ControlLogix Homing Issues

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Member
J
Join Date
Jan 2008
Location
Indiana
Posts
10
I am currently working on a 3 axis pick and place unit. The drives are Ultra 3000 controlled by CLX Sercos. When I powerdown and power backup the positions of the servos are 0. I have the homing set absolute in the setup tab therefore I believe the drive should retain the position during a power down. Does anyone have any suggestions?
 
It sounds to me like your motors don't have absolute encoders on them. If the encoders aren't absolute encoders then the absolute home function won't do what you want. In fact, you shouldn't need to do an absolute home unless you have disconnected and reconnected the motor and load.

What is the part number of the motor you are using?

Keith
 
This might happen if your TAG.AbsoluteReferenceStatus for any of your axis is false. Which can be fixed by re-referancing your servos again, which ever one's status is false
 
The motor CAT NO is MPL-A1530U-VJ44AA. I will check the the reference status tag. When I power up I use the MRP motion direct command to reference the motor. I basically jog the motor to position an issue the MRP command to zero the motor.
 
MRP is your problem

after executing MRP, the AbsoluteReference bit goes off. That is normal behavior.
From the knowledgebase (id 31789):


clearing the absolute reference flag
trnsp.gif
Answer Cure: The only times that the 1394/2094/8720 drives clear the absolute reference flag are:

1) Reset to Defaults through Drive Explorer
2) Execution of a non-absolute home
3) Execution of an MRP command
4) Transitioning from CP3 to CP4 if the encoder is not an SRM, or SRS with an unwind of 1 rev (this is not in the 1394)


Catalog Number: RSLogix5000;ControlLogix M08SE
DocFullNum: P117546487
Revision:
Fixed in Revision:
Package:
Modual:
 
Just to re-center the discussion, why is the axis position reporting zero after a power-up. Assuming the motor has an absolute encoder and neither an MRP or MAH is issued, the axis position should report the current position returned from the absolute encoder. Assuming the axis hasn't been moved, this should be the position the axis was in when it was powered down. Even if tha axis was moved it should still be the correct position based on the last hardware reference. At the very least the position should be SOMETHING, even if it isn't correct. Zero seems like an add position.

I don't have any answers. However, you shouldn't need to reference every time you power up. This should only be done as a maintenance/set-up type of thing.

Keith
 
About 6 months ago I had a similar situation on CLX with Kinetics drive. The vendor supplied an incremental encoder instead of absolute. Instead of changing the encoder I have forced the HMI to display a warning stating that homing needs to be performed. The warning message would only come up if processor went through power up (after the main disconnect was off). I would also lock out the AUTO mode until the homing was actually performed. Since the main disconnect is only switched off about once a week, this was an acceptable solution.
 
Thanks for everyones help. I eventually had to write logic that issues the following commands in order while the drive is enabled.

1.MSF
2.MASR
3.MAH

This fixed the problem. If you have any questions let me know.
 
This subject will be near and dear to my heart in about 2 months since I will be using an MPL servo with absolute feedback with an Ultra 3000i. Given what you are saying I may have made a judgemental error.

I still have the question from my previous post. Why do you need to home in the first place? I would think the encoder position would still be valid since the last powerdown. If it's not, why use an absolute encoder in the first place? You should need to tie the encoder position tot he machine position once after installation. That relationship should hold until the motor/encoder is uncoupled from the machine.

Keith
 
In my particular situation the OEM never referenced the drives. The drives must referenced once using a MAH command. After this is done the drives will retain the position. I had to add logic that would allow maint. to reference a motor via the HMI if the motor was changed or the coupling on the motor shaft slipped. In that case I had to write logic that issued a MSF then MASR Then MAH while the drive was enabled. Hope this has helped.

Jesse
 

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