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  1. #1

    Fanuc spindle going nuts

    Hello

    I have a lathe ( Tsugami NU4B live tool ) fitted with a Fanuc A06B 6078 H112 Spindle servo motor and 0T control. Under MDI, Jog or Auto modes the Spindle is having a seizure.

    Symptoms:
    The spindle when instructed to move by any input will erratically turn. It jitters back and forwards by about 10 degree increments regardless of inputted spindle speed or direction. Load gauge maxes out due to input RPM and actual RPM being different.
    Under no input the spindle moves freely and is not dragging. C axis is not engaged.
    No alarms on the control or the spindle drive. A 408 alarm on the control and AL 19 on the driver appeared once but were cleared by a power cycle.

    Background:
    Purchased working second hand from a big manufacturer that had run the lathe almost every day since new. Meticulously well looked after and there is documented history of servicing down to the smallest washer and fitting. The lathe worked fine once installed and tested in the new location but came to power on today to start its first production and the spindle would not run.

    Trouble shooting so far:
    Suspected bad motor decoder. Cleaned and gap between encoder and gear tooth wheel checked and set.
    Suspected bad spindle decoder. Cleaned and refitted.
    Suspected bad cabling between spindle motor, decoders, driver. Re seated every connection. Alarm only shows then cable is not connected.
    Parameters checked and updated with the original perimeter backup from new. Including 900 parameter set.
    Called Tsugami and Fanuc. Both wanted to sell me new spindle drivers as they had never seen this type of problem.

    Questions:
    Has anyone seen such behavior from a Fanuc spindle before?
    Is the IGBT on the spindle driver easy to access?
    Can i run the machine with parameters set to ignore the decoders so i can check for faults.

    Tomorrows plan unless someone figures whats i am missing:

    Take out the driver and inspect the board. Check fuses and for blown components.
    Test every cable connected to the driver with a multi meter.
    Check the outputs of the decoders with a scope to see i am getting a signal.
    Check that all phases of the motor are working in the correct sequence and cables are connected.

    Please help. I am used to problem solving Fanuc hardware but this is driving me and my spindle NUTS!!!

    Mike

  2. #2
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    Re: Fanuc spindle going nuts

    Disclaimer: I am not a Fanuc tech, so my suggestions should be taken in the general sense only...

    If possible, I'd have a look at the control input signals going to the drive to see if it's being commanded wrong. I'd also have a look for cracked solder joints.

    I don't know how your spindle drive is commanded; but if it's +/- 10v analog; an intermittent connection somewhere in the chain (not necessarily in the wiring) could conceivably cause this behavior.

    Also - you could be aware of this, but I'm going to leave it here just in case (or for anybody else who reads this):

    https://youtu.be/xaELqAo4kkQ

  3. #3

    Re: Fanuc spindle going nuts

    Quote Originally Posted by __Britt View Post
    Disclaimer: I am not a Fanuc tech, so my suggestions should be taken in the general sense only...

    If possible, I'd have a look at the control input signals going to the drive to see if it's being commanded wrong. I'd also have a look for cracked solder joints.

    I don't know how your spindle drive is commanded; but if it's +/- 10v analog; an intermittent connection somewhere in the chain (not necessarily in the wiring) could conceivably cause this behavior.
    Thanks for the advice. Its is indeed a wired connection not a optical connection so i want to sniff all the signals but i am not getting a signal loss error on the drive. I will check so see if the data is getting through correctly without any corruption.
    The driver boards looked in perfect condition so breaks around traces or damaged components. I cant test the board under power. IGBT had no problems, i checked with a multi and then a diode checker. There was large capacitor on the board that looked very slightly bulged. I am suspecting it could be on its way out... Or the cause of the problem.

    The signals to the motor look crazy so where ever the problem comes from its before the motor wires. Motor its self was fine.

    Going by your advice i am guessing its a either a corrupted data stream from an encoder or the signal into the driver.

    I dont have a scope i can use to check the encoder pulses. Can i use a frequency counter on a multi meter?
    I know i should have a 1v sine wave and if i spin the motor with another motor i should be able to check for a consistent frequency out.

  4. #4

    Re: Fanuc spindle going nuts

    Replying to myself I know but...

    Looks like it's a problem with the motor decoder. I checked the servo parameter screen and there was no movement for the load, gain and position readings when turning the spindle by hand or under power.

    Will change the decoder and check the cable.

  5. #5
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    Re: Fanuc spindle going nuts

    There was large capacitor on the board that looked very slightly bulged. I am suspecting it could be on its way out...
    Classic issue on older equipment. I would strongly suggest replacing that cap (and any others that might be questionable). Electrolytic caps can develop leaks - even while the board is continuing to function - and the electrolyte is corrosive to the PCB. For collectors of older gear (ranging from test equipment from the 20's & 30's all the way through classic game consoles and personal computers) replacing backup batteries and capacitors is standard procedure for ensuring that the machines continue to survive. Manufacturing equipment - due to the environment that it lives in - has it particularly hard; with temperature & humidity swings, coolant in the air, etc.

    Looks like it's a problem with the motor decoder.
    Decoder, or encoder? The encoder would be the device that attaches to the motor; and turns the physical position (or movement, if it's an incremental encoder) into an electrical signal to be sent to the control. The decoder is whatever chip (or part of a chip, or set of chips, depending on the system design and age) that takes that signal from the encoder and turns it into something that the rest of the control can use. Typically, "the rest of the control" is a microprocessor or microcontroller of some sort, but sometimes it's discreet logic, an FPGA, or an ASIC.

  6. #6

    Re: Fanuc spindle going nuts

    Encoder (hall effect sensor). However the Japanese parts book describes the sensor unit as a decoder.

  7. #7

    Re: Fanuc spindle going nuts

    Just to add to the story so far for anyone who has the same problem.

    Testing the driver board under almost every circumstance it was found that the AL12 alarm comes up on the display for a half second on power up. You would miss it i you were not looking at the right time. I have seen AL12 before on drives and the solution was to swap the IGBT due to a blown transistor. However on retesting the IGBT it has no problems and putting a megga on the motor found no shorts. Having said all that the one time case of AL19 and the AL12 on start up is a definite sign of a dead driver and replacing the board is the only sensible thing to do.

    I put all this to management and they just threw money at the problem and ordered a new board. 8 hours later the board arrives and is ready to be swapped in. I am worried without finding the root of the blown driver will result in another blown unit. I still also have my suspicions about the encoder side of things...

    Will update with the final results.

  8. #8
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    Re: Fanuc spindle going nuts

    its the Tach // carbon or oil on the brushes or feedback from Tach...loose wire/plug.

    Good Luck on the mission

  9. #9
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    Re: Fanuc spindle going nuts

    > However the Japanese parts book describes the sensor unit as a decoder.

    Yeah... sounds like a bad translation... probably by someone who’s not very technical.

    Wild guess on my part... perhaps there’s a loose connection (or was... it might have been fixed during the trouble shooting process) that caused an inductive voltage spike, killing the IGBT? I’m trying to think of what can do that kind of damage to an already working system...

    Anyway... good luck with it! Hopefully it’s just a one-time failure.

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