Bipolar Stepper driver with just 2 DACs and 2+1 mosfets - possible?

I have a question regarding Stepper motor drivers:

Would it be possible to drive a Stepper motor using 2 DACs, with one side of the Windings of several motors connected to a high power mosfet driven by a reference voltage?
The remaining ports should be driven by a mosfet directly driven by the dacs.

The reference voltage should be half of the drain voltage so I get a bipolar voltage on the windings of the stepper.
I haven't found any circuit diagram implementing this, so I wonder if this is even possible...

If I understand you right, the answer is yes, but why bother? You're halving the applied voltage to the windings that way and thereby limiting the possible torque. The microstepping controller chips (such as the AllegroMicro's 3967) already do an H-bridge with regulated directional current through the windings.. which is sort of the same thing without the voltage limit.

patbob Wrote:
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> If I understand you right, the answer is yes, but
> why bother? You're halving the applied voltage to
> the windings that way and thereby limiting the
> possible torque. The microstepping controller
> chips (such as the AllegroMicro's 3967) already do
> an H-bridge with regulated directional current
> through the windings.. which is sort of the same
> thing without the voltage limit.


There are 2 reasons:
At first the dedicated stepper motor drivers are more expensive & are special ics which are not as commonly available.

The second reason is to try to do microstepping in software for better repeatability and accuracy. The way these stepper motor ics do microstepping induces some error in-between steps meaning that for example 3/16 steps aren't 0.1875 but rather something like 0.175 - 0.2 steps depending on the direction and how far the next full step is.

A lot of higher-end microcontrollers already include several high resolution d/a converters and would offer the possibility to compensate for these errors. (This could be relatively easy calibrated on the machine using calipers to measure the distance from a reference point)

On the voltage: Since this would have to be designed from the ground up anyway this could be taken into consideration from the start meaning that we would simply use a higher power supply voltage in the first place.
The idea is to think of the steppers more like 2 (or 4) phase synchronous motors rather than the traditional way. From what i've read 3 phase synchronous motor controllers use mosfets driven by a combination of frequency and amplitude modulation to archive continuous traction control.

If you think about it stepper motors are the same thing as 3 phase permanent magnet motors only that high power electrical engineering is considered something different. But it is not, there are steppers which have more beef than some small scale ac motors...