Re: Base Task Period

[Jon Elson <elson-at-pico-systems.com>]

Hassan Rabe wrote:

>Hi John,
>
>I wasn't sure about what the period is. so, when it's set to 50u S then that
>would mean 20,000 pulse per second
>or 20 KHz. I have  couple of questions:
>
>am using freqmode.o , and I need 20,000 pulses/inch because am using a 10
>microsteps/step drive
>the original set up was 2000 step/inch, am shooting for 30 inch /second
>linear speed.can i do that on a PIII 667MHz computer. am gonna be using the
>inch system
>  
>
30 Inch per SECOND?  At 20,000 steps/inch, that would need 600,000 
steps/second, or a step
every 1.6 uS!  That would need you to interrupt the CPU at twice that 
rate, or every 833 nS.
You can't do this in software.  The only thing I can think of that will 
do it is my Universal
Stepper controller, and it might take some firmware mods to allow the 
position counters to
count that fast (no big deal to do that, it is a totally artificial 
limitation).  See
http://jelinux.pico-systems.com/univstep.html for more info on this.

>1-what are the proper cycle time values in the [TRAJ]
>and in each one of the axis and how they are relate to each other.
>
There is some documentation on this in the EMC manual.  But, I think 
this is usually
5 - 10 times larger than the axis cycle times.

>2-am strugling with the way ...default velocity and acceleration value are
>calculated...or set up
>in a way to get the following error out of the way. or does the following
>error depends on the values of P, I, and D.
>  
>
Just set minimum following error equal to 10.0, that would be 10 user 
units (in or mm, whatever
you are set to).  Default and max velocity are in user units per SECOND, 
while feedrate is in
user units per MINUTE.  Darn confusing!  So, a feedrate (or jog rate) of 
60 IPM is 1 IPS.
If you want to do 30 IPS maximum, you need to set maximum velocity to 
30.  The screen
should show the jog rate go from 0 to 1800 (or the GUI will blow up 
because it wasn't
designed for such high rates).  Acceleration is in user units per second 
per second.  I have
no idea what acceleration is required to make an 1800 IPM machine 
practical, but this number
must not exceed the capability of your stepper motors and drivers.  Wait 
a minute!  You must
have made a mistake, you CAN'T do 1800 IPM with steppers, even if the 
machine only weighs
a couple of grams.  Anyway, the software step generation simply cannot 
handle steps at
this rate.  If you set the folloeing error threshold real high, it won't 
shut down, but the axes will
not run at the commanded rate, either, and some axis may be running 
thousands of inches
behind another axis under these conditions.

Jon