Libsensorimotor

Author(s):

Matthias Kubisch
kubisch@informatik.hu-berlin.de
Last update: March 2019

Clone the repository

Check that 'git' is installed:

git --version

and install if needed:

sudo apt install git

Clone the repository:

git clone https://github.com/suprememachines/libsensorimotor.git

and navigate to the newly created working directory:

cd libsensorimotor

Building the shared library

The build system used to create the shared library from sources is 'scons'. Check if scons is already installed on your system:

scons -v

To install scons type

sudo apt install scons

To build the shared library just type

scons

the build process should terminate with the message

'scons: done building targets.'

and you will find the freshly build library in the binary directory

bin/libsensorimotor.so

Using the sensorimotor python interface

Before you proceed with programming, make sure you have already set up your USB-to-Serial interface in low-latency mode (see below) and that each of the connected sensorimotors has a unique ID (see 'setting id')

Setting up Serial Devices

For setting up your USB-to-Serial/RS485 device to best work with sensorimotors you have to enable low latency mode on your machine. Low latency mode is needed to have the best performance on higher baudrates and with multiple motors attached to the bus. Otherwise some machines tend to buffer data packages and send them delayed which is NOT recommended when using sensorimotors, because the library needs a tight real-time loop here. In order to temporarily set your interface to low-latency mode type:

setserial /dev/ttyUSB0 low_latency

when e.g. /dev/ttyUSB0 is your device. If setserial is not installed on your linux machine type

sudo apt install setserial

If that works out as expected you can make this permanent when creating a udev rule file, e.g

sudo nano /etc/udev/rules.d/50-ttyusb.rules 

and put in

KERNEL=="ttyUSB[0-9]*", MODE="0666", RUN+="/bin/setserial /dev/%k low_latency"
KERNEL=="ttyACM[0-9]*", MODE="0666", RUN+="/bin/setserial /dev/%k low_latency"

The udev rules get activated when your reconnect your USB-to-serial device.

Further reading on how to write udev kernel rules see: http://www.reactivated.net/writing_udev_rules.html

Connecting Serial Devices

Despite the fact that each RS485 interface with 1Mbaud rate capability should work with the Sensorimotors, we have successfully tested communication with the following device:

• FTDI USB-RS485-PCB
• Digitus DA-70157

However we cannot recommend the DA-70157 device in ESD risky environments.

Connect the RS485 device only with 3 pins:

• Data+
• Data-
• GND

Do not power the sensorimotor from your machines USB Bus, since motor control will easily exceed the maximum current of your machine's USB port. Always use a stabilized 12V (DC) voltage source. Maximum current draw of a single Sensorimotor is up to 6000mA which obviously depends on your specific application, so be prepared and estimate your power needs in advance and avoid overloading weak power sources.

TODOs for documentation:

describe modes

• setting IDs

• position

• velocity

• hold/break

• voltage control

• setting limits

  • voltage limit
  • position limit