Need Help!- Adjusting AC velocity control

[Andy Fritz]

Fanuc AC axis control top board p/n is A20B-1000-0560/rev 11F

Here is the problem; I have this drive hooked to a lathe C axis red cap model 0 motor which makes high pitched noise and gets really hot at idle. I have meggered the motor and it seems fine, but I doubt the hundreds of megaohm readings I get. If I rotate the shaft by hand while checking between phase and chassi I get infinity on a good fluke meter, so at least there is a chance the motor is ok.

I have the maintenance manual for the control, a 10T and the maintenance manual for the valocity drive, but none make any recommendations for testing that the variable resistors are set up correct.

I bought the lathe recently so I have never had the motor run ok. I know the company who sold it have not used the C axis for the last 10 years. Maybe because they had the same problem. So I have no reference for the settings which could be way off. The manuals only suggest 40% on RV1 which is gain and this is where I have it. RV4 is 5V fine adjustment and I did lift this a little to get to exactly 5V.
I read somewhere that RV2 should be set for 0V while checking across Check pin 1 (0,687 * velocity command voltage) and CH3 (0V), so I noted that it was originally set at -0,11V, then I set it to 0V. This did not make much difference so I set it back to -0,11V. Next I checked the setting for RV3 on the X axis board ( same motor and drive ) which works fine and copied the 50% setting to the C axis. Did not make much difference either, but I played with it a bit. At one point there was actually a bit smoke coming from the C motor, but I dont know if this was due to the time it had been powered up or my attempts to set the RV3! There are a few more trimpots, but as the revision number is not the same between X and C boards I really have no clue and dont want to mess with them. I hooked up X motor and pulse coder from the C board and it sounded fine. next I hooked the C motor and pulse coder to the X axis drive and it was still noisy. So this would suggest that the settings on the C drive is at least semi-ok and the C motor or pulse coder is bad.
Still I would really like to know if there is a better approach to setting all trim pots the way the really SHOULD be set? Fanuc only writes "RV2 - RV4 have been adjusted before shipping"???

The best result I got from playing with the trim pots was to stop the noise while the motor is powered, but not mounted to the axis. If I then force the shaft radially by hand the squeeling noise comes back. Some time it stops when I stop forcing the shaft and sometimes it doesn't stop. In any case as the weight of turning the C axis is enough to make it squeel it always does when hooked to the axis. If I leave it squeeling for a few minutes I get alarms as expected.

I would like to try the X axis pulse coder on the C motor, but want to wait until I know the C top board is set up right.

I just bought a scope if that is needed.

All the best,

Andy

[CHAD LAWSON]

I am not familar with your specific equipment but it sound like either a hunting or tuning issue. The first thing i would do is set you scope up on the feed back signal and check the wave form. I would look for noise and specificaly the wave length and amplitude as i am assuming this is an analog feed back system. I would check both a and b phase and even look for the marker pulse. I also can tell you when dealing with some of this older equipment a newer fluke does not always do the trick. Many of our meters to day do not register a true zero volt. This being the case I use my o scope or would reccomend a simpson 260 for the pot adjustments. I am sorry if this does not help but not a specific model i am familar with. It sounds like you are fairly familar with electronics so you may be on the right track.

[Andy Fritz]

Thanks for your reply Chad,
The problem is that I am actually not very good with electronics.
I bought a scope as I knew it would be useful some day, so I guess I have to try to figure out how to use it! I think the encoder is analog, but the control converts the signal to digital. There are 4 sensor zones on the encoder disc. When a zone is active the encoder sends 3,5V through its zone specific wire. When not active the voltage is 0,7VDC. The position of the motor affects the combination of on vs off zones and each high/low signal is interpreted by the control as either a 1 or a 0.
I have tested the encoder alignment on the motor shaft. What I did not test was for noise in the encoder. I ruled out crosstalk in the motor and encoder wires as I swapped them when swapping drives. As the problems stayed with the motor/encoder combo I think the problem is either in motor or encoder.

Next I will try to find out how to see noise with the scope.

Cheers,

Andy