Re: [CAD_CAM_EDM_DRO] Digest Number 1042

On 11 Jan, CAD_CAM_EDM_DRO@egroups.com wrote:

> One thing that I hear from time to time is that the holding torque of
> some of these in-between steps is quite a bit less than in other
> steps, resulting in the increased likelyhood of missed steps. From
> what you've just explained, it seems to me that there isn't that much
> of a decrease... Am I correct in assuming that?

Well, there's in between and there's in between.

Some of the early half-step controllers ( and perhaps some of the
current controllers as well ) did not increase the current in the "one
winding on" state, and under these circumstances, there is about 1/2
the torque available every second step.

Until the advent of "chopping controllers", it was a bit
more difficult to control the current in a stepper. SOME means of
current control was required, if only to prevent the motor from
melting down. Most of the manufacturers used a simple resistor for
this task, thereby supplying the same current under all conditions.

Quarter step and finer controllers by their very nature require
adjustable current control, and once some sort of current control
electronics are in place, it is not too hard at all to pick the CORRECT
current values.

It is fundamental in the math of all but the four half step positions
that the winding currents required to get to a given position are also
the currents required for equal torque - or so the theory implies.

Given controllers supplying appropriate currents, I would not expect any
substantial change in torque from one step to the next.

There is a "however" to all of this, though.

Not every chopping controller ( which is to say, just about every modern
controller ) will work optimally with every motor. There is an
interaction between motor inductance and chopping frequency. It is
possible to use a motor of too high or too low an inductance for the
chopping frequency. In these cases, the current the controller "thinks"
it is supplying may not be what is actually being supplied. Under these
conditions, steps could be missed.

Unipolar steppers run in bipolar mode are somewhat prone to this
phenomena, as they typically have 4 times the inductance of an
equivalently sized bipolar motor. This is somewhat solveable by lowering
the chopping frequency. The lower frequency can result in the motor
overheating, so a decrease in the peak current supplied is adviseable.

Alan


--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta