TRANSDUCERS 



motor. A typical circuit arrangement is shown in Figure 33.33; a mechanical 

 output of 60 W is controlled by an electrical input of 10 W. The motor 

 output torque is roughly proportional to the field current. Such motors 

 are often structurally combined with tachometer generators ; Table 3 gives 

 an example of the characteristics of such a device. 



TABLE 3 



Motor-generator type 88 



Motor 



Generator 



Field current 

 Armature current 

 Torque 

 Speed 



Field current 

 Output voltage 



25 mA 

 0-8 amp 

 180g-cm 

 9,000 rev/min 



30 mA 



00 1 V per rev/min 



CIRCUITS FOR USE WITH STATIC TRANSDUCERS 



As has already been noted, static transducers need an external supply of 

 power for their operation, and special circuits to present their output in a 

 convenient form. There are certain fundamental points common to many 

 of the circuits which will be discussed first. 



It is desirable that the transducer and its circuit should where possible 

 be symmetrical and balanced. This balance should appear not only in the 

 circuit diagram but also in the mechanical construction of the transducer. 

 Such balanced arrangements are inevitably less subject to the effect of 

 outside disturbances, less liable to drift, and often more linear than their 

 unbalanced counterparts. 



The second point concerns only transducers fed from a.c. It is essential 

 for faithful reproduction that the frequency of the energizing supply be 

 considerably higher than any frequency which it is required to study: 

 otherwise, when the output of the transducer is rectified and smoothed to 

 feed, say, an oscillograph, some information of interest will also be removed 

 by the smoothing process. While in theory an energizing frequency of 

 twice the highest signal frequency is all that is necessary, the smoothing 

 filters needed are so complex that the use of this theoretical frequency is 

 not practicable. At least ten times the maximum signal frequency is 

 recommended to energize a.c. transducers. 



Circuits for variable resistance transducers 



All circuits which are used with variable resistance transducers are 

 variations of the Wheatstone bridge circuit shown in Figure 33.34. Some- 

 times all the components of the bridge are not obvious; R^ may be the 

 transducer, R.^. ^ resistor, while R^ and R^ may be part of the shift circuits 

 of the measuring oscillograph. It is desirable in order to achieve balance 

 that /?! and R^, should be of the same order of magnitude, and of similar 

 construction. It is often possible to make the transducer provide both 

 Ri and R^, as in the liquid or wirewound potentiometer. When R^ is, for 



500 



