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Charge-Injected Device on the Cutting Edge

Photonics Spectra
Feb 2001
Daniel C. McCarthy

Cutting lumber in a mill is not like running a board from your hardware store through a joiner. Trees taper as they grow and are neither perfectly round nor always straight. As in any other industry, sawmills must optimize yield, which requires automated controllers to minimize scrap while quickly producing uniform products from nonuniform materials. For an automated system to edge these raw materials, it must know the size and shape of the unedged board.

Silvatech Corp., which makes production equipment for the forestry industry, applied laser triangulation to solve this problem. In its Edger Optimizer System, galvos direct a laser line across the top surface of the plank. A camera aimed at the point where the laser's central axis meets the board determines the board's thickness and surface characteristics based on the location and shape of the reflected laser line. The system determines and adjusts the position of logs to control their cut pattern with an accuracy down to 0.015 inches.

The system is composed of four lasers and three cameras. This combination prompted Silvatech to select Thermo Cidtec Inc.'s 2270-model charge-injection-device cameras. "We're looking for the center of a very bright spot in an otherwise dark field," explained Mike Hughes, the company's vice president of engineering. For CCD cameras, this can cause blooming in the image, but the contiguous pixel structure of charge-injection devices resists this problem.

Silvatech also benefited from the 2270's rapid scan feature. Both CMOS and charge-injection devices may be configured to selectively image specific lines or combinations of lines within the image. This random addressing of pixels highlights only the areas of interest, speeds up the frame rate and reduces the amount of data for processing.

"You can do random addressing with CMOS, but they're not fast enough for this application," Hughes said. "To do that, you have to empty the camera and then integrate light again. We couldn't clear CMOS images quickly enough."

Thermo Cidtec's charge-injection device, however, skips over the unimaged lines, cutting the conventional 60 ms required to read each line to a 267-ns scan. This helped Silvatech increase the accuracy, throughput and yield of its system.

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