Displacement Measurement

What is an optical proximity sensor?

The optical proximity sensor can be used to detect the position, so basically they are position sensors. This is a type of sensor that can convert signals generated by light emission into electrical signals. The response of the optical receiver would vary according to the wavelength. This is also a contact-free position sensor and its sensing range is up to 10 meters. The major part of an optical sensor is a light source which could be an LED and a detector. The optical proximity sensor can detect materials such as solid, liquid, powder, and opaque. The optical proximity sensor has an electronic part that would evaluate and amplify the detected signal. These sensors are widely used in automated systems because they are smaller in size. This device can detect the distance and presence of an object by using light.

The light source in this device could be infrared or visible light, and the generated light frequency will be able to detect by the light sensor and it must not be generated by the nearby devices. The most common type of light source will be infrared light because they have high intensity. The infrared light is less susceptible to interference. If we need to get reliable operation from an optical proximity sensor then we must consider certain factors, they are the optical proximity sensor must be operated at a sufficient operating margin, and we must use this sensor with a contamination warning signal.

How is optical proximity sensor constructed?

The major part of an optical proximity sensor is an emitter and a receiver, according to the type of applications reflectors and fiber optic cables are used.  The emitter would house the infrared light source and a receiver is used to detect the light, the receiver could be a photodiode or photo transmitter.

What are the types of optical proximity sensors?

Through beam sensor

This type of optical sensor has a separately assembled receiver and transmitter and it has a good sensing range. The emitter would emit the light and it would reach the receiver and the object that interrupts the beam would be detected. This type of optical sensor has a good operating range, and it could detect an object at a distance of up to 50 meters and this sensor can operate in bad environmental conditions.

What are the advantages of through beam sensor

  • Wide operating range
  • It can detect small objects which are situated at a large distance
  • Positioning accuracy is better
  • Good reliability due to the permanent light when it is not operating

What are the disadvantages of through beam sensor?

  • The emitter and receiver would need separate electrical connections
  • It is not suitable for transparent objects

Retro reflective sensors

In this type of optical sensor, the light emitter and receiver would be installed in a single housing. This sensor uses an extra reflector, the operation of this sensor is similar to the through-beam in that a target object that passes in front of the sensor would block the light being received. While in this type of sensor the light that is blocked is the light that is returning from a reflector and because of this it doesn’t need any additional wiring for the remotely located receiver unit.

What are the advantages and disadvantages of retro reflective sensor?

The advantages are installation is really simple, good range and good reliability. The major disadvantage is that it won’t be able to detect transparent, bright, or shiny objects.

Diffuser sensor

In this type of sensor, the emitter and the receiver are in the same housing, the object would reflect a small amount of emitted light and this would activate the receiver the output is then switched through or switched off. The distance of the switching is related to the object reflectivity.

What are the advantages of the diffuser sensor?

  • Frontal detection is possible in the direction of the light beam
  • Less wiring because the emitter and the receiver is in the same housing
  • No need for an additional reflector

What are the disadvantages of the diffuser sensor?

  •  It won’t detect the transparent object
  •  Low reflective target is not properly detected

What are the advantages of optical proximity sensor?

  • Greater sensitivity
  • There is no electromagnetic interference
  • Wide dynamic range
  • Multiplexing capabilities
  • Both point and distributed configuration
  • It can sense all kinds of materials
  • Long life
  • Quick response
  • It can be used in a vacuum

What are the disadvantages of optical proximity sensor?

  • The lens could be contaminated
  • The color and reflectivity of the target would affect the sensing range
  • These sensors could be exposed to contamination such as dust and lubricants while operating

What are the applications of optical proximity sensors?

  • Alarm system
  • Computers
  • Copy machines
  • It is used in light fixtures that turn on automatically in the dark
  • Synchros for photographic flashes


post-graduate in Electronics & communication.

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