Re: SUGGESTION ABOUT NEW 3 AXIS DRIVER

Hi Jon,

> The two halves of each phase windings are wound on the same iron
> core, so they have strong magnetic coupling. The supply
> voltage still has to ramp down the previous magnetic flux before it
> can begin to apply flux in the reverse direction.
> ....
> Jon

Ahh, right. Fortunately, when using a microstepping unipolar drive,
the current will have been ramped down to zero (or close to it)
following a sine wave pattern before the other coil has to be turned
on. It doesn't have to slam one coil off, then the other on (like
the old days), the transition can happen reletively smoothly.
Again, a major problem is at high speeds, when the "ramping" has to
occur fast. The unipolar drive only has the body diode to disipate
power through (and that can cause alot of heat), while the bipolar
drive can use synchronous rectification, and turn on the low
resistance FET to let current flow back to the supply.

OK, so the unipolar still has to disipate more power, but when
microstepping, I think (but presently do not have emperical evidence
for) the results will be alot closer to a bipolar drive than the old
unipolar L/R drives, especially since higher voltages can be used
without suffering the consequences of using the fat resistors.

Of course the description above only compares bipolar and unipolar
drives that maintain a sine wave current flow rather than switching
to a full step type of pattern at high speeds. Inexpensive chipsets
(whether bipolar or unipolar) do not have the luxury of easily
modifying the wave form like the good (and more expensive) bipolar
drives.

Jeff