AC Vector Drive + Encoder, Motor's shaft oscilates at set up speed 5

[robertoesc2001]

HI, thanks for this helpfull forum

I'm asking for your help to solve this aparent problem we have on a brand new mill tube forming machine, the man who originaly designeed the electric part, just left, living us to solve any further problem, besides that i really like the chalenge and do have some electric and electronics hability.

The system consists of (2) Baldor 18h Ac flux VFD's controling two diferent motors, 15hp and 5hp, coupled directly to one 1024 ppm dynapar shaft encoder each, and the speed reference(+/- 10 vcd) generated by a Machine Motion Control module from GL controls.

The machine is still on its starting up and tests period, more than 1 year so far!
Diferent copper tube forming and welding problems had ocurred since then. So i dont have any reference to say the electric motor controls were working pretty good before.

The last idea the production guys had, was to try the stroboscopic lamp to discard any vibration/oscilation problems on dies, pulleys, belts, etc, so they got to the two motors shaft and found this oscilation at the set speed on the two independent motors, it ocurred with and without the main belt on them (with and without load), i guess it was about +/- 1/6" and suddenly jumps to +/- 1/8" on the motor's (4")pulley's external diam. To be shure, we tryed the strobo lamp on one Lathe and on one CNC Machine Center Spindle, they where exactly steady fixed at the set-up speed. So Im wonder if this would be posible with the combo we have on this new machine, I havent had experience on closed VFD-motors before, nevertheless it was posible on the CNC spindle wich has a VDF drive and encoder on it too, not to say i read some threads here that says so, regarding a VDF with the encoder feedback.

Tests where made in the 5hp VDF unit and were as follows:

1) Control speed signal is normaly over 5 volts and under osciloscope has a little amount of noice on it, we twisted and ground shielded, nothing happened. (Perhaps we should try to put a small disc capacitor if recomended.)

2) Encoder signals where also twisted and shield grounded, the osciloscope between A+ and A- shows a +/- 200ns period (time) variation on each pulse. This made me think on the acumulation, when mult. by 1024 pulses and perhaps this is the electronic measurement of the phisical oscilation.

3) The control speed and encoder signals where separated and kept 90º angle from supply and motor lines as Baldor Manual recomends.

4) i havent disasembled the motor's guard and by that niether verified the encoder's coupling to the motor, as I could read from other treads on this forum, it could be the problem, will doit on monday.

5) some relevant parameters on the VDF are set as follows;
control base speed=1826RPM
feedbackfilter=4
current prop gain=100
current int gain=50kz
speed prop gain=10
speed int gain=1hz
speed dif gain=0
position gain=31
slip freq.=.47h
stator X1=4.4ohm
prop gain#1=15000
int gain#1=5000

Motor data is as follows:
motor voltage=460; motor rated amps.=6.5A; motor rated spd.= 1750; motor rated freq.=60hz; motor mag amps.=2.91A; encoder counts=1024ppr;

Sorry for the amount of information above, I'll apreciate your help if posible, first to let me know if this oscilation will be posible to be eliminated or diminished, Any ideas will be very apreciated.

Thanks for your time.
Roberto

 

[robertoesc2001]

sorry Curt, I was following so close what you said, that I forgot to mention correctly, "I hope the cause are not about the two system loops, trying to adjust speed simultaniously.",
It was an amazing explanation amigo!
Thanks again
Roberto

 

[robertoesc2001]

Hi Guys
Just to up date you:
On the first today test with the strobo gun, no changes, and a cooler day, it was about 80 F in town (normally is above 95 F) the two systems look fine, so i was directed to think about the motor temperature issue. But 30 mins later, the failure started show, and incrementing as motors continued running. So I let the motors to got hot, and even with a heath gun on them, and perform the autotune function, with and without load the respective parameters, as the manual explains. I noticed how some of the Vector parameters changed automatically. Then i let the motor get cool, and test again, noting happened. Secondly, i moved the int speed gain, to cero, as you recomend, and still nothing happened. Third attempt was to mess the control parameters as ITsmoked recomended and,no change.
The Fourth test was controling from Keypad, no external control on it, and it worked great, a steady 1140 RPM speed was reached, no oscilation!!
So, I think this is a control signal noice, or a external control loop simultaneously "fighting" as Mr. Wilson suspect, so what do you think?, to install a .1 mfd disc capacitor at the VFD speed control terminals would be a good idea?, i hadnt one at that moment, but cant wait to doit in the morning and to discard this cause. Perhaps, with diferent capacitor's values.
Mr Wilson, it was indeed a good experience, to auto tune up the motor, and see the changes on the control loop parameters, with a hot motor and a cool one.
BTW you and Sed2 are right, this model, Baldor 18H doesnt have motor temperature compensation parameter.
Will keep you posted
Thanks
Roberto

 

[cswilson]

Roberto:

If I understand you correctly, when you put a constant voltage into the drive from the keypad, the drive behaves well, cold or hot. This tells me a couple of things. The problem is very unlikely to be noise, unless somehow, the MMC's analog output is much more noise susceptible than the keypads. This further means that a capacitor isn't going to help (and will probably hurt, because it will just add destabilizing phase lag).

So it seems now that the key issue is the interaction of the MMC with the drive. You are going to have to figure out what the MMC is doing, or attempting to do. I expect that it is closing a position loop based on encoder feedback and some kind of commanded position trajectory. Normally, this should be a pretty simple job if it is outputting a velocity command, especially when just trying to generate a constant-velocity trajectory.

Does the MMC have a mode where you can simply force an open-loop output? I have found this type of thing invaluable in debugging these kinds of problems. In your case, this should just confirm what you found with the keypad, but it probably will rule out issues with the quality of the MMC analog output signal.

It looks like you are reporting that the system works well "cold" but not "hot". Now, old analog jocks tend to go looking for analog signal drift when this happens (and I can't entirely rule it out), but more likely it is an effective gain change somewhere in the loop that makes the system go a bit unstable -- I do suspect rotor time constant changes here.

The curious thing in this case is that the velocity loop seems to be fine with these changes (if I read you right), but not the position loop. Usually, it is the velocity loop that misbehaves first. So I would still investigate what both the drive and controller are doing with regard to position loop control, and whether they could be "fighting".

Curt Wilson
Delta Tau Data Systems

 

[ozmosis]

Roberto
It sounds like you may have found the problem: noise on the Analogue.
I'd just like to say thanks for your informative post and the way you've taken the information, tried it and actually come back and told us what is going on. It makes a refreshing change. We folk on Eng-Tips are engineers with a passion for engineering, we try and help people where we can and are honestly interested in how things work or, as it turns out, why things don't. It helps enormously to get good information at the start and during and is always interesting when we see light at the end of the tunnel.
Your experience will also help others I'm sure. And when you get peole like Curt adding their knowledge, we all learn.

 

[cswilson]

As I think about it a bit more, the keypad would most likely be setting a velocity command purely digitally, so it does not necessarily rule out analog input noise. Still, it does not smell to me like noise is the issue.

Somehow you will definitely want to put a constant voltage into the drive and see what the velocity stability is. The results of this test will let you know what you have to look at next.

Curt Wilson
Delta Tau Data Systems

 

[robertoesc2001]

Thank you guys for the ideas and words Keith,
Ok, today i just went ther with some .1 and .01 mfd tantalum capacitors, and started to filter the signal at the velocity control terminals, what we found was a more stabilized speed, but not as steady as from keypad, but the improvement was absolute, the oscilation is so little now, that i needed a couple of guys'opinions to pereceive due to my exposition to the strobo falshes. First test was with .1 mfd in both drives same time, to eliminate posibility of at some point on MMC, and we got some improvement, and then added the .01mfd (paralel with the .1 mfd) and got noticeable better. So Perhaps I need couple of .001mfd, in this way to filter higher noice signals. Ok, that was what i thought there, now im reading your posts and make me reconsider as you explain. Any way, next round will be next monday, tomorrow will be out of town, but will check this forum for shure.
Guys, thank you all, it is being a real pleasure to meet people like u, so willing to help others in that sincere and direct way on their profesion, with the only interest described so well by Keith.
We keep in thouch till this fault is over or we/i learn the most of it.
Roberto