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Servo tuning: velocity gain

baran3

Cast Iron
Joined
Jan 28, 2007
Location
Littlestown PA
what specific types of performance changes occur when the velocity gain is lowered for a servo motor. I reduced the gain by approximately 1/3 to solve a recurring oscillation problem and want to get some idea of what to expect, performance wise, when i put the machine back together. I changed thrust bearings, not much oil was getting to the ballscrew so i believe that may be the issue. However, it leaves good finishes and can't spend the money dealing with a ballscrew now... so, i decided if this works i'll just go with it.
 
I probably shouldn't even be answering because this is over my head, but isn't that an adjustment for how quickly the servos take off and stop while moving from point to point?

If so, I adjusted this once on my Fadal, and it was much snappier. Seemed real peppy when running from point to point, except the Z axis made a slight clunk when dropping down towards my part, so I adjusted it back to original settings.
 
Well, I'll risk oversimplifying to help. I expect someone with a bit more experience will correct me or add.

Gain is the servo reaction time. When too high the servo is unstable and will react to noise readily, when turned down it doesn't react to velocity changes quickly enough and becomes sluggish, often causing finish problems. This is the P in PID.

I is the stiffness. This controls how much movement the motor makes before countering that movement with a reaction. When this is too high it will sound like the motor is cogging, brrrr. When too low the axis will be sloppy and may not hold tolerance. You can think of this as the holding torque of a non-moving axis. This is the I in PID.

The last parameter is the undershoot/overshoot, or the tolerance to velocity changes. Not all drives will give control over this parameter. It contributes to the ability for the axis to hit a repeatable value every time. If the value is too low, the axis will overshoot the target point and may not repeat very well. When the value is too high, the axis may undershoot the target point because it prematurely halts axis movement; reacts too quickly. This is the D in PID.

The fastest part of the loop is usually the gain, then the I, then the undershoot/overshoot. Often the stifness is set lower than the others, with the u/o set high to ensure accuracy. The gain is adjusted until you get a hum, then back down. On digital drives you adjust until the drive spazes out at the slightest velocity command, then back it down.

On DC drive, you can readily tune them quickly with just your ear. Wait for the hum, dial back 1 to 1.5 steps. For I or Current, adjust it up until you get a rough sound from the motor and back it down until it smooths out. For u/o you need to use a precision dial indicator or take test cuts. Adjust it up until the finish stops improving, then back down to the last setting.
 
Perry is on target for most of what he said, but I would differ with a few points. The velocity part of the feedback loop has the highest bandwidth (fastest). What it does is compare the difference between the rate of change of the setpoint and the velocity of the axis. The difference is used as one part of the PID loop. In PID terms, velocity is D for differential. That is because the controller is electronically taking the derivative of the position input to determine velocity. The P part of the loop (proportional) compares the commanded position to the actual position. The output is proportional to the difference of those two. This part of the loop has the second highest bandwidth (second fastest). Finally, is the integral part of the PID loop. This part integrates the proportional part of the loop (unless you have a two encoder setup). This is the least bandwidth (slowest) in the loop. What it does is force the loop to converge to the commanded position, because the integral of any difference continues to get bigger as long as there is a difference. That drives the loop to converge.
 
Perry - please don't think I was being critical - I think your experience will be valuable to baran3.

To add a point to the original question, the velocity gain parameter helps to stabilize the loop. When this gain is too low, you will experience slow settling or even oscillation. So use care in lowering the velocity gain.
 
i did start the machine back up after replacing the thrust bearings. just grabbing the flex coupler and placing rotational force on it, it starts vibrating just as it did before. when machining it didn't vibrate all the time, but it was fairly frequent in my opinion. i don't want to keep chasing a solution and throwing money at it at this point if i live with a tuning adjustment. I actually brought in a maintenence company, thinking that it would be case closed they will solve my problem. Turned out to be a waste of money. So I reduced the velocity gain and now when you apply rotational force to the screw you can feel it correct its location without oscillating. I was just curious how this would affect the performance of the machine and if i should possibly be adjusting a combination of the gains.
 
what might cause a high pitched singing on the y axis on a Mori Hori (16 i) with questionable maintenance history?... along with high servo load.
Take care of the high load (mechanical or motor) and the whining will go away?
 
Adjustments are not designed to fix problems. They are designed to compensate for manufacturing tolerances. You have a problem. Something is worn out or broken. Fix that. Messing with any adjustment to fix a problem only makes things worse.
 
Google is your friend. Search for Ziegler–Nichols. I basically use only the P part of it but that gets one started.
My machine is very touchy about D and I get almost no benefit out of I. I have FF0, FF1, FF2 to play with; which gives me offset (not used)
feed forward comp and the derivative of feed forward comp. I can get following errors of maybe 5.0E-4 at 100-200 ipm but a nasty spike at each end of travel from backlash. (well used machine). It gets better at normal machining speeds.
 
You may be correct that adjustments compensate however if someone was in the control previously and messed up the settings it may be good reason to address them. If anything they should be looked at. All three of our fadals have some differing settings and each is usually calibrated for the specific machine in certain cases. I bought a 3016L that had been messed around with by a maintenance guy in Denver who didn’t know what he was doing causing far more issues. Some 4020 stock settings were in play, in Utrecht turret factor, orientation, gain, etc.., and once things were calibrated properly the problems quickly went away.
 








 
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