# Vertical Landing for Micro Air Vehicles using Event-Based Optical Flow

## TL;DR

This paper introduces an innovative event-based optical flow method integrated into a quadrotor's control system, enabling fast and accurate landing maneuvers by overcoming the limitations of traditional frame-based cameras.

## Contribution

It presents the first integration of event-based optical flow estimation into a flying robot's control loop, improving computational efficiency and speed for landing maneuvers.

## Key findings

- Accurate divergence estimates over a wide range of speeds.
- Enables very fast landing maneuvers.
- Validates the method with real event sequences.

## Abstract

Small flying robots can perform landing maneuvers using bio-inspired optical flow by maintaining a constant divergence. However, optical flow is typically estimated from frame sequences recorded by standard miniature cameras. This requires processing full images on-board, limiting the update rate of divergence measurements, and thus the speed of the control loop and the robot. Event-based cameras overcome these limitations by only measuring pixel-level brightness changes at microsecond temporal accuracy, hence providing an efficient mechanism for optical flow estimation. This paper presents, to the best of our knowledge, the first work integrating event-based optical flow estimation into the control loop of a flying robot. We extend an existing 'local plane fitting' algorithm to obtain an improved and more computationally efficient optical flow estimation method, valid for a wide range of optical flow velocities. This method is validated for real event sequences. In addition, a method for estimating the divergence from event-based optical flow is introduced, which accounts for the aperture problem. The developed algorithms are implemented in a constant divergence landing controller on-board of a quadrotor. Experiments show that, using event-based optical flow, accurate divergence estimates can be obtained over a wide range of speeds. This enables the quadrotor to perform very fast landing maneuvers.