Real Time Pose Control of an Industrial Robotic System for Machining of Large Scale Components in Aerospace Industry Using Laser Tracker System

Abstract

The high demand of efficient large scale machining operations by concurrently decreasing operating time and costs has led to an increasing usage of industrial robots in contrast to large scaled machining centers. The main disadvantage of industrial robots used for machining processes is their poor absolute accuracy, caused by the serial construction, resilience of gearings and sensitivity for temperature changes. Additionally high process forces that occur during machining of CFRP structures in aerospace industry lead to significant path errors due to low structural stiffness of the robot kinematic. These errors cannot be detected by means of motor encoders. That is why calibration processes and internal control laws have no effect on errors caused by elastic deformation. In this research paper an approach for increasing the absolute accuracy of an industrial milling robot with help of a Laser Tracker system during machining tasks will be presented. To measure the position and orientation of the robot tool center point (TCP) a specific adapter is mounted on the milling spindle near the TCP to provide a 6DoF measurement. Via a real time interface pose data can be obtained in millisecond cycles and is used for calculating the current path errors of the robot. The implementation of an additional controller in the manufacturers CNC allows the correction of the programmed trajectory so that the machining path will match its specifications.

Robots in machining

“…Work on real time pose control of an industrial robot using 3-DoF and 6-DoF laser tracking has also been reported by Moeller et al [99]. Fig.…”

“…External guidance (Fig. 29) makes the machining robot unsusceptible to external or constant process forces, thermal expansion and calibration errors [99].…”

Hybrid compliance compensation for path accuracy enhancement in robot machining

“…The model is modified by adding virtual joints at the end effector to take into account the compliance of the force/torque sensor (9), the spindle and its holder (10) as well as the tool (13). The accurate orientation of the tool is calculated by an Euler rotation from coordinate system (11) to (12).…”

“…One possibility is the monitoring and correction of the TCP position, which is realized by very cost-intensive position measuring technology, e.g. laser trackers [8,12,15]. Second encoder systems, which measure the axis position on the load side in addition to the motor side system, can be used to compensate torsional compliance and gear backlash [7,9].…”

Hand-eye calibration method with a three-dimensional-vision sensor considering the rotation parameters of the robot pose

“…It requires the machining robots not only to have good machining performance but should be equipped with relevant measuring system to assist the robots to locate the machining position of workpiece. [1][2][3][4] The commonly used measuring system in industry can be divided into contact measurement and noncontact measurement. Due to the complex and irregular surface profile of the workpiece, the contact measuring system is difficult to meet the needs of industry.…”