Re: bills thoughts and comments.

> From: Leslie Watts <leswatts@...>
> The intended control system is LM628 based (Tech80 5638). I have written
> c control programs for this many times in the past and it has worked ok.
> Motor update is 250 microseconds and trajectory commands can be sent at
> 1 khz if you pull some tricks.

This sounds very interesting. I had a strong interest in the LM628/629
chips. I got samples, lots of documentation, etc. from National Semi.
The one thing I couldn't figure out was how to keep 2 (or more) of
them synchronized, especially on 'chained' moves, where 2 axes are
making many very small moves, as in circular interpolation, etc.
I called Nat semi, they told me they'd just been bought by Fairchild,
and were real busy at the moment. But, anyway, they had no info
on synchronizing multiple axes. For plain linear moves, you just
give the 'go' command as close to simultaneously as possible, and
it should do fine. But, on a multi-axis chained move, if one axis
gets a bit ahead of the other, you want the slow axis to speed up,
or the fast axis to slow down, just a little. But, you don't want the
faster axis to go into 'ramp down to stop' mode, or it will really
muck up the motion. So, I never built a system with the
LM629 chips, because I couldn't be sure this problem wouldn't
come to haunt me.

> I am concerned with interrupt latency and the lack of feedforward
> in the LM628/pc though. The hard realtime capability of rtlinux EMC
> looks very appealing as does the first and second order feedforward
> algorithm. The high feedrates and and low following error requirements
> I have demand a high performance control. Buying an expensive
> manufactured control is not an option.

Yes, it really seems to work quite well. You can get graphs out of the
system to help in tuning the parameters.

> - making 7' machine ways to .001" with only a level, 12x18 surface
> plate, and hand angle grinder

That sounds like QUITE a trick!

> - designing and building preloaded ball screw support bearings

I need to replace a preloaded pillow block on my X axis, because
it is just too light for the job it is now being required to do. I have
some angular contact bearings, but I'd like to hear your ideas.

> - Surplus sources for linear motion components (Chicago area)

Always interested in where to get good parts from.

Thanks,

Jon