Safety light beam basics : Working principles 

 Principles of operation:
    we differentiate Safety Light Beams into 4 principles of operation: 

 Working principles 

•   The sender/receiver system
•    The twin concept
•   The active/passive design
•   Vision sensors        

• Our standard technology is the sender/receiver system. 

        The sender unit emits a light beam which is detected by the receiver unit. This principle covers a range up to 90 meters.   

       
safety light curtain

•        The twin concept is nice to handle. 

       

Sender and receiver are integrated in one housing so you just take two of a kind and twist one upside down. Now both senders emit the beams to each other’s receiver.   The twin concept simplifies logistics by using the same unit twice. 

•        Active/passive systems

      combine a single sender and receiver in one housing called active side together with mirrors on the passive side that deflect the beam from sender to receiver. This solution has only one opto-electronic device so it is cost efficient. 

•       The vision sensor

            is especially made for small machines. While the device is mounted in a corner a reflecting tape edges the protective field sending back beams to the sensor. Besides the installation of only one sensor another advantage is that the abrasion-proof tape offers a huge flexibility due to its easy attachment on round and uneven surfaces.   

Safety light beam basics: Minimum distance calculation

         For a correct installation the minimum distance between the machine and the Active Opto-electronic Protective Devices (short AOPD) is crucial. 

       It can be determined with  S=(K*T)+C  formula. 

       The first part of it is this approach speed K multiplied by the stop time of the machine T. In case of light curtains an approach speed of 2000 mm/s is mostly valid, in the multi beam case 1600 mm/s is the correct value. The fix intrusion distance C has to be added.  

       What does C actually mean? 

          Let’s take a look at the light curtain. Decisive is, how far can the operator reach into the hazardous area without being detected. This depends on the selected resolution D as shown in the formula ( C= 8*(d-14)). In the case of a 30 mm resolution we receive a value C of 128 mm ensuring hand detection. With the finger protection resolution of 14 mm, C becomes zero. 

         Using a Multi Beams system a whole arm can reach through the light beams. Only bodies can be detected. For that reason C is significantly larger: 850 mm. 

        A single beam system in combination with further measurements requires 1200 mm intrusion distance.

          Summarized: The better the resolution the smaller is C and the shorter is the minimum distance.

 Safety light beam basics : Before operation

       Dangerous situations may still remain after installing a safety light curtain. Reaching through and reaching over situations may be eliminated extending the minimum distance. 

        For reaching under and standing behind situations other solutions have to be taken into account. 

active optoelectronic protective device

        To check a reaching over situation the height "a" of the hazardous area and the height "b" of the upper edge of the detection zone are relevant. With the given values the additional intrusion distance C reaching over can be identified on the table. The resulting minimum distance Sro reaching over has to be compared versus the Srt value.The bigger value is leading. 

        In reaching under situations crawling under the Active Opto-electronic Protective Devices (short AOPD)  has to be avoided. In case of access prevention the maximum gap between the first beam and the floor shall be 300 mm max. Other distances from the floor underlie special requirements. A gap between Opto-electronic device and hazardous movement that allows the operator to step between the device and the machine should be avoided too.

        We name it “Step-behind” situation. In the following we will have two possibilities to prevent harm of the operator. 

        With the restart interlock the measure maintains stopped once the sensor is interrupted. 

        The operator has to leave the hazardous area to reset the from a safe distance. Only then the machine can be restarted Accidents may happen when the second person appears at a spot while the operator is still located in the hazardous area. 

        That's why the reset is only permitted in totally visible areas. In case no total visibility is given an additional horizontal device detects the operator in the entire area. An electrical connection between the vertical and the horizontal device is called cascading. 

         After the installation the next task is the adjustment. Supported by a laser beam initially the sender is roughly aligned to the receiver. Guided by LEDs the alignment is fine-tuned at the receiver. The more LEDs are on the better the alignment. A test rod check has to be executed to test the effectiveness of the protective measurement.