Misbehaving Axis

Hello All,

I have a servo axis that is causing me some problems. The PLC is a CLX 5555, communicating through a SERCOS card to the motor controler which is an Ultra 3000 (2098-DSD-020-SE), the motor part number is missing at the moment. Using RSlogix 5000 of course.
The setup works well most of the time-- it is doing a simple pick (from one of 10 positions) and place (in one position). The machine has been running fairly regularily from October with no great issues. However, twice in this time the axis has created trouble from startup by flying at high speed to one end of the axis, hitting the end, and damaging the belt drive on the axis (A FESTO DGEL axis). On both occasions, it happened after a local power down of the station (part of a long line). On restart, the head should home itself if needed, then proceed to the next pick/place position. Instead, the head moved at a higher speed, accelaration, and torque than any programmed, away from the home position until a collision occured. I can't seem to recreate the problem or understand what is happening.

This is my first AB servo axis, it was set up by another more knowledgable person (thank you Donncadgh), so I can't really say for certain that I have enough experience to eliminate all possibilities on this one. Can anybody tell me what may be happening/ how to solve it?

A related issue I have is the setting up of hard overtravel limits for the axis. A bit late now, but needs to be done. At the moment, there are no hard or soft overtravel limits. One of those things that caused problems at the beginning, was left until more time could be given to it, then was forgotten about until too late...
The axis is working away with no end limits, but reduced torque. I have sensors wired up for hard travel limits, wired (correctly) back to the controller. In the axis properties window limits tab, I put a check in the box for "Hard Overtravel", download the program, and the axis will fault at the MSO instruction every time. The error shown is error 20: "Tried to execute on an unconfigured axis". Have tried normally open and normally closed sensors, have tried playing with all of the settins in the axis properties dialog box. nothing works until I remove the checkbox in the "Hard overtravel" limits box, then all is ok again.
Am I missing something? Does this need to be done from the beginning? How do I configure the the axis? Help?


I aslo have a problem with a ghost short circuit, but that is for another thread...


Thanks in advance.

What version of RSLogix 5000?

Is your axis set-up as Linear or Rotary positioning mode(on the conversion tab)?

Any chance you can post the program, or at least a stripped down version with nothing but the axis configuration information?

RSLogix 5000 Version 12.02.00

Axis is set up as Linear, home to Switch then Marker.

File was too big to attach, even after scrapping most of it, so I uploaded it to the Allen Bradley folder in Downloads (Misbehaving Axis).

1. The problem is very intermittent. Therefore it is unlikely to be a program/firmware fault.

2. The high speed runaway is a classic "loss of feedback symptom".

3. I little niggle somewhere in the back of my head tells me AB may have had a crook batch of "drive/motor" cables in recent times. All details ellude me at the present, but for troubleshooting purposes I would begin by replacing them. (If AB do identify the cable as belonging to the suspect batch they will swap them out.)

Philip is correct on the bad batch of feedback cables. We had to replace all of our cables due to feedback problems that were caused by these bad cables. From what I recall, it was a disgruntled employee that screwed up a batch of cables. AB will reimburse you if they are of the correct "lot" number and you send them back to them.

I had a similar fault with an AB servo and Festo linear actuator, it turned out to be a bearing failure on the actuator, the servo was using so much torque to maintain its set speed that it couldn't slow down and over-traveled. We had to replace the actuator eventually but in order to keep the machine running until the new part arrived we found that slowing it down worked.

Originally posted by The_Wanderer:

On both occasions, it happened after a local power down of the station (part of a long line). On restart, the head should home itself if needed, then proceed to the next pick/place position. Instead, the head moved at a higher speed, accelaration, and torque than any programmed, away from the home position until a collision occured.

Click to expand...

This doesn't sound like a feedback cable to me. It only seems to happen at axis power-up. It sounds like the CLX is not recognizing the fact that the local axis needs to be re-referenced. I'm not an expert on the AB SERCOS implementation, but I would be very surprized if the drive didn't maintain it's actual position value (as opposed to the motion controller). When you cycle power on the axis you may be losing the actual position value. When you enable the axis there is a HUGE position error and the motion controller responds accordingly.
How do you determine if an axis needs to be homed? You may want to look into that a little. You should also be able to define a maximum allowable position error tolerance that will trigger a fault if you get too far out of position. This might help.

Keith

i had same problem with CLX, M02AE controllers and AB 1394 drives.
problem was loose connection on the 5VDC PSU and servo was loosing position but not faulting as expected.
as a result we had few nice big accidents...
torquing those terminals fixed problems permanently.

I've heard that Yaskawa servos used to have a similar problem (runaway due to intermittent loss of feedback signal) in some early servodrive models. They had it fixed by means of firmware upgrade long time ago.

This of course has nothing to do with AB Ultras and CLX, but the picture looks quite similar: either a bad cable or intermittent loss of feedback power.

I too, have experienced this phenomenon on a Parker Compumotor 6K4 controller. All of the previous suggestions should be looked at as well as noise. My problem was the result of a little 24 vdc ice-cube relay that induced a 76 vac p-p spike on the 24vdc circuit feeding the servo controller each time the relay dropped out. This would result in enabling/disabling all four axis, and in some instances ucontrolled runaways at max velocity/torque. Turns out, it was the only inductive load in the panel that the electricians failed to install a diode across the coil. Installed the diode and the noise (and the problem)went away.

kamenges said:

I'm not an expert on the AB SERCOS implementation, but I would be very surprized if the drive didn't maintain it's actual position value (as opposed to the motion controller). When you cycle power on the axis you may be losing the actual position value. When you enable the axis there is a HUGE position error and the motion controller responds accordingly.

Click to expand...

I have no clue what the problem is, but to add specific hardware advice to Kamenges advice, the Ultra 3000 SERCOS Drives can have a 5V power supply wired to it, to maintain logic power when the AC dropgs out.

This 5V power supply is VERY sensative. If the AC is off, and all you see is a little dot in the corner of the eight segment display then that means the 5V power supply isnt doing its job, you need to tweak the 5V power supply voltage higher until the eight segment display continues to display a value (0-4).

This could explain the axis losing its position during a power loss.

Good Luck!

Hello All,

Just an update-- I called our AB distributor and asked if there was any chance of the cables being bad. He said he had not heard of any recall and would get back to me and then promptly went on vacation... I bugged him when he got back, of course.

Turns out that the cables were part of the bad batch and so replacements are on their way. Hope that solves it! All well and good, but who pays for the 2 days of messing around finding the problem, calling the sales rep, etc.?

Thanks for the lead and the responses.

Jeff

The_Wanderer said:

He said he had not heard of any recall and would get back to me and then promptly went on vacation.

Click to expand...

The old standard response.

Glad to hear you got a solution in the works.

We recently had a brake go bad on a similar setup (CLX, SERCOS, AB drive). The hardest part was convincing AB that they shipped us a failed unit. Once they were convinced they turned around a replacement pretty quickly.

Its Baaaaaacckk!

We have replaced the cables, even laying in new trunking to keep them fully away from any noise. We have installed nice big filters on all the other noisy bits in the panel.

Last night the bloody thing crashed again. 7 hours of production lost while they repaired the damage. I am getting blamed because of "poor programming", which to me doesn't add up. Bad programming would show up more often than 3 times in 6 months in my mind, but I could be wrong. //moaning session deleted//

I will check the power supply/ connections when I get down there, but does anyone have any other sugestions? I have exhausted my knowledge on the subject and have no answers. I have been told that I just "HAVE to fix it this time!"...

JohnW said:

I had a similar fault with an AB servo and Festo linear actuator, it turned out to be a bearing failure on the actuator, the servo was using so much torque to maintain its set speed that it couldn't slow down and over-traveled. We had to replace the actuator eventually but in order to keep the machine running until the new part arrived we found that slowing it down worked.

Click to expand...

Have you checked this? I seem to remember that it wouldn't home properly either.