Understanding Pressure Compensation in Hydraulic Systems

Blog | June 19th, 2019

Pressure compensation systems operate a little like feedback-based electronic systems. That’s not exactly a surprise, not when hydraulics circuits can become every bit as complex as any electronically based wiring. At any rate, the similarities are there for everyone to observe. When an output change occurs, a fraction of that change is sent back to govern the control unit, be it a preamplifier or a hydraulic actuator.

Hydraulic Pressure Regulation Principles

Sticking with fluid-based systems, as a load pressure varies, a fractional amount of that energy is dispatched back to the flow controlling mechanism to perform its pressure compensation duties. That’s how post-compensation hydraulics systems work. Of course, to realize this design, a load sensing pump or actuator must accept a feedback sensing line. For example, sticking with pump architectures for a moment, a piston pump would receive the feedback pressure. From here, the line would connect directly to the pump swash plate. As the equipment load varied, that signal would make the swash pate perform its pressure compensating assignment. Again, this is how post-compensation systems operate. Having become outmoded over time, pre-compensation technology has long since taken over.

Talking About Pre-Compensation Benefits

Known also as “flow sharing,” post-compensation configurations have limitations. Basically, there are some disadvantages to having a fixed ratio pressure drop. Equipment responsiveness is also sometimes limited when controlled by load sensing lines and springs. Instead of the LS spring, pre-compensation devices adopt the following principle. With a different architecture, this pressure compensation circuit measures a load-generated pressure differential over individual valve spools or orifices. Sure, there’s less chance of a flow sharing issue when individual valve orifices are monitored in this way, but now each of those monitored valves is “blind” to the functions of its neighbours. No matter, when or if a system pump becomes load saturated, the pressure compensation mechanism will responsively access more pump power, perhaps by adjusting the pump swash plate, or perhaps by adjusting a pilot signal on some other hydraulic pump type.

There are two principal forms of pressure compensation. If a heavy load threatens to swamp the system pump, load sensing feedback lines and springs adjust pump performance. That’s how a post-compensation circuit operates. For pre-compensation systems, the compensator monitors pressure differentials across each valve orifice. Fortunately, new waves of advanced mobile hydraulics devices are now adding pressure compensation options, which include porting controls for pre or post-compensation controls. That leaves the designer free to weigh the pros and cons of both solutions. For one thing, does a designer really want to introduce flow sharing? If not, turn instead to pre-compensation configured load controls.

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