Diode laser based velocity sensors for industrial applications

Theis Faber Quist Iversen

Research output: Book/ReportPh.D. thesis

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In this thesis the use of dynamic speckle patterns and interferometric techniques is investigated for measuring the motion of diffusely scattering, rigid objects that are illuminated by a coherent light source. The focus is on measuring pure object translation and pure rotation. Principles and methods from spatial filtering velocimetry are employed to perform the measurements. A novel measurement principle is developed for three dimensional translational velocity measurements based on imaging speckle interferometry. A plane object is illuminated by a single beam of coherent light and an image of the illuminated part of the object is formed. The introduction of a coherent, angular off-set reference wave in the image plane introduces a fringe pattern in addition to the speckle pattern. It is observed that the fringe pattern translates in response to out-of-plane translation of the object, while the speckle pattern generally translates in response to in-plane object translation. This simple geometrical modification of the incidence angle of the reference wave allows measurement of out-of-plane translation, retaining the directional information, without the need for optical frequency shifting equipment used in conventional heterodyning configurations. Furthermore, the optical imaging geometry, connecting the object plane and observation plane, enables the measurement of local out-of-plane motion within the image itself. To understand the interaction of the spatial filters with the intensity distribution and thus the impact on performance, both numerical and analytical models of the intensity dynamics are developed. The numerical model is developed with the specific task of evaluating the performance of rectangular apodized spatial filters that are designed to measure all three velocity components. Using the results of the numerical simulation, an optimum system configuration, utilizing the fringe orientation flexibility to eliminate cross-talk between the spatial filters measuring the in-plane and out-of-plane velocity components, is identified. The analytical model is based on the ensemble averaged intensity crosscorrelation function, derived using complex ABCD formalism, and is developed for general system studies of translational and rotational velocity measurements. Using the results from the numerical and analytical models, spatial filters are applied to extract the velocity of an object translating in three dimensions. Specifically, a detector array arrangement is applied to measure the in-plane translation and an integrated optical spatial filter based on a lenticular lens array is applied to measure the out-of plane translation. In the case of an in-plane rotating cylindrical object the analytical model predicts, under certain conditions, a spatially dependent displacement rate of the fringe pattern that depends on the image coordinate. This is exploited to extract the in- plane angular velocity of using spatial filters implemented as a specialized set of detector arrays. The integrated optical spatial filter is used as a receiver in free space miniaturized translational sensors for cursor control solutions. The optical spatial filter consists of an array of cylindrical micro-lenses that, in combination with a focusing lens and detector arrangement, forms the spatial filter. The optical spatial filter is part of a single optical unit, combining an additional optical spatial filter and a transmitter used for beam shaping. The optical unit used in the miniaturized sensor is acting in combination with an application specific integrated circuit (ASIC) fitted with appropriate detector arrays facilitating a two dimensional velocity measurement of in-plane translation of a rigid object (surface). Particularly, the beam shaping transmitter optic is redesigned for optimum performance using a Fourier optical diffraction model. Furthermore, a ray tracing model is developed for the receiving part of the optical unit. The model shows a previously not recognized effect of aberration in the focusing lens that limits the number of lenslets contributing constructively to the signal. Furthermore, a source of unwanted passbands in the transfer function of the receiver is identified using a non-sequential ray tracing model. Prototypes, implementing the recommendations proposed by these models, were manufactured. The results of performance tests of the manufactured prototypes are presented and the results are discussed. A method for characterizing the radius of curvature and array period of the micro- optical cylindrical lens array, used in the integrated optical spatial filter, is developed. The method is based on observing the far-field diffraction pattern of the cylindrical lens array when illuminated by a coherent light source. The diffraction pattern obtained in the far-field of the illuminated lens array reveals information about the average radius of curvature of the lenslets in the array via the diffraction efficiency while the angular separation between individual diffraction orders is measured to obtain the lens array period. Special emphasis is placed on the potential for minimizing the measurement cycle time, robustness and reliability of the method. Scanning probe microscopes are used to validate the accuracy of the proposed method. The scattering properties of fingertip tissue structures known as dermal ridge patterns are investigated. A fingertip is placed in contact with a planar dielectric surface and illuminated by coherent light under total internal reflection conditions. The experimental observations provide a basis for further investigation into the associated effects on observed speckle pattern statistics and a theoretical model for the intensity crosscorrelation function is outlined and discussed. The observations made outlines possible advantageous properties that may be exploited to develop ultrathin touch sensitive sensors for use as cursor control devices in form-factor critical applications, such as e.g. mobile phones.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherTechnical University of Denmark
Publication statusPublished - Aug 2011

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