Exploiting individual wheel actuators to enhance vehicle dynamics and safety in electric vehicles
Mats Jonasson
Doctoral Thesis
TRITA-AVE 2009:33 ISSN 1651-7660 ISBN 978-91-7415-387-3
Postaladdress Visitingaddress Telephone Internet RoyalInstitute ofTechnology Teknikringen8 +4687906000 www.ave.kth.se Vehicle Dynamics Stockholm Telefax SE-10044Stockholm +4687909290 c Mats Jonasson 2009
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This thesis is focused on individual wheel actuators1 in road vehicles intended for vehicle motion control. Particular attention is paid to electro-mechanical ac- tuators and how they can contribute to improving vehicle dynamics and safety. The employment of individual wheel actuators at the vehicle’s four corner re- sults in a large degree of over-actuation. Over-actuation has a potential of ex- ploiting the vehicle’s force constraints at a high level and of controlling the vehicle more freely. One important reason for using over-actuated vehicles is their capability to assist the driver to experience the vehicle as desired. This thesis demonstrates that critical situations close to the limits can be handled more efficiently by over-actuation. To maximise the vehicle performance, all the available actuators are sys- tematically exploited within their force constraints. Therefore, force constraints for the individually controlled wheel are formulated, along with important re- strictions that follow as soon as a reduction in the degrees of freedom of the wheel occurs. Particular focus is directed at non-convex force constraints aris- ing from combined tyre slip characteristics. To evaluate the differently actuated vehicles, constrained control allocation is employed to control the vehicle. The allocation problem is formulated as an optimisation problem, which is solved by non-linear programming. To emulate realistic safety critical scenarios, highly over-actuated vehicles are controlled and evaluated by the use of a driver model and a validated com- plex strongly non-linear vehicle model. it is shown that, owing to the actuator redundancy, over-actuated vehicles possess an inherent capacity to handle actuator faults, with less need for extra hardware or case-specific fault-handling strategies.
1wheel actuator=vehicle component which enables control of wheel kinematics and/or forces
in the tyre’s contact patch.