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Fundamentals of Photoelectric Sensors

Industrial sensors are the eyes and ears of the new factory floor, and they come in all sizes, shapes, and technologies. The most common technologies are inductive, capacitive, photoelectric, magnetic, and ultrasonic. Each technology has unique strengths and weaknesses, so the requirements of the application itself will determine what technology should be used.

Photoelectric sensors are readily present in everyday life. They help safely control the opening and closing of garage doors, turn on sink faucets with the wave of a hand, control elevators, open the doors at the grocery store, detect the winning car at racing events, and so much more.

A photoelectric sensor is a device that detects a change in light intensity. Typically, this means either non-detection or detection of the sensor’s emitted light source. The type of light and method by which the target is detected varies depending on the sensor.

Photoelectric sensors are made up of a light source (LED), a receiver (phototransistor), a signal converter, and an amplifier. The phototransistor analyzes incoming light, verifies that it is from the LED, and appropriately triggers an output.

Photoelectric sensors offer many advantages when compared to other technologies. Sensing ranges for photoelectric sensors far surpass the inductive, capacitive, magnetic, and ultrasonic technologies. Their small size versus sensing range and a unique variety of housings makes them a perfect fit for almost any application. Finally, with continual advances in technology, photoelectric sensors are price competitive with other sensing technologies.

Sensing Modes

Photoelectric sensors provide three primary methods of target detection: diffused, retro-reflective and thru-beam, with variations of each.

Diffused Mode

In diffused mode sensing, sometimes called proximity mode, the transmitter and receiver are in the same housing. Light from the transmitter strikes the target, which reflects light at arbitrary angles. Some of the reflected light returns to the receiver, and the target is detected. Because much of the transmitted energy is lost due to the targets angle and ability to reflect light, diffused mode results in shorter sensing ranges than is attainable with retro-reflective and thru-beam modes.

The advantage is that a secondary device, such as a reflector or a separate receiver, is not required. Factors affecting diffused mode sensing range include the target’s color, size, and finish because these directly affect its reflectivity and therefore its ability to reflect light back to the sensor’s receiver.

Retro-reflective mode

Retro-reflective mode is the second primary mode of photoelectric sensing. As with diffused mode sensing, the transmitter and receiver are in the same housing, but a reflector is used to reflect the light from the transmitter back to the receiver. The target is detected when it blocks the beam from the photoelectric sensor to the reflector. Retro-reflective mode typically allows longer sensing ranges than diffused mode due to the increased efficiency of the reflector compared with the reflectivity of most targets. The target color and finish do not affect the sensing range in retro-reflective mode as they do with diffused mode.

Thru-beam mode

Thru-beam mode—also called opposed mode– is the third and final primary method of detection for photoelectric sensors. This mode uses two separate housings, one for the transmitter and one for the receiver. The light from the transmitter is aimed at the receiver and when a target breaks this light beam, the output on the receiver is activated. This mode is the most efficient of the three, and allows the longest possible sensing ranges for photoelectric sensors.

Thru-beam mode sensors are available in a variety of styles. The most common includes one transmitter housing, one receiver housing, and one light beam between the two housings. Another type is “slot” or “fork” photoelectric sensors that incorporate both transmitter and receiver into one housing, with no alignment required. Light grids are arrays of many different transmitters in one housing and many different receivers in another housing, which, when aimed at each other, create a virtual “sheet” of light beams.

Fiber optic sensing

Fiber sensors guide the light from the transmitter through either plastic or glass cables called fiber optic cables. In applications involving small targets or unfavorable conditions, fiber optic cables may be the optimum solution. Fiber optic cables allow either diffused mode or thru-beam mode sensing.

Application Specific Photoelectric Sensors

In addition to the standard modes of operation for photoelectric sensors, several application specific sensors also exist. These sensors are used to solve many non-traditional photoelectric applications, such as detecting changes in a target’s color, porous targets, and invisible mark.

Source: Pepperl+Fuchs

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