Re: Close loop steppers

Ian Wright wrote:

> From: "Ian Wright" <Ian@...>
>
> Hi,
>
> I did understand what Arne is trying to achieve in principle and, in fact,
> he is just trying to make an electromagnetic DRO without the readout
> however, I can forsee a couple of major problems with just a simple system
> as described.

I brought home the 8 large stepper motors and indexers friday. As I had mentioned in previousposts I had planned on servo for my
gantry mill (I have a Tech80 servo card an have written
and tested software) but these are just too good not to try. Four of the units are new
and were never even powered up.The other four were only used in the lab briefly.
This will save me a couple thousand dollars if it works out. The 1500 in oz units
are a good torque and inertia match to my system except for the Z axis for which
they are overkill.

So I have been machining motor mounts this weekend.

From previous experience I know running the system open loop is a bad idea.
At least with the mass i'm pushing.
So I'm hunting for an encoder feedback scheme as well. Even in half step mode
the resolution is too course for PID; one (half) step equals .0005" linear travel.
I generally use the old ten times rule with servos- encoder resolution about
ten times desired accuracy. Just because it is a stepper doesn't mean one is
freed from the same stability issues as feedback servo systems.

But I have no stepper software.

I think the simplest stable control system would be to add missing steps during
the next rapid or other noncutting portion of the cycle. An all axes off command
would be issued if any axis exceeded a certain error.I think for each stepper
I need several bits of digital I/O and a timer counter channel. I don't want to
reinvent the wheel at this point. Does anyone have software that does this
already?

I did not quite understand Arne's idea with the shift register but would
like to see another post explaining further.

I have also noted that a two phase stepper and encoder can be configured to
become a brushless servo. I found this information at:

http://www.cs.uiowa.edu/~jones/step/highlevel.html

--
Leslie M.Watts
Illinois Tool Works
Corporate Advanced Technology group
(847) 657-4559
http://www.netcom.com/~leswatts/leswp.html