TRANSDUCERS 



Capacitance transducers have many properties in common with inductance 

 transducers ; thus they may easily be constructed so as to have a mechanical 

 impedance suitable for any particular purpose. In addition, they too are 

 fundamentally non-linear and must therefore be operated over a restricted 



/ 



Fixed and 

 moving 

 condenser 

 plates 



Moving 

 plate \ 



Insulator 



/ 



Fixed 

 plates 



Tension 

 /w/ire 



Flexible 

 /diaphragm 



Insulator 



Fixed 

 plate 



(a) (b) (c) 



Figure 33.18 Variable capacitance transducers 



range if linearity is desired. Transducers of this type are potentially extremely 

 sensitive ; with close electrode spacing (and, of course, correspondingly good 

 workmanship) movements of 0-01 fj, may easily be detected. When the 

 moving electrode is controlled by a stiff spring to give a high mechanical 

 impedance such a transducer can be used to measure force with very little 

 deflection and a high resonant frequency. 



Miscellaneous static transducers 



In this section two transducers will be described which while basically of 

 the variable resistance type achieve this variation in such an unusual way 

 that they are best classified as miscellaneous. 



The mechanoelectronic transducer (RCA Type 5734) is illustrated in 

 Figure 33.19. The anode of a very small triode valve is flexibly mounted on 

 a metal diaphragm and is continued outside as an operating shaft. Sideways 

 deflection of this shaft varies the anode-cathode spacing, and hence the 

 anode current of the valve. 



"With the mechanical input applied to the end of the 3 mm long operating 

 shaft, the compliance of the transducer is 10~' cm per dyne (I fi per g wt). 

 This gives a very high mechanical input impedance well suited to the measure- 

 ment of force. The resonant frequency of the transducer alone is 12 kc/s, 

 although this is inevitably reduced by any mass attached to the operating 

 shaft. 



The circuit used is shown in Figure 33.20; it will be seen to be a variant 

 of the Wheatstone bridge. The sensitivity of the transducer in this circuit is 

 about 1 mV per dyne (1 V per g wt) applied at the tip of the operating shaft. 

 By using a longer lever on the operating shaft this sensitivity can be increased, 

 but only at the expense of lower input impedance and resonant frequency. 



Mechanoelectronic transducers are inevitably rather delicate, both mechani- 

 cally and electrically, and it is essential before such a device is used to 

 consult the makers' literature^ to find out the recommended precautions. 



490 



