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IndustryArena Forum > MetalWorking Machines > Cincinnati CNC > Strange: Z-axis BDS4 with Z-Axis servo stuck (Arrow 750)
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  1. #1
    Join Date
    Jun 2022

    Strange: Z-axis BDS4 with Z-Axis servo stuck (Arrow 750)

    Dear all,

    I am investigating this issue for quite some time now on a Cincinatti Milacron Arrow 750.

    The Z-axis is not moving with the Z-axis BDS4 servo amplifier ("driver").

    However, Z-axis BDS4 driver is moving X and Y-axis perfectly fine (they are not as heavy). This tells me that the Z-axis BDS4 driver should be working OK.

    Also, X and Y BDS4-drivers move Z-axis (slowly, but they do, up & down with about the same speed - could be a setting in the software). This confuses me! It doesn't make too much sense to me. So I took a lot of measurements, found out that the servo PWM-signals (Ma,Mb,Mc) of X and Y-axis BDS4's are around 51% duty cycle whereas the Z-axis PWM-signal's duty is only around 30-43% max.

    Could the lower PWM duty cycle be the ONLY reason? If that would be the case, then why the X-axis BDS4 driver (currently on Z-axis servo) is only moving slowly at 51% PWM duty? According to the owner of the machine, everything is automatically being lubricated, although the machine was standing for a few years now.

    This is a "Goldline Brushless P.M. Servomotor" by Cincinatti Milacron on the Z-axis.

    The DC-resistance of the windings of the Z-servo is 3x 1.7 Ohm whereas for X and Y-servos the DC-resistance is 3x 4.5 Ohm. This could be correct as the motor should be more powerful and all 3 are equal.

    We have taken out the mechanical brake at the back of the servo to rule out issues with that (for all our tests).

    The Z-axis can now be turned even by hand from the back of the servo with a pair of pliers.

    The tool holder is sitting on a block of wood. As we were able to lift up the Z-axis just now, using the X-servo amp (BDS4), we left a gap of about 0.5-1 cm, then turned off the power. The Z-axis dropped on the wood immediately (as expected). So why the Z-axis BDS4 driver wouldn't move the Z-axis down at least?

    So I guess all that is left now:

    1) doubt the owner about the machine being fully lubricated, although still moving by X/Y-BDS4's and by hand + able to drop by itself
    2) the 3 windings of the Z-servo are shorted in a strange way so all 3 show same low resistance of 1.7 ohms each
    3) the magnet inside the Z-servo lost its magnetic fields over that past nearly 30 years

    More measurements in the attached table (resolver, DC bus etc.).

    Hope someone could give me some advice on this please.

    Thank you!


  2. #2
    Join Date
    Jun 2022

    Re: Strange: Z-axis BDS4 with Z-Axis servo stuck (Arrow 750)

    We have just taken out the Z-axis servo. It DOES move with the Z-axis BDS4 now, but has very low torque.

    At machine idle, servo powered up, you can relatively easily turn it by hand. When its moving, you can stop it by hand. Both shouldn't be possible that easily.

    What could cause this? Problem with BDS4 or Servo? A switch inside the BDS4 maybe? Or a setting in the A2100 software?

    P.S. BDS4 is enabled, 3 green LEDs ON, no RED LEDs on the outside, all looking good.
    P.P.S. Command Scale is NOT fully CCW but fully CW, which means the full amplitude control voltage should go into the BDS4 (see simplified schematic + section 5.3.1 in BDS4 Installation & Setup Manual).

    Any ideas?

  3. #3
    Join Date
    Jun 2022

    Re: Strange: Z-axis BDS4 with Z-Axis servo stuck (Arrow 750)

    The problem is with the BDS4 of the Z-axis.

    In fact we just swapped servos of X and Z axis. Now the low torque Z-axis servo has a high torque with the X-Axis driver (BDS4) and vice versa, the high-torque X-Axis servo now has the same low torque with the Z-axis driver (BDS4).

    I have opened a new threat to troubleshoot the BDS4.

    Well, that was an interesting monologue... don't you think?

    Hope all my measurements taken benefit some of you folks!

    You're welcome!

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