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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.

The increased number of heavy trucks on today's highways, along with the extended driving hours, resulted in increased demand for improved driving conditions and prompted concern about the dynamic pavement loads. The dynamic pavement loads are one of the major causes of pavement deterioration. Passive suspensions, while being very reliable and easily implementable, fall short of satisfying the various conflicting design requirements. The overwhelming improvement of ride quality resulting from the use of active suspensions seems to have overshadowed their effect on tire generated pavement damage. An in-plane tractor-semitrailer model is used to evaluate the relative performance of fail-safe active and passive suspensions. Both full state feedback and limited state feedback are used in the design of the active suspension.