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Author Topic: Dugong Tuning Problems, Positioning Error  (Read 2034 times)

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Dugong Tuning Problems, Positioning Error
« on: February 13, 2014, 09:15:12 AM »
 Good morning all,

I've got a rather frustrating problem with a pair of Dugong DG2S-0080 servo drives driving a pair of Kelling 34-170 brushed servos. The issue is that my positioning error is never centering on final position. This system is on a lathe. The Z axis has a 1000 count line driver quad encoder, and the X axis has a 2500 count line driver quad encoder. Both have been confirmed to be good (actually, the 2500 count encoder is brand new). There is no appreciable noise in the system which I've confirmed with a scope. I have tuned and tuned and re-tuned again these drives. The only way I can get the position error to stay somewhat close to 0 (+/- 10 counts) is to turn the Kp and Kd gains up to the point where things start getting "buzzy". The critical thing to note here is that NO MATTER WHAT THE Kp and Kd tuning, Ki inputs (integral, error correcting over time) does absolutely nothing to the curves. This doesn't seem possible to me since it should just be math driving the transistors in the drives, but if I didn't know better I'd say the integral term in my drives was broken. The configuration software for the Dugong drives allows inputs of Ki from 0 to 65500. The response graph at Ki=0 and Ki=65500 look nearly identical with neither of them eventually landing on 0. I have these same drives in my mill with some older DC servos but identical encoders and I have never had this problem.

Any thoughts would be very much appreciated.


Offline Hood

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Re: Dugong Tuning Problems, Positioning Error
« Reply #1 on: February 13, 2014, 01:31:47 PM »
Why not swap a drive over from Mill to Lathe and see if you get the same issue, if you do then its not the drive, if you dont then its the drive.
Re: Dugong Tuning Problems, Positioning Error
« Reply #2 on: February 13, 2014, 01:47:55 PM »
 Certainly something I'd thought of Hood but #1 I don't want to take down the mill since it is usually busy, and #2, the mill has been running fine and I don't want to risk throwing off the "chee". I just ordered a Granite Devices servo amp with dual motor control capabilities....given the complete lack of technical response I've gotten from either Dugong or their US distributor I think I'm going to start shying away from them. In the mean time I've got to finish a job on the lathe so I'll try and band-aid things along for a few more hours of run time.

What is utterly puzzling to me though is how the Ki function in the drive can be "dead"....?

Re: Dugong Tuning Problems, Positioning Error
« Reply #3 on: February 19, 2014, 12:48:48 PM »
Just to close this out, Dugong finally got back to me and sorted out the problem....

Integral Limits!!

Looking back on it now, the solution was obvious. The integral limit is set as a default to 0 with a maximum value of 65500. All other tuning I've done with Dugong drives I've never had to mess with the integral limit, and the tuning manual says to leave it alone.

Well, this time I needed to bump the limit up to around 15000 to get Ki to respond. With that all sorted the servos are happily finding 0 each time and I'm getting a positioning accuracy better than 0.0005.

Servo Tuning once again gets the better of me.