Re: [CAD_CAM_EDM_DRO] DC motors to servo's

Hi Rod,

A proper servo motor has very little torque ripple and 'cogging'. Try
turning an average DC motor by hand and it feels lumpy as the poles pass
the magnets. The controller has to compensate for these lumps in the
torque. As no control loop is perfect you will get a slight variation in
the speed as it does so. Increasing the resolution of the encoder can
help but there are limits.

One way to help reduce the problem is to gear the motors down more. If
you gear the motors 2:1 you will reduce the speed variation more than
half. Of course the motor will have to spin twice as fast so you
compromise the maximum feed rate.

Ordinary DC motors also have much higher inertia so you cannot achieve
the sort of acceleration that a good servo can.

Can you get a cheap DC motor to perform like a $$$ servo motor? Of
course not. However for hobby/small workshop use I believe they are a
viable option. As long as you allow them to rev and don't expect
ridiculous feed rates I think the errors will be small enough to make no
difference.

Originally my mill had a resolution of .001"/step. Even with
microstepping drives you could only expect the motors to be within about
1/2 step of the desired position.

The servos will have a resolution of .0000625"/count (800 counts/rev
with 2:1 reduction) so I can have a following error of up to 8 counts
and still be as accurate as the steppers. I have yet to find out if this
is good enough. If not I can always change the encoders. I am only using
200PPR because that is what I happen to already have.

Les


sargossa_99 wrote:

>
>
> I'm just wondering how accurate a servo can be made using this
> method? Will you be able to get similar repeatability and accuracy
> with this type of servo?
>
> Thanks
>
> Rod
>