Gaining steps...maybe? with Servo Drive

[guynamedbathgate]

I am scratching my head on this one so I thought I would consult the Hive Mind. I am having an issue where my Z axis on my Mill is gaining steps. If I move the z head down 1" I get exact motion. If I move the z axis up, I get between 1.008" and 1.015" of movement (not lost steps, but gained steps). I do not think it is slop in the system because the error is cumulative and based on the feed rate. the slower I move the Z the greater the error. If I do a test program that moves the head +1" then -1" and runs the cycle 50 times. at 40 IPM I get a total accumulated error of about .015" if I run the same program at 5 IPM I get a total accumulated error of .05"-.075" or more!!

I am using Gecko Servo Drives and the servo's are from Industrial Hobbies. I am using Bob Campbell s break out board as well.

If anyone has any test suggestions I am open.
So far I have tried
- re tuning the servo drive with no effect.
- check the ball screw and bearing to make sure it was not moving in and out.
- Backlash on my Ball nut is approximately .003" (its a heavy mill head so preloaded nuts dont really work so good)
- adjusted the motor acceleration way down to be sure I wasnt loosing steps that way.

I have had this machine for 5 years and it generally has run well, this is a new development I cant seem to sort. anyone got any ideas.

Chris

[Hood]

Have you added anything electrical recently that could be producing noise on the encoder lines?

Dont know much about the Geckos but is there any way in software to monitor the encoder counts?
Hood

[guynamedbathgate]

hmmm,
I think I am getting some noise for sure. I have been getting a number of unexplained E stops lately, as well. Dont really know how to account for it though.
I will also say this. My house was struck by lightning a few weeks ago as well, which fried another controller I had in my shop (left the computer on, but not the controller and fried a Bob Board), but not this one. Luckily this one was off and seemed to be OK. I dont know what kind of a current spike or damage could occur when the power supply was off and the computer turned off as well, but I suppose its entirely possible that did something to it. Again, wouldnt know what to test to be sure without just replacing each component and seeing if that fixes the problem. That seems like an expensive way to check things though.

Chris

[Hood]

I would try swapping components around rather than replacing them, for example swap X and Z drive and see if it follows or not. Swap wires around on BOB etc etc.
Hood

[Hood]

Also meant to ask, is the cutter always lower with the error? Could be losing steps on the way up due to extra weight with gravity and possibly a slightly binding axis. Moving faster may give it more momentum to get through the sticky patch where moving slower doesnt.

What amount would the largest amount you have seen it out of position be in encoder counts?
Hood

[guynamedbathgate]

No, the cutter is higher. Thats why I am so baffled. I swapped out the Gecko driver with a brand new one I had sitting around and it made no difference. Like I wrote before. On the way down, the motion is accurate, exactly 1" down every time. On the way up though, I get an accumulative overshot so that I end up with the cutter moving higher and higher out of alignment. very strange. I dont see why the encoder would only error in one direction.

CB

[guynamedbathgate]

its a 1000 line encoder. and the machine runs at 20,000 steps per inch. so I'd so over 2,000 lines off at least. I just kept running it and running it to see how much of an error would accumulate. there seems to be no end to it.

Chris

[Hood]

Would seem it would have to be noise on the Step/Dir then or surely the Geckos would fault?
Hood

[stirling]

but why would noise only affect one direction? If it was on the step line then surely it would affect both directions. If it was on the dir line then sure a LOW direction signal is (I think) more susceptible to noise, but it would (theoretically) momentarily reverse the intended direction and therefore look like LOST steps albeit predominantly in one direction.

Sorry - no help I'm sure - just bouncing thinks.

Ian

[Tweakie.CNC]

Sorry to say this but if you have had a lightning strike then any of your electronic equipment could be suspect (whether it was actually switched on at the time or not). Any active components within that equipment, logic, transistors etc could have been injured and thus not performing correctly - when components are dead it is perhaps easy to identify a fault but when they are just injured well..........
Unfortunately I think you need to look toward replacing some stuff. 

Tweakie.