650 



BELL SYSTEM TECHNICAL JOURNAL 



Thus the over-all error becomes 

 Fi — Fo ^^ 



l-^-±^^]F,, 



1 + M 



or 



Fi - Fi = —— Fi . 



1 + M 



If the added transmission path is so designed that 



1 



Ma - 



M2 



(22) 



(23) 



FORWARD-ACTING 

 TACHOMETER 



y SYNCHRO PAIR 



C.T. 



INPUT 



e, 



FEEDBACK 

 TACHOMETER 



M2 



OUTPUT 



-jcjORte2 



'FOLLOW-UP 

 LOOP 



|_ AD DE D_ TR AN SM ISSjO N_ PATH 



St(ei-e2) 

 juoR'te, 



FOR ZERO DYNAMIC ERROR, R'^== K* 



Fig. 20 — Forward-acting tachometer system. 



then Fi = F^ , and the dynamic error vanishes. Thus the desired form 

 of the added transmission depends only upon the m portion of the loop 

 characteristic. It will not be possible to satisfy the condition given by (23) 

 exactly, especially at the higher frequencies where noise enhancement and 

 parasitic effects will become increasingly important. However, it is often 

 possible to obtain the proper form for juo over the range of frequencies re- 

 sponsible for the bulk of the dynamic error. If /Xo has the proper frequency 

 characteristic but is too large by 10%, for instance, it may be seen from (22) 

 that there still remains a 10/1 increase in dynamic accuracy. 



The foregoing method is especially applicable when n^ represents the trans- 

 fer characteristic of a motor-drive system employing tachometer feedback, 

 as shown in Fig. 20. Here the basic input-output comparison is obtained 

 by means of the synchro pair, while a tachometer coupled to the input shaft 

 provides the error-reducing signal. Thus the transmission Ha is equal to 

 joiR't , where R't is the tachometer transfer resistance. The expression for 



