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Photogrammetry and imaging for measurement

UAV for landform measurement, mapping and digital elevation models

High resolution digital elevation models (DEMs) are increasingly produced from photographs acquired with consumer cameras, both from the ground and from unmanned aerial vehicles (UAVs). However, although such DEMs may achieve centimetric detail, they can also display systematic broad-scale error that restricts their wider use. Such errors which, in typical UAV data are expressed as a vertical 'doming' of the surface, result from a combination of near-parallel imaging directions and inaccurate correction of radial lens distortion. Using simulations of multi-image networks with near-parallel viewing directions, we show that enabling camera self-calibration as part of the bundle adjustment process inherently leads to erroneous radial distortion estimates and associated DEM error. This effect is relevant whether a traditional photogrammetric or newer structure-from-motion (SfM) approach is used, but errors are expected to be more pronounced in SfM-based DEMs, for which use of control and check point measurements are typically more limited. Systematic DEM error can be significantly reduced by the additional capture and inclusion of oblique images in the image network; we provide practical flight plan solutions for fixed wing or rotor-based UAVs that, in the absence of control points, can reduce DEM error by up to two orders of magnitude. The magnitude of doming error shows a linear relationship with radial distortion and we show how characterization of this relationship allows an improved distortion estimate and, hence, existing datasets to be optimally reprocessed. Although focussed on UAV surveying, our results are also relevant to ground-based image capture. © 2014 John Wiley & Sons, Ltd.


AIRBUS LANDING GEAR DEFLECTIONS - FAL Final Assembly Line (2012)

COMMERCIAL PROJECT: Restricted public information. 3-month project, August - October 2012.

Airbus UK commissioned UCL to deliver a measurement plan for landing gear deflection measurements on the A350 Final Assembly Line in Toulouse. This request was an extension of proposals for measuring landing gear deflections in the AST project (separately listed). The actual measurements were to be carried out by the commercial service company G2Metric and G2Metric were involved in the final evaluations in Toulouse.

The essential elements of this proposal were:

  • Evaluation of requirements at Airbus Filton (UK)
  • Reconnaissance visit to Toulouse
  • Documentation and presentation of proposal in Filton

The UCL team and the commercial service provider were in agreement on a general approach to combine photogrammetric networks with laser tracker measurements. The project deliverable was a guideline to Airbus for specifying a contract with G2Metric to carry out the proposed measurement plan. This was subsequently successfully implemented.

  • Collaboration: Stuart Robson, Stephen Kyle, AIRBUS

AIRBUS METROLOGY FOR DEFORMATION 2012 - AST (AIRBUS STATIC TEST) 

Airbus metrology for deformation 2012 – AST (Airbus Static Tests).

Airbus metrology for deformation 2012 – AST (Airbus Static Tests)

COMMERCIAL PROJECT: Restricted public information. The project took place January - June 2012.

An aircraft’s wings and landing gear are subject to very high loading and consequent bending and deformation. The extent of the bending and deformation must be understood, and be within the design parameters, before a new aircraft can be permitted to fly or modifications implemented. Finite Element Modelling (FEM) is a technique used to predict deformations under load, but like any modelling method it must agree with independent measurement, so comparisons are required. Specific load tests on the ground, known as static tests, were planned for mid-2012 in order to validate two wing designs and one landing gear design. In preparation for these, Airbus UK commissioned UCL in early 2012 to specify measurement methods and strategies suitable for multi-point measurement on the wings and landing gear under the proposed load levels. As well as FEM validation and accurate determination of deflections under specific loads, another objective was to avoid the use of “string potentiometers” as tools for measuring wing bend. These physical contact devices are wire cables on reels, fixed at one end and connected to the wing at the other. As the wing deflects under a static load, the cables reel and unreel. This rotation, measured by potentiometer, is a direct measure of the linear movement at the point of attachment. Single-direction displacement measurement, physical attachment, multiple wires and limited numbers of sample points are all disadvantages of this method. Non-contact, 3-dimensional multi-point measurement, including detailed surface buckling, were required alternatives. From the outset it was clear that photogrammetry, capable of dimensionally accurate, wide-area 3D “snapshots” of large numbers of targeted points, was a likely tool to solve the problem. Laser tracker measurements could also be advantageously included. Although they only single point measurement devices, they could dynamically check critical points to a very high 3D accuracy. Despite these apparently clear options, a full review was made of potentially usable methods in portable 3D metrology in order to confirm that photogrammetry was indeed the best option.

The essential components in this project were:

  • Evaluation of existing technology to choose the best options
  • Determination of the requirements at Airbus for displacement measurement to deal with: wing displacements and buckling, comparison with string potentiometer measurement, landing gear displacements
  • Design and simulation of photogrammetric solutions for the above
  • Specification of final solutions

The proposed wing bend solutions were subsequently successfully implemented.

Collaboration: Stuart Robson, Stephen Kyle, AIRBUS


AIRBUS SURFACE METROLOGY 2011 - FAB (FreeForm Airbus)

COMMERCIAL PROJECT: Restricted public information. The project took place over the period mid-2010 – mid-2012.

The earlier LDA project (2010 - separately listed) had identified critical issues in the operation, comparison and performance evaluation of portable optical surface scanning systems:

  • Limitations in surface scanning accuracy, in particular: Errors caused by tracking of line scanners, Errors due to properties of measured surfaces, e.g. polished/matt, black/white
  • Difficulty in comparing scanning systems due to inconsistent and limited specifications
  • Limitations in guidelines for the performance evaluation of systems

As a result, Airbus UK commissioned a follow-on project from UCL and the UK’s National Physical Laboratory (NPL) to assess the surface scanning requirements at Airbus in detail, and devise effective procedures for evaluating different surface scanning systems. The primary objective of this project was to specify reference artefacts which would enable different surface scanning systems to be evaluated and compared for accuracy and for their ability to measure typical surfaces used in Airbus manufacturing processes. Following acceptance of the specifications proposed by the project partners, two artefacts were subsequently manufactured by NPL for internal use at Airbus. These were supplied with documentation for its effective usage.

LiDAR tiles. Inserting and retrieving files on a cloud-based database is compared to native file system and cloud storage transfer speed.

Collaboration: Stuart Robson, Stephen Kyle, AIRBUS


AIRBUS SURFACE METROLOGY 2011 - LDA (Low Drag Aircraft)

COMMERCIAL PROJECT: Restricted public information. A short 4-month project in early 2010.

Lower drag improves an aircraft’s performance and efficiency in many ways, such as reduced fuel costs and a smaller environmental impact. Low drag aircraft need low drag wings for which accurate shape and a high level of surface smoothness are significant factors in reducing drag. For this commercial project, Airbus UK commissioned UCL and the UK’s National Physical Laboratory (NPL) to deliver an in-depth review of portable surface scanning technologies relevant to the measurement of wing shape and surface quality, both on the ground and in flight. The project required a technology review, the generation of concept proposals and the scanning of sample surfaces. The technology review provided a more in-depth analysis of scanning systems than the NGCW project (separately listed). The deliverables were in two parts:

Part 1 – capture the State of the Art (SoA) in surface form measurement covering:

  • Static and on-ground surface form measurement
  • In-flight measurement of surface form and key dimensions.

Part 2 – evaluate metrology for production

  • Test measurements and proposals for metrology selection, evaluation and integration
  • Proposals for metrology development and in-flight testing

Key findings from this short-term project were:

  • Identification of errors due to the tracking of line scanners and the properties of the measured surfaces (e.g. polished/matt, black/white)
  • Inconsistency in manufacturers’ systems specifications which make comparison difficult
  • Limitations in procedures for performance testing of surface scanning systems

A follow-on project was therefore initiated to evaluate the key findings in more detail. See the FAB project for more information.

  • Key publication: Freeform surface measurement under review at Airbus UK, PowerPoint presentation delivered at the Large Volume Metrology Conference (LVMC), Nov. 2010.Note that the LVMC has now become the European Portable Metrology Conference (EPMC) and the presentation is available as a download from archives now at www.epmc.events

Collaboration: Stuart Robson, Stephen Kyle, AIRBUS


AIRBUS METROLOGY 2009 - NGCW (Next Generation Composite Wing)

COMMERCIAL PROJECT: Restricted public information. A 4-month project from October 2008 to February 2009.

Airbus UK commissioned this multiple UK-partner project to deliver a set of State-of-the-Art (SoA) reports concerning 3D metrology relevant to the manufacture of next generation composite wings. Partners were: Commercial: Metris (now Nikon Metrology), Rolls-Royce; · University: UCL, Bath, Nottingham, Loughborough; Labs: National Physical Laboratory (NPL). This project generated 11 separate reports as deliverables. These reports covered a spectrum of dimensional metrology systems and techniques which were potentially relevant to metrology-assisted manufacture and assembly of the next generation of composite wings. The UCL team contributed to 4 of these reports which concentrated on portable 3D metrology in the following theme areas:

Structure mapping

  • Techniques used to locate hidden parts and features
  • Examples: Offset probes, 6D probes, CMM arm relocation, through-skin sensing

Process and metrology integration

  • Generic metrology-enabled processes Detailed descriptions of specific technologies
  • Analysis of performance, strengths and weaknesses
  • Best practice to integrate measurement systems into processes Example processes: Nikon’s adaptive control, Hexagon’s T-Mac

Large-volume photogrammetry

  • Generic instrumentation providing sub-mm accuracies at scales > 10m State of the art capabilities of large volume photogrammetric systems, e.g. Targeting, illumination and imaging systems,Automated image measurement, network adjustment and self calibration
  • Examples of system applications
  • Relevance of the VDI/VDE Guideline
  • Illustrative simulation of system capabilities in an environment > 10m
  • Future developments and enhancements

Localized photogrammetry

  • Generic instrumentation providing sub-mm non-contact accuracies at scales < 10m
  • State of the art capabilities of large volume photogrammetric systems, e.g. Targeting, illumination and imaging systems, Automated image measurement, network adjustment and self calibration
  • Examples of system applications
  • Relevance of the VDI/VDE Guideline
  • Illustrative simulation of system capabilities in a environment < 10m
  • Future developments and enhancements

Collaboration: Stuart Robson, Stephen Kyle, AIRBUS