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  1. #13
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    Aug 2009
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    Re: Fanuc 0T-A Z axis run away on power up

    Does your machine have a Shear Ring on the Z axis Screw. There would be a Limit Switch on it. Shear Rings are the last line of defense from major crashes...

    Or maybe its a Clutch type
    Click image for larger version. 

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  2. #14
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    Re: Fanuc 0T-A Z axis run away on power up

    Quote Originally Posted by machinehop5 View Post
    Does your machine have a Shear Ring on the Z axis Screw. There would be a Limit Switch on it. Shear Rings are the last line of defense from major crashes.
    I hadn't noticed any shear rings on the Z axis screw but I may have to remove the covers to see them. Looking at the schematic more carefully I believe I need to find switches S271 and S273, they are the Ref X and Ref Z. The part list indicates they are Balluff BNS 519-99-D-11. I haven't noticed them but the hunt is on.

    Attachment 419250

  3. #15
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    Re: Fanuc 0T-A Z axis run away on power up

    Well it appears the schematic and my machine vary slightly. I do not have any single switches. The X axis has a two double switches and the Z axis has a single quad switch. It appears to be decel and limit in each extreme. The schematic only shows 6 switches not 8. I did verify all the limit switches are appearing or toggling in diagnostics. I'm confident the limit switches are working.

    I hooked connection 16 (+15V release) back up to the Z axis drive and re-attempted to power.

    I'm back to the original problem from day one. I press the power, the Z axis moves forward about 0.75", and generates an Error 420. The manual states this is Z axis position deviation amount during stop is greater than setting value. See the photo, you can see the movement in the way oil.

    When I had the #16 release wire off I attempted to use the MPG to move the Z axis very slowly. The counter would move BUT the motor would not, and would generate the same error. The axis drive works fine on the X axis when I swapped them. The X and Z motors are the same. Could I have a brush issue on the Z axis motor?

    Click image for larger version. 

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  4. #16
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    Re: Fanuc 0T-A Z axis run away on power up

    i will go for power cable between motor and servo drive,check it closely or change it

  5. #17
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    Re: Fanuc 0T-A Z axis run away on power up

    Quote Originally Posted by zavateandu View Post
    i will go for power cable between motor and servo drive,check it closely or change it
    I disconnected the drive from the Fanuc velocity controller and hooked up the battery tester set at zero volts. When I enable the drive it moves. I've adjusted pot1, which appears to be the setpoint (sollwert in German?). With some fiddling I can get it to reverse or move forward depending on the adjustment but I can't seem to prevent the movement. Maybe I need to adjust something else? I've attached a screen shot of the pots and their description in German (from the Russian manual). Anyone familiar with German terms?

    Attachment 419418

    A tech familiar with these machines sent me these details when he sent me the battery box details a few months ago.
    Typically the drive needs to be ready and the velocity loop needs to be tuned first before the full closed loop would act normally.
    this means two things to start with:
    - the offset needs to be adjusted so that the motor is not creeping with the drive enable on and analog setpoint switched to 0V.
    - next the drive step response needs to be adjusted so that it reaches the desired speed relatively quickly but no overshooting.
    - if the axis runs away uncontrolled after enabling and no adjustments would stop it, this would be a case of bad tach generator feedback.
    After the drive is acting ok using the battery box, the analog output and the drive enable from the NC control can be reconnected.
    - if you keep getting v-ready off at this point, something else is wrong in the system related to emergency stop and drive enable logic.
    - when the axis becomes active and holding position the tuning needs to be completed in closed positioning loop.
    - if the axis worked ok with the battery box but running away in closed loop the position feedback is bad, either missing or opposite direction.
    - the actual pos. loop gain must be adjusted to match the proposed loop gain (adjust the tach feedback pot to fine tune this)
    - the actual pos loop gain need to match between any two axis involved in circular interpolation otherwise the circles comes out oval.
    - after the pos loop gain shows correct on both axes, the velocity loop P-gain can be turned up slowly until the motor starts vibrating, then back down to about 80% of that. I know this does not sound very scientific but this is kind of of rule of thumb for field adjustment to achieve reasonably good drive response quickly without a lot of measurements.
    - there is a lot more to drive tuning and a lot more can be done in a lab environment but all the above is necessary for a good start in a reasonable time frame.

  6. #18
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    Re: Fanuc 0T-A Z axis run away on power up

    I hooked up the oscilloscope to the tachogenerator input on the drives today. The Z axis encoder is sending some crazy noisy signal, nothing like what I'm seeing on the X axis. I suspect the tachogenerator is bad or I have a bad wire someplace.

    Manual states the Tachogenerator is part #T2435. Now I need to see if it can be cleaned or replaced.

  7. #19
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    Re: Fanuc 0T-A Z axis run away on power up

    I tore into the tachogenerator today. I didn't look dirty but I decided take it apart and check everything. The brushes had very little wear. Commutator wasn't too tarnished. Wires all tested good. Cleaned and re-assembled. The only difficulty was re-installing the snap ring that retains the stator on the shaft. In order to install I had to compress a belleville spring washer. I ended up turning a simple tool to drive the snap ring on while compressing the spring with a bar clamp. Turning the motor by hand I measured consistent voltage.

    Attachment 419530Attachment 419532

    After hooking everything back up I could see a huge improvement. Previously the motor would jump forward about 3/4 of an inch (~19 mm), now it only moves about 1/4 inch (~6 mm). I was able to rough tune it in until the carriage only moved a little, and one time the machine actually started error free. Yeah, all I need to do is to tune the DC axis drive. Perfect job for the morning.

    Now if I could only find a manual for the Stromag CVT drives in English.

  8. #20
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    Re: Fanuc 0T-A Z axis run away on power up

    I have been able to get the drive adjusted rough enough to start the machine but when I attempt to move the axis, even slowly, it lunges about 2" (50mm) in the wrong direction. One time I was able to get the motor to move more slowly but still reversed. Anyone experienced with Stromag drive tuning?

    UPDATE: After I posted I realized I hadn't swapped the axis drives to see if the problem followed the card. Turns out the Z axis is still crazy with the known good X axis drive. It appears the issue is likely on the Z axis motor, tachogenerator, encoder, and/or wiring. At least now I can stop turning pots for a while and trace wires.

    The encoder is a Heidenhain ROD 426A. It's 5V TTL incremental with 1500 PPR. I checked all the connections from the encoder to the AMP connector. One wire was loose and I resoldered it but no change in behavior. Could a bad encoder cause this issue?

  9. #21
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    Re: Fanuc 0T-A Z axis run away on power up

    Today I decided to check the brushes on the Z axis motor. This required pulling the motor out of the machine. Brushes looked fine so I cleaned out the carbon dust and decided I'd test it on the X axis using the open loop battery box. Hookup is fairly simple. 0V (red & black) is hooked up to 1 & 2. Enable (yellow & green) is hooked up to 16 & 17. Encoder loop from velocity controller 21 & 22 must be removed. 5 & 6 are the tachogenerator voltage.

    Attachment 423830

    Unfortunately the Z axis motor ramped right up to 2500 rpm on the X axis. The X axis motor had barely moved in open loop. Obviously there was something different about the two motors. After carefully comparing the two I found a short on one pole of the tachogenerator to the body. Oddly both motors generated similar +/- voltages when tested. I used a little liquid electrical tape to correct the short. After the repair the Z axis motor behaved properly on the X axis.

    Attachment 423832

    Following the motor repair I swapped the axis drives. The Z axis drive performed correctly on on the X axis. At this point I am confident both motors work along with both axis drives are good but there is clearly something not right with the Z axis wiring. I suspect the tachogenerator wiring. The voltage looks ok but I didn't have time to check for a short to ground. That's the next thing to check. I guess.

    I'm getting closer...

  10. #22
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    Re: Fanuc 0T-A Z axis run away on power up

    After many months of working on this lathe I have finally been able to get it fully startup. Turned out the tachogenerator connections were cold and only held in place by the heat shrink. Good enough to see a voltage with a multimeter and watch it with the scope but not enough current for the axis drive to behave. Resoldered the connections and the axis drive finally started working. Some genius had soldered them on the outside rather than using the solder cup. :devious:

    I verified both X and Z axises both move, the spindle runs nice and quite, turret rotates as it should, hydraulic chuck clamps and un-clamps correctly. Now all I have to do is fine tuning. Yeah!

    Thanks for all your help and suggestions. I have learned a TON about DC servers and I'm a lot more comfortable working on them.

    Attachment 423936

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