Re: [CAD_CAM_EDM_DRO] Question about Servo Drives

John Dammeyer wrote:

> Hi Jon,
>
>
>>Also, this is yet another synchronous-antiphase drive, which is
>>hard on the motor, especially if it is low inductance. Adding
>>an L-C output filter would help reduce current ripple, but it
>>makes the above problem worse, of course.
>>
>
>
> What is a "synchronous-antiphase drive" as opposed to some other type of
> drive? How is it hard on the motor?

The Gecko 320 series also uses this scheme. It is where the two
half-bridges are working in mirror fashion, when one is high,
the other is always low. This forces a triangle wave current
through the motor at the PWM frequency, and also produces a lot
of EMI which can affect step/direction and/or encoder signals.
If the motor's inductance is high, this triangle wave current is
not great, but with low-inductance motors the current can become
large, heating both the motor and the drive.

Also, this triangle wave current makes it hard to filter out, as
it takes large filter components to handle the heating.

An alternative is "sign-magnitude". By default, the motor is
shorted, and current recirculates through the motor's
inductance. When drive power needs to be applied, one of the
half bridges switches to "high" for the required pulse width,
then back to ground to recirculate the current. Since the
current doesn't reverse every PWM cycle, the magnitude of the
current is smaller, and a filter can be added to remove the EMI
and smooth the motor current even more. This is the scheme I
use in my servo amps. It took some development to get the kinks
out of it, but they work very well.

Jon