OT: Odd behavior in a servo valve.

Valve is Atos DLK ZOR-TE-140-L73

This is a direct drive proportional with OBE Operating a 430mm (17") cylinder at 290 bar (4200 psi)

I have tried three different valves, including two brand new valves and all behave the same way.

When I slew command to the valve with the hydraulic pump off the valve spool position feedback follows the command very tightly, both for positive and negative command.

When I slew command with the pump turned ON then the valve spool tracks command up to about 25%. Then the spool stays at ~25% even if command increases up to 100%. If command decreases then spool tracks just fine below 25%.

If I put command anywhere between 25% and about 60% and turn the pumps off then the spool position immediately shifts to match command. If I turn the pumps back on spool position drops back to 25%.

If I put command anywhere above 65% and turn the pumps off spool position will them move to match command and when I turn the pumps back on it will remain there and track command up and down just fine all the way to 100%.

If I start at 100% command and 100% spool position and turn the pumps on and slew command to 0 then the spool will track command almost perfectly going down but will not track going back up.

This condition holds for both positive and negative valve commands.

Its almost like the valve doesn't have enough power to move the spool through the mid-range but I've checked the power supply and there is no voltage drop off.

I've checked the controller analog output to the valve and it is fine, same with its analog input for the position feedback.

I have several identical systems and none of the others behave this way.

Any ideas?

Do you have any pressure relieve in you hydraulic system? Is it possible that you have not enough pressure? Looks like its not valve related, but either your analog command or the pressure level.

You probably need a two stage valve. What you are fighting is flow forces. I have seen many single stage valves start doing weird stuff after the pressure gets above 1800 psi. A two stage valve has always fixed the problem.

I bet the valves will work fine if the pressure is reduced but then the rest of your system won't work. Oh well.

How is the valve piloted? Internal or external? I always have to check my new valves when they come in. Some have plugs in the pilots and some don't.

This definitely sounds like a flow limit situation. The data sheet lists 70 l/min max which as Peter states is where the flow forces (Bernoulli forces) overcome the coil forces. What flow is the pump putting out?

If/When you go two stage, watch for a limit on the pilot pressure to the valve. For example the Bosch Rexroth 4WRLE valve is rated to 300 bar but the pilot is only rated to 250 bar so an external pilot supply is required to operate at maximum pressure.

Thanks for the tips Peter and Norm. That helped put me on the right path.

This unit was functioning fine until a few days ago and there are a couple dozen of these units in operation so I know the valve is the right one for normal conditions, so I had to find why there was a higher than normal flow here.

From prior troubleshooting I knew the cylinder didn't have any problems, so I went hunting for other flow paths. I had noticed that the rod pressure of the piston was lower than I expected it to be, however I was able to still develop some rod pressure.

On the older systems there is a legacy bang-bang valve that is parallel in the circuit to the servo valve. The servo valve is mounted right on the cylinder but the legacy is on a different manifold block. The valve is never used now and the controller outputs are unconditionally latched off. The valve was left in place to plug the manifold ports.

I went to check out the legacy valve and found a brand spanking new shiny valve mounted to the manifold. The new valve had a j-spool in it (a,b,t common in center position). It should be a closed center spool. Problem found. Below the valve was a p.o. check plate for the A port to the cylinder blind end. Since this is a force control application we were still able to rapid advance the piston and then develop and hold force, but the rod side is short circuited back back to tank through the J-spool. Very little pressure is needed to retract the piston so it would still retract, it just wouldn't reach normal velocity which is what caused me to look at the servo valve performance in the first place. The short circuit back to tank meant a much higher than normal fluid flow, hence the spool symptoms I described in the OP.

It seems that about last Thursday or Friday Bubba changed out that valve for some reason. I don't know why he thought the valve needed changing since it is not used. At any rate, the problem has been permanently eliminated - the legacy valves are gone from all the older machines that still had them and the ports are blanked off with a blanking plate.