Re: [CAD_CAM_EDM_DRO] Re: Is this servo usable for position control?

grantfair2001 wrote:

> --- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> > grantfair2001 wrote:
> >
> > > --- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> > > > grantfair2001 wrote:
> > > >
> > > > > I have 3 Japan Servo DC servomotors . . . If I count the tach
> > > pulses can I
> > > > > use this to establish positions reliably?
> > > >Are these brush motors or brushless DC. If 2-wire brushless DC,
> > > > then it will only turn
> > > > one direction, so that is also useless for positioning
>
> > > Is there an easy way to determine if it has brushes?
> >
> > Hmmm, not so easy. You might try using a DVM in ohmmeter mode
> > across the wires. if you get very low readings (10 Ohms or less)
> > with both polarities, it is almost certainly a brush motor. If you
> get
> > high reading one or both ways, then it is a brushless DC motor, and
> > cannot be reversed (without changing circuit wiring inside the
> motor).
>
> I got about 200 ohms with both ohmmeter polarities.
>
> I connected it to 15 volts and then reversed power supply polarity,
> and it worked both ways. Does this mean it is a brush motor?

Almost certainly, yes.

> The tach put out a nice sine wave, about 1 V p-p.
>
> Assuming it is usable forward and reverse, then I can tell which
> direction it is going depending on what polarity voltage I am telling
> it to use.

No, a common misconception. A position servo is a 4-quadrant
device, so not only does the servo amp supply power to the motor
when accelerating, but the amp DRAWS power from the servo when
decelerating. There is a point near zero speed where the voltage on the
armature is opposite to the direction of motion, because the resistance
of the armature is dropping more voltage than the back EMF from
its velocity. So, your idea sounds good, but in fact won't work.

There is a reason that servos have used some form of quadrature position
measurement since the 1960's.

Jon