Re: [CAD_CAM_EDM_DRO] Big Servo Drives

roboticscnc wrote:

> Hi Group,
> Forgive my ignorance as I'm not an electrical engineer, but after
> reading the comments on this subject I'm wondering if it would be
> helpful to have an external power board with overkill mosfets and
> anything else necessary to remove back emf, etc. This way you could
> use any servo driver to do the positioning function with the output
> only switching larger mosfets - keeping high voltage / current noise
> away from the encoder signals. The external power board could then be
> designed for the correct power without dictating which servo driver
> you have to buy.

With proper design, you don't have much problem with this sort
of thing. On my original analog servo amp, I did some testing
of smoothness at low speeds. The movement broke into stick-slip
movement at .01 IPM. I calculated the tachometer voltage at
that speed, it was 7 micro-Volts! Obviously, I could not
measure such a low voltage. But, if I shorted out the
tachometer with a screwdriver, the servo amp would fault
immediately, proving that it was receiving that tiny voltage
and that it was enough to keep the servo loop closed. These
signals ran less than 1 inch from the power stage at certain
points, which were switching up to 5 A at 70 V, and switching
fast, under 100 ns. So, that is 70 v / 10^-7 s = 7 x 10^9 V/s,
a pretty high dv/dt. I did spent a lot of time laying out that
board, even bringing sensitive signals in with the PCB
equivalent of a twisted pair cable.

In theory, it would be possible to build a power booster for a
generic PWM servo amp, but you'd need to tap into their current
measuring circuits to allow the current limit and fault sensing
to work at the higher current level.

But, I think practically, if you are going to have to build the
power stages, driver stages, current limit and fault logic, you
might as well add the additional 10% and make it a complete,
stand-alone servo drive.

Jon