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FRIEND::Motion Planning

A scene from the Mapped Virtual Reality (MVR) for a FRIEND task: move to the inside of a refrigerator.
Primary task of the motion planning is to find a collision free trajectory. Nevertheless, in service robotics, other aspects must be additionally taken into account like safety and smoothness of the trajectories. In order to suffice all these requirements for FRIEND a sensor-based motion planning for the manipulator was developed. The procedure is based on Cartesian space information [19], and therefore computationally efficient as well as with a high precision. The object grasping frames are calculated on-line which increases the flexibility of the system during execution. All implemented algorithms are suitable for real-time applications in service robotics but also for industrial usage.

The motion planning is performed based on a Mapped Virtual Reality (MVR). This 3D-model of the environment is built from the information within the world model which was perceived by the sensors. Before any motion of the robot arm is performed the trajectory from the current to the goal configuration is calculated within this 3D-model. During motion on-line collision checking is done, since obstacles can move during motion. Obstacles are in this case all objects of the environment, which are not included in the current motion, also in some tasks the user in the wheelchair. It is important that there is at any time no danger for the user.

At the wrist of the manipulator a force-torque-sensor is installed. The information from this sensor is used to detect collisions or for fine-tuning during manipulative operations, e.g. when the robot arm should put a gripped object in a small opening. This ensures robustness during execution.

In general the trajectories calculated from the motion planning algorithms are robot like, i.e. clipped and jerky. In order to enhance this and make the movement of the manipulator more pleasant to the user the trajectories are smoothed and the quality is enhanced[20]. Therefore the used robot arm is helpful. Since six degrees of freedom are sufficient to define a 3D pose in the environment (three degrees for position and three degrees for orientation) and FRIEND's manipulator has seven degrees of freedom, one degree of freedom can be chosen optional, i.e. the manipulator can turn its elbow joint by 360 degrees without changing gripper's pose. This can be used to find the best trajectory from a start to a goal configuration among an infeasible set of possible configurations. This seventh degree of freedom is also used to solve and avoid dead-locks during the motion process and to keep a minimal distance to obstacles.
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