forward-biased semiconductordiode changes by about 2 mV per °C.These devices are inexpensive, linear andeasy to use but are limited to a temperature range from about −50 °C toabout 150 °C by the semiconductor materials used.
LightSensors • Sensors for measuring light intensity fall into two main categories: thosethat generate electricity when illuminated and those whose properties (forexample, their resistance) change under the influence of light. We
LightSensors • Photovoltaic • Light falling on a pnjunction produces a voltage and can therefore be usedto generate power from light energy. This principle is used in solar cells. Ona smaller scale, photodiodes can be used to measure light intensity, sincethey produce an output voltage that depends on the amount of light falling on them. A disadvantage of this method of measurement is that the voltageproduced is not related linearly to the incident light intensity. • Photoconductive • Photoconductive sensors do not generate electricity, but their conduction ofelectricitychangeswithillumination. • The photodiode described above asa photovoltaic device may also be used as a photoconductive device. If aphotodiode is reverse biased by an external voltage source, in the absence oflight it will behave like any other diode and conduct only a negligible leakagecurrent. However, if light is allowed to fall on the device, charge carriers willbe formed in the junction region and a current will flow. The magnitude ofthis current is proportional to the intensity of the incident light, making itmore suitable for measurement than the photovoltaic arrangement describedearlier. • A third class of photoconductive device is the light-dependent resistoror LDR. As its name implies, this is a resistive device that changes itsresistancewhenilluminated.
Force Sensors • StrainGauge • The resistance between opposite faces of a rectangular piece of uniform electricallyconducting material is proportional to the distance between the facesand inversely proportional to its cross-sectional area. The shape of such anobject may be changed by applying an external force to it. The term stressis used to define the force per unit area applied to the object, and the termstrain refers to the deformation produced. In a strain gauge, an applied forcedeforms the sensor, increasing or decreasing its length (and its cross-section)and therefore changing its resistance.
Force Sensors • Piezoelectric • Piezoelectric materials have the characteristic that they generate an electricaloutput when subjected to mechanical stress. Unfortunately, the output is nota simple voltage proportional to the applied force, but an amount of electriccharge which is related to the applied stress.
Sound Sensors • A number of techniques are used to detect sound. Since sound representsvariations in air pressure, the objective of the microphone is to measurethese variations and to represent them by some form of electrical signal(often in the form of a varying voltage).
Sound Sensors • Carbonmicrophones • Carbon microphones are one of the oldest and simplest forms of sounddetector. Sound waves are detected by a diaphragm which forms one sideof an enclosure containing carbon particles. Sound waves striking thediaphragm cause it to move, compressing the carbon particles to a greater orlesser degree and thus affecting their resistance. Electrodes apply a voltageacross the particles and the resulting current thus relates to the sound strikingthedevice.
Sound Sensors • Capacitivemicrophones • Capacitive microphones are similar in operation to the carbon microphonedescribed above except that movement of the diaphragm causes a variationin capacitance rather than resistance. • Moving-coilmicrophones • A moving-coil microphone consists of a permanent magnet and a coil connectedto a diaphragm. Sound waves move the diaphragm which causes thecoil to move with respect to the magnet, thus generating an electrical signal.Moving-coil devices are probably the most common form of microphone. • Piezoelectricmicrophones • The piezoelectric force sensor described earlier can also be used as a microphone.The diaphragm is made of piezoelectric material which is distortedby sound waves producing a corresponding electrical signal.
