Re: Steppers, bi- vs uni ?? who cares ?

> > Unipolar vs Bipolar.
> Thanks for your answer. I wonder Do I (or anyone else)
> really need to care or know.
>

<HUGE SNIP>

>
> What is the advnatage of Unipolor over Bipolar, given
> everyhthing else is the same? Torque, size, Amps,.....?????
>
> [Supposedly, bipolar stepper systems give you more speed, at a certain
> expense in torque, since you only use half the motor's windings (I

hope I

> got that right). It takes a somewhat more complex system, but it's
> generally supposed to be worth it.]
>
> Andrew Werby
> www.computersculpture.com
>

Look at your machine, and calculate the forces needed to do the work.

then look at motors and methods of actuating an axis.

if you determine you need "X" pounds of force, then you can figure a
small motor with a 3:1 belt drive on a 20 TPI leadscrew will increase
delivered power and you can use a smaller motor.

For screws or direct drive, adjust your gains based on using ball
screw with it's lower drag than ACME or threaded rod.

For gear rack or open ended bet as a rack or any of those styles, your
distant per revolution is more than your motor (threads by design are
much less) so you speeds are way up, but the forces needed from the
motor go way up. Where a screw multiples the power, the rack divides it.

AFTER (not everybody gets that 'after' part) you can look at motors
and drivers. in some cases, there are no choices, if you need an 800
oz-in stepper you need to use a Gecko. if you need 50 oz-in, then you
can use pretty much anything. there is a range where each design works.

there is also a range were steppers and servos would cross and either
could be used. and then there is a place where servos shine.

I truly believe that those who failed with steppers on their mills
missed something and the system was out of tune in some manner. There
are some who have steppers on bridgeports and get similar results to
others with servos. But, servos on bridgeports work 99% of the time
and steppers have a poor reputation...

Alas, Mariss sees that with a closed loop stepper, the single largest
problem with steppers, (lost steps) goes away.

Steppers deliver most of their power at low speeds, where a mill needs it.

A plasma or router that is flying around at hundreds of inches a
minute are prime for servos.

There are clear cut choices, but there is a vast middle ground where
the choice is not as clear.

As a grasshopper who cares little for the engineering reasons why a
motor works or what goes on in the motor, then you can ignore all that
and pick from what you want that will work.

a very rough market breakdown (forgive me if I left out most of the
players)

The Lini-Stepper requires you to buy all the electronical gizmo's and
program the microcontroller.

Hobby CNC offers a kit where you get everything but need to solder all
the gizmo's.

Xylotex, Gecko, Industrial Hobbies, all offer completed major
components for your assembly.

Some offer complete driver kits consisting of the major components
that you just install in a cabinet.

and Ballendo offers something that is almost plug-n-play.

-ACT-2-

OK, so you want to outfit your Sherline with motors and ask 'what
drivers' someone says Xylotex, but you say what about HobbyCNC?
others say industrial hobby.

then someone says the SLA7062 driver chip that HobbyCNC uses makes the
motors really noisy.

others say the A3977 chip in the Xylotex will smoke when powered with
too much voltage.

so.... somewhere someone says,.... " that driver has a problem with
micro stepping at times because of it's mid-band resonance alogrithm
or lack thereof.... and padawan, the circle is complete.

First you ask how to avoid the technical discussion, then you learn
the ways of the forces and you want to know the technical bits.


Dave