TRANSDUCERS FOR CONTINUOUS ROTATION 



very nearly all the charge of Q is transferred into it. The charge transferred 

 per second is thus nVC^, i.e. the mean current flowing in ^2 is nVC^. The 

 mean voltage across 7?2 will be nVC-^R^, the output is thus proportional 

 to shaft speed. For accuracy it is necessary that the time-constant C^R^ 

 should be much less than (say 1/10 of) the dwell-time on the contacts; this 

 ensures adequate charging. Further, the output voltage must be kept low 

 compared with V, as otherwise all the charge is not transferred to C2. 



Both generator and capacitor tachometer suffer from ripple on their 

 outputs ; some smoothing is often necessary. 



Electromechanical rotating transducers 



In this section two examples will be considered of the vast number of 

 'instrument motors' which may be used to convert electrical inputs into 

 rotating outputs. 



The simplest arrangement uses a motor with a constant (permanent or 

 wound) field, the armature being driven from the electrical input. Here 



Input 



n^wt^sw- 



Motor field 



Motor armature 

 fed from source 

 of constant direct 

 current 



Figure 33.33 Circuit for split-field motor 



the speed of the motor will be roughly proportional to the input voltage, 

 and the torque to the input current. Constant- voltage and constant- current 

 feeds thus give approximately constant-speed and constant-torque mechanical 

 outputs, illustrating again the interaction of electrical and mechanical 

 impedances. While large motors may have their armatures fed from 

 batteries of large valves, the spht-field arrangement (see below) is more 

 convenient. However for low-power applications it is usually possible to 

 drive the armature of a midget motor (of the type used in toys) directly 

 from a valve or, better, a transistor. One motor which has been success- 

 fully used in such an arrangement requires 12 V 0-4 amps to produce its 

 full output of 1/1,000 H. P., and will run at speeds up to 9,000 rev/min. 

 A large output valve or a power transistor will provide this power easily. 

 For larger motors it is usual to use the electrical signal only to control 

 the motor power, not to provide it, by varying the excitation of a wound 

 field. This field is commonly balanced, giving rise to the term 'split-field' 



499 



