LINEAR SERVO THEORY 



649 



been increased by the factor aj,„/w,„ over that obtained without the use of 

 local feedback, assuming identical follow-up loop characteristics (/ijS) for 

 the two cases. The ratio col/com thus directly measures the feedback 

 eduction of static and low-speed errors of the follow-up system due to 

 torque disturbances. In practice the resulting increase in static accuracy 

 may be of the order of 10 to 100 times. 



3.6 Error Reduction by Non-Feedback Means 



In situations where the noise associated with the input signal is small, it 

 may be desirable to reduce the dynamic errors obtained with a given servo 

 system by the use of forward-acting equalization external to the loop. 



r 



r|(t)| F, F,-F2 



ADDED 

 PATH 



VQ? 



IMo'^i 



f2(t) 



^2 



FOR ZERO DYNAMIC ERROR, Ma = 



M2 



Fig. 19 — Forward-acting error compensation. 



That is, the dynamic error characteristic may be computed, and the servo 

 input or output modified by supplementary networks in such a fashion as 

 to reduce the over-all error. 



An illustrative arrangement, which is suitable when the input member is 

 accessible,'^ is shown in Fig. 19. For convenience the servo is taken to be a 

 simple follow-up system having ^ = -1. The m circuit is shown divided 

 into two parts, mi and y.2 . 'i ypically, mi may be the transfer stiffness of a 

 synchro pair (Fig. 3b), and M'j the transfer characteristic of a motor-drive 

 system. The normal dynamic error component for such a loop, omitting 

 the dotted line, has been shown to be l\/{\ + ^i). If an additional signal 

 HaFi is obtained from the input member and injected into the system as 

 shown by the dotted line, then 



F2 = 7— — Fi + — — ti , 

 1 -f /x 1 + M 



M + MaM2 



Fx 



1 + M 

 " This is not the case for a radar trackkig loop, for instance. 



