AB Servo Run away conditions

Good morning,
Does any one have experience with Servos going to a position that wasnt called for? This is AB servo with an absolute encoder. I am using AB kinetix 6500 with a L33ERM compactlogix controller. I have had 1 axis run full speed into the dead stops 3 times in the last 2 years. Twice in the last month. I have been told that it happens while homing the machine. All servos are absolute, so I perform a MAM to call the home positions. The home position is based off a recipe file (all home positions are in the middle of the travel). I checked all recipes and they all have a valid home position. The home position is only adjustable through the HMI. When we hit the deadstops we jump a tooth on the belt, and cause all positions to be off.

I am going to setup software limits to try and prevent it, as well as a limit test before the MAM that calls the home position.

I am just curious if anyone else has had this issue, and knows the root cause.

Thanks,
Kenneth

My two thoughts are:
1. You have a programming issue that commands it to go there (are all your MAM's absolute? Do you ever MAJ?)
2. You have a program issue with the servo actual position definition. You are saying this happens during "homing". From a servo perspective, homing usually means redefining the servo position. ie move to a position or prox, stop, and now call this position 0.00(or something else) regardless of what the servo thought it was before. If you are redefining the position by a MAH command or by a MRP command, then I would suspect that to be where things go wrong. If your "homing" just mean that it moves to a certain start position and you are not redefining the servo actual position, then I would lean back toward my first statement.

One thing to check....when it crashes (doesn't sound like it happens often), what does the servo show as the actual position? If it thinks it is in a "good" position, then you have a MAH/MRP issue and software limits will not help you.

Overtravel prox's are the safest bet.

I am not performing an active home (MAH). I am performing a MAM to "home" the machine to a predefined location based on the loaded recipe.

I have not personally seen it crash.

All motions are absolute.

Hard wired limit switches on travel to stop the motion would be a start. Then when it occurs you hopefully wouldn't damage anything and could record things like what position the servo was at when it tripped the limit switch, and what the command position of the MAM was. That may help you determine what is causing the runaway while also stopping damage to your equipment.

Hi

I know its not actual needed with the absolute encoder but I always use a home limit prox or switch.

If I am changing recipe, after an e-stop, power up of plc etc i would home to the switch and then move to where is required. This system also allows for someone to change the belt or change the motor and just home the system and away it goes

I know it's not the problem but it would stop this from happening and save some damage to your system

Donnchadh

I had a problem with an AB Sercos drive. Turned out the AB Drives rep sold us the wrong cable for the motors encoder. Would work fine for a while, then go nuts all by itself. Even an Motion Axis Off command would not stop it. Sheered off a couple stops before we finally figured it out.

It's always a good idea to run a limit check before any move - for your troubleshooting it might be worth the commanded position to a holding register if it fails the limit check.

Any chance that you run the move immediately after loading the recipe and there's a race condition where the move is commanded before the commanded position is read out from the recipe?

I assume this is a production machine, running 8-24 hours/day, 5-7 days/week. If so, I would expect a programming problem to manifest far more often than 3 times in two years.

Does the machine hit the deadstop on the same side of home every time this happens?

Could contamination, from a washdown or other source, be an issue?

In my experience, an intermittant servo runaway is usually caused by a physical problem - poor connection at the motor, contamination, something along those lines.

kjacoby said:

All servos are absolute, so I perform a MAM to call the home positions.

Click to expand...

Here is your issue the Servo is absolute however the drive train connected to the motor is not. It can change - for lots of valid reasons - eg maintenance, something getting caught in the belt tooth causing a tooth jump.

I am going to setup software limits to try and prevent it, as well as a limit test before the MAM that calls the home position.

Click to expand...

Software limits are useless here as they are testing for internal positioning errors assuming that the physical axis is homed

Use physical hard limits or Verify the axis homing on a power up using an external reference (eg Prox as kekrahulik suggested)

Until you have verified the home position only use MAJ and MAH. Reason the position feedback is not valid.


What I do is
on Power up perform a homing routine to an external reference (sensor that is only on at one of travel)
Where possible I verify that homing reference during normal operation looking for tooth belt jump. If detected then Stop the process and re home.