Basic setup

Motor used

For this project we are using a DCX16S GB KL 12V DC motor, ENX16 EASY Absolute SSI encoder, and GPX16 A 21:1 gearbox, all of them made by Maxon motor.

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Electronic Speed Control (ESC)

We are using a FingerTech Robotics TinyESC v2.4 (specifications), a bi-directional brushed motor controller that can take battery voltage from 6.5 to 36 volts, and output a continuous current of 1.5A (3.0A peak). It includes undervoltage, overcurrent backwards polarity, and a BEC (battery eliminator circuit) protection.

The tinyESC is equipped with a 2-pin calibration connector, a 3-pin JST connector for your SIG, 5V, and GND lines to access a receiver, two bare lead wires for M1 and M2 to connect to your motor, and another set of two bare lead wires for Batt and GND to connect to a battery.

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Microcontroller

The motor that makes the Hokuyo LIDAR spin around its axis is controlled by a Teensy 3.2 board, with a 72MHz ARM Cortex M4 microcontroller.

Programming environment setup

In order to program the Teensy, you need to download the Arduino IDE, and the Teensyduino add-on.

Then, you'll need to add your Linux user to the dialout group, by running the following command:

sudo usermod -a -G dialout $USER

Next, copy the udev rules of the Teensy, so that you can always have read/write permissions on the serial port it gets assigned when you plug it in:

cd $DD2465_WS/src/spinning_lidar/spinning_lidar_motor_control/
sudo cp 49-teensy-udev.rules /etc/udev/rules.d/

Configuration of rosserial-arduino

NOTE: Before proceeding make sure that you have followed the procedure described in the Installation section.

Before uploading the code into the Teensy you'll need to build locally the rosserial_arduino dependencies. This is because we have some custom ROS messages and services for manipulating the motor remotely.

Run the following commands to build the required dependencies:

cd ~/Arduino/libraries/
rm -r ros_lib/
rosrun rosserial_arduino make_libraries.py .

If everything went well, you should be able to scroll up, and see the following message printed:

Exporting spinning_lidar_motor_control

  Messages:
    MotorState,

  Services:
    TurnMotorOnOff,ChangeTargetVelocity,

Teensy motor control code

The Arduino files are located in spinning_lidar_motor_control/teensy_motor_control/.

The main file is teensy_motor_control.ino, and it has two headers:

• motor_control.h: this is where all the code related to motor control is stored (PINs setup, encoders and PID)
• ros_comm.h: this is where all the ROS-related code is stored (node handler, publishers and services).

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ROS motor control interfaces

Teensy serial interface node

To send command to the motor and read its state, this node has to be running. To launch it, run:

roslaunch spinning_lidar_launch motor_control.launch

Motor state topic

From the Teensy we publish a topic /spinning_lidar/motor_state that contains the current angle and velocity of the sensor, the desired speed, and if the motor is stopped or not.

header: 
  seq: 107
  stamp: 
    secs: 1512996391
    nsecs: 184720022
  frame_id: "/laser_axis"
stopped: True
curr_angle: 0.0
offset_angle: 0.0
curr_vel: 0.0
des_vel: 2.35619449615

Motor on/off service

The service /spinning_lidar/turn_motor_onoff can be called to start/stop the motor.

Example:

rosservice call /spinning_lidar/turn_motor_onoff "stopped: false"

Velocity change service

The service /spinning_lidar/change_motor_vel can be called to start/stop the motor and change the desired velocity.

Example:

rosservice call /spinning_lidar/change_motor_vel "stopped: false
rot_vel: 3.00"