Exploiting individual wheel actuators to enhance vehicle dynamics and safety in electric vehicles

Doktorsavhandling, 2009

This thesis is focused on individual wheel actuators1 in road vehicles intended
for vehicle motion control. Particular attention is paid to electro-mechanical actuators
and how they can contribute to improving vehicle dynamics and safety.
The employment of individual wheel actuators at the vehicle’s four corner results
in a large degree of over-actuation. Over-actuation has a potential of exploiting
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 systematically
exploited within their force constraints. Therefore, force constraints
for the individually controlled wheel are formulated, along with important restrictions
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 arising
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 complex
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.