Re: Basic Servo Motor Question...

Dan,

A quadrature encoder gives direction as well as speed information.
You can recognize them because they need a minimum of 4 connections;
2 for power and the channel A and B outputs. Often there is a fifth
connection which is called an index output.

1) The best situation is if the stall torque is listed.
2) Next is if the stall current or armature resistance is listed.
3) Last, no info except fo no-load speed.

For (2) multiply stall current by rated voltage and divide the result
by 4. That will be the motor peak power output in Watts. Re-arrange
Eq. 3 so it reads S=5407*W/N. Make W your above-calculated Watts and
N your no-load speed in RPM. S will be your stall torque in oz-in.

For (3) you need to accurately measure the armature resistance.
Divide the rated voltage by your measured resistance to calculate the
stall current. Use it in (2).

Your applied torque would have to be worked out knowing what will be
the maximum force you expect you will need at what maximum speed.
From this calculate the mechanical watts required to do this work.
This will be what your motor has to deliver as continuous power.

You can use a tachometer to measure a motor's no-load speed. Some RPM
ratings are at rated load. It will be less than no-load speed. Unless
it explicitly says "no-load RPM", you may wish to check.

For the last question, I have no idea or experience.

Mariss

--- In CAD_CAM_EDM_DRO@egroups.com, djr@n... wrote:
> Mariss,
>
> Thanks for the reply. What is a "quadrature" type encoder? How can
I
> tell if the surplus motor has it? The line count (if you mean
number
> of slots in the wheel) is about 500 from memory... it has very fine
> slots on about a 3" wheel (from memory). I will take a picture or
> two
> of the motor/encoder and post them. I looked at post 9342 and have
a
> few questions... You give the following formula:
>
> W=(1 – T/S)N*T/1351.8
>
> What is the best way to measure torque applied (T) and stall torque
> (S)? Is there a device to help me do this? I imagine no-load RPM
is
> the RPM rating on the motor.
>
> How many watts should I shoot for when considering the sherline
mill?
>
> Thanks
> - Dan
>
>
> --- In CAD_CAM_EDM_DRO@egroups.com, "Mariss Freimanis"
> <geckohall@h...> wrote:
> > Dan,
> >
> > If you are planning to use a STEP and DIRECTION input DC
servodrive
> > then a tachometer is not necessary. It will derive all PID
> > information from the encoder.
> >
> > It sounds like you have a good motor there. Make sure the encoder
> is
> > a "quadrature" type. What is the line count?
> >
> > Regards to what you can get out of the motor; please refer to
> message
> > #9342. It goes into a little detail on that subject.
> >
> > Mariss
> >
> >
> > --- In CAD_CAM_EDM_DRO@egroups.com, djr@n... wrote:
> > > I want to jump in head first in a CNC servo sherline mill
> > conversion,
> > > but I am really ignorant when it comes to DC servo motors and
> > > drivers.
> > >
> > > I see that some servos have slotted encoder wheels with optical
> > > detectors and/or tachs and/or brakes. I can see what the
slotted
> > > wheels and brakes do... the tell position and stop the thing,
but
> > > what
> > > is the need for tachs? Can't you tell position AND speed with
> the
> > > optical detectors? What motor stats are important for me if I
am
> > > going to use a servo motor with the sherline? Does stall
current
> x
> > > volts (watts) tell me something? What RPM range is good?
> > >
> > > I was browsing a local surplus store this past weekend ran
across
> a
> > > servo motor with one of those slotted wheels and an optical
> > detector
> > > circuit board attached still in the anti-static wrap. It was
> made
> > by
> > > Japan Servo and it's rated at 24v 3000 RPM. It is about 3" in
> > > diameter and 5" long. I don't have any other data on it. How
> can
> > I
> > > determine if this motor is a good candidate for the sherline?
At
> > 12
> > > volts it really seems to have a good bit or torque.
> > >
> > > Thanks in advance for any answers... Pointers to "servo CNC
> motors
> > > and drivers for dummies" websites would also be great!
> > >
> > > Thanks
> > > - Dan