Different Reset Alternatives
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Supervised Manual Reset
When a light curtain/light grid is interrupted it will give a stop signal to dangerous machines within the risk area it protects. For a new start of the machine the light curtain/light grid has to be reset. This is done with the reset button (Figure 1) which is placed where it cannot be reached from within the area which is protected. There are high requirements on the reset function - neither a short circuit nor a component fault shall give automatic reset. When the reset button has been affected the outputs are activated and the machine can initiate.
Supervised Time-Reset
During time-reset unintentional reset is prevented when someone is inside the risk area. To reset the light curtain, button 1 (Figure 2) must first be pressed and afterwards button 2 must be pressed within a chosen time (e.g. 8 seconds). This is especially important when one cannot see the entire area that is protected by the light curtain.
Automatic Reset
Automatic reset is used when the light curtain is used for area monitoring. When the light curtain is actuated this indicates that a robot is in the area. The robot is stopped if a person enters the same area through a gate (Figure 3). When the light curtain is free again the control unit will be reset automatically.
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Figure 1
Reset button with light indication.
Figure 2
Button 1 is pressed and afterwards, within a chosen time (e.g. 8 seconds) button 2 is pressed for resetting the light beam.
Figure 3
An interrupted light beam b indicates that the robot is situated in area A. In this position it is possible to walk in through the gate to area B without stopping the robot.
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Bypassing
Bypassing may be needed for different reasons. One of the most common reasons for bypassing is during in and out feeding of material on a conveyor, auto industrial trucks, etc. Another common application is bypassing while passing with a three-position device to the risk area.
Important aspects for bypassing are that is should be safe, not be activated by mistake and be difficult to defeat. In other words, it should give a reliable bypassing when a loading carrier comes but not allow a human to pass. To achieve the highest safety level a dual and supervised bypassing system is needed - usually with at least two independent signals. If this is done with sensors, it is recommended they be of different kinds, because the probability of them both malfunctioning for the same reason (e.g. common mode failure) is very small. An example of a solution is to use a mechanical limit switch and a photocell sensor.
To avoid deliberate defeating/manipulation of the bypassing sensors/signals a safety relay or a safety PLC is connected, thereby monitoring that both sensors are activated and deactivated in every bypassing cycle.
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