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Combine 25 years of electrohydraulic component, production, and system experience with control theory into a new valve design incorporating both nozzle-flapper and solenoid components, and you obtain an innovative, differential pressure control pilot valve (PCP). Utilizing a single component that integrates many functions, this invention reduces a complicated valve into a simple low-cost assembly, yielding a multi-purpose (patent-pending) electrohydraulic servovalve. This PCP has been combined with several patented boost stages to yield pressure control servovalves (PCS) and flow control servovalves (FCS) that can be utilized in construction, industrial, and agricultural environments. The new, simplified and symmetric PCP/PCS can drive a pump directly with an option of built-in electronic rotational feedback. The PCP generates servovalve performance at less than proportional valve costs. These controls can be utilized in both open and closed-circuit arrangements.

Many mobile vehicles require four-wheel steering modes in order Co enhance mobility in tight spaces. Not only is four-wheel steering required, but three modes are also desired for a variety of mobility problems of off road vehicles. Typically three modes of steering are required: 1.) Standard steering; 2.) Coordinated; and 3.) Crab. The rear steering operation should be automatic and proportional allowing the operator to give full attention to the front steering and other vehicle operations. Also, switching from one steering mode to another must be simple and quick. Through the use of inexpensive analog electronics and proportional hydraulic components, a reliable servo steering system capable of three modes can be constructed. This paper will consider the following areas of design: 1.) Hydraulic design requirements and considerations; 2.) Electronic design requirements and considerations; and 3.) System operation and set-up.