Sensor Parts

IMU, GPS, encoders, and other sensor parts for Donkeycar

Sensor parts provide additional data about your vehicle’s state and environment.

IMU (Inertial Measurement Unit)

Measures acceleration, rotation, and orientation.

Location: donkeycar/parts/imu.py:9

Supported Sensors

• MPU6050: 6-axis (accelerometer + gyroscope)
• MPU9250: 9-axis (accelerometer + gyroscope + magnetometer)

from donkeycar.parts.imu import IMU, SENSOR_MPU6050, SENSOR_MPU9250

imu = IMU(
    addr=0x68,                      # I2C address
    poll_delay=0.0166,              # Poll interval (60Hz)
    sensor=SENSOR_MPU6050,          # Sensor type
    dlp_setting=0                   # Digital low-pass filter (0=disabled)
)

Installation:

# For MPU6050
sudo apt install python3-smbus
pip install mpu6050-raspberrypi

# For MPU9250  
pip install mpu9250-jmdev

Enable I2C:

sudo raspi-config
# Interface Options -> I2C -> Enable

Usage:

# Add to vehicle
V.add(imu, 
      outputs=['imu/accel_x', 'imu/accel_y', 'imu/accel_z',
               'imu/gyro_x', 'imu/gyro_y', 'imu/gyro_z', 'imu/temp'],
      threaded=True)

# Or run directly
accel_x, accel_y, accel_z, gyro_x, gyro_y, gyro_z, temp = imu.run()

Outputs:

• Accelerometer (m/s²): Linear acceleration on X, Y, Z axes
• Gyroscope (rad/s): Angular velocity on X, Y, Z axes
• Temperature (°C): Die temperature
• Magnetometer (MPU9250 only, μT): Magnetic field on X, Y, Z axes

Digital Low-Pass Filter:

# Common DLP settings (gyro/accel bandwidth)
# 0: Disabled (8kHz/1kHz)
# 1: 184Hz/188Hz
# 2: 94Hz/98Hz
# 3: 44Hz/42Hz
# 4: 21Hz/20Hz
# 5: 10Hz/10Hz
# 6: 5Hz/5Hz

imu = IMU(sensor=SENSOR_MPU6050, dlp_setting=3)  # 44Hz filter

Testing:

cd ~/mycar
python -c "from donkeycar.parts.imu import IMU; import time; imu = IMU(); [print(imu.run()) or time.sleep(0.1) for _ in range(10)]"

GPS

Global positioning from NMEA-compatible GPS modules.

Location: donkeycar/parts/gps.py

GpsNmeaPositions

Converts NMEA sentences to UTM coordinates.

from donkeycar.parts.gps import GpsNmeaPositions
from donkeycar.parts.serial_port import SerialPort, SerialLineReader

# Open serial port
serial_port = SerialPort(
    port='/dev/ttyUSB0',  # Or /dev/ttyAMA0 for Pi GPIO
    baudrate=9600,
    timeout=0.5
)

# Read NMEA lines
line_reader = SerialLineReader(
    serial_port,
    max_lines=10
)

# Convert to positions
gps_reader = GpsNmeaPositions(debug=False)

# Add to vehicle
V.add(line_reader, outputs=['nmea/lines'], threaded=True)
V.add(gps_reader, inputs=['nmea/lines'], outputs=['gps/positions'])

Outputs:

• List of (timestamp, x, y) tuples
• x, y in UTM coordinates (meters)

GpsPosition

Combined GPS reader that returns the latest position.

Location: donkeycar/parts/gps.py:54

from donkeycar.parts.gps import GpsPosition
from donkeycar.parts.serial_port import SerialPort

serial_port = SerialPort('/dev/ttyUSB0', baudrate=9600)
gps = GpsPosition(serial=serial_port, debug=False)

V.add(gps, outputs=['gps/timestamp', 'gps/x', 'gps/y'], threaded=True)

NMEA Parsing:

Supports GPRMC and GNRMC sentences:

$GPRMC,003918.00,A,3806.92281,N,12235.64362,W,0.090,,060322,,,D*67

Parsed to:

• Latitude/Longitude in degrees
• Converted to UTM (x, y) in meters
• Timestamp from sentence

Hardware Setup:

# For USB GPS
ls /dev/ttyUSB*  # Find device

# For GPIO serial (disable console first)
sudo raspi-config
# Interface Options -> Serial Port
#   -> "Would you like a login shell accessible over serial?" No
#   -> "Would you like the serial port hardware enabled?" Yes

# Check
ls /dev/ttyAMA0 /dev/serial0

Installation:

pip install pynmea2 pyserial utm

Testing:

# Raw NMEA output
cat /dev/ttyUSB0

# Test with script
cd ~/mycar
python -m donkeycar.parts.gps --serial /dev/ttyUSB0 --baudrate 9600

Encoders

Measure wheel rotation for odometry and velocity.

Note: The encoder.py module is deprecated. Use tachometer.py and odometer.py instead.

Rotary Encoder (Legacy)

Location: donkeycar/parts/encoder.py:128

GPIO-based rotary encoder (deprecated in favor of donkeycar.parts.tachometer.Tachometer).

from donkeycar.parts.encoder import RotaryEncoder

encoder = RotaryEncoder(
    mm_per_tick=0.306096,  # Distance per encoder tick
    pin=13,                 # GPIO pin
    poll_delay=0.0166,     # Poll rate (60Hz)
    debug=False
)

V.add(encoder, 
      inputs=['user/throttle'],
      outputs=['enc/velocity'],
      threaded=True)

Requires: pigpio

sudo apt-get install pigpio python3-pigpio
sudo systemctl enable pigpiod
sudo systemctl start pigpiod

Calibration:

1. Measure wheel circumference (C) in mm
2. Count encoder ticks per revolution (T)
3. Set mm_per_tick = C / T

Outputs:

• velocity (m/s): Current speed
• distance (m): Total distance traveled

Tachometer (Modern)

For current speed measurement, use:

from donkeycar.parts.tachometer import Tachometer, GpioEncoder

encoder = GpioEncoder(pin=13)
tach = Tachometer(encoder, meters_per_tick=0.000306096)

V.add(tach, outputs=['enc/velocity'], threaded=True)

Odometer (Modern)

For distance measurement:

from donkeycar.parts.odometer import Odometer

odometer = Odometer(tachometer=tach)

V.add(odometer, 
      inputs=['enc/velocity'],
      outputs=['enc/distance'])

Ultrasonic Sensors

For obstacle detection and distance measurement.

Location: Custom implementation needed

import RPi.GPIO as GPIO
import time

class UltrasonicSensor:
    def __init__(self, trigger_pin, echo_pin):
        self.trigger = trigger_pin
        self.echo = echo_pin
        GPIO.setmode(GPIO.BCM)
        GPIO.setup(self.trigger, GPIO.OUT)
        GPIO.setup(self.echo, GPIO.IN)
        
    def run(self):
        # Send trigger pulse
        GPIO.output(self.trigger, GPIO.HIGH)
        time.sleep(0.00001)
        GPIO.output(self.trigger, GPIO.LOW)
        
        # Measure echo
        while GPIO.input(self.echo) == GPIO.LOW:
            start = time.time()
        while GPIO.input(self.echo) == GPIO.HIGH:
            end = time.time()
            
        # Calculate distance
        duration = end - start
        distance = duration * 34300 / 2  # cm
        return distance
    
    def shutdown(self):
        GPIO.cleanup()

# Usage
sensor = UltrasonicSensor(trigger_pin=23, echo_pin=24)
V.add(sensor, outputs=['ultrasonic/distance'])

LIDAR

For 2D/3D environment scanning.

Location: donkeycar/parts/lidar.py (if available)

# Example for RPLidar
from rplidar import RPLidar

class LidarPart:
    def __init__(self, port='/dev/ttyUSB0'):
        self.lidar = RPLidar(port)
        self.running = True
        self.scan_data = []
        
    def update(self):
        for scan in self.lidar.iter_scans():
            if not self.running:
                break
            self.scan_data = scan
    
    def run_threaded(self):
        return self.scan_data
    
    def shutdown(self):
        self.running = False
        self.lidar.stop()
        self.lidar.disconnect()

Configuration

In myconfig.py:

# IMU
HAVE_IMU = True
IMU_SENSOR = 'mpu6050'  # or 'mpu9250'
IMU_ADDRESS = 0x68
IMU_DLP_CONFIG = 0

# GPS
HAVE_GPS = True
GPS_SERIAL_PORT = '/dev/ttyUSB0'
GPS_BAUDRATE = 9600

# Encoder
HAVE_ODOM = True
ENCODER_TYPE = 'GPIO'  # or 'ARDUINO'
ENCODER_PIN = 13
MM_PER_TICK = 0.306096
ENC_POLL_RATE = 60

# Ultrasonic
HAVE_ULTRASONIC = False
ULTRASONIC_TRIGGER_PIN = 23
ULTRASONIC_ECHO_PIN = 24

Data Recording

Add sensor outputs to TubWriter:

# In manage.py
inputs = ['cam/image_array',
          'user/angle', 'user/throttle', 'user/mode',
          'imu/accel_x', 'imu/accel_y', 'imu/accel_z',
          'imu/gyro_x', 'imu/gyro_y', 'imu/gyro_z',
          'gps/x', 'gps/y',
          'enc/velocity']

types = ['image_array',
         'float', 'float', 'str',
         'float', 'float', 'float',
         'float', 'float', 'float', 
         'float', 'float',
         'float']

Using Sensor Data in Models

For IMU-augmented models:

from donkeycar.parts.keras import KerasPilot

class ImuPilot(KerasPilot):
    def run(self, img_arr, accel_x, accel_y, accel_z, gyro_x, gyro_y, gyro_z):
        # Use IMU data along with image
        return self.inference_from_dict({
            'img_in': normalize_image(img_arr),
            'imu_in': np.array([accel_x, accel_y, accel_z, 
                                gyro_x, gyro_y, gyro_z])
        })

Common Issues

IMU Not Detected

# Check I2C
sudo i2cdetect -y 1
# Should show 0x68 or 0x69

# If not found, check connections and enable I2C
sudo raspi-config

GPS No Fix

• Ensure antenna has clear view of sky
• Wait 30-60 seconds for initial fix (cold start)
• Check NMEA output: cat /dev/ttyUSB0
• Verify GPS LED is blinking

Encoder Not Counting

• Check GPIO pin connections
• Verify pigpio daemon is running: sudo systemctl status pigpiod
• Test with multimeter or oscilloscope
• Check for proper pull-up/pull-down resistors

Serial Port Permission Denied

# Add user to dialout group
sudo usermod -a -G dialout $USER
# Log out and back in

# Or temporarily
sudo chmod 666 /dev/ttyUSB0

Next Steps

• Keras Models - Use sensor data in AI models
• Data Stores - Record sensor data
• Custom Parts - Create custom sensors