A Simpler Method for Calibrating Surface Metrology Systems

Surface metrology has become an essential part of any high-tech production line, where extremely precise optical instruments are used to ensure that surface characteristics are up to standard. These instruments are frequently used past their capacity meaning erroneous measurements can occur and, due to their highly complex nature, calibration can take weeks and is almost impossible for non-experts.

However, research into optimising these calibration methods has recently been carried out by Dr. Claudiu Giusca, which will allow non-experts to calibrate the machines over a far shorter timescale. AZoM spoke to Dr. Claudiu Giusca about his new calibration method, how he developed it and the integral role that MountainsMap^® processing software played.

What issues are associated with the current methods of calibrating instruments used for surface metrology?

The instruments used for surface metrology are extremely complex as they are required to make nanoscale measurements or to observe small deviations in surfaces. The technologies used in surface metrology instruments are relatively mature as they are either based on a contact method, which was developed in the early twentieth century or on purely optical methods which were developed during the advent of computers.

When these instruments were first conceived they were considered too sensitive for roughness measurements, as researchers didn’t yet have optical sensors to capture the information. Nowadays, these machines are hugely popular in the field of process control as they are capable of rapidly collecting vast quantities of data. However, now the question has become not how can we collect the data? But, instead, what is the quality of this data?  

If these machines were only used to measure compliant parts and compliant surfaces, then no problems would be encountered. However, this would defeat their purpose for quality control. Instead, the machines are pushed to their limits which can result in erroneous results and it is difficult for factory users, who are non-experts, to determine where these errors are coming from.

This is where calibration comes in, which aims to characterise these machines in a very simple way. To do this, you must create a plan of how you test them. The testing methods currently used are complicated, only really achievable for the instrument manufacturers themselves or those in academia and it is difficult to summarise the results in a simple and meaningful way.

I’m currently trying to determine the simplest way to calibrate these machines, which will allow me to create procedures that can easily be applied by non-experts on the factory floor.

Surface metrology is an important part in all industrial processes, however as the equipment used is sophisticated calibration methods can be complex. Image Credit: Dmitry Paikin/Shutterstock.com

Please could you introduce our readers to your new calibration methods? What parameters are you focusing on?

Whilst I was working at the National Physical Laboratory, where I was researching dimensional and surface texture measurements, I was working on a specific standard - ISO/TC 213. As I was working on this standard I immediately realised that if you wanted to create a calibration standard based on these specific standards it would take a week or two just to calibrate one machine.

From there I tried to create a simpler calibration framework. This involves developing a simpler mathematical model, with less input parameters (also called metrological characteristics) and ones that can easily be tested, which allows for the accurate calculation of the uncertainties associated with measurements.   

How did you develop your calibration methods?

We systematically went through the different metrological characteristics and catalogued each of them, making note of how efficient and effective the measurement of each parameter was.  

We then discussed this method at the international level and we published to get acceptance on the global stage.

What role was MountainsMap^® playing in your research, and what techniques were you integrating with it?

One of the advantages of MountainsMap^® is that it allowed us to quickly visualize the experimental results. As part of our research we wanted to demonstrate that you can estimate metrological characteristics (parameters) with tools that are ready available for industrial users rather than use, for example, specialized analysis software such as MATLAB. MountainsMap® helped us to show how easily implementable the methods are.

In terms of techniques, we validated our methods with lots of different types of instruments. We used a confocal scanning instrument, a couple of white light interferometers, coherence scanning interferometers and we've looked at contact stylus instruments. MountainsMap® is able to read specific files from different instrument manufacturers meaning it allows us to compare the results within the same visual platform.

MountainsMap^® software was used to analyse and compare results from many different kinds of surface measuring instruments

Why did you choose MountainsMap^® instead of developing your own calibration application?

We were already collaborating with MountainsMap^®, as we considered its editor Digital Surf the market leader in surface texture analysis. It was logical for us to work with that platform because we wanted to show that it can be applied very quickly with already available tools.

We also like how results are visualized with MountainsMap^® and when it comes down to applying your operators, it is intuitive.

What impact do you expect your improved calibration methods to have?

The biggest impact is that these methods have been adopted into the ISO standardization framework which affects, to a certain extent, everybody carrying out surface texture measurements who wishes to be ISO compliant. So, I think all industries, as far as I'm concerned, will have been impacted.

We’ve been approached by many working in industries such as automotive and aerospace, who were seeking training in surface measurement system calibration. There are also other governmental bodies that were interested in seeing how we might be able to apply these methods to calibrate their own machines and to understand their capabilities.  

Where can our readers find out more about your research and MountainsMap^® ?

The best place to look would be the three papers that I published in Measurement Science and Technology, which summarise our method. The papers further explain the role of the metrological characteristics and how one is able to use these methods. They also demonstrate all of the imaging and data processing that is done using MountainsMap^®.

If you’re interested in finding out about MountainsMap^® you can visit the Digital Surf website.

About Dr. Claudiu Giusca

Dr Claudiu Giusca is Senior Lecturer in Sensor Technology for Precision In-process Measurement at Cranfield University, working on metrology aspects related to Advanced Manufacturing.

Previously, Claudiu worked at the National Physical Laboratory, where he developed expertise in the metrology associated with optical surface topography measuring instruments, surface texture and micro-coordinate measurements, design and build of primary instrumentation and in-line optical 2D/3D hybrid systems for roll-to-roll applications.

Claudiu has been active in developing calibration ISO standards for surface topography measuring instruments and co-authored and published in prestigious metrology and engineering journals.