Brake-by-wire (BbW) systems are one key technology in modern vehicles. Due to
their great potential in the areas of energy efficiency and automated driving,
they receive more and more attention nowadays. However, increased complexity and
reliance on electric and electrical components in BbW systems bring about new
challenges. This applies in particular to the fault tolerance of the brake
system. Since drivers cannot form a fallback layer of braking functions due to
the mechanical decoupling of the brake pedal, known BbW concepts provide a
redundant system layer. However, driving is significantly limited in the event
of a failure in the BbW system and is only possible under certain restrictions.
The reason for that is a further possible failure (double point of failure
scenario), which can result in a significant loss of braking performance.
To improve the availability level of the braking functions, a principally new
redundancy concept for the double point of failure scenario is presented. This
allows for a less restricted driving operation when the BbW system is subject to
failures. For this purpose, a central electric motor (CEM) and modified
electronic parking brake (EPB) actuators are used on the front axle of a vehicle
to form a separate redundancy layer. Strategies for deceleration and wheel slip
control are developed for the individual actuators as well as for their
simultaneous operation. The performance of the different brake modes is
evaluated in road tests. The analysis shows that especially the combined
operating mode of the CEM and EPB leads to high deceleration levels and robust
operation in low road friction conditions.
