Making light work of machine safety
systems can be specified to reliably detect a finger, for example, while medium resolution screens can be set to signal an arm or foot/ ankle. Low-resolution systems would be used for perimeter and access guarding applications, detecting when someone has stepped right through the curtain. While high and medium resolution curtains can be oriented in any direction, low-resolution systems are usually only configured in a vertical orientation.
LEDs can be programmed enable many more safety features to be incorporated as part of the curtain system. These include built-in redundancy, self-test and restart interlock, thereby improving failsafe operation while providing greater flexibility and lower cost. Omron Automation offers a wide range of models in its F3SG-R series of safety light curtains, meeting both Type 4 and Type 2 specifications. Finger protection models range from 15 beams and a protective height of 160 mm up to 207 beams and a protective height of 2.08 m. With a beam gap of 10 mm, object resolution is 14 mm
The use of advanced electronic controls together with facilities such as a smart muting actuator, allows the system to be easily configured and reconfigured to suit different production lines and/or work pieces.
In certain medium resolution applications, material or tooling may need to be fed into the machine through the light curtain without stopping the machine. A blanking feature can allow the light curtain to be programmed to ignore objects of a predetermined size, or permanent stationary objects
such as tooling or conveyors that obstruct a part of the field. Importantly, today’s infrared LEDs are more reliable, and thermal issues are better understood, extending the lifetime of the components. Microprocessor control and the ease by which
Figure 1: Safety light curtains control access to potentially hazardous areas in an industrial environment. The highest resolution (center) will detect the presence of a finger. Medium resolution (right) will sense a hand, while still lower resolution can be set to sense an arm or a whole person. (Photo credit: Omron Automation)
Features and flexibility
Early light curtain designs had drawbacks. They could be difficult to install effectively for certain types of equipment. Today’s designs are more flexible in terms of mounting options. Smaller curtains, configured vertically and horizontally, can be made to fit into tighter spaces within a machine framework. This not only helps avoid blind spots and dead zones, but can also allow operators easier access to the equipment while still being protected from hazards. Some machines require intermittent access by the operator, either in certain areas, or at certain times. A muting function is available on many systems that allow the light curtain, or sections of the screen, to be bypassed or the sensor unit disabled while the machine is in a non-hazardous part of the cycle, such as an upstroke.
Safety light screens for industrial machinery are typically constructed to meet IEC61496- 1 or -2 international standards requirements, and described as Type 2 or Type 4. Type 2 is for lower risk applications where an accident would result in a slight injury. These include the guarding of small assembly equipment, automated production equipment such as pick and place machines, table-top robotic workstations and small packaging machines. Type 4 light curtains are required for higher risk applications that could result in serious injury or death. These light screens have high levels of fault tolerance through redundancy and constant monitoring, plus many other safety features, and are generally constructed in more robust
housings. Typical applications include mechanical and hydraulic power presses, molding presses, stamping, forming, riveting, and other automated assembly machinery. Once the appropriate standard has been determined, a number of other factors must be considered. For example, the minimum separation distance from the sensing field to the hazard is determined by the hand/body speed, machine response time, safety light curtain detection capability and response time. The size of the object to be detected will determine the resolution of the optical system, which is a function of the beam diameter and spacing of adjacent beams (Figure 1). High-resolution
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