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This paper discusses the design of an electrohydraulic force loading system. This system is able to produce any desired arbitrary forces. As an example, the loading system is designed to load force to a hydraulic position servo. It is shown that if the loading system is designed by using a combination of a servo valve with critical center and a hydraulic cylinder to realize a force feedback close-loop system, the position system may generate a large additional force to frustrate this loading system to work properly. By using a bypass adjustable throttle valve across the two flow lines of the loading cylinder, it is verified by theoretic analysis and simulation that the additional force generated by position system can be greatly reduced. Because an asymmetric cylinder is used as the loading actuator, modelling of its characteristics is also carried out.

A hydraulic system is inherently nonlinear and sometimes its performance is greatly affected by those nonlinear factors such as deadband, saturation, hysteresis etc. For a system characterized with badly nonlinear behavior, the predominent nonlinearity must be well determined and compensated, otherwise the performance of the system will suffer. As an example, a hydraulic proportional valve controlled rotary motor servo is analyzed in this paper. Through theoretical analysis and experimentation result, a deadband is determined as the predominant nonlinearity in such system and parameters of the deadband are estimated by using system identification method. A nonlinear controller consisting of a linear pole-placement control algorithm and a deadband compensator is designed for the rotary hydraulic system.

Nonlinearities in a hydraulic system, such as deadband, saturation, friction, hysteresis etc., are usually not known exactly. If they are not properly compensated for, the performance of the system will suffer. This paper discusses the identification and control of this type of nonlinear hydraulic systems. The emphasis is on the deadband. A method is proposed in this paper to estimate its characteristics, and a piecewise-linear preload element can be used to compensate for the deadband in the design of the controller. This combination results in the closed-loop behaviour of a system with a deadband being the same as that of its linear portion.