Tread on Me

THE LOWLY SAFETY MAT HAS COME A LONG WAY IN SOPHISTICATON.

By Edward P. Moser

You might think of safety mats as devices that are so simple in design that, well, you don't even think about them. But the lowly (literally, not figuratively) devices have come a long way in sophistication.

Take Larco's MAX series control unit for safety mats as an example. The Brainerd, Minn.-based company has devised a system that can monitor up to 10 mats linked in a series through a junction box. Each unit has selectable auto-reset or monitored manual reset. The machine-mountable units can accommodate 115 VAC (alternating current), 230 VAC and 24 VDC (direct current).

“The mats' well-defined area of protection make [the system] ideal for applications requiring a safety zone that is more than a thin vertical plane of protection, as you would have with a light curtain,” says Joe Schultz, sales manager at Larco.

In general, safety mats consist of an electrical switch, encased in a durable outer vinyl shell, often made of polyvinylchloride. The switch closes when pressure, such as the footstep of an employee, is applied to the surface of the mat, and opens when pressure is removed.

The mats typically work by connecting four wires to a safety mat controller, says Lou Schubert, product manager for pressure sensitive safety products at Fremont, Calif.-based Scientific Technologies, Inc., a maker of mats, light curtains, contact strips and other safety items. Each four-wire mat operates on a low-power DC signal. A signal is transmitted through the upper and lower plates separately via the two wires connected to each plate. The mat controller monitors signals through the safety mat.

When sufficient pressure is applied to the active mat area, the conductive plates touch, causing the output relays in the controller to de-energize, and a stop signal is issued to the machine. If a wire separates from a plate, breaks, or disconnects from the controller, the controller's output relays “de-energize,” and a stop signal is transmitted.

Safety mat controls employ sophisticated circuitry to constantly check the status of the mats as well as the circuitry itself. Wiring shorts and breaks and internal faults are monitored; redundant circuits monitor key safety functions.

When operators perform repetitive operations, safety mats may “minimize operator activity and fatigue by eliminating the need to manually open a guard or activate a safety switch,” says Schubert.

And Schultz adds that safety mats are incredibly durable, “capable of withstanding forklift traffic and complete submersion in fluids.”

“These devices offer incredible freedom and flexibility when compared to other guarding methods, such as fences, restraints and barriers,” says Schubert.

LIGHT MAKES RIGHT

Light curtains-virtual barriers that form a sensor-emitting “wall” around potentially risky work areas-are becoming more prevalent in the packaging environment. And one company with a somewhat unusual market niche is Smartscan Inc. The company, with U.S. headquarters in Livonia, Mich., is gearing its light curtains to work in conjunction with the growing market for robotic palletizers.

Smartscan general manager Paul Budesheim says there's a demand for palletizers with greater automation. Thanks to their speed and flexibility, robotic palletizers are increasingly becoming a viable solution for end-user companies. But with this change comes new safety issues.

“Changing a process on a [traditional] palletizer can involve considerable work,” Budesheim says. “However, with a robotic palletizer, you can press a button to touch off a subroutine. You might have one process for bags and another one for boxes. You can program the computer for anything you want.”

The sticking point, though, is that it's very difficult to permanently and completely fence off a palletizing operation. End-of-line packaging machines-which include palletizers, depalletizers, pallet lifts, robotic handling systems, stretch-wrapping machines, stackers and destackers-operate automatically, and have openings large enough to allow the passage of pallet loads. Therefore, the potential for unauthorized access exists, as a conveyor puts humans and robots in possibly dangerous proximity. A physical curtain offers one solution, but it involves constant opening and closing. Another solution is a photoelectric light curtain.

“We've designed a light curtain to determine the difference between a person and a pallet,” says Budesheim. “It allows a pallet to move in and out, but it shuts the system down when it detects a person. Robots are dumb animals. Most robots still cannot reliably distinguish between humans and pallets. You have to have a protection between a safe and dangerous area. And a danger area is anywhere a person can interact with machinery and get hurt.”

Ashland, Va.-based Flexicell is one OEM that uses Smartscan light curtains with its palletizers.

“We integrate light curtains into our systems and cells to provide a non-physical barrier to the work area,” says Thomas Halish, the company's vice president of engineering. “This prevents the user from entering, but allows pallets to enter and exit. Every palletizing cell we produce has at least one, and up to 10, curtains.”

Halish says the curtains do require good alignment and must be securely mounted. And, he adds, “because the beams are not visible, untrained personnel can inadvertently stop the system by accidentally block[ing] the curtain.”

The SmartScan 8000 Series light curtains have two-, three- and four-beam modules that can scan a perimeter up to 100 feet. To increase efficiency, the 8000 series also has a so-called “mute-initiating” system. If a pallet passes through an entry or exit point, the light curtain temporarily mutes its signal, allowing the pallet to enter or leave the area without bringing the packaging system to a halt. But if a person enters the entry or exit area, the light curtain simply shuts the machinery down.

Edward P. Moser is a business and technology writer based in Alexandria, Va.