How to Operate an Automatic Door Using Infrared Photoelectric Sensors

An automatic door is not complete without infrared photoelectric sensors to control it. Photo eyes provide a non-contact method to detect people or objects passing through the door and may be used in conjunction with a controller to open the door.

Choose a quality set of industrial infrared photoelectric sensors. It is imperative that sensors are used on an automatic door, not just to open the door, but to prevent the door from closing with a person or object underneath. Photoelectric sensors are ideal for this application because they do not require the door to make contact with the subject to detect their presence. Ultimately, the manufacturer should provide wiring diagrams and installation instructions to help interface sensors or other input devices with the door controller, but the following instructions should give you a general idea of how they should be used.

Mount the photoelectric sensors in a through-beam configuration. This will save you a lot of trouble later. Do not try to mount the photo eyes on one side and use a reflector on the other because you will find yourself constantly cleaning the reflector. Infrared photoelectric sensors provide a stronger and much more reliable beam when they are mounted across from each other. If the photoelectric system that you are using includes an external amplifier, compare the voltage ratings for the photoelectric system and the PLC. If they are a match, connect the output from the amplifier to the door's PLC input card.

Configure the photoelectric system to output all the time, but then take away that output when the photo eyes detect something. This is commonly listed by the manufacturer as a feature known as "Normally Closed", "Light Operate", or "Make Switching." Program the PLC to interpret an output from the photoelectric system as a "false" condition and the absence of that output as a "true" condition. When the PLC reads a "true" condition, it should open the door, and when the condition returns to "false" it should close the door. This does two things, it opens the door when an object is detected and also if the photoelectric system experiences a fault or loss of power.


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How to Connect a Three-wire DC Sensor to a PLC

An application calls for a three-wire DC sensor to report to a PLC. How should they be connected together?  Check the input card of the PLC and find the voltage rating. Does it operate on DC voltage? Also determine whether the input needs to be sinking (NPN) or sourcing (PNP.) Look at the sensor and make sure the voltage rating and PNP / NPN type is the same. If there is a mismatch, refer to the manufacturer's instructions for both devices to determine compatibility.

If the PLC's input card is rated for DC voltage and it is a sinking (NPN) type input and also if the sensor operates on DC voltage in the same range and its output is sinking (NPN), then the two should be compatible. Connect the PLC to the power supply, but do not turn the power supply ON yet. Then connect the sensor's positive source voltage wire, which is normally brown, to the positive terminal of the power supply. Connect the negative or neutral sensor wire, normally blue, to the power supply as well, but on the negative terminal. Then connect the sensor's signal wire, normally black, to the PLC input card at the number address that you will assign as the sensor in the PLC program. When the sensor is activated, it will connect the PLC output to neutral or ground, which the PLC will interpret as a "true" condition and respond as programmed.

If the PLC's input card is rated for DC voltage and it is a sourcing (PNP) type input and also if the sensor operates on DC voltage in the same range and its output is sourcing (PNP), then the two should be compatible. Connect the PLC to the power supply, but do not turn the power supply ON yet. Then connect the sensor's positive source voltage wire, which is normally brown to the positive terminal of the power supply. Connect the negative or neutral sensor wire, normally blue, to the power supply as well, but on the negative terminal. Then connect the sensor's signal wire, normally black, to the PLC input card at the number address that you will assign as the sensor in the PLC program. When the sensor is activated, it will connect the PLC output to the positive terminal of the power supply, which the PLC will interpret as a "true" condition and respond as programmed.

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