Peter Kinnell

Performance Assessment of a New Variable Stiffness Probing System for Micro-CMMs.

When designing micro-scale tactile probes, a design trade-off must be made between the stiffness and flexibility of the probing element. The probe must be flexible enough to ensure sensitive parts are not damaged during contact, but it must be stiff enough to overcome attractive surface forces, ensure it is not excessively fragile, easily damaged or sensitive to inertial loads. To address the need for a probing element that is both flexible and stiff, a novel micro-scale tactile probe has been designed and tested that makes use of an active suspension structure. The suspension structure is used to modulate the probe stiffness as required to ensure optimal stiffness conditions for each phase of the measurement process. In this paper, a novel control system is presented that monitors and controls stiffness, allowing two probe stiffness values (“stiff” and “flexible”) to be defined and switched between. During switching, the stylus tip undergoes a displacement of approximately 18 µm, however, the control system is able ensure a consistent flexible mode tip deflection to within 12 nm in the vertical axis. The overall uncertainty for three-dimensional displacement measurements using the probing system is estimated to be 58 nm, which demonstrates the potential of this innovative variable stiffness micro-scale probe system.

Soluble Abrasives for Waterjet Machining

The addition of hard abrasives to the jet in waterjet machining can improve machining rate, however, embedding of particles in machined surfaces is a limitation, which results in reduced fatigue life, and limits the application of well adhered subsequent coatings to the surface. In this study, softer soluble abrasives were investigated as a potential solution. Soluble abrasives yielded a higher material removal rate compared to plain waterjet, although were not as effective as traditional hard abrasives. Soluble abrasives reduced grit embedment on all four workpiece materials. A post-machining surface cleaning operation demonstrated that any remaining soluble abrasive could be removed.

An evaluation of cleaning methods for micro-CMM probes

To support the manufacture of complex products made of multiple micro-parts, a traceable and accurate 3D-measurement system with sub-micron accuracy is required. This paper investigates the use of a micro electro mechanical system (MEMS) tactile-micro-probe for 3D-coordinate metrology on the micro-scale. It will show that probe-tip contamination may be a significant source of error for such tactile-probing systems. Even in a clean-room environment, there are many types of contaminants that may affect dimensional measurement accuracy. These mainly come from repeated contact between probe tip and artefact material. To address this issue a number of cleaning methods were utilized to remove contaminants from the MEMS tactile-micro-probe sensor. Scanning electron microscopy was used to assess the efficacy of each cleaning method. Initial findings on different cleaning methods including ultrasonic, plasma, micro-beam melting, electron beam melting and cryogenic cleaning have shown that the latter is a feasible process.

Characterization of a micro-engineered selective strain-coupling structure using Raman spectroscopy

This paper reports the physical characterization of a novel micro-electro mechanical system (MEMS) packaging structure using micro Raman spectroscopy. The structure is designed to reduce the effects of unwanted residual packaging strain on a resonant strain gauge device. It does this by maintaining the resonator alignment and selectively coupling the resonator to strain in one degree of freedom only. Previous work has demonstrated the ability of the structure to maintain resonator alignment. In this work, the ability of the structure to provide selective coupling is determined experimentally using micro Raman spectroscopy. The experimental results are shown to agree well with the predicted performance of the structure.

Development of CO2 snow cleaning for in situ cleaning of µCMM stylus tips

Contamination adhered to the surface of a µCMM stylus tip compromises the measurement accuracy of the µCMM system, potentially causing dimensional errors that are over ten times larger than the uncertainty of a modern µCMM. In prior work by the authors, the use of a high pressure CO2 gas stream was demonstrated to achieve significant cleaning rate for a range of contaminant without damage to the stylus tip surface. This paper explores the practical challenges of achieving effective stylus tip cleaning in situ on µCMM systems. Two types of snow cleaning approaches were evaluated for their coverage of cleaning, thermal impact and gas flow forces. This work then presents a novel multi-nozzle prototype system using pulsed snow streams to achieve cleaning coverage over the entire stylus tip, and balances forces from the snow streams reducing drag force imparted by the gas stream to levels comparable to the probing force of µCMMs, as well as allowing automated cleaning procedure integrated into a µCMM system.

Plastic deformation of micromachined silicon diaphragms with a sealed cavity at high temperatures

Single crystal silicon (SCS) diaphragms are widely used as pressure sensitive elements in micromachined pressure sensors. However, for harsh environments applications, pure silicon diaphragms are hardly used because of the deterioration of SCS in both electrical and mechanical properties. To survive at the elevated temperature, the silicon structures must work in combination with other advanced materials, such as silicon carbide (SiC) or silicon on insulator (SOI), for improved performance and reduced cost. Hence, in order to extend the operating temperatures of existing SCS microstructures, this work investigates the mechanical behavior of pressurized SCS diaphragms at high temperatures. A model was developed to predict the plastic deformation of SCS diaphragms and was verified by the experiments. The evolution of the deformation was obtained by studying the surface profiles at different anneal stages. The slow continuous deformation was considered as creep for the diaphragms with a radius of 2.5 mm at 600 °C. The occurrence of plastic deformation was successfully predicted by the model and was observed at the operating temperature of 800 °C and 900 °C, respectively.

Characterizing the influence of surface roughness and inclination on 3D vision sensor performance

This paper reports a methodology to evaluate the performance of 3D scanners, focusing on the influence of surface roughness and inclination on the number of acquired data points and measurement noise. Point clouds were captured of samples mounted on a robotic pan-tilt stage using an Ensenso active stereo 3D scanner. The samples have isotropic texture and range in surface roughness (Ra) from 0.09 to 0.46 μm. By extracting the point cloud quality indicators, point density and standard deviation, at a multitude of inclinations, maps of scanner performance are created. These maps highlight the performance envelopes of the sensor, the aim being to predict and compare scanner performance on real-world surfaces, rather than idealistic artifacts. The results highlight the need to characterize 3D vision sensors by their measurement limits as well as best-case performance, determined either by theoretical calculation or measurements in ideal circumstances.

Evaluation of the Capabilities and Damage Risk of Cleaning Methods for Micro-CMM Stylus Tips

The dimensional accuracy of a micro-CMM is significantly affected by contamination adhered to the stylus tip during use. Contaminant particles can cause dimensional errors that are orders of magnitude greater than those reported in the literature. To reduce such errors, this study evaluates the suitability of three cleaning methods (brushing, laser cleaning and snow cleaning) for removing surface contamination on a micro-CMM stylus tip. The cleaning capability of each method is experimentally investigated. Due to the fragile nature of the styli, possible damage (mechanical and thermal) to the tip is assessed. Overall, snow cleaning was found to possess higher cleaning capability and lower risk of damage than the other two methods.

The suitability of lightfield camera depth maps for coordinate measurement applications

Plenoptic cameras can capture 3D information in one exposure without the need for structured illumination, allowing grey scale depth maps of the captured image to be created. The Lytro, a consumer grade plenoptic camera, provides a cost effective method of measuring depth of multiple objects under controlled lightning conditions. In this research, camera control variables, environmental sensitivity, image distortion characteristics, and the effective working range of two Lytro first generation cameras were evaluated. In addition, a calibration process has been created, for the Lytro cameras, to deliver three dimensional output depth maps represented in SI units (metre). The novel results show depth accuracy and repeatability of +10.0 mm to -20.0 mm, and 0.5 mm respectively. For the lateral X and Y coordinates, the accuracy was +1.56 μm to −2.59 μm and the repeatability was 0.25 μm.

The Unpredictable Errors of Micro Tactile Metrology - Factors Affecting Stylus tip Contamination

Abstract In 3D tactile micro-metrology the contamination of probing devices is a major problem that affects the accuracy and repeatability of measured dimensions. Despite a large body of research in the field of micro CMM and micro probe design there is limited research which has been done so far to explain and tackle this problem. In this work, experimental probing on a range of materials using a micro coordinate measuring machine was conducted to investigate the mechanism of stylus tip contamination. In addition the effects of surface finish on the build-up of stylus tip contamination were also studied. The results provide practitioners with guidelines which allow for the likely build-up of stylus tip contamination to be minimized based on sample material type and surface finish.