Re: emc help

Hello,
I thank you for the intelligent answer.
I have access to equipment that puts out of the normal hobbist range
and have considered using emc as a free front end for a retrofit.
Since it would be legal to do the retrofit and supply the hardware
and give a copy of emc with the machine.
Regards,
Greg
-- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:

>
>
> Greg wrote:
>
> >Hello
> >I guess having been an cnc enginneer for the last 10years,I tend

to

> >assume people understand what i mean.
> >I all the thing you have said are on the surface correct but in
> >reality are completely wrong.
> >
> >The servo update time which in emc's case is determined by the pc

and

> >limited by the communication to the stg board,as well as the max
> >encoder input rates and maximum accurate control of the d/a

converter.

> >
> >
> OK, most people run the servo update rate at 1 KHz. This is

adequate

> for many
> applications. But, there's nothing enforcing this. EMC has

definitely

> been run
> at 10 KHz before. My boards that connect through the parallel port

is

> limited by the speed of the communication over the IEEE-1284

channel.

> But, a 133 MHz Pentium classic can do the 3-axis cycle, plus all

the digital

> I/O and supervisory functions in under 100 uS. So, it could

probably run

> a 5 KHz update rate, and allow enough time for other tasks in the
> non-realtime
> section. A 333 MHz Pentium II does it in 50 uS, so even that

pretty old

> CPU could do 10 KHz updates well. Eventually you hit the wall on

the

> parallel port, and the CPU can't push bytes any faster. I think

that would

> hit at about a 20 or possibly 25 KHz update rate.
>
> A PCI board would likely be able to close the servo loop at 50 KHz

or

> better. Up in that neighborhood you start getting into the realm of
> DSPs that can bypass all the folderol of OS's, memory management
> and complicated bus-bus bridges, and turn around an I/O read request
> in under 100 nS.
>
> >So the realistic rate cannot be 999ipm ,especially with the
> >limit of .0001 resoulution or 10000 counts an inch.
> >At 400 ipm or 6.6666 ips is equal to 66khz of encoder,which makes
> >800ipm a max for a standard 150khz encoder(assuming a little head
> >room)
> >
> >
> Well, now, we weren't talking about encoders, we were talking about

the

> software. If you NEED to go faster, you need faster encoders. My

boards

> have a very artificial limit of about 300,000 counts/second imposed

by

> the digital filtering of the encoder signals. That seemed to be a
> reasonable
> limit for the application.
>
> >
> >Then there is the case of how fast the processor can close the
> >servo loop,which become even harder with each additional axis.
> >Which is why i asked what the limit is when you make a multiple

axis

> >translation.
> >
> >Then we come to the case of point to point moves ,Or high speed
> >contouring,This is even a worse strain for the processor as you

have

> >trajectory changes every few thousandths(determined by your step
> >distance and step over),All this will bring most cnc's to there

knees

> >long before 400ipm(i have had fanuc's programmed at 999ipm with

the

> >overide at 200% that couldnt go 50 ipm).
> >Not mention the accell/deaccell limits of the motors and drives.
> >
> >
> Yes, in the hobby world, talking about much over 100 IPM is just

blather.

> It takes REALLY expensive drives to make a metal-cutting machine
> move at the rates modern commercial CNC equipment do.
>
> >So i ask yet again what is the max realistic feed rate that Emc

will

> >support for the following:
> >1 a simple one axis move
> >2 a 3 axis move
> >3 a point to point contour move
> >
> >
> Well, I really can't tell you. I USE a milling machine driven by

EMC,

> and I know what the performance of the servo drives is pretty

accurately,

> and EMC has no problem driving this machine to its limits. But,

that is

> NOT much, because it is a Bridgeport with relatively wimpy servos
> (1/8 Hp continuous, about 1/2 Hp peak). This is entirely adequate

for what

> I do, but others may need more performance. I am completely

confident

> that EMC will perform well up to 100 IPM no matter what you throw
> at it. Others have reported some serious problems with the

lookahead

> of the trajectory planner when you try to runa router at 400 IPM.

There

> is an advanced trajectory planner that was developed a few years

ago that

> should fix these problems, but it has a pesky software bug that

causes it

> to hang in certain cases. Experts are looking into it, but haven't
> found the
> problem yet.
>
> I still believe that extremely high feedrates can be controlled

accurately

> by EMC, but the trajectory planners suffer from some problems with
> high feedrates for extended periods. It is not a case of the

machine

> running with large errors or slowing down, it is a case of the

machine

> suddenly jerking to a stop. The old scheme just couldn't keep up,

the new

> one hangs due to that bug. My understanding is that the new planner
> can handle extended runs, like tens of thousands of blocks of G-code
> at 400 IPM, as long as you don't set up the conditions that trigger

the

> thing to hang. I think it is Les Newell that has the router

running at 400

> IPM, but I'm not absolutely sure that is the right name.
>
> Jon