The use of linear cameras and the operating principle of WebCHECK.
For the control of products in continuous motion, the most natural camera technology uses the acquisition with linear cameras, often referred to as "scanners". These cameras have sensitive elements arranged in a single direction and create the final image using the movement of the product to be controlled.
There are numerous advantages of this technology for the automatic inspection described. The geometry of the one-dimensional sensor allows a very high resolution, both in the direction transversal to the motion (the direction of the pixels themselves of the sensor) and longitudinally to the motion (due to the considerable acquisition speed that can be reached). The arrangement of the control area, a thin strip perpendicular to the motion of the product, makes it possible to create constant lighting with ease, by using linear lamps. In this way, it is relatively simple to obtain uniform lighting, compared to a matrix acquisition that requires lighting on a two-dimensional area.
Types of detection of linear cameras
During the shooting with linear cameras, the sensors are arranged transversely to the direction of motion. The adoption of one or more cameras, together with the choice of the appropriate optical devices, allows to have the right resolution (minimum defect-detection capability) as a ratio between the overall width of the product to be inspected and the total number of photo-sites of the cameras.
The images of the product to be inspected can be obtained in two different configurations:
Detection by transmission, by arranging the illuminators on the opposite side to the cameras. It is used when the product is transparent (e.g. glass) or semi-transparent (e.g. plastic film and paper) and defects are visible due to the difference in light transmission (holes appear lighter, spots or inclusions appear darker) .
Detection by reflection, arranging the illuminators on the same side of the cameras, inclined appropriately. It is mainly used when the product is opaque (eg. metal sheet) and defects are visible due to the difference in reflection, the variation in shape or finish of the product surface;
The detected images are processed to extract the characteristics of interest to control and arrive at a completely automatic judgment. The processing takes place continuously, with the phases perfectly in parallel and with dynamic algorithms; it is for this reason that the distinction of the various phases has the logical meaning of separation of functions, but not of execution in subsequent moments in time.
The first phase of analysis consists in recognizing and separating the defects contained in the product. Defects are always highlighted as a local variation of the image with respect to the normal background of the product, generally as lighter or darker "spots". To consider an anomaly as a defect, a series of characteristics are evaluated:
- contrast of the defect with respect to the background;
- Defect area;
- Shape coefficient (squat or elongated);
- Orientation (mostly horizontal or vertical);
- Filling coefficient (regular or rugged);
For each of these characteristics it is possible to define criteria of acceptance limits, below which the product is still considered good.
Defect judgment of the product
The next phase of analysis consists in the classification of the detected defects, to obtain an automatic and univocal judgment on the quality of the product. Defects are first of all divided into categories, in order to better represent the testing necessities. It is possible to define the thresholds between classes, for each of the parameters of defect characterization. For each class it is possible to define the maximum number of acceptable defects, in absolute terms for the entire product unit (roll or reel to be checked), or relative to a predetermined length unit (e.g. number of defects for every 10 meters of product ). It is possible to divide the inspection width into an arbitrary number of strips, even of different widths. The detected defects, and the relative thresholds, are valid for each strip. The presence of numerous possibilities for regulating the system does not mean that its use is complex; the parameters are managed in a "clear" form, that is, they are expressed in the same language and with the same criteria known by the line operator, the adjustments are also interactive and it is generally possible to verify in real time the effect of the parameter itself on performance control.
- Defect reporting: the presence of defects is identified in real time, by means of digital signals, with sound or light warning, defect marking, or immediate stop of the machine. For each defect class it is possible to define one or more of the actions described, allowing the maximum flexibility of use of the control system.
- Reports: WebCHECK stores information on the product and its quality; these data can be printed at any time, on labels or printouts, or exported to external supervision systems.
- The identification of the inspected product can take place with the following data (all alphanumeric):
- Product code
- Inspection code
- Lot number
- Number of Units (roll, reel)
- Inspection date and time
- Meters produced, number of strips
- Total defects found (divided into various classes)
- Defects found for each strip (divided into the various classes)
- List of defects, for each strip, with indication of:
- progressive distance from initial product
- defect class
The resolution of a machine system control system expresses the ability to detect the defects , both in terms of size (spatial resolution) and contrast; obviously this detection capability can be adjusted as needed, by intervening on the quality parameters described above.
- In the direction transverse to motion, the overall resolution depends on the number of pixels of the camera. In a basic configuration, WebCHECK adopts cameras of 2048 pixels per line; configurations with different resolution or with multiple cameras in parallel are also available. The spatial resolution is obtained by dividing the width to be inspected by the total number of pixels available. It is therefore possible to use the system for very different product widths; products from a width of 4 mm up to about 4000 mm have been inspected, with defects ranging from a few microns to a few millimeters in size.
- In the longitudinal direction of motion, the global resolution is virtually infinite (it depends on the total length of the product); the spatial resolution, on the other hand, depends on the processing power of the processors used (processing frequency expressed in lines per second) and on the speed of the advancement of the product. The specific spatial resolution value is obtained by dividing the line speed by the processing frequency, WebCHECK allows it to detect up to over 20,000 lines/second.
- As a contrast resolution, 8-bit resolution monochrome cameras are used in almost all applications. Each pixel can therefore assume a value between 0 (black) and 255 (white). It is possible to use color cameras, with 24 bit global resolution. Each pixel
Inspectable materials and detectable defects
Transparent and opaque plastic films
(holes, inclusions, lumps, tears, spots)
Continuous glass (float) or in sheets
(bubbles, inclusions, breaks, spots)
(holes, inclusions, lumps, tears, spots)
Paper and similar
(holes, inclusions, lumps, tears, spots)
Raw or uniform color fabric
(holes, stains, tears)
(holes, oxidations, spots, striae)
WebCHECK consists of the following parts:
- Image shooting system, consisting of cameras and illuminators: The system provides a standard 1 b / w camera of 2048 pixels, however it is possible to request configurations with multiple linear cameras (up to 4 per system, with the possibility of parallelizing the number of systems) and use color cameras. The lighting can occur by transmission through the product (lamps on the opposite side of the cameras), or by reflection on the surface of the product (lamps on the same side of the cameras).
- Image processing and system management unit: Based on the use of industrial computers and specific high-power hardware; all the devices meet the criteria of high industrial reliability. The system operates in full compliance with all applicable safety regulations.
- Synchronization system with product progress: The system can work freely, without any synchronization with the product, or it can be synchronized with an encoder or pulse counter, to correlate the detected defects with the progressive distance from the initial inspected product.
- Defective product marking system: One or more ink jet markers may be included, which indicate the location of defects directly on the product; marking is typically done near an edge of the product.
- Control station mechanics: The system can include the mechanics necessary for complete integration with the production line, in particular the support, protection and adjustment devices of the cameras, illuminators, synchronization and marking devices.
- Meccanica della stazione di controllo: Il sistema può comprendere la meccanica necessaria alla completa integrazione con la linea di produzione, in particolare i dispositivi di sostegno, protezione e regolazione delle telecamere, illuminatori, dispositivi di sincronizzazione e di marcatura.