Exploring force allocation control of over actuated vehicles

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: As the concern for environmental changes and diminishing oil resources grows more and more, the trend of new vehicle concepts now includes full electric or partly electric propulsion systems. The introduction of electric power sources enables more advanced motion control systems due to electrification of the vehicle's actuators, such as individual wheel steering and in wheel hub motors. This can enable a control methodology that uses different chassis control strategies into a system that will be able to fully utilise the vehicle. Due to this, future vehicles can be more optimised with respect to energy consumption, performance and active safety.Force allocation control is a method that distributes the wheel forces to produce the desired response of the vehicle. In order to evaluate if this methodology can be implemented in future series production vehicles, the aim of this work is to explore how force allocation control can be utilised in a real vehicle to improve vehicle dynamics and safety.In order to evaluate different approaches for generic vehicle motion control by optimization, modelling and simulation in combination with real vehicle experiments will be needed to fully understand the more complex system, especially when actuator dynamics and limitations are considered. The use of a scale prototype vehicle represents a compromise between development cost, efficiency and accuracy, as it allows realistic experiments without the cost and complexity of full vehicle test. Moreover since the vehicle is unmanned it allows studies of at-the-limit situations, without the safety risks in full vehicle experiments.A small scale prototype vehicle (Hjulia) has been built and equipped with autonomous corner module functionality that enables individual control of all wheels. A cost effective force allocation control approach has been implemented and evaluated on the prototype vehicle, as well as in vehicle simulation. Results show improvement of stopping distance and vehicle stability of a vehicle during split-m braking. The aspects of vehicle dynamic scaling are also discussed and evaluated, as it is important to know how the control implementation of small scale prototype vehicles compares with full size vehicles. It is shown that there is good comparison between vehicles of different scales, if the vertical gravitational acceleration is adjusted for. In Hjulia, gravity compensation is solved by adding a specific lifting rig.Studies of vehicles considering optimal path tracking and available actuators are also made to evaluate control solutions of evasive manoeuvres at low and high friction surfaces. Results show differences in how the forces are distributed among the wheels, even though the resulting global forces on the vehicle are approximated to be scaled by friction. Also it is shown that actuator limitations are critical in at-the-limit situations, such as an obstacle avoidance manoeuvre. As a consequence these results will provide good insights to what type of control approach to choose to handle a safety critical situation, depending on available actuators.The built prototype vehicle with implemented force allocation control has shown to be a useful tool to investigate the potential of control approaches, and it will be used for future research in exploring the benefits of force allocation control.

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