Similar to other process manufacturers, sawmills work around the clock to make quality products from raw materials with a minimum of waste. They want to maximize the capital invested in the factory, use it at full capacity and accommodate flexible manufacturing practices. In an ideal world, factories would operate in a mode where each machine center runs within its process capability, and the operators would know immediately when something drifts out of tolerance, so it could be corrected quickly.
Before sensors became part of process control, dimension monitoring was done by an operator who would bring a caliper to each machine center, pull and measure boards in multiple places, log the measurements on paper and repeat this process a few times per shift. In some cases, it would be necessary to lock out machine centers, possibly impacting production, in order to gain safe access to the measurement samples. Later, the measurements were taken back to an office where they were typed up, and dimension accuracy was analyzed. However, the sample size was too small to be effective, and the information arrived too late because the machine continued running in the meantime.
One of the sawmills owned by Canfor in Prince George, British Columbia, Canada, was determined to eliminate this problem (Figure 1). The mill's size checks used to occur once every two hours or so. If five boards were cut out of a log, the checkers might only measure the bottom two and overlook a problem in the third or fourth saw. "We used to waste a lot trying to find the source of problems," says Ray Wium, quality control supervisor at Canfor's Prince George sawmill.
In a sawmill, there are generally four to six breakdown stages as a log is cut into smaller pieces and planed. If the machinery is not set up correctly or isn't operating properly, for instance, if the cutting tools are wearing, then the company could end up with rail cars full of slightly undersized or oversized product.
The repercussions are far more costly if the problem is not discovered until after the product is packaged and shipped. If a customer in Japan rejects a shipment because the boards are too small, the company will need to dispose of them in that region, most likely selling them for pennies on the dollar. If they are too large, arrangements must be made to have them cut down to size.
"If you are doing 1.2 million board feet per shift as some mills do, you can't measure a lot of wood with a pair of calipers because you'd need an army of people," says Nick Barrett, president and technology development partner at system integrator SiCam Systems, Surrey, B.C. "We deal with thousandths of an inch of recovery in the sawmills, so if they mismanufacture by 0.01 of an inch when they cut a million board feet in a shift, that quickly adds up to many thousands if not millions of dollars over time."
Recent technology advancements are bringing this situation under control. "New three-dimensional machine vision systems allow process manufacturers to monitor and control their product dimensions automatically, whether that product is lumber, meat, tires, aluminum truck frames or loaves of bread," Barrett explains. "The systems use 3D sensors to conduct hands-free, real-time dimension monitoring of incoming, in-process and outgoing processes; perform process capability analytics; and generate alerts to the process control system when the process deviates from tolerance."
Barrett adds that 3D machine vision adds value beyond what you might initially conceive. "It improves quality by ensuring that the products are produced within specification limits. It allows problems to be solved quickly, which improves throughput and reduces waste (Figure 2). It increases profitability by maximizing the usage of raw materials and minimizing downtime. And it promotes safety by automating risky tasks and avoiding the need for tag-outs."
Many companies are familiar with two-dimensional machine vision, and some have tried to use this technology to solve three-dimensional problems, says Terrance Hermary, co-founder and CEO of British Columbia-based Hermary Opto Electronics. SiCam builds its measurement solutions based on the Hermary Opto scanner. "Two-dimensional measurements are flat, often black-and-white or grayscale, and are subject to variations in ambient lighting," Hermary explains. "On the other hand, sensors that measure in 3D are like a pair of eyes, adding depth and intelligence to the scan."