8-34] STABILIZATION LOOP MECHANIZATION 469 



because the gyros are not on the antenna. In some cases it is necessary to 

 cage the gyro gimbals during the search mode to prevent damage and to 

 keep the gimbals near neutral position in readiness for the tracking mode 

 of operation when it is initiated. 



The location of the gyro mounting involves a problem that is often 

 overlooked. Usually the gyros cannot be mounted directly on the actuator 

 output shafts because of space limitations. In fact it is desirable to have 

 the gyros mounted on the antenna dish, and the necessarily light, compact 

 structural members between the actuator drive shaft and the gyro base have 

 compliance, mass, and resonant frequencies which limit stabilization loop 

 performance. Usually there is more than one resonant frequency which 

 can affect the performance of the control loop; although electrical networks 

 may be used to partially cancel the detrimental effects of these frequencies, 

 they are extremely complex if more than one resonant frequency is to be 

 attenuated. The most economical method of avoiding this difficulty is to 

 design the antenna with the resonant frequencies ten times the loop band- 

 width or to limit the bandwidth and performance of the stabilization loop. 



In Fig. 8-37b an integrating gyro is used in each stabilization loop as a 

 device for comparing the antenna rate with the commanded rate and for 

 providing an electrical signal proportional to the difference between these 

 rates. This signal is amplified and passed through appropriate compen- 

 sating networks to drive the antenna through the power amplifiers and 

 actuators designed for the search loops. Theoretically, any gyro could be 

 used as an integrating gyro; practically, it is difficult to provide sufficient 

 stability, loop gain, and damping to prevent the gyro gimbal from moving 

 into its mechanical stops during antenna transient motions unless a large 

 amount of viscous damping is provided at the gimbals. Therefore, HIG 

 gyros are commonly used as integrating gyros although they may be used 

 as rate gyros also. 



Note that in the integrating gyro configuration, antenna rates are not 

 measured directly. They are assumed to be proportional to the rate loop 

 command which is the track loop radar error signal. As is discussed in 

 Chapter 9, the track loop has a low bandwidth and removes high-frequency 

 signals incident to antenna vibration from the measured rate signal. This 

 is not a great advantage, because the rate signal is actually measured 

 through a low pass filter in any configuration in order to remove system 

 noise. 



In Fig. 8-37a and b the amplifier is assumed to have compensating 

 networks to convert the measured or calculated component characteristics 

 to the derived loop transfer ratio shown in Fig. 8-42. The transfer function 

 of the compensating elements can be derived as indicated in Fig. 8-36. 



The configuration of Fig. 8-37c has been described in Paragraph 8-31 and 

 the design details are beyond the scope of this text. However, it should be 



