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Fixed-displacement pump, variable-displacement motor configurations are rarely utilized for hydrostatic transmission designs, but they can achieve several advantages over more traditional systems. Developing a hydraulic drivetrain to accomplish high efficiency at high torque is often difficult because variable-displacement pumps are less efficient at low displacements. However, a system that utilizes modern electro-hydraulic valves and an electrically controlled variable-displacement hydraulic motor is easy to modulate and achieves its peak efficiency at low speeds. During the 2005 ASABE (American Society of Agricultural and Biological Engineers) ¼ Scale Tractor Student Design Competition, one such system was compared to conventional hydrostats and belt driven continuously variable transmissions. The fixed pump, variable motor system proved to be easy to operate. An electronic controller was used to modulate motor displacement to maintain constant engine speed.

An accurate nonlinear model of a typical magnetic actuator is included in a new SPICE simulation of an electrohydraulic system. The actuator model is obtained by electromagnetic finite element analysis. The resulting nonlinear magnetic flux linkages and forces as functions of airgaps and currents are included in the SPICE model. The SPICE model is able to include electronic driver circuits and models of the hydraulic circuits, which are shown to interact to determine the closing time of electrohydraulic valves.