Moravian instruments, Inc., source: https://www.mii.cz/art?id=626&lang=409, printed: 06.06.2020 18:38:47
|Machine vision systems currently get rid of the former reputation of complicated, unreliable and expensive technology. The development in image sensors area and especially the current CPU performance approached the possibilities of machine vision to user's demands. So the number of applications is growing rapidly. Growth in the number of usage is largely caused by the growing demands on so-called production quality, which is possible to achieve only by inspection of every produced item. It is necessary to eliminate the human factor for that kind of inspection, because „nobody's perfect“. The machine vision is often not only the best, but also the only possible way to implement the required inspections.|
Unlike other areas of industrial automation, the particularity of machine vision applications is high number of unsuccessful solutions. Despite all the profession advances, the design of machine vision is relatively very difficult to understand and respect the basic principles of imaging and work with image data. The most important is the first phase of the proposal, that means determination the geometric arrangement of the inspection system, camera choice, lens, illumination and selection of appropriate hardware and software for image processing. If you make a major mistake here, the probability of failure is very high.
We must make the most important decisions first. The issue is quite extensive and this article may include only a tiny fraction.
General task configuration
Choice of spatial configuration relatively determines the type and number of cameras and illuminating units and the requirements on the lens focal length. We must be sure, from what direction and distance will the camera capture the scene, we must also set sufficient cameras resolution, possibly need to use multiple cameras. It is needed to decide about the way of illuminating and its color at the same time – this is connected with the elimination of distractive light and therefore the shading proposal and the possible need to use color or polarizing filters in the camera.
Hardware and software image processing
An essential factor here is surprisingly not the cost of final solution – both concepts are about equally expensive. Especially important is the requirement on computing performance, flexibility and variability of software. Smart cameras do image processing themselves and are externally equipped with binary outputs, enabling to signalize the result of the process. Usually do not allow free programming, it is only possible to configure them through serial line or Ethernet connection. They are usually equipped with specialized signal processors or low consumption RISC processors with tact in hundreds of MHz and simple real-time operating systems. These facts already point to their limitations. Smart cameras are equipped with only a few basic tools for image processing and are suitable only for simple tasks. On the other hand, many tasks are usually dealt with surprisingly simple means, and here the integrated cameras suit. Estimate the situation in advance requires a lot of knowledge, sense and experience. Where it is necessary to cope with such variable scene, responding to changes in positions and shapes of objects, lighting changes or to solve complex and performance-intensive algorithms, we quickly run into limits that are firm and insurmountable. Effort to solve by smart cameras tasks beyond their means, is the reason for many failed projects.
Connecting the camera to a standard PC is a necessary choice for complex applications, but also for simpler applications leaves us more room to correct any inaccuracies in the initial estimate of requirements. Performance of modern processors dramatically exceed even the best smart cameras and embedded computer does not have to take the form of a large box with several fans. In addition, many typical operations with the image data can be accelerated by parallel processing on multiple cores simultaneously. Some software systems, and the spearhead of these technologies is e.g. VisionLab machine vision system, can exploit the massive parallel performance of current graphics processors. While today's CPUs have up to four cores, GPUs divide the calculation e.g. to 240 cores. Such a system is capable with image in real-time to perform until recently unthinkable operations.
Even with more than adequate programming environment we sometimes encounter the absence of the required functions or its poor performance. The possibility of adding custom code can give us peace. And even if it did not help to meet our requirements, at worst, we can change the entire software system for machine vision and avoid failure in dealing with contract. More room for operating is always handy.
In many cases it is sufficient when the only output in visual inspection system is a binary output, signalizing a faulty product, more often machine vision systems do not operate separately from the rest of the world, but is required their integration into enterprise information systems.The programming environment should allow the inclusion of a visual inspection into wider context of operating and visualization system, should transfer all data network, including image data, communicate with the PLC, cooperate with SQL servers, HTTP servers etc.
Further, suppose we propose a system where the cameras are connected to a standard PC. It remains to choose the right camera, lens and illumination.
There are so many criteria for selecting the camera, we can consider the CCD or CMOS detector, chip size and resolution, monochrome or color sensor. In the case of the color sensor may be a single-chip implementation, three-chip color design or sequential shooting with black and white sensor and color filters. Camera connection may be analog or digital. In the case of a digital interface we have a choice of Ethernet / IP, USB or Firewire. Digital cameras can be run either with a fixed frame rate, can be externally triggered, can run free with the accumulation of light, may provide variously compressed data stream or can provide unbiased raw data, etc. – when selecting the camera, really confusing number of criteria plays the role.
Let's simplify the selection for the purposes of machine vision. Above all, we must choose the appropriate point resolution – for the purposes of measurements in image the resolution determines the size of the measured object and demand for its measurement accuracy. One pixel must theoretically correspond to the measurement accuracy. This theoretical accuracy and reproducibility of measurement is reduced by the effect of noise and unwanted image artifacts, necessarily occurring in the lossy compression of image data. Sometimes we can use statistical methods to reach the Sub-pixel measurement accuracy, and generally without the knowledge of things but it is not possible to design a system like this. In the case of single-chip color cameras has to be taken into account approximately half the linear resolution ability. In color mosaic, usually from four pixels, two pixels are green, one red and one blue. This brings not only the lower resolution ability, but also as e.g. image cross-shift in different color channels.
In addition to resolution, we must choose connection and type of camera. We probably reject an analog camera for machine vision needs. Digital cameras are usually connected at a bigger distance via Ethernet and at a short distance through the USB. The principle of cameras digital connection itself is not the guarantee of image quality. The cameras are generally designed very similarly - the vast majority of digital CCD camera includes similar integrated camera controller, that digitizes data from the CCD, balances color, interpolates colors from the Bayer mosaic and lossy compresses data to MJPEG or MPEG4 stream. Considering the compromisingly reduced characteristics of integrated image processor, the quality of these operations is always visibly limited and the resulting image is so burdened with significant undesirable artifacts. Therefore, already at the stage of conceptual design of machine vision we must have a very precise idea of what image quality we need. Purity, stability and accuracy of image certainly is not essential for all types of applications, sometimes it is surprising that visual inspection works based on the presence or absence of a few blurry spots in the pre-defined positions. But for more complex applications it is the image quality what makes stable and therefore successful operation of inspection system decisive. Best image quality achievable bring cameras, which provide RAW image data. The image is not transformed by these cameras, color balanced, interpolated nor compressed. It delivers unmatched image precision, which is available in the connected computer, and there can be processed without any compromises limiting its quality. High bitrate can be sometimes an obstacle between the camera and computer. In short-distance connections these requirements are perfectly solved by the USB 2.0 interface with bitrate up to 480 Mb/s (one cable at 5 meters and up to 30 meters when using active extension and hub).
Choice of lens and visual angle is on of the most important decisions in designing a machine vision system. Common types of lenses reflect the image into area with so-called perspective projection. This compels us to deal with features of projective imaging of three-dimensional scene into two-dimensional area of image sensor's surface. Lens field of view is in this case formed by viewing truncated cone. Rectangular area of the image sensor further reduces this cone into viewing pyramid. Its peak is called the focal point of the projection. When converting a scene image inside viewing pyramid into image area, there is considerable loss of information. Each half-line passing through the focal point of the projection is in the image area represented by a single point.
Even the theoretical assumption of a perfect lens with linear transfer from angle to position and planar two-dimensional scanning of the pattern, we have to deal with distortions of image geometry due to perspective errors. Imagine a picture of dark dots on a light background with a constant spacing of dots in the axes x and y. To achieve a constant spacing of dots provided accurate perspective projection even in the projected image, the pattern must be scanned from the inner surface of the spherical area. When the planar pattern spots in the projected image will move away from each other depending on their distance from the optical axis.
Loss of spatial information in perspective projection complicates e.g. the exact measurements of sizes of three-dimensional objects. Without prior knowledge of taken subjects shapes we are not additionally able to correct these errors. Even with knowledge of the object's shapes, the correction of projective errors requires identification of objects of machine vision software equipment, therefore a high level of image understanding is required. In addition to normal lens with a perspective projection, there are also special lenses with orthographic projection. These lenses do not display a scene with a focal perspective, but with a perpendicular parallel projection. So, the size of the displayed items are always the same, regardless of their distance. That sounds like a good solution to all problems with accurate measurements in the image, but it has one problem. In this type of projection the size of the lens input surfaces must be similar to the area of scanned scene. These so-called telecentric lens are then very large and expensive. The principle of telecentric lenses is quite simple. With aperture diaphragm located in plane of image main point (outbreak lenses) all the rays coming from other directions than parallel to the optical axis are blocked.
In order not to end all problems, significant limitations of measurement accuracy in the image can cause the geometric distortion of image field of lenses, and they occur even at telecentric lenses. By distortion we mean the difference between the theoretical position of a pixel, which derives from the principle of projection and the actual position of point displayed by an actual lens. At the actual lenses there is never a transfer between the angle position (or distance) of an object from the optical axis and between the image distance of the object in the image area completely linear. Angle transformation at distance has quadratic character, but more often, the cubic polynomial. When using with several-megapixel cameras, even a very good lens has usually radial distortion in the range of units to tens of pixels. In some applications, such as when we read texts, codes or count components, it does not have to matter. In applications where is the required precision of measurements, the lens quality becomes a crucial factor.
While with the principles of projection we cannot do anything, distortion of image field can be corrected by software and we can achieve outstanding sub-pixel accuracy with standard lenses. The problem may be computational complexity of correcting algorithms. For example, VisionLab system carries out these corrections using the GPU with quite minimal impact on computers workloads.
While in the preceding paragraphs it is possible to estimate the right solution intellectually very well in advance, and often to accurately calculate, the correct choice of lighting requires considerable experience and often a lot of experimentation. Especially if the scene consists of transparent, shiny or embossed unremarkable objects, lighting design is the key of overall success. We must choose the type, number and position of lighting units and the color of their light. Often it is necessary to solve blocking an unwanted light from the surroundings by shading and color filters in the camera. The polarizing filters can contribute to a significant reduction of unwanted reflections .
Cheap lighting can be solved e.g. by using fluorescent tubes, possibly even without electronic ballasts, but the camera must be able of sufficiently long exposure times. More quality and better parameterizable lighting provide light-emitting diode units, whose price has been reduced so much that is usually not an obstacle to deploy them. If operating the lighting during the activity is needed, e.g. setting brightness, colors or initialize flashes, it is a great advantage to have an option of operating the lighting units directly from cameras.
If we did not leave anything substantial in the previous steps, it remains only to choose appropriate software for image processing and understanding (and of course everything to configure and program) and the success of the contract should no longer be endangered.
DataCam digital industrial camera offer interesting properties for use in machine vision systems. It is a low-noise CCD cameras, which provide clean raw image data in sixteen bit dynamic pixel brightness. Are connected to a computer via USB cable, from which are simultaneously powered. Cameras excel in image quality and clarity.
Each of these cameras can directly control up to four DataLight lighting units, which are available in the form of a circular illuminators, area illuminators, flash illuminators and illuminated panels. At DataLight units, we can choose color and emissive angle of diodes and possibly the presence and type of diffuser.