Arduino

Serial protocol

We use a binary protocol for the communication between the Intel NUC and the Arduino Nano. The protocol uses COBS encoding for packet framing. Each packet consists of a flag indicating the message type and data. The data length is defined for each message type differently.

On the Arduino side the communication is being handled through the PacketSerial library and on the Intel NUC side through the ROS package arduino_communication. The communication is performed at 115200 baud.

Message types

We defined the following message types:

enum class MessageType :uint8_t {
    DEBUG,
    INFO,
    WARN,
    ERROR,
    SPEED_CMD,
    STEERING_CMD,
    LED_CMD,
    STEERING_ANGLE,
    TICKS,
    IMU,
    VOLTAGE,
    HEARTBEAT,
    IMU_CALIBRATION
};

Type	Info
DEBUG	Logging information with severity DEBUG. Will be broadcast as ROS_DEBUG in ROS
INFO	Logging information with severity INFO. Will be broadcast as ROS_DEBUG in ROS
WARN	Logging information with severity WARN. Will be broadcast as ROS_DEBUG in ROS
ERROR	Logging information with severity ERROR. Will be broadcast as ROS_DEBUG in ROS
SPEED_CMD	int16_t Value between -1000 and 1000 defining the PWM signal to send to the speed motor
STEERING_CMD	uint16_t defining the PWM signal to send to the steering servo motor
LED_CMD	char* LED command (turn left, turn right, etc.)
STEERING_ANGLE	uint16_t Steering angle feedback between 0 and 1024
TICKS	uint8_t Captured encoder ticks from the speed motor between one time interval of 0.01s
IMU	uint8_t[23] MPU6050 data consisting of orientation, acceleration, and rotational velocity
VOLTAGE	Averaged battery voltage from readings in the last 3 seconds.
HEARTBEAT	Heartbeat command that must be send at least every 30ms else the motor will stop
IMU_CALIBRATION	Perform MPU6050 calibration during this time no input will be handled and the car must be stable (takes > 5 minutes)

ROS Node

The ROS node arduino_communication manages the communication with the arduino. All of the above message types are mapped to subscribers and publishers in this node. The heartbeat is send to the Arduino at 100hz.

Publishers

Topic	Type	Description
~voltage	autominy_msgs/Voltage	Battery voltage [V]
~ticks	autominy_msgs/Ticks	Number of ticks from speed motor in 0.01s
~steering_angle	autominy_msgs/SteeringFeedback	Steering angle feedback voltage from steering servo motor [V]
~imu	sensor_msgs/IMU	IMU data from MPU6050 (if enabled)
~imu/temperature	sensor_msgs/Temperature	IMU temperature from MPU6050 (if enabled)

Subscribers

Topic	Type	Description
~steering	autominy_msgs/SteeringPWMCommand	Steering motor command in PWM Signal duty
~speed	autominy_msgs/SpeedPWMCommand	Speed motor command in PWM Signal duty

Services

Topic	Type	Description
~calibrate_imu	std_msgs/Empty	Calibrates the MPU6050 (if enabled). Takes ~5 minutes and the car must be stable during calibration.