An appliance manufacturer has a machine that polishes the stainless steel exterior on the finished doors of appliances to a fine brushed look on one of its popular designs. The polishing head’s speed is powered by a servovalve hydraulic system that allows them to maintain the selected speed of the brushes. The servovalve control electronics incorporate a feedback signal from the hydraulic motor to ensure a constant speed for the brushes, regardless of the pressure applied.

This process had been in place for several years, and other than occasional maintenance fixes, worked well. They did a good job of maintaining the ISO cleanliness code and temperature of the hydraulic oil. Oil samples always came back reflecting good maintenance practices they had in place.

A problem developed where the maximum brush speed could not be achieved, but the circuit for advancing and applying pressure worked just fine. See the accompanying circuit.

The coils of the servovalve were wired in parallel, and when the resistance of each was measured, they both appeared to be fine. Technicians checked the signal coming out of the amplifier card to the servovalve, and it also seemed fine. They then replaced the servovalve with a new one after verifying the continuity of the cable going to the valve.

The system worked fine part of the time, but not consistently. Any idea what could be wrong?