By Lou Schubert
Scientific Technologies Inc.
Edited by Leland Teschler
Pressure-sensitive safety mats can eliminate the need to manually open a guard or activate a safety switch. The elimination of these activities also helps to improve machine productivity by integrating safety devices into the machine's control circuitry.
Stepping onto a safety mat holds open the safety circuit until all personnel step off the mat. This makes safety mats suitable for where there's extended time devoted to manual setup, repair, or maintenance. Operations can only be reenergized by the use of a manual reset switch (restart/interlock mode) that must sit outside the hazardous area. This prevents the possibility of the machinery restarting when the mats are clear.
A safety mat may be compared to a large, normally open switch that is integrated into a safety circuit. When the mat sees a pressure or weight exceeding a specified minimum amount, a top conductive plate touches a conductive plate on the bottom of the mat. When this contact closes, the safety circuit sends a stop signal to the machine's primary controller. The controller puts the machine into a "safe" or stopped state before anyone can get close enough to either be injured or contaminate the processes taking place.
Several safety mats can be wired in series to protect a large area. As with any "safety-rated" device, its design must comply with international standards. This includes redundant circuits and a design that will not allow any single point of failure to render the device inactive resulting in an unsafe condition. A safety mat is designed so that it will fail in a safe mode. If a wire breaks or is separated from one of the plates, or if the mat somehow gets disconnected from the safety controller, the safety controller will deenergize its output relays and send a stop signal to the machinery controller. A similar sequence of events happens if a safety mat is damaged or shorts out. The controller will not restart until the damaged mat has been replaced.
In the case of safety mats from Scientific Technologies (STI), a four-wire system connects the safety mat to the safety mat controller. Conductors are encased in a molded insulating material, typically polyvinylchloride (PVC). The STI safety mat has a raised "traction dot" surface to minimize the possibility of slipping.
Sizing safety mats
Safety mats have no way of physically keeping people out of hazardous areas. So the layout and size of the mat becomes a function of how long a machine takes to reach a safe state and the time a person takes to reach the machine. This is referred to as the "safe minimum distance."
This distance, S, is calculated from:
S = (63 ips T ) + (47.2 in. – 0.4 H) where H represents the distance above the horizontal plane or the thickness of the safety mat, in.; and T= overall stopping time. This is the sum of times t1 and t2, where t1 = the sum of times from when a safety mat is actuated to the time the system achieves a safe state, and t2 = the maximum time that the guarded machine requires to reach a safe state or stop. This calculation typically results in an area much larger than most people would estimate based on a visual inspection.
In cases where the safety mat mounts on the floor, H has a value of zero. This reduces the equation to: S = (63 ips T ) + 47.2 in.
The 63-ips figure represents an estimated walking speed of just over 3.5 mph. The length of a machine operator's stride affects the necessary length of a safety mat. This dimension comes from EN 999, which sets the stride equal to 750 mm or 29.5 in. Designers need to calculate the minimum safe distance from each potential approach.
Installation and use
Since most areas will not conform to a standard size, mats of various sizes can be mounted together and wired in series to cover the entire area. Mats must mount on an area that's flat and free of debris. Anything left under the mats can potentially cause a short between the mat plates or a hole or tear, which can let moisture get between the plates.
Cables for mats need to be kept out of harm's way. In STI mats, a two-part ramp trim both provides for mounting and integrates a wireway into the aluminum base for routing wires to the controller. Installations employing multiple mats can use two-part "active" joining trim that lets the wiring from two mats run in the same wireway.
Finally, it's important to determine that the safety mat won't see an environment that will cause problems. If chemical spills are a possibility, check the mat's resistance to make sure it will resist their effects.
In some situations a safety mat may cover an area exceeding the minimum safe distance, S. For example, in the case of a robot arm, the mat will need to extend beyond the arm's maximum reach. In any event, S must include the minimum detection zone C to accommodate a person's stride.