A Study on Altitude Control of an Autonomous Vehicle (First Report)

Abstract

A guidance and control system for an autonomous vehicle of a cruising type with a rudder and elevator is studied, which is intended to investigate the surface of ocean ground continuously. In order to collect underwater information continuously taking pictures or measuring chemical characteristics, it is required to keep low altitude from the surface on the condition that it keeps the safety distance.
In the paper, a simplified altitude control system to keep predetermined height is investigated observing the distances to the surface by several narrow ultra sonic fan beams in different directions. The control system is consist of two hierarchical control blocks, i. e. a guidance block to give attitude reference angle calculated from the observed distances to keep desirable altitude, and a local attitude control block to realize the reference angle keeping the stability of the vehicle. As the attitude control system, a linear feedback elevator angle control system with velocity saturation is adopted, taking account of the system dynamics.
In the guidance block, pitch correction angle is calculated from the observed distances. Three different algorithms, linear, polynomial filtering, and mixed type, are proposed. And the system performance of each is evaluated from various surface conditions by simulation study using simplified two dimensional model in vertical (pitch) plane. Non-symmetric smoothing of the correction angle is also introduced to keep the safety distance for rough surfaces. From the study, it is assured that the proposed control system can control the vehicle to keep the predetermined altitude for most cases within 20% margin, and basic feasibility of the system is verified.