This thesis presents optical metrology systems using laser and position-sensitive detectors (PSDs). In this area, a new diffraction strain measurement technique is developed, implemented, analyzed and characterized. Related research work on the optical metrology components is also included, dealing with position-sensitive detectors analysis and laser-beam applications.
The research in optical metrology systems using lasers and PSDs comprises development of a novel whole-field strain measurement technique and its implementation as Multipoint Diffraction Strain and Tilt Sensor using Moire Interferometer and Multichannel PSD. All the different variations of diffraction strain sensors as developed by different researchers up to date, measure single point strain on the sample only. Concurrently, the whole- field strain measurement techniques like Moire Inteiferometry and ESPI have their own shortcomings like lengthy fringe processing and image subtraction algorithms. Additionally, they face obvious degradation in their spatial resolution at lower strain values owing to the sparse fringes. On the contrary, the developed system has the unique feature of direct and near-real-time diffraction-based strain and tilt measurement at a large array of points on the sample surface. The system uses simultaneous position tracking of smaller beamlets diffracted differently from different regions of the component surface under test. Using this setup, whole-field strain and tilt distribution patterns over the component body can be obtained immediately without the need for fringe processing. Furthermore, the system also has capability of measuring rotation and shear strain, making it a truly versatile whole-field strain sensing system. Simultaneous strain and tilt measurement at more than one thousand points is being reported in this research. Complete microcomputer-based implementation of this system is described. Detailed characterization and analysis of the developed system is presented along with some important development decisions. The system offers a promising combination of features like direct and fast calculation of whole-field strain with a fine spatial resolution and good sensitivity. These make this technique suitable for precision applications in structures, mechanics and micro-mechanics, while offering a considerable saving in time and equipment cost.
Work on Optical Metrology components starts with a unique analysis of position-sensitive detectors used in noisy industrial environment, where several other light sources also coexist and thus produce unpredicted effects on PSD output. Many researchers of other photo-sensors have analyzed their performance in presence of these stray noises, while this problem has many times been outlined in context of PSDs too. Nonetheless, detailed performance analysis of PSDs with stray optical noises is very much needed. For this purpose, these stray noises are described and modeled with respect to the operation of PSDs. Then the response of the detectors is analyzed in the presence of these spurious signals. The experimental results are compared with the results from the proposed mathematical model and it is observed that the measured performance is within a fraction of a percent of the calculated one. The analysis of systematic errors encountered during data collection is also presented. The study is expected to be very useful for the accurate and precise use of PSDs based sensors in the industrial units like production lines, workshops and others.
Following this, the work on laser beams analyzes their use in a novel material processing technique. Laser welding of galvanized steel sheets in lap configuration is the challenging problem being investigated for more than two decades, originating due to the difference in the melting points of steel and zinc. Earlier solutions are either insufficient for desired results or too cumbersome to be realized in practice. For this purpose, dual laser beams method is being discussed modld and analyzed, involving a precursor beam and a higher-power welding beam. The first beam cuts a slot, thus making an exit path for the zinc vapours, while the second beam performs the needed welding. The work also presents some experiments performed on the shop floor using this method, along with the metallurgical analysis from laboratory showing successful absence of zinc in the weld area. Owing to its simpler approach and lesser time consumption, this technique is expected to be very attractive in terms of workshop implementation and welding throughput.