Re: [CAD_CAM_EDM_DRO] Bridgeport CNC Mill???

On 10/01/2011 08:32 PM, Jon Elson wrote:

> David G. LeVine wrote:
>> > Higher performance products (like the Mesa Electronics
>> > <http://www.mesanet.com/> stuff) has 20 KHz sample rates
> While the Mesa PCI line may be able to sample encoder position at a 20 KHz
> rate, the full servo loop of EMC2 would be hard pressed to complete the
> whole
> process in a guaranteed 50 us (20 KHz). That includes the motion
> driver, PID
> and trajectory planner/interpolator.
>
> Jon

Since the Mesa stuff (and I only chose that as an example) can do most
of this in the FPGA, EMC2's speed is not a question. It is not normally
significant if there is a 1 mS delay between the end of one move and the
start of the next EXCEPT when doing cuts with many straight line
segments. You misread (or I misunderstand) the Mesa specs, they count
encoder positions at 2 million counts per second, they do the servo loop
in the FPGA at 20 KHz (50 uS), so EMC2's limitations do not matter as much.

That is not an attempt to complain about the Pico stuff, but use of
another manufacturer as an example. What you are saying, in fact, is
that you can provide similar performance for a lower price, and that is
not a bad thing.

I was pointing out that EMC2 by itself would allow a lower cost, lower
performance way to test things before deciding if adding additional
hardware was necessary. If EMC2 (or Mach) provides acceptable
performance, no extra hardware is needed, if it doesn't, try to figure
out if some high end (and expensive) solutions will be able to give
sufficient performance for the job. THEN figure out how cheap you can go.

It is often reasonable to figure out the basic limitations of a system
before investing in high end solutions, if it needs 20 million counts
per second, Mesa (or Pico) will not going to be a good choice, if it
falls between EMC2's and Mesa's (again or Pico's) capabilities, it is
doable with that technology. If it works at full speed with EMC2 (or
Mach), why change things?

The trick is to find where the end result is possible without spending a
huge amount of money (and effort), then spend the money wisely.
Spending $1,000 per axis when $100 is reasonable is pretty silly.

Dave 8{)

--
/"Political Correctness is a doctrine, fostered by a delusional,
illogical minority and rabidly promoted by an unscrupulous mainstream
media, which holds forth the proposition that it is entirely possible to
pick up a turd by the clean end."/
(quoted from http://www.wattflyer.com/forums/showthread.php?t=30060)

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