Bioinspired Autonomous Visual Vertical Control of a Quadrotor Unmanned Aerial Vehicle

Abstract

Near ground maneuvers, such as hover, approach and landing, are key elements of autonomy in unmanned aerial vehicles. Such maneuvers have been tackled conventionally by measuring or estimating the velocity and the height above the ground often using ultrasonic or laser range finders. Near ground maneuvers are naturally mastered by flying birds and insects as objects below may be of interest for food or shelter. These animals perform such maneuvers efficiently using only the available vision and vestibular sensory information. In this paper, the time-to-contact (Tau) theory, which conceptualizes the visual strategy with which many species are believed to approach objects, is presented as a solution for Unmanned Aerial Vehicles (UAV) relative ground distance control. The paper shows how such an approach can be visually guided without knowledge of height and velocity relative to the ground. A control scheme that implements the Tau strategy is developed employing only visual information from a monocular camera and an inertial measurement unit. To achieve reliable visual information at a high rate, a novel filtering system is proposed to complement the control system. The proposed system is implemented on-board an experimental quadrotor UAV and shown not only to successfully land and approach ground, but also to enable the user to choose the dynamic characteristics of the approach. The methods presented in this paper are applicable to both aerial and space autonomous vehicles.

Monocular distance estimation with optical flow maneuvers and efference copies: a stability-based strategy

“…This is indicated for the (approximations to) the values of K z at which the system becomes unstable (w 1 = -). 1 The constancy of this relation depends on the constancy of the robot's mass, which applies for landing quad rotors but not for spacecraft. A spacecraft will loose more and more mass when landing, hence aggravating the effect in equation (13).…”

“…The key to this success is that flying insects follow straightforward strategies based on optical flow observables. First, it was found that honeybees keep ventral flow (defined as x [1,24,26,28]. These strategies either enforce a decreasing time-to-contact (as humans do for car braking [30]) or keep the divergence constant (as honeybees do for straight landings [3]).…”

Survey on Computer Vision for UAVs: Current Developments and Trends

“…On-board the UAV were also mounted a sonar and an optic flow sensor for altitude, position and velocity control. In [44] a novel strategy for close distance to the ground VTOL-UAV manoeuvring like hovering around, landing and approaching a target has been described. The framework of the time-to-contact (tau) theory has been implemented for autonomous navigation.…”

Vertical landing for micro air vehicles using event‐based optical flow

“…Ventral flows may also be used for hover stabilization to augment visual vertical control. 29 In recent aerial robotic applications, mainly visual observables based on vertical motion were applied, allowing control of vertical dynamics independent of horizontal motion. One of these observables is flow-field divergence D, i.e., the ratio of vertical velocity to height.…”