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Re: [Emc-users] Latency - was Re: Which video card/driver for LinuxCNC? charles green Tue Feb 21 04:00:44 2012

the allocation of computing resources seems to be a recurring theme with pc 
based machine controllers.
 
while i have wished for an accesible pocket calculator gui more than one time 
while standing in front of a machining center console, i have almost never 
wanted to surf the web or send emails while doing so.
 
the enhanced machine controller spans a disparity between its intended function 
and the common usage of it's supporting hardware.  maybe there is a workaround? 
 when i installed linux cnc packages, one of the steps was drive partitioning, 
in case there was another OS already there, with some kind of neutral zone for 
exchange of data when switching the pc from one dedication of control to a 
different dedication of control.  why not do the same thing with an enhanced 
machine controller?  in one mode, the pc has the sole function of machine 
control and user interface for machine control.  in the other mode, the pc has 
the common functions like internet and anything else in an OS's routine employ.
 
i only have a very basic understanding of pc construction, so there might be 
some sort of fatal flaw in the idea of a dedicated machine control operating 
mode for a pc.  however, my suspicion is that such operation has not been 
pursued for reasons of convenience of having a do-it-all pc operation mode.  
this may not always be advantagious.  for example, imagine waking up in the 
morning, turning on the coffee maker, showering, and then finding that the 
coffee pot is empty because the coffee machine had a problem downloading the 
most recent bean roast sensing algorithm.
 

--- On Mon, 2/20/12, Jon Elson <[EMAIL PROTECTED]> wrote:


From: Jon Elson <[EMAIL PROTECTED]>
Subject: Re: [Emc-users] Latency - was Re: Which video card/driver for LinuxCNC?
To: "Enhanced Machine Controller (EMC)" <[EMAIL PROTECTED]>
Date: Monday, February 20, 2012, 9:54 AM


Roland Jollivet wrote:
>
> My question is; if one built a headless system(no video) and disabled
> drivers wherever possible, would such a system be fast enough to run DC
> servo's and read the encoders on the parallel port?
>   
You have to figure this out for each case.  What is the maximum encoder 
RPM at the desired rapid
feed rate?  (Don't forget belt reduction.)  What is the base thread rate?

For example, 60 IPM, 1000 cycle/rev encoder and a 2:1 belt reduction to 
a 5 TPI
ballscrew.  So, that is 60 * 5  * 2 * 1000 * 4 (encoder quadrature) / 60 
secs/min =
40,000 counts/second.  Well, that is a bit iffy, a count every 25 us.  
And, remember
that if the encoder exceeds the rate at which the computer can count the 
pulses,
you get a servo runaway, which is a lot worse than just losing position with
a stepper.

Now, if 30 IPM is OK, and you have 250 cycle/rev encoders directly coupled
to the 5TPI screws, it should work fine, but that is a lot of compromises to
avoid a better interface.

I generally think that running steppers via software step generation is 
not a
great thing to do, and running servos via software encoder counting is far
WORSE! 

So, now you are suggesting using TWO computers instead of using 
purpose-built
hardware to interface to the motion system!  Now, you have to have TWO
hard drives to back up, and TWO computers to boot and shut down cleanly.
There may be times such an arrangement is desirable, but I really think
it is the wrong approach.

Jon

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