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import optoforce

A package which simplifies connecting to and reading from optoforce sensors, using Python

This is mainly here to accompany my blog posts on communicating with embedded systems using python, where the optoforce sensor is an example. I hope this package is simple enough to serve as a template for others. I see that there are other packages on GitHub which help with this

I don't imagine it'll have many users, since the optoforce website redirects to another company which doesn't even mention them, and that company's optoforce page is pretty blank. BUT I use one, so perhaps there are others?

Installation

python -m pip install optoforce

Usage

From a python script:

# 16 byte frame/single-channel 3 axis force sensor (OMD-45-FH-2000N)
from optoforce import OptoForce16 as OptoForce
from optoforce.status import no_errors

with OptoForce(speed_hz=100, filter_hz=15, zero=False) as force_sensor:
    measurement = force_sensor.read(only_latest_data=False)
    assert measurement.valid_checksum
    assert no_errors(measurement.status)

    do_stuff_with_force_readings(measurement.Fx, measurement.Fy, measurement.Fz)

A call to OptoForce.read() returns a measurement packet (a NamedTuple) containing force readings and other potentially useful data. For the specifics of each sensor model, see Reading16, Reading34 and Reading22 in optoforce/reading.py. For example OptoForce34 returns force readings as Fx1, Fy1, Fz1, Fx2, ... (not Fx, Fy, Fz) as there are multiple channels

It's still a little verbose, so you may want to define shortcuts for your particular application. For example, if you don't care about anything except the vertical force:

with OptoForce() as force_sensor:
    read_fz = lambda: force_sensor.read(only_latest_data=False).Fz
    while True:
        print(read_fz())

If you haven't read the force force sensor in a little while and want to get all the packets waiting in the buffer, use:

measurements = force_sensor.read_all_packets_in_buffer()

Or from the command line, to log to a file:

$ python -m optoforce --filename force-data.csv

If you want to detect and handle sensor errors as reported in the status word, look at the doc string in optoforce/status.py. It was written for completeness after I finished my use for the OptoForce, so it hasn't been tested!

OptoForce models supported

Possibly,

  • 16 byte frame/single-channel 3 axis force sensor (OMD-45-FH-2000N)
  • 34 byte frame/multi-channel 3 axis force sensor (4 channels)
  • 22 byte frame/single-channel 6 axis force/torque sensor

but I only have access to the 16 byte frame model, so I can't test the other two. The 34 byte model has been reported to work too. I imagine this means that the 22 byte model will be fine, but the torque readings aren't scaled as I don't have that datasheet :/

Sources

OptoForce General DAQ - USB,CAN,UART - v1.7.pdf was used to implement this module

The force scale parameters are from SensitivityReport-PFH0A052.pdf

A friend mentioned that I might not be allowed to share those docs, since the company is quite secretive, and I unfortunately haven't seen them online

Common bugs

If you get permission errors when trying to open the serial port and you run linux, try running the code below (source)

$ sudo chmod 666 /dev/ttyACM0  # replace with your serial port

Permission errors can also happen on Windows -- you'll know that's the case when you get an error which includes Original message: PermissionError(...). That can happen when something else is using the device. In that case, making sure no other program is connected to the device should work. You could do that by unplugging the sensor, and plugging it back in again.

Developing

See python-template for some general tips on Python package development.

Run unit tests using pytest. You should see output similar to,

$ python -m pytest
======================================== test session starts ========================================
platform linux -- Python 3.9.6, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
rootdir: /home/alex/dev/optoforce, configfile: pyproject.toml, testpaths: tests
collected 6 items                                                                                   

tests/test_all.py ......

========================================= 6 passed in 0.02s =========================================

If you have matplotlib installed, you can run a more visual test using

$ python test-plot.py

Plot of sensor data

Publishing a new version

Install flit, which makes publishing packages ridiculously easy. You can install it as part of your development environment, or in a base/systemwide environment, as it's just used for publishing but isn't a requirement of the package itself.

Next, increase the __version__ number in optoforce/__init__.py. Then, create a (local) tag for the commit and publish:

# make a local tag of the current commit
$ git tag v0.0.1
# push local tag to remote repo
$ git push origin v0.0.1
# generate files into dist/ and upload them to pypi
$ flit publish