Remedy For Overrun?

I still cant seem to get my cylinders to stop passing my target positions. They are 48"stroke 2.5"bore cylinders. we are running 3 on this system. I have experimented with the plots and tried everything I can think of and I do not seem to be getting any change. I do not really believe it is with the tuning. My gains are very low for the size system it is. I think my proportional gain is around 17 and my actual velocity is still trying to oscillate a bit. the position plot looks great and is actually showing a max.002 underrun at any position of the plot making a 1.5’’ absolute movement at 12" per sec with the accel and deccel both set at 20… The system isnt moving a load at this time. Just the cylinder rods are moving. Could this be the reason for such low gain settings? Again it would make moves within .003" very accurately with a max being around .008". When I started adding some math expressions that run all the time and a few in some of the user programs. the overruns just kept getting worse. .030-.120". There isnt much programming at all. Most of my logic and programming are done on another plc that writes to the variables to start programs.

I called tech support a few days ago, and I do not remember who I spoke with but he said that it could be due to my cycles times and that im using temsonic g series pwm transducers with 4 recirculations. And they could be taking to long to update the positions. Therefore I could be getting a feedback position then possibly another as the rmc starts the program to make the move. which makes sense. Not being able to swap these out is there a way to remedy this if it is the cause of my issues with a program or expression. could you just simply figure the move you want to make with a trigger and delay the move?

Also am I waisting my time other than learning the controller by trying to tune the axises perfectly now. Since they are moving no load, and when they are installed they will be moving a load varying from 200 lbs to 5000 or more?
As soon as I find my usb card I will try and get the plots and program send to tech support If I cant remedy the issues with what I learn on here.
Please forgive my stupidity about all this as it is the only time I have ever done any motion control. And again thanks for all the help and comments.

I forgot to add.
All of my position tolerances and and bits are set to be .001". I can trigger a move. and the axis will physically move .030-.080" to far and the rmc still tells me that I am in position.
Also all of my program moves are made with the cylinder extending. I can make the same move with the cylinder retracting and the overrun seems to be less than .010".

To deal with the feedback update taking longer than the loop time, you can increase the loop time. 2 msec should be long enough for 48" with 4 recircs. When tuning with no load, it is usually easier to get good control, so now is the time to aim for good control and solve any problems. It’s tough to give any advice without seeing plots. What is the valve you are using?

We are using 3 Moog 661 series servo valves. I don’t have the exact number on me at the time. I tried to increase the loop time but there was no change. I do believe I fixed the problem tho.

I took a millisec signal from the master plc I am using to make the RMC run my expressions constantly. I also tried making an internal timer in the rmc off of the millsec counts and got nearly identical results.

I put a very slight delay before each motion command just to be on the safe side.

I think Im going to stick with the one from the master plc I am using so that I can also use the signal between the two on my HMI panel to show me that they are still communicating. Ive ran the system off and on for 3 days now this way and am getting results within .003". The worst being around .005". Which is great!

I dont think the motion control part of it is going to give me any problems. I do believe all my trouble was coming from the programming. I am still slightly confused as to why the expression I am using would take so long to finish, and update. Could it be because it uses several variables that are always updating and changing, Half are internal to the RMC and the other half come form the HMI panel. There may be a total of 8.

Anyway it is working and performing better than expected! And the system install is planned for this weekend so we will see how it all goes.
I will post how everything works out.

And thank you for the fast responses to my posts on this forum and the help I received when I called.

What do you mean by a millsec signal? What is it and what does it do?

Was it only the Actual Position that overshot the requested position, or was it the Target Position?

It would be real nice to see the project and the plots, otherwise, it’s impossible to be of much help.