Re: Update on closed-loop to the computer with Gecko 320

Jon,

Looks interesting. You can opto-isolate the encoder to the G320. This
allows you to:

1) Make the encoder outputs available to be read by the PC.
2) Allow the PC +5V to power high-current (>50mA) encoders.

Use a dual high-speed opto-isolator (HCPL-2531). Place the opto LEDs
as pull-up loads on the encoder channel outputs. Bias them at 20 to
25mA to permit a 5mA collector current needed for the G320 (the HCPL-
2531 has a transfer gain of 20%).

Place both emitters on ENC-, take the collectors to the CHANNEL A, B
inputs. Have a 470 ohm resistor from ENC+ to ENC-.

I assume you have done something like this in your scheme. If you
feel you are up to modifiying SMT components (a low-power microscope,
tweezers and a hot-air rework station helps), I can send you a
circuit with landmarks to modify the drive to have a proportional
loop response only. Maybe that will help.

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
> Since several people were interested in this, I thought I should
> send an update. The problem is that the Gecko 320 (and 340)
> are closed-loop servo drives, but the standard connection doesn't
> allow the CNC computer to see the encoder position. There are
> several advantages to getting the position info back to the CNC
> computer. One is to have more flexible following error limits,
> such as a sliding scale proportional to velocity. Another is
> to be able to make plots of commanded vs actual position during
> various moves, to verify the accuracy of the servo movements.
> Another advantage is that you can switch between manual and
> CNC operation while maintaining the coordinate system, and
> using EMC as a DRO when in manual mode.
>
> I have verified that this works, and that EMC can control the servo
> motor with very stable loop response. This was a concern in that
> it adds several more layers of control loops to the existing servo
> loops in the Gecko drive. I believe that I have actually gotten
much
> better servo response with this system, too. Since the I and D
terms
> are adjustable, and velocity and acceleration feed-forward are
> also available, it is more flexible. I have made a sample plot
> of a stop from moderate speed, using an Ametek servo motor and
> a 36 V power supply.
> http://jelinux.pico-systems.com/images/servo.png
>
> The encoder was powered from my USC board, and I tried a
> magnetically-coupled isolator to convey a copy of the encoder
> signals to the Gecko drive. That did not work, as the encoder's
> rise and fall times were apparently too slow for that isolator.
> I will try it with a fast opto-isolator shortly.
>
> But, the big news is that the entire scheme works, and the servo
> response is stable, and the tuning is only slightly harder than
usual.
> (The more knobs, the harder anything is to set up. The Gecko 320
> has 3 pots on it that interact with the software-set variables.)
>
> Jon