LINEAR SERVO THEORY 651 



M2 is given approximately by (21.2) as \/Jq)Ri . Thus, by (23), Rt c-^ Rt 

 for substantial cancellation of the dynamic error (at frequencies small com- 

 pared with ojm). That is, the output voltages of the two tachometers must 

 closely annul each other when the input and output shafts are travelling 

 at the same speed. Since the tachometers may be closely alike and excited 

 from the same supply line, it is comparatively easy to keep their transfer 

 ratios closely matched. In practice an error reduction of 20/1 is readily 

 maintained by this method. 



The error compensation scheme described above does not change the 

 loop characteristic /u/S of the basic servo loop, and thus does not create new 

 stability problems. Its use to obtain high servo accuracy is desirable 

 when the input noise is small and when a high loop gain is difficult to obtain 

 because of parasitic elements or equipment complexities. 



