RE: [CAD_CAM_EDM_DRO] Question about Servo Drives

Hi Jon,

> 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.

To be truthful I always thought this was the only way it was done. With
extra tricks like enabling the opposite high side drive FET because it
switched on faster than the intrinsic body diode. But the PWM
controller had to be fairly sexy to do that. I think the current crop
of dsPICs are clever enough for that but I don't know if that's
compatible with the IR2184 drivers.

In either case, I searched through my box of surplus motors and found a
servo that looks to be big enough to run the lead screw on my South
Bend. It appears to be a Dunkermotoren GR 63x25 which means it's a 24V
motor, good to 3300 RPM, 14Ncm continuous with 2.7A and 108 Nm starting
at 18A. Demagnetising current is 24A so as long as I set up the
UHU-Servo correctly I can avoid toasting it. And it has a quadrature
encoder on the back so things are looking good.

There's a toothed pulley press on the shaft but I don't recognize the
pitch. A bit of heat and a puller should take care of that. The shaft
is 8mm do I can get a standard set of SD pulleys for 3:1.

This should allow me to play. I'll worry about a hobby project to
replace the ATMEL on the UHU with a dsPIC later when the ELS is done.
(Real soon now!!!!).

John Dammeyer

>
> Jon