A Multi-mode Transverse Dynamic Force Microscope - Design, Identification and Control

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this record.The transverse dynamic force microscope (TDFM) and its shear force sensing principle permit true non-contact force detection in contrast to typical atomic force microscopes. The two TDFM measurement signals for the cantilever allow, in principle, two different scanning modes of which, in particular, the second presented here permits a full-scale non-contact scan. Previous research mainly focused on developing the sensing mechanism, whereas this work investigates the vertical axis dynamics for advanced robust closed-loop control. This paper presents a new TDFM digital control solution, built on field-programmable gate array (FPGA) equipment running at high implementation frequencies. The integrated control system allows the implementation of online customizable controllers, and raster-scans in two modes at very high detection bandwidth and nano-precision. Robust control algorithms are designed, implemented, and practically assessed. The two realized scanning modes are experimentally evaluated by imaging nano-spheres with known dimensions in wet conditions.Engineering and Physical Sciences Research Council (EPSRC

Optimisation of a nano-positioning stage for a Transverse Dynamic Force Microscope

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.This paper describes the optimisation of a nano-positioning stage for a Transverse Dynamic Force Microscope (TDFM). The nano-precision stage is required to move a specimen dish within a horizontal region of 1 μm × 1 μm and with a resolution of 0.3 nm. The design objective was to maximise positional accuracy during high speed actuation. This was achieved by minimising out-of-plane distortions and vibrations during actuation. Optimal performance was achieved through maximising out-of-plane stiffness through shape and material selection as well optimisation of the anchoring system. Several shape parameters were optimised including the shape of flexural beams and the shape of the dish holder. Physical prototype testing was an essential part of the design process to confirm the accuracy of modelling and also to reveal issues with manufacturing tolerances. An overall resonant frequency of 6 kHz was achieved allowing for a closed loop-control frequency of 1.73 kHz for precise horizontal motion control. This resonance represented a 12-fold increase from the original 500 Hz of a commercially available positioning stage. Experimental maximum out-of-plane distortions below the first resonance frequency were reduced from 0.3 μm for the first prototype to less than 0.05 μm for the final practical prototype

Real-time sliding mode observer scheme for shear force estimation in a transverse dynamic force microscope

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.This paper describes a sliding mode observer scheme for estimation of the shear force affecting the cantilever in a Transverse Dynamic Force Microscope (TDFM). The vertically oriented cantilever is oscillated in proximity to the specimen under investigation. The amplitude of oscillation of the cantilever tip is affected by these shear forces. They are created by the ordered-water layer above the specimen. The oscillation amplitude is therefore a measure of distance between the tip and the surface of the specimen. Consequently, the estimation of the shear forces provides useful information about the specimen characteristics. For estimating the shear forces, an approximate finite dimensional model of the cantilever is created using the method of lines. This model is subsequently reduced for its model order. An unknown input sliding mode observer has been used to reconstruct the unknown shear forces using only tip position measurements and the cantilever excitation. This paper describes the development of the sliding mode scheme and presents experimental results from the TDFM set up at the Centre for Nanoscience and Quantum Information (NSQI) at Bristol University

Collaborative Robotics Strategies for Handling Non-Repetitive Micro-Drilling Tasks Characterized by Low Structural Mechanical Impedance

Mechanical micro-drilling finds widespread use in diverse applications ranging from advanced manufacturing to medical surgery. This dissertation aims to develop techniques that allow programming of robots to perform effective micro-drilling tasks. Accomplishing this goal is faced with several challenges. Micro-drills suffer from frequent breakage caused from variations in drill process parameters. Micro-drilling tasks afford extremely low feed rates and almost zero tolerance for any feed rate variations. The accompanying robot programming task is made difficult as mathematical models that capture the micro-drilling process complexities and sensitive variations in micro-drill parameters are highly difficult to obtain. Therefore, an experimental approach is adopted to identify the feasible parameter space by carrying out a systematic characterization of the tool-specimen interaction that is crucial for understanding the robotic micro-drilling process. The diameter of the hole to be drilled on a material is a primary defining factor for micro-drilling. For the purposes of this dissertation, micro-drills are defined as having a diameter less than or equal to 1 mm. The Sawyer and KUKA collaborative robots that meet the sensitive speed requirements have been chosen for this study. A regression analysis revealed a relationship between feed rate and reaction forces involved in the micro-drilling process that matched the underlying mathematical model of the tool-specimen interactions. Subsequently, this dissertation addresses the problem of destabilization in robotic micro-drilling caused by the low impedance of the collaborative robot’s cantilever structure. A semi-robotic method that combines force-controlled adaptive drill feed rate and human-assisted impedance enhancement strategy is developed to address the destabilization problem. This approach is inspired by the capability of humans to stabilize unstable dynamics while performing contact-based tasks by using selective control of arm mechanical impedance. A human-robot collaborative kinesthetic drilling mode was also developed using the selective compliance capability of the KUKA robot. Experimental results show that the Sawyer and KUKA robots can use the developed strategies to drill micro-holes of diameters up to a minimum of 0.6 mm and 0.2 mm, respectively. Finally, experiments involving drilling in different materials reveal the potential application of the collaborative robotic micro-drilling approach in composite repairs, micro-channels, dental drilling, and bone drilling

Development and Implementation of Novel Bristle Tool for Surface Treatment of Metallic Components

Despite advances in paints and coatings technology, protective coatings are prone to eventual corrosion, degradation and/or failure. Consequently, a corrosive layer will develop that can undermine the performance and integrity of structural components. Therefore, both the corrosive layer and defunct coating must be periodically removed, and an acceptable level of surface cleanliness and texture must be obtained prior to the reapplication of new paint. Currently, an array of processes and equipment are used for efficiently cleaning and conditioning metallic surfaces, such as grit blasting, needle guns, and a variety of non-woven and coated abrasive tools. This research investigates the method termed the bristle blasting process. The process utilizes a specially designed rotary bristle tool, which is dynamically tuned to a power tool spindle that operates at approximately 2,500 rpm. The present research suggests that the repeated collision of hardened bristle tips with a corroded steel surface results in both the removal of a friable corrosive layer and simultaneous exposure of fresh subsurface material. Surfaces generated by the bristle blast process are shown to mimic the visual cleanliness and anchor profile that is characteristic of grit blasting processes. One particular application evaluated during this research was offshore pipeline refurbishment and pre-treatment of weld seams prior to the application of protective coatings. Comparative analysis was done with conventional methods of surface treatment on the basis of visual cleanliness, surface profile generation and coating adhesion strength. The results obtained suggest that this novel technology performs better than the existing conventional power tool methods and is on an equal par with grit blasting methods. Moreover, the bristle blasting process is eco-friendly and does not use or generate hazardous waste, thereby providing a green approach to corrosion removal and surface preparation of steel components