Energy Management and Control of Electrical Drives in Hybrid Electrical Vehicles

Abstract: Hybrid vehicles have attracted tremendous attention during the last years.
Increasing environmental concern and a steady increase in fuel prices are key
factors for the growing interest. Hybrid vehicles, which benefits from having
at least two different energy converters and two different energy storage
systems, have proven to have significant potential to improve fuel economy
without reducing the performance of the vehicle. However, the extra degree
of freedom inherited by the use of two energy sources on-board the vehicle,
gives rise to a more complicated energy management control.
The first part of the thesis treats the subject of energy management in hybrid
electrical vehicles. The gain in fuel consumption and the minimization of
emissions are highly dependent on the performance of the control strategy. A
rather simple heuristic control strategy presented in the literature is
optimized. Heuristic control strategies are often referred to as hard to tune,
and none optimal. However, the result presented in the thesis shows that the
strategy is easily tuned, robust and has no significant cycle-beating trait.
Dynamic programming is used to obtain a global optimal solution to the
control problem. The result of this global optimization is then used as a basis
for evaluating the real-time heuristic control strategy and serves as a lower
bound for the fuel consumption for a given cycle. A comparison of fuel
consumption for the two control strategies shows that, though being quite
simple, the heuristic control strategy gives a relatively near-optimal result.
The second part of the thesis is devoted to the development of an electrically
driven rear axle for a HEV in collaboration with SAAB Automobile. A rear
drive unit, consisting of an electrical machine, planetary gear and a
differential, was provided by SAAB. Focus is on control and thermal
modeling of the electrical machine. A simple and effective field weakening
controller, giving fast field weakening performance is proposed. The fast field
weakening performance is important in a HEV since the battery voltage
undergoes rapid variations, during accelerations. In addition to this, the FWC
minimizes the torque-per-current ratio by, for a given torque, using the
current combination yielding the minimal stator current. In addition to this,
a thermal model based on several thermal measurements is proposed and
validated against data. The thermal model forms the basis for the derivation
of an over temperature controller, preventing the machine from over heating.