I just finished an extended four day long troubleshooting job, involving a Galil motion controller, and Allen Bradley servo drive. The customer was experience erratic behavior of the servo motor, on a linear cut-to-length application. The servo drive is at the end of a large machining and assembly line at a wood window mfg. facility.
When I arrived at the plant, the first day was spent becoming acquainted with the system, as it is a very complicated setup, with part of the control locally programmed at the process, and part of the control coming from a main PC at the head of the line.
The machine had worked for ten years, and now suddenly the servo motor was acting very strange, going forward and reverse at times, running into the end-of-travel hard stops, and not responding to command at other times. I began the research by contacting the machine OEM, who advised to connect to the controls and check for any errors. Upon doing so, there were no errors or alarms present in the controls.
I spent the second day, talking with service reps from the motion control mfg’s and each claimed that there was was no problem with the controls. Then I began to focus on the communication cable that connected the Allen Bradley servo drive with the Galil motion controller. I removed the cable and inspected the ends, and noticed that one of the connectors did not look good, a couple of the connector pins were pushed back into the housing.
The morning of the third day I located a replacement connector at an electronics house in a neighboring city, and went to pick it up. Upon replacing the connector, the servo began to act consistently, always going in the reverse direction. Now the challenge was to find out why it was going in reverse, even though no command to move was being given by the control.
The morning of the fourth day, I noticed that the servo drive was being given an analog signal to move in the negative direction, even with the motion controller was powered off. The analog signal was brought to the drive over the cable that I had just repaired one end with a new connector. So I inspected the other end of the cable and took the connector apart. I found that one of the wires was in the wrong place, and it was the reference ground wire of the analog signal to the servo drive. This was giving a “floating” reference speed control to the drive, without a proper reference to ground.
I spliced the wire into the right position on the connector, and the servo began to work correctly. This brought up the question. HOW had the machine worked for ten years with the cable assembled incorrectly??? Perhaps someone had messed with it, and didn’t tell anyone. I believe the most logical explanation was that the analog signal had found a path to ground when the machine was new, and now that the connectors were aging, it had lost the path to ground, and would no longer work. Once we got the ground wire in it’s proper place, it worked just fine.