Nosman

Unstable cycle time of motor controlled by TwinCAT2 (Rexroth PC)

6 posts in this topic

Hello,

I'm using a Rexroth PC with TwinCAT2 to control a system. On the machine, we have an integrated motor which controlled via Profibus. (https://schunk.com/de_en/gripping-systems/product/16750-0306870-prh-050-050-pb-54/).
The problem is that the period of time for the motor to move to the home position is not stable. After restarting the PC, the duration for homing can achieve my target. However, after several weeks of running, the homing time will be higher and not achieve the defined target.

I have tried to: disconnect the motor cable --> no improve the cycle time.

                        redownload PLC file --> no improve the cycle time.

The only way to improve the cycle time of homing is by restarting PC (or System manager)

Do you have any idea/comment for my problem?

Thank you in advance!

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Am I reading this correctly.  You push the HOME button and the time it takes the system to reach home varies and gets longer the longer the PC runs.

I assume it as at the same position each time you start to home it?

Can you place an external tachometer on the motor shaft and confirm the speed is consistent?

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Thanks BobLfoot for your advice.

You're right, the position for each time is the same. The speed variable of the motor is fixed in program. However, after an amount of rotation, the actual speed is slower than the original value. The issue can solve by restarting system manager, but I can't explain why? 

 

 

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So just to be thick headed on my part.  The Speed Variable is unchanged, but the tach read speed gets slower.

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obviously something changes after while... but there is not enough info...

can you record variables at powerup and after a while when speed seem to drop? how is the code structured? is there a leak of resources (memory, CPU time, scan time, etc.) . what is the observed behavior? is the physical motion really slower? is there something that causes motion to be executed later (due to interlock etc. how do you check? motion execution depends on a whole range of things - not just programmed velocity. this is why one should check everything including programmed acceleration, jerk, changes in configured load values (inertia or whatever), control gains etc. there could be something else that gets activated and not switched off such as tracking or gearing with another axis. there are literally 100s of settings that could be the reason...

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I am not familiar with this system, but with the info provided, it seems that the motion planner is being affected over time.  This should not be the case, as a motion control system should be more deterministic in nature.  Is there a utility built into the system that allows you to chart the motion profile as it is running?  If so, I would reboot the system and record the planned vs actual motion profile of the home routine.  Then run it again when you see the system slow down.  That would show you the differences.  Once you can definitively show the differences, you can involve the techs at Rexroth.  If you are seeing this type of change, they are probably aware of a shortcoming of the system and can show you how to remedy it.  Please post your progress here.  Your responses may help in the future.

 

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