Gert Goch

Review of non-destructive measuring methods for the assessment of surface integrity: a survey of new measuring methods for coatings, layered structures and processed surfaces

Abstract Even though the near-surface areas of precisely manufactured parts represent only a few percent of the workpiece’s material volume, they influence its functional behaviour, quality and lifetime significantly. Therefore, sensors and measuring principles able to detect material changes, damage and process influences in the near-edge zone are of increasing importance. However, they have to meet industrial purposes concerning accuracy, measuring time, robustness and costs. In addition, they should work non-destructively and should offer the capability for integration in a production line.

Automatic Segmentation of the Papilla in a Fundus Image Based on the C-V Model and a Shape Restraint

For computer aided Glaucoma diagnostics it is essential to robustly and automatically detect and segment the main regions, e.g. the papilla (optic nerve head), in a fundus image. In this paper an effective method for automatic papilla segmentation based on the C-V model and a shape restraint is proposed. The method is a combination between the C-V model using level sets and the elliptic shape restraint for papilla segmentation. The combination of the level set framework with a shape restraint ensures that the evolving curve stays an ellipse. Experiments verify that the method shows a good performance in detecting the papilla shapes and computing the shape feature parameters within a broad variety of fundus images. The experiment results also show that the method is robust to noise and object deformity

Comparison of Conventional Light Scattering and Speckle Techniques Concerning an In-Process Characterisation of Engineered Surfaces

Abstract It is well known that several features of engineered surfaces can be determined by light scattering techniques. However, the characterisation of surface microtopography by scattered light sometimes only yields a qualitative assessment or does not directly correspond to common roughness parameters. The latter shortcoming may even lead to ambiguity, so that these techniques are hardly accepted for industrial quality inspection. Nevertheless, the technological progress in the field of optoelectronics (laser diodes, CCD- and CMOS-cameras, digital image processing) allows a compact realisation of novel arrangements. In this contribution, light scattering and speckle techniques for a measuring range from 1 nm to about 10 μm (rms-roughness) will be introduced. Conventional light scattering methods relevant to industrial applications are known as “angle resolved scattering” (ARS) and diffuseness measurement. The speckle techniques include the method of polychromatic speckle autocorrelation and the doubly scattered light approach, which have been recently developed by the authors. The different parametric methods will be compared with respect to their in-process capabilities and the correlation between optical output parameters and statistical parameters of the surface profile under investigation. Furthermore, it will be shown that 3D-quantities of engineered surfaces can be obtained from the measuring data.

A Process Oriented Approach to Automated Quality Control

Abstract To guarantee a constant quality of manufactured products, it is necessary to optimise the process parameters immediately when deviations of the workpiece quality have been observed. Established methods of quality management are only able to register quality deviations (like the statistical process control) or to analyse them offline with the help of experts (like failure mode and effects analysis). The presented approach develops a process oriented automated quality control for manufacturing in two steps: First, a local quality controller stabilises the product quality to reference values in the pace of workpiece production. In order to obtain the control law for the process parameters, a learning approach is applied. In the second step, the information exchange between the local controllers is established parallel to the workpiece flow in order to obtain an optimised global process. As an example, the method is applied to quality control in the turning process.

Requirements for the Application of Speckle Correlation Techniques to On-Line Inspection of Surface Roughness

Abstract Conventional surface roughness measurements using profilometric instruments (either mechanical or optical) are standardized and extensively applied in industry. However, these techniques are not suitable to an on-line inspection of industrial machining processes. If an optically rough surface leading to a diffuse scattering is illuminated by coherent light, a speckle pattern occurs showing a granular spatial intensity distribution of the scattered light. In general, the averaged intensity distribution depends on the distribution of the local surface slopes. However, if two speckle patterns at different laser wavelengths are compared via correlation techniques, a measure of the root-mean-square (rms) roughness (Rq) can be obtained, based on certain statistical assumptions. This paper contributes to the application of speckle correlation techniques to the inspection of machined surfaces. Surface statistics dependent on the manufacturing processes are checked. Optical arrangements of a measuring device are introduced. Furthermore, results showing the limits of speckle correlation are presented.

Acoustic tonometry: feasibility study of a new principle of intraocular pressure measurement.

PurposeAn accurate measurement of intraocular pressure (IOP) is still essential for detecting, following-up, and treating glaucoma. The objective of the interdisciplinary project GlauPhon was to prove a new noncontact tonometry principle that analyzes the acoustic oscillation of the eye. MethodThree enucleated porcine eyes were infused via the optic nerve with a saline solution. The IOP was adjusted by varying the height of the infusion bottle. A speaker closed one end of a cylindrical pressure chamber and an eye was fixed to the other side. A PC sound card induced the speaker to oscillate by generating a rectangular signal (20 Hz). A pressure sensor recorded the oscillating pressure within the chamber. For each IOP a calculation was performed that characterizes the attenuation profile. ResultsEach series of measurements revealed an evident dependency between the amplitude difference and the IOP. The highest signal belonged to low IOP levels and it decreased with increasing IOP. The correlation of the mean acoustical signal with the given IOP showed a highly significant correlation coefficient (r=−0.98). As a result, the measured oscillation parameters are strongly dependent on the exerted IOPs. ConclusionsThe experiments verified the presumed relation between the acoustic oscillation of the eye and the IOP. Nevertheless, further developments are necessary for converting the oscillation parameters into reliable IOP values, to construct a tonometry device for clinical trials.

In-process control of cutting depth during turning

During the turning process of a ring, the clamping force of the chuck causes circumferential deviations in the ring wall thickness and diameter. In order to minimise the wall thickness deviations, an experimental set-up for an in-process control of the cutting depth during turning is being developed. The set-up comprises a fast tool unit, an ultrasonic measurement system and a global control unit. In preliminary tests, the ultrasonic measurement proved suitable to acquire the changes in wall thickness. First turning experiments show that the developed fast tool unit and the applied control strategy fulfil the requirements for an in-process compensation of wall thickness deviations during standard turning conditions.

Non-Destructive and Non-Contact Determination of Layer Thickness and Thermal Properties of PVD and Sol-Gel Layers by Photothermal Methods

Abstract Thin layers in combination with metallic substrates are often used for moulding tools. The layer properties depend on the coating parameters and technology (e. g., sol-gel, PVD, CVD). This paper presents the photothermal measurement of layer thickness and thermal properties of PVD and sol-gel layers which are needed for the development of moulding tools. The influence of optical parameters and surface roughness on the results is discussed.

Nondestructive and contactless evaluation of surface coatings and adhesion defects by photothermal radiometry

For more than a double photothermal measurement techniques offer the suitability for a non-destructive and contactless evaluation of coating thicknesses and for the investigation of boundaries between a surface layer and its base material. In the following sections, we want to describe in detail the use of the photothermal radiometric technique (which is the most promising photothermal method for an in-line manufacturing control) on different interfaces embedded in an optically opaque workpiece. Additionally, we will demonstrate the effect of adhesion defects on the photothermal signals, or, vice versa, how the obstructed propagation of thermal waves can be explained by thermal contact resistances when delaminations or disbondings are located between a coating and its substrate. For this purpose, we mainly analyze very accurately the frequency dependence of the photothermal phase signal. Theoretical calculations and some experimental verifications on painted and on galvanically plated metal substrates will be presented. In summary, the photothermal measurement techniques allow to characterize subsurface adhesion defects possibly leading to a failure of a used component. Therefore we suggest that the photothermal instruments are usable for production control.

Simulation of light scattering for surfaces with statistically distributed subwavelength cavities

This paper deals with an efficient computation method for scattered light intensity distributions, which occur, if a nanostructured surface is illuminated with a monochromatic laser beam of several millimeters in diameter. The minimization of the computational amount is an essential precondition in connection with the development of powerful design tools for laser optical surface measuring methods, which derive structure characterizing attributes from structure dependent scattering effects. The presented approach differs from concepts based on near-field solutions of the Maxwell equations (finite element methods (FEM), finite difference time domain methods (FDTD)) or approximation methods for the near-field (Discrete Dipole Approximation (DDA), Generalized Multipole Technique (GMT)) as the near-field is not computed. Instead, an electrically equivalent model based on pre-computed radiation sources like Huygens point sources, dipoles, quadrupoles, etc. is used, which for standard geometrical nanostructures (cylindrical holes, spheres and ellipsoids) leads to the same far-field distributions as the conventional methods. In order to simulate the scattered light by an arbitrary surface it is divided into subwavelength geometries, which can be substituted by electrically equivalent dipole radiation sources. The far-field is calculated with a numerical scalar method. The computational effort is much smaller compared to algorithms based on the solution of Maxwells equations.