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

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

On Tuesday 10 September 2002 04:52, Ray Henry wrote:
> Hello Robin
>
> Good to hear from you and that you are moving toward Linux as a central
> part of your machine shop.
>
> I really want to take you up on your offer to write something about
> kinematics for the handbook and I'll get back to you on that before long
> but I'm not certain that kinematics is what you need.
>
> Kinematics are the computation of motor motion based on the relationship
> between the motion that a motor controls and the coordinate system in
> which the g-code was written.  For a router with three axes 90 degrees
> from each other we use trivkins which is a perfect correspondence between
> a motor and one of the axes in cartesian space. 

however .. as I pointed out this does not actually reflect reallity .. there 
is mechanical interaction between the axes.

> Most of our kinematic
> applications are to hexapods where cartesian motion is generated by a
> matrix of strut lengths and angles between struts.  You can see one
> example of this at

understood .. in this case the transforms between joints and pos are very 
strong ... the functions to transform x,y,z into joints drastically affect 
the mapping to the joints .. but surely the principle is the same ( you have 
joint[0] = Ax.sin(something); (a very strong transform)
I need 
joint[0] = x + (0.002 * y);

although different ... the principle is the same .. the kinematics simply map 
the cartesian positions onto the real world mechanics ... I appreciate what 
htey were originally conceived to do .. in principle my machine is something 
with *almost* trivkins, just not quite.

> I am certain that there would be a kinematic solution to your axis droop
> problem but I haven't a clue where to start.  We do have several on the
> list who are using routers for one thing or another and I think that they
> may help us with this problem.

<aol>me too

I'm just going to go and hunt down the correction table code for the single 
axis corrections you mentioned (because table driven is basically what I want 
to do (ie a map of the offsets every 100mm or so and interpolate between 
them)) and see where I get to. In an ideal world I'd even get the router to 
calibarate itself by probing a straight edge ...

I am howeve absolutely certain this is NOT a GCODE post processor 
application. this is simply a matter of mapping machine commands onto a 
weakly transformed reallity. It's definitely a 'machine space' problem rather 
than a user space problem. Imagine what would happen with several different 
machines in the shop .. different sets of GCODE for each one .. running the 
worong job on the wrong machine etc ... mmm .. no thanks :)

> Look ahead works two ways, at the highest level, it will read several
> thousand lines ahead on my mill here.  At this level it is only reading
> the code and looking for gross errors.  At the motion control level it
> works about 200+ ahead of the current motion command.  

<snip> sounds fine ... I'll quit worrying about that then.

-- 
Robin Szemeti