CIRCUITS FOR USE WITH STATIC TRANSDUCERS 



example, a strain gauge, R^ may be a similar 'dummy' gauge, connected 

 with similar wires and situated in the same environment, but not subjected 

 to strain. 



The bridge may be fed with either d.c. or a.c, except when using a liquid 

 potentiometer, when a.c. is usually used to avoid polarization of the electrodes. 

 No difficulty arises when using d.c, as when the bridge is unbalanced one 

 way or the other the output signal is positive or negative. With a.c. energi- 

 zation the output signals on the two sides of balance are 1 80 degrees out of 



~° Energizing'^ 

 voltage 



Figure 33.34 Wheatstone bridge circuit 



phase with each other. With simple rectification this phase distinction 

 disappears, leaving the direction of unbalance ambiguous. This may be 

 resolved in one of two ways: (a) the bridge may be deliberately unbalanced, 

 so that the output never goes through zero; (b) a phase-sensitive rectifier 

 (Chapter 6) may be used. While (a) is very simple, (b) confers advantages 

 in simplicity of smoothing circuits and in overall signal-to-noise ratio. 



Circuits for variable reactance transducers 



It is again possible to use suitable bridge circuits^^ with variable capaci- 

 tance or variable inductance transducers ; all the considerations of the pre- 

 vious section about balanced design, use of phase-sensitive rectifiers, etc. 

 apply. It is essential, of course, to use a.c. energization. The differential 

 transformer is an example of a concealed bridge circuit; the bridge elements 

 are the reluctances of the elements of the magnetic circuit, the output being 

 derived from the flux in the 'diagonal' member. 



Variable reactance transducers can conveniently be used in an entirely 

 different type of circuit. The variable element forms part of a tuned circuit, 

 whose resonant frequency is thus varied by the mechanical input. This 

 frequency is then determined continuously and presented in a suitable 

 form as the output of the transducer system. The variable tuned circuit 

 may conveniently be used as the controlling element of an oscillator, the 

 changes of whose frequency are converted into proportional voltages by 

 a discriminator circuit of the type used in frequency-modulation radio 

 receivers. Two examples will be given; many other circuits will occur to 

 the reader who studies f.m. receiver design practice^^'^*. 



501 



