Re: stepper pulse rate idea

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

Ray Henry wrote:

>List
>
>Those of you who have applied the EMC to steppers that run from the parport 
>know that you have to design into the machine a compromise between speed and 
>resolution.  
>
>This compromise is fixed in part by Jon's PPMC stepper rate generator card.  
>This card goes a long way toward expanding the range of stepper powered 
>machines to which the EMC can be applied.  (this is not just a blatant 
>advertisement but an attempt to clarify where the rest of this post is going 
>or not going.)  I'm thinking here of a direct parport driven system.
>
>What I'd like you to think about is a two scale pulse train for each axis.  
>One pulse train would run at a low rate for high speed travel, the other at a 
>high rate for low speed resolution.  Let me suggest a couple of ini variables 
>because that may be the easiest way to think about this.
>
>INPUT_SCALE_RAPID =  5000 0000
>OUTPUT_SCALE_RAPID = 5000 0000
>
>and 
>
>INPUT_SCALE_FEED = 50000 0000
>OUTPUT_SCALE_FEED = 50000 0000
>
>As I see it we would not change the resolution of the axis hardware itself.  
>We would change the resolution that the EMC sees or imagines to se out there 
>at the machine.  I think this thought experiment has software, hardware, and 
>value components.
>
>
>1 - Software
>
>What kind of a nightmare would we create for the software if we ran the 5000 
>step scale for any rapid move and the 50000 step scale for any feedrate move? 
>
What if you were in the middle of the place where the two systems match 
up, ie. some
multiple of ten real steps plus 5 more?  The rapid feed intervals are 
every ten real steps.
So, the real position is, say 5005 steps off zero, then you do a rapid 
feed of 10000 steps.
Do you end up at 15000 or 15005 steps?  Who keeps track of all this?  Yikes!
What happens if you switch from rapid mode to normal mode while the 
machine is
still moving - as is clearly legal in a program like :
N1234 G00 X12.5
N1235 G01 F5.5 X13.25

I'm sure, with very close coordination between the drive designer and the
software designer, you could make this work.  But, you;d have to keep
your heads totally in sync at all times, or there would be some condition
where steps would be gained or lost when switching back and forth.

My board can spit out 300000 steps/second EASILY, and ramp right up from
zero to max, and then back down again.

Jon