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average current PWM control stepper drivers

Discussion in 'Electronic Design' started by Adam Seychell, Dec 1, 2003.

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  1. I am looking into stepper motor controller circuits and wondering why
    no one makes a true bipolar current drivers. The drivers available use
    peak current PWM control and sense current between the bridge and
    ground, thus giving a unipolar current sensing.

    What about the following bridge circuit;

    +V --*---------*
    | |
    FET FET
    | |
    | |
    *--MOTOR--*
    | |
    | |
    FET FET
    | |
    *---- *--R3--*--R6----
    | | | | |\ |
    R1 | R2 |_|- \ |
    | | | | \_|_
    GND--*---------* -|+ /
    | | | /
    | |
    -------R4--*--R5---GND


    The op-amp will sense average current of the motor winding in both
    positive and negative direction independent of the bridge state.
    Average PWM current could be achieved by taking the current sensing
    and current programmed signal into an error amplifier. The error
    amplifier output is then compared to a triangle wave to produce PWM
    control just as in switch mode power supplies. With bipolar current
    control the motor would give better control during breaking periods.
    Any thoughts ?

    Adam
     
  2. Rich Grise

    Rich Grise Guest

    Probably because for a stepper, they're not needed. You just
    drive it full-on for however long a step needs to be. All the
    acceleration and braking is done in software, by timing the
    drive pulses. At least that's the way I understand it. (and
    was a fundamental assumption in my stepper-driver design work.)

    Cheers!
    Rich
     
  3. John Larkin

    John Larkin Guest

    An interesting case is when there's motor current but both bottom (or
    top) fets are on; this is a common situation. The opamp reports zero
    current, and the wrong current in certain other duty-cycle situations.

    I've done microstepping with a current sensor floating on the actual
    nasty, noisy motor current leads, and it was ugly. But the stepping
    was beautiful. Maybe one of the new cheap Hall-effect current sensors
    would allow simple, true coil current sense.

    John
     
  4. Genome

    Genome Guest

    Yes indeedy, that's the way it is done. Do a google for the UC3637 motor
    control IC. I think the newer version is the UC3638. Have a hunt for the
    relevent app notes on TI's website.

    Also used for micro-stepping of stepper motors, there's an app note about
    that somewhere too.

    AND
     
  5. Russell Shaw

    Russell Shaw Guest

    Full-on pulsing is only good for low performance, low rate
    steppers where the leakage inductance can be ignored.
     
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