Light curtains are a convenient and unobtrusive automation-safety component to guard machines and reduce accidents. In many cases, light curtains can complement or even replace chain-link fencing and other hard mechanical guards while imparting additional system functions. In fact, many machine-tending tasks (such as loading raw material into machines and unloading finished goods at the machine output) are only possible with light curtains. The adaptability of these safety components also suits facilities that operate traditional industrial robots near work cells populated by manufacturing personnel — for significantly reduced risk of injury.
Figure 1: When properly installed and periodically tested, correctly operated light curtains are simple and valuable components for protecting workers from harm. The pair of yellow light-curtain posts in this tradeshow demonstration are behind a clear acrylic pane that wouldn’t be present in a real application. (Image source: Design World)
As mentioned in a previous Digi-Key article on light-curtain basics and fundamental parameters, muting is just one of many settings to be specified during the design process. Other interrelated light-curtain parameters include resolution, positioning, stopping distance, EN ISO 13849–1 Type classification, and mirroring for perimeter control.
Light curtain muting setup, operation, and testing
Initial setup and calibration are critical to proper light-curtain operation. To facilitate this installation process, most light-curtain manufacturers today integrate their components with LEDs. Simple red-and-green LEDs work by indicating (by glowing green) when the emitter curtain and the receiver curtain are properly aligned or (by glowing red) need additional adjustment. Installation personnel should ensure that:
- All the LEDs associated with all the light curtain’s active areas are glowing green (indicating that they’re set up for proper operation)
- The distance from the light curtain to the hazardous machinery or robotic axes is within values provided by the light-curtain manufacturer as well as applicable ISO, OSHA, and/or ANSI standards
Installation should be done in two steps. First, the light curtain should be set up while the hazardous machine or robotic design being guarded is completely unpowered — to follow proper lockout/tagout procedures. Then (once independent light-curtain operation is verified) installation personnel should connect the curtain to the work zone’s emergency stop (e-stop) and retest the system with the guarded equipment powered and operating in safe mode.
Engineering SLC4 light curtain” alt=”Specification and Use of Muting Functions on Light Curtains”>Figure 2: The light curtain shown here is engineered to safeguard an access point on a compact production machine’s equipment. It comes in variations to guard heights of 160, 240, and 320 mm across 0.1 to 2 m — and in resolutions of 14 mm (for finger detection) or 24 mm (for hand detection). End-to-end sensing prevents blind zones. (Image source: Banner Engineering)
Of course, light curtains aren’t suitable for all machine-guarding applications. For example, they’re problematic in settings that involve highly reflective surfaces near the active machine area. They can also malfunction in environments subject to very smoky processes — or those where there’s a lot of particulate in the air. In such settings, equipment guarded by light curtains may (if their perimeter is breached) take an excessively long time to come to a safe stop. The issue might also cause equipment to eject material out of the defined active area. That’s why it’s key that design engineers designing safety systems for smoky and dusty environments carefully consider whether light curtains will effectively protect personnel … or if physical machine guards are warranted.
Good practice dictates that light curtains should be given a quick validation test at the start of each shift to ensure proper operation. In addition, more thorough semi-annual inspections should be conducted. These inspections should quantify any deterioration of operation — especially those arising in the form of slow response times or obscurement of the work zone’s active area. The latter is most relevant where there have been changes in the machine set up to accommodate the manufacture of new product lines. Where such changeovers are very dramatic, they can warrant relocation or even replacement of the light curtains around the work zone … and adjustments to the light curtains’ resolution and other settings to ensure sufficient worker and machine protection.
Special situations involving light curtain muting
Very large work zones usually necessitate the ganging of light curtains. These are placed in series to expand the protected area’s perimeter. Such installations usually require that the orientation of the light curtains alternate so that the emitter post of one curtain is back-to-back with the receiver post of the next curtain in the line. This emitter-receiver-emitter-receiver post sequence avoids crosstalk.
Of course, in these arrangements (as with any light-curtain installation) technicians should ensure that the emitter and receiver posts of every light curtain are oriented the same way top to bottom so that the active area is covered as expected and maintains its intended resolution.
Where a guarded area requires light-curtain corners, two options are hinged post assemblies (that have two straight lengths joined by a joint to wrap around corners to 90° or greater) and the use of corner mirrors. With the latter, three sides of a work zone can be guarded with an emitter post, receiver post, and two tall mirrors. In short, the emitter post points toward one corner stationed by a mirror oriented 90° towards a second mirror at the next corner. The emitter’s signals travel from the first mirror to the second and onward to the receiver at the guarded area’s fourth and final corner. This is a simple way to enclose a larger area, though the tradeoff is that each mirror reflection is not 100% efficient … so the signal diminishes with each reflection. Design engineers would do well to check manufacturer specifications for these setups; usually, the signal strength diminishes about 8 to 15% with each reflection.
One last note regarding work zones that are very large: Most have the potential for passthrough situations. These occur when an operator walks through the light-curtain boundary and triggers a machine stop or safe mode … but is still able (if there are the right controls inside the guarded area) to restart the hazardous machine. If that happens, the operator will essentially stand undetected by the light curtain within the hazardous work zone — and vulnerable to injury or worse. A common solution to prevent such situations is to omit or disable start switches and controls on machines and include a hard-reset switch outside the guarded area — typically on an operator panel near the work cell’s HMI. This effectively forces machine operators to exit the hazardous work zone to restart the equipment.
Light curtain muting and the related functions of blanking
Some work zones require periodic suspension of the light-curtain function. Called muting, such suspensions are usually during a safe portion of the machining process that also happens to require manual machine-operator intervention. For example, muting is indispensable when operators must load pallets into a palletizer’s input area. Muting is also commonly used to let operators unload items from a robot cell’s output station.
In short, light-curtain muting is carefully coordinated by a series of presence detectors expecting materials of a certain size and shape. The presence detectors must be triggered in a certain sequence to temporarily inactivate the light curtain portion to be breached by the items. Once they’re through and the task necessitating the muting operation is completed, the light-curtain guarding function is restored.
Figure 3: No matter the application, a light curtain’s muting function is engineered to only allow material to move through the curtain … and not personnel. Timing diagrams can quantify the specifics of a light curtain’s muting function. (Image source: Design World)
A function relating to muting is that of light-curtain blanking. Here, it’s the light curtain itself and not presence detectors that trigger the function. In short, the light curtain’s beams remain active and temporarily allow breaching by objects entering the restricted work zone — without triggering a machine shutdown. Or in a blanking variation called floating blanking, the light curtain’s electronics establish a blanked region plus a modest tolerance band around that blanked area — to allow passthrough of goods without forcing the operator to place items with overly challenging precision.
Light-curtain blanking is commonly used during machine operations that need finished parts to exit the work zone.
Design engineers sizing and specifying light curtains should document all standards applicable to the machine or robotics being guarded — including industry-specific requirements as well as those from regional and occupational governing bodies. Once established, these standards can inform the correct setup and testing routines for a given light-curtain installation … and indicate whether confounding conditions or machinery type preclude the use of light curtains.