Bob Ammerman wrote:
>
> Roman,
>
> Very impressive idea.
>
> Do I understand correctly:
>
> -- The PWM PIC has no responsibility other than genearating the PWM signals.
>
> How does the PWM PIC know what value to output at any given time?


The system was needed for a low cost 8th step
microstepping stepper motor driver.

I needed to control the current in two phases,
ie two h-bridges, so that is 9 possible current
"sets" of combinations, (9x2 matrix) which can
give all 8 microsteps. I wanted to avoid a hardware
solution as that would eliminate tuning for
torque and degree compensation with large motors.

The PIC chip (16F84) has 3 pins for the 3 industrial
standard step/direction/enable inputs. It then
drives the 4 halves of the 2 h-bridges directly via
4 output pins, each through a simple RC integrator.
Using hardcoded PWM loops of differing frequencies
allowed fine tuning of the PWM voltages. I also used
32 actual PWM settings, to give all 4 quadrants
so there is no phase direction switching needed.
The PIC runs very simple firmware to step forward
or back, and also can select full/half/4/8 steps
via dipswitches. Options include auto current
reduce mode and boost and software anti-resonance
(not really needed with 4 or 8 ustep though!)

Using 4 PIC pins and 4 RC networks means that I
don't need much external hardware, otherwise it needs
some pins for PWM and others for phase direction
switching.

The power stage uses hardware current feedback,
so the PIC only provides a reference voltage, the
hardware then adjusts current to match. This is
lightning quick and gives fabulous motor performance
compared to chopper systems that rely on L of the
motor in the feedback loop.

I have had arguments re the chopper/linear issue
before, but the motor performance of a microstepping
linear driver and a big motor is something else! :o)

I always use hardware current feedback as it provides
a lot of safety. In the event that the two h-bridge
halves might both be turned on, the whole bridge will
still limit to the correct current. Also will limit
to the correct current if one of the 4 switches in the
h-bridge goes short circuit.
-Roman

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