Peak Detector. Minimum Detectable Z Step. Dr. Josep Forest Technical Director. Copyright AQSENSE, S.L.

Transcription

1 Peak Detector Minimum Detectable Z Step Dr. Josep Forest Technical Director

2 Peak Detector Minimum Detectable Defect Table of Contents 1.Introduction Layout Results Conclusions...9

3 Copyright 2008 AQSENSE, S.L. All rights reserved. This document may not in whole or in part be reproduced, transmitted, transcribed, or stored in any electronic medium or machine readable form, or translated into any language or computer language without the prior written consent of AQSENSE, S.L. Disclaimer While every precaution has been taken in the preparation of this manual, AQSENSE, S.L. assumes no responsibility for errors or omissions. AQSENSE, S.L. reserves the right to change the specification of the product described within this manual and the manual itself at any time without notice and without obligation of AQSENSE, S.L. to notify any person of such revisions or changes.

4 1. Introduction 3D Surface Inspection deals with the analysis of surface defects in 3D, like bumps, dents, scratches or rough edges. Laser triangulation is a well known, widely used 3D acquisition technology for many industrial applications in industries such as automotive, aircraft, space, etc. This technology is currently in use for 3D surface inspection, as well as 3D metrology applications, among others. Detection accuracy and resolution are two crucial concepts that must seriously be considered when choosing the 3D laser triangulation based acquisition system for a particular 3D surface inspection application. The AQSENSE peak detector improves the laser stripe detection accuracy compared with the traditional weighted center of gravity detectors by applying special digital filtering and interpolation techniques to ensure that the detection of the maximum intensity point is performed at a maximum (theoretical) accuracy of 1/64th of a pixel. This document describes the results of our own experiments to determine the minimum detectable step in depth. That is, the minimal difference in Z that our laser stripe detection techniques are able to see. 2. Layout The tests were done using the following hardware: MV D1024E 3D01 Photonfocus camera (implementing the Peak Detector) Lenses 35 and 50 mm. Standard quality, unbranded Lasiris Stocker Yale 10mW 635 nm 501L 30º laser emitter Gage blocks (certified by Metrology & Quality Services Ltd)

5 Figure 1: Acquisition configuration at different angles The test measurements were done using different camera laser angles, i.e. 30, 60, 70, and 90 degrees (see figure 1). The distance between the camera and the gages was 475 mm giving a field of view of 130mm when using the f=35mm lenses and 90mm with the f=50mm lenses. Figure 2 shows the set of gages used for the test. As can be seen, the surface is shiny polished to the point that they could be used as mirrors. Even though this surface finish is not appropriate for laser triangulation, the optical system and the camera exposure time were adjusted to guarantee a minimal acceptable image quality of the laser stripe. Figure 2: A picture of the set of gages The standard, calibrated gages were put together next to each other, forming steps. The gages thicknesses were 1.000, 1.005, 1.010, 1.020, 1.050, 1.100, and mm, and their manufacturing tolerances were in the order of 10 4 mm, as certified by Metrology & Quality Services Ltd.

6 A laser line was projected onto the gages (see figure 5) and the detected profile was acquired. Figure 3 shows a picture of the laser line as projected onto the gage set at a 90 degree angle between camera and laser, together with a plot of the data. The average value of each step is clearly distinguished, even in the case of the left most two gages, Figure 3: Below: The laser line as projected onto the gages. Above: A plot of the profiles detection where the difference in height is 5 m. The average of multiple tests showed even better distinctions between the and the 1.005mm gages (see figure 4). Figure 4: A plot showing the difference between detections of 1 and mm gages When the steps were clearly distinguished in average, the height step between the "distinguishable" gages was taken as the minimal detectable defect size. It is worth noting that the detection of less specular (or reflective) surfaces will be even better than the best

7 detection we could have with this shiny surface (the noise level will decrease), so the distinction between small steps in height will appear more clearly. Figure 5: Projection of the laser emitter onto the gage set. Our peak detector uses a specialized FIR filter stage where the order and coefficient values can be "customized" such that lower noise levels can be achieved, hence smaller height differences could be detected. However, laser speckle noise as well as the sensor noise are the factors that limit the detection of smaller defects. In our opinion, even if the sensor noise level (black current, thermal, etc...) is significantly lower and the sensitivity is increased, without a speckle free or reduced speckle light source or a special optical arrangement it is not feasible to detect variations in height smaller than 4 to 2 m with this field of view. In any case, we have not been able to perform smaller defect detection tests with the equipment available in our facilities. 3. Results According to the previously mentioned method and criteria, a number of tests were undertaken using different camera laser angles, by scanning profiles from the gage set (see figure 6). These tests resulted in the tabular data shown in tables 1 and 2, where the minimal detectable step in height is reported for every different angle between camera and laser, in an arrangement similar to that shown in figure 1.

8 Figure 6: 3D Reconstruction of the gages (1mm, 1.02mm, 1.04mm, 1.06mm, 1.08mm, 1.10mm) Angle camera laser Step Size (μm) Table 1: Minimum detectable height step using f=35mm lens Angle camera laser Step Size (μm) Table 2: Minimum detectable height step using f=50mm lens Note that the minimal defect has remained at 5 microns for a 90 degrees angle between camera and laser, although the field of view has decreased from 130 to 90 mm with the use of f=35mm or f=50mm lenses, respectively, the different noise sources including laser speckle and image sensor noise being the limiting factors that prevent the system from seeing smaller height steps with normal laser and optical components.

9 4. Conclusions The AQSENSE peak detector performance has been checked using standard, certified, calibrated gages, providing a clear portrait of what's the minimal z step that can be sensed with a particular camera, for different fields of view and for different camera laser angles. Although the gage surfaces are difficult ones because they are highly specular to the point that they look like mirrors, very small height variations (down to 5 microns) could be sensed. The fact that with this specular surface we achieved such good results, make us think that they are representative enough for a wide variety of surface types. Only a particular implementation of the peak detector has been tested. However, since the peak detector can be tuned by recalculating the FIR filter coefficients, even worse surface types (like skin, translucent, oily, etc...) can be detected more accurately, so special versions of our peak detector can be produced for specialized scanning of particular surface types. Carrying out some action on the laser speckle noise as well as an increase in image sensor sensitivity, or even employing another type of light source with the property of being sharply focussed, should increase the detection capability so that smaller steps in height could be sensed.