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

[Dave Engvall <dengvall-at-charter.net>]

Robin,
Good for you thinking about emc rather than XX.

There is already table driven code for correction on the same axis but your
problem needs to relate correction in Z to X (and maybe Y). This would look
like a possible post-processor application (table driven or maybe
calculated).  Calibrating it other than assuming a loaded beam might be a
problem.  Just thoughts off the top of my sometimes rather blank head.

Dave


On 9/9/02 2:40 PM, "robin szemeti" <robin-at-rszemeti.demon.co.uk> wrote:

> 
> 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 ...??
>