TRANSDUCERS 



synchronous link are approximately 100, 200 and 600 g-cm. Several 

 receivers may be driven from one transmitter, provided that the total torque 

 transmitted is not excessive. 



Magslips operating on 50 c/s supplies are commonly wound for 50 V, 

 but units for other frequencies and voltages are available. A very wide 

 range of different windings is available for special purposes, but manu- 

 facturers' literature^" must be consulted for details. 



Mechanoelectrical rotating transducers 



It will by now be apparent that all the transmitters of synchronous links 

 are m-e transducers, transforming information about shaft position into 

 electrical form. It will also be apparent that the electrical information is 

 in a most inconvenient form : that of the ratios between three voltages. For 

 example, although the output of one phase of a magshp may be phase- 

 sensitive rectified to give a voltage proportional to the sine of the angle 

 of rotation, it is unusual to use the electrical output of a single m-e transducer 

 as a guide to the position of a shaft. Such transducers, used in pairs, 

 commonly provide information about the difference in position between 

 two shafts, but this will not be discussed here. 



n rev/sec 



n rev/sec 



Output 



Output 



Ca) (b) 



Figure 33.32 Commutated capacitor tachometer: {a) output independent of 

 direction of rotation; {b) output clianges sign as rotation reverses 



One form of rotating transducer which is in common use is the analogue 

 of the moving-coil m-e transducer described earlier; it produces a voltage 

 output proportional to the speed of rotation of a shaft. Such rotating 

 velocity transducers are called 'tachometers'. 



A d.c. generator with a constant field gives an output voltage propor- 

 tional to its speed, and which reverses in sign as the direction of rotation 

 changes. The constant field may come either from a permanent magnet or 

 from a field winding fed with constant current. Linearity of a high order is 

 possible with a well-designed machine provided that negligible current 

 is drawn from the output. 



A small and simple device, capable of good accuracy as a tachometer, 

 is the commutated capacitor tachometer, commonly known as the 'bucket 

 machine'^^. The circuits of Figure 33.32 show: (a) a device whose output 

 does not change sign with direction of rotation; (b) a rotation-sensitive 

 device. The capacitor C^ is charged once per revolution through the pro- 

 tective resistance R^. Cg is made much larger than Cj so that in due course 



498 



