Re: Half step/ Full step switching mid traverse

[Jon Elson <jmelson-at-artsci.wustl.edu>]

Doug Fortune wrote:

> This may be a silly question.
>
> I have read or assumed that EMC on a 500 Mhz
> processor can do upwards of 100,000 steps/sec
> Is this correct?
>
> However, reading the docs on my newly arrived
> Superior Electric Slo-Syn 230-T  bipolar driver
> module reveals it can do a max of either 10,000
> full steps/s or 10,000 half steps/s.
>
> Presumably if 100,000/s is possible, and 10K is
> my limit (assuming I am in halfstep mode for
> higher resolution), then I could possibly increase
> the speed (up to double that) by switching to
> full step mode mid-movement.  Of course EMC
> will have to control this, because as soon as it
> commands full step mode, it will have to instantly
> halve the rate of pulses it is putting out.
>
> So my questions:
> #1  - is 10,000 pulses/sec a real limitation? (I don't know
>          my final ratios of motor to traverse yet).   In any case,
>          for non-cutting rapids presumably you want it as fast
>          as possible.

I doubt that 10,000 steps a second, or even 10,000 half-steps
a second is very reasonable for standard steppers attached
to a machine tool table.  That is 25 revs/sec or 1500 RPM on
a 200 step/rev motor.  I have seen steppers go this fast with a
VERY light load and a high-quality damper, but I doubt that
you can get one to go this fast without extremely fine acceleration
control and a monstrous stepper driver.  The inductance of the
phase windings of a stepper have very large inductance, and
the current in these windings would have to reverse direction
at a rate of 5000 times a second (in full step mode at 10 KHz).
Running even a 2 V/Phase stepper with a 100 V DC power
supply would likely not be enough to go this fast.

> #2 - are other motor drivers (servo, uni/bi polar stepper) limited
>          to rates MUCH  less than what EMC can emit?

Servos have no limit like this.  You set the 100% DAC voltage
to command the max velocity of your system, and EMC does all
the computations to work it out.  If you want 1000 IPM rapids,
EMC will certainly handle it, although the low end might suffer
with the 12/13-bit DAC on the Servo-to-Go card.
I don't like all these artificial limitations, so I went to great
trouble to set
up servos on my Bridgeport conversion, and I've been very happy with it.

I have the max velocity set for 95 IPM right now, but I could go
faster with a little bit of tweaking.  I know others with bigger motors
and servo amps have run faster.

> #3 - if so, can the performance of these systems be doubled
>         by the use of the above algorithm (if the motor drivers
>         support both full & half step)?

Rather unlikely.  The motor tops out due to inductance vs. the power
supply voltage, and that is a fundamental limit.  Half vs. full steps
may
alleviate the loss of sync, and allow a slightly higher speed.  A
bi-polar
drive on some motors makes a vast difference, often doubling the speed
before stalling.  But, it is still unlikely you can get anything NEAR
1500
RPM out of a standard stepper.

> #4 - How difficult/feasible is the modification to EMC?

I would say it is VERY difficult, because it sounds like a GREAT way
to get lost steps, switching back and forth between half and full
stepping,
and making the step rate compensate for the switch.

Find out what your motors and driver will ACTUALLY do, before
dreaming of 10,000 steps/sec.

Jon