Reliable assembly of products using an impedance controlled manipulator

This paper describes the development of a robotic assembly system for the assembly of consumer products in the food and personal care industry. Most commercially available manipulators are equipped with PID motion controllers which are suited for pick-and-place operations. Here we considered assembly of products under conditions of geometric deviations, a task for which PID controllers are less suited. The feasibility of such assembly tasks was investigated using an industrial manipulator provided with an impedance controller. Results indicate that a proper set of product related impedances enables the system to cope with relatively large positioning errors, resulting in more reliable assembly. Background In several industries the demands on quality and flexibility have led to the use of robotic systems. The foods industry is no exception to this. Robots are applied extensively in the packing and palletising of products. Technical limitations and cost aspects have sofar prohibited the use of robots in the manufacturing, i.e. assembly, of consumer products. The use of robots for the assembly of products is not a novel idea. Robots performing assembly tasks can be found in e.g. the automotive industry (gear-boxes) and electronics industry (video players). However, the foods industry imposes different requirements on robotic assembly: − high capacities, which lead to short cycle-times (typically 0.25-1.0 sec); − products consisting of a small number of parts (typically 2-4); − the physical characteristics of the parts vary from rigid (e.g. glass jars) to floppy (e.g. carton boxes); − the geometrical properties of the product and the packaging material are variable and these variations are relatively large; − the need for a hygienic manufacturing environment. The goal of the research described in this paper is the development of a robotic assembly system fulfilling these requirements. Problem definition The task for the system will be to carry out an assembly task involving two parts: a container and its closure. The task consisted of three subtasks as depicted in figure 1: 1. pick up the closure and move to a position above the container; 2. move downward and close the container; 3. move back towards the picking position for a new closure. Place-position Pick-position 0.04 m 0.20 m