What are photo-electric sensors?

A photoelectric sensor, or photo eye, is a device used to detect the distance, absence, or presence of an object by using a light transmitter, often infrared, and a photoelectric receiver. They are used extensively in industrial manufacturing. There are three different functional types: opposed (through beam), retro-reflective, and proximity-sensing (diffused).

Photoelectric sensors use a beam of light to detect the presence or absence of an object. This technology is an ideal alternative to inductive proximity sensors when you require long sensing distances or when the item you want to sense is non-meta

A self-contained photoelectric sensor contains the optics, along with the electronics. It requires only a power source. The sensor performs its own modulation, demodulation, amplification, and output switching. Some self-contained sensors provide such options as built-in control timers or counters. Because of technological progress, self-contained photoelectric sensors have become increasingly smaller.

Remote photoelectric sensors used for remote sensing contain only the optical components of a sensor. The circuitry for power input, amplification, and output switching are located elsewhere, typically in a control panel. This allows the sensor, itself, to be very small. Also, the controls for the sensor are more accessible, since they may be bigger.

When space is restricted or the environment too hostile even for remote sensors, fiber optics may be used. Fiber optics are passive mechanical sensing components. They may be used with either remote or self-contained sensors. They have no electrical circuitry and no moving parts, and can safely pipe light into and out of hostile environments.

How Photoelectric proximity sensor works

The advantages of this mode is that the sensing range is more. The disadvantage is 2 parts need to be mounted separately and for long range the installation could be difficult because the emitter has to be put with the detection range of the detector.

Diffuse Reflective: In this case the emitter and detector are put in the single package in such a way that their field of view cross. Here the emitter continuously emits the light. When the target comes within the operating range of the sensor the light from the emitter is reflected off the target and detected by the detector.

The advantages of this are low cost, easy installation. The disadvantages are short sensing distance. The sensing distance depends on target size, surface and shape.

Retro-Reflective: the main components of this sensor are the emitter, detector and the Retro-reflector. The emitter and the detector are in the same package. The Retro-reflector is placed little far from the sensor. The light from the emitter is reflected off the Retro-reflector and detected by the detector. When the target passes between the sensor and the Retro-reflector the beam is not reflected back to the detector. Here the problem can be that the beam could reflect from the target itself. For this the polarising filter is used in the sensor. Hence only the light reflected by the retro-reflector is detected by detector

The advantages of this are low cost and easy to install. The disadvantages are separate retro-reflector to be used, Cannot be used to detect small objects as the target has to block the entire beam from emitter, performance could be affected in case of dirt on retro-reflector, problems in detecting clear targets.

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Coutesy: Wikipedia, Sensors and Actuators