- November 18, 2015
- Cognex Corporation
- Case Study
Cognex helps AMS Solutions‚Äô IdealSTICK vision-based stick inspection system improve stick reject detection at the sawmill stacker.
November 18, 2015 - Automated stick placing machines at the sawmill stacker require a system to detect defective sticks that jam in the machine or are otherwise unfit to be used in the stacking process. Sticks that are broken or are not the correct size to fit in the machine must be reliably rejected to maintain continuous production at the sawmill stacker. There are a number of methods currently used to detect these defective sticks. Some older systems rely on a person to manually inspect and reject sticks before they enter the machine but these manual systems are costly, a safety concern and not as reliable as an automated system. Other systems use a series of photo-eyes to automatically measure certain stick characteristics at a few locations but are unable to reliably detect all of the relevant defects due to limited information. Other more advanced systems use a 3-D scanner to measure a small section of the stick but due to the high cost of multiple scan heads and the complexity of the system it is unfeasible to scan the entire stick.
AMS Solutions has developed the IdealSTICK vision-based stick inspection system that cost-effectively evaluates geometric defects over the entire stick at rates up to 300 LPM. Utilizing a series of lasers and plan view image analysis using the Cognex In-Sight platform, IdealSTICK is able to evaluate sticks for thickness, width, length, bow, crook, twist, breakage and overall shape. The IdealSTICK system has been successfully deployed at multiple locations since the summer of 2012.
After lumber products such as 2x4s or 2x6s are cut to size in a sawmill, they are stacked into packages before being sent to a kiln drying process that heats the wood to a specified temperature in order to remove moisture and eliminate any insect infestation. The lumber is stacked in tiers in each package and each tier is separated by small boards called sticks, a specialty wood product used to maintain spacing during the kiln drying process. Depending on the specific requirements of the mill, sticks come in many sizes but are typically 4 to 5 feet long, 1.5 to 2.5 inches wide and 0.75 inches thick. Sticks provide air circulation around each tier of lumber, allowing for even heating and moisture reduction. The sticks are continually cycled through the sawmill, kiln and planermill and can be reused for up to a couple of years before they deteriorate to a point where they must be disposed of.
Before automatic stick placing systems were developed, people would manually place the sticks between every tier as the package was built and dispose of broken sticks as required. With automatic stick placing systems, the sticks are loaded into hoppers located along the length of the packages and automatically drop sticks at the required locations. These hoppers are automatically refilled using an overhead pusher system with diverters that route sticks into the hoppers. To feed the overhead pusher system, the sticks are first singulated by an unscrambler system and then transported by a lugged chain which maintains each stick in its own lug space until it is handed off to the overhead pusher chain. While being transported in the lug chain section, the sticks are inspected and rejected if required.
One proposed solution to the problem of detecting defective sticks utilizes lineal scanning technology. In a typical configuration, multiple scanners would be positioned around the sticks on the lugged conveyor. Each scanner would capture a 3-D image of a portion of the stick and the multiple images would then be combined in a computer to provide a complete 3D profile of the stick. The cost of this approach is easily justified in log and board optimizer applications but isn’t cost effective for stick inspection. The result is that most sawmills have continued to use less than ideal stick inspection methods.
“Generally, today’s vision systems are divided into two groups: PC-based and self-contained smart camera vision systems,” said Richard Vetter, Vice-President of AMS Solutions. “We decided to go with a vision system because they are generally easier to configure, validate, and maintain.” Because they are solid state, smart camera vision systems intrinsically provide a more stable platform than Microsoft® Windows®-based alternatives, requiring less maintenance. Being configurable rather than programmable, smart camera vision systems also make it easier to accommodate future changes.
“We selected the Cognex 5603 vision system because it provides 1600 by 1200 pixel resolution which is sufficient to inspect the entire top surface of the stick to approximately 0.050” accuracy and because its high speed processor can easily complete the inspection operation within the time requirement,” Vetter said. “We also like the ruggedness of the 5603 including its rugged die-cast aluminum case, sealed M12 connectors, and protective lens cover that provides IP67 and IP68 rated protection against dust and moisture.”
The IdealSTICK system is calibrated by simply capturing an image of a stick with the desired dimensions. The operator stops the conveyor, places this “ideal” stick in position to be viewed by the vision system and pushes the calibrate button. From this point on, the vision system compares each stick to the one used for calibration and rejects sticks whose dimensions vary too much from the ideal stick. The vision system program was developed using the Cognex spreadsheet interface. This interface makes it possible to program a vision application by dragging and dropping vision tools into a matrix without ever writing a line of code.
A photoeye detects when a stick is in position below the camera and triggers the camera to take the picture. AMS programmers used vision tools available in the Cognex programming software to analyze the stick and generate a Pass/Fail result. The result of the analysis including a picture of the inspected stick is displayed in real-time using an industrial touchscreen PC mounted on the scanner frame and running the Cognex Vision View software. The Vision View software was used to create a dynamic real-time user interface that can be used not only to monitor the current piece being analyzed but also to review the previously rejected sticks. The review mechanism allows the operator to easily see why the system rejected previously inspected sticks.
“The first IdealSTICK systems have been in operation since the summer of 2012 and are performing very well,” Vetter concluded. “The customers are very happy with the inspection systems and our expectation is that an increasing number of sawmills will recognize the advantages of vision-based stick inspection including the relatively low cost, high accuracy and the ability to function reliably in the sawmill environment.”
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