Asking for help with motion control

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Getting a trend of the control output, target position and actual position is a must. For a velocity control application substitute velocity for position. Sometimes is is handy to plot all 5 items which is the default on our software.


There is so much that can be determined by just looking at the trend.


If the trend is long, such that there are commands issued during the trend, it is good to have another line or pen that moves when a move command issued. For instance, after every motion command set a variable unique to that command and plot it. This way it is easy to tell when the commands were issued.


Also, it is best to update the trend as fast as possible. Much can happen between two samples 10 milliseconds apart.
 
The first thing I ask a customer when they say the axis isn't doing what they think it should, is to tell them to trace it. Most of the time it is doing what it is being told to do. In other words, it is following the motion profile, but the profile isn't programmed properly. This often happens with camming applications. I recently experienced the opposite of that when converting a customer from one brand of servos to Yaskawa. Did the onsite startup, tuned like I normally do, and the customer ran the machine (large plasma table). They said the machine was "thumping" when starting and stopping motion, which didn't happen with the other servos. I traced it, and actual velocity was following commanded almost perfectly. I told them to add s-curve if they wanted to make the moves smoother.
 
If I had to venture a guess, Peter is answering a PM?

I often do what you are suggesting Peter, add pens for events that indicate when to expect something to happen. I also try to set up a situation where I can control when the event takes place, to keep the trend data as short as possible. Obviously, this can't always be done during production.

In rockwell land, for some reason, milliseconds isn't shown on the trend by default, this has to be selected. Not sure how this is even an option.
 
If I had to venture a guess, Peter is answering a PM?
No, there is another thread where someone is complaining that their motor overshoots.

In rockwell land, for some reason, milliseconds isn't shown on the trend by default, this has to be selected. Not sure how this is even an option.
Rockwell PLCs cannot update that fast over the back plane. Sometimes important things get missed between the samples.

I once was hired by Rockwell to help debug an M02AS system. This was about 15 years ago. The systems was a flying shear. The problem was that the HYD02 couldn't synchronize to the line and the customer was ****ed at Rockwell. The M02AS trend could only update every 10 ms at that time and the trend looked very noisy. I went to the customer site with an RMC100 that sample 1024 times a second . I found 2 problems within 30 minutes of hooking up the RMC. There was a bad bearing in the machine that would bind then release. The second problem was that the Balluff SSI rod updates were occurring an irregular intervals. Both problems made it impossible for the M02AS to use the derivative gain for tight synchronization. This let Rockwell off the hook. I still have the old RMC100 plots from that project.
Getting a better SSI rod was easy. Fixing the bad bearing in the machine took a few weeks. Eventually the machine ran fine.
 
OT, but didn’t you write the firmware for these cards.?
Yes, that was why I was called. The local Rockwell guy was unfamiliar with the M02AS and was unable to figure out what was wrong due to the trend being unusable due asynchronous feedback and to the slow and asynchronous updates . There wasn't anything wrong with the M02AS. The Control Logix CPU weren't as powerful as they are now.
 

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