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Significance of Sensors and Transducers in Mechatronics System

The purpose of this paper is to give significance of various sensors and transducers in mechatronics system. In all the mechatronics system is the need to measure some physical quantities such as displacement, speed, position, velocity, motion, pressure, temperature etc. This paper includes terminology of performance parameter and different types of sensors and transducers with application.


I. INTRODUCTION
A sensor is an element in a measurement system that acquires a physical parameter and changes it into a signal. The sensing element is in contact with the object or process and provides the output, which depends upon the variation in the process, or experiences a change, which is related to process variables being monitored. Most sensors work by converting some physical parameter in to an electrical signal.
Transducer is an element in measurement system that converts one form of energy into other. Hence a transducer provides the means of conversion between two physical variables in a process.
Mechatronics basically refers to mechanical electronic systems and normally described as a synergistic combination of mechanics, electrical, electronics, computer and control which, when combined, make possible the generation of simple, more economic and reliable systems. [1] II. PERFORMANCE PARAMETERS 1) Range and span The range of transducer defines the limits between which the input can vary. The span is the maximum value of input minus minimum value of input.

2) Error
Error is the difference between the results of measurement and the true value of the quantity being measured.

3) Accuracy
The accuracy defines the closeness of the agreement between the actual measurement result and a true value of the measurand. It is often expressed as a percentage of the full range output or full-scale deflection.

4) Sensitivity
Sensitivity of a sensor is defined as the ratio of change in output value of a sensor to the per unit change in input value that causes the output change.

5) Nonlinearity
The nonlinearity indicates the maximum deviation of the actual measured curve of a sensor from the ideal curve. Nonlinearity (%) = Maximum deviation in input ⁄ Maximum full scale input

6) Hysteresis error
The hysteresis is an error of a sensor, which is defined as the maximum difference in output at any measurement value within the sensor's specified range when approaching the point first with increasing and then with decreasing the input parameter.

Fig.2 hysteresis error [2] 7) Resolution
Resolution is the smallest detectable incremental change of input parameter that can be detected in the output signal. Resolution can be expressed either as a proportion of the full-scale reading or in absolute terms.

8) Stability
Stability is the ability of a sensor device to give same output when used to measure a constant input over a period of time. The term 'drift' is used to indicate the change in output that occurs over a period of time. It is expressed as the percentage of full range output.

9) Dead band/time
The dead band or dead space of a transducer is the range of input values for which there is no output. The dead time of a sensor device is the time duration from the application of an input until the output begins to respond or change.

10) Repeatability
It specifies the ability of a sensor to give same output for repeated applications of same input value. It is usually expressed as a percentage of the full range output: Repeatability = (maximum -minimum values given) X 100 ⁄ full range 11) Response time Response time describes the speed of change in the output on a step-wise change of the measurand. It is always specified with an indication of input step and the output range for which the response time is defined.  Photodiode is a solid-state device which converts incident light into an electric current. It is made of Silicon. It consists of a shallow diffused p-n junction, normally a p-on-n configuration. When photons of energy greater than 1.1eV (the band gap of silicon) fall on the device, they are absorbed and electron-hole pairs are created. The depth at which the photons are absorbed depends upon their energy. The lower the energy of the photons, the deeper they are absorbed. Then the electronhole pairs drift apart. When the minority carriers reach the junction, they are swept across by the electric field and an electric current establishes.

III.CLASSIFICATION OF SENSORS AND TRANSDUCER
Photodiodes are one of the types of photo detector, which convert light into either current or voltage. These are regular semiconductor diodes except that they may be either exposed to detect vacuum UV or X-rays or packaged with a opening or optical fibres connection to allow light to reach the sensitive part of the device. Fig.3 Construction of photo diode detector Figure 3 shows the construction of Photo diode detector. It is constructed from single crystal silicon wafers. It is a p-n junction device. The upper layer is p layer. It is very thin and formed by thermal diffusion or ion implantation of doping material such as boron. Depletion region is narrow and is sandwiched between p layer and bulk n type layer of silicon. Light irradiates at front surface, anode, while the back surface is cathode. The incidence of light on anode generates a flow of electron across the p-n junction which is the measure of light intensity.

Resistance temperature detectors (RTDs) :
RTDs work on the principle that the electric resistance of a metal changes due to change in its temperature. On heating up metals, their resistance increases and follows a linear relationship as shown in Figure 4. The correlation is Rt = R0 (1 + αT) where Rt is the resistance at temperature T (°C) and R0 is the temperature at 0°C and α is the constant for the metal termed as temperature coefficient of resistance. The sensor is usually made to have a resistance of 100 Ω at 0 °C  Figure 5 shows the construction of a RTD. It has a resistor element connected to a Wheatstone bridge. The element and the connection leads are insulated and protected by a sheath. A small amount of current is continuously passing though the coil. As the temperature changes the resistance of the coil changes which is detected at the Wheatstone bridge Differential Pressure Sensor (DPS): Differential pressure flow meters have a primary and a secondary element. Generally speaking , the primary element is designed to produce a difference in pressure as the flow increases. There are many different types of primary element, the most common being the orifice plate, venturi, flow nozzle and Pitot tube.
The secondary element of the flow meter is the differential pressure transmitter. It is designed to measure the differential pressure produced by the primary element as accurately as possible. In particular it is important that the differential pressure measurement is not affected by changes in the fluid pressure, temperature or other properties such as ambient temperature. The output signal from and industrial DP transmitter is likely to be 4-20mA.