456 REGULATORY CIRCUITS 



equal, the gyro gimbal moves at a rate determined by the amount of viscous 

 damping. The motion is detected with a sensitive transducer, often a 

 microsyn, and transformed into a proportional voltage. This voltage is 

 amplified and applied to the antenna actuator in a direction to reduce the 

 rate error. Aircraft rates are also compensated in the same way. 



A principal difference between the integrating gyro and the rate gyro 

 control loop is that the steady-state error in the integrating gyro loop is 

 zero" for a constant rate command while the steady-state error in the rate 

 gyro loop has a finite value proportional to the rate command and inversely 

 proportional to the d-c loop gain. However, this steady-state error in the 

 rate loop can be reduced to a negligibly small value without great difficulty. 

 The integrating gyro must have compensating networks to make it stable, 

 whereas the loop with the rate gyro may not need compensating networks, 

 depending on the degree of performance required. Practically, some 

 compensation is usual in both forms of the stabilization loop. 



Another difference in performance between stabilization loops using the 

 integrating gyro and those using the rate gyro is incident to the saturating 

 characteristics of the gyro. Large steady-state rates do not usually saturate 

 the integrating gyro, because the gyro gimbal is in the forward path of the 

 loop and its motion is proportional to the rate error, which is small. Even 

 if it should saturate, it does not change the loop performance appreciably, 

 because changes in the forward gain of a feedback control loop do not affect 

 the overall loop characteristics or its performance significantly. On the 

 other hand, the rate gyro saturation does occur for large, steady rates and 

 its measuring and performance characteristics effectively change. This 

 is not desirable, nor even allowable in most cases, because the rate gyro 

 must measure antenna space rates accurately for the fire-control computer, 

 it must provide a stabilizing signal proportional to the antenna rate at all 

 times to prevent drift, and it must have the proper transfer function to 

 provide track loop stability. This may be particularly serious for systems 

 using guns because the large random space rates induced in the antenna 

 during periods of gun fire cause rate gyro saturation. Consequently, it is 

 necessary to provide rate gyros with linear rate measuring ranges far in 

 excess of those needed to measure the aircraft space rates for the fire-control 

 computer. Unfortunately, a large, linear measuring range reduces the 

 accuracy of the gyro in the important low rate region. However, with 

 missiles for armament and jet aircraft as an antenna platform, this problem 

 is not serious. 



In recent years, HIG gyros, '^^ fluid damped and hermetically sealed, have 

 been commonly used in antenna stabilizing loops. They are extremely 



S'^This neglects the effects of actuator stiction which can make the loop nonlinear and 

 produce a small error. 



^Hermetic Integrating Gyros developed at the Massachusetts Institute of Technology. 



