Friday, July 17, 2009

Photo Electric Switches

How do garage doors know when something is in the way?
How do elevators know where to stop?
How does the store keeper know when someone enters a store?
How does a car wash know where the car is?
How does a manufacture count the cans on an assemble line?


Photoelectric switches use a light source to detect an object. The switch then changes it's logic or switch to make an action, like an alarm or movement of a device.

The photoelectric switch uses either visible light or infrared light as it's source. The object being sensed should be reflective for a diffuse switch or the use of a reflector is required and the object breaks "the beam" as in an elevator or garage door switch.

A thru beam system utilizes a transmitter and receiver. the Transmitter sends out the light beam and is directed at a receiver unit. Both units need to be powered and interconnected together for the system to work. Advantages are the longest field of view is achieved and the most secure sensing is realized via polarization. The main disadvantage is that each unit must be wired and interconnected together. A "thru beam" system can also be utilized using a reflector. this is called retro-reflective. The retro-reflective system has the advantage over a thru beam of all the logic and power supply being located in one box and one area. It is also more secure like the thru beam system by using polarizing filters that turn the light and only allow the correct "polarization" to be sensed by the receiver. The retro-reflective unit as the receiver and transmitter housed in a single unit. Thru Beam and Retro-reflective systems are used for counting, sorting and sensing the presence of an object or person for safety or security reasons.

The Diffuse system relies on the object being reflective, these are usually used in an industrial setting sensing a product or color code or marking on a product or box. Like the retro-reflective unit the transmitter and receiver are in the same unit. The distance is usually quite short so that the reflected light is able to be sensed. The target does not usually have the reflective qualities of a reflector. A diffuse system is used when a thru beam or retro-reflective cannot be used or when sensing color or a bar code.

All Photo Electric systems need an output to communicate with the PLC or system logic. Most use NPN or PNP logic but some have built in relays (1 amp usually) to give the output signal. The signal can go to a computer, PLC or directly to the system like a motor or contactor or alarm.


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