Re: true closed loop vs. hobby closed loop

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:

> The Gecko drives are known to be within +/- 128 counts of commanded
> position, but you don't know where. With software closed loop, the
> CNC program DOES know what the error is at any time. EMC allows you
> to have 2 terms for following error. One is a lower limit at

standstill,

> and can be set VERY small, perhaps 2-5 encoder counts. The other

limit

> is proportional to velocity. This allows you to have a larger

following

> error limit when moving fast, and especially at rapid feed rates,

while

> keeping a small limit at the low cutting feedrates, where it

matters.

I interpret this to mean that if I use smdromod.o and the Gecko
drive, that I can set the following error to basically be a MAXIMUM
of 128 counts. With 2000 ppr, and a .200" lead screw, that's 0.0128"
of allowable error. Doing profiling I guess I could see that being a
bit much.

> You really need to hook up the fault indication from the Gecko

drives so

> it can stop ALL axes and alert the CNC program.

Mariss has the circuit to do just that posted in the files section.

> Another advantage of true closed-loop is that the machine can be
> switched back
> and forth between closed-loop CNC and manual DRO mode, with the

screen

> always showing the true position. If your machine has handwheels,

this can

> be a nice feature.

On the Gecko G201/G210 if I were to ground terminal 7 (DISABLE) then
you should be able to drive the system manually, using the encoder to
feed the DRO, just as you say, correct?

Sorry for the basic level questions, but I just got EMC working on my
laptop this weekend (WOO HOO!) and I am now going about the business
of getting my Bridgeport Series 1 CNC retrofitted with new drives
(same old motors). Anyone have any luck fitting handle to the old
Bridgeport CNC machines? I can easily get one on the Z drive. The Y
drive I couls maybe see how I can modify the belt cover and put a
handle in. What about the X?

Thanks
Kevin