An autonomous vehicle is a vehicle that is capable of navigating and driving with no human intervention whatsoever through the utilization of various sensors and positioning systems. The possible applications of autonomous vehicles are widespread, ranging from the aerospace industry to the mining and military sectors where the exposure of human operators to the operating conditions is hazardous to their health and safety. Automobile accidents have become the leading cause of death in certain segments of the world population. Removing the human driver from the decision-making process through automation may result in significantly safer highways. Although full autonomy may be the ultimate goal, there is huge scope for systems that aid the driver in decision making or systems that take over from the driver under conditions where the human driver fails.
The aim of the longitudinal control system to be implemented on the Land Rover test vehicle in this study is to improve the vehicle’s safety by controlling the vehicle’s longitudinal behaviour. A common problem with sports-utility-vehicles is the low rollover threshold, due to a high centre of gravity. Rather than modifying the vehicle to increase the rollover threshold, the aim of the control system presented here is to prevent the vehicle from exceeding speeds that would cause the vehicle to reach its rollover threshold.
In order to develop a control system that autonomously controls the longitudinal degree of freedom, a model of the test vehicle (a 1997 Land Rover Defender 110 Wagon) was developed in MSC.ADAMS/View and validated experimentally. The model accurately captures the response of the test vehicle to supply forces as generated by the engine and demand forces applied through drag, braking and engine braking. Furthermore, the model has been validated experimentally to provide reliable simulation results for lateral and vertical dynamics.
The control system was developed by generating a reference speed that the vehicle must track. This reference speed was formulated by taking into account the vehicle’s limits due to lateral acceleration, combined lateral and longitudinal acceleration and the vehicle’s performance capabilities. The control system generates the desired throttle pedal position, hydraulic pressure in the brake lines, clutch position and gear selection as output. The MSC.ADAMS\View model of the test vehicle was used to evaluate the performance of the control system on various racetracks of which the GPS coordinates were available. The simulation results indicate that the control system performs as expected.
Finally, the control system was implemented on the test vehicle and the performance was evaluated by conducting field tests in the form of a severe double lane change manoeuvre. The results of the field tests indicated that the control system limited the acceleration vector of the vehicle’s centre of gravity to prescribed limits, as predicted by the simulation results.
Dissertation (MEng)--University of Pretoria, 2013.