Impact of combined longitudinal, lateral and vertical control on autonomous road vehicle design

Abstract

This paper is to address several points related to the design of Autonomous Road Vehicles (ARV) which are understood as those equipped with automatic control systems to perform some manoeuvres completely automatically or to a certain extent with some assistance from the driver. Emphasis is put on the improvement of vehicle driving stability and string stability through vehicle control systems design. The performance of an ARV depends largely on two factors: its physical structure and performance of its control systems. Physical structure determines a vehicle's limit for acceleration, deceleration and cornering capabilities. Control system performance can be measured by driving stability and string stability. Vehicle design problems discussed in this paper is restricted to the components (sensors, actuators and communication systems) relevant to the control systems. Control system design can be divided into two levels. The upper level includes the part from vehicle dynamics to the desired forces above the suspension system. The lower level includes all the actuators (throttle, brake, steering, and active suspension systems). Driving stability is mainly determined by vehicle dynamics and road situation. It can be abstracted as a 6DOF rigid body to dynamically interact with the road surface. To consider driving stability, the coupling between three latitude (longitudinal, lateral and vertical) motions and three rotational (yaw, pitch and roll) motions needs to be taken into consideration. However, to consider all the couplings would lead to a highly nonlinear model which causes difficulties to control design and implementation. It is shown that a 5DOF model with only roll motion relative to the road ignored is feedback linearisable. Similarly, one can prove that this is true if pitch motion is ignored, with roll motion taken into consideration. String stability is a concept for longitudinal control motion, which describes the dynamic interaction between vehicles in the same lane with short inter-vehicle distances. Practical string stability depends on control design method, following strategy, signal processing and data fusion. Driving stability in the longitudinal motion is only a necessary condition for string stability.