When Weyerhaeuser identifies a safety measure that can be improved, the need is documented, and steps are taken to put better safety measures in place. Such a need arose within Weyerhaeuser's Containerboard Packaging business where corrugated paper transfers between work centers.

The work centers employ automated transfer carts, or “auto carts,” which are automated guided vehicles (AGVs) mounted with conveyors. Auto carts transfer stacks of corrugated paper blanks to work centers that convert the corrugated material into containers of different sizes. They move back and forth between lines to make sure that corrugated material from the correct source is delivered to the correct work center.

The auto carts, which move according to control signals provided by onboard PLCs, in the past incorporated a system of mechanical bumpers to make sure that no human was injured by a moving cart. However, mechanical bumpers still need to come into contact with people before the control system could force the cart to stop. These “pinch points” caused several ankle injuries, and prompted Weyerhaeuser to search for a solution.

To solve the problem, Glen Lawson, Weyerhaeuser's specialty group manufacturing manager, turned to a proven technology resource: its system integrator for almost a decade, Concept Systems Inc. of Albany, Oregon. Concept Systems was charged with developing a non-contact system for alerting the carts to stop as they approach obstacles.

Concept Systems had been building experience with laser ranging systems, and had announced its time-of-flight laser radar based LengthScanPro solution about two years earlier. Designed for the forest products equipment market, LengthScanPro measures the length and diameter of logs to help sawmill operators make better cutting decisions.

Similar technology has since been used to develop a canned “safety scanner,” where much of the flexibility is stripped from the laser radar system to suit the stringent performance requirements of safety-rated products. The on-board electronics/intelligence of safety scanners are designed to serve a relatively specific function, rather than to allow remote data access or data manipulation. As such, they are ideal for Weyerhaeuser's need.

Designing a control system that is safe is inherently difficult, particularly when you're working with older equipment that wasn't designed with safety at the forefront. One design challenge was the need for the new laser system to work with a variety of OEM auto carts containing different PLCs. For example, the auto carts at Weyerhaeuser's Aurora, IL facility are different mechanically and electrically from the Anaheim, CA auto carts. Hence, the system needed to be extremely adaptable to minimize custom design work for each cart.

As he configured the laser safety scanner for use on the auto carts, Concept Systems' engineer developed a circuit to detect and notify the scanner of the direction of the cart's motion. The system was also designed so that the lasers' scan fields can be adjusted on the fly, via encoder feedback, in order to avoid triggering the stopping mechanism when a fixed known object is in proximity to the cart. These parameters prevented nuisance shutdowns that might occur when the cart is moving away from an object, or near an object like a wall.

The engineer configured each laser scanner to track two scanning zones: a “warning zone” that signifies that the cart may be getting too close to an object (the controller then sounds an audible alarm and slows the cart's speed); and a closer-in “shutdown zone” that causes the cart to stop when an object is too close for safe operation.

Weyerhaeuser was pleased. “By being able to use a single design for multiple installations, Concept Systems was able to retrofit both of our plants, improving safety for less cost than if we had bought brand new carts,” said Lawson.

Various members of the technical staff at Concept Systems contributed to this article.