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  1. #1

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    Servo to encoder tuning

    I have a 1990 Sabre 1250 with an 850MC with an x-axis servo motor that I put new bearings in. I marked the coupler location for the encoder and put it back together successfully however since I've had this machine i've always notices a high pitch coming from this servo when its just sitting still, the y and z do not do this so this leads me to believe the encoder could be tuned in better. After doing the bearings I turned the machine on and loosened the 4 bolts that hold the encoder on and push the slack one way or the other and I can change the tone of the noise so I'm sure there is a procedure for tuning the 4 potentiometers on the encoder. I've been searching but haven't had any luck finding anything yet. Does anyone here know the procedure or have a document they could send my way. Any help would be great.
    Thanks, Joe

  2. #2
    Al_The_Man's Avatar
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    Is this a Brushless servo? If so it most likely has the commutation tracks on the encoder and these have to be aligned with the stator windings.
    I have posted the method to do this a couple of times here, you need a double beam scope to do it, and a means of powering the encoder on the bench.
    Al.

  3. #3

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    Thanks for the speedy response Al. Yes it is a brushless. I have a double beam scope and I'm sure I can come up with a way to power the encoder I have a BK Precision 1635 variable power supply. I've been searching on here trying to find the procedure, I'll keep looking, if you could otherwise post a link to one of your previous posts with it.
    Thanks,
    Joe

  4. #4
    Al_The_Man's Avatar
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    You need to identify the H1,H2,H3 hall effect equivalent outputs of the encoder, and the three stator windings.
    The three stator windings are each connected to a 1k -10k resistor whose other ends are connected together to provide a virtual star point for the scope common.
    The probe of one channel is connected to the A phase.
    The other scope input is connected to the H1 or HA hall output.
    You need to back feed the motor around 200rpm, I use a small DC motor for this.
    Then you need to align the two traces so that the H1 straddles the A phase positive going peak.
    The PDF shows the alignment for all three tracks.
    As long as you have identified the right pair, then it should be OK.
    Al.
    Attached Files Attached Files

  5. #5

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    Al, I have for pots on my encoder board an A-0 B-0 C-0 and Z would u use them to tune each of the hall outputs then I'm assuming not sure what the Z is for.
    Thanks,
    Joe

  6. #6
    Al_The_Man's Avatar
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    I have never known these to be tunable, or even need to on the drives I have used?
    I don't know what they would be?
    Al.

  7. #7

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    Here is a picture of the 4 pots, I'm making a coupler now to turn the motor with a dc motor I have to do the alignment like you said, got my 10k resistors and scope ready to go, just have to figure out which wires I need to be looking at to see H1, H2, H3 and I will be set.
    Attachment 204852
    Attachment 204854

  8. #8
    Al_The_Man's Avatar
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    For some reason i took it to be a Siemens 850 control not a Fanuc, this changes the whole picture, they not only are AC sinusoidal powered, they have their own proprietary commutation.
    CN1-2-4-8 are the four commutation signals.
    These encoders are automatically aligned with the motor by marks on the encoder bracket, there is an alignment check that can be done, it requires a automotive battery to aling the armature and a test jig to monitor the CN signals for all 1's. I made up a little LED powered board to make it simple.
    I can see if I can dig it out.
    Al.
    Attached Files Attached Files

  9. #9
    Al_The_Man's Avatar
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    Here is the method from the servo manual.
    I made a simple board using 4 2N7000 FETs and LED's with 330ohm resistors for checking the CN outputs.
    Al.
    Attached Thumbnails Attached Thumbnails Servo to encoder tuning-encalign.pdf  

  10. #10

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    The only problem I see with the procedure you have in the book they show the encoder being on a 3 bolt patter that has slotted holes for the adjustment, Mine has 4 with no slots, there is a tiny bit of play with them just cracked loose but not much probably only .010" or so. I wish I had a manual for this servo motor like the one you posted the pages out of that might have the info I need. Is there a place to obtain such a thing?
    Joe

  11. #11
    Al_The_Man's Avatar
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    even if you cannot cannot rotate the encoder the bit pattern should be the same when the armature is oriented properly with the battery, you can use a meter to sheck the voltage on the CN-1 to CN-8
    Al.

  12. #12

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    I finally had time to take the motor and hook it up on the bench to test the output of C1-8 I applied the voltage to the u and vw leads and the motor locked into a postition and I checked the c1-8 outputs and indeed they are not correct. I tried 8 times to take the encoder off and move the coupler on the end of the motor shaft but I cannot get the right output of C1-8. C2 is the one I cannot seem to get to get correct with the others. If anyone has a service book or knows where to get one for this type of servo I'd sure appreciate it or has the procedure to do this. Its a Fanuc A06B-0512-B501#7073 with A860-0315-T101 2000p encoder. Al, thanks for all your help in getting me this far, I wish I had the type of encoder that your book has it sure would simplify this I think.
    Joe

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