With the growing concern for environmental change and uncertain oil resources, the development of new vehicle concepts will in many cases include full or partial electric propulsion. The introduction of more advanced powertrains enables vehicles that can be controlled with a variety of electric actuators, such as wheel hub motors and individual steering. With these actuators, the chassis can be enabled to adjust its properties depending on the driving situation.
Manoeuvring of the vehicle, using for example electric propulsion, braking, suspension, steering and camber control may also allow a variety of combinations which, if properly utilised, can increase the outer limits of vehicle performance and safety. The fact that the vehicle has a greater number of actuators than required to control a certain number of degrees of freedom is called over-actuation. Since there is a great need for energy optimised vehicles, energy efficient control is also required. For this reason, this work is about the allocation of wheel forces can improve safety, performance and energy efficiency in future electrified vehicles in different driving situations.
Studies of optimally controlled vehicles show that performance, safety and efficiency can be improved by utilising available actuators in over-actuated vehicles. Path tracking and optimal actuator control signals are evaluated in evasive manoeuvres at low and high friction surfaces. The results show how the forces are distributed differently among the wheels, even though the resulting global forces on the vehicle are similar. Optimal control of camber angles and active suspension show that vehicle performance and safety can be greatly improved. The limits of tyre forces can be increased and better utilised in a way that a passive system is unable to achieve. Actuator performance is also shown to be important, however even low actuator performance is shown to be sufficient to improve vehicle performance considerably. Energy efficiency is also improved as unnecessary vehicle motions are minimised during normal driving and wheel forces are used in a better way.
Simplified algorithms to control available actuators, such as wheel angles, vertical actuation and propulsion torques, have been developed, based on the analysis of the results of the optimisation studies. Analyses of the impact of these simplifications have been made. For the cases studied, it has been shown that it is possible to get significantly better performance at reasonable levels of actuator performance and control complexity. This helps to simplify the introduction of this technology in electrified vehicles.
Control allocation is a method that distributes the wheel forces to produce the desired response of the vehicle. Simplified control allocation algorithms are proposed that allocate wheel forces in a way that resembles the behaviour of the optimisation solutions. To be able to evaluate the applicability of this methodology for implementation in vehicles, a small-scale prototype vehicle with force allocation control possibilities has been designed and built. The vehicle is equipped with autonomous corner module functionality that enables individual control of all wheels regarding steering, camber, propulsion/braking and vertical loads. Straight-line braking tests show that force allocation can be used in a real vehicle and will enhance performance and stability even at a very basic level, using few sensors with only the actual braking forces as feedback.
In summary, this work has contributed to a better understanding of how the allocation of wheel forces can improve vehicle safety, performance and energy efficiency. Moreover, it has contributed to increased understanding of how vehicle motions should be modelled and simulated, and how control strategies for over-actuated vehicles can be made more suitable for implementation in future electrified vehicles.
Stockholm: KTH Royal Institute of Technology, 2014. , x, 44 p.
Cao, Dongpu, Ph. D.
Stensson Trigell, Annika, ProfessorJerrelind, Jenny, UniversitetslektorJonasson, Mats
Finansierat av SHC, Swedish Hybrid Vehicle Centre. QC 20141114