Page - 119 - in Proceedings - OAGM & ARW Joint Workshop 2016 on "Computer Vision and Robotics“

5 1 3 6 2 4 7 rotary static Figure 2: Mechanical realization and parts of the powertrain. 1© drive motor and break, 2© steering motor, 3© powertrain electric, 4© steering gear, 5© optical absolute rotary steering encoder, 6© 90◦ gear, 7©chaindrive. 3.1. MechanicalRealizationof thePowertrainsandn-WheeledDriveKinematics As each car-like vehicle, the robot needs at least three degrees of freedom (DoF). The classic kine- matic realization of service robots are differential drives, in opposite we decided to implement a n-wheeled steering to combine tractive power, maneuverability, and scalability of the robot. Hence, weproposeakinematicencapsulatedpowertrain thatcanbeequippedwithorwithoutamotor for the steering or tractive power. The wheel can be realized as free running wheel without any motor, can be equipped with a single motor for pure tractive power, or as fully powered, independent steerable wheel (cf. Fig. 2). Most of the already realized systems use wheel hub motors [2, 3]. Wheel hub motors need a wired connection from the static part to the rotary part. That connection constrains the number of possible wheel turns respectively the maximum steering angle and makes the inverse kinematic complex, be- cause thealgorithmshave toconsider theprior steeringmotions. Weapproachacable free rotarypart that allows infinitewheel turns inorder to remove theseconstraints for the trajectory planing. Vehicles that are equipped with more than one steerable wheel, need a interconnected steering that fulfills the Ackerman-constraint [4]. Summarized, the perpendicular line of each wheel has to inter- sect at one point. However, pure mechanical realizations of steering systems go hand in hand with comparable complicated mechanical constructions. Hence, we replace the mechanical connection by a electronic connection and an intelligent control that is able to handle the steering maneuverer inde- pendent from the amount of steerable wheels, based on their position in the kinematic constellation. Based on the Ackerman-constraint and the “instantaneous center of curvature”PICC, the necessary steering angle θn and different velocities vn of the single wheels can be calculated with (1)-(4c). The approached equations result automatically in valid trajectories and steering angle configuration ifPICC is linearly interpolated. Figure 3 depicts an exemplary kinematic configuration and depicts thenomenclatureused for theequations. Differentdrivebehaviors fordifferent infieldusecasesasa 119