Intelligent Modular Actuator Concept
The concept of Wheel Robots is derived from robotics and Mars rovers, with all the actuators integrated in-wheel. Each actuator is controlled by a separate local control unit, with a central control computer communicating with these units to coordinate the vehicle’s motion. Together with the integrated in-wheel steering mechanism and in-hub traction motors, an extended steering angle of 95° to -25° degrees is realised. As a result, the ROboMObil is able to rotate around its own vertical axis (centrically and eccentrically) and move sideways. With the independent control of four wheel-steering actuators, four electric wheel hub motors and two friction brake actuators, the vehicle dynamics variables of yaw rate, side slip angle and vehicle velocity can be decoupled and independently controlled. These four Wheel Robots are integrated in two axle modules, which house also the drive inverters and electrical distribution components.
The ROboMObil is designed guided by the module concept, consisting of four primary modules as seen in the figure. Other than the already mentioned front and rear axle modules, there is additionally the body module, which holds the driver compartment together with onboard computers and vehicle body sensors. Beneath the body module lays the primary high-voltage battery pack, which is inserted from the bottom of the vehicle.
The permanent magnet synchronous motor (PMSM) in-wheel architecture is critical for the modular wheel module concept and allows the wheels to be steered through an extended angle range. The motors are designed for a maximum vehicle velocity of 100 km/h and are each capable of a peak torque of 160 Nm.
The individual wheel steering consists of a rotary electric actuator mounted on the traction motor housing. Compared to conventional steering actuation via a tie-rod, this direct rotational actuation of the steering axis allows a very large steering angle range. A PMSM motor connected to a backlash-free harmonic drive transmission is used for the actuation.
Electrohydraulically actuated friction disc brakes supplement the braking performance of the in-wheel traction motors. Currently, concepts are being studied to optimise the cooperative braking function between the traction motors and the friction brakes.
A double wishbone geometry is used for the ROboMObil suspension, which combines proven geometric properties together with a possibility of mounting the outboard steering actuators with a large range of wheel rotation angle. The semi-active damper and the spring are actuated via a pushrod-rocker kinematic. The electro-rheological damper mounted in ROboMObil enables a fast adaption of the damping force within a large range. This is used for damper control to mitigate the well-known conflict between ride comfort and road holding. For the Wheel Robot, this is particularly challenging due to the large wheel mass resulting from the in-wheel traction motors and steering actuator. In the future road obstacle information extracted from the 360 degree view provided by the stereo cameras will be used to improve the control algorithms. Vehicle dynamic simulations in Dymola were used extensively in the layout and design of the suspension to virtually validate the design and obtain the required loads. In addition, the simulation is employed in the development of algorithms to control the vertical dynamics of the ROboMObil using the semi-active damping units.