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A Comprehensive Guide to Light Curtains: Safety Applications and Emerging Trends

POSTED 01/23/2024  | By: John Lewis, A3 Contributing Editor, Tech B2B Marketing

The future use of light curtains will be significantly influenced by emerging trends. These include integration into interconnected systems for improved efficiency and data provision, advanced sensing for precise object detection, enhanced connectivity for seamless safety system integration, and intelligent features to boost system reliability and reduce downtime.

Light curtains, also known as safety light curtains or optical safety devices, are crucial components of industrial safety measures. They are designed to detect the presence of objects or individuals within a defined area, thereby preventing access to hazardous areas or stopping machine motion when a person or object enters a protected zone.

Key Components of a Light Curtain System

According to Matt Mohr, safety product manager at Omron Automation Americas, a light curtain system typically includes the following components:

Transmitter – The transmitter emits an array of infrared light beams across the protected area. An infrared light source, such as an LED or laser diode, generates the light beams.

Receiver – The receiver is positioned opposite the transmitter and contains light-sensitive elements, such as photodiodes or phototransistors, that detect the presence or interruption of the light beams.

Control unit – The control unit is responsible for processing signals from the receiver and determining if any objects or individuals have entered the protected area. It typically includes a microprocessor, safety relays, and other circuitry to analyze the received signals.

Mounting brackets – Mounting brackets are used to securely install the light curtain system in the desired location. They provide stability and ensure proper alignment between the transmitter and receiver.

Interconnection cables – These cables connect the transmitter, receiver, and control unit, allowing for the transmission of signals and power between the components.

Alignment aids – Alignment aids, such as alignment indicators or laser alignment tools, may be included to assist in aligning the transmitter and receiver during installation.

Integration interfaces – Light curtain systems often include integration interfaces, such as safety relays or safety controllers, to interface with the machine’s control system and to enable safe machine stoppage or control based on a detected presence or interruption of the light beams.

Derek Charge, global product manager for safety, sensing, and industrial components at Rockwell Automation, explains, “A key component of a safety light curtain is the output signal switching devices (OSSDs), which are typically solid-state transistor components used to switch a safety relay to control the machinery causing the dangerous motion. However, the most obvious components in a safety light curtain system are two devices commonly called sticks, one operating as a transmitter, transmitting invisible beams of infrared light, and the other operating as a receiver, which has photodetector components to receive the transmitted beams.”

Figure 1: Light curtains create an invisible barrier of light beams. When something or someone interrupts one or more of the beams, a safety response is triggered. (Image courtesy of Omron Automation Americas.)All these components work together to create an invisible protective barrier of light beams. When an object or person interrupts one or more of the light beams, the control unit detects the interruption and triggers a safety response. Also known as AOPDs (active opto-electronic protective devices) or ESPE (electro sensitive protective equipment), light curtains offer optimal safety, yet they allow for greater productivity and are the more ergonomically sound solution when compared to mechanical guards.

The Role of Light Curtains in Safety

During operation, the transmitter in a safety light curtain emits a series of infrared light beams across a protected area. The receiver, positioned opposite the transmitter, detects the presence or interruption of the light beams. When an object or person enters the protected area and interrupts one or more of the light beams, the receiver detects the interruption and sends a signal to the control unit.

The control unit processes the received signals and determines if any objects or individuals have entered the protected area. If an interruption is detected, the control unit triggers a safety response to prevent potential harm, such as stopping machine motion or activating an alarm. The response is typically based on predefined safety parameters and can be customized to suit specific application requirements.

“Light curtains are effective safety measures due to their non-contact detection, high resolution, fast response time, flexibility, integration capabilities, and reliability,” Mohr explains. “Non-contact detection allows for easy access to the protected area while maintaining safety. With high resolution, they offer precise detection and reduce the risk of false triggers.”

Fast response time ensures quick detection and immediate light curtain safety response. Light curtain sensors can be customized and adjusted to suit specific application requirements, making them versatile. They can also be integrated with other safety devices, enhancing overall safety and coordination. Light curtain sensors are designed to withstand harsh industrial environments, ensuring their reliability and durability.

“Safety light curtains are primarily a Category 4, SIL 3, PLe safety component, used where a door or other barrier is not optimal for the application’s access requirements, but they also fall within the stop-time capabilities of the hazard they are protecting,” says Steve Lange, a technical application engineer at Schmersal Canada.

“Light curtain technology is a very efficient safety measure, as the electronics are designed to be self-checking (for errors and faults) and even measuring the outputs for issues such as shorts,” explains Rod Karch, a safety application engineer at Leuze Electronic, Inc. “This well-tried and proven technology meets many industrial and safety standards, including failing in a safe state when any malfunction occurs and continuously monitoring every possible fault.”

According to Dawn Etta, a senior application engineer at Schmersal USA, a defined field of arrays can monitor the opening for a walkway, a point of operation, or an area around a machine. The protection field is defined by its height and width. The protected height is the range between the first and last infrared light beams of a light curtain. The protected width, or operating range, is the distance between the emitter and receiver units. 

“For the detection of body parts, a distinction is made between finger, hand, and body protection,” Etta explains. “EN ISO 13855 sets the biometric data for finger protection to 14 mm, for hand detection to 30 mm, for leg detection up to 70 mm, and for body detection to over 70 mm. Safety light grids are generally used to detect the penetration of the entire human body.”

The Benefits of Light Curtains in Safety

Light curtains offer numerous advantages over traditional safety measures, such as physical barriers or guards. They provide enhanced accessibility by allowing easy access to the protected area without obstructing visibility, enabling efficient material handling and maintenance tasks. Light curtains are highly flexible, as they can be easily adjusted and customized to suit specific application requirements, making them suitable for a wide range of industries, such as automotive, intralogistics, machine tool, lab automation, and packaging.

“Light curtains offer improved visibility and ergonomics by not obstructing the view of the machine or process, enhancing operator comfort and safety,” says Mohr. “With their fast response times, light curtains ensure immediate detection and rapid safety response, minimizing accidents and reducing machine downtime. They are cost-effective, eliminating the need for physical barriers or gates and reducing installation and maintenance costs.”

Light curtains can be seamlessly integrated with other safety devices and control systems, enhancing overall light curtain safety functionality. Industries such as automotive manufacturing, packaging and material handling, CNC machining, and pharmaceutical and food processing benefit greatly from light curtains due to their flexibility, accessibility, and quick response times. These benefits enable efficient and safe operations while maintaining productivity in these industries.

“A great example would be a machine press for bending or cutting sheets of metal,” says Karch. “Many of these presses apply tens or even hundreds of tons of force when cycling, and this type of application potentially exposes an operator to risk redundantly as the press is cycled.”

One traditional safeguard for this type of machine is hard guarding, which helps keep an operator’s hands, arms, and body away from the press. Another option is two-hand operation, designed to keep the press from cycling without two hands on push buttons, ensuring that the operator has both arms away from the press.

“Both safeguards have merit and are used today,” Karch continues, “but there are drawbacks. Hard guarding does not always allow easy access to the area of operation. It limits movement, which may have an impact on production, including changing from one style product to another. Two-hand control is a great safeguard, but this control may still rely on the operator ensuring that the press is clear before initiating the cycle.” 

A safety light curtain will not allow a machine to cycle if, for instance, an operator has an arm in the field protecting the hazardous area. Additionally, light curtains have very fast response times — fast enough to be used very close to the hazard and still stop a machine cycle if an operator breaks the protective barrier plane.

In addition to their speed, light curtains offer an easy way to enter hazard zones, according to Lange. “For example, instead of stepping off a forklift to open a door, you can drive through the curtain, or you can just walk through a light curtain if your hands are full. It is beneficial to use light curtains in robotics cells, case packers, or anywhere there are large items coming in and out regularly.”

Hazards and Risks That Light Curtains Protect Against

Light curtains are designed to protect against various hazards and risks in industrial environments. Specific hazards they protect against include:

  • Contact with moving parts – Light curtains can detect the presence of a person or object in the hazardous zone and trigger a safety response, such as stopping the machine or activating an alarm to prevent contact with moving parts.
  • Pinch points and crush hazards – Light curtains can be positioned to detect the presence of body parts or objects in pinch points or areas where crushing hazards exist. They can initiate safety measures to prevent accidents and injuries.
  • Access to restricted areas – Light curtains can be used to control access to restricted areas, ensuring that only authorized personnel can enter. They can detect unauthorized entry and trigger appropriate safety responses.
  • Machine start-up during maintenance – Light curtains can be used to ensure that machines do not start up unexpectedly during maintenance or repair work. They can detect the presence of personnel and prevent accidental machine activation.

Mohr cites numerous instances when light curtains have prevented accidents and injuries:

  • A light curtain in a manufacturing facility detected the presence of an operator’s hand near a press machine and immediately stopped the machine, preventing a potential crushing injury.
  • A light curtain detected a person entering a restricted area in a packaging line and activated an alarm, alerting the operator to the unauthorized access and preventing a potential safety breach.
  • In a CNC machining center, a light curtain detected the presence of a tool during a tool change operation and prevented the machine from starting, avoiding a potential collision and damage to the tool or machine.

Categories of Light Curtains

Karch explains that Leuze Electronic, Inc. separates light curtains into three categories: point-of-operation guarding, area guarding, and access guarding.

  • Point-of-operation guarding – Point-of-operation guarding is specific for finger/hand protection close to the hazard. The light curtain is typically mounted vertically for this type of guarding. Hazards might include cutting knives, rollers, metal stamping heads, or conveyor rollers or chains.
  • Area guarding – Area guarding is typically designed for foot/ankle/leg protection in a designated area, to ensure that no one enters or is present there. “Either [it] would shut down the machine or not allow restart,” Karch says. “An example would be if an operator needs to interact with the process, to load and unload parts, such as with the automotive welding of car parts.”
  • Access guarding – The third type of protection, access guarding, involves body protection. Light curtains that provide access guarding are designed to replace hard guarding, such as short fences. These light curtains keep individuals out of specific areas that may pose a hazard, such as the area around an automated stamping machine that does not require normal operator interaction. 

Charge says that light curtains are “used on presses, forming and sawing operations, robot cells, conveyors, palletizers, and other work cells, where they protect against amputation, crush injury, cuts, and death.” Lange agrees, noting, “Safety light curtains protect against hazards that need to be stopped very quickly and that have a high frequency of traffic. Great examples of where light curtains have prevented accidents are presses, because the operator is very intimate with the machinery and can get complacent around this large machinery, which can easily end their work career or life.”

Varieties of Light Curtains

There are several types of light curtains available in the market, each with its own functionality and application. According to Mohr, the main types of light curtains are:

  • Basic light curtains  These light curtains have transmitter and receiver units that emit and detect infrared beams. They are typically used for simple presence detection and can be used in various applications where non-contact detection is required.
  • Muting light curtains – Muting light curtains are designed for applications where materials or objects need to pass through the protected area without interrupting the production process. They have additional sensors, or muting arms, that temporarily disable the safety function when certain conditions are met, such as when a pallet or object is being transported.
  • Hand protection light curtains – Hand protection light curtains are specifically designed to protect operators’ hands from getting too close to hazardous areas, such as machine openings or pinch points. They have higher resolution and sensitivity to detect small objects, such as fingers, and to trigger a safety response to prevent injuries.
  • Area-scanning light curtains – Area-scanning light curtains use advanced technology to create a 3D safety zone. They can detect the presence of objects or people in a defined area, even one comprised of complex shapes or irregular surfaces. These light curtains are suitable for applications where a more precise and flexible detection zone is required.
  • Safety light grids – Safety light grids are similar to light curtains but consist of multiple beams arranged in a grid pattern. They are used for perimeter guarding or access control applications, where a larger area needs to be protected.

Karch explains, “Leuze Electronic offers two main types of light curtains: standard, which have switching outputs or measurement output data for non-safety-related applications, and then safety light curtains.”

A safety light curtain has an array of optical transmitter and receiver elements, creating an invisible two-dimensional barrier. The barrier is designed to identify objects of a certain size, a process referred to as safety resolution. Safety light curtains come in many different safety resolutions and lengths, based on what is needed for the application.

“With respect to safety light curtains, you have only a handful of types: basic, cascading, advanced, and application specific,” Karch says. “All have a safety function: turn off if something breaks the protective field or if a malfunction occurs.” 

Functionality and Features of Light Curtains

Light curtains offer a wide range of different functionalities. According to Karch, “Basic functionality is an on/off light curtain. These light curtains would be the bulk of what is sold in the world. Cascading light curtains are still a basic unit but consist of several light curtains in an array, which may be used for an L shape, consisting of one vertical light curtain and one horizontal light curtain. Cascading is primarily used for the approach to a hazard (vertical) and the potential of standing behind the light curtain (horizontal). The advanced light curtain provides functions such as floating and fixed blanking, muting, and other options. ... Application-specific products might include cold applications, ATEX rated, IP69K, shock-resistant, process gating, and more.”

Schmersal offers several series of safety light curtains. They feature a compact (28 x 33 mm) rectangular extruded housing for added durability and LED-illuminated end caps for signaling.

“We have a unique bluetooth interface that provides secure data transmission up to five meters to smartphones and tablets,” Etta explains. “The operational data displayed in the safety light curtain assist app can help with alignment during installation, troubleshooting faults, and planning service and maintenance. Our light curtains are available with functions such as start/restart interlock, fixed or floating blanking, contactor control, double reset/acknowledgment, beam coding, muting, cyclic operation, and multi-scan.”

Selection and configuration of the various integrated functions are made via a simple push button and do not require costly setup devices or separate software. “We also have a full set of accessories, such as vibrational dampeners, deflection mirrors, mounting posts, and protective housings, including an IP69-rated housing,” Etta says.

Certain special functions allow safety light curtains to be used in complex applications. For example, with the muting function, the infrared beams of a light curtain can be interrupted by material automatically entering or exiting a hazardous area without the machines inside this area being shut off. This is accomplished by the safety light curtain monitoring a set of muting sensors, which must be interrupted in specific sequence by the moving material, allowing the temporary muting of the monitored infrared array. The muting sensors are installed such that a human cannot physically interrupt them in the correct pattern to initiate a mute. Thus, every time someone interrupts the beams of the safety light curtain, a stop function is initiated for the hazardous area.

“Safety light curtains typically fall into either the point-of-operation control (POC) or perimeter access control (PAC) type,” says Charge. “Safety light curtains have a range of functionalities. They can be as basic as on/off.  They also have more advanced features. One of these is blanking. This allows a single beam to several beams across the span of the light curtain to be set to allow or ignore interruptions. This is useful for applications where there are material in or out feeds. It’s also useful where there is a fixed piece of machinery, such as a conveyor, permanently blocking beams.”

Figure 2: Point-of-operation control (POC) light curtains have beam spacing, or resolution, capable of detecting a finger (14 mm) or a hand (30 mm). These types of curtains are placed close to the hazard, where personnel frequently interact with the machine as part of a process — for example, feeding parts or material into a machine. (Image courtesy of Rockwell Automation.)

Another feature is muting. This is the temporary automatic suspension of the protective function of the safety light curtain. Cascading is another advanced feature. This is the ability to interconnect pairs of safety light curtains. It reduces overall system wiring and simplifies the safety circuit. According to Charge, this results in the lower overall cost of a multiple safety light curtain system.

Figure 3: The beam spacing (resolution) of a perimeter access control (PAC) light curtain allows for the detection of a whole body. These curtains can be used to create a safety perimeter around robot cells and machines that don’t require frequent interaction during normal operation, such as conveyor and pelletizing systems. (Image courtesy of Rockwell Automation.)Such product configurability allows a safety light curtain system to protect multiple sides of a machine. It also adds flexibility of positioning for various applications. Additionally, some systems are stand-alone while others have more advanced communications capability, allowing them to be added into the enterprise network.

The choice of light curtain depends on the specific application requirements, such as the size and shape of the protected area, the level of sensitivity needed, and the presence of any specific hazards. It is important to select the appropriate type of light curtain with the right features and functionality to ensure effective safety measures in different industrial environments.

 

Figure 4: Light curtains offer different degrees of protection. In terms performance, ESPE can be either Type 2, Type 3, or Type 4; Type 4 offers the highest level of performance with respect to fault tolerance. In terms of safety performance, or integrity, there are two standards: (EN) ISO 13849-1 and IEC/EN 62061, which provide comparable levels of protection. PLe and SIL 3 are the highest levels. (Image courtesy of Rockwell Automation.)

Key Factors to Consider When Choosing a Light Curtain

When choosing a light curtain for a specific application, several factors should be considered. Mohr provides a step-by-step guide to help with selecting the appropriate light curtain:

  • Identify the hazards – Determine the specific hazards and risks in the application. A thorough risk assessment considers factors such as moving parts, pinch points, restricted areas, and any other potential safety concerns.
  • Assess the application requirements – Consider the size and shape of the protected area, the distance between the light curtain and the hazard, and the level of sensitivity needed for detection. Also determine if any special features, such as muting or hand protection, are required.
  • Determine the resolution – Evaluate the level of precision required for detection. If you need to detect small objects or body parts, a light curtain with higher resolution and more beams per inch may be necessary.
  • Consider the response time – Determine how quickly the light curtain needs to detect and respond to potential hazards. Fast response times, typically in milliseconds, are crucial for minimizing the risk of accidents.
  • Evaluate integration capabilities – Determine if the light curtain needs to be integrated with other safety devices or the machine’s control system. Consider compatibility with safety relays, safety controllers, or other safety components.
  • Assess environmental conditions – Consider environmental factors, such as temperature, humidity, vibrations, or exposure to dust and moisture. Choose a light curtain that is designed to withstand the specific conditions of your application.
  • Consult safety standards and regulations – Ensure that the selected light curtain complies with relevant safety standards and regulations in your industry or region.
  • Seek expert advice – If you are unsure about the appropriate light curtain for your application, consult with safety experts or manufacturers who can provide guidance based on their expertise and experience.

Light Curtain Applications

Light curtains are used in industries and environments where safety and hazard prevention are crucial. According to Mohr, common applications for light curtains include:

  • Machine guarding – Light curtains are widely used for machine guarding to protect operators from hazardous areas, such as presses, robotic cells, or assembly lines. They trigger an alarm or ensure that the machine stops if a person or object enters the protected zone.
  • Material handling – Light curtains are employed in material handling applications, such as conveyor systems or automated warehouses, to detect the presence of objects or personnel in restricted areas. They help prevent collisions, jams, and accidents during material transfer.
  • Packaging and sorting – Light curtains are utilized in packaging and sorting applications to detect the presence or absence of products, control the flow of items, and ensure proper positioning or alignment of packages.
  • Access control – Light curtains are used for access control in restricted areas, such as clean rooms, laboratories, and hazardous zones. They allow authorized personnel to enter while preventing unauthorized access.
  • Robotic safety – Light curtains play a crucial role in ensuring the safety of collaborative robots (cobots) by creating a protective zone around them. They detect the presence of humans and trigger safety measures to prevent collisions or injuries.

In terms of unique or innovative applications, there are several instances where light curtains have been creatively employed, according to Mohr. For example:

  • Ergonomic safety – Light curtains have been used to create virtual ergonomic boundaries around workstations, helping operators maintain proper posture and avoid repetitive strain injuries.
  • Intrusion detection – Light curtains have been utilized in security systems to detect unauthorized entry or intrusion into restricted areas, triggering alarms or notifying security personnel.
  • Sports and entertainment – Light curtains have been employed in interactive installations and gaming applications, where they detect the movement of players or objects to trigger specific actions or responses.

These are just a few examples of how light curtains can be applied in unique and innovative ways beyond their traditional safety applications. Charge notes that slim-profile systems can be built into machines rather than added onto machines.

“In addition, muting is used extensively in conveyorized processes and palletizing applications,” Charge explains. “Muting allows material to move through the safety light curtain’s sensing field without stopping the machine. However, if a person attempts to pass through the safety light curtain, they will be detected, and the machine will stop. The differentiation between material and personnel is determined by using muting sensors. These muting sensors create a particular switching sequence together with the safety light curtain as the material passes through the process.”

The versatility of light curtains allows for creative solutions to various safety and detection challenges in different industries. One unique Leuze safety light curtain application is in cold storage. Karch explains, “Because the Leuze light curtain (MLC) is rated to minus 30 degrees Celsius, it is one of the few light curtains that can be put directly into a cold storage environment. Additionally, the Leuze MLC-SPG is our smart process gating product. It allows for material transport (pallets, crates, and other material) through the light curtain via conveyance while keeping personnel out. Most light curtains require separate muting sensors to provide this muting function. The SPG does not, which minimizes the length of the conveyor using the MLC-SPG technology ... and provides more reliable conveyance by eliminating nuisance trips.”

More regions and industries across the globe are adopting safety light curtains to benefit from the operating efficiencies they offer while providing the highest safety protection to personnel.

Emerging Trends in the Field of Light Curtains

Emerging trends in the field of light curtains are set to have a profound impact on their future use and will make light curtains more versatile, efficient, and effective in preventing accidents.

“Integration with Industry 4.0 will enable light curtains to become an integral part of interconnected systems, providing real-time data and enhancing operational efficiency,” Mohr predicts. “Advanced sensing technologies will enhance detection capabilities, allowing for more precise and reliable object differentiation. Enhanced connectivity and communication will enable seamless integration into safety and control architectures. The incorporation of intelligent functionality will improve system reliability and reduce downtime through self-monitoring and proactive detection of issues. Additionally, the combination of light curtains with augmented reality will enhance situational awareness and reduce human error. These trends collectively will revolutionize the use of light curtains, making them more versatile, efficient, and effective in ensuring safety and preventing accidents in various industries.”

Karch sees safety communication protocols becoming a standard, meaning that much more device information will be available. “This information will be available over CIP Safety™ (AB), PROFIsafe (Siemens), and TwinSAFE (Beckhoff) PLC proprietary protocols,” Karch explains. “Additionally, open protocols, such as IO-Link Safety and ASi-5, may become available. This would allow the devices to provide information on temperature, dirty/clean status, end of life/maintenance, real-time diagnostics, ease of configuration for blanking, and more.”

Charge continues along this line, noting that most traditional safety devices do not provide data and that extracting data when it is available is time-consuming. “There is a growing trend for smart devices in industry. Safety light curtains are available with CIP Safety over Ethernet/IP™. CIP Safety is an application extension to the Common Industrial Protocol (CIP™), a media-independent communication protocol providing users with a unified communication architecture throughout the industrial enterprise,” explains Charge. “CIP and CIP Safety are built on ‘the black channel’ principle, where it is assumed that the host network is unreliable, and to mitigate this, error detection is built into the protocol of the connected devices, which will go to a known safe state if error is detected.”

As industry evolves and Industry 4.0 technologies become more pervasive, customers increasingly rely on information from intelligent safety components to minimize unplanned downtime and increase efficiencies. Ethernet is the network of choice for obtaining information from intelligent safety devices due to Ethernet’s ease of design, installation, and configuration.

“Customers are increasingly demanding that all intelligent products, including functional safety devices, be Ethernet based, simple to configure, and integrate seamlessly with Logix-based controllers using Add-On Profiles (AOP),” says Charge. “CIP Safety over Ethernet/IP can communicate over existing enterprise-level Ethernet networks and the internet.”

With respect to CIP Safety light curtains, users can see exactly which beams are tripped on a light curtain to better understand the cause of a downtime event. Or they can take a sample of the light screen every 100 milliseconds to track profiles of products running through it and identify potential quality issues. These smart products combine simpler connectivity and powerful data for smarter, higher-performing production.

The availability of this data not only helps keep people and processes safe but also significantly expands productivity. Smart safety devices provide better visibility into operations and help users understand process states, environmental conditions, and other factors that affect safety and productivity.