3 Axis USB Rotary Encoder Feedback

Apparently Peter was able to use an optical 'encoder' connected to a parallel port pin to control spindle speed.

My system has three off-the-shelf USB rotary 2-channel optical encoders, xyz on a mill, along with three USB hardware interfaces. The mill is currently running open-loop.

So I would like to input the position values into the Axis GUI and do some feedback operations such as limits for motor stall, correct for missing steps, etc.

There is a gcc Linux compatible C/C++ library for the encoders using a linked in library to allow reading the encoder position, set up event handlers, etc. It is simple enough to get the data using the library and display it in real time as the mill runs. The nice thing about this interface is that it has absolute position and knows which direction the encoder is spinning - without having to count each step.

The question is, how to input the data into the Axis interface? Could I just store it in a file and use their event handler to update the file, then read it somehow into EMC, or could it even be integrated directly into EMC as either an event handler or as a readPositon call?

Note: I have an older version of the encoder but it is dual-channel just like the newer one to discern direction.

This is how to get the values from the USB device:
Library to link in -lphidget21
Header file: #include <phidget21.h>
C file Encoder-simple.c in download phidget21-c-examplesNNN.tar.gz

Here is a link to the various downloads:
Documentation: www.phidgets.com/documentation/Tutorials...d_Standard_C-C++.pdf
Example Code: www.phidgets.com/downloads/examples/phid....1.8.20120216.tar.gz
GNU library for Linux: www.phidgets.com/downloads/libraries/lib....1.8.20120216.tar.gz
Encoder info: www.phidgets.com/products.php?product_id=1057

Thanks

Alfa

Andy,

tried
less /proc/bus/input/devices

per the article, but they didn't show up. Installed libusb-dev as well with no joy.

Is this ability now available with the new linuxCNC vs EMC?

lizard wrote:

My system has three off-the-shelf USB rotary 2-channel optical encoders, xyz on a mill, along with three USB hardware interfaces. The mill is currently running open-loop.

So I would like to input the position values into the Axis GUI and do some feedback operations such as limits for motor stall, correct for missing steps, etc.


Alfa

The problem with this is that USB is not a real time mechanism, and in some modes can only
process one message per frame (1 ms). If the device sends position updates automatically,
this may be once per frame, but it won't be synched with the LinuxCNC servo thread rate.
So, some servo cycles will get a reading, and some won't. So, there could be a +/-
1 ms jitter in when these encoder position readings arrive for processing. Also, at
least so far, there is no real time driver for USB, so it will be VERY hard to get
timely position updates to the motion system of LinuxCNC.

Jon

lizard wrote:

Encoder info: www.phidgets.com/products.php?product_id=1057

Alfa

Ah, there is is in their datasheet, update rate 125 samples/second.
What "1 ms time resolution" means is a mystery, but I guess it means it sends
the USB packet in the first USB frame after the measurement was taken.
I can't see how this can be used to correct for lost steps or any other
motion assurance function.

Jon

Jon ,

yes i'd agree with you , even with all the timing issues with usb , I don't think you'd get the data through in a reliable and consistant time frame

As this is a stepper system it isn't for the feedback loop - doesn't have to be realtime.
I'll try to see it in the HAL as andypugh suggests.
Another option might be to disconnect the phidget USB cable and talk to the encoders directly.