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Teensy MCU

Teensy Setup

The Teensy 3.2 microcontroller (MCU) is used to get the ticks from the encoders attached to the motors and send this information (counts) as a message over the /diffbot/ticks_left and /diffbot/ticks_right ropics. For this rosserial is running on the Teensy MCU which allows it to create a node on the Teensy that can communicate with the ROS Master running on the Raspberry Pi.

To setup rosserial on the work PC and the Raspberry Pi the following package has to be installed:

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sudo apt install ros-noetic-rosserial

To program the Teensy board with the work PC the Arduino IDE with the Teensyduino add-on can be used. Other options are to use PlatformIO plugin for VSCode. How to install the Arduino IDE and Teensyduino is listed in the instructions on the Teensy website. Here the instructions to setup Teensyduino in Linux are listed:

  1. Download the Linux udev rules (link at the top of this page) and copy the file to /etc/udev/rules.d. sudo cp 49-teensy.rules /etc/udev/rules.d/
  2. Download and extract one of Arduino's Linux packages. Note: Arduino from Linux distro packages is not supported.
  3. Download the corresponding Teensyduino installer.
  4. Run the installer by adding execute permission and then execute it. chmod 755 TeensyduinoInstall.linux64 ./TeensyduinoInstall.linux64

The first step can be used on the work PC and the Raspberry Pi to enable the USB communication with the Teensy board. Step two of these instructions are only necessary on the work PC to actually program the Teensy board.

Note

Make sure to download the Arduino IDE from the website and don't install it from the Ubuntu repositories.

The following video shows installation process, more instructions to setup the Arduino IDE can be found in the ROS wiki.

To check if the connection to the Teensy board works use these commands on the Raspberry Pi:

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$ lsusb
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 003: ID 16c0:0483 Van Ooijen Technische Informatica Teensyduino Serial
Bus 001 Device 002: ID 2109:3431 VIA Labs, Inc. Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

And to see on which serial port it is connected use:

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$ ls /dev | grep ttyACM0
ttyACM0

If the output is empty it might be the case that the board is connected to another port like ttyUSB0.

Encoder Program

When installing Teensyduino new example programs are provided. One of them is to test Encoders. The code for the motor encoders uses it as basis together with a pubsub example from rosserial:

TODO link to code encoders.ino

After the program is flashed to the Teensy board it can be tested with the following procedure:

  1. Start a ROS master by executing roscore in a new terminal.
  2. Create a rosserial node using rosserial_python package:
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$ rosrun rosserial_python serial_node.py _port:=/dev/ttyACM0 _baud:=115200
[INFO] [1602784903.659869]: ROS Serial Python Node
[INFO] [1602784903.692366]: Connecting to /dev/ttyACM0 at 115200 baud
[INFO] [1602784905.809722]: Requesting topics...
[INFO] [1602784905.824418]: Note: publish buffer size is 512 bytes
[INFO] [1602784905.829712]: Setup publisher on /diffbot/ticks_left [std_msgs/Int32]
[INFO] [1602784905.839914]: Setup publisher on /diffbot/ticks_right [std_msgs/Int32]
[INFO] [1602784905.856772]: Note: subscribe buffer size is 512 bytes
[INFO] [1602784905.861749]: Setup subscriber on /reset [std_msgs/Empty]

In case of the following error, probably the wrong program is flashed to the Teensy board:

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[ERROR] [1602782376.724880]: Unable to sync with device; possible link problem or link software version mismatch such as hydro rosserial_python with groovy Arduino

Note

Note that the rosserial node needs to be stopped to flash new sketches to the Teensy board.

Each DG01D-E motor has two signal pins for its built-in encoder. For these, the Teensy pins 5, 6 are used for the left encoder and 7, 8 are used for the right one, see also the Teensy pinout.

Teensy 4.0 Pins.

TODO add connection schematic

With one motor encoder connected to pins 5, 6, echo the /encoder_ticks topic:

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rostopic echo /encoder_ticks

Rotating a wheel attached to the motor shaft 360 degree (one full turn) will increase the first value of the encoders array:

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---
header: 
  seq: 190323
  stamp: 
    secs: 0
    nsecs:         0
  frame_id: ''
encoders: [0, 0]
---
header: 
  seq: 190324
  stamp: 
    secs: 0
    nsecs:         0
  frame_id: ''
encoders: [230, 0]
---
header: 
  seq: 190325
  stamp: 
    secs: 0
    nsecs:         0
  frame_id: ''
encoders: [350, 0]
---
header: 
  seq: 190326
  stamp: 
    secs: 0
    nsecs:         0
  frame_id: ''
encoders: [480, 0]
---
header: 
  seq: 190327
  stamp: 
    secs: 0
    nsecs:         0
  frame_id: ''
encoders: [540, 0]

The found value 540 for a full turn of the wheel is important for the hardware interface.

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