Safety Mat

What is a Safety Mat

A safety mat, formally a pressure-sensitive mat, is a presence-sensing safeguard that detects a person standing within a defined area on the floor. Functionally it behaves as a large normally open contact integrated into a safety circuit. When a load greater than a specified minimum is applied to the surface, two conductive plates inside the mat make contact, the circuit changes state, and a stop signal is sent to the machine control to drive the hazard into a safe condition before a person can reach it. The mat is one member of the presence-sensing family alongside safety light curtains and area scanners, but it is the only one that protects a horizontal plane the operator physically stands on, which makes it the natural choice for floor-level access guarding around robot cells, press brakes, and palletizers.

It is important to distinguish the device from a simple electrical switch. A qualified safety mat is a certified machine-safety component: it must respond to a defined actuating force, within a stated response time, repeatably, across its full operating temperature range, for at least a million cycles, while reliably ignoring water, oil, and dirt on its surface. Those guarantees are what separate a true safety mat from a decorative pressure mat or a doorway activation mat used to open automatic doors. The governing product standard is ISO 13856-1, published in Europe as EN ISO 13856-1, which superseded the earlier EN 1760-1 series.

A complete pressure-sensitive protective device has three parts, and confusing them is the most common cause of an unsafe installation. First is the sensor, the mat itself, which generates a signal when an actuating force deforms its surface. Second is the signal processing and output switching, housed in a separate control unit or safety relay, which converts the sensor signal into a monitored safety output that the machine controller can trust. Third is the wiring scheme that connects the two, either two-wire resistive or four-wire, and which must match the control unit. The mat without the control unit has no diagnostic capability and no safety rating; the integrity of the whole function lives in the pairing.

The working principle is mechanically simple. Most industrial mats contain two parallel current-carrying steel plates held a small distance apart by insulating spacer strips. Body weight pressing the top plate down closes the gap and creates an electrical short between the plates. In a four-wire mat this cross connection is detected directly by a dual-channel control unit. In a two-wire resistive mat a monitoring resistor sits at the far end of the loop, a small standing current flows in the safe state, and actuation collapses that current as the plates short, which the control unit reads as a demand. Either way the change of state is what stops the machine.

Application scale runs from small access mats of a fraction of a square meter guarding a single robot loading station, up to tiled fields covering many square meters in front of a long press line or an automated guided vehicle docking bay. Because a single mat element is limited in area, large zones are built from multiple mats wired in series, with the control unit monitoring the whole chain as one safety function. The engineering task is never simply to buy a mat; it is to size, lay out, and seam a field of mats so that every reachable approach path to the hazard crosses an active sensing area before the person is close enough to be harmed.

Historically, pressure-sensitive floor safeguarding grew out of doormat-activation technology and matured into a regulated category as machine guarding standards tightened through the 1990s and 2000s. The harmonization of EN 1760-1 and its replacement by EN ISO 13856-1 gave designers a single test regime for actuation force, response time, static load, and environmental endurance, which is why those four metrics dominate every modern datasheet and form the backbone of the chapters that follow.

Switching Technologies and Types

Safety mats are classified first by their wiring and monitoring scheme, because that choice dictates which control unit you can use and how faults are detected. The two dominant schemes are four-wire and two-wire resistive. A secondary classification covers the physical construction, primarily whether the mat uses an aluminum edge profile for forklift traffic or a fully molded polyurethane profile. The table below compares the two switching technologies on the points that matter during specification.

Four-wire technology runs two separate conductive plates back to the control unit as two independent channels. In the safe state the two channels are isolated. Stepping on the mat shorts the upper plate to the lower plate, creating a deliberate cross connection that the dual-channel control unit recognizes as the actuated state. Because the unit watches both channels, it can distinguish a genuine footstep from a single broken wire or a welded contact, which is exactly the diagnostic coverage that a Category 3 architecture demands. Four-wire mats are the long-standing industrial default and are produced by ASO, Schmersal, Mayser, Tapeswitch, and others.

Two-wire resistive technology places a terminating resistor at the far end of the mat loop, with common values of 1.2, 8.2, or 22 kilohm. The control unit drives a small monitoring current through the loop. In the safe state the resistance equals the terminator value; actuation shorts the plates and drops the resistance toward zero, while a cut cable opens the loop to near-infinite resistance. Any reading outside the expected window is flagged as a fault. The advantage is that several mats can share a single loop with less cabling, which suits long lines of tiled mats; the constraint is that the control unit must be configured for the specific resistor value the mat uses.

On the construction axis, two families recur. The first uses an aluminum edge or ramp profile bolted to the floor, which both fixes the mat and forms a sloped lip that forklifts and pallet trucks can drive over without catching the edge; Schmersal markets this as the SMS 4 style. The second molds a polyurethane actuating profile directly to the active surface for a lower, fully sealed unit, marketed by Schmersal as the SMS 5 style and offered with studded, sand-coated, or checker-plate anti-slip surfaces by ASO and others. The electrical principle inside both families is identical; the difference is mechanical durability and how the seams between tiled mats are closed.

A related distinction worth noting is between presence detection and command actuation. A safety mat used for guarding must hold the machine stopped for as long as the person remains on it, and must require a deliberate reset before motion can resume. This is different from a doorway mat that simply pulses a door opener. Only the former is rated as a safety component to ISO 13856-1, and only the former is appropriate around hazardous machinery.

Control Units and Functional Safety

The mat is only a switch; the safety integrity of the installation lives in the control unit, often called a safety mat monitoring module or safety relay. A bare mat cannot detect that one of its conductors has been severed or that an output contact has welded shut. The control unit supplies the diagnostics, the dual-channel evaluation, and the controlled reset that together let the combination reach a usable performance level. This is why a general-purpose relay must never be substituted for a dedicated mat module: the module is the part that earns the safety rating.

The functional-safety performance of the mat-plus-module function is expressed two ways. Under ISO 13849-1 it is rated as a Performance Level from PL a to PL e, tied to a Category (B, 1, 2, 3, or 4) describing the fault-tolerance architecture. Under IEC 62061 the same function is rated as a Safety Integrity Level, SIL 1 to SIL 3. Most machine risk assessments for floor guarding call for Category 3 Performance Level d, which a properly wired mat and a dedicated dual-channel module achieve. The control unit also implements the requirements of IEC 60204-1 for the electrical equipment of machines, including the stop function and reset behavior.

A safety mat module typically accepts a dual-channel input (two normally closed channels in the resistive case, or the cross-fault pair in the four-wire case), runs self-checking circuitry, and drives redundant safety outputs. The table below lists representative control-unit characteristics drawn from published module datasheets, illustrating the order of magnitude an engineer should expect when reading a spec sheet.

A critical safety behavior the module enforces is the re-start inhibit interlock required by ISO 13856-1. After a person leaves the mat, the safety output must not return to the ON state on its own; it may only re-enable after a deliberate reset signal, and a reset held continuously before or during actuation must be ignored. The intent is to prevent a machine restarting while someone is still inside the guarded zone but has briefly stepped off the sensing area. The choice between automatic reset and manual reset is therefore a risk-assessment decision, not a convenience setting: manual reset with the reset button placed where the whole hazard zone is visible is the safe default for areas a person can fully enter.

Representative modules include the Schmersal SRB 301HC series, which evaluates SMS mats to Category 3 of the machine-safety control standard, and the Banner SM-series relays, which monitor one or several four-wire mats and provide redundant safety outputs with self-checking and selectable reset. Mat-monitoring controllers from Mayser, Pilz, and Rockwell (MatGuard) perform the same role. The non-negotiable rule is that the module and the mat must share the same wiring scheme and, for resistive mats, the same terminating resistor value.

Construction, Materials, and Layout

The body of an industrial safety mat is a fully molded elastomer that encapsulates the conductive steel plates and their insulating spacers into a single sealed unit. The two materials in common use are polyurethane and PVC. A molded polyurethane mat is impervious to oil, water, coolant, and most chemical contaminants, which is why protection to IP65 is the norm for floor-level units. Polyurethane offers higher abrasion and chemical resistance for harsh production environments, while PVC is a lower-cost option for cleaner indoor settings. A representative element is about 14 millimeters thick including its surface covering and weighs roughly 25 kilograms per square meter, so a multi-tile field is heavy and is normally bolted to the floor through edge profiles.

Surfaces are always anti-slip and come in several textures: a studded structure, a sand coating, or an aluminum checker plate, with black and high-visibility yellow as standard colors. The surface texture is not cosmetic. It must keep footing secure when the mat is wet with coolant, and it must not so stiffen the top layer that a light footstep fails to reach the actuation threshold. The table below summarizes the material and mechanical figures an engineer should confirm on the datasheet.

The single most overlooked design issue is the inactive edge, or dead zone. The conductive plates do not run all the way to the molded rim, so a narrow border, commonly about 16 millimeters wide, will not switch even under full body weight. On a single mat this border is harmless. But when several mats are butted together to cover a large zone, two adjacent inactive borders combine into a strip a person could stand on without being detected. ISO 13856-1 addresses this directly: joints and junctions between sensors must still meet the actuation requirement when checked with the Ø80 mm test piece, so seams have to be closed with overlapping trim, interlocking profiles, or carefully dimensioned ramp edges. During layout the rule is that no reachable approach path to the hazard may cross an undetected border.

Mat position relative to the hazard is governed by the safety distance, not by convenience. The distance from the leading edge of the sensing area to the danger point is calculated as S equals K times (T1 plus T2) plus an intrusion allowance, where K is the assumed approach speed, taken as 1600 mm/s for a walking person, T1 is the response time of the mat and its control unit, and T2 is the machine stopping time after the stop signal. A mat that responds quickly still cannot be placed close to a machine that coasts for a second; the field must be large enough that the machine stops before the person crosses the remaining gap. Connection to the control unit is typically through a robust cable, for example a 5 mm PVC-sheathed lead carrying two cores of 0.5 mm squared or four cores of 0.34 mm squared, routed and protected so it cannot be crushed by traffic crossing the mat.

Key Specification Parameters

Reading a safety mat datasheet means checking the numbers ISO 13856-1 defines, because they are directly testable and directly safety-relevant. Six parameters drive the decision: actuating force, response time, static load capacity, operating temperature range, ingress protection, and mechanical life. The table below collects the ISO test thresholds together with representative commercial values so the two can be read side by side.

Actuating force is the force that must be applied through a defined test piece to switch the mat to its OFF (safe) state. ISO 13856-1 verifies this with circular test pieces pressed slowly onto the sensing area at a maximum speed of 2 millimeters per second. For mats detecting persons over 35 kilograms, the Ø11 mm and Ø80 mm pieces must trigger at no more than 300 newtons and the Ø200 mm piece at no more than 600 newtons; a child-detection variant for persons over 20 kilograms adds a Ø40 mm piece at 150 newtons. Commercial mats commonly actuate near 150 to 300 newtons through the Ø80 mm piece. That figure is far below an adult body weight, which is roughly 700 newtons for a 70 kilogram person, so a single footstep reliably switches the device even before full weight transfers.

Response time is the interval from the test piece first touching the sensing area to the start of the OFF state at the output. ISO 13856-1 caps the manufacturer-stated value at 200 milliseconds across the operating temperature range and tests it with the test piece descending at 0.25 meters per second. A bare mat element often responds in under 25 milliseconds, but the connected control unit adds its own delay of roughly 20 to 50 milliseconds, and it is the combined system figure that enters the safety distance calculation. Always use the documented mat-plus-module response time, never the mat element alone.

Static load capacity confirms the mat survives heavy standing or stationary loads without losing function. ISO 13856-1 applies 2,000 plus or minus 50 newtons through the Ø80 mm test piece for 8 hours and requires the output to change state within 2 minutes of removal, and separately applies 750 newtons through the Ø11 mm piece for 8 hours with permanent deformation limited to 2 millimeters after recovery. Operating temperature range, commonly minus 10 to plus 55 degrees Celsius for indoor industrial mats and as low as plus 5 degrees for some units, matters because actuation force and elastomer stiffness shift with temperature; the stated response time must hold across the whole range.

Ingress protection, typically IP65 per IEC 60529, governs whether the mat tolerates washdown, coolant, and dust without the conductive plates corroding or shorting falsely. Mechanical life, generally specified at more than one million actuation cycles, sets the realistic service interval for a mat in a high-traffic loading station. A further set of figures comes from the control unit, namely its supply voltage, mat loop current of roughly 10 to 100 milliamps, safety output ratings, recovery time around 350 milliseconds, and reset mode; these belong to the module datasheet but must be read together with the mat because the two are specified as a pair.

Selection Decision Factors

Specifying a safety mat is a layered decision. Most mistakes come not from picking the wrong mat but from skipping the risk-assessment and layout steps that determine whether any mat can protect the zone. Follow the sequence below, which doubles as an RFQ template.

1. Risk assessment and required performance level: Determine from the machine risk assessment whether the function needs ISO 13849-1 Category 3 Performance Level d, the usual target for floor guarding, or a different level. This sets the diagnostic and architecture demands on the mat-plus-module pair before any product is chosen.
2. Zone geometry and safety distance: Map every reachable approach path to the hazard. Compute the safety distance S equals K times (T1 plus T2) plus the intrusion allowance, using K of 1600 mm/s for walking, the system response time T1, and the measured machine stopping time T2. The sensing field must extend at least this distance from the danger point on every approach.
3. Mat field size and tiling: Since a single mat is limited to roughly 1.5 square meters of active area, size the number of tiles needed and plan the seams. Confirm that the combined inactive edges at every joint are closed so no undetected strip lies on an approach path.
4. Switching technology: Choose four-wire for the traditional default and simplest control-unit matching, or two-wire resistive when many mats chain over long runs; for resistive, fix the terminating resistor value (1.2, 8.2, or 22 kilohm) to the control unit.
5. Mat construction and surface: Select aluminum edge or ramp profile where forklifts or pallet trucks cross the mat, or a molded polyurethane profile for a lower sealed unit. Choose the anti-slip surface (studded, sand-coated, or checker plate) for the floor condition, wet or dry.
6. Environment ratings: Confirm the operating temperature range covers the installation, the ingress protection (typically IP65) suits washdown or coolant exposure, and the static load and drive-over ratings match the heaviest traffic the mat will see.
7. Control unit pairing: Specify a dedicated safety mat monitoring module rated for the same wiring scheme, with dual-channel evaluation, cross-fault detection, the correct supply voltage, adequate safety output rating, and the reset mode (manual reset is the safe default for fully enterable zones) the risk assessment requires.
8. Certification and documentation: Require ISO 13856-1 (EN ISO 13856-1) test evidence for the mat, the ISO 13849-1 or IEC 62061 rating for the combined function, IEC 60204-1 conformity for the electrical equipment, and the CE marking, plus regional approvals such as cULus where applicable.

One last dimension that is easy to defer is serviceability and maintenance. A floor-level mat takes abuse: it is walked on thousands of times a day, driven over, and washed down. Plan for periodic functional testing, confirm that spare mats and the matching control unit remain available from the supplier, and verify the cable entry and edge profiles can be replaced without lifting the whole field. Suppliers with documented mat-and-module families and local stock, including ASO Safety Solutions, Schmersal, Mayser, Tapeswitch, Banner Engineering, Pinnacle Systems, Pilz, and Rockwell Automation, reduce the risk of an obsolete mat leaving a guarded zone unprotected after a few years of service.