Re: DC motor as replacement for Stepper motor

Hi,

Let's try to clear some of the fog surrounding this subject. What I
would suggest is "stepper or servo" is in fact two sides of the same
coin.

To show that, the "black box" concept is a useful thought construct
here.

Let's assume that you have two black boxes. Unknown to you is one box
contains a step motor while the other contains a servomotor. Each box
also contains the drive electronics; a full step drive for the
stepper and a 200 count encoder and servodrive for the other. Both
boxes also contain a PC (they are big boxes).

All you can see is each has a motor shaft protruding at one end and
motion command wiring protruding from the other end. The input format
(say G-Code), allows you to move the output shafts as you choose.

The question is: can you tell one box apart from the other?

The rules are you cannot look inside, both output shafts have the
same torque and power. You can only observe what the output shafts do
in response to various input commands. You can apply a load to see
what happens.

Logic says if you cannot tell them apart, then they must be the same.


Now you have to ask "what exactly is a servo?" Both shafts move in
1.8 degree increments of motion, both resist being moved from those
positions with equal reluctance.

A servo is any system which detects the difference between the
commanded output and where the outut actually is (error), amplifies
and inverts this difference, then adds the result to the command
input (negative feedback).

A step motor does this intrinsically while a DC servomotor needs
external devices (encoders) and control electronics (PID loop).

A step motor's rotor rests a magnetic null location (commanded
position) when its phase coils are energized. Any external torque
load moves the rotor (actual position). The difference between the
two is the error. This error generates a proportional restoring
torque (negative feedback), until it exactly balances the external
torque. This is equivalent to a PD servo loop.

A servomotor equipped with a 200 count digital encoder does the same
thing. Like the stepper, it has 200 encoder-set null locations where
it can rest. Just like the step motor, it will resist being moved
from a location by generating a restoring torque.

The only important difference is since a servomotor must have an
encoder anyway, it can also be used to indicate a "stall". This is
a "freebee" because the loss of servo-lock would result in motor
runaway if it were not shut down.

As long as more is not asked of either motor than it can deliver,
both will perform with equal reliability.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "Smoke" <smoke@t...> wrote:
> I think what I don't really understand here is HOW the motor is
positioned
> relative to the motor.
> Smoke,
> >
> > This might help. The motor is driven to the "beat" of the
> > encoders' "drum". In other words, the encoder is setting the
target,
> > to which the motor position is driven...
> > Ballendo