ABSTRACT
CONTENTS 2.1 Role of Transducers in Food Processing.................................................. 28 2.2 Classification of Transducers ..................................................................... 30
2.2.1 Self-Generating Type ...................................................................... 30 2.2.2 Variable Parameter Type................................................................ 30
2.2.2.1 Change in Resistance or Conductance.......................... 31 2.2.2.2 Change in Capacitance .................................................... 31 2.2.2.3 Change in Magnetic Properties ...................................... 31
2.2.3 Pulse or Frequency Generating Types......................................... 32 2.2.4 Digital Transducers ......................................................................... 32
2.3 Self-Generating Transducers ...................................................................... 33 2.3.1 Piezoelectric Transducers............................................................... 33 2.3.2 Thermocouples ................................................................................ 36
2.3.2.1 Thermocouple Materials.................................................. 38 2.3.2.2 Reference Junction Compensation................................. 39 2.3.2.3 Thermocouple Installations............................................. 39 2.3.2.4 Thermocouple Wire Insulators ....................................... 40
2.3.3 Photoelectric Transducers .............................................................. 41 2.3.3.1 Photo-Emissive Transducer............................................. 41 2.3.3.2 Photoconductive Cell ....................................................... 42 2.3.3.3 Photovoltaic Cell............................................................... 43 2.3.3.4 Photodiode and Phototransistor .................................... 44
2.3.4 Magneto-Electric Transducer......................................................... 44 2.3.4.1 Electrodynamic Transducer............................................. 45 2.3.4.2 Electromagnetic Transducer............................................ 46 2.3.4.3 Pulse Generator Type....................................................... 46
2.3.5 Radioactive Transducer.................................................................. 47 2.3.5.1 Penetrating Power ............................................................ 47 2.3.5.2 Half Life ............................................................................. 49 2.3.5.3 Half Distance ..................................................................... 49
2.4 Variable Parameter Type ............................................................................ 51 2.4.1 Resistive Transducer ....................................................................... 51
2.4.1.1 Potentiometer .................................................................... 52 2.4.1.2 Resistance Strain Gauge .................................................. 54
2.4.1.3 Diaphragm Pressure Sensor............................................ 60 2.4.1.4 Load Cell ............................................................................ 63 2.4.1.5 Resistance Temperature Detectors ................................. 63 2.4.1.6 Thermistors ........................................................................ 66 2.4.1.7 Semiconductor Temperature Sensors ............................ 69 2.4.1.8 Hot Wire Anemometer..................................................... 71 2.4.1.9 Resistive Humidity Sensors ............................................ 72
2.4.2 Inductive Transducers .................................................................... 72 2.4.2.1 Variable Inductance Type ................................................ 73 2.4.2.2 Variable Reluctance Type Sensors.................................. 74
2.4.3 Capacitive Transducer .................................................................... 77 2.5 Digital Transducers ..................................................................................... 80
2.5.1 Direct Digital Encoder.................................................................... 80 2.5.2 Frequency, Pulse Encoder .............................................................. 80 2.5.3 Analog-to-Digital Encoder............................................................. 81 2.5.4 Analog-to-Digital Converter ......................................................... 81 2.5.5 Digital Encoders .............................................................................. 82
2.5.5.1 Digital Shaft Encoder ....................................................... 82 2.6 Selection of Transducers ............................................................................. 83 2.7 Actuating and Controlling Devices .......................................................... 84
2.7.1 Actuators .......................................................................................... 85 2.7.1.1 Electrical Solenoids........................................................... 86 2.7.1.2 Electrical Motors ............................................................... 86 2.7.1.3 Fluid Control Valves ........................................................ 86
2.7.2 Actuating Motors ............................................................................ 88 2.7.2.1 DC Motors.......................................................................... 88 2.7.2.2 Control Motors .................................................................. 89 2.7.2.3 Stepper Motors.................................................................. 92
2.7.3 Final Control Elements................................................................... 96 2.7.3.1 Mechanical Types.............................................................. 97 2.7.3.2 Electrical Types.................................................................. 98
2.7.4 Pneumatic Control Devices ........................................................... 98 2.7.4.1 Baffle-Nozzle Detector ..................................................... 99 2.7.4.2 Pneumatic Amplifiers .................................................... 100
2.8 Conclusion .................................................................................................. 103 Further Reading ................................................................................... 103
Transducers and sensors are the basic input devices for industrial process measurement and control. In some industries automatic control devices are not adopted, either due to lack of necessity or lack of cost-effectiveness. In such situations, transducers might be used only for routine checks of various
process parameters such as oven temperature in a small-scale biscuit factory. In this case, obviously, a microprocessor or a computer-based automatic controller is not cost effective. Hence, observing the temperature of the oven by a thermocouple-based temperature indicator is sufficient. This implies that the measurement is taken manually so that the dough does not get overbaked or underbaked.
