Re: [CAD_CAM_EDM_DRO] 90v DC motor as servo?

Comparing case dimensions of similar PMDC motors would be close enough I
think. I have found pretty similar values between manufacturers. You don't
need to disassemble them
even though barium ferrite magnets are highly resistant to demagnetization.
Alnico magnets might be affected some
but I have never heard of any problems in dc motors other than
demagnetization with very high motor currents.

Since you mention hamfests you must have some electronics knowledge. What we
are trying to do here is get an appropriate mechanical impedance for the
motor. Like electrical impedance it has energy loss components (friction)
and reactive or energy storage components (mass and springyness) that
correspond to resistance, inductance,
and reciprocal capacitance in electrical systems. We use moment of inertia
because we are in polar coordinates.
This is all a linear approximation but is still fine for most
motions. An exception is extreme stick slip or saturating a
magnetic structure.

All motions can be described as a combination of frequencies
or cyclic motions.

It is most economical to have the load mechanical impedance
at or somewhat higher than the motor mechanical impedance.
In the Kollmorgen program this is entered as "inertia ratio"
as the motor is of primarily inertive mechanical impedance at low
frequencies.

As an example a typical 1/2 hp 1750 rpm PMDC motor direct driving a 5
turn/inch screw impedance matches about a 1000 lb load. It's polar moment is
about .2 oz-in/sec^2. The torque must be enough to overcome friction though.
Would it work for a 100 lb load? Yes, but most of the current would be used
to overcome the motors own inertia rather than the load when
accelerating.Might be fine if you don't need high acceleration or are making
something for a one time deal with surplus components. A lathe carriage will
not require high accelerations anyway. The load is mostly frictional with
sliding ways. Look at the small motors Hardinge uses for that purpose.

I don't think you even need a closed loop servo for a lathe carriage unless
you are threading.... in that case you need to know spindle position and
servo lock to that.

You can see the trade-offs of servo amp size, motor size,
and gearing for a given load by playing around with the program.



Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html

----- Original Message -----
From: "Chris Stratton" <stratton@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Monday, May 21, 2001 11:13 AM
Subject: Re: [CAD_CAM_EDM_DRO] 90v DC motor as servo?


> > Yes gear reduction changes the effective inertia forces.
> > System performance can be easily calculated with free
> > motion control software like that at
> >
> > http:///www.kolmorgen.com
> >
> > You need to know the motor inertia, torques, power, etc.
> > If you cannot find a spec a fair inertia approximation can be arrived at
by
> > measuring rotor dimensions and assuming it is a solid cylinder of steel.
> > Some of it is air and some is copper but
> > it is very roughly a wash. The software will calculate the inertia.
>
> Rotor dimensions might be hard to measure - do PM DC motors have the
> same issue with loosing magnetization when dissasembled that steppers
> do?
>
> Baldor has inertia figures for their motors on the web. Of course the
> one I picked up at a hamfest this weekend was not a baldor, but it was
> cheap and should be usefull for something around the shop if it proves
> unsatisfactory for this project.
>
> Perhaps I'll compare the case dimensions of mine to the similarly
> rated baldor and assume the rotor dimensions are similar if the cases
> are.
>
> Chris
>
> --
> Christopher C. Stratton, stratton@...
> Instrument Maker, Horn Player & Engineer
> 22 Adrian Street, Somerville, MA 02143
> http://www.mdc.net/~stratton
> NEW PHONE NUMBER: (617) 628-1062 home, 253-2606 MIT
>
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