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

>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.

That sounds more like a 'disable' state than EStop. An Estop system
generally MUST remove power from the bits that move, that is what it is
there for. There are some newer specifications for industrial system that
can have a more relaxed EStop requirements, so on a large machine you don't
have to remove power from everything to access a small area.These have all
sort of extra safety gizmos and doubly redundant systems to make sure people
don't get hurt or killed though and are not likely to be seen in the home
shop.



If your servos/steppers/spindle motor/etc still have power applied you are
NOT in an EStop state. Be safe the life you save may be your own.



Jeff Birt

Soigeneris.com



From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Jon Elson
Sent: Friday, December 17, 2010 12:06 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: 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





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