Robust sensing of human proximity for safety applications

This paper presents a robust capacitive proximity sensor that facilitates the safety features of power tools. Power tools (e.g. pneumatic metal forming machines) enable easier and faster work but they can also cause severe injuries, if not operated carefully. However, accidents and related injuries can be prevented to a large extent by providing an automatic shut-off functionality. In order to realize such a functionality, a sensing mechanism which detects the presence of a human in the dangerous working areas of the power tool is necessary. A capacitive sensor which is suitable for this application has been developed. In most of the cases, the tools are made of metallic parts and have good electrical conductivity. Typically, the tools have a fixed part and a movable part. In the proposed scheme, the proximity sensor is attached to the movable part of the tool. Depending on the position of the movable part, changes of the electric field distribution across the electrodes will occur. Hence, comparatively large changes of the sensor capacitance are introduced compared to a capacitance change for the presence of a human hand. The current position of the movable part and the prior knowledge of the capacitance values for a vacant condition are used to solve this issue. The practicality of the proposed sensor system is demonstrated with detailed simulation studies and results from the measurements conducted with a prototype sensor installed on a typical metal forming machine.