How to Design a Metrology System for Measuring Industrial Robots

Abstract

Robots typically have high levels of positional repeatability (around 30-50µm). However, due to the design and mechanical limitations of industrial robots they are not inherently accurate. Although all robots exhibit different accuracies depending on manufacturer, options, size, application, etc. we can broadly say that typical accuracy is around the 1-2mm mark.

The positional accuracy of the robot is only one part of a manufacturing tolerance stack; the robot needs to understand where the workpiece is located relative to a mutual frame of reference. Traditionally this is accomplished through manually teaching the position of the workpiece and then utilising hard-tooling and fixturing to ensure the workpiece is repeatably located relative to the robot. This is expensive both in terms of tooling and time as additional commissioning is required.

The good repeatability has – in many cases – been misunderstood as accuracy and as such serial axis robots are increasingly being utilised for demanding machining or adaptive processes. These processes require a close correlation between the simulated (offline) environment to the physical reality, however there are multiple sources of error that must be addressed and considered before the widespread adoption of robotics can be realised.

The traditional approach to improving a robot or process accuracy with external feedback utilises either local sensors, portable metrology systems or a combination of the two. This can be a workable solution for the right application, however, to realise the full-potential of data-driven manufacturing a more holistic dataset is required. A global understanding of the robot end-effector in 3D space can be provided using an external metrology system; typically, a portable metrology system is utilised to understand the application further. However, these systems have shortcomings which make them impractical as long-term solutions.

Given the growing need for advanced robotics and the failings of current metrology systems, we have developed our own system: IONA, from the ground-up to meet this challenging application.

This presentation details the journey INSPHERE has undergone in developing IONA. IONA represents the culmination of our experience in tackling challenging robot applications and processes with existing portable metrology systems. The hardware and software have been developed from the ground-up to measure, improve and monitor robot processes. Enabling measurement in-process of the end-effector(s) and fixture(s). As such, we have created a system for use by production, automation, & CNC machining engineers, and not solely for metrology engineers. This change of audience and specific application has driven unique design requirements.

The presentation includes: i) the current challenge/problem & improvement strategies ii) metrology system requirements/rationale iii) resulting system design and results.