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CHAPTER 6 U N I M A P SENSORS AND TRANDUCERS 6.1 INTRODUCTION U N I M A P OBJECTIVES To introduce the basic concepts in measurement systems To define sensor terminology To identify sensor applications To present the need for microsensors 6.2 MEASUREMENT SYSTEMS Input Signal U N I M A P = Measurand Sensor (Input Tranducer) Physical Quantity Chemical Quantity Eg. displacement, presure Eg. Gas concentration Sensor : a device that converts a non-electrical physical or chemical quantity into an electrical signal. 6.2 MEASUREMENT SYSTEMS (cont……) U N I M A P System Boundary Display Input Signal Detect Modify Record Output Signal Transmit Fig. 6-1: Functional block diagram of a measurement 6.2 MEASUREMENT SYSTEMS (cont…) System Boundary U N I M A P Input Signal Sensor Processor (input transducer) Actuator Output Signal (output transducer) Fig. 6-2: Basic components of a measurement or information-processing system Processor : a device that modifies the electrical signal coming from the sensor without changing the form of the energy that describes the signal. Actuator or output transducer : a device that converts an electrical signal into a physical or chemical quantity. 6.3 CLASSIFICATION OF SENSING DEVICES Table 6-1: Classification of sensors by signal form. Form of Signal U N I M A P Measurands Thermal Temperature, heat, heat flow, entropy, heat capacity. Radiation Gamma rays, X-rays, ultra-violet, visible, infra red, micro-waves, radio waves. Mechanical Displacement, velocity, acceleration, force, torque, pressure, mass, flow, acoustic wavelength and amplitude. Magnetic Magnetic field, flux, magnetic moment, magnetisation, magnetic permeability. Chemical Humidity, pH level and ions, concentration of gases, vapours and odours, toxic and flammable materials, pollutants. 6.3 CLASSIFICATION OF SENSING DEVICES (cont……) Table 6-1: Classification of sensors by signal form. U N I M A P Form of Signal Measurands Biological Sugars, proteins, hormones, antigens. Electrical Charge, current, voltage, resistance, conductance, capacitance, inductance, dielectric permittivity, polarisation, frequency. 6.3 CLASSIFICATION OF SENSING DEVICES (cont……) Table 6-2: Classification of the human senses . U N I M A P Human Sense Sight Hearing Signal Radiant Mechanical Measurand Sensing Device Analogue Device Intensity and Rods and Photographic wavelength cones in film, photodiode, of light retina Phototransistor Intensity and Cochlea in Microphone frequency of inner ear sound Smell Chemical Odorants Olfactory receptor cells in nose Electronic nose 6.3 CLASSIFICATION OF SENSING DEVICES (cont……) Table 6-2: Classification of the human senses . U N I M A P Human Sense Touch Signal Mechanical Measurand Pressure, Sensing Device Nerves force Analogue Device Potentiometers and LVDTs (simple touch), optical gauging and tactical arrays (complex touch) Taste Biological Proteins Taste buds in tongue 6.3 CLASSIFICATION OF SENSING DEVICES (cont……) Table 6-3: Classification of some common actuators. U N I M A P Function Display Actuator Light emitting Signal Radiant diode Principle Current generation of photons Visual display unit Radiant Fluorescent screen Liquid crystal Radiant Transmittance of display polarised molecular Crystals Transmit Loudspeaker Mechanical Generation of sound Aerial Radiant Generation of radio wave Electric motor Mechanical Generation of motion 6.3 CLASSIFICATION OF SENSING DEVICES (cont……) Table 6-3: Classification of some common actuators. U N I M A P Function Record Actuator Signal Thermal printer Thermal Ink is melted Magnetic recording Magnetic Magnetisation of thin head Principle films on computer disc Laser Radiant Ablation of material on optical disc 6.4 Ideal Sensor Characteristics and Practical Limitations U N I M A P A sensor in its simplest form may be regarded as a system with an input x (t) and output y (t). (a) Self-exciting (b) Modulating External xd drive System Input x (t) SENSOR Output Input y (t) x (t) SENSOR Output y (t) + yd Fig. 6-3: Basic representation of self-exciting and modulating sensor systems 6.4 Ideal Sensor Characteristics and Practical Limitations (cont……) U N I M A P A self-exciting sensor has its output energy supplied entirely by the input signal x (t). The general equation that describes a self-exciting sensor system is y (t ) F ( x(t )) (6.1) where F(x(t)) is the characteristic relationship that describes the behavior of a self-exciting sensor. 6.4 Ideal Sensor Characteristics and Practical Limitations (cont……) U N I M A P Eg : A thermocouple → input signal = the difference in junction temperatures ΔT(t) and the output = e.m.f Φ(t) in volts. (t ) F (T (t )) In the case of the modulating sensor, the system equation can be written more explicitly as y(t ) F ( x(t ) xd ) where the external supply signal xd(t) should ideally be stationary and noise free. (6.2) 6.4 Ideal Sensor Characteristics and Practical Limitations (cont……) U N I M A P The ideal sensor not only has a linear output signal y(t) but it should instantaneously follow the input signal x(t), whence y (t ) S .x(t ) ym Sensor output Slope, S yd (6.3) xd = 0 0 The slope S is usually referred to as the sensitivity. xd ≠ 0 Sensor input xm-xd Fig. 6-4: Ideal input-output relationship of self-exciting and modulating sensors. General properties of a good sensor are: 1) Optimum measurement accuracy 2) Good durability U N I M A P 3) Ease of calibration and reconditioning 4) High sensitivity 5) Good reproducibility 6) Long term stability 7) Fast response 8) Continuous operation 9) Insensitivity to electrical and other environmental interference 10) Low fabrication, operation and maintenance cost