B. Transducers
Transducers are devices that transform values of physical variables into equivalent electrical signals-e.g., thermocouples and strain gauges. Two broad categories of transducers are passive and active. A passive transducer supplies output energy entirely or almost entirely by its input signal-e.g., thermocouples. An active transducer, on the other hand , requires an auxiliary source of power to supply a major part of the output signal, while the input signal supplies only an insignificant portion-e.g., strain gauges. Some of the basic transducers are as follows.
1. Variable-Resistance or Potentiometric Transducer
A variable – resistance transducer converts either linear or angular displacement into an electric signal. This may be in the form of a moving contact on a slide wire or on a coil of wire, through either linear or angular movement (Fig.1). Depending on the type of resistance wire wound, the change may be linear, logarithmic, exponential, etc. Pressure can be converted to a displacement through mechanical methods so that the device can be used in force and pressure measurements. These provide high electric efficiency; however, life is limited due to wear, and noise will develop due to wearout of the element.
2. Linear Variable Differential Transformer
A linear variable differential transformer (LVDT) is a special case of a differential transformer. In an LVDT, three coils are arranged linearly with a freely movable magnetic core inside the coils (Fig.2). An ac input voltage is applied on the center coil, and the output voltage from the two end coils depends on the magnetic coupling between the core and the coils. This coupling, in turn, depends on the position of the core. Thus the output voltage of the LVDT is an indication of the core displacement. There is a 180 phase shift from one side of the null position to the other. This transducer provides continuous resolution and low hysteresis, but it is sensitive to vibration.
3.Capacitive Transducer
The capacitance between the two parallel plates is given by C=9.85