An optical flow module estimates velocity by analyzing the apparent motion of features in sequential images captured by a downward-facing camera, often combined with a distance sensor to convert angular motion into linear velocity.

Core Principle

Optical flow modules operate by detecting motion in the visual field. A downward-facing camera captures consecutive images of the surface below. The module tracks the displacement of visual features (such as texture patterns or particles) between frames. This displacement, measured in pixels, represents the angular motion of the image relative to the sensor's body axes. Forward movement causes rotation of the ground image about the Y axis, while lateral movement causes rotation about the X axis, due to the camera's perspective .

Integration with Distance Measurement

To convert angular motion into linear velocity, optical flow modules often use a distance sensor, such as LiDAR or ultrasonic sensors. The distance from the sensor to the ground allows the system to scale the angular displacement into real-world speed, enabling accurate velocity estimation even in GNSS-denied environments like indoors or underground .

Data Processing and Output

The optical flow sensor outputs angular measurements in radians, which are then fused with other velocity data sources in the flight controller or navigation system. The sensor communicates via protocols like MAVLink or I2C, publishing messages such as OPTICAL_FLOW_RAD for angular flow and DISTANCE_SENSOR for altitude. These data streams are used to maintain position, stabilize flight, or navigate autonomously .

Alternative Optical Flow Techniques

In industrial applications, optical flow can also be measured using Laser-Two-Focus (L2F) technology. Here, two laser beams detect light-scattering particles in a fluid. The time delay between pulses generated as particles pass through the beams is inversely proportional to particle velocity, analogous to measuring vehicle speed between two marks on a road . While this is more common in fluid flow measurement, the principle of detecting motion via sequential optical signals is conceptually similar.

Summary

An optical flow module combines image capture, feature tracking, and distance measurement to estimate velocity. By analyzing the apparent motion of the ground or particles, it provides real-time velocity data for navigation, stabilization, and control in environments where GPS is unavailable, making it essential for drones, robotics, and industrial flow monitoring .