Re: Low cost Servo Controller

Leslie Watts wrote:

> From: Leslie Watts <leswatts@...>
>
> Jon Elson wrote:
>
> >
> > Well, I think you're nuts. Unless you are using a huge Vertical Machining
> > Center, your machine isn't stiff enough to give this sort of accuracy, especially
> > if you are accelerating hard up to 120 IPM.
>
> I am nuts! ;^)
>
> I designed this machine using finite element analysis to be very stiff.
> It is a space frame construction. The minimum static stiffnes at the
> spindle is about .002"/100lbs but the Q is very high and it may require
> active damping. The moving gantry is about 800 lb.

This sounds VERY difficult! When the 800 Lb gantry moves, something
is going to deflect up to .016", it seems.

> I will try to get pictures up on the web site.

I've GOT to see this!

> Understand this is not a hobby machine;
> it is an uphill battle to manufacture anything from wood in a small
> shop in the U.S. and one has to continually find ways to improve
> productivity.I need an extraordinary control system on a budget and I
> hope EMC will be it.
>
> The only hard acceleration is likely to be inside corners. Following
> error, over/undershoot, and heat removal are the main limiting factors
> I see that limit speed.
>
> I am also very concerned about crashes. I have a system of watchdog
> timers, shock absorbers, and independent speed detectors that will
> protect against end of travel stuff but nothing really in hardware
> to protect against hitting the work. All I can do is slow down the
> rapids. Even that doesn't protect against a velocity mode runaway.

Well, I've been running a converted manual mill with EMC, and my own
servo amps for over a year, now. All crashes have been due to
'idiot at the switch' sort of things, like watching carefully on the right side
as the vise crashes into the spindle from the left. I even hit the wrong button
and brought the quill down on my hand, once. I sat there for a second
trying to decide if there was a malfunction or an operator error, and finally
decided it was the latter, pushed the RIGHT button, and got my hand
released. I've never had any sort of runaway, other than those caused
by incorrect part programs that weren't checked at slow speed before
running them at full speed.

Jon