CNC spindle tachometer sensor suggestions

Discussion in 'General Electronics Chat' started by bluebrakes, Feb 19, 2013.

  1. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
    245
    7
    I'm currently building a spindle tachometer for my CNC milling machine so that there is feedback back into the computer of the spindle speed.

    Since there is no real way of putting an encoding disc on the shaft, I would like to use reflective paint on the shaft, which reflects light from an IR LED back to a photo transistor.

    My question is, what sort of circuit would you recommend for this?

    I have plenty of Op Amps such as the LM393 and 741 but finding any good examples of people doing the same thing has been quite limited.

    [​IMG]

    Any suggestions?
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
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  3. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
    245
    7
    thanks for the data sheet, quite pricey, but looks as if it will make a neat job.
     
  4. BobWhite

    New Member

    Apr 14, 2012
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  5. bluebrakes

    Thread Starter Active Member

    Oct 17, 2009
    245
    7
    not really, otherwise I'd use some of my hall effect sensors that I have kicking about the place. I did think about sticking one on above the motor's cooling fan blades but the damn thing is plastic. Not really practical to be sticking the optical sensor in places like that.

    Anyway, I was wondering if I could pass this circuit through your eyes to see whether there is any way to improve it or whether you think it would be good enough?

    As it is being simulated in multisim at the moment, I've stuck a function generator as the signal input (phototransistor eventually) and the oscilloscope as the output to the PC input on the breakout board.

    [​IMG]
     
  6. JayArr

    New Member

    Mar 11, 2013
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    Drill a hole in the shaft and the hall effect sensor will detect it.
     
  7. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    You can buy split gears that clamp onto a shaft for sensing speed with a Hall device or something like it. About 30 years ago, Airpax sold that stuff, not sure about now.

    You can also buy reflective tape that has alternating black and white stripes for sensing with opto devices. I don't recall who makes it though.:(

    You can measure the motor's back emf and control/monitor with that.

    A commutator on a brush motor makes lots of noise proportional to shaft speed.
     
  8. JohnInTX

    Moderator

    Jun 26, 2012
    2,339
    1,022
    http://www.optekinc.com/datasheets/OPB760THRU770SERIES.PDF these have conditioning circuits built in.

    http://www.optekinc.com/datasheets/OPB740.pdf these are just focused pairs.

    Optek has lots of different reflective sensors. If it were me, I'd go for something with the filtering and level sensing built in. If you decide to do it yourself you'll need a comparator with hysteresis on the sensor and possibly some filtering against ambient light.

    Once you get the pulse out, measure the period and you can calculate the RPM.

    You might have to experiment with the paint a bit. Not all black paints are 'black' in the IR spectrum and will reflect IR pretty well, fouling up your encoding.
     
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