Hi, and 2 questions (kinematics and look-ahead)

[robin szemeti <robin-at-rszemeti.demon.co.uk>]

Hi,

I'm  in the process of upgrading from something nasty in DOS to EMC under 
linux for my 3 axis router and plasma cutter 
(http://www.rszemeti.demon.co.uk/cnc2.htm if you want some tacky photos) thus 
allowing me to rid myself of the last bit of Microsoft software in the shop.

So ... firstly Kinematics ... 
I've been reading through the various [blah]kins.c packages and trying to 
make head or tail of the nomenclature ... much is spoken of 
'inverseKinematics' and joints .. and no doubt the documentation would mean 
something to me if I understood the terms ... I naiivley imagined hte 
kinematics to be in the order of  

<pseudocode>

convert(xin,yin,zin){
  xout = fx(xin,yin,zin);
  yout = fy(xin,yin,zin);
  zout = fz(xin,yin,zin);

  return (xout,you,zout); 
}
        
</pseudocode>

or something similar, using refernces etc as appropriate ... translating the 
desired x position into a 'real' x position

Take my router for example .. it 'sags' in Z along the X axis by 20 thou' due 
to flex in the beams, I'd really like to take that out of the Z axis as the X 
axis moves along ... trivial really .. but sadly after reading through the 
[blah]kins.c files I'm no wiser ... I'm familiar with C C++ and Perl (and a 
bit of lisp on occasion too) so its not the language its just the core 
concepts of whats going on thats confusing me here.

A simple pointer would be much appreciated. I'll even write up a 'kinematics' 
section for the FAQ if I can understand enough of it to make it of use to 
others!

Secondly, lookahead ... reading through the NGC intepreter stuff I was not 
entirely clear how many steps were passed ahead to the emcmot stuff .. as you 
probably know the majority of 3D work consists of linear moves along say the 
Y axis with Z going up and down to follow a 3D surface, at the end of the 
pass the X axis moves along say 1/4" and then the pass comes back along the Y 
axis ...  the GCODE for this is thousands of tiny line segments with a sudden 
X axis right angled corner at the end ... unless there is considerable 
lookahead for several hundred steps the machine is going to come to an out of 
limits condition as it hits the right angle at the end ... 

The reason I ask this is because the plasma cutter will often have multiple 
small line segments leading up to a right angled corner and then a 90 degree 
turn ... it would be disastorous if it over shot the corner (or worse rounded 
it off) so just how many lines of lookahead is there ...?? 


-- 
Robin Szemeti