Computed tomography (CT) is an aspiring contact free measurement method which allows the complete geometry of objects to be determined. This includes the inner and outer geometry and the surface texture, all of which are typically not fully accessible by other measurement methods. There are a broad range of applications for CT, which include macro and microfabrication, the automotive and telecoms industries, and additive manufacturing.
To support dimensional metrology in advanced manufacturing in the future, this project will develop traceable CT measurement techniques for dimensions and surface texture. Open issues regarding traceability, measurement uncertainty, sufficient precision/accuracy, scanning time, multi material, surface form and roughness, suitable reference standards, and simulation techniques will be addressed through the project’s objectives.
Over the past few years, CT has increasingly been used for dimensional measurements of both the inner and outer geometry of workpieces, such as cavities and parts in mounted assemblies, which originate from macro and microfabrication, the automotive and telecommunication industries and additive manufacturing, etc.
Despite the rapidly increasing number of applications in industry, the measurement errors of most CT systems are considered to be too high and need to be reduced substantially, by a factor of 2 – 8, to the order of 10 µm even when mid size parts (approx. 1000 cm3) are measured. The traceability of the results is yet to be established and methods to determine the measurement uncertainty also need to be developed. The time required to perform CT measurements and data evaluation needs to be reduced from hours to minutes if CT is to be more widely used in industry.
Guidelines and standards, such as standardised test procedures and specifications, are required for a fair and competitive market and to support users of industrial CT. The German standardisation committee VDI/GMA 3.33 has developed a few guidelines (VDI/VDE 2630 series) on dimensional measurements using industrial CT. At the moment, an international standard defining acceptance and reverification tests for CMS using the CT principle is under development by ISO TC213 WG10, which will become part of the ISO 10360 series. This project will provide input to standardisation bodies regarding inline CT and multi material measurements.
These needs are underpinned by the report “Strategic Analysis of Computed Tomography Technology in the Dimensional Metrology Market” published by Frost and Sullivan in 2015. The key areas identified in the study for developing a broader use of industrial CT in industry are “Capabilities to improve measurement resolution”, “Support for multi material complexity” and “Reduced measurement time (scanning and reconstruction)”. In addition, the EURAMET roadmap and the EURAMET Strategic Research Agenda suggest a strong need for improved CT methods.
The specific objectives of the project are:
- To develop traceable and validated methods for absolute CT characterisation including the correction of geometry errors by 9 degrees of freedom (DoF). This will include the development of reference standards, traceable calibration methods and thermal models for instrument geometry correction, as well as the correction of errors originating in the X‑ray tube and the detector in order to improve CT accuracy.
- To develop improved and traceable methods for dimensional CT measurements with a focus on measurements of sculptured / freeform surfaces, roughness, and multi‑material effects including supplementary material characterisation.
- To develop fast CT methods for inline applications based on improved evaluation of noisy, sparse, few, or limited angle X‑ray projections, reconstruction methods. This will be undertaken using a reduced number of projections from well‑known directions and include enhanced post‑processing.
- To develop traceable methods for uncertainty estimation using virtual CT models and Monte‑Carlo simulations. Batch simulation and evaluation capacities will be improved. The determination of accurate model parameters is necessary for a reliable uncertainty estimation and this will therefore be performed for different CTs and it will be systematised. Corrections for several artefacts will be developed. Uncertainty will be estimated by Monte‑Carlo based simulation and verified using the calibrated standards developed in WP1.
- To facilitate the take up of the technology and measurement infrastructure developed in the project by the measurement supply chain (accredited laboratories, instrumentation manufacturers), standards developing organisations (e.g. ISO TC213 WG10, VDI‑GMA 3.33 Technical Committee Computed Tomography in Dimensional Measurements) and end users (e.g. plastic manufacturers, automotive, telecommunication, medical and pharmaceutical industries and metrology service providers).
Progress beyond the state of the art
CT has great potential for the quantitative evaluation of industrial components, compared with conventional measurement technologies, due to its capability to measure both external and internal features simultaneously and non‑destructively. Over the last few years, CT has been increasingly used for dimensional measurements of both the inner and outer geometry of industry workpieces. However, both the quality and speed of the measurements hamper the industrial uptake of the technology.
This project will develop CT as a next generation dimensional and surface metrology tool for industrial applications. Both the quality and efficiency of the measurements performed using CT will be significantly improved in the project in order to meet industrial requirements. This includes an improvement of the accuracy by a factor of 2 - 8 and a reduction of the measurement time to a few minutes or less.
This project will establish a full 9 degrees of freedom in situ metrology CT system, investigate geometrical errors as well as thermal stability, X‑ray tube and detector‑based effects and the corresponding corrections. This project will also investigate state‑of‑the‑art correction methods for CT image forming artefacts, such as cone beam artefacts, metal artefacts, beam hardening artefacts, scattering artefacts and this project will also look into the robustness, efficiency, and standardisation of these correction methods. Well parameterised virtual CT models and Monte‑Carlo simulations will be developed to predict the measurement errors and to determine the measurement uncertainty to ensure confidence in the measurements performed using CT.
This project will pioneer the work needed on surface measurements using CT. The focus will include not only the measurements of surface form, but also the evaluation of surface texture parameters using CT based on ISO 25178. The influence of reconstruction methods and filtering will be studied and a range of surface parameters will be tested for the characterisation of the surface texture of advanced manufactured components e.g. additively manufactured hydraulic components.
Conventional CT scans that take hours to perform cannot meet the industrial requirements for inline measurements. The filtered back projection reconstruction algorithm – being one bottleneck of conventional CT – requires thousands of projection images to be assessed. This cannot be undertaken in industrial production lines and the algorithm also suffers from measurement artefacts induced e.g. by beam hardening and X‑ray scattering. This project will improve the speed of CT measurements, to the order of minutes or less, whilst maintaining the quality of the reconstruction by using compressed sensing reconstruction algorithms to cope with sparse, noisy data from a reduced number of projections or limited angles of projections from measurements. Modern machine learning techniques and theories will be used to optimise the parameters used in reconstruction.
This project is expected to significantly benefit advanced manufacturing, such as casting and additive manufacturing, by reducing the consumption of materials and improving the quality of products. This project will significantly expand the applications of CT technology. This also includes new fields of application in industry where today’s CT techniques are not feasible. It will also support the international standard organisation ISO TC 213 in setting up a specification and testing regime for CT for dimensional applications and it will enable the application of CT in surface metrology.
To develop traceable and validated methods for absolute CT characterisation including the correction of geometry errors by 9 degrees of freedom (DoF). This will include the development of reference standards, traceable calibration methods and thermal models for instrument geometry correction, as well as the correction of errors originating in the X ray tube and the detector in order to improve CT accuracy.
This project provides industry and the scientific community with methods to characterise CT geometry, either based on calibrated reference standards or traceable calibration measurements (e.g. interferometry). Therefore, more than ten reference standards optimised for different tasks were developed and are being characterised. Corresponding software tools were developed and are ready to characterise CT geometries. To study the influence of temperature variations on CT geometry, temperature measurement systems have been installed on three CT systems. Those measurements and simulations are used to develop effective mitigation strategies that will be made available. Information on the influence of complex error sources, such as the X-ray tube and the detector will be disseminated, which will enable their influence on the measurement uncertainty to be estimated. Strategies to mitigate X-ray tube drift show promising improvements for high resolution CT scans. These results represent a fundamental contribution to substantially improve the accuracy of dimensional CT by a factor of 2 - 8 and to establish traceability. Furthermore, they will enable three partner NMIs to provide new CT based calibration services.
To develop improved and traceable methods for dimensional CT measurements with a focus on measurements of sculptured / freeform surfaces, roughness, and multi material effects including supplementary material characterisation.
This project will enable traceable CT measurements of sculptured / freeform surfaces to be undertaken through the development of strategies for data registration and data filtering, which will enable comparison with tactile reference data. A freeform artefact has been selected and, once measurement tasks have been defined, will be used for comparative CT and tactile measurements.
The project will also establish a numerical approach to extract and analyse surface texture data from CT, which will be in compliance with the standard approach of surface roughness analyses. The limitations of roughness measurements undertaken using CT will be investigated and new feature based statistical methods will be developed for surface topography.
This project will demonstrate traceable CT measurements of multimaterial objects that are typically composed of two materials. Multimaterial objects with very different (e.g. plastic / steel), and with small to moderately different (e.g. ceramics / aluminium) absorption coefficients were investigated. The versatility and limits of common, single-threshold surface determination algorithms were investigated, to establish boundaries for their useful application to multimaterial objects. One starting point of these investigations was to develop procedures to reduce metal artefacts in medical CT. Supplementary material characterisation will be undertaken using synchrotron CT. Following an extensive review of the literature, promising metal artefact correction methods have been compared. Metal artefacts can obscure parts of a scanned volume, but most methods to date focus exclusively on qualitative, not quantitative aspects.
To develop fast CT methods for inline applications based on improved evaluation of noisy, sparse, few, or limited angle X ray projections, reconstruction methods. This will be undertaken using a reduced number of projections from well known directions and include enhanced post processing.
An open source metrology software toolkit will be developed to handle sparse and noisy data with a focus on quantitative evaluation. Different CT modalities, measurement procedures and data processing chains will be studied that will allow dimensional CT measurements to be completed within minutes instead of hours.
To develop traceable methods for uncertainty estimation using virtual CT models and Monte Carlo simulations. Batch simulation and evaluation capacities will be improved. The determination of accurate model parameters is necessary for a reliable uncertainty estimation and this will therefore be performed for different CTs and it will be systematised. Corrections for several artefacts will be developed. Uncertainty will be estimated by Monte Carlo based simulation and verified using the calibrated standards developed in WP1.
This project will provide a methodology to adapt CT simulation software to real CT systems. The methodology (published as a scientific paper) will describe how to determine accurate parameters to enable the simulation software to match the performance of a real CT device. These results will lead to a validated approach for estimating the uncertainty of dimensional CT measurements using Monte-Carlo simulations. The impact of different CT artefacts on dimensional CT will also be summarised in a report. These results will address important steps in the establishment of the traceability of dimensional measurements using CT.
The methodology will be incorporated into the simulation software aRTist, which is commercially available from BAM. This will comprise improved tools to enable the simulation to be matched with the characteristics of real CT devices. The uncertainty will be estimated based on the simulation. Tools are currently under development for planning and running large Monte-Carlo simulation batches on distributed systems, and for use with external data evaluation software. A first version of this additional software tools in aRTist is available for testing. It allows us to design large simulation jobs of Monte-Carlo experiments with deterministic and statistical parameter variations. An interface to external evaluation software has been defined and tested between aRTist and two different CT reconstruction programs.
Additionally, a generic virtual CT model has been implemented and will be used to assess the influence of specific system and measurement parameters. Reference objects, two with a size below of 30 mm and one above it, have been produced, calibrated, and modelled to support the model parameter determination. Algorithm-based correction methods for uncertainty improvement are in development. A correction filter for blooming artefacts has been finalised.
To facilitate the take up of the technology and measurement infrastructure developed in the project by the measurement supply chain (accredited laboratories, instrumentation manufacturers), standards developing organisations (e.g. ISO TC213 WG10, VDI GMA 3.33 Technical Committee Computed Tomography in Dimensional Measurements) and end users (e.g. plastic manufacturers, automotive, telecommunication, medical and pharmaceutical industries and metrology service providers).
The take up of the CT technology will be supported by:
- knowledge transfer via a stakeholder committee, a project website, publications in peer-reviewed journals and trade magazines, and presentations at international conferences. To ensure knowledge transfer and to accelerate the uptake of project outputs, there will be frequent and close contact with stakeholders, including those in the stakeholder committee, end users in industry and standardisation bodies.
- input to international and national standardisation. It is expected that the project will accelerate the establishment of ISO standards within the field of CT for geometrical measurements of monomaterial and multimaterial objects for the benefit of the whole CT community. For this, partners will inform ISO TC213 WG10 “Coordinate measuring machines”, ISO TC 261 “Additive manufacturing” and several national committees.
- training lectures at major trade fairs (e.g. CONTROL fair), training sessions for partners, and workshops for stakeholders to disseminate the knowledge obtained to a broader audience. The involvement of CT system manufacturers and end users from industry in the project will also have a positive impact on the uptake and exploitation of the project’s results.
Additionally, industrial case studies will be used to demonstrate the CT technology, and new calibration and measurement services will be introduced by the project’s NMIs.
Impact on industrial and other user communities
The results of this project are expected to be used by a broad range of end users in industry: manufacturing industry (in particular manufacturers of plastic parts fabricated by injection moulding), microfabrication (e.g. watch parts), automotive (e.g. cast parts, electronic components, fuel injection components), telecommunication (e.g. fibre‑optic and HF connectors), medical (e.g. ophthalmology, dental implants), pharmaceutical (e.g. lab on a chip), and metrology service providers. This will also have an impact on the manufacturers of CT systems, as it will make it possible for them to increase their business and growth, as well as to improve their market position.
The improved measurement accuracy and significantly reduced measurement time will be of particular interest for industrial and other CT users. The latter will make inline CT possible and this will provide 100 % quality control. A further industrial impact will be created by the possibility of carrying out roughness evaluations based on CT data.
Impact on the metrological and scientific communities
The main impact for the metrological and scientific communities will be the traceability of the CT measurements and the increased accuracy. The results will enable the NMIs and calibration service providers to introduce CT calibration services. This project will create impact through the uptake of the accuracy improvements by users from outside of the consortium that need to improve their hardware and software.
Impact on relevant standards
This project will enable a better comparison of CT systems by using enhanced standardised test procedures. In addition to only including the geometrical measurement of existing or developmental monomaterial objects in the standardised test procedures, this project will also deliver procedures that take multimaterial objects and surface roughness evaluations into account.
Longer-term economic, social and environmental impacts
The major long-term economic impact of this project will be an increased market penetration of European manufacturers of advanced precision parts and systems, e.g. in the automotive and healthcare industry, who require advanced measurement capabilities for quality control and development. This will be delivered by the advanced measurement capabilities of CT, e.g. the measurement of the complete workpiece geometry, including inner and hidden geometries, in a short measurement time. These capabilities will also accelerate the development of new production technologies for electro‑ and mechanical components, e.g. additive manufacturing, as well as improving the competitiveness of European measuring instrument manufacturers. Additional financial impact will also result from the use of the improved parts, e.g. reduced health costs from new high tech medical products, or reduced fuel consumption from improved fuel injection or battery cell systems.
Furthermore, the increased use of industrial CT systems, which will result from this project, will strengthen the market position of European CT manufacturers. Four of the top 5 participants in the total CT dimensional metrology market (Nikon Metrology, Werth, Yxlon, Zeiss) come from Europe.
Higher quality, longer lasting products will also improve the technical safety of household appliances, transport facilities and medical products, which will have a direct impact on human health and quality of life. The examples investigated in this project include healthcare products for insulin injection and LEGO toys that have inspired and motivated the creativity of several generations of children.
The increased use of CT will speed up product development and production processes, and it will reduce the energy used in the manufacture of physical prototypes. The use of traceable XCT measurements in quality assurance and quality control will result in the earlier detection of scrap parts with an increased possibility to reuse materials and components and it will also reduce waste. Furthermore, the possibility to analyse and test samples and products without cutting them will lead to a reduction in the amount of waste.
The emissions of combustion engines are strongly dependent on the dimensional characteristics of fuel injection systems. This project will result in better measurements of these parts leading to reduced tolerances.
Providing the European manufacturing industry with a strong tool such as traceable CT, will help the industry to keep their production in Europe and to reduce the amount of outsourcing to Asia for example. In general, European production facilities are ‘greener’ and more ‘environmental friendly’ than most production plants in Asia.