We present a novel coordinate measurement system based on a combination of frequency scanning interferometry and multilateration. The system comprises a number of sensors (minimum of four) that surround the measurement volume. Spherical glass retro-reflectors act as targets that are used to define the points in space to be measured. The sensors all measure the absolute distance to all targets simultaneously. The resulting distances are then used to compute the coordinates of the targets and other systematic parameters such as the sensor locations. Initial experimental comparison with a commercial laser tracker has shown that the proposed system is capable of achieving coordinate uncertainties of the order of 40 μm in a measurement volume of 10 m × 5 m × 2.5 m. The system is self-calibrating, inherently traceable to the international system of units (the SI) and computes rigorous coordinate uncertainty estimates.
The next generation of ground-based extremely large telescopes of 30 m to 100 m aperture calls for the manufacture
of several hundred sub-aperture segments of 1 m to 2 m diameter. Each annulus of the overall aperture is formed from
separate elements of the appropriate off-axis conic section (usually a paraboloid). Manufacture of these segments
requires a systematic approach to in- and post-process metrology for all stages of manufacture, including the grinding
stage, despite the fact that the resulting ground surface is generally not amenable to optically reflective measurement
To address the need for measurements on such 1 m to 2 m telescope segments, a swing arm profilometer has been
constructed as part of a collaborative project between University College London (UCL) and the UK National
Physical Laboratory (NPL). The current swing-arm profilometer is intended as a proof-of-concept device and has the
capability to measure concave and convex surfaces of up to 1 m in diameter with a minimum radius of curvature of
1.75 m for concave and 1.25 m for convex surfaces. Results will be traceable to national length standards.
Principles of the swing-arm instrument will be described together with the mechanics of the arm design, its bearing
and adjustment arrangements and surface probe options. We assess the performance requirements of 20 nm RMS form
measurement accuracy in the context of the tolerances of the selected profilometer components, the error budget, and
preliminary system measurements. Initial results are presented with a Solartron linear encoder. We also plan to mount
optical sensors on the end of the arm as an alternative to traditional contact probes. Initially these will include an
Arden AWS-50 wavefront curvature sensor and a Fisba μ-phase interferometer. The method of attachment of the
Arden AWS-50 is outlined. The swing arm profilometer is to be located at a specialised facility, the OPtiC Technium,
Denbigh, North Wales, where it will form part of a tool-kit of metrology and polishing devices for researching the
production of large aspheric surfaces.
A CMM has been developed which operates over a working volume of 50 × 50 × 50 mm, and achieves an uncertainty in 3D probing of ~100 nm. This miniature CMM is based around the concept of a metrology frame, mounted on a host CMM, with a miniature probe system held on the host CMM's ram. The probing system is rigidly connected to 3 orthogonal mirrors, the positions and rotations of which are measured using 3 dual axis interferometers (length, angle) and 3 dual axis angular sensors. Corrections for the mis-alignments of the interferometers, flatness errors of the mirrors and the performance of the miniature probe system are all determined in situ, by reference to the calibrated laser wavelength. This process performs a full error map of the CMM and requires only two artefacts: a precision sphere and a good quality optical cube. The error map is used online to determine the 3D position of the probe tip, based on measurements of the interferometers and angle sensing systems. The CMM is fully programmable and operates as a normal CMM, albeit with considerably improved accuracy. The design, manufacture and calibration of the CMM are described, followed by examples of measurements made with the machine and a determination of the uncertainty sources. This CMM is designed as the first step in bridging the gap between conventional (millimetre scale metrology) and nanometrology.
Conference Committee Involvement (2)
Recent Developments in Traceable Dimensional Measurements III
31 July 2005 | San Diego, California, United States
Recent Developments in Traceable Dimensional Measurements II
4 August 2003 | San Diego, California, United States