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  1. #1

    Z axis home problem

    Hi.

    I have a probbem with Z axis home.
    On the screen, when it is supposed that Z home has finished, it appears alarm # 126 Z AXIS NOT COMPLETED. in blue, or # 272 Z AXIS ORIGIN RETURN UNFINISH . in yellow.

    Can someone give any idea where to check, to find the problem?.

    Thanks in advance.

  2. #2
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    I believe this problem occurs when the control does not see the Z pulse coming from the encoder after it hits the dog switch. Does the axis slow down when it hits the zero return dog switch? If so try to swap the encoder on the end of the motor with another motor.

  3. #3
    Opps!. I forgot to mention this machine is a 1990 VQC 15/40.

    Yes,the Z axis slow down when it hits the zero return dog switch.

    I wil try to swap the encoder on the end of the motor with another motor.

    Thanks you.

  4. #4
    The problem I think is the encoder, because I have opened it, and found a plastic scratched disc and some plastic chips inside...

    The encoder can be repaired, polishing that plastic disc?

    Thank you.

  5. #5
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    You can try to clean the disc but usually the encoder must be replaced.

  6. #6
    Quote Originally Posted by onthebumper View Post
    You can try to clean the disc but usually the encoder must be replaced.
    I have cleaned the disc, but no polishing.
    At least something works because , turning by hand the encoder´s shaft , (wich is conected to the machine) I have readings on the screen.

    So I am wondering if the encoder is Ok ..how can I chek it?

    Thank you!

  7. #7
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    Something to Check

    HI There;

    Go to your machine parameters for the axis soft limits. Positive and negative limits.
    Record the numbers for both parameters. For the Neagtive type in
    (Negative) -9999999 and for the (Positive) type in 9999999. Do not turn the machine off. Try and home the Z axis. If it will home then change the values back to the original values. Power the machine off then back on and try homing again.
    If the Z axis continues to move slow all the time when trying to home then your delcelaration limit switch may be bad. You may also try doing a grid shift according to instructions in the maintenance manual that came with your machine.


    Larry (MTSS)

  8. #8
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    By rotating the encoder and seeing the readings change on the screen only means your A and B phases are working properly. It does not mean the Z phase is working. The Z phase is used for home position return and only outputs a signal once per revolution. The fact that you can move the axis without alarming also says the A and B phase are good.

    When you are zeroing out the machine the axis hits the dog switch which decelerates the axis and starts to look for the Z phase and stops once it finds Z and marks off home position. I suspect in your case the control/drive is not seeing the Z phase and that is why it is alarming.

  9. #9
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    Just thinking out loud.

    It's been a while, but you hit the dog at speed, then decel and travel at creep speed across the dog but it doesn't look for Z phase until the switch comes off the other end of the dog. After the switch extends it stops at the next Z phase plus offset. My experience has been if the limit swich is stuck in the axis will creep until it servo lags out or hits a hard limit and never stops until then. His machine sounds like it decels and does eventually stop which don't make sense. If it doesn't see the Z phase it should keep on rotating looking for it correct?

  10. #10
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    I believe the control or drive knows how many pules per revolution the encoder is (set by parameter). So for instance if it is a 25,000 pulse per revolution encoder and the control or drive sees that it has gone a full 360 degrees or 25,000 pulses without seeing a Z pulse then it alarms out. This of course is a guess but I'm pretty sure I've seen this problem fixed by encoder in the past.

    By the way I agree with your explanation of the dog switch that is does wait for the dog to come off the switch before looking for Z.

  11. #11
    Ok, too much work around here. But I promise that as soon as possible, I will follow yours recommendations.

    Also, as I have Z + /- readings on the screen , I could check with a dial indicator the Z axis position going up an down.

    Thank you for your interest in help me.

  12. #12
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    This machine is not equiped with Sony Magnascales is it?

  13. #13
    Quote Originally Posted by Leelund View Post
    This machine is not equiped with Sony Magnascales is it?
    No, it has not scales.

  14. #14
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    Aren't the resolvers on the end of the axis ballscrews? I think the one on the motor is just used for speed reference for the motor.

  15. #15
    Quote Originally Posted by Leelund View Post
    Aren't the resolvers on the end of the axis ballscrews? I think the one on the motor is just used for speed reference for the motor.
    It is mounted at the end of the ball screw, bolted with the electrical brake.
    It has a Mitsubishi plate at the top, but inside there is a Tamagawa inscription.

  16. #16
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    Tamagawa Seiki makes some of the encoders for Mitsubishi.

  17. #17
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    Is the encoder that was attached to the ball screw thru the brake assembly the one you took apart? When you go at the home dog at rapid speed and the axis decels, does it keep creeping and never stop? Or does it stop like it has gridded up and when you try to go to auto you get the alarms? I'm still a little confused on how it is trying to home. If this is a M2 control with TRA amplifiers and DC servo motors there is no hard limit so the axis should keep going until it sees a Z phase. If it misses the first grid because of a switch sticking or slow to pop out it will go to the next grid thus you have "jumped a grid" and your machine will off by whatever the grid pitch is. If the switch is never seen after coming off the dog it should keep creeping until the ballnut hits the end bracket cushions and servo lags(red alarm) and estops(another red alarm). Sorry we are no help as yet but need more specifics as how the machine is acting. Like bumper said, if you know how to reset your home positions, take an encoder from another axis and see if the problem follows the encoder or stays with the drive. That will tell you real quick whether you need an encoder or not.

  18. #18
    After some months of searching, I found an equal encoder to replace on my VQC 15/40 M-32 Z axis.

    Bought it on e-bay, and the seller assured me that was taken from a running machine .

    I had to replace the battery with a phone battery, but apparently there was no loss of parameters.

    The problem now, is that I see on the screen alarm 55 Z SERVO LAG EXCESS. when it try to do HOME 1

    Doing some test, I connected the encoder and turning the shaft by hand very gently, the spindle goes up or down according to the encoder´s direction of rotation. But if i turn slightly more fast, appears the above-mentioned alarm.

    Any sugestions?
    Is there any parameter related to the servo motor or encoder that must be cheked?


    Thanks in advance.

  19. #19
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    I, don't know about that encoder you go. I would change back to the old encoder and see if you get the same alarm. At least before you could move the axis. Are you sure you received the same model encoder?

  20. #20
    Quote Originally Posted by onthebumper View Post
    I, don't know about that encoder you go. I would change back to the old encoder and see if you get the same alarm. At least before you could move the axis. Are you sure you received the same model encoder?
    I bougth the same encoder, it is a Tamagawa FA Coder 5017 N56...and the piece number are both the same, and also it was taken of the Z axis from a runing machine.

    If I try with the old encoder, or another from Y axis the alarms are differents.

    I suspect it is a problem related to comunication betwen te encoder and the servomotor.

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