Re: COMPLETE 3 axis CNC electronics package (plug-n-cut, ready to go)

Alan,

Nice post.

But it's only fair to mention that Bipolar DOES still have a couple
advantages over Unipolar; even when both are chopper style drives.

A bipolar drive will provide more torque to a given motor than
unipolar. This is why we see the "same" size 23 double stack 3amp, 1
ohm motor nowadays labeled with either 280 oz.in. (bipolar) or 200
oz.in. (unipolar).

It's also true that bipolar drivers are more efficient with respect
to the input power required. This is due to their ability to
recirculate "unused" power back into the supply.

Finally, the bipolar drive requires fewer wires, so too fewer
connections to pay for or go bad.

Now, against that is the de facto truth that for a given PRICE, you
can get a chopper unipolar drive with higher power supply
capability, and higher motor amperage capability.

So IMO it all pretty much averages out; both are good choices.

We have our own uni-chopper drive board (like the HobbyCNC version
only smaller and more suited for our drive boxes), AND we have a
bipolar chopper board (like the xylotex and THS drives) on the way.

In the meantime, we can support most any of these 3rd party boards--
or Gecko's-- NOW with our "OEM" driver box. (The white one on the
www.cncresource website.)

Ballendo

P.S. Still amazes me how quickly what once was acceptable becomes
insufficient; how "testosterone" seems to have invaded DIY-CNC...
I have machines with 150 oz.in motors going 240 IPM, carrying 3HP
routers. Sherline's with "only" 116 oz. in. have been making money
for me for over a decade. Yet many folks now seem to think that if
you don't have the biggest, fastest...

I just read about someone shearing parts off their machine cuz the
new 450 oz. in. motor was "untamed". It's a "monster" mentality <G>
They may want to shoot holes or "blow up" their weeks of work, but I
prefer to KISS.




--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Alan Marconett" <KM6VV@...>
wrote:

>
> Hi John,
>
> Bipolar or Unipolar doesn't matter! But before I get flamed, let

me explain

> that either mode can be CHOPPED. Chopped is what you want!

Chopped mode

> allows the current in the stepper windings to build up quickly,

and then be

> cycled on/off to maintain the current at the desired rate until

being shut

> off for the next step.
>
> Unipolar drives got a bad reputation from the days when they were

driven

> with L/R drivers (think BIG POWER resistors). These made for slow

drives,

> and wasted much current. Not so with choppers! Unipolar drivers

can

> actually be simpler, as only 4 Darlington power transistors or

FETs are

> needed; vice two H-bridges, each with 4 power devices (simplified
> description).
>
> And guess what? The motors are really just about the same, only

the NUMBER

> of wires brought out for the ends of the windings is different!

The old

> unipolar scheme often brought out only 5 wires. 4, 6 or 8 wires

are

> commonly seen now. Modern stepper motors do have a significant

improvement,

> they use modern magnets and laminated RECTANGULAR cores (although

some look

> more like hex).
>
> 200 step motors have 1.8 degrees per step. Some take it a little

further,

> 400 steps, .9 degrees per step. This is inherent to the stepper

it's self,

> the number of poles. MICROSTEPPING uses varying currents in the

windings

> (sin/cos) to effectively give MORE steps. 8 microsteps gives a

smoother

> step through rotation, and thus lessens the tendency for stepper

motors to

> stall due to resonances in the slower speeds. Altering the

current phase in

> relation to the mechanical position also minimizes the

resonances. For a

> better explanation, see the white papers on the Gecko web site.
>
> Alan KM6VV
>
> >
> > 3 amp unipolar chopper driver board,
> >
> > Okay I am a late bloomer, but Somehow I thought
> > Bipolar or microstepping was "better".
> > IN a few sentences why would somone want to build a
> > unipolar device when the majority or the items I see
> > for sale are bipolar etc.
> > I have a perfefctly fine bechtop- miling machine that
> > I have about given away beacuse it had unipolar motors
> > and I coukld not get any responses about the drivers
> > etc?
> > So what really give with this Bipolar, Unipolar,
> > Microstepping, .9 degrees 1.8 degrees.??????????
> > John
> >
>