Re: [CAD_CAM_EDM_DRO] what the world needs now ... backlash aware stepper motor driver

Jon Elson wrote:

>
> Doug Fortune wrote:
>
> > While pondering how to cnc convert my bench
> > mill ....
> >
> > I thought it might be an idea for someone to
> > build backlash aware stepper motor drivers.
> >
> > Such a beast would have dip switches to indicate
> > at setup time what the backlash was (in steps), and
> > upon detecting any reversal of direction, quickly
> > counts up through that selected number of steps
> > to take up the backlash.
>
> The problem with this is that in most cases, the driver does
> not know what top speed and acceleration are acceptable
> to the stepper motor without losing steps. The CNC control
> knows this info, however. So, it means that top speed and
> acceleration have to be built into the stepper driver as well,
> with lots more dip switch positions.

I agree (its not perfect), but it has these advantages in its
favor:
- only a few steps will be called for
- upon axis reversal, by definition the velocity has gone
zero (so top speed doesnt matter).

The acceleration is a concern, but by definition of backlash,
only the motor/motor-coupling/and leadscrew have to be powered
(not the whole gantry and none of the cutting forces)... this
makes considerably less demands on the ability of the motor
to meet a rapid train of anti-backlash steps. By their
nature, steppers usually jump instanteously to some minimum
pulse rate, rather than accelerate at low rates from a
standstill. Yes, maybe another few DIP settings might be
necessary to convey this minimum rate info to the driver.

And of course, the gcode interpreter/controller can always
add a few more pulses of backlash compensation for fine tuning
(ie the driver hardware does 90% of the work).

In the case of any linear move, a slight pause can be added
to give the backlash aware stepper motor driver time to emit
some pulses if absolutely necessary.

In the case of a circular interpolation, the reversing axis
in question is moving very slowly anyway.

Doug Fortune
http://www.cncKITS.com