Re: [CAD_CAM_EDM_DRO] RE: Stepper questions

On Wednesday 03 May 2006 01:05 pm, Jon Elson wrote:

> The two halves of a stepper phase coil (on 6- and 8-wire motors) DO
> interact. They are usually wound on the same coil bobbin, and act as two
> windings of a transformer. If you have twice the turns, then the inductance
> is squared. Because you only need half the current in full winding mode, it
> only takes twice the voltage to turn the current around in the same time.
> But, at the same voltage, it takes 4 times longer to turn the current
> around. That's why the half-winding scheme is often used on a 6-wire motor
> with a bipolar drive.

I really wasn't clear on the reasoning behind that until you posted
this... :-)

<...>

> It depends entirely on how the drive manages the collapse of the field in
> the winding that is being turned off. Providing an effective scheme to
> dump the inductive energy quickly can greatly improve the running
> performance. But, that adds a lot of parts. The most common technique is to
> place diodes to return the current to the power supply, but this basically
> leaves the coil short-circuited. Putting some resistance in series with
> those diodes will increase the voltage on the transistors, but will make the
> field collapse faster.

Not disagreeing with what you're saying here, but that sure is
counterintuitive, that adding resistance in that context would make the
field collapse faster...

> But, then the resistors will get hot. Especially in a chopper-type drive,
> they will be carrying the circulating current nearly all the time while the
> motor is holding position, and get REAL hot. So, you need more transistors
> to shunt the resistors except when moving above some speed. Eventually, it
> becomes obvious why a bipolar drive is the easiest way to get a simple, high
> performance drive.

Is the above stuff referring to a unipolar setup then?

> And, as for microstepping, if it doesn't do microstepping, it isn't worth
> having! (My opinion, of course! But, if you've worked with a Gecko G201
> drive, not much else even comes close! And, a full-step, or even half-step
> unipolar, L/R drive is so FAR from a Gecko that it is just a really bad
> joke.)

Maybe I'll get there, eventually. But for now I've gotta work with what I've
got, which is a pile of TTL (and some CMOS, though not nearly as much as
I'd like), some bipolar parts, a few darlingtons, and even a small number
of FETs...

So I'll get something going that'll at least let me move things, to start
with. Which is okay, as I'm in no particular hurry here, having a pretty
full plate as it is.

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ablest -- form of life in this section of space, a critter that can
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