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erratic behavior on servo machine
« on: August 23, 2006, 11:01:58 AM »
After rebuilding my X axis, I am getting wierd test results when trying to get a proper backlash number. I'm not new to cnc and realize most problems like this are machanical, but I have been meticulous in putting the axis back together. I have had this axis down to .001" backlash but now it's in the area of .010 and not consistant! I even seem to gain steps sometimes!

Before I take it all apart again, I'd like to ask:
Is it possible there is something wrong in any of the following areas:
Gecko or Gecko tuning
encoder or wiring connections
Servo motor

In other words is it possible that anything other than mechanical problems, (screws, nuts, ways, bearings, pulleys, belts,) can be causing the problem.

Thanks,
Jerry

Offline Chip

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Re: erratic behavior on servo machine
« Reply #1 on: August 23, 2006, 02:38:54 PM »
Hi, Jerry

If it was working before you rebuilt the X axis.

I dought the Gecho's or tuning, If thay are wired correctly and not trip-ing out.

The only way thay could be do-ing that is if the Reset Drive is enabled all the time

The first thing I would do is Zero out your Backlash setting for the X axis or turn it off and under View /  Save current settings.

Then restart Mach re-check backlash X is Zero or off.

Then re-check the Steps Per inch number in one direction, be sure the backlash is out by running it in that Dir. some first and verify that

step's required are the same as Config / Motor Tuning /  Steps Per are the same number.

Then run it back the other way 1 inch at a slow feed rate,  determin the backlash and reset it and turn it back on.

Make sure that your not in Constant Velocity Mode, seems to me there not compatable with each other.

Is your encoder mounted on the servo motor, if not check for backlash there also.

Just some thought's Hear, Chip
Re: erratic behavior on servo machine
« Reply #2 on: August 24, 2006, 11:18:58 AM »
The steps per inch are correct. This same basic setup has been operating for a few years. The Gecko is however new and I find adjusting it much different from the original; it seems much more sensitive. The directions for adjustment that come with the Gecko are very difficult for me to understand. I don't have a pulse generator or oscilloscope.

Using a long stroke indicator, and setting step jog to .100" I can jog back and forth or go one direction several times. Using this method, I can adjust the backlash fairly close, but not exactly. I get erratic readings to the extent of a couple thou. If I keep playing with it I suddenly get a jump in one direction or the other of about 8 to 10 thou. and then that seems to be the spot that the dial oscillates about for further tests. It acts as if the indicator has moved but I'm pretty sure it hasn't.
I tried another indicator on another base and I get the same results. Other than the indicator moving I'm thinking something else erratic in the communication between the Gecko and encoder is happening.

I think I'll try some actual machining and see what's up.
If it won't cut right I guess I'll start changing components, a long slow process, and pain in the butt.

Jerry
Re: erratic behavior on servo machine
« Reply #3 on: August 24, 2006, 11:23:21 AM »
PS:

Does anyone know if it's possible for the gain on the Gecko to be set so low that it misses steps sometimes?

Offline Chip

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Re: erratic behavior on servo machine
« Reply #4 on: August 24, 2006, 12:50:43 PM »
Hi, Jerry

It sounds like it's getting into oscillation, eather the Gain is to high or Damping is set to low.

Are you replacing a bad Geco or is this a different control your replacing with a Geco control.

Thank's Chip
Re: erratic behavior on servo machine
« Reply #5 on: August 24, 2006, 01:00:17 PM »
I have never seen that.. but I have seen encoder counts that have gotten lost due to noise on the encoder lines.

Hope that helps
Brian
Fixing problems one post at a time ;)

www.newfangledsolutions.com
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Offline Chip

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Re: erratic behavior on servo machine
« Reply #6 on: August 24, 2006, 01:23:59 PM »
Hi, Brian

It can be noise this is true, but a mis-match in gain & dampin will cause it also.

If pin 5 is tyed to 5 volt's after reset thay will also auto-reset wth position loss.

Jerry take the wire off the # 5 terminal check the wire for power, when you turn on the machine, there should be 0 volts at power on, 5 volt's at reset /

drive enable,   and 0 after reset / drive enable released, if this the case the enable is wired properly, power off reatach the wire. if not the drive will be in

reset all the time and can cause problems.

I havent seen you mention any drive faluts  at all in tuning, there is usely some drive fault's when adjusting.

Too early to tell.

Thank's Chip

« Last Edit: August 24, 2006, 01:43:00 PM by afn09556 »
Re: erratic behavior on servo machine
« Reply #7 on: August 24, 2006, 04:24:35 PM »
I just spent a couple hours "dorking" with it.

 Chip, I first changed the Gecko before I decided that I had mechanical problems.
I don't think The gain is too high because I've tried it at zero gain and very low settings. At some settings it goes smoothly in one direction and rattles in the other until it faults.

Brian, I did notice that I did not re-connect the shield to ground, maybe that's it.

Pin 5 is not connected to power, permanently.

After working with it for a while and trying to "turn" with it the problem has gotten much worse and is in one direction.
I started with a .625" rod, using wizard, tried to take it to .575 in .01 cuts. Ended at a measured .545!!!
Adjusted axis tried to go to .525, ended at .480, adjusted again, etc
Final try was when the diameter was down to .380 and this time it wiped out the stock in two cuts!
Between successive tries I made changes in the Gecko gain but other than getting a fault when too high, I didn't see much difference.

So, either I'm gaining steps going in, or losing them coming out.

As I sit here writing this and thinking about what you have written, I'm thinking it is noise.
The signal and motor power are both shielded but the shields are not grounded, and they are bundled.
Tomorrow I'll fix that and if it doesn't help I'll change the encoder. (Grrrrrr)

Thanks so much for all the pointers,
Jerry
Re: erratic behavior on servo machine
« Reply #8 on: August 25, 2006, 11:39:26 AM »
Be sure that you hook only one end of the shield to ground!
Fixing problems one post at a time ;)

www.newfangledsolutions.com
www.machsupport.com
Re: erratic behavior on servo machine
« Reply #9 on: August 25, 2006, 12:11:18 PM »
Friday Morn:

I was wrong about the grounding yesterday. The signal shield is grounded and the motor power is not shielded, but a twisted pair.
I wrote a short program to enable me to quickly reproduce the problem; half dozen rapids and feeds. Good test, saw huge gains in one direction.
Tested the par port by switching leads, no change.
Re-checked grounding of signal shield, small wire but definitely grounded.
Decided to change Geckos still another time, and just on a whim I unbundled the wires and physically separated them by several inches. I powered up and ran the test.

GREAT BALLS OF FIRE! It returned to zero dead on! Accident? Ran it ten more times and didn't get any error I could read on the indicator. I might try it now with a .0001" indicator just to wallow in my happiness.

So it's definitely noise, but I don't know why the shield is ineffective. I'll work on that at my liesure.

Again, many thanks to Brian and Chip,
Jerry