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(Issue 19, 2011)
Safety Light Curtains
By Lenny Filipkowski,
Safety light curtains are an advanced method of safeguarding personnel around hazardous machines through the use of photoelectric technology and a concept known as Control Reliability. Safety light curtains are a great alternative to other traditional guarding methods such as mechanical barriers, sliding gates and pull-back restraints; they reduce operator fatigue and offer flexibility and freedom to the operator as well. (Figure 1)
How do they work?
Safety light curtains consist of a transmitter and a receiver. A photoelectric transmitter projects an array of synchronized, parallel infrared light beams to a receiving unit. When an object breaks the beams of light, the logic circuit in the light curtain sends a signal to the control system of the machine and stops the hazardous movement of the machine. (Figure 2)
The transmitter contains light emitting diodes (LEDs) which emit pulses of invisible infrared light when energized by the light curtains timing and logic circuitry. These light pulses are sequenced; one LED is energized after another and they are modulated or pulsed at a specific frequency. The receiving units are designed to detect only the specific pulses and frequency from the matched transmitter unit.
In order to meet the OSHA and ANSI requirements for control reliability, the safety light curtains are continuously doing self checks; if the self checks detect any fault, the light curtain immediately sends a stop signal to the machines control system. The light curtain then becomes locked in a safe condition until the faulty component is replaced and an appropriate reset is done.
Safety light curtains also have a set of two relays in order to provide a redundant circuit. If one relay fails, the second will still provide the stop signal. There are two basic types of outputs for light curtains: relays with force-guided contacts, and electronically cross-monitored and self checking solid-state devices know as OSSD (Output Single Switching Device).
In what applications are they used?
Light curtain applications are typically categorized by the type of guarding required. Light curtains are used in guarding machines such as mechanical and hydraulic power presses, molding presses, stamping, forming and automated assembly machinery. Safety devices are typically selected to protect the operator's finger or hand from the pinch point of the machine.
If there is a perimeter or boundary defined by a machine, robot or other equipment, then a perimeter guard style light curtain may be selected.
The main difference between a light curtain designed to protect a finger versus a hand, and one designed to protect a perimeter, is the spacing of the actual beams. A unit designed to protect a finger utilizes 14mm spacing between each beam while one for hand protection is designed with 30mm between the beams. Perimeter style light curtains can have beam spacing from 300mm to 500mm.
How do I select the proper light curtain system for a given application?
Several points must be addressed to make the proper selection; some of these factors include:
- The size of the object or body part being detected or protected will determine what resolution of beam spacing is required; 14mm for finger or 30mm for hand. (Figure 3)
- Determining the height and length of the area and the protected field to be guarded will determine the required maximum protective height of the light curtain system.
- The time required for the machine or process to stop after an interruption of the light curtain system will determine the required response time needed from the light curtain system.
- The maximum distance or range needed to separate the emitter and the receiver in the application must be considered.
- What type of load will the light curtain safety outputs control? Are they sufficient for the control circuitry? This will determine the current capability required of the safety signals. Do you need semiconductor style outputs or heavy duty relay outputs for direct control of a motor starter?
- What safety level or safety control category does the application's risk assessment indicate is required? Can a basic type 2 rated light curtain unit suffice or do you need the more robust type 4 units for the most protection and reliability for the application? All are points for consideration when determining the proper light curtain system.What other functional features can safety light curtains perform?
Safety light curtains can be used in a variety of different modes which can range from providing a stop signal to a machine's safety control circuit to more sophisticated functions such as muting, fixed blanking, floating blanking and presence sensing device initiation. These functions can be part of the logic circuitry in the light curtain units themselves, or certain functions such as the muting and presence sensing device initiation can be handled by an external safety control relay module.
What is muting?
Muting is the provisional and automatic overriding of the light curtain safety output function during normal, uninterrupted machine cycle operation. This function allows the light grid to be interrupted by some part of the machine or material being processed without stopping the operation or process.
An example of this would be a palletizing system within which the palletized product must be allowed to pass through the opening protected by the light curtain, while the entry of a person must stop the machine in a safe manner.
Muting is typically accomplished by using an external safety control relay module and additional sensing devices such as safety limit switches or photoelectric sensors in conjunction with an external signaling light to show when the muting function is activated.
What is fixed blanking and floating blanking?
Fixed blanking is when a fixed set of adjacent light beams are set to be permanently inactive to allow product or part of the process to enter the sensing area without deactivating the light curtain safety outputs.
Floating blanking is when a set number of adjacent beams are allowed to ignore the presence of an object within their portion of the protected field. Unlike the fixed blanking method where there is a specific set of inactive beams, floating blanking allows the set number of adjacent beams to "float" within the protective field. This allows the object to be ignored to move within the protected field without deactivating the light curtain safety outputs.
These functions are typically designed into the logic in the safety light curtain units themselves and require some sort of programming or dip switch settings.
When the application allows for the use of safety light curtains, the functions and benefits allow theprotected machine to run more safely and more productively than many other protection devices. The most important, consideration when choosing a brand of light curtains is to make certain they meet all requirements of the relevant safety standards such as EN/ISO 13849-1:2008 (former EN 954-1), and EN/IEC 61496-1:2004/-2:2006 type requirements. Also, it is important to verify these have been tested and certified by an independent, recognized third party such as TUV in order to make sure they are compliant to the relevant standards for safety light curtains.
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FYI, Issue 19, 2011