If you’re not familiar with “Street,” mad props could also mean “Irate Respect.” Well, not really, but it’s still fun to say. It’s more like “Abundant Respect.” Regardless, it’s not like I just threw down a double pump, reverse slam-dunk off a breakaway three on one, and I’d been given mad props for my mad hops. Anyone who’s met me knows I have the vertical leap of a granite countertop (but I’ve got a mean drop step and hook shot).
The “props” I’m actually referring to are proportional valves. Proportional valves are electrically actuated variable output hydraulic valves. The most common “prop valves” are directional valves. Directional valves change the direction of hydraulic flow, such as to and from an actuator or to dump pump flow. A proportional directional valve is usually a spool valve, and that spool can remain un-shifted, shift fully open or position itself anywhere in between. It can also shift variably in the opposite direction, metering flow out the other work port (if there is another work port).
The accuracy (and cost) of proportional valves can vary dramatically. Proportional cartridge valves can be had for a hundred bucks or run up into the tens of thousands of dollars for high response, high flow valves. The less expensive cartridge valve versions simply use a variable current into their coil to change the position of the spool. The problem with that system is they have relatively poor accuracy, repeatability and no method to compensate for things like oil viscosity changes or flow forces.
A rung up in the directional prop valve ladder might be ISO industrial valves, like a D03. The range is vast, as they can be simple current-input only or fancy versions with linear position transducers and on-board electronics. A D03 proportional valve is more accurate than a cartridge valve, but the problem is that we don’t ever really know how far the spool has shifted. What we can do is add a linear position transducer to measure the spool movement. Then with electronics, the position of the spool is fed back in to the controller, which in turn adjusts current to the valve to keep the spool (and flow) where we want it.
Proportional valves can control more than just direction. Pretty much every other valve in a hydraulic system is available with proportional control, including pressure reducing valves, relief valves, flow control valves and even variable pump controls.
A proportional pressure-reducing valve can adjust pressure in a sub-circuit of a system anywhere within its spring range via a command from a PLC. Proportional relief valves achieve a similar result, but control pressure upstream of the valve, rather than downstream of the valve like a reducing valve.
A less expensive method of creating a proportional directional valve is to use a standard bang-bang valve, but with a proportional flow control at the inlet (or P port) of the directional valve. Instead of the directional valve having variable input, it just shifts wide open and uses the proportional flow control to dictate input flow.
Proportional valves aren’t plug and play, however. There are various design criteria to be understood, auxiliary components to compliment them and knowledge of how to set them up. I’ll discuss more of the specifics next month. In the mean time, I’m gonna go practice my crossover dribble…
Josh Cosford is a certified fluid power hydraulic specialist with The Fluid Power House (Cambridge) Inc. Contact him at firstname.lastname@example.org or call (519)-624-7109.