SAFETY MATS: A SENSE OF PRESENCE

These safety systems maintain full visibility of and access to the work area while offering freedom, flexibility and reduced operator fatigue when compared with traditional guarding methods such as interlocked fences, pullback restraints or perimeter barriers.

UNIVERSAL SAFETY MAT SYSTEM
An Omron STI Universal Safety Mat system (UM series safety mat combined with an MC3, MC4, or MC6) offers a simple method for guarding personnel around hazardous machines. A safety mat system offers freedom, flexibility, and reduced operator fatigue when compared with traditional guarding methods such as interlocked fences, pullback restraints or perimeter barriers. Full visibility of and access to the work area is maintained.

The operation of the safety mat is easy to understand. The safety mat is a simple, normally open switch. When a specified minimum weight is applied to the safety mat the “switch” closes. This sends a signal to the safety mat controller, which in turn, sends a stop signal to the guarded machine. Each safety mat presents four wires to the safety mat controller. This provides the redundancy required to monitor the wires for open circuits due to incorrect wiring or physical damage to the safety mat wires.

Additionally, safety mats can also simplify routine tasks such as machine set-up, maintenance and repair. Typical applications for safety mats include welding robots, assembly machinery, material handling, packaging machinery, punch presses and robotic work cells.

Theory of Operation. Multiple safety mats may be wired in series to form a complete floor-level guarding system. Each 4-wire safety 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 signals through the safety mats are monitored by the safety mat controller.

When the safety mat is not exposed to sufficient actuating force, the signals are unimpaired. The output relays in the controller are energized permitting the guarded machine to run. When sufficient pressure is applied to the active safety 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 should break, separate from a plate, or become disconnected from the safety mat controller, the output relays in the safety mat controller will de-energize and a stop signal will be sent. Should the safety mat be punctured and the plates short together in a similar manner as being stepped on, the controller will not restart until the punctured mat is replaced.

The first and by far the most important consideration is the calculation of the safety distance. There is a minimum safety mat size that should be placed between a worker and a hazardous motion. Many users will “eyeball” the application, look at the area where a machine operator would stand and say, “that looks like it needs a 24 in wide safety mat.” It may not be enough.

In standard B11.19 the American National Standards Institute ANSI) states that, “The safety mat device shall be located such that the operator cannot reach the recognized hazard before hazardous motion of the machine has ceased.” Unfortunately, the ANSI standard stops there. In order to figure out how much safety mat you need between a machine and a worker, it’s necessary to refer to the standard EN 999. This standard provides a formula for determining the correct safety mat size for a specific hazard. The formula is similar in nature to another calculation, which some readers may be familiar – the mounting distance formula for a safety light curtain.

The mat formula reads as follows:

S = (63 in/sec x T) + (47.2 in – 0.4H)

where:

S = The minimum distance from the danger zone to the detection zone in inches. This calculation assumes that the approach of a person toward a dangerous area is at walking speeds.

T = t1 + t2

T = The overall system stopping time.

t1 = The maximum time between the actuation of the safety mat and the controller output relays being in the de-energized state.

t2 = The response time of the machine being guarded. The time required to stop the machine or remove the risk after receiving the output signal from the safety mat system.

H = The distance above the reference plane (floor) in inches. When mats are mounted on the floor, H = 0.

As an example, let’s say the stopping time of the guarded machine is 0.300 seconds. The response time of the mat system is 0.025 seconds. The safety mat is to be mounted at the reference plane (floor).

S = (63 in/sec x T) + (47.2 in – 0.4H)

or

S = (63 in/sec x (t1 + t2)) + (47.2 in – 0.4H)

Therefore:

S = (63 in/sec x (0.300 sec + 0.025 sec)) + (47.2 in – (0.4H x 0.0in))

S = (63 in/sec x 0.325 sec) + (47.2 in – 0.0 in)

S = 20.5 in + 47.2 in

S = 67.7 in or 1720 mm

This calculation specifies that the distance starting at the mat edge furthest from the hazardous location should be at least 67.7 in. This also implies that the floor area, from a distance of 67.7 in to the hazardous location must be guarded by mats or other means, such as fencing, to prevent any undetected access to the hazardous location. Using this formula will usually result in a larger safety mat specification than most people would estimate.

Minimum Detection Zone. The distance a person walks is generally a factor of the walking speed and the stride length. As we have already seen, the positioning of safety equipment, determined by these calculations assumes a person is walking. The stride length affects the minimum size of the mat detection zone. The EN 999 standard has determined that this dimension is equal to 750 mm or 29.5 in. Thus, the minimum size of a safety mat, measured toward the direction of movement should be at least 29.5 in, and is identified as “C” in the figure.