Why does the Manipulator Drift While Frozen?
This is the first indication of poor hydraulic fluid quality. The movement of the arm is due to ‘servo valve silting’. The human eye cannot see less than 40u (40 micron). The material that causes silting is less than 5u; therefore, oil that appears clean may actually be very dirty. And oil that appears dirty is VERY dirty. To test whether silting is the problem, run the manipulator and observe a single joint for movement. When you see the joint start to drift or hunt, operate the joint quickly back and forth a few times. If the joint is steady for some time but then gradually starts to drift again, the oil quality needs to be improved. An oil cleanliness program should be implemented that included oil quality analysis. Oil quality should exceed ISO 4406 16/14/11. It should also be noted that oil fresh out of a 55 gallon drum will not meet the requirements for servo controlled systems. Oil must be filtered before use.
