CERES :: Browsing by Author "Charrett, Thomas O. H."

Browsing by Author "Charrett, Thomas O. H."

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Open Access
2D spatially-resolved depth-section microfluidic flow velocimetry using dual beam OCT
(MDPI, 2020-03-27) Hallam, Jonathan M.; Rigas, Evangelos; Charrett, Thomas O. H.; Tatam, Ralph P.
A dual beam optical coherence tomography (OCT) instrument has been developed for flow measurement that offers advantages over microscope derived imaging techniques. It requires only a single optical access port, allows simultaneous imaging of the microfluidic channel, does not require fluorescent seed particles, and can provide a millimetre-deep depth-section velocity profile (as opposed to horizontal-section). The dual beam instrument performs rapid re-sampling of particle positions, allowing measurement of faster flows. In this paper, we develop the methods and processes necessary to make 2D quantitative measurements of the flow-velocity using dual beam OCT and present exemplar results in a microfluidic chip. A 2D reference measurement of the Poiseuille flow in a microfluidic channel is presented over a spanwise depth range of 700 μm and streamwise length of 1600 μm with a spatial resolution of 10 μm, at velocities up to 50 mm/s. A measurement of a more complex flow field is also demonstrated in a sloped microfluidic section.
Open Access
3D planar velocity measurements, using Mach-Zehnder interferometric-filter-based planar Doppler velocimetry (MZI-PDV) and imaging fibre bundles
(2008-12-31T00:00:00Z) Lu, Z. H.; Charrett, Thomas O. H.; Ford, Helen D.; Tatam, Ralph P.; David, Sampson; Stephen, Collins; Kyunghwan, Oh; Ryozo, Yamauchi
Three component planar flow-field measurements are made using imaging fibre bundles to port different views of the measurement plane, defined by a laser light sheet, to a single imaging head. The Doppler frequency shifts of light scattered by particles entrained in the flow are transduced to intensity variations using a Mach-Zehnder interferometric filter. The free spectral range of the filter can be selected by adjusting the optical path difference of the interferometer. This allows the velocity measurement range, sensitivity and resolution to be varied. Any laser wavelength may be used. A phase locking system has been designed to stabilise the interferometric filter.
Open Access
Absolute angle measurement using dual-wavelength laser speckle: theory and method
(Elsevier, 2023-11-30) Gibson, Sam J.; Charrett, Thomas O. H.; Tatam, Ralph P.
This paper presents a method utilising the speckle pattern formed by dual-wavelength illumination for the measurement of the two out-of-plane surface angles with respect to the sensor frame. Theoretical expressions are derived relating the observed speckle shift between patterns formed by two wavelengths for tilted surfaces with both on-axis and off-axis detector positions. These expressions are verified experimentally, showing RMS errors of between 0.5–1.0 . Finally, an on-axis implementation of the concept is presented using dual-wavelength illumination generated from two modes of a standard FP diode laser. Simplified expressions for the calculation of surface angles from measured speckle shift using this arrangement are presented, given in terms of three sensor constants; the responsivity or sensitivity of the sensor, C, and the zero surface tilt speckle shifts, Ax0 and Ay0. Experimental results using this sensor for a range of surface tilts between 0.0° and 7.5° showed an RMS error of 0.10° in θx and 0.19° in θy.
Open Access
Application and performance of laser speckle odometry applied to a mobile industrial robot
(SPIE, 2023-08-15) Charrett, Thomas O. H.; Gibson, Sam; Tatam, Ralph P.
In this paper we report the application of a laser speckle odometer to a mobile industrial robot in a typical factory floor environment. The suitability of typical floor surfaces and features is assessed in terms of the ability to form speckle patterns with sufficient signal to noise for correlation-based processing. All tested surfaces including concrete, rubber tile, dried paint and oil stains, and hazard tapes were found to be suitable. A comparison of the velocimetry sensor output to the industrial robot’s internal SLAM and wheel encoder data is presented with good agreement of < 0.3mm/s at tested speeds of up to 250mm/s. Finally, a comparison of speckle odometry to the robot’s internal SLAM based navigation will be presented using a laser tracker to provide ground-truth measurement data. Both techniques were found to perform similarly, with errors of up to 80mm when traversing a 16m square path of 4m sides. The laser speckle odometry was however found to perform significantly better over the initial sides of the path with a maximum error of < 10mm in comparison to < 47mm for the robot’s internal odometry.
Open Access
Automated interlayer wall height compensation for wire based directed energy deposition additive manufacturing
(MDPI, 2023-10-16) Qin, Jian; Vives, Javier; Raja, Parthiban; Lasisi, Shakirudeen; Wang, Chong; Charrett, Thomas O. H.; Ding, Jialuo; Williams, Stewart; Hallam, Jonathan Mark; Tatam, Ralph P.
Part quality monitoring and control in wire-based directed energy deposition additive manufacturing (w-DEDAM) processes has been garnering continuous interest from both the academic and industrial sectors. However, maintaining a consistent layer height and ensuring that the wall height aligns closely with the design, as depicted in computer-aided design (CAD) models, pose significant challenges. These challenges arise due to the uncertainties associated with the manufacturing process and the working environment, particularly with extended processing times. To achieve these goals in an industrial scenario, the deposition geometry must be measured with precision and efficiency throughout the part-building process. Moreover, it is essential to comprehend the changes in the interlayer deposition height based on various process parameters. This paper first examines the behaviour of interlayer deposition height when process parameters change within different wall regions, with a particular focus on the transition areas. In addition, this paper explores the potential of geometry monitoring information in implementing interlayer wall height compensation during w-DEDAM part-building. The in-process layer height was monitored using a coherent range-resolved interferometry (RRI) sensor, and the accuracy and efficiency of this measurement were carefully studied. Leveraging this information and understanding of deposition geometry, the control points of the process parameters were identified. Subsequently, appropriate and varied process parameters were applied to each wall region to gradually compensate for wall height. The wall height discrepancies were generally compensated for in two to three layers.
Open Access
Characterisation of a cryostat using simultaneous, single-beam multiple-surface laser vibrometry
(American Institute of Physics Publishing, 2016-06-28) Kissinger, Thomas; Charrett, Thomas O. H.; James, Stephen W.; Adams, Alvin; Twin, Andrew; Tatam, Ralph P.
A novel range-resolved interferometric signal processing technique that uses sinusoidal optical frequency modulation is applied to multi-surface vibrometry, demonstrating simultaneous optical measurements of vibrations on two surfaces using a single, collimated laser beam, with a minimum permissible distance of 3.5 cm between surfaces. The current system, using a cost-effective laser diode and a fibre-coupled, downlead insensitive setup, allows an interferometric fringe rate of up to 180 kHz to be resolved with typical displacement noise levels of 8 pm · Hz−0 5. In this paper, the system is applied to vibrometry measurements of a table-top cryostat, with concurrent measurements of the optical widow and the sample holder target inside. This allows the separation of common-mode vibrations of the whole cryostat from differential vibrations between the window and the target, allowing any resonances to be identified.
Open Access
Compensation strategies for robotic motion errors for additive manufacturing (AM)
(University of Texas, 2016-08-10) Bandari, Yashwanth K.; Charrett, Thomas O. H.; Michel, Florent; Ding, Jialuo; Williams, Stewart W.; Tatum, Ralph P.
It is desirable to utilise a robotic approach in additive manufacturing as Computer Numerical Control (CNC) is expensive and it has high maintenance costs. A robotic approach is relatively inexpensive compared to CNC and can provide much more flexibility, enabling a variety of configurations and easier parallel processing. However, robots struggle to achieve high positioning accuracy and are more prone to disturbances from the process forces. This paper attempts to characterise the robot position and velocity errors, which depend on the build strategy deployed, using a laser speckle correlation sensor to measure the robotic motion. An assessment has been done as to whether these errors would cause any problem in additive manufacturing techniques, where the test parts were built using the Wire+Arc Additive Manufacture (WAAM) technique. Finally, different compensation strategies are discussed to counter the robotic errors and a reduction of 3 mm in top surface profile irregularity by varying the wire feed speed (WFS) during the path has been demonstrated.
Open Access
Cost-effective vibration and displacement measurement using range-resolved interferometry
(British Society for Strain Measurement’s (BSSM), 2015-09-03) Kissinger, Thomas; Charrett, Thomas O. H.; James, Stephen W.; Tatam, Ralph P.
A recently developed range-resolved optical interferometric signal processing technique is applied to vibration and displacement sensing with fibre-based beam delivery. The technique is demonstrated to allow the simultaneous acquisition of high-quality, high resolution relative displacement measurements (ߪ ൌ ͵ ) based on interferometric phase evaluation along with coarser ሺߪ ൌ ͲǤͲͷ ) absolute displacement measurements. The interferometric relative displacement data can be used for vibrometry measurement and to yield high-quality derivative velocity and acceleration data suitable for position control applications. The absolute data can serve as an additional proximity sensor. The sensing approach employs cost-effective diode lasers, off-the-shelf digital processing hardware and a very simple optical setup, and can, due to the use of a collimated beam, operate over a wide range of working distances.
Open Access
Development of two-frequency planar doppler velocimetry instrumentation
(Cranfield University, 2006) Charrett, Thomas O. H.; Tatam, Ralph P.
This thesis describes the development of the two-frequency Planar Doppler Velocimetry (2n-PDV) flow measurement technique. This is modification of the Planar Doppler Velocimetry (PDV) technique that allows the measurement of up to three components of the flow velocity across a plane defined by a laser light sheet. The 2n-PDV technique reduces the number of components required to a single CCD camera and iodine cell from the two CCDs in conventional PDV. This removes the error sources associated with the misalignment of the two camera images and polarisation effects due to the beam splitters used in conventional PDV. The construction of a single velocity component 2n-PDV system is described and measurements made on the velocity field of a rotating disc and an axisymmetric air jet. The system was then modified to make 3D velocity measurements using coherent imaging fibre bundles to port multiple views to a single detector head. A method of approximately doubling the sensitivity of the technique was demonstrated using the measurements made on the velocity field of the rotating disc and was shown to reduce the error level in the final orthogonal velocity components by ~40 to 50%. Error levels of between 1.5ms-1 and 3.1ms-1 depending upon observation direction are demonstrated for a velocity field of ±34ms-1. The factors that will influence the selection of a viewing configuration when making 3D PDV measurements are then investigated with the aid of a computer model. The influence of the observation direction, the magnitude of the flow velocity, and the transformation to orthogonal velocity components are discussed. A new method using additional data in this transformation is presented and experimental results calculated using four-measured velocity components are compared to those found conventionally, using only three components. The inclusion of additional data is shown to reduce the final error levels by up to 25%.
Open Access
Dual Illumination Planar Doppler Velocimetry using a Single Camera
(International Society for Optical Engineering; 1999, 2003-12-31T00:00:00Z) Charrett, Thomas O. H.; Ford, Helen D.; Nobes, David S.; Tatam, Ralph P.
A Planar Doppler Velocimetry (PDV) illumination system has been designed which is able to generate two beams, separated in frequency by about 600 MHz. This allows a common-path imaging head to be constructed, using a single imaging camera instead of the usual camera pair. Both illumination beams can be derived from a single laser, using acousto-optic modulators to effect the frequency shifts. 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 that lies at an optical frequency away from the absorption line is not affected by passage through the cell, and provides a reference image. The other beam, the frequency of which coincides with an absorption line, encodes the velocity information as a variation in transmission dependent upon the Doppler shift. Images of the flow under both illumination frequencies are formed on the same camera, ensuring registration of the reference and signal images. This removes a major problem of a two-camera imaging head, and cost efficiency is also improved by the simplification of the system. The dual illumination technique has been shown to operate successfully with a spinning disc as a test object. The benefits of combining the dual illumination system with a three-component, fibre-linked imaging head developed at Cranfield will be discussed.
Open Access
Dual-channel OCT for velocity measurement in microfluidic channels.
(OSA, 2018-09-28) Rigas, Evangelos; Hallam, Jonathan M.; Ford, Helen D.; Charrett, Thomas O. H.; Tatam, Ralph P.
A dual-beam Optical Coherence Tomography system has been developed, using a bespoke dual optical fibre, to simultaneously image microfluidic channel structures and measure high velocity flows (presently 250μm/s) from a single optical access point.
Open Access
A fibre optic angle sensing tape for applications in robotics and automation
(SPIE, 2023-08-15) Pillai, Sivaji; Charrett, Thomas O. H.; Kissinger, Thomas; James, Stephen W.; Tatam, Ralph P.
A dual-fibre single-plane angle-sensing tape that utilizes optical Fibre Segment Interferometry and Range- Resolved Interferometry (RRI) for angle sensing is presented. The sensing tape facilitates the multiplexing of an array of angle sensors along its length and can be retrofitted into small robots and construction equipment. We demonstrate its application on three non-rotational joints of a small five-axis robot, describing the design, construction, measurement principle, and presenting measurement results. Preliminary data shows that the angles measured by the sensing tape exhibited agreement within a range of ±0.005° with the manufacturer-installed angle encoder of the robot.
Open Access
Fibre segment interferometry for dynamic strain measurements
(Institute of Electrical and Electronics Engineers, 2016-02-18) Kissinger, Thomas; Correia, Ricardo N.; Charrett, Thomas O. H.; James, Stephen W.; Tatam, Ralph P.
Using a novel range-resolved interferometric signal processing technique based on the sinusoidal optical frequency modulation of a cost-effective laser diode, a fibre sensing approach termed fibre segment interferometry (FSI) is described. In FSI, a chain of long-gauge length fibre optic strain sensors are separated by identical in-fibre partial reflectors. Targeted at dynamic strain analysis and ultrasound detection for structural health monitoring, this approach allows integrated strain measurements along fibre segments, removing the sensing gaps and sensitivity to inhomogeneities found with localized fibre sensors. In this paper, the multiplexing of six fibre segments, each of length 12.5 cm, is demonstrated. The sensor array can be interrogated at 98 kHz data rate, achieving dynamic strain noise levels 0:14 n Hz 0:5. The reflector fabrication is discussed, an analysis of linearity and noise performance is carried out and results from an exemplar experiment to determine the speed-of-sound of a stainless steel rod are shown.
Open Access
Fibre segment interferometry using code-division multiplexed optical signal processing for strain sensing applications
(Iop Publishing Ltd, 2013-09-30T00:00:00Z) Kissinger, Thomas; Charrett, Thomas O. H.; Tatam, Ralph P.
A novel optical signal processing scheme for multiplexing fibre segment interferometers is proposed. The continuous-wave, homodyne technique combines code-division multiplexing with single-sideband modulation. It uses only one electro-optic phase modulator to achieve both range separation and quadrature interferometric phase measurement. This scheme is applied to fibre segment interferometry, where a number of long-gauge length interferometric fibre sensors are formed by subtracting pairs of signals from equidistantly placed, weak back reflectors. In this work we give a detailed account of the signal processing involved and, in particular, explore aspects such as electronic bandwidth requirements, noise, crosstalk and linearity, which are important design considerations. A signal bandwidth of ±20 kHz permits the resolution of phase change rates of 2.5 × 104 rad s-1 for each of the four 16.5 m long segments in our setup. We show that dynamic strain resolutions below 0.2 nanostrain Hz-0.5 at 2 m sensor gauge length are achievable, even with an inexpensive diode laser. When used in applications that require only relative strain change measurements, this scheme compares well to more established techniques and can provide high-fidelity yet cost-effective measurements.
Open Access
Fibre-optic measurement of strain and shape on a helicopter rotor blade during a ground run: 2. Measurement of shape
(IOP Publishing, 2022-06-08) Kissinger, Thomas; James, Stephen W.; Weber, Simone; Mullaney, Kevin; Chehura, Edmond; Pekmezci, Huseyin H.; Barrington, James; Staines, Stephen E.; Charrett, Thomas O. H.; Lawson, Nicholas J.; Lone, Mudassir; Atack, Richard; Tatam, Ralph P.
Optical fibre strain and shape measurement sensors were deployed on a rotor blade during a full-speed helicopter ground run, with real-time data wirelessly streamed from rotor hub-mounted sensor interrogators. In part 2 of a 2-part paper series, two-dimensional direct fibre-optic shape sensing (DFOSS), using fibre segment interferometry-based interrogation is investigated. The concept of blade shape change visualisation over one rotation period using rotation displacement surfaces is introduced and the usefulness of DFOSS data to gain additional insights by determining operational modal frequencies independently for both horizontal and vertical vibration directions of the blade is demonstrated.
Open Access
Frequency division multiplexing for interferometric planar Doppler velocimetry
(Optical Society of America, 2014-05-30) Charrett, Thomas O. H.; Bledowski, Ian A.; James, Stephen W.; Tatam, Ralph P.
A new method of acquiring simultaneously the signal and reference channels used for interferometric planar Doppler velocimetry is proposed and demonstrated. The technique uses frequency division multiplexing (FDM) to facilitate the capture of the requisite images on a single camera, and is suitable for time-averaged flow measurements. Furthermore, the approach has the potential to be expanded to allow the multiplexing of additional measurement channels for multicomponent velocity measurement. The use of FDM for interferometric referencing is demonstrated experimentally with measurements of a single velocity component of a seeded axisymmetric air jet. The expansion of the technique to include multiple velocity components was then investigated theoretically and experimentally to account for bandwidth, crosstalk, and dynamic range limitations. The technique offers reduced camera noise, automatic background light suppression, and crosstalk levels of typically <10%. Furthermore, as this crosstalk is dependent upon the channel modulations applied, it can be corrected for in postprocessing.
Open Access
Frequency-division multiplexing for multicomponent shearography
(Osa Optical Society of America, 2013-01-20T00:00:00Z) Bledowski, Ian A.; Charrett, Thomas O. H.; Francis, Daniel; James, Stephen W.; Tatam, Ralph P.
A new method of multiplexing the speckle patterns needed in multicomponent digital shearography systems is presented. Frequency-division multiplexing (FDM) of the measurement channels is achieved by recording speckle patterns from objects illuminated by intensity-modulated sources. Each source is modulated at a discrete frequency, which is less than half of the camera frame rate, and a bank of images of the modulated speckle patterns is captured. This allows for pixel-by-pixel Fourier-based extraction of the individual speckle patterns from peaks in the power spectra. The approach is demonstrated with a two-component in-plane shearography instrument.
Open Access
Full-field interferometry using infinity corrected optics
(IOP Publishing, 2015-12-01) Charrett, Thomas O. H.; Tatam, Ralph P.
In this paper the construction of full-field (imaging) interferometers using infinity corrected optics commonly used in microscopy is discussed, with an emphasis on self-mixing interferometry configurations where the imaged light field is mixed with itself rather than a reference wave. Such configurations are used in speckle shearing interferometry, flow visualisation and quantitative flow measurement. The critical considerations for constructing path-length imbalanced full-field interferometers for these and similar applications are discussed, expressions are derived for key calculations and interferograms from example interferometers are presented. These include the concept of balancing the infinity-spaces of the two arms via the use of a glass block to minimise the optical path difference variation across the interferogram and ensure adequate sampling of the fringes on the detector. Further, the use of tilted glass blocks in single-pass and double-pass arrangements is detailed for the generation and control of spatial carrier fringes without extensive realignment of the interferometer, and for phase shifting.
Open Access
In-process range-resolved interferometric (RRI) 3D layer height measurements for wire+ arc additive manufacturing (WAAM)
(IOP, 2022-01-10) Hallam, Jonathan M.; Kissinger, Thomas; Charrett, Thomas O. H.; Tatam, Ralph P.
In this work a range resolved interferometry (RRI) instrument for absolute distance measurements is integrated into a wire + arc additive manufacturing (WAAM) system to provide in-process monitoring of layer height, and prospects for volume and profile monitoring are discussed. In this experiment titanium feedstock was used to create a 150 mm long, 13.5 mm high wall comprised of 11 WAAM deposited layers. The RRI in-process measurements are in very good agreement ($\lt$0.05 mm) with both mid-process, on-machine micrometre measurements taken by hand after each WAAM deposition, and post-process laser scanning measurements of the completed wall. The high depth of field allows direct referencing of the layer height measurements to the build plate making the measurement independent of the motion system and build plate bending, considerably lowering uncertainties. This, together with the capability for cost-effective in-process measurements in harsh environments, should make the proposed approach very interesting for routine use in WAAM systems.
Open Access
Investigation into the selection of viewing configurations for three-component planar Doppler velocimetry measurements.
(Osa Optical Society of America, 2007-07-01T00:00:00Z) Charrett, Thomas O. H.; Nobes, David S.; Tatam, Ralph P.
A method for the calculation of three orthogonal velocity components in planar Doppler velocimetry (PDV) using four or more measured velocity components (to the three typically used) is presented. The advantages and disadvantages are assessed by use of a Monte Carlo simulation and experimental measurements of the velocity field of a rotating disk. The addition of a fourth velocity component has been shown to lead to reductions in the final errors of up to 25%. The selection of viewing configurations for experiments is discussed by simulation of the level of errors in measured velocity components and investigation of the final level of errors in the orthogonal velocity components. Experimental measurements of the velocity field of a rotating disk are presented, demonstrating the effect of the viewing configuration on the final level of error.