Re: [CAD_CAM_EDM_DRO] Re: SUGGESTION ABOUT NEW 3 AXIS DRIVER

jeffalanp wrote:

>Hi Jon,
>
>You wrote:
>
>
>>They are Unipolar, which is another severe strike against them, as
>>most unipolar drives perform poorly when compared to bipolar.
>>
>>
>
>When you say *most unipolar drives* are you perhaps refering mainly
>to the old L/R type drives that use the big old fat ballast
>resistors? A unipolar drive (with no reference to Brewington drives
>capabilities) CAN be made that is PWM driven, thus allowing
>microstepping, which can help reduce a major concern with missed
>steps due to mechanical resosnance. Also when it comes time to
>reversing current flow, the other phase coil is used, so charge up
>time would be about half when compared to bipolar.
>

This is a misconception. 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. A bipolar drive applies the
reversed
power supply voltage directly to each phase winding. A unipolar drive has
several ways to deal with the voltage and current in the "turned-off" half
of the winding. The most common scheme is to have a freewheel diode
that sends the current back into the power supply. This effectively shorts
out the winding, and a shorted winding changes current only slowly.
To get faster charging of the winding, resistors, or Zener diodes can be
put in series, so a large voltage is developed across the winding, thereby
draining the magnetic field quickly. This, however, burns substantial
energy as heat every time the driver makes a step.

Jon

> The only major
>concern I can see of a modern PWM type unipolar microstepping drive
>is that it can not close a loop to allow current recirculation during
>off (decay) times (so maybe it still uses a little more current than
>bipolar).
>
>

Right, that's what I'm talking about above. But, it isn't just more
draw from
the power supply, it is HEAT produced in the driver, that has to be removed.

>What makes unipolar drives so horendous? Is it just their relation
>to the old L/R days, or am I still missing something.
>
>

The performance of the old L/R drivers was horrendous, even with large
power supply voltages. Maybe I've only seen newer ones with the least
sophisticated circuits for draining the current in the off windings, and
that's
why they were only a little better than an L/R drive.

Jon