A Generalized Control Model and Its Digital Algorithm for Aerospace Electrohydraulic Actuators

Shoujun Zhao,Xiao Xue,Keqin Chen,Guanghui Jing,Yingxin Zhao,Chuanwei Yin,Xiaosha Zhang

doi:10.3390/iecat2020-08500

Abstract

It is difficult to describe precisely, and thus control satisfactorily, the dynamics of an electrohydraulic actuator to drive a high thrust liquid launcher engine, whose structural resonant frequency is usually low due to its heavy inertia and its complicated mass distribution. A generalized model is therefore put forward for maximum simplification and sufficient approximation, where a second-order transfer function is used to model the heavy mass-spring nature of the large engine body outside of the rod position loop, another second-order transfer function with two zeros and two poles representing the hydro-mechanical composite resonance effect in the closed rod position loop. A combined control strategy is applied to meet the stringent specification of static and dynamic performances, including a notch filter, a piecewise or nonlinear proportional, integral and differential (PID) controller and a feed-forward compensation. The control algorithm is implemented in digital signal processors with the same software structure but different parameters for different aerospace actuators. Compared to other approaches, this one makes it easier to grasp the system resonance nature, and, most importantly, the traditional dynamic pressure feedback (DPF) is replaced with the convenient digital algorithm, bringing prominent benefits such as a simplified design, reduced hardware cost and inherent higher reliability. The approach has been validated by simulation, experiments and successful flights.

Highlights

The electro-hydraulic servo actuation is a well-developed technology
It was assumed that the driven load had sufficiently high structural resonances and w as modelled as a lumped mass directly attached to the piston rod or motor shaft, with the hydraulic natural frequency dominat ing the control loop
In aerospace applications, such as rudders, fins and engines, due to weight and space limitations, the structural stiffness is usually low and the model has to be modified. It was discussed in the classic book, where, only the dynamics at the motor shaft point was elaborated, with the dynamics at the load end left open for more work [1]

Summary

Introduction

The electro-hydraulic servo actuation is a well-developed technology. most of its physical understanding and mathematical modelling have been referred to the Hydraulic Control Systems by H.E. Inside the piston position Xp loop, a second-order transfer function with two zeros and two poles dominates, both poorly damped, representing the hydro-mechanical composite resonance effect arising from coupling between the load structural resonance and the hydraulic resonance, called “load effect”. With only an open loop gain of 15 rad/s and without any other compensation, given a series of sinusoidal commands, the system was tested to study the load resonance effect, with bode plots of XL and Xp shown in Fig.. The engine’s structural resonance dynamics can be obtained by directly subtracting the response of Xp at the piston point from that of XL at the load output in Fig., resulting a curve shown, together with a simulation by a standard second-order transfer function.

Discussions

Conclusions