Re: [CAD_CAM_EDM_DRO] Re: Has anyone made their own servos from DC Motors?

roboticscnc wrote:

>
>Hi group.
>I picked up a couple of 24 volt motors for electric scooters. One is 6
>amps and the other is 16 amps.
>The main concern I have is that the resolution of the magnets is very
>large and this dictates the number of positions the motor will stop at
>per revolution.
>

We're not talking about step motors, here. A DC motor (servo or not)
doesn't
have discrete positions it stops at, as long as we stay away from those
3-pole
Mabuchi motors in kid's toys. However, if you turn a DC motor by hand and
feel a cogging-like motion to it, that is a bad sign. That is magnetic
cogging,
the number of armature poles under the field poles changes through a
small rotation
and the varying field strength makes the motor cog. This is bad, as it
causes
the motor to move in a "lumpy" fashion at low speeds. The servo loop needs
to compensate for this variation, but it can only correct it so much.

When a constant low voltage is applied, either from a D cell or
adjustable power
supply, you should get a constant rotation rate. Even if there is no
magnetic
cogging, the design of the armature windings may cause some speed variation.
This is called velocity ripple.

Finally, when a constant current is supplied (a little harder to do with
simple equipment)
the motor should provide constant torque as it is slowly rotated. If
not, it has
torque ripple.

A good servo motor for a precision machining application should have
minimal amounts
of the three cogging problems mentioned above. Some ordinary DC motors
are ALSO
very good in this respect. Some others, however, can be quite awful.
it will at least
take a high resolution encoder to allow the control device to nearly
compensate for
these shortcomings.

Jon