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Debasmit Das 10115039 Batch : E3 Introduction A resistive sensor is a transducer or electromechanical device that converts a mechanical change such as displacement into an electrical signal that can be monitored after conditioning. Resistive sensors are among the most common in instrumentation. These Transducers do NOT generate electricity. Hence, they are called passive devices. The simplest resistive sensor is the potentiometer. Other resistive sensors include strain gauges, thermocouples, photoresistors and thermistors. Theory of Operation Resistance = (Resistivity * Length)/Area The resistance of a material depends on four factors: · Composition 𝜌𝑙 · Length 𝑅= · Temperature 𝐴 · Cross Sectional Area • To change the resistance of a material, you must change the value of one of the above factors. • When length is modified the change in resistance is direct. If you double the material’s length, it’s resistance doubles. When the cross sectional area is modified the change in resistance has an inverse effect, IE R = k/A. If you double the cross-sectional area of wire, its resistivity is cut in half. • But, Changes in composition and temperature do not change the resistivity of a material in such a simple way. Examples of Resistive Transducers Sliding contact devices Wire resistance strain gauge Thermistors Thermocouples Light Dependent Resistors (LDRs) Device Action Light Dependent Resistor Resistance falls with increasing light level Application Light operated switches Thermistor Resistance falls with increased temperature Electronic thermometers Strain gauge Resistance changes with force Sensor in an electronic balance Moisture detector Resistance falls when wet Damp meter Sliding contact devices There is a long conductor whose effective length is variable. One end of the conductor is fixed, while the position of the other end is decided by the slider or the brush that can move along the whole length of the conductor along with the body whose displacement is to be measured. When the body moves the slider also moves along the conductor so its effective length changes, due to which it resistance also changes. These devices can be used to measured linear as well as angular displacement. Construction of rotary and slider types The unit consists basically of a ‘track’ having a fixed resistance and a variable contact which can be moved along and make continuous contact with the track. If the track resistance is proportional to the length along the track (i.e. linear track), the output voltage will be proportional to the movement of the variable contact and the unit is suitable for use as a position transducer. The track may comprise a film of carbon formed on a substrate or may be a length of resistance wire wound on an insulator former. Applications: Potentiometer The Potential divider is the most obvious application. In its simplest form it is two resistors in series with an input voltage Vs across the ends. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. Potentiometers were formerly used to control picture brightness, contrast, and color response in Television sets. Low-power potentiometers, both linear and rotary, are used to control audio equipment, changing loudness, frequency attenuation and other characteristics of audio signals. Potential Divider Circuit Potentiometer Strain Gauge If a strip of conductive metal is placed under compressive force (without buckling), it will broaden and shorten. If these stresses are kept within the elastic limit of the metal strip (so that the strip does not permanently deform), the strip can be used as a measuring element for physical force, the amount of applied force inferred from measuring its resistance. This is the principle of a Strain Gauge. Gauge Factor The gauge factor is defined as: where ∆R is the change in resistance caused by strain, RG is the resistance of the undeformed gauge, and ∈ is Strain Also expression for Strain is, ∈ = ∆ L/L For metallic foil gauges, the gauge factor is usually a little over 2 Half Bridge Strain Gauge Circuit Unlike the Wheatstone bridge using a nullbalance detector and a human operator to maintain a state of balance, a strain gauge bridge circuit indicates measured strain by the degree of imbalance, and uses a precision voltmeter in the center of the bridge to provide an accurate measurement of that imbalance: Strain Gauge With no force applied to the test specimen, both strain gauges have equal resistance and the bridge circuit is balanced. However, when a downward force is applied to the free end of the specimen, it will bend downward, stretching gauge #1 and compressing gauge #2 at the same time: Applications : Strain Gauge Strain gauges are used to measure force and small displacements. They are used for analyzing the dynamic strain of complex structures. They are used to measure tension, torque etc. Types of strain gauges are: (a) Wire strain gauges (b) Foil strain gauges (c) Thin film (d) Semiconductor Thermistors Thermistors work on the principle that resistance of some materials changes with the change in their temperature. When the temperature of the material changes, its resistance changes and it can be measured easily and calibrated against the input quantity. The commonly used thermistors are made up of the ceramic like semiconducting materials such as oxides of manganese, nickel and cobalt. Thermistors can be used for the measurement of temperature, as electric power sensing devices and also as the controls for various processes. Thermistors The most common type of thermistor that we use has a resistance that falls as the temperature rises. It is referred to as a negative temperature coefficient device(NTC). A positive temperature coefficient(PTC) device has a resistance that increases with temperature. Thermistor Analysis The thermistor resistance-temperature relationship can be approximated by, 𝑅 = where: 1 1 𝛽( − ) 𝑇 𝑇 𝑅𝑅𝑒𝑓. 𝑒 𝑅𝑒𝑓 T is temperature (in kelvin), TRef is the reference temperature, usually at room temp. (25 °C; 77 °F; 298.15 K), R is the resistance of the thermistor (W), RRef is the resistance at TRef, b is a calibration constant depending on the thermistor material, usually between 3,000 and 5,000 K. Thermistor Analysis The graph of resistance against temperature is like this. The resistance on this graph is on a logarithmic scale, as there is a large range of values. Applications of thermistors Measurement of temperature Measurement of Difference of two temperatures Control of temperature Temperature compensation Thermal conductivity measurement. Measurement of Gas Composition Measurement of Flow Current-limiting devices replacement for fuse (PTC) for circuit protection as Thermocouple The thermocouples work on the principle of Seebeck effect, Peltier effect and Thomson effect. As per the Seebeck effect, when two dissimilar elements are joined at their ends the electromotive force exists at their junction. As per Peltier effect, the amount of electromotive force generated depends on the temperature of the junction While, the Thomson effect says that the amount of voltage generated depends on the temperature gradient along the conductors in the circuit. The voltage output from the thermocouple changes as its temperature changes or the temperature of the body in whose contact it is changes. The voltage output is calibrated against the temperature of the body that can be measured easily. Thermocouple is a very popular device used for measurement of temperature. Thermocouple Internal Circuit Types of Thermocouples A thermocouple is available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. There are high temperature calibrations R, S, C and GB. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple. Applications : Thermocouple Steel Industry Heating Appliance Safety Power Production : Thermoelectric Generation Thermoelectric Cooling Diesel Engines Gas Turbine Exhaust Temperature Measurement Temperature Variation of Resistive Sensors Light Dependent Resistor The light dependent resistor consists of a length of material (cadmium sulphide) whose resistance changes according to the light level. Therefore the brighter the light, the lower the resistance. Principle of Operation An LDR is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance. LDR Applications Smoke detection Automatic lighting Clock Radios Alarm systems Dynamic Compressors Solar Street Lamps Camera Light meters References Wikipedia http://www.eecs.berkeley.edu/~culler/WEI/labs/lab7 sensing/sensing.htm http://www.ce-transducer.com/Resistance.asp http://forum.onestopgate.com/forum_posts.asp?TID=3536 http://www.eee.metu.edu.tr/~koray/exp1.pdf http://www.brighthubengineering.com/hvac/53335variable-resistance-transducers/ http://ptuas.loremate.com/beee/node/5