py-opc-ng

A small python library to operate Alphasense OPC devices.

Supports most modern OPC devices: OPC-N2, OPC-N3, OPC-R1 and OPC-R2.

Installation

$ pip install py-opc-ng

Dependencies

The devices work with SPI interface. You can either connect them directly to a SPI bus using py-spidev (e.g. with the GPIO pins of a RaspberryPi) or use a SPI to USB device, like the one Alphasense provides, using the pyusbiss library.

Getting help

You can find full documentation, including API references, at http://py-opc-ng.readthedocs.io/

Usage

With a direct SPI connection:

from time import sleep
import spidev
import opcng as opc

spi = spidev.SpiDev()
spi.open(0, 0)
spi.mode = 1
spi.max_speed_hz = 500000
spi.lsbfirst = False

dev = opc.detect(spi)

print(f'device information: {dev.info()}')
print(f'serial: {dev.serial()}')
print(f'firmware version: {dev.serial()}')

# power on fan and laser
dev.on()

for i in range(10):
    # query particle mass readings
    sleep(1)
    print(dev.pm())

# power off fan and laser
dev.off()

or with a SPI to USB bridge:

from time import sleep
from usbiss.spi import SPI
import opcng as opc

spi = SPI('/dev/ttyACM0')
spi.mode = 1
spi.max_speed_hz = 500000
spi.lsbfirst = False

dev = opc.detect(spi)

print(f'device information: {dev.info()}')
print(f'serial: {dev.serial()}')
print(f'firmware version: {dev.serial()}')

# power on fan and laser
dev.on()

for i in range(10):
    # query particle mass readings
    sleep(1)
    print(dev.pm())

# power off fan and laser
dev.off()

Known issues

OPC N2 and N3 have a so-called autonomous mode where the device operates on its own saving data in the included sd-card. When you power on the devices and don't do anything for some time (about 65s for N3) the automatically enter this standalone mode and start acquiring data. See e.g. Chapter 9 in OPC N3 manual.

When this happens the device still listens for SPI commands but sometimes (most of the time?) fails to respond and behaves erratically.

When you try to access the device while in this mode you may get errors like:

>>> opc.detect()
ERROR:opcng:Error while reading bytes from the device: Received unexpected response 0x00 for command: 0x3F
ERROR:opcng:Something failed while reading byte sequence, expected size: 60, received: 0
ERROR:opcng:Could not detect a valid OPC device

I don't have a solution yet, see discussion in issue #1. The solution is do not let the device enter this mode. As soon as you power up your host or connect the OPC send some SPI command to prevent this autonomous mode. If it enters it before you have the chance to interact with it the best way out is to power cycle the device or reboot the host.

Not much of an issue as these are devices that should be operated continously so you should start them as soon as you boot your host.

Nonetheless, if you know a way to disable this mode please do let me know.

A note about the name

When this project was started the most popular library to operate Alphasense OPC devices was py-opc. At the time it only supported OPC-N2 and had a lot of code very specific to that device generation.

Adding support for next generation (hence the -ng) devices there seemed to require quite some effort. We wanted to abstract common characteristics of the different devices in a single interface right from the start. We also didn't want to support all the quirks they had for N2 different firmware versions as we were already moving away from N2 devices.

So we opted to start a completely new project. It's not a fork and doesn't share any code with py-opc.

License

This module is licensed under the GNU Lesser General Public License Version 3. Full text can be found in the LICENSE file.

Acknowledgement

This package was developed within the Cagliari2020 project with the support of Istituto Nazionale di Fisica Nucleare (INFN)