Ralph Tatam

Cranfield University, School of Engineering, Faculty Member

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Laser speckle velocimetry for robot manufacturing

Optical Measurement Systems for Industrial Inspection X, 2017

A non-contact speckle correlation sensor for the measurement of robotic tool speed is presented f... more A non-contact speckle correlation sensor for the measurement of robotic tool speed is presented for use in robotic manufacturing and is capable of measuring the in-plane relative velocities between a robot end-effector and the workpiece or other surface. The sensor performance was assessed in the laboratory with the sensor accuracies found to be better than ±0.01 mm/s over a ±70 mm/s velocity range. Finally an example of the sensors application to robotic manufacturing is presented where the sensor was applied to tool speed measurement for path planning in the wire and arc additive manufacturing process using a KUKA KR150 L110/2 industrial robot.

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In-process optical monitoring of contamination in an additively manufactured titanium alloy

IEEE sensors 2016 poster

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Particle image velocimetry: from two-dimensional to three-dimensional velocity mapping with holographic recording, object conjugate reconstruction (OCR) and optical correlation

Optical fibre-coupled displacement and angle measurements using range-resolved interferometry

A novel approach to displacement and angle measurements that uses range-resolved interferometry t... more A novel approach to displacement and angle measurements that uses range-resolved interferometry to interrogate multiple interferometers is presented. Spatially offset beams are multiplexed onto the same delivery fibre using a very simple setup comprising of two stacked beamsplitters and the phase evaluation of these interferometers allows simultaneous measurement of both displacement and angular changes of a target surface. The operating principle of this technique and key design concepts are discussed. This approach, with its simple and compact fibre-coupled optical setup and ease of alignment, expands existing applications of laser interferometry, particularly for space-restricted environments, and its cost-effectiveness will be key to allowing widespread usage

Dual sensing-light-sheet OCT for microfluidic PTV

A novel dual beam Optical Coherence Tomography (OCT) system is presented for microfluidics applic... more A novel dual beam Optical Coherence Tomography (OCT) system is presented for microfluidics applications. The dual sensing-light-sheet approach presented here achieves high velocity flow measurement because the rapid re-imaging by the second sensing-light-sheet allows for particle tracking between each image-frame of the pair. We have demonstrated measurements of microchannel flow using 10 μm particles in a 700 μm deep channel with peak velocities of approximately 0.28 metres-per-second (m/s), approximately two orders of magnitude higher than previously reported OCT systems operating at a few millimeters-per-second.

Highly linear, compact and cost-effective sensor probes for displacement measurements using range-resolved interferometry

We demonstrate a novel approach for interferometric displacement and vibration sensing using rang... more We demonstrate a novel approach for interferometric displacement and vibration sensing using range-resolved interferometry (RRI). The concept exploits the ability of RRI to reject the signal contributions due to multiple reflections that would otherwise lead to high levels of periodic non-linearity errors, allowing the use of very compact and cost-effective sensor heads based on gradient index (GRIN) lenses, with the presented approach thought to be widely applicable across many precision engineering applications

Multi degree-of-freedom position sensing by combination of laser speckle correlation and range-resolved interferometry

This paper reports on the development of an end-effector mounted, multi degree-of-freedom positio... more This paper reports on the development of an end-effector mounted, multi degree-of-freedom positioning sensor capable of measuring the (x,y,z) movements relative to an object. The instrument termed wPOS (workpiece Positioning Sensor), combines two complimentary non-contact optical techniques; laser speckle correlation (LSC) and Range-Resolved Interferometry (RRI). Laser speckle correlation is used to measure the in-plane position (x,y) relative to a reference point, while the RRI system provides an out-of-plane (z) absolute range measurement and correction of the in-plane LSC measurement for varying working distance. The concept and operating principles of the instrument is described along with details of the prototype sensor implementation and exemplar results showing accuracies of <30μm after 0.75m lateral travel and repeatability between measurements of ~7 μm.

Laser speckle velocimetry for robot manufacturing

Proceedings of SPIE, Jun 26, 2017

format_quoteThe sensor operates at ∼500 fps, measuring relative motion using laser speckle patterns to improve path characterization in manufacturing.format_quote

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Source Modulation Techniques for Static Deformation and Vibration Measurement in Optoelectronic Shearography

Source wavelength modulation in an unbalanced Michelson shearing interferometer is demonstrated a... more Source wavelength modulation in an unbalanced Michelson shearing interferometer is demonstrated as a means of phase modulation for optoelectronic shearography. This eliminates the requirement for an active phase-shifting component in the detection head. The technique is demonstrated for both static deformation and vibration measurements.

Time-division-multiplexed laser Doppler anemometry using pulsed laser diodes and optical fibers

Proceedings of SPIE, Sep 29, 1995

A time division multiplexed laser Doppler anemometer is reported, using a single high frequency p... more A time division multiplexed laser Doppler anemometer is reported, using a single high frequency pulsed laser diode as a source. Time division multiplexing requires a single detector channel, removing the need for multiple detectors and wavelength separation optics found in conventional CW laser Doppler systems. By incorporating optical fibers into the system to distribute the pulses into each channel and

Single camera three component planar velocity measurements, using two frequency Planar Doppler Velocimetry (2ν-PDV)

Proceedings of SPIE, Aug 18, 2005

The work presented here describes a method that allows three component velocity measurements to b... more The work presented here describes a method that allows three component velocity measurements to be made, quickly and non-intrusively, across a plane in a flow defined by a laser light sheet. The method, two frequency planar Doppler Velocimetry (2ν-PDV) is a modification of the Planar Doppler Velocimetry (PDV) technique, using only a single CCD camera, and sequential illumination of the flow using two frequencies, separated by about 700MHz. One illumination frequency lies on an absorption line of gaseous iodine, and the other just off the absorption line. The beams sequentially illuminate a plane within a seeded flow and Doppler-shifted scattered light passes through an iodine vapour cell onto the camera. The beam at a frequency off the absorption line is not affected by passage through the cell, and provides a reference image. The other beam encodes the velocity information as a variation in transmission dependent upon the Doppler shift. Use of a single camera ensures registration of the reference and signal images, which is the major problem in any spilt image system such as a two-camera imaging head, and cost efficiency is improved by the simplification of the system. A 2ν-PDV system was constructed using a continuous-wave Argon ion laser and acousto-optic modulators to produce two frequencies of illuminating laser light. This was combined with multiple imaging fibre bundles, to port three different views of the measurement plane to a CCD camera, allowing the measurement of three velocity components.

format_quoteThe system demonstrated effective velocity measurement from an axis-symmetric air jet with a theoretical exit velocity of 94 m/s.format_quote

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<title>Passive signal processing of miniature Fabry-Perot interferometric sensors</title>

Proceedings of SPIE, Nov 30, 1994

A passive signal processing technique of miniature (approximately equals 20 micrometers ) low fin... more

Objective speckle displacement resulting from the deformation of shaped objects

Proceedings of SPIE, Jun 22, 2015

This paper describes an extended theory of the displacement of the objective speckle pattern resu... more This paper describes an extended theory of the displacement of the objective speckle pattern resulting from displacement and/or deformation of a coherently illuminated diffuse object where the influence of the surface shape is included via the linear surface gradients. An experimental system capable of measuring the translational scaling factors, the ratios of speckle shift to object translations, to an accuracy of ± 0.02 and a repeatability of approximately ± 0.008 is described which was used to experimentally measure the speckle shift for a range of detector positions and surface gradients. The original expressions developed by Yamaguchi 1 and the new extended expressions 2 are then compared with experimental results for measurements on zero surface gradients, i.e. the mean surface lying in the x-y plane. The divergence of Yamaguchi's expressions from experimental results for off-axis detector positions that was first observed by Světlík 3 was confirmed, and the new expressions shown to successfully predict translational scaling factors for off-axis positions. The new expressions are then compared to the experimental results for a range of surface gradient magnitudes and directions, as well as detector positions both on and off-axis, and shown to successfully predict the observed speckle shift.

format_quoteTranslational scaling factors were measured with a precision of 0.1 pixel (1.4µm) across known translations, revealing measurement accuracy.format_quote

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<title>Simultaneous independent magnetic field and temperature measurement using optical interferometry</title>

Proceedings of SPIE, Feb 15, 1993

ABSTRACT

<title>Surface contouring using TV holography</title>

Proceedings of SPIE, Sep 1, 1991

ABSTRACT A fiberoptic electronic speckle pattern interferometer illuminated with a diode-laser so... more ABSTRACT A fiberoptic electronic speckle pattern interferometer illuminated with a diode-laser source has led to a novel system that allows shape measurement. The technique utilizes the modulation of the diode-laser wavelength to produce height contours of the object under study. This technology has been combined with a phase-stepping algorithm to produce a contour map of the object surface and, therefore, a representation of its shape. Results from a profiling study of a gas-turbine compressor blade using contours from 0.9 to 8 mm are presented.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Application of optical coherence tomography to non-destructively characterise rind breakdown disorder of ‘Nules Clementine’ mandarins

Postharvest Biology and Technology, Oct 1, 2013

ABSTRACT The feasibility of optical coherence tomography (OCT) for imaging histological changes a... more ABSTRACT The feasibility of optical coherence tomography (OCT) for imaging histological changes associated with the development of a progressive rind breakdown (RBD) disorder of ‘Nules Clementine’ mandarin (Citrus reticulate Blanco.) was investigated. The investigation utilised fruit with different levels of the disorder, carefully selected from a batch of fruit stored for eight weeks at ±8°C. Images of healthy and RBD-affected intact mandarin fruit were acquired using a Thorlabs OCT system based on a broadband 930 nm source. OCT provided high resolution 2D images of fruit rind to a depth of about 1.1 mm. Immediate and non-destructive acquisition of images showing histological and microstructural features of intact rind tissues was demonstrated. The oil glands stayed intact in unaffected fruit and gradually collapsed on RBD affected fruit. At advanced stages of the disorder, the collapsed oil glands became increasingly deformed and flattened. The study showed that OCT is a promising technique for immediate, real-time and non-destructive acquisition of images showing histological and microstructural rind features of ‘Nules Clementine’ mandarin fruit.

Absolute angle measurement using dual-wavelength laser speckle for robotic manufacturing

Optical Measurement Systems for Industrial Inspection XI

A non-contact optical technique employing dual-wavelength laser speckle is investigated for absol... more A non-contact optical technique employing dual-wavelength laser speckle is investigated for absolute angle measurement. The approach uses the separation of the speckle patterns formed by two closely spaced illumination wavelengths to determine the angle of a surface. Autocorrelation is performed on a single exposure containing both speckle patterns to find their relative displacement, which is directly related to the absolute surface angle. This absolute angle determination offers an advantage over previous techniques using laser speckle that require a reference image. The underlying theory linking the speckle pattern displacement and the surface angle is presented, along with a proof-of-concept sensor. Experimental results from the sensor confirm the validity of the theory, with measurements demonstrating a mean difference from applied angles of 0.136°.

A fibre optic angle sensing tape for applications in robotics and automation

Optical Measurement Systems for Industrial Inspection XIII

Long-term wavelength stabilisation of widely modulated lasers

Optical Measurement Systems for Industrial Inspection XIII

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