Safeguarding devices: light curtains, two-hand controls, mats and scanners

Guards keep people out. Safeguarding devices work differently: they detect a person approaching a hazard, or keep the person's hands occupied away from it, and make the machine stop or prevent it from starting. They are used where a guard would get in the way of the work, for example where parts are loaded by hand at the point of operation every few seconds. Because they rely on the control system and on the machine stopping in time, they must be selected, positioned and maintained with great care.

ANSI B11.19 sets performance requirements for safeguarding devices and gives the safety distance formula used for most machines. OSHA 1910.217 sets specific rules, including a safety distance formula, for presence-sensing devices and two-hand controls on mechanical power presses. Devices are typically built to IEC 61496 (electro-sensitive equipment), ISO 13851 (two-hand controls) and ISO 13856 (pressure-sensitive devices).

Types of safeguarding devices

Light curtains and the safety distance

A light curtain is one of the most common safeguarding devices. It consists of a transmitter and a receiver with an array of infrared beams between them. When an object larger than the curtain's resolution, also called detection capability or object sensitivity, interrupts the beams, the curtain sends a stop signal through the safety control system to the machine. Finger-detection curtains have resolutions of about 14 mm (about 0.55 in); hand-detection curtains about 30 mm (about 1.2 in); and body-detection systems use a few widely spaced beams.

A light curtain does not protect anyone by itself. It protects only if the machine stops before a hand that breaks the curtain can reach the hazard. That means the curtain must be placed far enough from the hazard. The safety distance depends on how fast a person moves, how long the whole system takes to stop and how far a hand can reach into the curtain before it is detected.

ANSI B11.19 gives the general formula, and OSHA 1910.217 uses a simpler version for presses:

Here Ds is the minimum distance in inches, K is the hand speed constant of 63 in/s, Ts is the stopping time of the machine, Tc the response time of the control system, Tr the response time of the device, Tbm an allowance for brake monitor wear on presses, and Dpf the depth penetration factor, the distance a hand can reach into the detection field before being detected. For light curtains, ANSI B11.19 gives Dpf from the object sensitivity S as roughly Dpf = 3.4(S − 0.276) inches for small sensitivities, and the manufacturer's data usually states it. OSHA 1910.217(c)(3)(iii)(e) for presses uses the simpler formula Ds = 63 in/s × Ts, where Ts is the stopping time measured at about 90 degrees of crankshaft rotation.

Other conditions for light curtains

A light curtain is only suitable if the machine can be stopped at any point in its cycle. Full-revolution clutch mechanical presses, which cannot be stopped once a stroke starts, cannot be safeguarded with light curtains. The curtain must also be installed so that no one can reach over, under or around it, or stand between it and the hazard undetected. Additional guards often fill the gaps, and a second horizontal curtain or a scanner may be needed to detect someone standing inside a large opening. Reflective surfaces near the curtain can bounce beams around an object, so installers follow the manufacturer's alignment and spacing rules.

Some applications use muting, which temporarily disables the curtain while material passes through, for example a pallet on a conveyor. Muting must be carefully designed so that it cannot be triggered by a person, using sensors that distinguish the shape and timing of the load. Blanking, which ignores certain beams so a fixed object or the workpiece can pass, increases the effective resolution and therefore the safety distance. Both are legitimate engineering techniques, but improper muting or blanking is a common way that light curtains are defeated.

Two-hand controls

A two-hand control requires the operator to press two buttons at the same time, within about half a second of each other, and to hold them until the hazardous part of the cycle is complete. Releasing either button stops the motion. The buttons must be far enough apart and shielded so that they cannot be operated with one hand, an elbow or a forearm, and they must be placed far enough from the hazard that the operator cannot release a button and reach the hazard before it stops.

OSHA 1910.217(c)(3)(vii)(c) gives this formula for two-hand controls on presses with part-revolution clutches, in inches, where Ts is the stopping time.

A two-hand control protects only the person operating it. If a helper stands at the back of the machine, or a second operator works the same press, each needs their own protection, often separate two-hand controls wired so that all must be pressed, or guards for the other sides. Two-hand controls are also easily defeated by taping or tying down one button, which is why anti-tie-down logic, requiring both buttons to be released and pressed again for each cycle, is required.

Pressure-sensitive mats and edges

Pressure-sensitive mats detect a person standing in an area, for example the floor in front of a machine or inside a cell, and stop the machine. They respond more slowly than a light curtain, and because a person can step quickly across them, they must cover a large enough area at the right distance. ISO 13855 uses a body approach speed of 1,600 mm/s and an allowance of 1,200 mm for mats. Pressure-sensitive edges and bumpers detect contact with closing doors, tables and mobile machines and reverse or stop the motion.

Safety laser scanners

A safety laser scanner rotates a laser beam and measures distance, building a map of a protective field around a machine or vehicle. When a person enters the protective field, the machine stops; a larger warning field can slow it first. Scanners are popular on robot cells and automated guided vehicles because zones can be programmed and switched for different modes. That flexibility is also a risk: zones must be configured by competent people, documented, protected by passwords and verified with test pieces after any change.

Common limits of all devices

Safeguarding devices share some limits that guards do not have. They do not stop parts, tooling fragments or chips from flying out, so a guard or shield may still be needed. They depend on the machine's stopping performance, which can deteriorate as brakes and valves wear. They depend on the control system being reliable enough for the risk. And they can be bypassed, muted, blanked or moved by people who find them inconvenient. Every device installation needs a verified safety distance, a reliable control system, regular functional testing, and supervision to make sure it stays effective.