Re: [CAD_CAM_EDM_DRO] DC motors to servo's (was Gecko burned)

sargossa_99 wrote:

> Info snipped from Les' post - my question at the end
>
>
>
>>Hi Keith,
>>
>>I will be using treadmill motors from surpluscenter.com, item number
>>10-1573. Carriage to the UK cost more than the motors but they are
>>still
>>lots cheaper than anything I can get in this country. Surplus center
>>were very helpful.
>>
>>The motors are designed to be foot mounted but they have a nice big
>>flange on them that can be drilled with the same bolt holes as the
>>current motors. They are smooth to turn by hand with only a little
>>'cogging' and will run down to about 1.5V.
>>
>>I intend to use 200CPR encoders(giving 800 steps/rev) and I will
>>probably gear them down 4:1, giving a resolution of 16000
>>steps/inch.
>>The Z axis will probably stay 1:1 because I don't fancy trying to
>>change
>>the belt :-)
>>
>>I will go with Rutex drives, not because I have anything against the
>>Gecko drives but because I have future plans that will make use of
>>the
>>SPI facility.
>>
>>I will let you know how I get on...
>>
>>Les
>>
>>
>
>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?
>
>

It depends. First, the treadmill motors were not designed to be
bidirectional, and there may be some brush shift to optimize for
one direction. Second, the motors may not have been optimized for very
smooth movement at low speeds, without cogging, torque ripple or
velocity ripple. But, the most important thing required for accuracy
and repeatability is the encoder. The motor is just a device for getting
the encoder to the desired position. So, in one sense, the motor is
almost a non-issue. As long as the encoder has sufficient resolution,
and the control loop has enough gain, then the motion will probably
be just fine. However, a motor that has a lot of ripple will impart some
of that ripple onto the movements, because servo gain is always finite.

The only other thing that might affect it is the motor inductance. Servo
motors usually are designed to minimize rotational inertia and
winding inductance, while that is not minimized in motors for other
applications. But, I suspect that the treadmill motors will probably
work fine.

Jon