How to Reach the Dynamic Limits of Parallel Robots? An Autonomous Control Approach

Abstract

Based on closed kinematic chains, parallel robots obtain favorable dynamic properties as well as high stiffness. Hence, their application can significantly enlarge the productivity of automated production processes. A control concept for tapping the high potential concerning low cycle times and high path-tracking accuracy is presented. The proposed approach adapts autonomously to changing dynamic parameters as varying payload. The autonomous behavior is achieved by combining an adaptive control approach with an adaptive, time-optimal trajectory planning concept and an online-trajectory adaption mechanism. Extensive experimental results prove the performance of the proposed approach. Note to Practitioners -Many applications in the field of production automation (material handling, assembly, etc.) require high operating speeds and accelerations. During the past years, parallel robots proved to be an efficient and suitable supplement to serial robots. Unfortunately, the promising possibilities of parallel robots often cannot yield profit because their dynamic potential is still not fully exploited. The payload/robot mass ratio of parallel structures is even higher compared to serial robots, where the influence of the payload on the impedance of the robot is negligible. By use of direct drives the influence of a variable payload cannot be ignored. A modified adaptive control concept, which adapts autonomously to changing dynamic parameters-as varying payload due to diversity of assembly processes-guarantees high tracking accuracy and therefore better process quality as well as accurate estimates of changing dynamic parameters and therefore better process quality. In addition the productivity of the process can be enlarged, if the full drive power can be used at each point on the path. Thus, a new adaptive time-optimal trajectory planning algorithm is used to exploit the dynamic potential of the direct drives and consequently to shorten the cycle times. The aim of time-optimal trajectory planning, as it is commonly understood, is the determination of the maximum velocity profile along a given path that complies with all given dynamic and kinematic robot constraints like limited drive forces/torques, limited path and/or drive velocities and limited path jerk. Combining the adaptive control scheme and the adaptive, time-optimal trajectory planning algorithm with an online trajectory adaption mechanism, a control concept is realized, which autonomously adapts to changing dynamic robot behavior. Using this new approach, the advantages of parallel robots-as well as serial robots with direct drives-can better be utilized. This is a necessary prerequisite for a larger extension of PKMs for industrial applications.