Re: [CAD_CAM_EDM_DRO] EMC, the basics...please

dougrasmussen@... wrote:

> Group,
>
> >From semi-lurking for a couple months I've picked up a little info on
>
> EMC, not a complete understanding, though.
>
> Could someone please summarize the software and hardware necessary
> for implementation of a servo (and maybe stepper) system. Like how
> does the EMC-resident pc communicate with the servos, serial,
> parallel port, add-in card, whatever. Is the machine control panel
> the pc's keyboard? How is I/O handled, spindle motor, coolant pump,
> feed rate override, etc?

The base system right now has 2 flavors, servo and stepper.
The servo system uses either the original Servo-to-Go card or the
newer STG-II card. You have your choice of 4 or 8 axes with these.
These cards run about $800, which makes it a significant part of the
expense. I am working on a replacement for the STG cards that will
be cheaper, more flexible and runs off an Enhanced Parallel Port.
It will also, with some software changes, be able to run all the
auxilliary
control (spindle motor, coolant, you-name-it) off the same port.
The encoder counter and DAC board hardware design is done,
and the driver is being written now. You can use any servo amp
you like, if it will take +/- 10 V velocity commands. Copely Controls
makes some cheap servo amps. See http://www.copleycontrols.com/
I have also made my own DC Brush-type servo amps, and they work
very nicely for me. I have made the PC board and a few other parts
available, but unless you are really cheap, you probably don't want
to build your own. But, you can see
http://ascc.artsci.wustl.edu/~jmelson/servo.html if you are interested.
You can see my EMC controlled machine at
http://ascc.artsci.wustl.edu/~jmelson/CNCconv.html
(Ignore that huge Allen-Bradley control there, that is a historical
relic!)

The stepper version uses parallel ports, but has some limitations, as
the CPU has to generate all the step pulses. This requires a huge
CPU load, or poor performance of the steppers. I will soon be working
on a hardware pulse generator to remove the load from the CPU,
allowing 100 MHz Pentiums to perform fine with steppers, and
achieve arbitrarily high step rates smoothly.

The PC keyboard is all we have right now. As an extension of my
Parallel Port boards, I intend to build a pendant controller, that
should
have things like :
Start, stop, pause, feed rate override, spindle start/stop, jog controls

and position displays. I kind of imagine a little thing that you put
across
your lap so you can slow down or stop if things ever go wrong, and
small enough to set on the machine table while picking up the location
of the part or fixture.

In both versions, a parallel port is used for the spindle, coolant, etc.

control. For steppers, right now, you need 2 parallel ports, one for
motion, the other for aux. functions. Feed rate override is done with
the
1-9 and zero keys, (for 10% - 90% and 100%), or a slider bar on
the screen can be moved with the mouse.

>
> Any rough estimates of costs based on something like a 650oz in
> system for a medium size knee mill, assuming a CNC-ready (ball
> screws) machine and purchasing new servos and amps as opposed to
> surplus stuff.

I can't really address the servo amp cost or stepper driver cost, but
the only other
stuff you'd need are a 100 MHz Pentium computer with 32 MB of RAM and
a 1 GB disk drive, and the Servo-to-Go card for servo systems. I am
hoping to
be able to make the parallel port stuff for $100 for the 4 axis encoder
counter,
$100 for the 4 axis DAC (16 bits resolution), and $50 for the
motherboard.
later, some digital I/O cards with everything on them would be done. By
this,
I mean that the AC out modules would plug onto the board, and the AC/DC
in
function would be built into the board. I'm not sure what this one
would cost, but
hopefully in line with the others.

> >From my casual lurking, it's hard to tell if there are any EMC
> systems out there currently making parts. Are there? If they are
> making parts, are they production quality, reliable systems or more
> hobby type?

Sure! My system is not a production environment, but I've made some
truly
aerospace-quality parts, like hydraulic valves. Mostly I make
instrument
panels for my products, and tools for internal use. The CNC system is
on
a 60 year old Bridgeport, and the ballscrews have developed some
backlash,
the ways need scraping, etc. But, EMC is a highly precise, reliable CNC

system, capable of whatever accuracy the iron itself can deliver. I
routinely
make parts to accuracy of .003", and with just a little attention to
tool deflection
.001" is easily attained. Really, the tool deflection and looseness in
the iron
is the only source of error, here.

Jon