Data Output from Sensors

Overview

Sensors in robotics produce raw or processed data in specific formats. Understanding these formats is essential for integrating and processing sensor data within robotic systems. The format can depend on the sensor type, communication protocol, and software stack (e.g., ROS).

Common Data Output Formats

1. Numeric Arrays

Many sensors output data as arrays of numerical values. For example:

  • IMU: [ax, ay, az, gx, gy, gz]
  • Magnetometer: [mx, my, mz]
  • GPS: [latitude, longitude, altitude]

2. Strings (Text)

Some sensors, such as GPS receivers, output data as ASCII strings, often using protocols like NMEA. Example: $GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47

3. Bitfields / Binary Formats

High-speed sensors (e.g., cameras or LiDARs) may output data in binary formats for efficiency. These formats require decoding based on sensor datasheets.

4. JSON / XML

Smart or IoT-enabled sensors may send data in human-readable formats like JSON: { "temperature": 24.5, "unit": "C" }

5. ROS Messages

In ROS-based systems, sensor data is encapsulated in standard message types:

  • sensor_msgs/Imu for IMUs
  • sensor_msgs/LaserScan for 2D LiDAR
  • sensor_msgs/NavSatFix for GPS
  • sensor_msgs/Image for camera frames
  • sensor_msgs/PointCloud2 for 3D LiDAR

Timestamping and Synchronization

Most sensor outputs include a timestamp or require one to be added for data synchronization. This is crucial in multi-sensor fusion applications (e.g., SLAM, sensor fusion with Kalman filters).

Units and Scaling

Sensor outputs must be interpreted with correct units:

  • Accelerometers: m/s² or g
  • Gyroscopes: rad/s or deg/s
  • Distances: meters or millimeters
Some sensors may require scaling factors based on calibration or configuration.

Real-Time Considerations

Sensor output rates vary significantly:

  • IMU: 100–1000 Hz
  • LiDAR: 5–20 Hz
  • GPS: 1–10 Hz
  • Cameras: 30–60 FPS
Understanding output frequency and data size is key for real-time processing and bandwidth management.

Conclusion

Different sensors produce data in diverse formats tailored to their application and design. Roboticists must understand these output formats to interpret, process, and fuse sensor data efficiently for accurate perception and decision-making.