RE: I-LPT & step/dir servo amps

Jon Elson wrote:

>
> Ughh! This whole idea of having step pulses controlling a servo is just
> making me sick! What a waste of good servo motors and encoders!
> I just don't believe that most of these systems have the timing resolution
> to move very smoothly over a wide range of velocities.
>
> Let's work out an example. Let's say we want resolution of .00005", and
> velocity up to 120 IPM, that is a step rate of 2 IPS x 20000 step/In =
> 40000 step/sec, or 25 uSec per step. Now, let's say we need to go at
> 119 IPM, that's 1.983 IPS x 20000 Step/In = 39666.66 Steps/sec or
> 25.21008 uSec per step. Hmmm, that's a small variation of about 210
> nS in the step rate. But, what if we needed 119.9 IPM? That is 25.02085
> uS, or about 21 nS. Can your program of choice deliver steps to 3
> (or more) motors with 20 nS resolution? Obviously, not. Can it do
> 210 nS? I suspect, still not, without custom timing circuitry to generate
> the step pulses. But, most of the programs output step pulses from the
> parallel port, directly from software. Most of these programs have a
> fixed interrupt clock, and either generate a pulse, or don't, every tick
> of that clock. I would suspect that this clock can't be much over 10 KHz
> for software step pulse generation. So, right there, your highest rate
> would be 10,000 steps/sec, and the next lower rate would be 5,000!
> That is a VERY coarse jump in step rate.
>
> With a 12-bit msgnitude plus sign velocity DAC, as used in the Servo-to-Go
> card, you get 4096 equal intervals of velocity. If you set max (4095) to
> be 120 IPM, then you get approximately 34 quanta on the DAC for each
> IPM. So, the servo routines have a choice between 120.0, 119.97, 119.94
> etc. This allows very smooth adjustments of velocity to keep the actual
> position close to the desired position at all times, even near
> max velocity.
>
> I really think this is where stepper systems fall down, and it is
> already in
> deep trouble before the signals have come out the parallel port. Namely,
> information has been lost, and there is no way to bring it back once it is
> lost!
>
> Jon

That's a good explanation; thanks. I'm only stuck on this idea because I'm
scared shitless of trying to get a Linux box together, and I'd like to run
servos. Indexer LPT is the only stepper package I've seen that can output a
high pulserate (100+ khz for 3 axes). I've been wondering myself how they
handle the interpolation, and I suspect they would have to use averaging to
get fine velocity resolutions. Maybe I'll call them and pick their brains a
bit.

Thanks,
Carlos Guillermo