LINEAR SERVO THEORY 



.645 



the necessity for designing the servo loop to match the type of input signal 

 to be transmitted, jiarlicularly for radar tracking systems where the "un- 

 wanted variations" are ever present. 



3.5 Use of Local Feedback 



There are many examples of the use of local or subsidiary feedback in 

 servo systems. The more common of these include feedback around vacuum 

 tube power amplifiers to obtain improved linearity and impedance proper- 

 ties, and over-all feedback around amplifier and motor-drive systems to 



1.2 



1.0 



0.2 



40 80 120 160 200 240 280 320 



00 -»• 



Fig. 17 — Frequency response of tracking servos. 



suppress frictional effects, increase output stiffness, and modify the inherent 

 frequency characteristics of the basic components." The tendency toward 

 "/3 circuit dependency" as previously discussed also produces greater con- 

 stancy of the stage transfer characteristics with time, temperature, etc. 



Perhaps the simplest and most useful kind of local feedback is negative 

 tachometer (velocity) feedback around motor-drive systems. This type of 

 feedback widens the transfer frequency band of the drive system by reduc- 

 ing its time-constant, and increases the linear speed range of the motor. 

 This may be illustrated by referring back to Fig. 7, which shows a typical 

 tachometer loop. Assuming the transfer ratio of the ampHfier to be a con- 



" In a slightly different class are the servo systems used to provide automatic frequency 

 and gain control in radio systems. 



