A next-generation optical surface form inspection instrument for high-volume production applications (original) (raw)
Related papers
A system for the dynamic industrial inspection of specular freeform surfaces
Optics and Lasers in Engineering, 2012
The inspection of moving specular freeform surfaces is an industrial challenge so far largely unsolved, even for the qualitative case, i.e. the mere determination of the presence of surface defects as opposed to the quantitative reconstruction of a surface. Products produced in high quantities therefore still have to be inspected manually which is labour intensive, expensive, monotonous and subjective. We propose a novel hardware setup and methodology to overcome this shortfall. The reflection of a line laser from a moving surface is captured on a translucent screen; surface defects show as gaps or bulges. Two methods to extract the resulting information are proposed and ways for its interpretation are shown. The proposed method is very cost effective and easy to implement. While limitations to surface geometry exist and absolute precision is not achievable, it is shown that the system is able to reliably detect, characterise and localise a range of typical surface defects on moving glazed ceramic tiles, our example application. The method is however applicable to a wide range of hybrid and specular surfaces.
The productivity rate of a manufacturing process is limited by the speed of any measurement processes at the quality control stage. Fast and effective in-line measurements are required to overcome this limitation. Optical instruments are the most promising methods for in-line measurement because they are faster than tactile measurements, able to collect high-density data, can be highly flexible to access complex features and are free from the risk of surface damage. In this paper, a methodology for the development of fast and effective in-line optical measuring instruments for the surfaces of parts with millimetre-to micrometre-size is presented and its implementation demonstrated on an industrial case study in additive manufacturing. Definitions related to in-line measurement and barriers to implementing in-line optical measuring instruments are discussed.
The precise distance measurement of fast-moving rough surfaces is important in several applications such as lathe monitoring. A nonincremental interferometer based on two mutually tilted interference fringe systems has been realized for this task. The distance is coded in the phase difference between the generated interference signals corresponding to the fringe systems. Large tilting angles between the interference fringe systems are necessary for a high sensitivity. However, due to the speckle effect at rough surfaces, different envelopes and phase jumps of the interference signals occur. At large tilting angles, these signals become dissimilar, resulting in a small correlation coefficient and a high measurement uncertainty. Based on a matching of illumination and receiving optics, the correlation coefficient and the phase difference estimation have been improved significantly. For axial displacement measurements of recurring rough surfaces, laterally moving with velocities of 5 m∕s, an uncertainty of 110 nm has been attained. For nonrecurring surfaces, a distance measurement uncertainty of 830 nm has been achieved. Incorporating the additionally measured lateral velocity and the rotational speed, the two-dimensional shape of rotating objects results. Since the measurement uncertainty of the displacement, distance, and shape is nearly independent of the lateral surface velocity, this technique is predestined for fast-rotating objects, such as crankshafts, camshafts, vacuum pump shafts, or turning parts of lathes.
Progress in the specification of optical instruments for the measurement of surface form and texture
Specifications for confocal microscopes, optical interferometers and other methods of measuring areal surface topography can be confusing and misleading. The emerging ISO 25178 standards, together with the established international vocabulary of metrology, provide a foundation for improved specifications for 3D surface metrology instrumentation. The approach in this paper links instrument specifications to metrological characteristics that can influence a measurement, using consistent definitions of terms, and reference to verification procedures. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/03/2014 Terms of Use: http://spiedl.org/terms Proc. of SPIE Vol. 9110 91100M-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/03/2014 Terms of Use: http://spiedl.org/terms Proc. of SPIE Vol. 9110 91100M-3 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/03/2014 Terms of Use: http://spiedl.org/terms
Surface Finish and 3D Optical Scanner Measurement Performance For Precision Engineering
3D optical scanners are rapidly replacing Cartesian CMMs due to their advanced metrological benefits, such as dense 3D point cloud data capture, portability and measurement speed. To support 3D optical scanner manufacturers and end-users, the National Physical Laboratory (NPL, UK) has developed a 3D optical scanner dimensional characterisation facility, which comprises: test artefacts, test procedures and equipment to examine the performance of 3D optical scanners. One of the facility's tests characterises scanner ability to measure surface finishes common to precision engineering. The test utilises a multi-faceted test artefact, to allow measurement of the test materials against highly Lambertian reference coupons through a series of fixed angles. Sample surfaces representative of materials typical of precision components manufactured by the aerospace, automotive and power generation industries were measured. Results for a white light fringe projecting 3D optical scanner show sensitivity to certain aluminium finishes and specular surfaces. Measurements of sandpaper samples suggest a trend of increasing point cloud density with decreasing surface roughness. Colour measurements show sensitivity to blue and green surfaces, while maximum point cloud density is measured for other coloured surfaces.
Simulation of Automated Visual Inspection Systems for Specular Surfaces Quality Control
Lecture Notes in Computer Science, 2007
This paper proposes the use of simulations as a design mechanism for visual inspection systems of specular surfaces. The system requirements and the characteristics of the objects involve a technological design problem for each of the solutions to be developed. A generic model is proposed. It may be adapted or particularised to solve specific inspection problems using simulations. The method results in a flexible low cost design, reducing the distance between the design model and system implementation in a manufacturing procedure. The proposed simulator generates model-based architectures. The paper shows the results on application of metallized automobile logos.
Large-aperture, equal-path interferometer for precision measurements of flat transparent surfaces
Applied Optics, 2014
The measurement of flat optical components often presents difficulties because the presence of parallel surfaces generates multiple reflections that confuse conventional laser-based interferometers. These same parts have increasingly demanding surface finish tolerances as technologies improve over time, further complicating the metrology task. Here we describe an interferometric optical system for high-accuracy noncontact evaluation of the form and texture of precision flat surfaces based on an equaloptical-path geometry that uses extended, broadband illumination to reduce the influence of speckle noise, multiple reflections, and coherent artifacts by a factor of 10 when compared to laser-based systems. Combined with a low-distortion, fixed-focus imaging system and 4-Mpixel camera, the 100 mm aperture instrument offers surface height resolutions of 0.1 nm over lateral spatial frequencies extending from 0.01 to 10 cycles/mm. The instrument is vibration resistant for production-line testing of flat optics such as glass hard disks for the data-storage industry and flat-panel-display substrates.
Fiber optic interferometry has been used to detect small displacements in diverse applications. Counting the number of fringes in fiber-optic interferometry is challenging due to the external effects induced in dynamic systems. In this paper, a novel interference fringe counting technique is developed to convert the intensity of interference data into displacements in the range of micrometers to millimeters while simultaneously resolving external dynamic effects. This technique consists of filtering the rough experimental data, converting filtered optical interference data into displacements, and resolving dynamic effects of the experimental system. Filtering the rough data is performed in time by using the moving average method with a window size of 400 data points. Filtered optical data is further converted into displacement by calculating relative phase differences of each data point compared to local maximum and local minimum points. Next, a linear curve-fit is subtracted from the calculated displacement curve to reveal dynamic effects. Straightness error of the lead screw driven stage, dynamics of the stepper motor, and profile of the reflective surfaces are investigated as the external dynamic effects. Straightness error is characterized by a 9th order polynomial function, and the effect of the dynamics of the stepper motor is fitted using a sinusoidal function. The remaining part of the measurement is the effect of roughness and waviness of the reflective surfaces. As explained in the experimental setup part, two fiber-optic probes detect the vertical relative displacements in the range of 1-50 mm, and the encoder probe detects 13.5 mm horizontal displacement. Thus, this technique can detect three order of magnitude different dynamic displacements with sub-micrometer resolution. The current methodology can be utilized in different applications which require measuring straightness error of lead-screw driven stages, large area surface profile of specimens, and vibration of actuators such as stepper motors.
A new test for optical surfaces
Off-the-shelf components can measure precision astronomical mirrors and solar-energy collectors as accurately as interferometric methods but much more cheaply. For an optical system to produce the best possible images, the optical surfaces of the mirrors and lenses must be polished to the correct shape to within a fraction of the wavelength of light (in most cases, this means a few tens of nanometers). Interfero-metry is usually used to inspect the mirror or lens and ensure that the correct shape has been achieved. However, the equipment is expensive and these tests are sensitive to alignment and environmental disturbances. We have developed a test system based on readily available consumer electronics that measures the slope of rays reflected from optical surfaces. It rivals interferometry in sensitivity to surface errors, yet is much less expensive. We call our system, whose performance is entirely software controlled, a software-configurable optical test system (SCOTS). We have used it to measure large precision mirrors—such as the Giant Magellan Telescope's (GMT) primary mirrors—and solar-energy concen-trators/collectors. It is functionally similar to interferometry, but directly measures slopes rather than surface heights. This approach can be more efficient and reliable for lens systems, and thus SCOTS is also a promising tool for evaluation of lenses. In its simplest configuration, only a laptop computer with a built-in camera is required. The laptop's LCD screen illuminates the surface and the camera detects reflected images that the computer then uses to calculate optical-surface gradients. SCOTS uses the same basic test geometry as phase-measuring or fringe-reflection deflectometry. 1, 2 It can be better understood as doing a traditional Hartmann test 3 with the light going through the system in reverse: see Figure 1(a) and (b). The CCD camera works as the point source and the screen has the function of the detector in a Hartmann test. Figure 1(b) shows schematically how the surface slope is measured and calculated. The computer or TV monitor is set up with its screen near the center of curvature and facing the mirror under test. If a single pixel is lit up on the otherwise dark screen, the image of the mirror, which is captured by the camera's CCD detector, will show a Figure 1. Comparison of the test geometry for (a) a Hartmann test and (b) our software-configurable optical test system (SCOTS). Alpha: Angle of incidence/reflection. Figure 2. (a) A segmented 3m, f/0.5 (focal ratio) paraboloidal solar con-centrator built at the University of Arizona. (b) Hardware setup of SCOTS located near the center of curvature of the solar concentrator.
A visual inspection system for quality control of optical lenses
International Journal of Physical Sciences, 2011
This paper proposes a quality inspection system for optical lenses using computer vision techniques. The system is able to inspect LED (Light-Emitting Diode) lenses visually and to validate their quality level automatically based on the defect severity. The optical inspection system applies the block discrete cosine transform (BDCT), Hotelling 2 T statistic, and grey clustering technique to detect visual defects of LED lenses. A spatial domain image with equal sized blocks is converted to DCT (Discrete Cosine Transform) domain and some representative energy features of each DCT block are extracted. These energy features of each block are integrated by the 2 T statistic and the suspected defect blocks can be determined by the multivariate statistical method. Then, the grey clustering algorithm based on the block grey relational grades is conducted to further confirm the block locations of real defects. Finally, a simple segmentation method is applied to set a threshold for distinguishing between defective areas and uniform regions. Experimental results show the defect detection rate of the proposed method is 94.64% better than those of traditional spatial and frequency domain techniques.