High-precision assembly automation based on robot compliance

Abstract

Industrial robots can be made compliant to the environment when the control loop gains are reduced, creating a so-called soft servo capability. This allows industrial robots to be used for assemblies with limited contact requirements. In this paper, we propose an assembly method using soft servo to perform certain assembly tasks where part location errors typically require the use of force control or remote center of compliance (RCC) methods. A typical industrial application, valve body assembly, was used to validate the developed method. This assembly was chosen because it is simple, yet requires compliance in all directions. Lab experiments were performed and the assembly operations were consistently successful enough to show that the developed soft servo strategy can perform certain assembly tasks with small part location errors. Therefore, the soft servo strategy may open a new door for low-cost industrial assembly. Experiments with force control were also performed to compare the performance between soft servo and force control. We found that the force control method is much more sensitive to environmental contact, that the contact forces can be controlled directly, and that greater part location errors can be tolerated. Conversely, assembly with soft servo may either fail to assemble the parts or generate bigger contact forces than allowed. Thus, applications of soft servo are more limited while force control can be successfully used in most all applications. Further investigation is needed to determine the practical industrial use of soft servo for particular types of precision assembly.