Hybrid compliance compensation for path accuracy enhancement in robot machining
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Hybrid compliance compensation for path accuracy enhancement in robot machining Felix Hähn1 · Matthias Weigold1 Received: 29 April 2020 / Accepted: 29 July 2020 / Published online: 14 August 2020 © The Author(s) 2020
Abstract Robot machining processes with high material removal rates lack of high path accuracy mainly due to the low stiffness of industrial robots. The low stiffness leads to process forces caused deviations of the tool center point (TCP) from the planned position of more than 1 mm in industrial applications. To enhance the path accuracy a novel hybrid compliance compensation is developed. It combines a force sensor and model based online compensation with forces of an offline simulation to instantly react to predictable high force changes e.g. at a milling cutter exit from the work piece. The method is applied to a KUKA KR 300 robot. A compliance model based on a forward kinematic with virtual joints is implemented on an external controller. Cartesian or axis specific compensation values are calculated and transferred to the robot via a control circuit. A compliance measurement method is developed and a force torque sensor is mounted to the flange of the robot. The system is validated in with Cartesian and axis specific compensation values as well as with and without pilot control. Keywords Robot-machining · Compliance compensation · Hybrid online/offline compensation · Compliance measurement
1 Introduction and state of the art For decades, industrial robots have been an integral and steadily growing part of industrial production chains. In the period from 2014 to 2018 sales of industrial robots have nearly doubled [13]. In addition to tasks in the fields of material handling and welding, applications that require high robot accuracy, in the range of a few tenths of a mm, are becoming established. These include tasks in the area of quality assurance, as well as machining of large components. The main reasons for the use of robots are the low investment costs for a large work area that can be expanded with little effort and the flexibility to combine several end effectors and processes. The low path accuracy and both static and dynamic stiffness of industrial robots, especially under changing process forces, are challenging for the machining process. External forces can lead to deviations of the TCP * Matthias Weigold [email protected]‑darmstadt.de Felix Hähn [email protected]‑darmstadt.de 1
Institute of Production Management, Technology and Machine Tools, Technical University of Darmstadt, Darmstadt, Germany
from the planned position of more than 1 mm with a heavyduty robot in industrial applications [19]. To increase the positional and path accuracy during machining with industrial robots, efforts to improve the construction of the robot, path planning methods and process parallel control strategies can be made. Concepts to increase the position accuracy of serial robots by changes in construction include the reduction of axes [1], hybrid drives with additional torque motors [6] an
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