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Interlocks could be called the worker bees of a safety system. They come in myriad varieties, and each has a specific function. They can be as simple as a mechanical interlock on a gate or electronic interlocks that are part of a broad safety network. Before making the investment, say sources, companies should know all their options and be able to determine which method is safest and cost-effective.
"People always used to ask, 'What's the cheapest way I can guard this machine?'" says Russ Wood, application engineering manager at Scientific Technologies Inc., Fremont, Calif. Wood usually responds: "Well, how much are your employees worth?"
THE BASIC TYPES
Mechanical contact systems involve elements that physically make or break contact, be it a key, pin or other element.
One form that has taken off in recent years has been the trapped-key interlock. The system involves various locks with keys that can't be turned or removed before certain steps are followed.
A simple system would include two keys, "A" and "B." Consider a manufacturing cell with traditional perimeter guarding or fencing, and a locked gate. The first step is the power-disconnect. A lock that controls the power holds key "A," which must be turned to disconnect power before it can be removed (hence the term "trapped" key). After that, key "A" can be inserted into the gate's lock which, in turn, releases the "trapped" key B. The technician can then turn key B, open the gate and take the key with him. This ensures no one but that technician can lock the gate and power up the manufacturing cell.
Another interlock style has recently emerged. Particularly suited for harsh environments, magnetic interlocks replace a standard lock's shaft with a powerful magnet. These non-contact interlocks work well if a gate may have, for instance "a sloppy door," says Wood, "where you need to tolerate a little misalignment of the actuator and the head," or in places where debris might hinder movement of the locking mechanism.
Yet another interlock variety involves safety limit switches. Note, Wood adds, that these switches should be installed the opposite way of conventional limit switches. "Typically, you push the plunger or the roller lever, and the contacts mate," he explains. "When you let go of those contacts, the contacts break. A safety limit switch should be installed exactly the opposite, so that when you press the plunger or roller, the contacts are physically broken, and when you let up on the actuating mechanism, the contacts mate. That's because you want the physical force of pushing down the plunger to break the contacts."
Another group of locks—electronic control interlocks—are practical in applications where, for example, a work area requires frequent access. They use solenoid controlled locks that contain safety circuits that monitor the gate's position.
CONTROL CONSIDERATIONS
As with all safeguarding elements, companies should conduct a risk assessment to find which kind of safeguarding (including interlocks) they need. (For more on risk assessment, see the May 2006 issue, available at www.fandmmag.com.)
"The kind of interlock is all dependent on the hazard and the result of the risk assessment," says Eric Hollister, applications engineer for Pilz Safety Automation, Canton, Mich. "The motivating principle in the U.S. is the term Control Reliability, specified by the ANSI B11.19 standard." That standard requires redundancy in safety control; if one fails, another must be there to perform safety functions.
Of course, older controls, he says, were sometimes not built with such redundancies and, at times, make it difficult to retrofit those systems with machine safeguards. This is particularly common with systems having older variable-frequency drives, Hollister says. Trying to integrate modern safeguards with these systems is "not fun, and it's not easy."
Regardless of equipment age, though, redundancy should follow through entire circuits. "Redundancy should carry all the way through your system," Hollister says, from the input through the logic control device interpreting the data, to the output and end contactor.
For this reason, "you're starting to see companies designing in safety from the start of the project," he adds. "The earlier you start to design in safety, the easier it is to integrate into your system."
NOT THE ONLY SOLUTION
Interlocks and barrier guarding often are seen as the most cost-effective way to integrate safety into a manufacturing cell. While often true, Wood says, companies should consider the entire gamut of safety devices. While laser scanners, light curtains and other devices may seem more expensive on the front end, the resulting productivity may tell a different story, he says. (Some light curtains now have resolutions down to 12 mm, small enough to detect a finger.)
"How often must you access a machine? Yes, interlocks are a perfect way to guard your machine. But they can get in the way if the system isn't properly designed," Wood says.
Editor's Note: For more information, visit Pilz USA, www.pilzusa.com; and Scientific Technologies Inc., www.sti.com.
author: By Tim Heston