THE CASE FOR REPLACEMENT OF PILOT VALVES WITH PILOT PUMPS IN HYDRAULIC CONTROL SYSTEMS

Traditionally, hydraulic servo power for high bandwidth primary stage actuation in electrohydraulic pumps and valves is provided by small pilot stage valves. These often operate in a pressure-control mode against a spring to effect position control of the main valve spool or pump swash plate. This paper presents the case for replacing these valves with small fixed-displacement, electronically speed-controlled pumps. Depending on the system, advantages can include more linear control, faster response, lower development and component cost, and higher energy efficiency. This paper presents comparative analysis of this concept for two common cases: a two-stage spool valve, and an axial piston load sensing pump. The first demonstrates that for systems of comparable flow capacity a pump-controlled-pump system will respond faster than a valve controlled system due to supplying its maximum flow for the majority of the response, as opposed to a valve controlled system whose flow reduces as pressures equalize. This system also exhibits a more linear response, easing closed loop control design. The second case study presents a pump flow control application, which demonstrates the pump-controlled-pump system can be more easily and safely designed using linearized control system analysis techniques. The linearized analysis correctly identifies the stability limits for the pump-controlled-pump, but dangerously overestimates the proportional gain limits for a valve-controlled system..