Re: First time EMC user (introduction)


So Jon, how does your universal stepper controller interface with EMC?  How does EMC tell the X axis to move some amount and that get translated through the parallel port to your controller?  Did you write a driver for EMC and your card?
 
Sounds interesting, and I could still use the Gecko drives.
 
Richard
 
 
----- Original Message -----
From: Jon Elson
Sent: Tuesday, October 01, 2002 11:21 AM
Subject: Re: First time EMC user (introduction)

Richard Everett wrote:
 Hi Jon, I think I am in big trouble.  I looked at the fanuc motor encoder on my mill, and it said 2500 counts/rev.  I thought to myself, wow!  2500 counts in quadrature, thats going to be a lot of steps/inch.  I loaded up emc with steppermod.o selected, and input and output set to 2500, velocity set to 5.  I hooked up the gecko 320 drive to the motor and parallel port, and directed emc to move 1 inch.  The motor rotated exactly 1/4 turn!  My dang encoder is TEN THOUSAND steps/rev in quadrature!!!  My ballscrews are 5 turns/inch, so I am going to need 50,000 steps/inch!!!!!  It can't be done Jim!
It's easy!  But, you can't do it in software.  You definitely need my universal stepper controller, if you want
to stay with a step/direction control of the motors.  See  http://pico-systems.com/univstep.html  for more info.
You could also look into my PPMC card set (  http://pico-systems.com/PPMC.html  ) for a direct servo
control setup.  Either way, however, you can have actual position feedback to EMC through these units.
  
I tried setting the input and output to 50000, and the program never even fully loaded.  It got past the 5 second intro graphic, and then just sorta sat there.  This is strange behavior....what is it doing, getting pre-scared of the ini file?  I changed the input and ouput in the ini file to 10000 and the program did actually load, after 20 seconds (normally takes about a second after the intro graphic).
 
It has to do with how EMC computes the time interval for the step pulse program.  It takes your maximum
velocity (in user units/sec) times the steps per user unit, to figure out how many interrupts per second you
need.  If you want 120 IPM, that's 2 IPS x 50,000 steps/inch = 100,000 steps/sec, and it needs 2 ticks
of the program, one to raise the step pulse, and one to lower it, for a 5 uS per period.
That is an awfully heavy interrupt load.  The old DEC Alpha CPU has all the timer stuff on-chip, so it
can actually work with that kind of load (maybe).  But, in PCs, the timer os off-chip, on the other side of a slow
bus bridge, and it may take 10 uS just to reprogram the timer for the next tick.  Anyway, this isn't
realistic, because there would be a 2:1 jump, from 50,000 steps/sec to 100,000 steps/sec, with no
intermediate speed.

My step rate generators run at 10 MHz, for a granularity of 100 nS.  It can comfortably crank out step
pulses at 300,000 steps/sec.  If you need more, I can make some small adjustments to get more.

I am thinking even if I move from this 350mhz K6 to a 1.4ghz athlon, I am still going to have big trouble getting rapids.
Your 350 Mhz should do fine with either of my board sets.
Would freqmod actually help this very much, or am I going to need to dump the gecko drive idea and try and get a servo amp that takes quadrature encoder inputs instead of a tach (I checked, there is definately not a tach in there).  I don't mind buying something, as long as it will work, and work at a decent speed.
Freqmod makes smoother trains of step pulses, at the cost of HIGHER CPU resources.  If you've already
run out of gas, freqmod won't help much.

A DC tach can be added very easily.  I had to add tachs to my motors to get the best performance.
It might be cheaper to add a tach (adapted from a Maxon or Escap mini DC motor) and use surplus
servo amps than to be forced to buy new servo amps with encoder feedback.  It depends, to some
extent, what pops up on eBay.  With those encoder resolutions, a tach is a lot less of an issue, in
performance terms, than it would be with a 10,000 count/inch setup.

Jon



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