25 June, 2017

Beware the use of counterbalance valves with electrohydraulic motion controllers

16 June, 2017

By Peter Nachtwey, Delta Computer Systems Inc.†Counterbalance valves are used for safety, holding loads and keeping loads from dropping when a loss of hydraulic pressure occurs. Sudden pressure loss can happen when a hose fails, for example, putting a lineman in danger who is high up on the business end of a bucket truck.


Counterbalance valves can also be used to limit the speed at which loads are raised or lowered, preventing the motion of an actuator from getting ahead of the fluid flow from the pump due to momentum created by the load. Counterbalance valves can also mitigate the effects of instability in some hydraulic systems.

Motion controller conflicts

When counterbalance valves are combined with servo valves controlled by closed-loop motion controllers in the same system , problems can develop. This is because a hydraulic motion controller operates on the assumption that the servo valve it is controlling is the only active valve in the system.

As discussed above, counter balance valves are semi active valves that act autonomously. They can open and close independently of the hydraulic controller and servo valve. This results in motion or lack of motion that isnít expected by the control loop model used by the hydraulic motion controller.

The job of the hydraulic motion controller is to continually work to reduce the error between the feedback that it is receiving from position sensors and the target value that represents the next desired state of the system. If the counterbalance valve is actively working to control flow on its own, the overall system wonít react the way the motion controller expects it to.

The motion controller tries to correct for what it perceives as errors caused by the counterbalance valve, and often over-corrects. The counterbalance valve compounds the problem because it doesnít respond to pressure changes quickly, because the pilot pressure set point must be above the load induced pressure.

Piloting the counter balance valve from the cap side of the cylinder often results in chattering in the motion of the cylinder while it is extending. This is due to the counterbalance valve opening and closing as the pressure on the cap side of the cylinder goes up and down. When the hydraulic motion controller starts to open up the servo valve the pressure on the cap side and rod side start to increase.

The motion controller is expecting the pressure on the rod side to decrease but it doesnít until the pressure on the cap side builds up to the point the pilot pressure opens the counter balance valve. During this time, the position error is increasing, so the motion controller increases the downwards signal to the servo valve. This increase in signal is much more than what is required if the counter balance valve was not obstructing flow. When the pilot pressure opens the counter balance valve the pressure on the rod side drops quickly so now the excess pressure cause by the controller compensating for the error causes the actuator to drop too fast often over shooting the target position. This causes an error in the other direction so the controller reduces the signal, reducing flow to the cap side of the cylinder. The reduced flow plus the downward motion cause the pressure on the cap side to drop quickly below the set point for the counter balance valve so it closes and the cycle repeats.




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