Determining resolution detection requirements for safety light screens
How do I determine whether I need high-resolution, medium-resolution or low-resolution detection capability in my safety light screen?
Responding is Mike Carlson, safety products marketing manager, Banner Engineering Corp., Minneapolis, MN.
Answer: Sensing capability – also known as resolution, minimum object sensitivity or detection capability – is the ability of an optical system to reliably detect an object anywhere within the sensing field. A light screen’s resolution is equal to one beam diameter plus the spacing between adjacent beams. This ensures an object of that size cross-section or larger will always be detected and issued a stop command. Safety light screens typically are grouped based on sensing capability and include high-resolution, medium-resolution and low-resolution.
To decide which resolution (detection capability) is needed for a particular application, the overall stopping time needs to be determined. Stopping time includes the safety light screen response time, any interfacing components and the stopping time of the machine. This will determine part of the total separation (safety) distance from the sensing field to the hazard. The second part of the total separation distance is dependent on the resolution. The larger the resolution, the farther an individual can reach into the sensing field before they are reliably detected; thus, the point of detection must be farther from the hazard. The means to determine this additional distance – the depth penetration factor – is dependent on a safety light screen’s resolution. As resolution increases, the Dpf decreases. Thus, when using a high-resolution safety light screen that can detect an object as small as a finger, expect the Dpf to be smaller than when using a low-resolution light screen.
High-resolution safety light screens typically are used in point-of-operation applications and positioned vertically to detect a finger or a hand approaching the hazard. High-resolution detection is used to minimize Dpf, resulting in a shorter separation distance. Light screens generally have a sensing capability of 14 mm to 20 mm to detect a finger and less than 40 mm or 60 mm to detect a hand or wrist, depending on the standard being followed (such as ANSI B11.19). An advantage of a high-resolution light screen is that it usually can be placed closer to the point of operation. In an assembly operation, this reduces the distance the operator must move in and out of the area, improving ergonomics.
Medium-resolution safety light screens may be oriented horizontally or vertically to detect an ankle, leg or torso in an area or perimeter guarding application. The optical sensing plane must be large enough, and positioned effectively, to prevent personnel from reaching over, stepping over or crawling under the sensing plane. A safety light screen with a resolution of 50 mm can reliably detect an ankle, but at 70 mm the sensing field should not be placed lower than 300 mm above the floor to ensure the individual’s lower leg is detected. Additionally, users can replace safety mats with horizontally mounted safety light screens, which help reduce maintenance and tripping hazards.
Low-resolution safety light screens – also known as “multiple beam systems” or “safety grids” – are used for perimeter and access guarding and should be positioned vertically to detect a torso or an entire body. Because these screens cannot reliably detect small objects such as a finger or hand, a Dpf equal to the length of an arm – normally 900-1,200 mm – must be included in the separation distance. Sensing capability for low-resolution guarding usually requires a minimum of two beams, spaced 600 mm apart, but typically requires three or four beams with the top of the sensing field 1,200 mm or greater above the floor to minimize separation distance.