Motion head scratcher.

moosehunter

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Sep 2010
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I'm new to this forum. Hopefully someone here can help me figue this one out.

I have a Control logix platform with a Kinetics 6000. I have two axis's. This system has been running pretty much error free for 5 years, until about a year ago. Since then I've had three different times that at least one of the axis's will run in the positive direction when it should be heading to home in the negative direction. This action does not fault the drive until the limit has been reached. This last time it happened the staff on shift at that time could duplicate the scenario. Everything in the program said it should not have moved at all and it should move in the negative direction when it does work right. I've had several people look at the program hoping an error would be found but we all seem to think it is programmed right. This is very infrequent, like I said earlier, three times in the last year. It's also a very low risk for injury but a high risk for equipment damage, which has alreay happened.

Any ideas?????
 
Have you checked the external devices ? Such as limit switches, wiring (look for a loose connection), etc.??
Over the years I've noticed that people that don't really understand what that "black box" does tend to always blame the program. Mostly when something of this nature happens, it's turned out to be an external device that isn't making a good connection. If you can get the equipment to duplicate the problem than you might be able to find it.
If you can program some "traps" into the logic, such as "if this then SET M100", or whatever works for that PLC, it could help. The PLC itsself can be your best friend in troubleshooting. Good luck.
 
WAG: Bad resolver/feedback cable? I have only seen a bad cable result in unexpected motion with the Elctrocraft drives/motors which evolved into the Allen Bradley Ultra 100, 200, then 3000 series standalone drives. Not sure if this root cause applies to a drive that is being controlled from a separate positioner like yours.

We would see a big jerk, usually followed by a position error fault, depending on how the drive was configured.
 
I would agree with OkiePC in that if the motor was doing some the controller didn't expect you should fault on position error, unless you have a very large positon error window configured.

You seem to be describing the fault in two different ways. In one case you seem to say the axis moves without any apparent reason. In the other case you seem to indicate that the axis is supposed to move but it moves in the wrong direction. Is it one or the other or both?

Keith
 
Check your home limit switch and its wiring. If your homing method is to seek the home limit switch, when you command a "find home" while the home switch signal is true, the axis should seek home in one direction. If the home switch is false when the find home command is asserted, the axis should seek home in the opposite direction. For that reason, the target for the home switch should be such that when motion in one direction trips it, the switch remains tripped as long as the axis is moving in that direction. Similarly, once the target clears the home switch, further motion in that direction keeps it cleared.
 
Check your home limit switch and its wiring. If your homing method is to seek the home limit switch, when you command a "find home" while the home switch signal is true, the axis should seek home in one direction. If the home switch is false when the find home command is asserted, the axis should seek home in the opposite direction. For that reason, the target for the home switch should be such that when motion in one direction trips it, the switch remains tripped as long as the axis is moving in that direction. Similarly, once the target clears the home switch, further motion in that direction keeps it cleared.

Good one Steve, I have seen that too. It ended up being a pinched cable and was very hard to find...
 
Can you give more details on how the fault was duplicated. Also is this strickly in the home comand? On most drives you send it a home comand and this is handled directly by the drive. You have several parameters you set that work with homing and these vary from drive to drive. IF it run for 5 years I seriously doubt the program is the issue. I would look for a shorted home prox, or an issue withe resolver. There are other posibilities but I would check here first.
 
Thanks for your responses everyone. Here's a bit more info and clarification on my original post.

When a home command is issued, either manually or after a normal cycle is complete, the axis is commanded to move in a negative direction until it reaches the home switch. What is frustrating here is that the drive does not fault when it moves the opposite direction until it reaches the overtravel limit. It really acts as if it needs to go in a positive direction to go home. I have replaced the motor once and had the the original motor sent in for evaluation and no problems were found with it. I have not replaced the cables but I would think if they were the problem I would see position error faults ot some other fault if they were bad. I have not replaced the home switch but maybe it's time to start throwing parts at it.
 
If the homing motion is smooth and normal looking other than the direction, then it is not likely to be cables/connectors.

This sounds more like Steve Bailiey's suggestion or something quirky with the way the home method is configured.

Is it configured to go to the home limit switch and then back off it by a certain amount?

Is the axis configured as linear?

Another thing that I noticed is that you said, "When a home command is issued, either manually or after a normal cycle is complete..." Are you required to home to a prox after every normal cycle? If not, then perhaps you can improve the situation by only homing when the machine is powered up, follwing servo faults, or when a home cycle is requested by the operator. I had a problem home input on one machine that was greatly improved by eliminating the home every cycle (every ten seconds 24/7!).
 
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If the homing motion is smooth and normal looking other than the direction, then it is not likely to be cables/connectors.

This sounds more like Steve Bailiey's suggestion or something quirky with the way the home method is configured.

Is it configured to go to the home limit switch and then back off it by a certain amount?

Is the axis configured as linear?

As Okie pointed to in alot of homing sequences you go to the homing sensor then back off until the senosr turns off. This is a fairly common pratice as it leads to faster and more accurate homing. THe downside is the possible senerio you have described. Basically the drive is looking for the edge of the sensor so it continues to run in the wrong direction. When the axis homes correctly check to see if the home sensor is on.

One thing I just thought of that happened to me several years back. We had a robot that had a Home and overtravel sensor. Basically if the robot hit the overtravel it would reverse direction and continue in the reverse direction until it made the home prox and then it would move off the home prox to get home position. Anyway the overtravel sensor would give a false high and the robot would reverse direction until it got to the over travel in the oposite direction.
 
The motor, accuator runs smooth with no issues.

Yes, it does run to home then move off the home switch to a postion.

The axis is configured as a servo.

In a 8 hour shift, under normal conditions, the axis will home maybe 15 or 20 times. Homing speed is 4 IPS. The operators can stop the machine, put it in manual and request a home at any time but only really do that if they E-stop the machine for some reason.

One other thing. There is another servo/axis on this machine with identical parts. The homing routine for both are mostly identical with a few minor exceptions. Like one has to home and be done before the other will home. The other axis I havn't had an issue with
 
I would monitor the state of the home limit switch, perhaps trend it. And possibly set up a fault bit that would alert the operator if the home limit switch was already "made" when homing was initially requested. You might go as far as inhibit the homing routine if that fault bit is set.

That bit of logic could be left as a trap to prove or disprove Steve's theory.
 
when it is running in the wrong direction is it a reduced speed? This would also be a clue as to the mode the drive is in.
 
I'm not an expert on motion control, but OkiePC asked the question "Is the axis configured as linear"....

This is very relevant - if the axis is configured as rotary, then homing can be configured to take the shortest path, so the homing direction will vary depending on where the axis is when homing in requested.

If the axis is configured as, and needs to be, rotary, then a configuration option is available to "fix" the homing direction, otherwise it may take "shortest route".
 

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