Vehicle Dynamics Control
From a vehicle dynamics perspective the ROboMObil can be seen as a highly over-actuated system. This leads to opportunities and challenges concerning vehicle dynamics, which arise when designing the control algorithms for four individual steering actuators, four individual in-wheel motors, two electro-hydraulic brake actuators and four semi-active dampers. At the DLR Robotics and Mechatronics Center these challenges are part of our current research activities like exploring the opportunities of an all-wheel steered vehicle, finding the optimal torque distribution to the wheel robots or improving the ride comfort and handling for vehicles with in-wheel motors, just to mention some. The control algorithms are hierarchically structured in low-level control close to the hardware running at high rates and high-level control with more abstracted views of the hardware running on lower rates. These are additionally grouped into replaceable modules. The geometric control, described below, is a comprehensive high-level algorithm, which uses several modules, like speed control or torque distribution to determine how ROboMObil interprets the driver commands.
Given the desired ROboMObil motion on the road, the challenge from a control viewpoint is to provide the correct actuator set-points. The vehicle should perform the desired motion as precisely as possible while external disturbances are rejected, safety is ensured and energy consumption and tire wear are minimized. The approach ROboMObil currently uses to define the steering actuator set-points is a feed-forward control algorithm based on an inverse geometric vehicle representation neglecting tire slip. Due to the limitation of the steering angles of the Wheel Robot, three modes of operation of the geometric control have to be distinguished: longitudinal driving, lateral driving and rotating around a predefined instantaneous center of rotation. These driving modes can only be changed during vehicle standstill to prevent uncontrollable vehicle states and guarantee the drivability of ROboMObil. The set-points of the traction motors and the brake actuators are defined by a vehicle motion controller supported by a subsequent torque distribution algorithm, which in a first step defines the proper drive torque of each wheel robot and in a second step allocates this drive torque to the electric motor and the brake. Further algorithms like “global chassis control” are investigated to extend the geometric control in the future.
Cooperative Braking and Slip Control
The motivation for combining an electro-hydraulic friction brake with the in-wheel motors for deceleration arises from their complementary properties with respect to response time, maximum quasi-static torque and the possibility to recuperate energy using the motors as generators. This brake system combines the high torque potential of the electro-hydraulic brake at lower bandwidths with the fast response time of the in-wheel motors having a rather limited torque potential. The restrictions on both components can be mitigated, allowing the overall brake system to achieve higher performance and efficiency. Within such a combined braking system the torque allocation algorithm becomes a key factor for the overall system performance.
Semi-active damper Control
Each Wheel Robot is equipped with a fast semi-active damper allowing the control algorithm to adapt the current damping force inside a wide range within milliseconds. The semi-active dampers mounted in ROboMObil are based on an electro-rheological fluid whose viscosity changes according to the strength of the applied electric field inside the damper. One challenge during the design of the control algorithm for the semi-active dampers is attributed to the transfer of the 16kW permanent-magnet synchronous motor into the wheel, which significantly raises the wheel mass. The conflict between ride comfort and driving safety gets even more challenging to handle this way. A passive suspension system could only deal with this conflict with compromise in comfort aspects to ensure driving stability. Semi-active suspension systems mitigate this restriction by allowing a continuous and fast adaption of the damper force. Currently control algorithms, explicitly combining passenger comfort and wheel control are investigated. These algorithms will be extended by obstacle information from the ROboMObil 360 degree stereo surround view used for environment reconstruction in the future.