Hybrid Multiple Control of Pump Displacement

Multiple controls of variable displacement pumps in open-loop applications are widely used. The first stage, common to all applications, is the pressure compensator, and flow and torque or speed compensators (or limiters) can be added as additional stages. The practical implementation of the control circuits is well known and documented in the fully hydromechanical or (more commonly in closed--loop hydrostatic transmissions) in the fully electrohydraulic option. The main concern is referred to the torque or speed limiter used as third stage in hydromechanical control circuits, as both of them show a number of drawbacks in handling the pump interaction on the mechanical side (i.e. interaction with the internal combustion engine). Aim of the paper is to show how an electronically controlled proportional servovalve, together with a properly suited (both traditional and based on fuzzy logic) control strategy featuring both torque and speed limitation, can overcome those limitation to a large extent, and can also be used as an add-on to existing pumps with double (pressure and flow) hydromechanical compensators.