Proximity sensors



 A proximity sensor is a small device which can detect the presence of any nearby object without any physical contact. These are more reliable because there is no physical contact between the sensor and detected the object.

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Working principle of the proximity sensor is similar to the IR obstacles detector consists of a LED diode and a potoelectric diode. It emits electromagnetic field or any infrared light If there is any change in an electromagnetic signal which results in presence of any object. Through the line

Through the line schematic, it can easily be understood that when the infrared light emits from the IR LED and falls on any element it reflected from that element and received by a photoelectric diode which breaks the circuit.

Types of  proximity sensors

1.inductive proximity sensor
2.capacitive  proximity sensor
3.ultrasonic proximity sensor
4.photoelectric proximity sensor


It works on the principle of the resonance and eddy currents. This sensor is also known as an electronic oscillator. These are basically used to detect the metallic object. It generally consists of an inductive coil made up of very fine copper wire. A capacitor is used to store the charge and a power supply to produce the electrical excitation. They are used currents induced by a magnetic field to detect the metallic objects. In the coil high-frequency magnetic field generated. Now, If there is any metallic object this inductive magnetic field produces a current into the metallic object which results to develop a new magnetic field and this magnetic field opposes the earlier magnetic field. The net effective change in the magnetic field is to changes in inductance of the inductive sensor. 
                                                The object which needs to be detected must be a conductive material and has low resistivity also does not necessarily ferromagnetic.
Application  of inductive type of  proximity sensors

1. wind power plant (wind turbine )

 Capacitive proximity sensors

Capacitive type of proximity sensor is similar to the inductive type of proximity sensor. The only difference is that capacitive proximity sensor produces electrostatic field instead of the electromagnetic field which is produced by an inductive type of proximity sensors. It can be used to detect both type of metal and non-metal objects. They use the variation in the capacitance to detect any nearby object.

Working principle of the capacitive type of proximity sensor

The working principle is based on the variation in the capacitance. It consists of plates, the threshold detector, oscillator and output circuit.
When we put the sensor in working condition an electrostatic field is generated by the capacitor plate and  if there is any object within that field then the capacitance will change as a result oscillator start oscillating. If the target object is very near to the sensor then the capacitance will increase and vice versa (As the target moves away from the sensor the oscillator  amplitude decreases )

Ultrasonic proximity sensor

The main application this is to measure the distance of any target(object) from the reference point. They are mainly used to detect the sound waves.

Working principle of the ultrasonic proximity sensor

It uses a piezoelectric transducer to detect the sound has an acoustic transducer which is vibrating at an ultrasonic frequency. The pulse is emitted in form of cone shaped beam and falls on the target object. These pulses are reflected from the target object and again received by the sensor. The sensor calculates the time between of travel and after velocity can easily be calculated by using basic formula distance = velocity of the sound wave in flow medium*time taken by the sound wave to reflect from the target object to the sensor destination 

Photoelectric proximity sensor 
This sensor is used to measure the distance, absence, or presence of any object.  It uses the light sensitive element to detect the object. It consists of an emitter and a reflector. The emitter emits the light which can be detected by the detector. Photoelectric sensors consist of a light source (usually a LED, light emitting diode, in either infrared or visible light spectrum) and a detector (photodiode). It can easily be used in the dusty environment because of high intensity of the light beam emitted by the emitter.

There are three type of photoelectric proximity sensor

1.retro-reflective photoelectric proximity sensor
2.through beam  photoelectric proximity sensor
3.Diffuse reflective photoelectric proximity sensor 

1. Retro-reflective photoelectric proximity sensor
This type of proximity sensor mainly consists of  three components
3.retro reflector
Emitter and detector are placed in a single housing (package) and a retro reflector placed at a small distance from the emitter detector housing (package). A reflector is also needed.

Disruption of the light between sensor and reflector is detected. It can detect the large separation between the sensor and the object because the emitted light totally reflected by the reflector. It can detect distance about 10 meters.

One reason for using a retro-reflective sensor over a through-beam sensor is for the convenience of one wiring location; the opposing side only requires reflector mounting. This results in cost savings in both parts and time. However, very shiny or reflective objects like mirrors, cans, or plastic-wrapped juice boxes can create a challenge for retro-reflective photoelectric sensors. 

2.Through  beam  photoelectric proximity sensor 

In this type of sensor emitter or transmitter and detector (receiver)  is separated with their housing (packaging). The purchase, installation, and alignment of the emitter and receiver in two opposing locations, which may be quite a distance apart, are costly and laborious. They have industrial and commercial applications. They detect obstructions in the path of garage doors.

3. Diffuse reflective photoelectric proximity sensor 

In this type of sensor target is acts as a reflector. The basic installation is same as the retro reflective proximity sensor.Main components are an only emitter, detector. The sensor sensitivity depends on the target material because it works as the reflector. if we use a non-reflective target like matte black paper then the sensor does not work because this target does not reflect any light wave however it absorbs all the light rays. So it is important to use the reflective targets.





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