M Reeves

Measurement of complex surface deformation by high-speed dynamic phase-stepped digital speckle pattern interferometry

We describe a high-speed digital speckle pattern interferometer incorporating a line-scan camera and a waveguide phase modulator for the measurement of complex deformation (vibration phase and amplitude) at audio acoustic frequencies. Experimental data show continuous phase-stepped recovery of out-of-plane surface deformation in one dimension, obtained at 100 kHz with 2pi/20-rad (0.02-mum) displacement resolution, for surface velocities of 3.2 mm s>(-1) .

Multipoint laser vibrometer for modal analysis

Experimental modal analysis of multifrequency vibration requires a measurement system with appropriate temporal and spatial resolution to recover the mode shapes. To fully understand the vibration it is necessary to be able to measure not only the vibration amplitude but also the vibration phase. We describe a multipoint laser vibrometer that is capable of high spatial and temporal resolution with simultaneous measurement of 256 points along a line at up to 80 kHz. The multipoint vibrometer is demonstrated by recovering modal vibration data from a simple test object subject to transient excitation. A practical application is presented in which the vibrometer is used to measure vibration on a squealing rotating disk brake.

Dynamic distortion measurements during laser forming of Ti¿6Al¿4V and their comparison with a finite element model

Abstract Laser forming is, potentially, an attractive flexible manufacturing technique for the controlled forming of aerospace alloys. Laser forming experiments using a continuous-wave CO2 laser were performed on coupons of material 80 mm × 80mm in area and 2mm thick, with sequential passes of the laser beam, at a surface scanning rate of 20 mm/s with 90 s of convective cooling between passes. A novel surface profilometer that was specifically developed to operate under the conditions of high vibration and stray light typically found in laser machining applications recorded transient surface shape changes during individual laser passes at frame rates of 4 and 0.2 Hz. A finite element model was developed using ABAQUS for the laser forming of linear bends in free Ti-6A1-4V sheets, with sequentially coupled thermal and elastic-plastic analysis incorporating temperature-dependent material properties. Transient heat source scanning was implemented to simulate the experiment. Good agreement was found between the experimental three-dimensional shape data and those predicted by the transient model. In particular, the formation of an unwanted ‘camber’ distortion perpendicular to the desired main bend was correctly predicted; its magnitude and temporal evolution throughout the three laser passes, and during the periods of convective cooling, agreed well with the experimental data. The model and the shape measurement technique will enable the future predictive controlled laser forming of more complex three-dimensional shapes.

Dynamic shape measurement system for laser materials processing

We describe a laser-based profilometry system that demon- strates high-resolution, dynamic surface height measurements in hostile conditions. The robust system measures transient distortions of continu- ous surfaces under the conditions of high stray light and mechanical disturbances that are typically found in laser drilling and welding. A co- herent fringe pattern is projected onto the object surface using a binary phase grating interferometer. A charge-coupled device (CCD) camera fitted with a narrow-bandpass filter, centered on the optical wavelength of the projected fringes, records images of the illuminated workpiece while rejecting unwanted light from the high-power processing laser. The fringe pattern in each image is analyzed by the Fourier transform method, en- abling dynamic shape measurement at the camera frame rate. The fringe order is identified uniquely by the dynamic projection of a laser spot aligned with a single fringe. An in-situ calibration produces height maps that are corrected for perspective and lens distortion errors. The unique measurement capability of the system has enabled the first dy- namic shape measurements to be made during laser forming. Results are presented for the laser forming of 80380 mm titanium alloy coupons, over which area a height accuracy of 612 mm was achieved.

The application of high-speed TV-holography to time-resolved vibration measurements

Abstract We describe an electronic speckle pattern interferometer (ESPI) system that has enabled non-harmonic vibrations to be measured with μs temporal resolution. The short exposure period and high framing rate of a high-speed camera at up to 40,500 frames per second allow low-power CW laser illumination and fibre-optic beam delivery to be used, rather than the high peak power pulsed lasers normally used in ESPI for transient measurement. The technique has been demonstrated in the laboratory and tested in preliminary industrial trials. The ability to measure vibration with high spatial and temporal resolution, which is not provided by techniques such as scanning laser vibrometry, has many applications in manufacturing design, and in an illustrative application described here revealed previously unmeasured “rocking” vibrations of a car door. It has been possible to make the measurement on the door as part of a complete vehicle standing on its own tyres, wheels and suspension, and where the excitation was generated by the running of the vehicles own engine.

High bandwidth multipoint fibre-optic vibrometer for transient measurements

We describe a laser vibrometer that is capable of making 256 simultaneous point measurements along a line. The ability to make simultaneous measurements is significant because in the case of multi-frequency or transient vibration modes it allows the measurement of vibration phase as well as vibration amplitude. For many applications such measurements cannot be obtained by any other means, and are of great interest to vibration engineers. In its present form, the vibrometer has a measurement bandwidth of 100 kHz, and measures displacements with a resolution of /spl lambda//25.

A system for dynamic shape measurements during laser processing

A laser-based 3D profilometry system is described which demonstrates high resolution, dynamic surface height measurements in hostile conditions. The system has been designed specifically to measure distortions under the conditions of high stray light and mechanical disturbances typical of laser forming (the bending of surfaces by thermally induced distortion) and welding. In our technique a CCD camera records images of the workpiece surface, onto which structured light is projected from an interferometer. A height map is generated from each image recorded in a sequence, allowing the dynamics of shape modification to be captured, quantified and studied.The system is capable of height resolutions of the order of 10J.m over 60 x 60mm areas and is presently finding routine use in the in-process study of laser forming. We present the first examples of time-resolved shape measurements during laser forming, which demonstrate the transient and asymmetric nature of shape formation in this process. We hope that such measurements, combined with a process model, will further our understanding of the process to the extent necessary to implement it for practical applications.A laser-based 3D profilometry system is described which demonstrates high resolution, dynamic surface height measurements in hostile conditions. The system has been designed specifically to measure distortions under the conditions of high stray light and mechanical disturbances typical of laser forming (the bending of surfaces by thermally induced distortion) and welding. In our technique a CCD camera records images of the workpiece surface, onto which structured light is projected from an interferometer. A height map is generated from each image recorded in a sequence, allowing the dynamics of shape modification to be captured, quantified and studied.The system is capable of height resolutions of the order of 10J.m over 60 x 60mm areas and is presently finding routine use in the in-process study of laser forming. We present the first examples of time-resolved shape measurements during laser forming, whic...