TRANSDUCERS 



In Figure 33.37 the applied force is imposed on two transducers mechani- 

 cally interconnected. Transducer A is an m-e transducer which detects 

 any movement of the assembly from its rest position. The output from A is 

 amplified and fed to the e-m transducer B so as to annul any movement. 

 This arrangement will be recognized as a feedback system. From the type 

 of analysis presented in Chapter 11 it follows that: (a) the transducer has 

 a very high mechanical input impedance, i.e. it does not 'give' appreciably 

 under the applied force; (b) the applied force is very nearly equal to the 

 force produced by the e-m transducer, and can therefore be measured from 

 the electrical input to this transducer; (c) the characteristics of the m-e 

 transducer do not affect the measurement. 



Since highly linear e-m transducers are easily constructed, and m-e trans- 

 ducers of high sensitivity can be made if linearity is not important, the 

 combination described has many practical advantages. 



An electromechanical transducer for controlled displacements 



It is difficult to design an e-m transducer whose output displacement is 

 accurately proportional to its electrical input if the range of displacement 

 must be large. It is even more difficult under these conditions to make the 

 movement independent of load (i.e. high output impedance). 



By an arrangement (Figure 33.38) similar to that of the previous section 



Output 

 shaft 



m-e 

 transducer 



Input 

 voltage 



Subtract 



e-m 

 transducer 



N 



Amplifier 



d 



Figure 33.38 Feedback system for controlled displacements 



it is possible to obtain a displacement proportional to an input voltage, 

 and virtually independent of the load. Now the m-e transducer must be 

 adequately linear, while the characteristics of the e-m one are more or less 

 unimportant. A suitable phase-advance stabilizing network A'^ will however 

 be essential to avoid oscillation of the system (see page 165); its exact design 

 will depend on the mechanical properties of the e-m transducer. 



The circuit will so adjust itself that the difference between the input 

 voltage and the output of the m-e transducer is very small; the displacement 

 of the whole system will thus reproduce accurately the waveform of the 

 input voltage. 



The velodyne 



This circuit^'^ is intended to give an output shaft-rotation, whose angular 

 velocity is proportional to an input voltage. It has two principal uses: 

 (a) to give a rotating motion where speed may be readily controlled (manually 

 or automatically) — this speed being virtually independent of load (i.e. high 



504 



