Re: [CAD_CAM_EDM_DRO] Re: Re: Re: GeckoDrives.. Can anyone Help?

Tom Eldredge wrote:

> As I understand it, the Gecko drives partially bridge the gap between
> step/dir and servos, but do not give the velocity control that true servo
> systems provide.
>
> It seems as though in order to get true velocity control, the encoder
> feedback needs to reach the control earlier in the processing stage.

No, the encoder is not enough. As I gave in an example yesterday,
I have 20000 encoder counts/inch, and can go down to .01 IPM
before the stick-slip friction becomes dominant. This is only 3.3 encoder
counts/second. That is not enough to give reliable velocity info to
whatever is controlling the motors. In my case, a DC tach gives feedback
to the servo amp up to a several KHz bandwidth no matter what the
velocity is. The servo amp then generates armature current to keep the
velocity constant.

> Are people using 320/340 gecko drives with EMC and DeskNCrt? How are they
> typically hooking up the encoders--to the drives and the control with the
> camtronics board?

I believe people are using the Gecko 320/340 with the encoders feeding
back ONLY to the Gecko drives, not an encoder counter for the CPU.

> Is DeskNCrt just as good or complete as the Lunix based EMC?

I can't answer that. I can make some comments about the interrupt response
time under RT-Linux, with EMC. I get less than 10 uS interrupt response jitter
on a 266 MHz Pentium-II. The CPU time required to complete 4 axes of
servo computations is about 50 uS on that CPU, indicating that you could
run a servo update rate of 10 KHz without killing the CPU.

I understand that DeskNCrt uses the standard character screen, which is
a limitation. EMC uses a 'GUI' interface to the screen, that is user-configurable,
in color, has defined fields for various parts of the display. Ie. the position
display is in larger characters, and change color when the axis is at the limit
position or has been homed. There are other blocks that display state with
color or reversed video. There are slider bars to display feedrate override.
You can select files to run from a 'browser' type menu. There is an
additional script that can be loaded into the GUI to preview the toolpath.
This avoids the situation where a program contains an error that will stop
the run in the middle. Since ALL interpreter functions are actually used during
the preview, if it runs in preview, it WILL run when motion is enabled.
(This is called the 'backplot' option.)
The advantage of being able to hop into the text editor, get files through the
network, use a calculator program, etc. while EMC is running is a great advantage.

Jon