Editorial Feature

What is a Photoelectric Sensor?

A photoelectric sensor is an instrument designed to detect the distance, absence, or presence of an object by using a light transmitter such as an infrared, and a photoelectric receiver. They are also referred to as photo eyes.

Photoelectric sensors are widely used in the industrial manufacturing sector as they provide a huge variety of problem solving alternatives in the industrial sensor market. Photoelectric technology has advanced a lot and a sensor is now able to detect a target less than 1 mm in diameter. Photoelectric sensors can be easily incorporated into an endless array of applications.

Photoelectric sensors contain the following components:

  • Emitter - A light source such as an LED or laser diode
  • Photodiode/phototransistor receiver - To detect the light source
  • Supporting electronics - For amplifying the signal relayed from the receiver.

Types of Photoelectric Sensors

The types of photoelectric sensors commonly found are mentioned below:

  • Laser photoelectric sensors - Lasers are sometimes used as sensor light sources. Laser photoelectric sensors are available in thru-beam, diffuse scan, and diffuse scan with background suppression versions. Lasers provide high intensity visible light, which enables simple assembly and adjustment. Laser technology makes it possible to detect extremely small objects at a distance.
  • Fiber optics photoelectric sensors – These sensors use an emitter, receiver, and a flexible cable that is full of tiny fibers meant to transmit light. When glass fibers are used, the emitter source is infrared light. When plastic fibers are used, the emitter source is visible light. Fiber optics can be adapted to thru-beam, retro-reflective scan, or diffuse scan sensors. Fiber optics works best for small sensing areas or small objects.
  • Remote photoelectric sensors – These are used for remote sensing and contain only the optical components of a sensor.

Working Principle

Photoelectric sensors provide three basic methods of target detection:

  • Diffused mode or proximity-sensing mode - In this sensing mode, the transmitter and receiver are placed in the same housing. Light from the transmitter strikes the target, which reflects the light at arbitrary angles. Some of the reflected light returns to the receiver, and the target is detected. Other variations to this mode include diffused convergent beam mode, diffused mode with background suppression, and diffused mode with mechanical background suppression.
  • Retro-reflective mode – Like in the previous mode, the transmitter and receiver are in the same housing. 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. Other variations to this mode include retro-reflective mode for clear object detection, retro-reflective mode with foreground suppression.
  • Thru-beam mode or opposed mode – The transmitter and the receiver are placed in two separate housings. The light from the transmitter strikes the receiver and when a target breaks this light beam, the output on the receiver is activated. This mode is considered as the most efficient of the three modes because it allows the longest possible sensing ranges for photoelectric sensors.

There are two operating modes followed by photoelectric sensors:

  • Dark Operate (DO) - Here the load is energized when light from the emitter is absent from the receiver.
  • Light Operate (LO) - Here the load is energized when light from the emitter reaches the receiver.

Applications

The following are the key applications of photoelectric sensors:

  • Detect changes in a target’s color, contrast and luminescence
  • Detect porous targets, and invisible markings on products
  • Detect the presence or movement of an object within a defined sensing area or zone
  • Detect the level of contents in a hopper
  • Check products passage in a rinsing process
  • Locate the position of an automated storage and retrieval system
  • Detect the presence/absence of contents in a bottle/ milk carton
  • Check the passage of cars on a conveyer
  • Check the seating of workpieces for an NC processor
  • Check positioning of cars in final assembly line
  • Check passage of engine block
  • Verify fill level of coffee in cans
  • Count bottles moving on a high speed conveyor
  • Detect missing labels on bottles
  • Ensure safely control at the opening and closing of garage doors
  • Enable turning on sink faucets with the wave of a hand
  • Control elevators, and open the doors at the grocery store
  • Detect the winning car at racing events.

References

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