Re: Stepper motor or servos?

Raynor Johnston wrote:

> Do I gather from the threads of discussion that the servo option has
> some advantages and is becoming more affordable than it has been when
> compared with the stepper as an option for ballscrew drives? What are
> those advantages?
>
> The main advantage is the wide range of speeds possible, and the fact
> that torque doesn't fall off
> at higher speeds. Oh, gosh, yes, the BIGGEST reason of all is that
> servo motors force you to use
> either rotary or linear position encoders, therefore, you can't lose
> steps. I have a somewhat
> primitive setup, and I can go from .01 IPM to 105 IPM, with smooth
> motion all the way.
> Below .01 IPM, the stick-slip friction of the bed causes motion to be
> somewhat jerky.
> If I had a slightly higher voltage supply, I could get up to 120 IPM
> or so for rapid traverse.
>
> There is a cost increase with servo motors. The motors are actually
> cheaper than steppers,
> at many surplus outlets, because the steppers are in more demand.
> But, you have to supply
> the position encoder, and in many cases (like mine) a DC tachometer,
> too. And, the servo
> amp is definitely more complex than a stepper driver. But the results
> are WELL worth it,
> and I am VERY glad I went this way. One of the other things that you
> get with servos
> is that the position resolution can be quite high, without sacrificing
> speed. I have resolution
> of .00005" on my X and Y axes. If I tried to have steps this size,
> rapid feed would barely
> be visible to the eye, but with servos, the CNC control doesn't care.
> This .00005"
> resolution isn't hard to come by, either. With a 5 TPI screw, that
> gives .2" motion per
> turn. A 1000 line shaft encoder gives 4000 counts per revolution, when
> counting all
> transitions. 4000 counts/rev x 5 Turns/inch = 20,000 counts/inch, or
> .00005" / count.
>
> Jon