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Going 3D with DMAC

July 18, 2003

The DMAC (Dimensional Metrology Awareness Club) Form Metrology Special Interest Group is concerned with the global characterisation of 3D surfaces using contacting and non-contacting techniques. DMAC, which is run by the NPL (National Physical Laboratory), on behalf of the DTI, has over 65 member organisations.

The activities of this group are key to organisations involved in the manufacture, test and application of a wide range of precision optical and mechanical components used in the consumer, defence, aerospace, automotive, and medical fields.

The prime objective is to bring together users and industrial providers of form metrology, instrumentation, and also to create links with academics active in the field. Through links with the National Standards bodies, awareness and implementation of form metrology standards will be improved with consequent benefits to UK industry. In turn, the group will provide the forum for both UK industry and academics to provide input into new standards as they are developed.

DMAC is a tailor-made forum for this topic because all key manufacturing sectors are represented in its membership. It is the Annual Conference, which attracts the largest number of members to meet, network and keep up to date on a range of dimensional measurement topics and this is the ideal DMAC event to disseminate and discuss form metrology issues.

At the 2002 DMAC Conference, one of the keynote speakers was Dr David Towers from Heriot-Watt University who described a laser based fibre-optic fringe projector, comparing the relative merits of this approach to fringe projection to the more conventional white light techniques. In particular, the depth of field of the projected fringes, system calibration and potential sensitivity of the approaches were compared. Specifically he has used this system to design a new cylinder head water jacket in collaboration with Land Rover Ltd and VAW Motorcast. David also presented details on the construction of the laser based fibre-optic system and the associated data processing methodology and calibration processes.

The challenge of form metrology for aspheric optics and free-form surfaces was described by Dr David Walker from the department of Physics & Astronomy, at University College London. The use of regular aspheric optics is increasing, due to the broader awareness of the benefits that they can bring in terms of less distortion. The technology for manufacturing aspheric (not completely spherical) lenses has improved. However, the ability to measure the form of what is produced is often the limiting factor. Moreover, as designs for truly free-form optics emerge, and engineering surfaces increasingly demand near-optical tolerances, the metrology challenge is compounded.

During the 2003 DMAC Conference a combined meeting of the Form and Surface Texture groups was held. Dr Graham Peggs (NPL) focussed upon the next generation of Extremely Large Telescopes with primary mirrors several tens of metres across. NPL and the Optical Science Laboratory at UCL have joined a project team led by Lund Observatory in Sweden. The team is planning to build the world's largest optical telescope, of 50 metres diameter, to be sited at the Roque de Los Muchachos Observatory, on the island of La Palma in the Canaries.

Light collecting-area and resolution are the fundamental assets of a telescope. Current 8-10 metre telescopes are making exciting discoveries in many areas of astronomy; particularly the large-scale structure and evolution of the universe, and detection of planets orbiting other stars. Nevertheless, these telescopes will inevitably mature and reach their technical limits over the next decade. Potential projects with immense popular appeal, such as imaging of extra-solar planets and searching for spectroscopic signatures of life, are tantalisingly close, yet beyond current facilities.

Dr James Andrew Gibson, 4D Technology and member of the Form Metrology group described the challenges ahead: " Metrology of large systems such as the new proposed ultra large aperture telescopes presents unprecedented problems when nanometre accuracy and/or sensitivity is required. At 4D Technology we are developing systems which provide `instantaneous` analysis so critical environmental problem such as vibration and thermal movement do not affect the measurement. The active assembly and alignment of large and complex multi-segment mirror systems to produce piston errors of nanometres and tilt errors of sub arc-seconds cannot be underestimated and will remain a prime challenge to metrologists for the forseeable future. The DMAC group has been, and will be, essential as a medium for information transfer and measurement development in this very demanding area. We have met most of our key potential customers at DMAC group meetings."

The race is now on to build the next generation of telescopes. These will reach fainter objects, and deliver the full diffraction-limited resolution (at the level of milliarcseconds) of the primary mirror through adaptively changing the shape of the optical elements to correct for the effects of atmospheric blurring. This will give an enormous boost in optical contrast to reveal more fine details of galaxies in formation and improve the chances of finding planets near bright stars. The Euro50 telescope has been conceived to meet these scientific challenges. It is based on a 50 metre diameter f/0.85 primary mirror, comprising 618 hexagonal aspheric segments 2 metres in size. A Gregorian deformable secondary mirror with 4000 actuators provides the first stage of adaptive correction and focuses the light onto auxiliary instrumentation.

A consortium comprising UCL, NPL, QinetiQ and Zeeko Ltd recently undertook a commissioned study into the mirror fabrication at the heart of the Euro50 project. The consortium put forward an imaginative plan for manufacturing the primary segments using the recently developed Zeeko/UCL polishing technology. Segment metrology is the major challenge, given the exquisite absolute-form accuracy required of the segments - 36 nm peak-to-valley. The study highlighted the danger of systematic errors creeping in, as was the case with Hubble Space Telescope - precise metrology, precisely wrong! A strong recommendation was put forward for metrology traceable to the national standard of length, with the caveat that traditional optical tests are seriously impeded by the enormous optical path-lengths - up to 170 metres in some cases. NPL proposed two solutions. Firstly, the multilateration concept (with an accuracy of <1 micrometre) would be suitable for characterising the primary segments in the earlier stages of fabrication. Secondly, a profilometric method could deliver the final traceable metrology of the primary segments. If all goes to plan, this would lead into a construction phase with completion around the year 2012, giving European astronomers the edge over US competition.

This article has given a taster of some of the topics covered in the past. DMAC is now opening its events to all those who wish to try out DMAC activities before joining. One of the DMAC members, John Garrett,Taylor Hobson described the benefits of membership; "At Taylor Hobson our area of expertise is in the measurement of form.   Our Talyrond instruments measure multiplane roundness, straightness and associated relationships such as cylindricity, squareness and parallelism. Our Form Talysurf instruments can compare a component's form with either a best fit or specific fit arc, conic section, or asphere. All of which are areas tackled by the DMAC group. We have group membership of the form metrology group and see the main benefits being the knowledge transfer between academia and the manufacturing industry, helping us identify the requirements of the future."

The Next Event

The next event, which will take place on 7 October, is dedicated to 'MultiDisciplinary Form Measurement' and will appeal to a wide range of manufacturing organisations. Perlos inTyne and Wear, which manufactures injection-moulded precision plastic parts and customized thermoplastic components, will host a tour of their site. There will also be presentations on different aspects of Form Metrology, Nabil Gindy, Professor of Advanced Manufacturing Technology at the School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham will present his work on the measurement of error during the machining of turbine blades and how these might be assessed using CMMs by measuring the fixture used during machining. Todd Stewart, from the Institute of Medical and Biological Engineering at Leeds will also be presenting on medical applications of form metrology.

To reserve a place on 'Multidisciplinary Form Measurement' please contact Melanie Williams on 0208 943 6121 or e-mail melanie.williams@npl.co.uk To obtain further information on DMAC view the DMAC website at www.npl.co.uk/dmac/

National Physical Laboratory (NPL)