How to select a safety light screen for industrial machinery
|Machine safety related reading|
Safety light screens, also referred to as safety light curtains, are optoelectronic devices used for guarding a machine hazard-a potential source of harm-and protecting personnel and equipment. Each screen is comprised of an emitter array and corresponding receivers that combine to create a sensing field with a specified resolution, which is the minimum size of an object that a safety light screen will reliably detect. Safety light screens detect the presence of an opaque object, such as a hand, arm or foot, when it interrupts the light beam.
Safety light screens vary greatly in the levels of protection they provide and functions they offer, delivering a flexible solution for a broad range of applications. Suitable applications should require some amount of access to the point of operation or guarded hazard and should feature a hazard that can be stopped within a short time by removing the electrical power or signal. Examples include mechanical and hydraulic power presses, molding presses, stamping, forming and automated assembly machinery.
When selecting a light screen as part of a robust machine guarding solution, various factors-such as the level of risk present, mounting options, resolution required and desired functions-should be carefully considered.
Level of risk
First and foremost, a safety light screen should be selected according to the level of risk presented by the application. The International Electrical Commission standard IEC 61496-1/-2 specifies requirements for the design, construction and testing of two types of safety light screens: Type 2 and Type 4. A risk assessment should determine which type of safety light screen is needed for an application.
Type 2 safety light screens provide a cost-effective solution for typically guarding lower-risk applications, where an accident would result in a slight injury. Type 2 light screens generally have a single microprocessor and use fault exclusion or FMEA as the main means of ensuring the integrity of the safety function. Type 2 light screens also have a larger Effective Aperture Angle (EAA), or field of view, than Type 4. Although the larger EAA facilitates easier emitter and receiver alignment, it also makes them more susceptible to optical short circuits. Type 2 light screens are ideal for applications with a limited risk of injury that still require some guarding, such as small assembly equipment, automated production equipment, table-top robotic work cells, pick and place machines, small packaging machines, equipment protection and supplemental safeguarding.
Type 4 safety light screens can meet OSHA or ANSI standard control reliability requirements and should be selected for use in higher-risk applications that could result in severe injury or death. These light screens achieve high levels of fault tolerance through redundancy and monitoring, are available in more robust housings, and offer a wider range of features.
Mounting and installation
Safety light screens must be installed between the machine hazard and personnel, in accordance with requirements specified in accepted standards, so that the machine can remove the hazard before personnel can reach it. An adequate separation (safety) distance-the distance from the sensing field to the hazard-must be determined by considering the hand/body speed, machine response time, safety light screen response time and safety light screen detection capability.
Housings come in many sizes and degrees of robustness and should be part of the selection process. Smaller cross-sections and low profile ("no dead zone") light screens can fit snuggly within the machine frame without creating gaps in the sensing field. Larger and more robust light screens can with stand physical impact from workpieces, such as sheetmetal.
Size of object: Detection capability
Detection capability is a specification that defines the size of the object that can be reliably detected by the safety light screen, and it helps determine the physical placement of the light screen. Also known as the resolution, the detection capability is the ability of an optical system to reliably detect an object. The detection capability is determined by the beam diameter and spacing of adjacent beams and is the basis of safety light screens classifications.
High-resolution light screens can detect a finger or hand and provide an ideal fit for applications that need to minimize the separation (safety) distance. Medium-resolution screens can detect an arm or an ankle and are good choices for area guarding. Low-resolution screens can detect a torso and are used for perimeter and access guarding applications. High- and medium- resolution light screens can be orientated in any direction, but low-resolution light screens can usually only be used in vertical orientation.
Selecting functions of safety light screens
– Output mode : Various functions can be combined to tailor a safety light screen to fit the needs of each application. For example, the output mode can be set to either automatic or manual reset (also known as trip or latch). With automatic reset, which is generally used for applications in which an individual is continually sensed, the light screen automatically resets once the interruption has been removed from the sensing field. Manual reset, typically used for applications where the operator actually passes through the sensing field into the guarded area, requires the light screen to be manually reset once the interruption has been removed. Once the light screen has been reset, either automatically or manually, the machine is allowed to be cycled by actuating the normal starting means.
– Scan rate : Most safety light screens have a function that allows the user to change the scan rate or the scan code of the beams of infrared light. This is to prevent interference from other sources of infrared light, such as other light screens, that could cause intermittent machine stoppage. The user must determine whether enabling this function increases the response time of the light screen, which would cause the separation distance to increase.
– Blanking : In certain applications, it may be desirable for high or medium-resolution safety light screens to ignore objects of a predetermined maximum size. This capability, called blanking, allows an object to interrupt a set number of beams without generating a stop signal or affecting the ability to protect personnel. There are several types of blanking. Reduced resolution allows every other or every two beams to be block, while floating blanking allows just one or two beams (that is, a given number) to be blocked. In fixed blanking, a "fixed" area must be blocked or cause a lockout. Exact blanking is similar to fixed blanking but is tied to the "exact" number of beams and not the location.
– Muting : Another function commonly associated with light screens is the muting function, which is the temporary automatic suspension of the safety light system during a safe portion of a machine cycle to allow material to flow into a machine or process. Common muting applications include manual unload and load operation during the safe portion of the machine cycle, as well as automatic material flow processes into and out of a robotic work cell .
Functions such as muting, blanking, automatic or manual (trip or latch output mode), as well as the desired resolution, mounting options and the level of risk should all be considered when selecting a safety light screen for a machine guarding application. Application requirements for optical safety systems are well defined within several standards and should be carefully followed to ensure safety for personnel and machinery.
– Mike Carlson is safety products marketing manager, Banner Engineering, www.bannerengineering.com/safety. Edited by Mark T. Hoske, editor in chief, Control Engineering , www.controleng.com.