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.

The worst-case scenario happens when a cylinder is mounted rod-down with a counter balance valve controlling flow out the rod side, and the cylinder is plumbed for regeneration of the fluid directly back to the cylinder cap. In this case, the servo valve and hydraulic control have no control of the flow out of the rod side.

†Self-piloting valves

Some people have suggested that counterbalance valves can be used if they are self-piloting, in other words, if they detect rapid fluid changes within the valve itself and control the pressure relief function accordingly. This is definitely a better option than piloting off the pressure in the other side of the cylinder, but the counterbalance valve still opens and closes without interacting with the motion controller and therefore can affect how the overall system works in normal operation.

If self-piloting counter balance valves are used it is best if the pilot pressure is set to just above the load induced pressure for best control. Even then, this may be a problem if the load changes causing the pilot pressure to exceed the opening pressure.

If the pilot set point is too low a heavy load may induce a pressure high enough to open the counter balance valve, resulting in load dropping. If the pilot set point is set too high the motion controller will see the counter balance valve as an obstruction, causing the controller to increase flow and pressure, but without any resulting motion until the pilot pressure is reached.

The pressure will increase rapidly but a delay still occurs when no motion results when the hydraulic controller is expecting motion. Since the motion controller responds in a millisecond or faster, and may see many milliseconds with no motion, the motion controller will increase the control signal in an effort to compensate for the lack of motion.

This effect may not be too severe and may be acceptable, but one cannot expect the motion controller to compensate perfectly when it has no feedback for what the counterbalance valve is doing and no means to change the pressure instantly to compensate for the extra pressure necessary to open the counterbalance valve.

Another point to consider is that if the counterbalance valve is closed, the pressure in the line between the counterbalance valve and the servo valve can change over time. Servo valves will experience some fluid leakage across the servo valve. The result is that pressure in the line between the counterbalance valve and the servo valve drops causing a small downward motion when the counter balance valve opens up as the oil flows to increase the pressure to oppose the motion.

The typical result is that the load will not move immediately while the pressure builds to the pilot pressure set point. Once it does, the load will drop a bit as the pressure in the line between the servo valve and counterbalance valve pressurises again.

Better methods of safety valving

The primary system goals are equipment and personnel safety along with load control and load-holding. A better method of protecting the system without affecting the motion controllerís servo control functionality is to use pilot-operated on/off check valves, with the pilot flow derived from a source of pressure that is always above the pressure that is required to move the cylinder during operation. This way the pilot valves are always open and not interfering with the motion controllerís closed-loop control of flow through the servo valve.

Energised-open blocking valves are another recommended option. These valves rely on an electric signal being present in order to keep the fluid flowing through them. If a fault occurs as detected by the motion controller, or power is removed in any way, the blocking valve will close, keeping the load from dropping.



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