Hello,

when controlling a brush DC motor with PWM (bidirectionally, thus H
bridge), there are basically two different methods. Method 1 has one leg of
the bridge always on and PWMs the other. The motor gets pulses of supply
voltage and in the intervals it freewheels through one of the switched off
legs (B in this case):

+ ------+-------+
        |       |
       A=On    B=Off
        |       |
        +---M---+
        |       |
       C=Off   D=PWM
        |       |
- ------+-------+

Method 2 PWMs all legs, which means that 50% duty cycle means standing
still (-PWM means inverted PWM):

+ ------+-------+
        |       |
      A=PWM   B=-PWM
        |       |
        +---M---+
        |       |
      C=-PWM  D=PWM
        |       |
- ------+-------+

Does anybody know the differences between the two methods in terms of motor
behavior? I'm mostly interested in the mechanical aspects like torque and
speed. Are there any good sources of information?

Thanks,
Gerhard

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