DYNAMIC MECHANOELECTRICAL TRANSDUCERS 



the transducer mechanisms generally differ only in their geometrical arrange- 

 ments in the two cases. Indeed the input of many so-called linear displace- 

 ment transducers is, in fact, applied at the end of a rotating arm ; the angle 

 over which this arm moves is arranged to be small so that its end moves in 

 an almost straight line. 



Transducers for continuous rotary motion, such as generators and motors, 

 are not fundamentally different, but for convenience will be considered in a 

 separate section. 



DYNAMIC MECHANOELECTRICAL TRANSDUCERS 



As explained above, dynamic transducers do not give a steady output for a 

 steady deflection: in other words, their response does not extend down to 

 zero frequency. It is important, however, to discuss the exact form of the 

 frequency response of a dynamic transducer, and to examine how the output 

 for a constant input displacement falls off as the input frequency approaches 

 zero. This behaviour is different for the two major types of dynamic 

 transducer. 



Piezo-electric transducers 



Certain crystalline materials, when subjected to mechanical stress, develop 

 surface electric charges. Thus, if a stress is applied to a piece of such material 

 provided with suitable electrodes, charges will appear on the electrodes, and 

 hence a voltage will be developed across them. This is the principle on which 

 piezo-electric transducers operate ; the disposition of the electrodes, and the 

 mode of application of the stress, will vary with the material and configura- 

 tion of the transducer. 



C 



Voltage 

 oc stress^ 



Figure 33.9 Equivalent electrical circuit of piezo-electric transducer 



Viewed from the output terminals, a piezo-electric transducer behaves 

 like a fixed capacitor carrying a charge (and hence a voltage) proportional to 

 the applied stress. Were it possible to connect the transducer to an indicating 

 device of infinite resistance (such as an electrostatic voltmeter), the indicator 

 would deflect when the stress was applied, remain deflected so long as it was 

 present, and return to its original zero when the stress was removed. The 

 frequency response of the system would then extend down to zero frequency. 

 However, in practice the transducer is likely to feed an amplifier or other 

 device with finite input resistance; the charges developed as the stress is 

 applied will gradually leak away through this resistance. So an indicator on 

 the output of the amplifier will deflect as the stress is apphed, and then 

 gradually return to zero with a time constant of CR, where C is the capaci- 

 tance of the transducer and R is the input resistance of the amplifier. 



All the characteristics of the transducer-amplifier combination, such as 

 steady-state frequency response and transient response, can be determined 

 by giving the transducer the equivalent circuit of Figure 33.9. 



481 



