Accuracy Degradation Analysis for Industrial Robot Systems

As robot systems become increasingly prevalent in manufacturing environments, the need for improved accuracy continues to grow. Recent accuracy improvements have greatly enhanced automotive and aerospace manufacturing capabilities, including high-precision assembly, two-sided drilling and fastening, material removal, automated fiber placement, and inprocess inspection. The accuracy requirement of those applications is primarily a function of two main criteria: (1) The pose accuracy (position and orientation accuracy) of a robot system's tool center position (TCP), and (2) the ability of a robot system’s TCP to remain in position or on-path when loads are applied. The degradation of a robot system’s tool center accuracy can lead to a decrease in manufacturing quality and production efficiency. Given the high output rate of production lines, it is critical to develop technologies to verify and validate robot systems’ health assessment techniques, particularly the accuracy degradation. In this paper, the National Institute of Standards and Technology’s (NIST) effort to develop the measurement science to support the monitoring, diagnostics, and prognostics (collectively known as prognostics and health management (PHM)) of robot accuracy degradation is presented. This discussion includes the modeling and algorithm development for the test method, the advanced sensor development to measure 7D information (time, X, Y, Z, roll, pitch, and yaw), and algorithms to analyze the data. INTRODUCTION In recent years there is a growing demand within the automotive and aerospace industry for greater robot accuracy [1, 2]. With the accuracy improvement in both position and orientation, the articulated robot arm can be applied to a much broader range of applications that were once limited to custom machines, including high precision assembly, two-sided drilling and fastening, material removal, automated fiber placement, and in-process inspection. Compared to custom machines, the robot’s articulated arm can span a relatively large working envelope capable of navigating along highly curved surfaces and into tight spaces. Since the robot’s mass is relatively low, the foundation (e.g., supporting structure) requirements of robots are minimal. Robot applications bring manufacturers benefits in both improving flexibility and reducing costs with these noted advantages. Robot accuracy is defined as the measurement of the deviation between the commanded and attained robot 6-D (six degree-of-freedom) position and orientation [3]. Accuracy can also represent the difference between commanded and actual velocities, accelerations, forces, and torques. Improving accuracy (i.e., lessening the difference between commanded and actual values) allows rapid deployments of industrial robot applications by rapidly transferring or downloading robot programs between two “identical” robot cells. It enables the quick replacement of a robot in a manufacturing system by reducing or eliminating re-teaching processes. High robot accuracy during manufacturing ensures that parts are precisely manufactured with predictable results even after changes are made to the process. High accuracy is also critical in data-driven applications, such as those applications developed using off-line programming methods [4]. High accuracy enables the use of offline programs to minimize the robot downtime (e.g., the timeconsuming task to train a robot to drill thousands of holes on an airplane’s fuselage). The market requirement for high designvariations and low-batch production has driven users and integrators to look more towards “off-line programming”. Using robots for in-process inspection or gauging is another application that calls for high accuracy of a robot’s pose because the robot is an influential part of the measurement operations [5, 6]. There are a large number of automotive and aerospace applications that currently utilize and could benefit from the flexibility of robotics with high accuracy to perform metrology on manufactured parts.