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Whining PWM motor control

Discussion in 'Electronic Design' started by Robert Latest, Aug 18, 2005.

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  1. Hi guys,

    I built a very simple PWM circuit to maintain a contant, unidirectional
    current through a small (24V/1A) DC motor from parts I had kicking
    around.

    The circuit consists of a low-side MOSFET switcher with .5R current
    sense in the source leg. An LM393 comparator compares this voltage to an
    externally set reference and asynchronously resets a flip flop when the
    sense voltage is greater than the ref. The flip flop is clocked "on"
    periodically by a 20kHz oscillator; its output drives the gate driver.

    Well, it works fine, actually.

    My problem is that the circuit doesn't switch at 20kHz but at much lower
    frequencies, which are quite unstable and vary greatly depending on the
    preset current limit and motor speeds, which of course creates
    intolerable noise.

    Before I start redesigning the circuit to employ a less direct form of
    feedback (not using the instant motor current itself to create the PWM
    ramp) I'd like to ask if this problem is common, and how it is usually
    dealt with. I've looked at data sheets of several PWM controller ICs and
    found pretty much the very same circuit I'm using.

    What are the fine points I have missed?

    I know that a certain "singing" of PWM controlled inductances such as
    stepper motors is common and hard to avoid. I'd just like to limit the
    noise to a tolerable level.

    Thanks,
    robert
     
  2. Apparently the current takes longer to rise than the though 50us.
    When the voltage is low the risetime to the current is longer.

    Do you have a diode over the motor ?

    Rene
     
  3. Guest

    Exactly it's whinning because of the frequency modulation technique
    method which you've employed is dropping the frequency well into the
    audible range. In order to lessen the frequency modulation you could
    make your reference a function of the output.

    Alternatively is there any reason why you couldn't use a 100Khz clock
    and let it drop?
     
  4. On Thu, 18 Aug 2005 11:21:11 +0200,
    Initially of course, but once the current is up it shouldn't drop so
    much. The motor has 5mH, supply voltage is 27V (which means it takes 4
    PWM periods to take the motor from 0 to 1 A).
    I do, and as much as I hate to admit it, it's a 4001. Can this alone
    kill the freewheel current in 50 us? The diode is going to be replaced
    with an appropriate one as soon as I place my next order for parts.

    Thanks,
    robert
     
  5. Jim Thompson

    Jim Thompson Guest

    Use a clock, so that it runs constant frequency, then PWM.

    The whine is probably the pole pieces flexing. I did a controller
    design for Bosch/Mercedes and passing the noise spec was the hardest
    part... The director of engineering stuck his head in the air duct,
    listened, and decided yes or no ;-)

    ...Jim Thompson
     
  6. On 18 Aug 2005 02:37:51 -0700,
    I'm actually using a fixed frequency; it's my circuit that made the
    decision to lowering the frequency at will.
    Yeah, I guess that's what they mean by "voltage mode PWM".
    Yes -- that it makes no sense. The time constant, given by supply
    voltage and motor inductance, stays the same.

    robert
     
  7. Is your control loop hunting?
    Could the load of the motor be pulling the reference voltage down?


    OR (a bit of a long shot)
    Is something oscillating above 20 Kc/s and giving an audible beat note
    with the clock?
     
  8. john jardine

    john jardine Guest

    [...]
    Yes it is common and a side effect of this method of control. ICs suffer as
    well so it's not some trivial construction aspect. Annoying thing is that
    the stepper IC datasheets make scant mention that this will happen, showing
    instead textbook chopper current profiles which have bugger all relation to
    reality.
    Depending on supply voltage/motor/current, the current 'ramps' are quite
    happy to flip down (sometimes quite stably) into various integer, 'sub
    ramp' rates. An acceptable level of ramp skipping will give a 'rustling'
    motor noise, longer term skips can be occasionally musical. Bad cases result
    in harsh fluctuating wideband noise as the the ramps flip from one ramp rate
    to another on a random cycle basis. Plays havoc with the motor ripple
    currents.
    It's vital that the those current sensing ramps are as free as possible from
    hf spikes, noise and the cross coupled switching noise from the other coil.
    A real cure can be interestingly convoluted.
    regards
    john
     
  9. I had this problem with a current controlled stepper motor drive,
    based on an integrated driver. The motor inductance was large enough
    that the current wouldn't fall below the current trip hysteresis in
    the next clock cycle. So the control skipped cycles and made noise.
    My solution was to add some of the clock circuit triangle wave to the
    current trip threshold, so that the difference of that and the current
    shunt voltage crossed each other (well more than the comparator
    hysteresis) twice each cycle, even though the current was essentially
    constant throughout the cycle. Instant silence.
     
  10. john jardine

    john jardine Guest

    Belay that.
    There was no mention of a stepper. I must be fixated with the damned things.
    Sod it.
     
  11. Joerg

    Joerg Guest

    Hello Robert,

    As Jim already said a fixed frequency is better. What I usually do if it
    has to be cheap (or in your case the parts should already be flying
    around in your garage): Take a Schmitt trigger such as the 74HC14 or
    CD40106, build an oscillator and then quench that with a FET or
    transistor that is controlled from the feedback point. IOW if the motor
    current wants to exceed your limit it throttles the oscillator's "on"
    period but not its frequency too much. Actually the frequency changes a
    bit but not a lot.

    If you want absolutly constant frequency you can build that oscillator
    and follow with a one-shot that can also be made from a Schmitt gate.
    Then quench that one-shot instead. There are six in a chip so you have
    plenty.

    Regards, Joerg
     
  12. Joerg

    Joerg Guest

    Hello Jim,
    They can be picky. This wasn't Daimler Benz but a big guy of a major
    automotive mfg was looking at a similar issue. Comment on one version:
    "Es macht ein unedles Geraeusch". You can't really translate that but
    roughly it would be "It makes a noise that is not noble".

    Regards, Joerg
     
  13. Joerg

    Joerg Guest

    Hello John,
    Have you ever done the opposite, like using the processor to accompany
    with the sound of a Harley sans muffler? That made quite an impression
    when doing the first demo.

    Regards, Joerg
     
  14. Jim Thompson

    Jim Thompson Guest

    "It makes a noise that is not noble".

    Like a fart ?:)

    ...Jim Thompson
     
  15. Joerg

    Joerg Guest

    Hello Jim,
    ROFL!

    It was more one of those cheapy ratchet sounds like on a child's toy.
    Not quite in line with the price category of the vehicles.

    Regards, Joerg
     
  16. Tim Wescott

    Tim Wescott Guest

    I hope that when faced with a spec that says "whatever he likes" you
    gave an estimate that said "however long I take".
     
  17. On Fri, 19 Aug 2005 00:24:22 GMT,
    Hmm, that sound exactly like what I'm doing already: I'm using a
    constant-frequency oscillator to periodically set a flip flop to "on"
    (the oscillator clocks a D-FF with D tied to High), and the comparator
    resets the FF when the sense current exceeds the limit.

    So I'm in fact running a constant frequency. The motor takes several
    cycles to get up from zero to 1A of course (f=20kHz L=5mH V=27V), but I
    had expected it to basically stay there via the freewheel diode and just
    take an additional sip of current each cycle. All I can see on my old
    analog scope is multiple-clock rising ramps all over the screen; I can't
    see if the PWM drive takes longer breaks between ramps.

    It all looks as if the motor loses too much current between pulses which
    may be due to my KNOWINGLY poor choice of freewheel diode. I'd like to
    put in a BYV28-100 but my supplier is out of them. Any better ideas for
    a few-amps fast recovery diode?

    Thanks,

    robert
     
  18. On Thu, 18 Aug 2005 08:00:46 -0700,
    That's what I'm alreay doing. See my more verbous reply to Joerg
    elsethread.
    Whatever it is, it has to go into the inaudible range. These are small
    motors, it should be doable.
    My sister's boyfriend's job used to be the acoustics of auto gears. He's
    moved on to motor acoustics now. They're really picky about what a car
    sound like on the inside.

    robert
     
  19. Jim Thompson

    Jim Thompson Guest

    Of course. Also made about a dozen trips to Germany in Business
    Class, once in First Class ;-)

    ...Jim Thompson
     
  20. Robert Latest wrote:
    (snip)
    At 27 volts, I would use a Schottky. They have a lot of capacitance,
    so no larger than necessary would be good.
     
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