Sensor Positioning Inside a Linescan Camera

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1 Sensor Positioning Inside a Linescan Camera Atmel innovates with its Aviiva TM cameras By Jean-Luc Diverchy, Camera Project Manager Summary In product inspection processes, companies often face problems related to camera settings and sensor positioning, particularly when several cameras operate together in one system, such as in paper or wood inspection. Atmel, a world-leader in imaging solutions, has innovated with a camera that enables users to overcome critical sensor positioning errors. Atmel Corporation 2325 Orchard Parkway San Jose, CA TEL (408) FAX (408) Web Site:

2 Table of Contents Introduction... 3 Sensor Positioning Difficulties... 3 Tilt Z: The Most Critical Parameter... 4 The Most Critical Camera Parts That Can Affect Sensor Positioning... 5 The Casing s Finish... 5 The Position of the Sensor... 5 The Position of the CCD Package... 5 A Typical CCD Package... 6 Atmel s Linear CCD Package and Sensor Positioning Improvement... 6 The Ideal Package... 6 Atmel s Linear CCD Package Development... 7 Conclusion... 8 Atmel s Aviiva Linescan Camera: The Industry s Best Sensor Alignment Performances... 8 References... 8 Editor's Notes... 9 About Atmel Corporation A IMAGE 01/04

3 Introduction In today s imaging industry, sensor positioning and any associated deviations play an important role in the results obtained by machine vision applications. Depending on the application, any number of cameras may be mounted on a gantry to inspect products moving along a conveyor. These cameras can cover a wide angle of view depending on the products to be scanned, and each camera often has its own particular settings and adjustments. If an adjustment needs to be changed on one of the cameras, the conveyor must be stopped and the adjustment made manually. Undoubtedly, stopping the inspection process, if even for a very short time, results in loss of money for the end-user. To reduce maintenance costs associated with camera adjustments, an ideal solution consists in using a linear camera, where the tilt between the sensor plane and the camera s optical references is more precise. Technically advanced applications such as paper or metal inspection, to name just two examples, require both the camera and high-precision optics to be compact. Minimal light to inspect product imperfections acutely and the necessity to position the camera and optics as close as possible to the inspected object are also important requirements of a flawless system. In these applications, the precision of the sensor in relation to the camera s mechanical optical references is an important time- and money-saving factor for the end-user anxious to avoid continual adjustments. This article describes a sensor architecture that results in both lower sensor positioning errors and better repeatability of sensor positioning while maintaining low manufacturing costs. The architecture provides improved accuracy on the x and y axes by a unique reference system on the side edges of the package that results in low registration errors, and on the z axis by minimizing the number of surfaces between the die s surface and the sensor s reference plane. This architecture has been developed internally by Atmel Corporation. Sensor Positioning Difficulties Figure 1. X and Y Axes Y axis First pixel center y + y Active area θ x,y Mechanical reference point x + x X axis As shown in Figure 1, x and y define the position of the first pixel from the camera s mechanical reference point. x and y are their respective deviations A IMAGE 01/04

4 If the sensor deviates too much from the mechanical reference point, one must adjust the way the camera is mounted so as to align the sensor with the inspected object. This adjustment must be made individually on each camera used in the system, which can be painstaking and time-consuming, incur high costs and increase both maintenance and setup time. Whenever a camera is changed or added to the system, one has to redo the same adjustment, resulting in delays in bringing the system back on-line. Furthermore, many web-based inspection systems use a narrow, highlyfocused line light for product illumination which exacerbates the problem, making the angular alignment error very critical. Figure 2. Z Axis z+ z CCD photosensitive plane Lens Tilt z In figure 2, z is the distance between the sensor and the lens. This distance is called the focal distance and z is the deviation from this focal distance (z). This deviation can be critical if the camera does not support a focal distance setting and if the scene is situated further than the hyperfocal distance. If such is the case, it could be quite impossible to adjust the focus. In industrial applications, however, the inspected object is not usually situated over the hyperfocal distance, so z is not critical and the lens setting is possible. Tilt Z: The Most Critical Parameter Tilt z is the angle between the sensor s photosensitive plane and the mechanical reference where the lens is fixed. This is probably the most critical parameter as you cannot change its setting, whether on the lens or the camera itself, unless you install an intermediary element, which could prove complicated. The tilt does not include focus of the entire sensor. Why is it so critical? The following example illustrates just how critical tilt z actually is. The depth of focus in visible light is approximately the blur diameter [B] multiplied by the Fstop (F# = aperture number). With a 10 µm pitch and an F#=2, the depth of focus is more or less 20 µm, so the sensor positioning must be within this +/-20 µm limit to avoid degradation. With the longer sensors this is a critical issue A IMAGE 01/04

5 The Most Critical Camera Parts That Can Affect Sensor Positioning The Casing s Finish The camera s casing is usually made of metallic parts. It is relatively simple to obtain precise dimensions between the different parts of the camera case, and in today s machining industry, a tolerance of 10 µm is not problematic. The casing s finish must also be considered. A painted finish can be quite thick and irregular it is difficult for a manufacturer to do a perfectly smooth and uniform painted finish and can contribute to inaccuracies in the alignment of the sensor. For its cameras Atmel has chosen an anodized finish, shown on the example below, which is perfectly smooth and uniform and can be reproduced easily. Figure 3. Atmel s Aviiva Camera The Position of the Sensor The sensor s positioning (a CDD in the case of the Aviiva camera) is usually problematic because of the lack of mechanical reference surface on its package. For example, fixing a CCD in DIL package on the case can be quite a challenge - unless you develop specific positioning tools, you ll find it nearly impossible to obtain an accurate alignment. The Position of the CCD Package The precision of the CCD package positioning is the main concern. The sensor is composed of a sensitive die, which has a certain thickness with its own variation. The die is glued, the glue also having its own particular variation. If the top surface of the glass is used as a reference, which is common, the thickness of the glass adds a third degree of randomness to the positioning. The build-up of these errors makes accurate positioning difficult A IMAGE 01/04

6 A Typical CCD Package Figure 4. Example of a Common DIL CCD Package Glass Die Glue Thickness of glass Thickness of glue Package Thickness of rear panel Pin Rear panel Controlling the regularity of the different dimensions in this type of package is a challenge for manufacturers as the measurements given by CCD manufacturers or CCD packaging manufacturers are not usually very precise. In most cases, this lack of precision drives camera manufacturers to either specify erroneous mechanical positioning specifications or implement expensive mechanical settings on the camera, which obviously increase the cost of the equipment. For example, using a DIL CCD package with the sensor glued to the rear panel typically gives more than a 100 µm deviation on the dimension of z. In some industrial applications where a high level of precision is required, this can be a barrier to obtaining accurate and reliable results. Atmel s Linear CCD Package and Sensor Positioning Improvement The Ideal Package An ideal CCD package should have precise x, y and z mechanical references whose settings are as close as possible to the CCD pixel size (see the explanation in the first paragraph of this article). These recommendations exclude a DIL package because of the lateral pins, which provide no possible lateral mechanical reference for fixing the sensor. Nor does a DIL package provide any other fixing possibilities A IMAGE 01/04

7 Atmel s Linear CCD Package Development Example of Atmel s Aviiva M2 linescan camera With these considerations in mind, Atmel developed a new CCD package. As the company manufactures and uses its own sensors, the engineers worked with packaging subcontractors to define a well-adapted package. Figure 5. Atmel s Linear CCD Package Glass Mechanical reference surface Die Glue Holes for fixing Side edges Atmel s package, which is a PGA type, has been studded to minimize the number of parts that pile up between the sensitive plane and the reference plane. You can see in Figure 5 that the mechanical reference surface is the same surface to which the die is bonded. This eliminates the variation in thickness of the rear face s layer, which is the most difficult element to manage. Figure 6. Reference Points on Atmel s Linear CCD Package Side edges Reference points Side edges Die y Holes for fixing x A IMAGE 01/04

8 Looking at the CDD package from the top (Figure 6), the mechanical references used to position the sensor inside the camera case (marked on the illustration by the arrows) are the same ones used in CCD manufacturing to position the die when it is glued to the inside of the package. Again, great pains have been taken to minimize sources of random positioning errors and to make the manufacturing process more economical. The precise mechanical positioning results in a simpler assembly task than with other packaging technologies. Additionally, this package has a large surface area on the bottom that results in very good thermal performance. Conclusion Atmel s Aviiva Linescan Camera: The Industry s Best Sensor Alignment Performances Thanks to this specific CCD package, Atmel s innovative Aviiva linescan camera provides excellent mechanical positioning performances: around 50 µm on the x and y dimensions, around 30 µm on the z dimension and 35 µm maximum (for longer sensors) on tilt z. These performances are among the best in today s professional imaging industry and this is the reason Atmel recommends using its linear cameras in applications such as paper and metal inspection or parcel sorting, where it is imperative that maintenance costs be reduced to a minimum and where the CCD s position in relation to the camera s mechanical optical references is a crucial point in the visual inspection of product defects. References 1. Smith, Warren J. Modern Optical Engineering. 3 rd ed. New York: McGraw-Hill, A IMAGE 01/04

9 Editor's Notes About Atmel Corporation Founded in 1984, Atmel Corporation is headquartered in San Jose, California with manufacturing facilities in North America and Europe. Atmel designs, manufactures and markets worldwide, advanced logic, mixed-signal, nonvolatile memory and RF semiconductors. Atmel is also a leading provider of system-level integration semiconductor solutions using CMOS, BiCMOS, SiGe, and high-voltage BCDMOS process technologies. Further information can be obtained from Atmel s Web site at Contacts: Jean-Luc Diverchy, Camera Project Manager, Atmel Grenoble, France jean-luc.diverchy@gfo.atmel.com Sylvie Mattei, Communication Manager, Atmel Grenoble, France sylvie.mattei@gfo.atmel.com Special thanks go to those who helped in writing and reviewing this white paper: Rémi Gerbelot, Camera Development Manager George Tallman, US Representative for Professional Imaging Florence Forthoffer, Camera and CCD Standard Product Marketing Laurence Petit, Technical Writer Atmel Corporation All rights reserved. Atmel and combinations thereof are the registered trademarks and Aviiva is the trademark of Atmel Corporation or its subsidiaries. Other terms and product names may be the trademarks of others A IMAGE 01/04

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