Guidelines for Specifying a Safety Light Screen
These allow quick and frequent access, provide protection for multiple operators, and allow better visibility of the hazard.
- By Mike Carlson
- May 01, 2010
From power presses to automated assembly machinery to robotic work cells, many applications require personnel to be in close contact with a machine hazard in order to perform a variety of tasks, such as hand-feeding work pieces or verifying a process. For these and other applications that require frequent operator access, safety light screens can be an effective choice to guard against machine hazards.
Safety light screens or curtains consist of an emitter and receiver pair. An infrared LED array emits invisible light beams to a phototransistor array, which detects the light beams -- creating an optical sensing field commonly referred to as the defined area or the protective field. Light pulses from the emitter are modulated, or pulsed, at a specified frequency, and the receiver's phototransistors are set to only detect that specific pulse, ignoring signals from external light sources. In this way, ambient light from the factory floor will not affect the performance of the safety light screen.
When all of the phototransistors detect light from their corresponding LEDs, the light screen is "clear" and the safety outputs allow the machine to continue operating. When an opaque object, such as a hand or arm, interrupts some part of the defined area, the light screen detects the presence and sends a protective (safety) stop signal to the safety-related controls of the machine. The machine immediately reacts, stopping the hazardous machine motion or removing the hazardous situation before the body part reaches the hazard.
The safety light screen used in high-risk situations interfaces to the machine control through two safety outputs -- individually called an Output Signal Switching Device (OSSD) -- that initiate the protective (stop) signal. How the OSSDs are interfaced with the machine control depends on the machine, the hazard, and the amount of risk reduction required. Typically, OSSDs control redundant intermediate devices --known as Final Switch Devices (FSDs) -- can switch high current, isolate DC circuits from AC circuits, and increase the number of protective (safety) stop circuits or safe switching points.
These stop circuits are most often normally open contacts wired in series from redundant, mechanically linked relays (i.e., the FSDs). The mechanical linkage between the contacts allows the FSDs to be monitored for certain failures. If a failure occurs, such as a welded contact, a monitoring function will turn OFF or keep the OSSDs OFF, thus preventing the machine from operating until the failure is repaired. This monitoring function is often a function of the safety light screen called External Device Monitoring (EDM). Safety outputs (OSSDs) should not be interfaced to any intermediate device, such as a standard PLC, that could fail and result in the loss of the protective (safety) stop command.
Advantages over Alternative Guarding Methods
Safety light screens offer greater flexibility than many guarding methods, such as hard guards, interlocking gates, and pull back/restraint devices. For example, compared to hard guards -- screens, bars, or other mechanical barriers affixed to the frame of the machine and intended to prevent entry into the hazardous area(s) -- safety light screens generally cost less for installation and maintenance when a large area needs to be safeguarded. They also allow quick and frequent access, provide protection for multiple operators, and allow better visibility of the hazard.
Interlocking gates or guards consist of a physical barrier interfaced with a machine control system so as to restrict access or prevent inadvertent access to the hazard. An interlocked gate or guard should be used in certain situations, such as when fluid, parts, or other material can be ejected from the machine, creating a hazard. However, interlocking gates or guards generally are not the best choice when frequent access is required. They can reduce the efficiency of the machine due to the time required to open and close the gate or guard to load/unload a part. They can also cause ergonomic issues if the gate or guard is large or heavy and can result in increased maintenance to ensure proper operation of the interlocking means.
Pull backs and restraint devices are safeguarding devices that attach to an operator's hands or wrists and either withdraw the operator's hands from the hazard area during hazardous operation or prevent reaching into the hazardous area. These devices are generally maintenance intensive and must be adjusted for each operator. Another drawback is that they provide protection for only one operator, versus a safety light screen that protects all individuals.
Resolution (Detection Capability)
Along with application flexibility, safety light screens offer versatile capabilities, such as a range of resolution options and functions that should be selected according to an application's requirements. In situations where a hazard needs to be stopped or removed, light screens must be installed between the machine hazard and all personnel. To provide effective safety, a minimum separation distance -- the calculated distance between the light screen's sensing field and the hazard -- must be determined that results in a greater time to access the hazard than stopping time. The separation distance or safety distance is dependent on the hand/body speed, machine and light screen response time, and resolution.
The resolution, or detection capability, is the ability of the safety light screen to reliably detect an object anywhere within the sensing field (defined area). The resolution is determined by a light screen's beam diameter (multiplied by two) plus the spacing between adjacent beams. This ensures that at least one beam will be interrupted by an object with a cross-section greater than that value. This value is used to determine an "adder" to the distance determined by the "time" it takes to stop the hazard, which is how fast the individual is moving and how fast the machine can react to a stop signal. The separation distance can be thought of as "the point at which an individual is reliably detected and how long it takes to stop." A separation distance that is too short sets up a hazardous situation in which the hazard can be accessed, and a separation distance that is too long can cause ergonomic issues and unnecessarily use up floor space.
Resolution is an important specification when choosing a safety light screen. While high-resolution (e.g., 14 mm) devices are capable of detecting fingers, they are more expensive. With medium- (e.g., 30 to 62 mm) or low-resolution (e.g., more than 62 mm) devices -- they detect only a hand, arm, or torso -- the cost goes down, but the separation distance increases because of the ability to reach toward the hazard before being reliably detected. A properly installed safety light screen protects all body parts equally well. The trade-off is cost versus the amount of floor space (separation distance) that will be required.
Another consideration concerning the choice of resolution is the sensing field orientation. High- and medium-resolution safety light screens can be positioned in any orientation but are often oriented vertically to detect fingers or hands approaching a hazard. For instance, a machine operator may be required to hand-feed and remove stock from stamping, punching, shearing, or assembly operations. In these applications, a safety light screen placed close to the point of operation reduces the distance the operator must move in and out of an area, improving ergonomics. Medium-resolution safety light screens are commonly positioned horizontally to detect an ankle or leg in an area guarding application. Low-resolution safety light screens normally are positioned vertically to detect the torso for applications such as perimeter or access guarding, in which large safety distances are not a concern but cost must be kept at a minimum.
Types of Safety Light Screens
The international standard for design and testing of safety light screens (curtains), IEC 61496-1/-2, groups these machine safeguarding devices into two "types" based on their safety performance and design criteria. Type 2 light screens offer a cost-effective option for applications that present a lower risk of injury but still require safeguarding, such as small assembly equipment. They can also be used as supplemental safeguarding to prevent the reset of a primary safeguard. Type 2 light screens may not offer redundancy -- defined as the duplication of components or circuitry that will provide the same function should a component or circuit fail -- and use fault exclusion, or FMEA, as the main means of ensuring the integrity of the safety function. Type 2 light screens also have a +/- 5-degree Effective Aperture Angle (EAA), or field of view. This larger EEA (than a Type 4 device) facilitates easier emitter and receiver alignment but also makes them more susceptible to optical short circuits. Due to this, Type 2 light screens have to be placed farther away from reflective surfaces.
For higher-risk applications that could result in severe injuries or fatalities, Type 4 light screens should be used. Type 4 light screens achieve high levels of fault tolerance through redundancy and monitoring and meet OSHA regulations and ANSI standards for control reliability. To determine whether a Type 2 or Type 4 light screen is needed for an application, conduct a risk assessment. Risk assessments help to determine the frequency of exposure, probability of injury, and severity of the potential injury. When results from an assessment determine high risk or are uncertain, a Type 4 light screen must be utilized. Operators should refer to standards such as ANSI B11, ANSI/RIA R15.06, or ISO 14121 (EN 1050) before starting a risk assessment.
Optional functions may be selected to meet the requirements of specific applications. For instance, there are typically at least two output mode options available. In "trip mode" (or automatic reset), the safety light screen automatically turns ON the OSSDs upon removal of the interruption from the sensing field. Trip mode can be selected for applications where the individual is continually sensed by the defined area (protective field). For applications where the operator can pass through the sensing field into the guarded area and no longer be detected, "latch mode" (or manual reset), which requires the safety light screen to be manually reset upon removal of an interruption to turn ON the OSSDs, should be selected.
As mentioned earlier, a function called External Device Monitoring (EDM) can monitor the devices that are under control of the safety light screen. The purpose is to prevent a failure from causing the loss of the safety function that would put individuals who believe they are safe at risk of injury. This function assists in complying with control reliability (OSHA & ANSI) or categories and performance level (ISO & EN) requirements. The EDM function can be accomplished by several methods. It can be a single channel or dual channel input, detect a slowing of devices' response times, provide diagnostic information, and usually can be enabled or disabled, to provide flexibility.
For applications that require material, such as long sheet stock, to be present within the sensing field but not issue a stop command, a blanking function, such as reduced resolution, may be beneficial. With reduced resolution, similar to floating blanking, a high- or medium-resolution safety light screen may be configured to ignore objects of a predetermined size so these objects can interrupt a set number of beams without generating a stop signal. However, when enabled, the sensing field must be placed at a greater separation distance because of the increased detection capability. Another type of blanking, "fixed blanking," features a fixed area or zone of the sensing field that allows material to pass. If the object is removed or moved for a different machine setup, the safety light screen will detect this event and prevent machine operation until the blanked area is reconfigured. Fixed blanking is helpful for machines that require a fixed object -- such as a feed table, tooling, or a conveyor -- to be continuously present within the light screen. The "hole" created by fixed blanking must be completely filled, or again the sensing field must be placed at a greater separation distance to ensure an individual cannot reach through and access the hazard.
Commonly confused with blanking, muting is a function that allows the temporary automatic suspension of the safety light screen function to permit materials or individuals to pass through the sensing field without causing a stop signal. The differences are that muting affects the entire sensing field (not just a few beams), occurs only during the non-hazardous portion of the machine cycle. and does not affect the separation distance. Muting is often used for manual unload and load operation during the safe portion of the machine cycle, as well as automatic material flow processes into and out of robotic work cells.
Safety light screens can provide an ideal safety solution for applications where the hazardous machine motion can be stopped relatively quickly -- anywhere in the machine cycle before the operator can reach the hazard. A variety of questions must be considered prior to selecting and installing a safety light screen. For instance, what amount or frequency does the operator need access to the process? Can a safety light screen be used to improve efficiency and safety? Will it be able to reliably detect intrusion of personnel into the hazardous area and prevent access to the hazard by reaching over, under, around, or through the sensing field? What resolution is required? What functions should the safety light screen provide?
The answers to these questions and others can determine whether a safety light screen is appropriate, as well as what characteristics and features should be part of the safeguarding solution.