title: Magnetically Actuated Hybrid Brake for Autonomous, Electrified Vehicles
Creators: name: Guckes, Lennart
Publisher: name: UNSPECIFIED
Year: 2023

description: Autonomous driving, the electrification of the powertrain and the reduction of emissions are three major development trends in today’s automotive industry. New mobility solutions such as electrified, autonomous shuttles for transporting people and goods in cities are being conceived and require a change of view in development, not only on the software level but also on chassis components. For braking systems, there are changes in the requirements for these vehicles. Existing test specifications for the performance of wheel brakes for passenger cars are primarily designed for the worst-case scenarios that can be caused by a human driver. Based on a stakeholder analysis as well as a use-case consideration new requirements are generated and new test specifications are developed, which are tailored to autonomous shuttles. Especially the restriction of the Operational Design Domain of the autonomous shuttle results in significantly lower thermal requirements for the wheel brake compared to conventional passenger cars. Nevertheless, high braking torques are still required due to high vehicle masses and the scenario of non-availability of the regenerative brake. Based on the identified requirements and development goals from the analysis, the solution space of braking systems in the state of the art is considered and the suitability of different brake concepts is discussed. Although current hydraulic concepts for passenger cars meet the dynamic and thermal requirements, they have little potential for reducing emissions and tend to corrode due to the increasing duration between actuations when regenerative brakes are in use. As an alternative, the focus of development lies on electromechanical concepts for individual wheel brake modules with electromotive actuation, which in the event of a fault only leads to the failure of one of four wheel brakes. Previous concepts of electromotively actuated brakes show a development conflict in the fact that many gear stages are necessary due to the high transmission ratios, which show a high efficiency variance over temperature due to their lubricants. In the concept space of electromechanical actuators, electromagnets can be found in addition to electric motors. However, only a few concept studies have been published regarding the state of the art and research, although electromagnetic actuation promises some advantages over electromotive actuation. These include less complex actuation, smaller installation space, and less mechanical complexity. Due to the revealed gap in the state of the art, an electro-magnetic concept is selected to be investigated for its suitability as a wheel brake for an autonomous shuttle. For this purpose, either holding magnets or solenoids can be used as actuators, with holding magnets promising significantly higher magnetic forces at small air gaps. For the investigations, a first design of a drum brake actuated by a holding magnet is prototypically realized. The concept comprises a combination of a magnetically actuated solid disc brake with a downstream duplex drum brake. The interconnection of the two brakes promises the utilization of the wheel rotation angle for actuation of the drum brake and a redundant actuator design through the use of two excitation coils in the mag-netic disc brake. To evaluate the suitability of the concept, actuator forces, dynamics, total torques and disc brake torques are measured in operation. The largest deviations from the design considerations occur in the torque curve of the brake over speed, which are investigated in more detail using a hypothesis- and test-based approach. In addition to investigations into the influence of magnetic field weakening effects, such as eddy currents, the coefficient of friction of the friction partners on the holding magnet actuator is also determined and identified as the main cause of the torque drop. A dependence on both the velocity and the axial force is examined. The investigation of the torque hysteresis of the overall braking system shows the difficulty in setting small torques, since the magnet has a high current demand to pass the initial air gap, which then generates medium torques when it hits the disc. A proposal for control by means of a current pre-control is also being tested and evaluated. Through the investigations carried out, the development conflict in the use of a holding-magnetic actuator concept for the actuation of a wheel brake is recorded, and through the hypothesis-based approach the causal effect of the torque drop is identified. The realized magnetic actuator concept fulfills the requirements for dynamics and the wish for low-emission, compact braking concepts, but does not achieve the required braking torques over the entire operating range of an autonomous shuttle. In the outlook a proposed conceptual solution of actuation by means of solenoid actuators is discussed, which promises to solve the described development conflict, but in turn requires a higher transmission ratio due to the lower force potential.