Jon N. Petzing

Surface measurement errors using commercial scanning white light interferometers

This paper examines the performance of commercial scanning white light interferometers in a range of measurement tasks. A step height artefact is used to investigate the response of the instruments at a discontinuity, while gratings with sinusoidal and rectangular profiles are used to investigate the effects of surface gradient and spatial frequency. Results are compared with measurements made with tapping mode atomic force microscopy and discrepancies are discussed with reference to error mechanisms put forward in the published literature. As expected, it is found that most instruments report errors when used in regions close to a discontinuity or those with a surface gradient that is large compared to the acceptance angle of the objective lens. Amongst other findings, however, we report systematic errors that are observed when the surface gradient is considerably smaller. Although these errors are typically less than the mean wavelength, they are significant compared to the vertical resolution of the instrument and indicate that current scanning white light interferometers should be used with some caution if sub-wavelength accuracy is required.

Nonperturbing measurements of spatially distributed underwater acoustic fields using a scanning laser Doppler vibrometer

Localized changes in the density of water induced by the presence of an acoustic field cause perturbations in the localized refractive index. This relationship has given rise to a number of nonperturbing optical metrology techniques for recording measurement parameters from underwater acoustic fields. A method that has been recently developed involves the use of a Laser Doppler Vibrometer (LDV) targeted at a fixed, nonvibrating, plate through an underwater acoustic field. Measurements of the rate of change of optical pathlength along a line section enable the identification of the temporal and frequency characteristics of the acoustic wave front. This approach has been extended through the use of a scanning LDV, which facilitates the measurement of a range of spatially distributed parameters. A mathematical model is presented that relates the distribution of pressure amplitude and phase in a planar wave front with the rate of change of optical pathlength measured by the LDV along a specifically orientated laser line section. Measurements of a 1 MHz acoustic tone burst generated by a focused transducer are described and the results presented. Graphical depictions of the acoustic power and phase distribution recorded by the LDV are shown, together with images representing time history during the acoustic wave propagation.

Recent Developments and Applications in Electronic Speckle Pattern Interferometry

Abstract Electronic speckle pattern interferometry (ESPI) is a wholefield non-contact optical metrology technique for displacement measurement, based on the optical physics of surface-generated laser speckle. Since its inception during the early 1970s ESPI has gradually evolved into different optical designs and has been applied to a range of engineering and non-engineering applications. Development of ESPI has continued during the 1990s with the introduction of new laser and optical technology into the interferometers, allowing further optimization and extending the potential applicability of the technique. This review considers the most notable developments of interferometer design and application that have occurred and been widely published during the 1990s, and examines the current and near-future direction of research into the technique.

Development of a STEP-compliant inspection framework for discrete components:

Abstract The measurement and inspection of manufactured parts play a vital role in manufacturing industry and is considered an integral part of the quality control. Though this is recognized, the state of the art in measurement and inspection of components is still recognized as a separate island of automation with no formal overall integration of standards and specifications. Although the introduction of probing has enabled automated inspection processes to become commonplace, the solutions for the in-process gauging at CNC machine tools and the programming of coordinate measuring machines is still vendor specific. The introduction of new standards for data exchange, such as STEP, AP219, and STEP-NC, has influenced the future direction of the inspection process. In light of current research in this field, a STEP compliant inspection framework for a component is presented. This framework aims to provide a capability to establish standardized measuring and inspection across the total CAx chain. It will facilitate the use of information downstream in the chain, such as an inspection workplan, workingsteps, and a mechanism to close the loop and feedback of inspection results to component model design. In order to achieve this integration STEP-NC (ISO14649) along with AP219 has been used in this research to provide the basis of a STEP-compliant inspection framework.

Application and assessment of laser Doppler velocimetry for underwater acoustic measurements

Abstract The majority of traditional methods for making underwater acoustic pressure measurements involve placing all or part of a measurement transducer in the acoustic field. A variety of optical metrology techniques have been developed in an attempt to reduce or remove any perturbing effects. An example of this is the use of laser interferometry which has been developed as the primary method of calibrating hydrophones in the frequency range 500 kHz – 20 MHz at the National Physical Laboratory (NPL). This technique involves suspending a thin Mylar pellicle in the acoustic field and recording the displacement of the pellicle surface using a Michelson Interferometer. This study details a comparison of a Laser Doppler Velocimeter (LDV) with the NPL Laser Interferometer, which gives a good correlation where agreement is within approximately 4% and 7% for two different power levels from a 500 kHz plane piston transducer and within 2.5% and 1% for the same power levels from a 1 MHz plane piston transducer. A novel, non-perturbing method of recording temporally resolved acoustic pressure distributions in water using an LDV is also described. The technique is shown to benefit from the consistent frequency response of the LDV detection system, such that the measured output resembles the drive voltage input to the transducer more closely than a similar hydrophone measurement. For the experimental arrangement described, the LDV system is shown to be sensitive to minimum pressure amplitudes of nominally 18.9 mPa /√ Hz .

Measurement of transient deformations with dual-pulse addition electronic speckle-pattern interferometry

We describe an electronic speckle-pattern interferometry system for analyzing addition fringes generated by the transient deformation of a test object. The system is based on a frequency-doubled twin Nd:YAG laser emitting dual pulses at a TV camera field rate (50 Hz). The main advance has been the automatic, quantitative analysis of dual-pulse addition electronic speckle-pattern interferometry data by the introduction of carrier fringes and the application of Fourier methods. The carrier fringes are introduced between dual pulses by a rotating mirror that tilts the reference beam. The resulting deformation-modulated addition fringes are enhanced with a deviation filter, giving fringe visibility close to that of subtraction fringes. The phase distribution is evaluated with a Fourier-transform method with bandpass filtering. From the wrapped phase distribution, a continuous phase map is reconstructed with an iterative weighted least-squares unwrapper. Preliminary results for a thin plate excited by an acoustic shock show the suitability of the system for the quantitative evaluation of transient deformation fields.

Wavefront divergence: A source of error in quantified speckle shearing data

Abstract Divergent laser illumination is commonly used in current designs of commercial electronic speckle pattern shearing interferometry (ESPSI) or shearography, for qualitative non-destructive testing (NDT) of material defects. The growing demand for quantitative out-of-plane (OOP) and more recently in-plane (IP) ESPSI, is determining the quality of optical system design and analysis software. However, little attention is currently being given to understanding, quantifying and compensating for the numerous error sources. Data describing the measurement inaccuracy due to the divergence of the object illumination wavefront for an OOP interferometer is presented. The errors are measured by comparing divergent object illumination with collimated illumination, with respect to illumination angle, lateral shear and shearing direction. Results indicate that the magnitude of the relative error increases by approximately a power function as the distance from the illumination source decreases.

Design and characterisation of ultrasonic cutting tools

Cutting of food products and other materials with ultrasonically assisted tools has demonstrated significant benefits including reduced wastage and improved cut quality. However, the success of the technology relies on careful design of the ultrasonically excited tools and transmission components. In this paper, the different challenges of tool design are discussed with reference to two cutting devices. The studies demonstrate that accurate characterisation of the vibration behaviour of the tool and an understanding of the effects and limitations on vibration responses of design modifications, allows tool performance to be enhanced in the design.

ESPI non-destructive testing of GRP composite materials containing impact damage

Recent developments in electronic speckle pattern interferometry (ESPI) pioneered by the authors have highlighted the potential for applying the technique to the study of mechanical damage in composite materials especially where non-contact, whole-field, real-time measurements are required. Intensity fringe patterns have been used as qualitative assessment tools for over 10 years. In this case, however, computer based fringe pattern analysis techniques are used in a novel way to extract autonomously both amplitude and phase data. Thus this paper demonstrates how intensity fringes and phase maps from phase stepped ESPI may be used to evaluate delaminations, cracks and other defects in glass-fibre reinforced polyester (GRP) pultruded panels in which the damage originated from instrumented falling weight impact tests. The experimental results show that both intensity fringe patterns and phase maps can be used to detect the internal damage. The intensity fringe approach enables the damage to be examined in real-time but with the disadvantage of limited visibility and readability. In contrast, the phase map approach is capable of showing the fine features of internal damage with higher visibility and readability providing post-processing is applied. In the current work the damaged areas are calculated and compared with those derived from conventional ultrasonic C-scan and sectioning techniques and show promising correlation. It is also demonstrated that there is a consistent correlation between impact event parameters and the extent of revealed damage. Finally of particular significance, in the broader scientific sense, is the fact that in the case of the phase mapping there is the potential to extract information from the computer which could quantify the level of damage and which could be used to alert, say a quality control operator, to the need to make further decisions. The paper therefore signals the real possibility of being able to produce an automated NDT analysis tool.

Sensitivity errors in interferometric deformation metrology

Interferometric measurement techniques such as holographic interferometry and electronic speckle-pattern interferometry are valuable for measuring the deformation of objects. Conventional theoretical models of deformation measurement assume collimated illumination and telecentric imaging, which are usually only practical for small objects. Large objects often require divergent illumination, for which the models are valid only when the object is planar, and then only in the paraxial region. We present an analysis and discussion of the three-dimensional systematic sensitivity errors for both in-plane and out-of-plane interferometer configurations, where it is shown that the errors can be significant. A dimensionless approach is adopted to make the analysis generic and hence scalable to a system of any size.