Development of a flexible and modular metrology system for measuring complex surfaces

The demand for customised optical devices is increasing tremendously. Such optical devices do not employ traditional designs like planar, spherical, or even aspherical shapes. Instead, modern lenses exhibit free-form surfaces with a large variety of gradients in all directions. Highly accurate and repeatable measurement of such lens surfaces represents a considerable challenge; therefore there is a pressing need to both improve the metrology systems used in the optical industry and to develop new generations of high-performance metrology systems that employ innovative measurement techniques.Workshops need fast measurement solutions for the rough surfaces produced in the early stages of a lens typical production chain. The last steps produce very smooth surfaces, usually ideally suited to interferometers. However, interferometers are physically not suited to the measurement of strong aspheres or free-form shaped objects. Therefore, research was undertaken to investigate a metrology solution applicable to all common surface types and roughness grades at any stage of the production chain.This PhD research presents a novel approach for applying the principle of a spherical coordinate measurement machine (SCMM) to lens metrology. SCMMs require the precise and repeatable alignment of all axes. Therefore, research was performed to investigate a novel method for generic axes alignment without the need for external tools. This method, with the enhanced SCMM approach, was then combined with research into suitable multi-sensor measurement modes, in order to adequately address the needs of all stages in the production chain. Coordinate measurement machines are subject to the influence of errors. Therefore, research was conducted to develop a novel user-interface and a patented device to analyse and compensate for errors of the applied rotational axes and the 3D-Scale. The mathematical models presented, enable a simple transfer to other types of SCMMs. Also, the researched processes, software tools and mechatronic devices may be generically adopted to other machines applying rotational axes. Therefore, in addition to providing advanced capabilities for high-accuracy measurement of lenses with complex morphologies; the results of this research and the new approaches developed may be employed with SCMMs more generally, in a wide range of industrial sectors

Aspheric geodesic lenses for an integrated optical spectrum analyser

Abstract available p. xiii-xi

A laboratory program to develop improved grazing incidence X-ray optics

Grazing incident double reflection X-ray telescop

Mechanics and micro-mechanisms of LCF and dwell fatigue in Ti-6Al-4V

All manufacturers of gas turbines, and indeed, of engineering systems, suffer from both managed and unexpected cracking issues, where components are retired from service. This PhD project was sponsored by a group in Rolls-Royce plc concerned with fractog- raphy and failure investigation, with the purpose of asking whether dwell, high cycle and low cycle fatigue, stress ratio or block loading can be distinguished from fracture sur- faces. Therefore, the central theme of this thesis is the examination of fracture surfaces of a Ti-6Al-4V alloy and relating them to the fatigue loading regimes. The as-received material was unidirectional rolled and exhibited a nearly equiaxed microstructure with a strong {11 ̄20}⟨0001⟩ texture. A series of LCF and dwell experiments were performed on unnotched cylindrical samples to investigate initiation behaviour under both loading con- ditions. The formation of fatigue striations was investigated with corner crack specimens. The fatigue initiation behaviour was substantially different between continuously cy- cled samples and specimens that experienced long dwell periods at high stress. Cracks in continuously cycled samples typically initiated at the surface by facet formation when cycled at 92% of the yield stress. At the same cyclic stress but with a 2 minute dwell pe- riod, the cycles to failure reduce and multiple subsurface crack initiation occurred. Areas with cleavage-like failure each with a single initiation facet would form across the gauge length. The cleavage like areas were identified as regions where grains were preferentially oriented with the c-axis along with the loading direction (macrozones). The crack prop- agates faster through these grains, causing the crack to grow along the rolling direction. The initiation facets were typically tilted about 32◦ towards the loading direction. The facets tilted around the normal direction that suggested that initiation facets are formed outside the macrozone. EBSD measurements of the initiating area confirmed that dwell initation occurred at the edges of macrozones. In dwell initiation facets the basal slip systems were most likely to be active under tension, while LCF initiation facets showed highest shear stresses on ⟨a⟩ type pyramidal planes. The Schmid factors of dwell facets were generally lower than those of neighbouring grains. This supports the load shedding mechanism based on the Stroh model. Propagation facets under continuous cycling were only seen around the initiation point, with increased tilt angles of about 38◦. Cracks propagate in α titanium by the formation of fatigue striations. Above a prop- agation rate of approximately 100 nm per cycle each striation was formed by a single load cycle. Due to the fact that they form in stage II of crack propagation they are referred to as Paris striations. The 1:1 ratio broke down when the crack propagated at lower rates and then a striation was the result of up to several hundred load cycles. These were referred to as non-Paris striations. The dislocation analysis below the striations showed slip bands on prismatic planes. Outside the Paris region slip bands were only seen along one plane. The slip bands were tilted at nearly 30◦ towards the crack growth direction and intersect with the surface. In Paris striations slip bands were seen along all prismatic planes. The observed slip band spacing correlated with the striation separation. A model for formation was proposed where striations form by extrusion of material onto the sur- face due to localised glide along the slip bands. Slip activity can not be fully reversed when the crack closes due to an instant oxide layer forming on new created surfaces. For striations to form only one slip band needs to be active. Additional slip bands form in the space between to allow for a homogeneous deformation. The effect of load ratio R, frequency and waveform on the striation profile was ex- amined using AFM. The ratio of striation height to separation (H) was about 0.12 and s constant for any ∆K values. The shape of the striations was investigated by compar- ing the slope and flank length on either side of the striations. On average the striations showed a steeper rise and longer fall. A change in R ratio, frequency or waveform did not influence the H value but did influence the striation shape. The flank length was s thought to be proportional to the distance that dislocations glide per cycle. The relative length of the striation during load removal increased with (i) higher R ratios or (ii) longer time under stress by either a lower test frequency or a trapezoidal waveform. The ratio of slopes associated with the striation rising and falling was influenced by the applied waveform.Open Acces

The Influence of Secondary Processing Conditions on the Mechanical Properties and Microstructure of a Particle Reinforced Aluminium Metal Matrix Composite

The influence of secondary processing conditions on an aluminium metal matrix composite, comprising of an AA2124 matrix and 3 Jlm particulate SiC reinforcement at 25 volume percent was investigated. The metal matrix composite (MMC) was extruded at three different temperatures, 350???????C, 450???????C and 550???????C, at a ratio of20:1 and at three different ratios, 5:1, 10:1 and 20:1, at a temperature of 450???????C. It was subsequently solution heat treated and naturally aged. A mechanical property assessment was carried out using standard tensile and rotating bend fatigue test methods to determine the properties of the material extruded under each condition. A novel technique using a Focussed Ion Beam (FIB) Microscope was developed to prepare polished specimens and microtextural analysis was performed by FIB imaging. Additionally, techniques were successfully established, through the use of FIB milling and polishing, to provide site-specific electron transparent films, permitting detailed examination ofthe microstructure with a transmission electron microscope. Material extruded at 550???????C exhibited a lower yield strength than material extruded at 350???????C and 450???????C, which was attributed to grain coarsening and recrystallisation. Evidence of recrystallisation was found during texture analysis by X-Ray diffraction, where there was a reduction in the intensity of the fibre texture in the extrusion direction. The phenomenon was also observed during irticrostructural analysis work, where recrystallised grains at grain boundaries were observed. Higher extrusion ratios offered a small improvement in tensile properties, due to an enhanced fibre texture within the microstructure. Microtextural examination gave evidence of the existence of both high angle grain and low angle grain boundaries for the material extruded at 350???????C. It is believed that a subgrain structure was partially transformed during extrusion, through subgrain rotation, leading to the formation of high angle grain boundaries. This'microstructure was found to offer the optimum mechanical properties.Imperial Users onl

Design, control and error analysis of a fast tool positioning system for ultra-precision machining of freeform surfaces

This thesis was previously held under moratorium from 03/12/19 to 03/12/21Freeform surfaces are widely found in advanced imaging and illumination systems, orthopaedic implants, high-power beam shaping applications, and other high-end scientific instruments. They give the designers greater ability to cope with the performance limitations commonly encountered in simple-shape designs. However, the stringent requirements for surface roughness and form accuracy of freeform components pose significant challenges for current machining techniques—especially in the optical and display market where large surfaces with tens of thousands of micro features are to be machined. Such highly wavy surfaces require the machine tool cutter to move rapidly while keeping following errors small. Manufacturing efficiency has been a bottleneck in these applications. The rapidly changing cutting forces and inertial forces also contribute a great deal to the machining errors. The difficulty in maintaining good surface quality under conditions of high operational frequency suggests the need for an error analysis approach that can predict the dynamic errors. The machining requirements also impose great challenges on machine tool design and the control process. There has been a knowledge gap on how the mechanical structural design affects the achievable positioning stability. The goal of this study was to develop a tool positioning system capable of delivering fast motion with the required positioning accuracy and stiffness for ultra-precision freeform manufacturing. This goal is achieved through deterministic structural design, detailed error analysis, and novel control algorithms. Firstly, a novel stiff-support design was proposed to eliminate the structural and bearing compliances in the structural loop. To implement the concept, a fast positioning device was developed based on a new-type flat voice coil motor. Flexure bearing, magnet track, and motor coil parameters were designed and calculated in detail. A high-performance digital controller and a power amplifier were also built to meet the servo rate requirement of the closed-loop system. A thorough understanding was established of how signals propagated within the control system, which is fundamentally important in determining the loop performance of high-speed control. A systematic error analysis approach based on a detailed model of the system was proposed and verified for the first time that could reveal how disturbances contribute to the tool positioning errors. Each source of disturbance was treated as a stochastic process, and these disturbances were synthesised in the frequency domain. The differences between following error and real positioning error were discussed and clarified. The predicted spectrum of following errors agreed with the measured spectrum across the frequency range. It is found that the following errors read from the control software underestimated the real positioning errors at low frequencies and overestimated them at high frequencies. The error analysis approach thus successfully revealed the real tool positioning errors that are mingled with sensor noise. Approaches to suppress disturbances were discussed from the perspectives of both system design and control. A deterministic controller design approach was developed to preclude the uncertainty associated with controller tuning, resulting in a control law that can minimize positioning errors. The influences of mechanical parameters such as mass, damping, and stiffness were investigated within the closed-loop framework. Under a given disturbance condition, the optimal bearing stiffness and optimal damping coefficients were found. Experimental positioning tests showed that a larger moving mass helped to combat all disturbances but sensor noise. Because of power limits, the inertia of the fast tool positioning system could not be high. A control algorithm with an additional acceleration-feedback loop was then studied to enhance the dynamic stiffness of the cutting system without any need for large inertia. An analytical model of the dynamic stiffness of the system with acceleration feedback was established. The dynamic stiffness was tested by frequency response tests as well as by intermittent diamond-turning experiments. The following errors and the form errors of the machined surfaces were compared with the estimates provided by the model. It is found that the dynamic stiffness within the acceleration sensor bandwidth was proportionally improved. The additional acceleration sensor brought a new error source into the loop, and its contribution of errors increased with a larger acceleration gain. At a certain point, the error caused by the increased acceleration gain surpassed other disturbances and started to dominate, representing the practical upper limit of the acceleration gain. Finally, the developed positioning system was used to cut some typical freeform surfaces. A surface roughness of 1.2 nm (Ra) was achieved on a NiP alloy substrate in flat cutting experiments. Freeform surfaces—including beam integrator surface, sinusoidal surface, and arbitrary freeform surface—were successfully machined with optical-grade quality. Ideas for future improvements were proposed in the end of this thesis.Freeform surfaces are widely found in advanced imaging and illumination systems, orthopaedic implants, high-power beam shaping applications, and other high-end scientific instruments. They give the designers greater ability to cope with the performance limitations commonly encountered in simple-shape designs. However, the stringent requirements for surface roughness and form accuracy of freeform components pose significant challenges for current machining techniques—especially in the optical and display market where large surfaces with tens of thousands of micro features are to be machined. Such highly wavy surfaces require the machine tool cutter to move rapidly while keeping following errors small. Manufacturing efficiency has been a bottleneck in these applications. The rapidly changing cutting forces and inertial forces also contribute a great deal to the machining errors. The difficulty in maintaining good surface quality under conditions of high operational frequency suggests the need for an error analysis approach that can predict the dynamic errors. The machining requirements also impose great challenges on machine tool design and the control process. There has been a knowledge gap on how the mechanical structural design affects the achievable positioning stability. The goal of this study was to develop a tool positioning system capable of delivering fast motion with the required positioning accuracy and stiffness for ultra-precision freeform manufacturing. This goal is achieved through deterministic structural design, detailed error analysis, and novel control algorithms. Firstly, a novel stiff-support design was proposed to eliminate the structural and bearing compliances in the structural loop. To implement the concept, a fast positioning device was developed based on a new-type flat voice coil motor. Flexure bearing, magnet track, and motor coil parameters were designed and calculated in detail. A high-performance digital controller and a power amplifier were also built to meet the servo rate requirement of the closed-loop system. A thorough understanding was established of how signals propagated within the control system, which is fundamentally important in determining the loop performance of high-speed control. A systematic error analysis approach based on a detailed model of the system was proposed and verified for the first time that could reveal how disturbances contribute to the tool positioning errors. Each source of disturbance was treated as a stochastic process, and these disturbances were synthesised in the frequency domain. The differences between following error and real positioning error were discussed and clarified. The predicted spectrum of following errors agreed with the measured spectrum across the frequency range. It is found that the following errors read from the control software underestimated the real positioning errors at low frequencies and overestimated them at high frequencies. The error analysis approach thus successfully revealed the real tool positioning errors that are mingled with sensor noise. Approaches to suppress disturbances were discussed from the perspectives of both system design and control. A deterministic controller design approach was developed to preclude the uncertainty associated with controller tuning, resulting in a control law that can minimize positioning errors. The influences of mechanical parameters such as mass, damping, and stiffness were investigated within the closed-loop framework. Under a given disturbance condition, the optimal bearing stiffness and optimal damping coefficients were found. Experimental positioning tests showed that a larger moving mass helped to combat all disturbances but sensor noise. Because of power limits, the inertia of the fast tool positioning system could not be high. A control algorithm with an additional acceleration-feedback loop was then studied to enhance the dynamic stiffness of the cutting system without any need for large inertia. An analytical model of the dynamic stiffness of the system with acceleration feedback was established. The dynamic stiffness was tested by frequency response tests as well as by intermittent diamond-turning experiments. The following errors and the form errors of the machined surfaces were compared with the estimates provided by the model. It is found that the dynamic stiffness within the acceleration sensor bandwidth was proportionally improved. The additional acceleration sensor brought a new error source into the loop, and its contribution of errors increased with a larger acceleration gain. At a certain point, the error caused by the increased acceleration gain surpassed other disturbances and started to dominate, representing the practical upper limit of the acceleration gain. Finally, the developed positioning system was used to cut some typical freeform surfaces. A surface roughness of 1.2 nm (Ra) was achieved on a NiP alloy substrate in flat cutting experiments. Freeform surfaces—including beam integrator surface, sinusoidal surface, and arbitrary freeform surface—were successfully machined with optical-grade quality. Ideas for future improvements were proposed in the end of this thesis

Design and Applications of Coordinate Measuring Machines

Coordinate measuring machines (CMMs) have been conventionally used in industry for 3-dimensional and form-error measurements of macro parts for many years. Ever since the first CMM, developed by Ferranti Co. in the late 1950s, they have been regarded as versatile measuring equipment, yet many CMMs on the market still have inherent systematic errors due to the violation of the Abbe Principle in its design. Current CMMs are only suitable for part tolerance above 10 μm. With the rapid advent of ultraprecision technology, multi-axis machining, and micro/nanotechnology over the past twenty years, new types of ultraprecision and micro/nao-CMMs are urgently needed in all aspects of society. This Special Issue accepted papers revealing novel designs and applications of CMMs, including structures, probes, miniaturization, measuring paths, accuracy enhancement, error compensation, etc. Detailed design principles in sciences, and technological applications in high-tech industries, were required for submission. Topics covered, but were not limited to, the following areas: 1. New types of CMMs, such as Abbe-free, multi-axis, cylindrical, parallel, etc. 2. New types of probes, such as touch-trigger, scanning, hybrid, non-contact, microscopic, etc. 3. New types of Micro/nano-CMMs. 4. New types of measuring path strategy, such as collision avoidance, free-form surface, aspheric surface, etc. 5. New types of error compensation strategy