Rotational Drive Unit for the DLR bimanual Haptic Device - Integration of Safety Features

Kurzfassung

In the scope of this thesis, a three-layered safety concept for the rotational Drive Unit of the bimanual Haptic Device was developed and partially implemented. It has to be ensured, that for the use of the Drive Unit no hazards can occur as there is an human operator directly involved in the action. In addition, it has to be ensured, that the chair's working space is well defined and can not be exceeded. Otherwise, the risk of cables shearing off occur. Therefore, there is the need of several safety features. For each layer safety features can be identified. The most important safety-layer consists of Mechanical End Stops (third layer). This layer is the most important one as in case its safety features' functions become inoperative, no other layer is able to replace the important core characteristics. Mechanical end stop blocks were designed to be mounted below the chair in order to mark the very end position of the chair's working space. The design is done in a way, that even in case the highest possible loads occur, the mechanical end stop blocks will not break down. This characteristic is achieved by a bolt calculation ensuring the screws are able to withstand the occurring loads. In addition, it is ensured, that even in case of an impact, non of the two Light Weight Robot arms connected with the Drive Unit will break down due to too high decelerations. This is achieved by rubber buffers mounted on the mechanical end stops which break the system slowly in contrary to a hard end stop. Furthermore, considerations of an appropriate voltage limitation limiting the angular velocity of the rotating system is conducted. The second safety-layer consists of a Sensoring System. Inductive-based proximity switches are selected and positioned in order to be mounted on each side of the chair. These sensors detect the end position of the chair and trigger certain events in case of detection. The second sensor acts as an end stop. Positioned directly in front of the rubber buffers which are mounted on the mechanical end stop, the motor's current supply is cut off in case the chair reaches its position. The first sensor is positioned in front of the second one in a way that there is enough time left for the motor to turn the system's direction of rotation, even in case of a maximal angular velocity of the rotating system. The selection of these sensors was conducted by applying methods of Systems Engineering. Several core-technologies were properly examined and rated by a cost-utility analysis. Finally, the inductive-based sensor technology ranked best and was therefore selected. The first safety-layer consists of the analysis of data from a Force-Torque-Sensor (FTS) developed at DLR. Considerations for plausible values show that torques referring to the z-axis (up-down direction) are probably not higher than 15.31Nm in case an human operator with a total mass of around 75kg is sitting upon the chair. This value can be assumed as a limiting value for the meausured torques around the z-axis by the FTS in action of the Drive Unit.