Re: [CAD_CAM_EDM_DRO] How do you compare steppers to servos

Steppers don't have "springiness" like this. They lock on a step pretty
tight.
A servo, if you put an arm on it, is probably less capable of holding an
exact position, as the encoder would sense motion and try to drive in
the opposite direction to correct it, but it would be confused as to how
MUCH to drive it.

In fact servos are NOT 100% step-by-step accurate on the counts, as the
control system must deal with issues of inertia and allows some leeway
for correction. IIRC, the servo system is inherently less accurate in
this regard, but not on a level that matters for a router, really.

In any case, this is not how drives are characterized, by
"springiness". There's primarily just torque and speed issues. Step
accuracy typically does not come up. Even if you were to direct-drive a
2 TPI-lead ballscrew with a stepper, 200 steps/in is 0.002" accuracy
which is pretty good- and that doesn't even consider microsteps yet!

The closed-loop behavior of servos is great for reliability; stepper
stalls are awful- but a properly tuned system will NOT stall (or lose
steps).

I've heard the low-end torque (and thus acceleration) of steppers tends
to be superior, but the servos spin faster. Of course, as long as it's
not direct-drive, gear ratios can change things, but the fact remains
that the servo's power will always be biased in favor of the high speed
range, whereas steppers are biased in favor of the lower speed. Either
one can cause design problems for the device, but that's just part of
the design process.

Danny

On 2/25/2011 8:39 PM, Les Newell wrote:
> As you say virtually all motors rely on magnetic fields. My point is
> that steppers are not rigid. They are springy due to these magnetic fields.
>
> Servos are naturally even more springy but the electronics compensate
> for this. Steppers being open loop have no compensation. Try it
> yourself. Attach a bar to the shaft of a stepper and apply a force to
> that bar. It will move. As long as you stay within +/- 2 steps it will
> return to the correct position as soon as you remove the force. If you
> exceed the motor's holding torque the motor will then jump position and
> you have lost steps.
>
> If you repeat the above experiment with a servo it will also give a
> small amount until the electronics compensate and fight back. If you
> exceed the system's maximum torque the drive will still keep fighting up
> until you fault the drive due to over current or excessive following error.
>
> Les
>
>
> My point is that contra
>
> On 26/02/11 01:52, Jeffrey T. Birt wrote:
>>> A stepper works by making the rotor follow a
>>> magnetic field generated by the coils.
>>
>> As does any electrical motor. I'm not I follow the point your trying to
>> make.
>>
>>
>
>
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