Re: [CAD_CAM_EDM_DRO] EMC, Was Re: CAD experiment entry

timgoldstein wrote:

> For the sake of those just getting involved, I want to point out that there are different approaches to servos. One is driven like a stepper with pulses being sent to the drive with each step telling the drive to move the motor 1 increment (usually an encoder pulse) and then the drive electronics tracking the movement and adjusting accordingly. In this setup it is open loop as far as the computer is concerned. So the controller program does not actually know where the axis being moved really is. A very viable and low cost servo approach that gives you the acceleration and speed benefits of a servo. I have my Bridgeport series II (known by all the old timers on the group as Bridgett) running Gecko G320 drives which are this style.
>

Right. The problem with Gecko-style drives is you have no way to know
what the following error is. The computer sends the steps, and ASSUMES
the drive has put the motor there. The old G320 had a fixed error limit
of +/- 128 encoder counts, which could be quite a large error. I had a
friend locally who had a G320 machine with high-resistance motors and
only 36 V supply, and it would not return to the servo null. You could
turn the motors 10-15 degrees and they would just sit there. Sometimes,
after seconds, they would go "grrnk-grrkn-grrnk" and move back closer to
the null point. This was not what I'd call tight servo control. Some
other drives like the Rutex at least had a way to hook up a computer and
get tuning and performance info out of the drive. (Not that I'm
recommending Rutex here, they have other problems.)
Another problem with the step/dir approach is you can't ever E-stop the
machine without having to rehome. I usually estop my machine whenever I
walk away from it, just click a button to bring it back online.

>
>
> Speaking of hexapods, does anyone know of any hobby priced version or achievable DIY approaches? They are fasinating machines. If you don't know what one is you need to Google it. Maybe someone can post some links and we can talk about the pluses and minuses.
>
>

I think the more-easily built ones are called hexaglides. They have a
plate with several linear slides mounted to it in a plane. There are
links between these slides and the platform. This avoids having to make
extremely good telescoping rods.

>
> But, I talked to Art about it once. Not sure if he would admit to it in public, but what he told me was every time he added a button or widget to the screen he sold more licenses.
>
>

Well, adding a FUNCTION that wasn't available before certainly can
expand usage in new areas. We do it in a different area, many of the
smaller enhancements are accomplished by finding new and ingenious ways
to reconnect stuff in HAL to bring out things to display on the
customizable part of the GUI. I added an MPG with a filter to prevent
the coarse MPG counts from "buzzing" the servos, and a spindle speed
display with filtering to stabilize the displayed value, and a
rate-of-change limiter to the spindle speed, mostly for use when rigid
tapping, all in HAL scripts, without touching a single line of the EMC
code base. HAL is the big new thing when EMC became EMC2, the
brainchild of John Kasunich. It is a scripted language and framework
that connects all the major components of EMC2 together, and allows you
to make new connections for all sorts of functions.

>
>
> I have to disagree on this. I have full closed loop servo machines, open loop servo machine, and stepper machines. They all work and make fine parts. Different capacity and performance, but all capable of making a to tolerance piece.
>
>

Steppers are fine for things where the speed ratio is modest and the
forces are quite predictable. Metal machining is just not that
predictable, so the only way to avoid position loss --EVER-- is massive
overkill. I'd mich rather have a wimpy machine that TELLS me when it is
overloaded than one that will just try to bull its way through concrete.

> Jon, can you also post a link to where someone can get more details and pricing on your products for servos?
>
>

Sure, http://pico-systems.com/oscrc4/catalog/ is my web store, and most
of the stuff there has links to the "back pages" with more
detailed descriptions. The PPMC (analog servo interface) has been
relatively unchanged since 2002, the Universal Stepper Controller
has been out since 2002, and my PWM controller and matching servo amps
have been out since 2004.

Jon