TRANSDUCERS WITH FEEDBACK 



A circuit suitable for use with the capacitance transducer manometer 

 (page 505) is given in Figure 33.36. The manometer capacitance determines 

 the frequency of the oscillator K^ and Fg. This frequency is converted into 

 a voltage output by the 'gated beam discriminator' V^^^. An output of 

 50 mV per cm of water pressure (representing 10 V per pF capacitance 

 change) is obtainable, with very good stabiUty. The operating frequency 

 is about 1 Mc/s; such high frequencies are necessary with transducers of 

 low capacitance or inductance, as in order to achieve high sensitivity the 

 transducer should form the major part of the tuning reactance. 



The exact mode of operation of the discriminators described is beyond 

 the scope of the present work ; the references quoted should be consulted 

 for further details. 



TRANSDUCERS WITH FEEDBACK 



The principles of negative feedback have been described in Chapter 11 

 where it was shown that its use can bring the following advantages: (a) 

 improved linearity ; (b) improved frequency response ; (c) change of input 

 or output impedance. 



Negative feedback may similarly be applied to transducers with equivalent 

 advantages. It is not possible to cover all applications of negative feedback 

 to transducers, but examples will be given which illustrate the principles. 

 As in all feedback circuits, care is needed to avoid continuous oscillation 

 due to a large number of phase shifts in the loop. This effect and its cure 

 are discussed on pages 165 and 166. 



A self-balancing mechanoelectrical transducer for force measurement 



It has been noted that an m-e transducer measuring applied force must 

 have as high a mechanical input impedance as possible. This can be 



Input 

 shaft 



m-e 

 Transducer 



e-m 

 Transducer 



HZ> 



B 



Amplifier 



J 



Output 

 Figure 33.37 Feedback force-transducer — block diagram 



achieved with any transducer if a stiff spring is placed across it : the sensi- 

 tivity will then be low, and considerable amplification will be needed. In 

 the device to be described this amplification is used to improve the properties 

 of the transducer by the application of negative feedback. 



503 



