Finger and Hand Detection

Safety Light Curtains

Type 4 safety light curtains protect personnel from injury and machines from damage by guarding points of operation, access, areas and perimeters. Banner Engineering offers easy-to-use, heavy-duty Type 4 safety light curtains with several resolution options available to detect fingers, hands, arms, or ankles. Our heavy-duty light curtains are built to withstand harsh industrial environments.

How to Choose

• 360° visibility with illuminated end cap • Advanced diagnostics • 14 mm and 30 mm resolution • Cascading capability • Zone indication and weak beam indicators • End-to-end sensing from 300 to 1800 mm with a 12m range If the following are needed: • Compact • Integrated muting If the following are needed: • Greater defined area, up to 2380 mm • IP69K environmental rating • End-to-end sensing • Full length zone status indication If the following are needed: • Up to 18 m sensing range • Latch/reset

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What Is a Light Curtain and What Does It Do?

A light curtain, or safety light curtain, is a presence-sensing device that protects operators from hazardous equipment. At the most basic level, it is an invisible barrier made of multiple infrared beams sent from one device, an emitter, to a second device, a receiver. If these infrared beams are broken by a person or object, the machine stops. But there’s a lot more to protecting operators than just breaking a beam of invisible light. 

Light curtains typically have two main machine guarding duties: access guarding or perimeter guarding. When a light curtain is used for access guarding, it protects specific access points to a hazardous area. This area can be large or small, depending both on the area that needs to be safeguarded as well as what may trigger the light curtain. When used for perimeter guarding, larger light curtains are set around a machine or other piece of equipment and prevent access to the surrounding area. This takes up more floor space, but allows greater flexibility in approaching equipment. 

Applications in which light curtains may be a preferred protection method include: 

  •  Guarding against moving parts on machines 
  •  Avoiding pinching or crushing hazards 
  •  Controlling personnel access to restricted areas 
  •  Managing items on packaging and sorting lines 
  •  Preventing machine startup during maintenance 

Because light curtains offer non-contact machine guarding, they typically last much longer than mechanical devices like door switches, which can wear over time due to repeated locking and unlocking. Also, less floor space is needed because there’s no physical door needing room to swing open. 

Why Is Resolution So Important?

When discussing different levels of protection, such as hand or finger, what we are really talking about is the smallest size of object a light curtain can detect any place in the sensing field. This is called the resolution, which is measured from the outside of one beam to the far side of an adjacent beam. Any object larger than the resolution will not be able to pass through the light curtain without breaking the beams and causing a fault.  

Safety light curtains are available in many different resolutions. Smaller resolutions (14 millimeters to 40 millimeters) are used for machine guarding, in which light curtains protect a specific access point to the machine. Larger resolutions (50 millimeters and greater) are used for perimeter guarding, which use less floorspace than physical guards, such as fences, to protect the area around equipment. Common resolutions include: 

How Light Curtains Operate

No matter which type of safety light curtain you are using, they all essentially function the same. Each curtain requires two devices: an emitter and a receiver. Both devices must be paired with each other so they can communicate. They must also be aligned with each other in order to function (this can be aided using alignment indicators, such as those on Banner’s S4B Series of heavy-duty safety light curtains). The emitter sends an array of infrared light beams to the receiver, which is tuned to recognize only these specific light beams. This minimizes the effects of ambient light.

What Happens After Light Curtain Beams Are Broken?

Once the light curtain has been tripped, the safety interlock needs to be reset so the equipment can begin operating again. This may be manual or automatic, depending on the application, the light curtain capabilities, and the design of the machine guarding system being used.

In an automatic reset, the light curtain determines that the beams are no longer blocked, then turns its two output signal switching device (OSSD) outputs ON. This is an efficient way of restarting equipment, though it should only be used when a person cannot be inside the guarded area, such as finger or hand guarding access points to small machines. For situations in which a person could potentially be inside the protected area when a hazard is restarted, a manual reset is typically used. This requires an operator to press a button located outside the protected area, keeping the operator out of harm’s way as the equipment is powered back up. When pressed, the button sends a signal to the relay or controller, which tells the light curtains to turn the OSSD outputs ON and again let power flow to the machine but does not start the hazardous motion. The normal machine start process is required to restart the hazardous motion after the safety circuit is turned back on.

Special Light Curtain Functions

Sometimes light curtains need to allow objects to pass through without shutting off equipment and interrupting workflow, such as preventing operator access to a conveyor while still allowing pallets through, or maintaining operation when a fixed object would otherwise trip the system and halt production. These optional functions are referred to as muting and blanking. Other applications may also benefit from cascading, connecting light curtains in a chain.

Light Curtain Muting

Muting lets a safety system temporarily ignore the breaking of a light curtain when objects or materials need to pass through a protected area without halting the production process. This is often used to allow objects through a light curtain, such as goods on a conveyor moving from one area to another.

To accomplish this, a pair of muting sensors are mounted so they detect objects as they come to the light curtain. If the sensors detect an object, the light curtain is momentarily disabled as the object passes through. After the object has passed through the light curtain, sensors detect the object again and re-enable the curtain. However, a person reaching through the light curtain will trigger the machine to stop.

Cascading Light Curtains

Certain applications may be well-served by using cascading light curtain technology, connecting pairs of light curtains in a series with other light curtain pairs to reduce wiring and connections to a safety controller. Typical uses of cascaded light curtains are for creating U-shaped sets of light curtains placed around a piece of equipment for three-sided operator protection, L-shaped pairs that protect both the entrance to a machine as well as the approach area, and perimeter guarding of hazardous areas.

Each successive light curtain added to a cascade adds its own detection delay time, slowing safety response times and requiring the curtains to be set back farther from the hazard in order to achieve required safety levels. However, Banner Engineering’s safety light curtains are designed to minimize this extra response time, adding as little as two milliseconds for each additional curtain. This allows Banner’s safety light curtains to be placed closer to the hazard while still maintaining proper protection, creating a smaller footprint and saving valuable floor space.

Light Curtain Blanking

Similar to muting, blanking also lets certain objects pass the light curtain. In this case, blanking only disables adjacent light beams in a certain part of the curtain’s field, ignoring either moving or stationary items of a certain size.

There are two types of blanking: fixed and floating resolution. In fixed blanking, a predetermined set of beams is disabled, allowing an object to remain in a portion of the light curtain’s sensing field while the rest of the light curtain continues operating. This is useful when a piece of fixed equipment would normally block beams.

Reduced Resolution

Reduced resolution allows the light curtain to ignore one blocked beam anywhere along the light curtain while still triggering a safety response if two or more beams are blocked. This function is useful in applications such as horizontal guarding or material feed, where small objects, falling scraps, machine components, or passing material may temporarily interrupt the sensing field.

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