Compensation of Massive Thrust Chamber Positioning Servomechanisms

Compensation efforts directed at the stabilization and control of the Saturn S-II engine actuation system have given rise to nonlinear compensation philosophies that are also applicable to other types of hydraulic positioning servos. The analysis presented in this paper shows by theoretically and analog simulation techniques that a lag-lead form of compensation is superior to the presently popular dynamic pressure feedback (DPF) for reducing system step position errors in the presence of a large stiction/cou-lomb gimbal friction component. This result is important because nonlinear friction positioning errors are directly related to the magnitude of the resultant vehicle attitude limit cycle.

An analog computer nonlinear system simulation was used to verify the analytical results discussed herein. Using the ITAE criterion, step and ramp responses of the system were compared to evaluate the DPF and laglead networks. The gains and time constants of the competing networks were varied in a systematic manner, and the superiority of the lag-lead network was clearly indicated.