STATIC MECHANOELECTRICAL TRANSDUCERS 



magnetic field, the output voltage is accurately proportional to the velocity 

 of the coil. Such a system is illustrated in Figure 33.13; it will be seen that it 

 is similar to the ordinary moving-coil loudspeaker. 



With a moving system weighing several grammes and a suspension 

 compliance of about 10~^ cm per dyne (10 f,i per g wt), the usual moving-coil 

 transducer has a somewhat inconvenient mechanical impedance. When 

 used for measuring velocity its compliance is so low that undue loading of 

 the preparation will occur, while as a force transducer its resonant frequency 

 is uncomfortably low. However, when loading effects are of little importance 

 a moving-coil transducer provides an accurate measurement of velocity with 

 an output which may be as much as 1 V per cm/sec. Transducers of this type 

 can readily be adapted from commercial moving-coil loudspeakers. 



STATIC MECHANOELECTRICAL TRANSDUCERS 



Dynamic transducers, being self-generating, derive all their electrical output 

 power from the mechanical system under observation. Static transducers, 

 however, require an external source of electrical power; they control 

 mechanically how much of this power is delivered at the output. Theoreti- 

 cally they can therefore cause no loading of the mechanical system whilst 

 delivering an indefinitely large electrical output. For this reason they are 

 more generally useful than dynamic transducers. 



The circuit element which is mechanically controlled may be resistance, 

 inductance or capacitance; static transducers will be discussed under these 

 three headings. The special circuits used with some of the transducers will 

 be described in a later section. 



Variable resistance transducers 



Transducers using variable resistance occur in three forms: (a) wiping 

 contact on resistive track ; (b) moving electrode in liquid resistance ; (c) defor- 

 mation-sensitive resistive element. These types have very different charac- 

 teristics and will be considered separately. 



Resistive track transducers — In this type of transducer the wiping contact 

 may move either round a circular resistive track or linearly along a straight 

 track. The circular type is eminently suitable for use with circular motion, 

 but requires levers or a rack when used for linear motion. On the other 

 hand, such devices are readily available in the form of the ordinary radio 

 potentiometer, and, in refined forms, are capable of the highest accuracy of 

 any type of transducer. Linear variable resistances will usually have to be 

 constructed for a particular application, but then may be applied directly to 

 the measurement of linear movement. 



For low- and medium-accuracy work, the potentiometers described in 

 Chapter 20 are suitable ; the degree of accuracy to be expected is discussed 

 there. It is possible, however, to obtain potentiometers with a linearity 

 rather better than OT per cent; they are known as 'cam-corrected' potentio- 

 meters. These devices are basically very high-grade wirewound potentio- 

 meters, but with the addition of an adjustable face-cam by which the 

 inevitable small inaccuracies may be corrected at, typically, eight points. 

 This correction procedure, which is of course carried out by the manufacturer, 



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