Nikon Metrology

Robot Accuracy: Considerations to make when using robots in high accuracy applications and programming robots off-line

Case Story, Robot Metrology

Image courtesy of GKN and StEM

This paper gives an overview of the niche field of high accuracy robotics, particularly in the area of moving from virtual world (3D CAD robot simulations) to production – where the tolerance requirements are less than 1mm.

We explain the construction and reference systems for the robot and its environment, and the main sources of error between nominal and reality. This includes practical examples of robot measurements.

In summary of sources of error we show an “Absolute Accuracy Robot” specified as being accurate to within 1mm of nominal. Different manufacturers have different tolerance limits around this, and it also depends on robot size (where larger robots are generally less accurate). To help put this in context, here is an extract from ABB Robotics IRB6640 range of robots, “The difference between a virtual robot and a real robot can be typically 8-15 mm, resulting from mechanical tolerances and deflection in the robot structure. The Absolute Accuracy concept bridges this gap with a complete accuracy concept for the entire robot lifetime, ensuring a maintainable accuracy of approx 0.5 mm in the entire working range.” (https://library.e.abb.com/public/e69d8dd25cd7d36bc125794400374679/AbsAccPR10072EN_R6.pd). We verified this specification was correct (Average 0.5mm; 97% within 1mm; max 1.2mm). We also measured another “new” standard robot measured to similar tolerance. In this case it was a standard Fanuc M710i – without absolute accuracy option – but smaller/lower payload (max error approx. 1mm, average error 0.4mm – improved to average error 0.3mm with calibration).

We share the effect of temperature (running a robot “hard”) increasing change in position of around 1mm (some robot vendors offering motor cooling systems to compensate for this). We show the effect of putting a robot on a rail causing a step degradation of several millimeters. There is data on applying forces to robot – moving it generally between 1->2mm, and when using these forces the potential to “skid” and the use of metrology to understand and tune these effects – although the behavior under load is non-linear and therefore difficult to quantify – certainly over time as the machine wears.

The difference in performance between static position accuracy (we call “destination”) and path accuracy (we call “journey”) are explained.

After explaining all of these “challenges” when trying to use robots in high accuracy applications, we present some solutions. On the research side a brief summary of our work on the COMET project – in particular with Delcam and Lund University is given, for both off-line compensation and on-line compensation for machining tasks. On the commercial side we explain Adaptive Robot Control (ARC) – where we control the position of a robot to 0.1mm, with final drilled hole positions within 0.2mm (it’s not just the robot that’s involved in the process tolerance budget). We also explain our solution for robotic trimming with final cut path accuracy <0.5mm. [Note: these are max errors 99.7% (+/-3σ) – not average errors as some people quote].

To end we introduce a relatively new theme of multi-sensor systems to perform high accuracy robotic production – where sensors measure “as built” condition, and this information is used intelligently to adapt the nominal 3D CAD process to produce quality parts and assemblies.

Read the full paper