Re: Home brew servo board

["Darrell Gehlsen" <darrell-at-machinemaster.com>]

Jon,
The "Hunting" problem could be handled by running the output of the VCO
through a "Divide By" chip. Use the output of the VCO for feed back and the
output of the Divide By chip to run the stepper driver. Program the Divide
By for the necessary dead band.
Darrell


> Darrell Gehlsen wrote:
>
> > I just had a thought for an add on for running steppers from EMC.
> > If you use the servo board to feed +/- 10 volts to a Voltage Controled
> > Oscilator and drive the stepper driver with the output of the VCO that
would
> > give you a smooth step signal.
>
> This is just slightly short of brilliant!  The only reason it ISN'T
brilliant
> is that it has been done before.  But, it might just completely solve
> the frequency / pulse jitter problems with EMC!  The only tricky part
> of this is to make a circuit block that converts a BIPOLAR signal
> to frequency (step) and direction signals, and works very cleanly
> around zero.  A simple analog scheme might tend to hunt maddeningly
> near zero - in fact I would expect it to.  A better approach would
> be an all digital scheme.  You have a fast digital counter, and a
> preload value tells it how often to produce a step.  There would be
> a bit (or count value) that prevents it from stepping, and a direction
> bit.  This is pretty close to what freqmod is doing, BUT, a chip can
> do this a WHOLE lot faster than a CPU.  So, if you had a 10 MHz
> clock, you would have the ability to program step pulses in 100 nS
> units, about 100 times finer than with freqmod.  The chip could
> also have a position counter built into it, so it keeps track of relative
> motor position.
>
> I have been doing some designs for a 4-channel quadrature encoder
> counter and parallel-port interface one one chip, using the $18
> Xilinx Spartan XCS10 series chips.  I think we could make a
> stepper version with at least 2 channels of this type circuit on
> one of these chips.  The software would look essentially like
> the servo version of EMC.  The one gotcha is that an emergency
> stop, or any other fault condition would cause an abrupt stop,
> and the motor position would no longer be in sync with the
> position counter.
>
> > You could then either use an encoder for feed
> > back to EMC or for an open loop system just feed the VCO output back to
EMC.
> > Now all you have to do is "tune" the PID for your steppers. What I
haven't
> > figured out yet is if a VCO will accept the -10V for input or if you
would
> > have to convert the signal some way.
>
> I would recommend that anyone building such a system would
> put quadrature encoders with a cycle count at least 1/2 the
> number of steps/rev they will be using on their motors.  In
> quadrature, that will give 2x the steps/rev of the motor.  Just
> a little dead zone in the feedback (like 2 counts) would eliminate
> hunting.
>
> I'd be glad to work on this as soon as I get the analog output and
> quadrature modules working, if there is sufficient interest.
>
> Jon
>
>
>