problem with servo. flakey in one direction

[Daniel Wilcox]

I have a hardinge HNC converted to run on EMC2. I kept the stock servos, amps, power supply, and resolvers. I am using a pico systems resolver to encoder converter for both axi.
The lathe was working great for a while, and eventually i started getting joint following errors for the x axis. I noticed it was only in one direction (x-), and when it did it, the movement was pretty rough. If i went x+ for a second and went back to x-, it was usually ok. I slowed down the velocity so i could still run parts and it helped for a while, but now it's to the point where it's almost unrunable. Even at like 10ipm, it'll still fault. It does not appear to be anything mechanical.. if i turn the leadscrew by hand, it works great. Also, i tried changing the belt and it did not help. I do not think it is the resolver or the pico board (i tried swapping the pico board with the z axis and it didnt follow the board). The machine is still holding a good tollerance, and it doesnt run away. I'm thinking either amp or servo. Since it's only in one direction, i'm wondering if maybe it's a brush? (brushed DC). Has anyone had this problem and figured out what it was? I know i can swap amps and see if it follows the amp, but i figured i'd ask first. I'm going to attempt to go pull the brushes to check them out.
Thanks in advance,
Dan

[Daniel Wilcox]

well, i pulled the brushes out, and they looked good. However, there was some black grease built up on them, so i wiped them off. It was only on one edge of each brush, so maybe it was enough to catch the next pole and make it run rough in that direction. Also, i noticed one of the brush caps wasnt tight. I put it back together, and it seemed rough in both directions and faulted in the same direction a couple of times, then it smoothed out. I cranked it up to 200ipm (the max) and i ran it back and forth a ton of times and it seems smooth and hasnt faulted. I'm wondering if the first faults were because i may not have put the brushes back in the same way they came out and they just needed some revs to seat properly, or maybe there was still some of that grease in there and it needed to make a few revs to wipe it off. Does any of this seem plausable?
I'm not going to call it fixed untill i run it for a day or two without a fault. If it is indeed a definite improvement like it seems, then at least i have a good place to start. I was tapped out at like 40ipm before and it was still faulting alot.

[Daniel Wilcox]

well i spoke too soon. it went to crap again right after i went out there after typing this.

[mike_Kilroy]

hmmm... no other takers so I'll give u my 2 cents worth... dc motor may mean u have a tachometer in the motor too. tach brushes are more apt to cause probs in 1 dir like u describe..... can u get to it and try carefully to clean its brushes/comm? what is brand and part no of motor and drive? from ur comments I would not think the drive is the issue but more likely the motor and more likely even the tach. let us know.

[Daniel Wilcox]

Originally Posted by mike_Kilroy hmmm... no other takers so I'll give u my 2 cents worth... dc motor may mean u have a tachometer in the motor too. tach brushes are more apt to cause probs in 1 dir like u describe..... can u get to it and try carefully to clean its brushes/comm? what is brand and part no of motor and drive? from ur comments I would not think the drive is the issue but more likely the motor and more likely even the tach. let us know.

hmmm didnt even think about that! Thanks.

The motors are Electro Craft. Not sure on the tach, but it may be difficult to take apart. It's a pretty small unit, and kinda looks like a little rc car motor. I'll take a look at it though.

BTW, this is a pretty low mileage machine. The controller died 10+ years ago from what i heard, and before that, it was only used for prototyping. The ways are PERFECT, and the paint is still in pretty damn good shape. Basically what i'm getting at is i doubt it would be the motor, but i could definitely buy it being a bad tach.

[Al_The_Man]

If you have the original drives and they are using tach input then they could most likely be Westamp or Glentek, these often suffered component change with age, one item was the 10 turn pots for tuning.
I usually ended up fitting Advanced Motion drives, they can be run in the Torque mode and eliminate the tach feedback altogether.
Generally you also get tighter control.
They are usually plentiful on ebay and the BLDC models can be used just as well with DC brushed motors.
Al.

[Daniel Wilcox]

Originally Posted by Al_The_Man If you have the original drives and they are using tach input then they could most likely be Westamp or Glentek, these often suffered component change with age, one item was the 10 turn pots for tuning. I usually ended up fitting Advanced Motion drives, they can be run in the Torque mode and eliminate the tach feedback altogether. Generally you also get tighter control. They are usually plentiful on ebay and the BLDC models can be used just as well with DC brushed motors. Al.

the original amps are GE, but the tachs are nameless.

I was alwas told that velocity mode was best for EMC2, is this false?

I took the tach apart and cleaned the brushes and comm, but everythnig looked good. working on putting it back together now.

[Al_The_Man]

The drive manuf. I use such as AMC and controller such as Galil Motion recommend the torque mode in CNC usage, I found found quite an improvement when switching over from the old velocity mode drives.
Did you try swapping the drives?
Al.

[Daniel Wilcox]

Originally Posted by Al_The_Man The drive manuf. I use such as AMC and controller such as Galil Motion recommend the torque mode in CNC usage, I found found quite an improvement when switching over from the old velocity mode drives. Did you try swapping the drives? Al.

i havent swapped the drive yet, still trying to get the tach back in. I got it in, but now it throws a following error in both directions. I'm not sure if this is due to the fact that i cleaned the brushes or if it's because the tach is missaligned. I'll have to play with it a bit. At any rate, i cant afford to replace any drives or motors at this point.

[Daniel Wilcox]

well i played with the tuning a little, and it seems to be helping. I'm back up to 200ipm. I'm going to see if i can dial it in a little better and give her a run.

Could it be that it needed to be retuned simply because the brushes might have been dirty?

[Daniel Wilcox]

just ran a part, and it went smoothly. No problems whatsoever at max velocity. This is the first time in quite a few months that it has done that, and actually the first time in a couple weeks that it made it through without getting an error even at 30ipm.
I'm not calling it fixed until i run it for a week or two, but this is definitely progress.

The weird part is that it got ferrors in both directions after cleaning the tach, and it also started ocillating a little when stopped. Tuning got it smoothed out, but i find it weird that it needed it just from cleaning, and it didnt even look dirty in the first place.

Thanks for the help guys!

[Doctor Tsugami]

What you have is pwm drives (pulse with modulated) these require tuning sometimes as they age, like all of us. I have some manuals on sweo and west amp drives they date back to the late 80's so let me know if you would like any information , most drives use the same adjustments just located at different places on the boards and the set up proceedure should be very similar good luck