March/April 2012
Factory Automation

Flexible safety solution

Safety light screens offer advanced features and flexible machine guarding over alternative safety methods

Fast Forward

  • Safety light screens offer improved flexibility, cost-effectiveness, and ease of installation over alternative machine safeguards.
  • Available in multiple models, with durable housing options and enhanced diagnostic capabilities, safety light screens are a reliable safeguarding device for a wide range of demanding applications.
  • By altering mounting, distance, and resolution, safety light screens can perform point of operation, area guarding, perimeter guarding, and access guarding to meet diverse safety requirements.
 
By Mike Carlson

 
Factory
Within most manufacturing environments, production requires a working balance of machines and personnel. While this interaction is necessary, it can also be dangerous, with billions of dollars a year expended on machine-related accidents in the workplace. According to an American Society of Safety Engineer's fact sheet, "Indirect costs of injuries may be as much as 20 times the direct costs, including costs from accident investigation, low employee morale, retraining workers, repairs, and production delays. Studies and case history indicate for every $1 invested in a safety and health program, $3-$6 are saved because injuries, illnesses, and fatalities decline, medical costs and workers' compensation costs decrease, and productivity increases."

Due to the wide range of hazards present in any industrial plant, facility managers need safety solutions that are cost-effective and flexible. Safety light screens provide a flexible safeguarding solution with versatile mounting options, numerous cascading capabilities, and broad applications suitability.

Safety light screens (or curtains) are optoelectronic devices that can detect the presence of opaque objects, such as a hand, arm, or foot, upon entering the sensing field. The emitter/receiver pair is comprised of two basic components, an LED array, which emits infrared light beams, and a phototransistor array that detects the corresponding beams. The emitter modulates the light at a specified frequency and "code" that the phototransistors detect, allowing the internal logic to accept only that particular pulse of light and to ignore signals from external light sources. This precludes factory floor ambient light from affecting the performance of the safety light screen.

Guarding points of access to machine hazards, safety light screens will prevent or halt machine operation when an object is detected within the sensing field, protecting personnel from injury and equipment from damage. When all the phototransistors detect light from their corresponding LEDs, the light screen is "complete" and the safety outputs turn ON, which allow the machine to operate. However, once a hand or arm interrupts the beams of light, the light screen detects the presence, which triggers a stop signal to the safety-related controls of the machine. The machine immediately reacts, stopping the hazardous machine motion or removing the hazardous situation, until the object is removed and the sensing field is "complete" once again.

Safety light screens vs. alternative guarding methods

While there are a multiple effective safety solutions available for safeguarding machinery, safety light screens offer a variety of advantages over alternative methods. In comparison to interlocked guards, fixed hard guards and pull-backs, safety light screens provide benefits, such as enhanced flexibility, unobtrusive and ergonomic operation, as well as minimal set-up and maintenance requirements-making them suitable for a large number of applications.

Interlocked guards and fixed hard guards protect employees from harm by providing a physical barrier that restricts access to and prevents accidental contact with hazards. They guard against material, tooling, and other items from being ejected from the machine. By properly interfacing with the machine control system, when the interlocked guard is opened, a stop signal will halt or prevent machine operation until the guard is in its fully closed position again. While effective, this safeguard does not allow effortless human/machine interaction inherent in safety light screens, preventing them from being an ideal solution for applications that require frequent access. Further, the amount of time needed to open and close the interlocked guard can reduce efficiency. Additionally interlocked guards are sometimes large and heavy, which can cause ergonomic concerns.

Another common machine guarding strategy is to install fixed hard guards around a hazard. While providing excellent protection against hazards, hard guards do not possess the combination of safety and flexibility found with safety light screens. Hard guards essentially create a permanent obstacle between personnel and machinery. Though they can be removed for maintenance, hard guarding must be in place during machine operation, which prevents any access and often limits visibility.

In addition to interlocked guards and fixed hard guards, pull-backs and restraint devices are also used to protect against work-related injuries. These safeguarding devices attach to an operator's hands or wrists, operating by either withdrawing the operator's hands from the hazardous area during operation or preventing reaching into the hazardous area. These devices are generally maintenance intensive and require constant adjustment, as they must be modified for each operator. Another drawback is they provide protection for only one operator versus a safety light screen that protects all individuals. For these reasons, pull-backs and restraint devices have become less common in the last 20 years, as safety light screens have enjoyed an explosion of popularity.

Flexible safety solution

When safeguarding against machine-related hazards, safety is not a uniform solution. In order to accommodate various safeguarding needs, safety light screens can offer versatile options, with differing features and characteristics. Available with two distinct performance capabilities, Type 2 and Type 4 safety light screens (as described by IEC 61496-2), they allow users to solve applications that have differing levels of required risk reduction. 

Typically containing only one microprocessor, Type 2 light screens mainly rely on fault exclusion and design practices to ensure the integrity of its safety function. Fault exclusion is the ability to minimize or eliminate known possible failures or faults through design, selection of components, or implementation of additional measures. Type 2 light screens have a larger field of view, which makes them easier to align, but also increases their susceptibility to suffering "optical short circuits." Optical short circuits can create a desensitized area within the sensing field that would allow an object to pass undetected by bouncing the light beams off of reflective surfaces. While this causes additional installation concerns, the Type 2 safety light screens can provide a cost-effective solution for guarding lower-risk applications where an accident would result in a slight injury, such as small assembly equipment, automated production equipment, and equipment protection.

Alternatively, Type 4 safety light screens achieve high levels of fault tolerance, which enables them to provide continuous, reliable operation through redundancy and self-monitoring. Generally available with more sophisticated safety features and meeting OSHA and ANSI requirements for control reliability, Type 4 safety light screens should be selected for use in higher-risk applications that require personnel safeguarding, as they could result in severe injury or death.

 Safety Light

Advancements in light screens

In order to adapt to ever-changing requirements, safety light screens have continued to advance, providing sophisticated levels of safety and customizable features for a comprehensive machine safeguarding strategy. Historically, these devices have been offered in large robust housing designed to take a beating, but newer space-saving, compact profiles are now available to meet the demands of smaller machines. Plus, different housing finishes are also available to satisfy esthetic or environmental requirements, such as ESD-resistant static-sensitive applications.

To facilitate ease of use, safety light screens are designed with the installer in mind, offering less complicated installation and configuration processes. For example, systems with a built-in muting function do not require a third box, which minimizes wiring requirements, thereby allowing simplified installation and reduced labor. In some systems, setup can be performed without a PC, hand-held programmer, or other device, reducing the number of items to purchase, train individuals on, and keep track of. Further, innovative light screens can effortlessly be configured for reduced resolution and fixed blanking when objects like tooling need to be ignored.

With mounting flexibility and the option of cascading systems together, safety light screens provide an ideal fit for a wide range of safeguarding applications. By allowing multiple systems of any length or resolution to be connected in a series, a single comprehensive safety solution allows for continuous detection without overlapping light curtains or protruding cables.

Safety light screens now offer enhanced diagnostic capabilities, providing performance and status information. Additionally, through auxiliary outputs, such as Status, Reset, and Mute Device, data can be relayed to external indicating devices immediately, communicating system status to personnel.

Additionally, safety light screens are engineered to withstand a wide range of harsh and demanding conditions. For instance, light screens are constructed to be highly immune to EMI, RFI, ambient light, weld flash, and strobe lights and come equipped with factory burned-in emitter and receiver circuitry for reliability, dependability, and vibration tolerance.

Implementing safety light screens

With various mounting configurations, versatile use and multiple cascading options, safety light screens solve diverse applications and comply with machine and safety requirements. Before implementing safety light screens, it is important to know what type of hazards it is guarding, the necessary resolution, and which configuration to use. By understanding the many available application possibilities, such as, point of operation, area guarding, perimeter guarding, and access guarding, manufacturers can optimize machine safety with the use of safety light screens.

Given the nature of their operation, safety light screens should be used for applications where the hazard can be halted within a short time frame. The stop is achieved by removing electrical power to a machine control element, such as a hydraulic valve. They should not be utilized in applications where materials can be ejected from the machine, for hazards with extreme temperatures, with machines that cannot be stopped immediately, or with those with inconsistent response times.

Users should also determine the appropriate resolution or detection capability. Numerous tightly-spaced beams comprise high-resolution light screens, which are designed to detect smaller objects, such as a finger or hand. Medium-resolution light screens feature less-densely spaced light beams and can easily detect a hand or ankle, while low-resolution safety light screens use only a few beams and can reliably identify the presence of larger objects like a torso.

In order to select the appropriate safety light screen, users should consider other design features besides resolution. Manufacturers should know the environment in which the safety light screen will be expected to operate, the height and size of the defined area, the number of access points to the hazard, the electrical supply and the combined response time of the safety light screen, and the machine generating the hazard. These factors will determine the resolution, placement, and configuration required for the appropriate level of risk reduction.

Point of operation

The largest number of safety light screen applications is for point-of-operation safeguarding. Typically, these applications involve a hand-feed process, such as feeding sheet metal into a power press where the individual is continually detected by the safety light screen. Resolutions of 40mm or less are generally used depending on the amount of safety distance between the sensing field and the hazard. For a short distance, a resolution of 14mm or 25mm is a good choice. 

Area guarding

Area guarding is ideal when continually sensing individuals within a hazardous area is required. Typically in these applications, the sensing field is horizontal (or less than 30 degrees) to the floor and is meant to detect a foot, ankle, or thigh-rather than identifying a finger or hand. The initial point of detection for an individual is generally positioned farther away from a hazard in order to prevent the risk of reach-over. Often, the horizontal sensing field of an area guard is located behind other safeguards. These safeguards are used as the primary safeguard, responsible for stopping or removing the hazard, while the area guarding device is used to prevent start or restart.

Perimeter guarding

Perimeter guarding is used in applications where personnel can enter and pass through the sensing field of the safety light screen. Perimeter guarding can be cost-effectively accomplished with a medium-resolution light screen or a multiple beam system designed to detect the torso of an individual. Installed around the perimeter or one/two side, once the sensing field is interrupted, the machine(s) will immediately stop operation. This allows workers to enter and freely move about the area, making this ideal when workers have to access an area with multiple hazards.

Access guarding

Access guarding is used to protect doorways, cell entries or exits, and machine access points. By placing a safety light screen at these entryways, access guarding usually only requires a medium-resolution light screen or a multiple beam system to provide proactive, cost-effective safety. Access guarding is generally short range, typically less than 30 feet between emitter and receiver.

Safety light screens are an established safety solution, providing flexible safeguarding for a broad scope of applications. Looking for safety light screens with easy set-up, intuitive operation, and reliable performance will provide a cost-effective solution for diverse industrial applications. Additionally, users can refer to the standard, ANSI B11.19, "Performance Requirements for Safeguarding," for requirements and recommendations in the application of safety light screens, perimeter guarding, muting, and other safeguarding design and application solutions.

ABOUT THE AUTHOR

Mike Carlson (DEng@bannerengineering.com) is a safety products marketing manager at Banner Engineering.