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Balancing a grinder with accelerometers

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  • Balancing a grinder with accelerometers

    I have been trying to sharpen drills on my bench grinder lately and have been irritated by the drill chattering all over the place because of the vibration due to out of balance. I remember having fun using the balancing rig in my vibrations class in college, so I went to reproduce it.

    There are great tutorials and products for doing static balance:
    - Oneway grinder balancer
    - Various homebrew equivalents (or better) like this one

    I did it with accelerometers because:
    1 - It sounds fun
    2 - I would like to do this with a microcontroller mounted on the grinder at some point (not today), so it is a simple process, like a tire balancer
    3 - From a theoretical perspective, it should be more accurate than static balance because there is no error of mounting the wheel after balancing because it is balanced in place
    4 - I will learn something

    Let's be clear -- #1 above is the real reason I am doing this #4 is also a nice fringe benefit.

    The setup:
    - One of my grinders is a on old Craftsman grinder I like that I inherited from my uncle. It's dear to my heart, hence no Baldor.
    - I'm using a grinding wheel on one side and a Scotchbrite wheel on the other side for deburring.
    - An accelerometer on the back of the grinder to read acceleration. I used this accelerometer
    - I made an aluminum flange to mount the balance weights and also to sense the 0 degree position of the grinding wheel. I used this sensor.
    - Hot glue gun
    - Fasteners in the shop for balance weights
    - Digital scale from the kitchen to measure balance weights
    - Oscilloscope to read signals
    - Excel spreadsheet I made to calculate balance weights
    - Towels to put under the grinder to keep it from skittering around on the workbench
    - Netduino - used only for supplying power to the sensors right now


    Overall setup


    Accelerometer mounted on back of grinder


    Balancing flange and index sensor. The index sensor is hot glued on the end of the block of aluminum for prototyping. The blue tape is to minimize the c-clamp handle from clanging around. It was surprisingly clear to see on the measurement.

  • #2
    The general procedure is to:
    - Take a baseline vibration measurement
    - Add a trial weight and measure the resulting vibration
    - Calculate the effect of the trial weight and then calculate the needed weight to eliminate the vibration

    This is a simple, single plane balancing scheme and I think it should be fine for bench grinder use.


    The picture above shows a yellow trace of the accelerometer output and a blue trace of the index pulse (sharpie mark on the flange, read by the optical sensor).
    I get a 360 mV reading at 5.8 msec (117 deg)


    Then I added a trial weight at 0 degrees and got an amplitude of 232 mV at 5 msec (101 deg).

    I calculate the needed weight to be 1.24 units of mass relative to the first mass at -36 deg. Here is the result.

    The resulting vibration is in the noise. I think I can see a vibration in there at 10 mV. In any case, this is 2.8% of the original value, which I think is pretty good.

    Too bad these numbers are a bit optimistic...
    Last edited by Erik Brewster; 08-02-2012, 06:50 PM. Reason: grammar

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    • #3
      Erik--

      So how did the grinder's sound or feel change?

      Also, which axis were you reading?
      For just a little more, you can do it yourself!

      Comment


      • #4
        The biggest problem I ran into is that there was a big 2 times per revolution vibration. A pure out of balance condition should be a once per revolution sine wave. Here is a nice example from before I balanced the grinder:


        Note that there is one sine period per index pulse. Perfect. The problem is that I took this measurement with the motor off. I ran the motor up, turned the switch off and immediately took the measurement. The motor is basically going full speed at this point.


        Here is a picture of the measurement right before I turned off the motor. Note that there is a significant 2 times per revolution vibration. It goes away completely the instant I turn off the power. It goes from a pretty good balance with the power on to absolutely spookily silent and smooth as soon as the power is off.

        Any ideas? I'm thinking I'll take the motor apart to find the problem, but honestly, I don't know what I'd be looking for.
        Last edited by Erik Brewster; 08-01-2012, 11:20 PM.

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        • #5
          Erik--

          It might be a resonance. Try damping different part of the machine by grabbing them tightly.
          For just a little more, you can do it yourself!

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          • #6
            Originally posted by ed_h View Post
            Erik--So how did the grinder's sound or feel change?
            Like I said in a following post, it's super smooth and silent now that it's balanced. It had a pretty noticeable vibration before. Enough that it was tough to grind with great precision. The Scotchbrite wheel was the big offender, though both had some out of balance. All the pictures here have the Scotchbrite wheel balanced and I am focusing only on the grinding wheel. Now that it is smooth, it is mostly a whooshing of air sound, when the motor is off.

            Originally posted by ed_h View Post
            which axis were you reading?
            I am reading the vertical axis. For some reason, the axis front to back (z on my accelerometer) had strangely low gain that made it tough to measure. I understand that the up and down axis (X axis on my board) can pick up a torque vibration. I suppose I should go and remount the board to use the X axis in the front to back configuration to avoid picking up any torque vibration.

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            • #7
              Originally posted by ed_h View Post
              It might be a resonance. Try damping different part of the machine by grabbing them tightly.
              Interesting point. One thing I forgot to mention is that is clearly not a second harmonic of the out of balance -- it is just slightly a different higher frequency than twice the rotor imbalance. The rotor imbalance locks exactly to the index trigger. The "twice rotor speed" is just a bit faster than twice the rotor speed. I take that to mean that it is some vibration from the electrical lines, which should be a bit faster than the rotor speed, due to induction motor slip.

              I did go and grab the machine. If I held it in the air, the twice per rev vibration reduced about 30%.If I pressed down, the vibration increased until I pushed with 30 lbs.-ish, where it started reducing. That seems to make some sort of intuitive sense.

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              • #8
                If I'm reading your screen right, you have about a 120 Hz signal. Certainly could be a torque reaction. May not be much you can do. Add mass, maybe?
                For just a little more, you can do it yourself!

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                • #9
                  To make reading the plots easier, the blue index pulse is at 58.something Hz. That's 3500 RPM or so. I remounted the accelerometer so that it would read front to back through the axis of the shaft. This should remove the torque couple. It read about like the vertical axis when I held it in the air, so that seems consistent.

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                  • #10
                    How does the unit run without the grinding wheels? I have almost the same grinder and had a balance problem and it was a cheap wheel. Install one wheel and run the unit and determine which wheel is the problem. Interesting project.

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                    • #11
                      Originally posted by Erik Brewster View Post
                      To make reading the plots easier, the blue index pulse is at 58.something Hz. That's 3500 RPM or so. I remounted the accelerometer so that it would read front to back through the axis of the shaft. This should remove the torque couple. It read about like the vertical axis when I held it in the air, so that seems consistent.
                      How many axes are you sensing? What does it look like on other planes?

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                      • #12
                        2/rev on motors is often "oil canning" of the stator from the rotating magnetic field, as you get a peak from both the positive and negative phase of the current waveform. That would explain why it goes away during coasting.

                        Davis
                        Davis

                        "Nothing is impossible for the man who doesn't have to do it himself"

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                        • #13
                          Originally posted by interrupted_cut View Post
                          2/rev on motors is often "oil canning" of the stator from the rotating magnetic field, as you get a peak from both the positive and negative phase of the current waveform. That would explain why it goes away during coasting.
                          Davis
                          I'm not familiar with "oil canning." Is that runout of the rotor? Bending of the shaft under speed? Other? It seems like some things could be fixed...

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                          • #14
                            I found a B&K paper on balancing that seems really useful:
                            http://www.bksv.com/doc/bo0269.pdf

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                            • #15
                              Before you assume that your 120 Hz signal is an indication from the accelerometer, consider this:

                              Your sensing circuit is not shielded. That is an ELECTRIC motor that operates by generating an electromagnetic field and your sensor circuit board is certainly inside of the outskirts of that field. It could be that it is just picking up that field.

                              Try taking the sensor board off of the grinder but HOLDING it in roughly the same position WITHOUT TOUCHING THE GRINDER. Turn the motor on and see if that 120 Hz signal is still present. If it is, it is not from the accelerometer, but is instead just picking up the field in the air. Notice I said "HOLDING" it. Keep it isolated from the grinder and from the bench.

                              I have often noticed that an oscilloscope probe can be used to detect AC fields just by connecting the end of the ground lead to the tip of the probe and using the supposedly grounded loop formed in this manner to find a field. Certainly your circuit can do it also.
                              Last edited by Paul Alciatore; 08-02-2012, 03:05 AM.
                              Paul A.
                              SE Texas

                              Make it fit.
                              You can't win and there IS a penalty for trying!

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