Characteristics, accuracy and reverification of robotised articulated arm CMMs

VDI article 2617 specifies characteristics to describe the accuracy of articulated arm coordinate measuring machines (AACMMs) and outlines procedures for checking them. However the VDI prescription was written with a former generation of machines in mind: manual arms exploiting traditional touch probe technologies. Recent advances in metrology have given rise to noncontact laser scanning tools and robotic automation of articulated arms – technologies which are not adequately characterised using the VDI specification. In this paper we examine the “guidelines” presented in VDI 2617, finding many of them to be ambiguous and open to interpretation, with some tests appearing even to be optional. The engineer is left significant flexibility in the execution of the test procedures and the manufacturer is free to specify many of the test parameters. Such flexibility renders the VDI tests of limited value and the results can be misleading. We illustrate, with examples using the Nikon RCA, how a liberal interpretation of the VDI guidelines can significantly improve accuracy characterisation and suggest ways in which to mitigate this problem. We propose a series of stringent tests and revised definitions, in the same vein as VDI 2617 and similar US standards, to clarify the accuracy characterisation process. The revised methodology includes modified acceptance and reverification tests which aim to accommodate emerging technologies, laser scanning devices in particular, while maintaining the spirit of the existing and established standards. We seek to supply robust re-definitions for the accepted terms “zero point” and “useful arm length”, pre-supposing nothing about the geometry of the measuring device. We also identify a source of error unique to robotised AACMMs employing laser scanners – the forward-reverse pass error. We show how eliminating this error significantly improves the repeatability of a device and propose a novel approach to the testing of probing error based on statistical uncertainty

Analysis of a robot arm with exoskeleton for coordinate Measurement in complex environments.

In partnership with Metris UK we present the modelling and development of a revolutionary Robot Coordinate-measuring Arm (RCA). The RCA combines the automation capability of traditional Coordinate Measurement Machine (CMM) methods with the mobility and part accessibility of an articulated arm, resulting in a versatile and powerful tool for coordinate measuring applications with a target accuracy of sub-100µm. The RCA exploits novel, patented technology to accelerate repetitive 3D inspection jobs. A highly accurate 7-axis articulated arm is housed within a robotized exoskeleton driven by electromotors. We utilise Denavit-Hartenberg parameters to model the robot links and investigate the accessible configurations of the exoskeleton under the effects of arbitrary offsets and twists. We further examine the configuration space of a 2-axis version of the system, its singular points and the singularities of the mapping to the space of controls. We demonstrate the compliance of the RCA to accepted engineering standards and discuss improvements in the calibration and error-correction of the unit, which have reduced residuals by a factor of two. We identify singular poses as a large cause of uncertainty and propose solutions to circumvent them. This work is supported by the UK Technology Strategy Board under the EPSRC-managed scheme “Gathering Data in Complex Environments”