A REAL-TIME ROBOT ARM COLLISION AVOIDANCE SYSTEM

A data structure and update algorithm are presented for a prototype real-time collision avoidance safety system simulating a multirobot workspace. The data structure is a variant of the octree, which serves as a spatial index. An octree recursively decomposes three-dimensional space into eight equal cubic octants until each octant meets some decomposition criteria. We use the N-objects octree, which indexes a collection of 3-D primitive solids. These primitives make up the two seven-degrees-of-freedom robot arms and workspace modeled by the system. Octree nodes containing more than a predetermined number N of primitives are decomposed. This rule keeps the octree small, as the entire world model for our application can be implemented using a few dozen primitives. As robot arms move, the octree is updated to reflect their changed positions. During most update cycles, any given primitive does not change which octree nodes it is in. Thus, modification to the octree is rarely required. Incidents in which one robot arm comes too close to another arm or an object are reported. Cycle time for interpreting current arm joint angles, updating the octree to reflect new positions, and detecting/reporting imminent collisions averages 30 ms on an Intel 80386 processor running at 20 MHz.