Sensors

Hi viewers,I hope that almost all of us are very familiar with the sensors because it is all most used in every day applications today just I want to make you remember about the sensors.

 

 Sensor is mainly defined as a device that detects certain external stimuli and responds to it in a distinctive manner. these sense condition of the process variable and produce an output
which reflects the same condition.And it can be also defined as the device which measures a physical quantity and converts it into signal which can be read by the instrument or an observer.

Uses:-
As I mentioned earlier it is used in every day objects like touch sensitivity elevator buttons (tactile sensors)
and lamps which dim or brighten by touching the base. There are infinite applications of the sensors of which most of the people are never aware, a few of them include cars, machines,
aerospace, medicine manufacturing and robotics.

The sensors may be classified as :
1. Proximity sensors
2. Pneumatic sensors
3. Light sensor

1. Proximity sensors:-
   A Proximity sensor consists of an element that changes eitherits state or an analog signal when it is close to it,but often not actually touching an object..
 Magnetic,electrical capacitanc, inductance and Eddy current methods are particlarly suited to design a proximity sensor.
Common applications for proximity sensors are:
1. Counting moving objects
2. Limiting the traverse of a mechanism.

The proximity sensors conatins three types, they are:-

• Eddy current proximity sensors.
• Capacitance Proximity sensors.
• Inductive proximity sensors.

• Eddy Current proximity sensors:-

Working principle:
When a coil is supplied with an alternating current an alternating magnetic field produce. If there is a metal object in close proximity to this attending magnetic field, the eddy currents are
induced in it. The eddy currents themselves produce a magnetic field which distorts the magnetic field responsible for their production. Consequently, the impedance of the coil changes and so
the amplitude of the alternating current. This change, at some present level, can be used to trigger a switch.

Advantages:
1. small in size.
2. Relatively inexpensive.
3. High flexibilty.
4. High sensitivity.

•  Capacitance Proximity Sensors:-

 It consists of a simple plate with the object acting as other plate.
As the object approaches the sensor,the separation between the plate of the capacitor and object changes which becomes significant as the object is close to the sensor.

• Inductive Proximity sensor:-

An inductive proximity sensor consists of a coil wound round core. when the end of the coil is close to a metal object its inductance changes. This  change can be monitored by its effect on resonant circuit and   the change used to trigger a switch.

It can only be used for the detection of metal objects and is best with ferrous metals

2. Pneumatic Sensors:-

Pneumatic Sensor

These sensors involve the use of compressed air, displacement or proximity of an object being transformed into a change in air pressure.

Operation:

Low pressure air is allowed to escape through a port in front of the sensor. This escaping air, in the absence of any close by object, escapes and in doing so also reduces the pressure in the near by sensor output port. However, if there is a close by object , the air cannot so readily escape and result is that the pressure increases in the sensor output port. The output pressure from the sensor thus depends on the proximity sensors.

Pneumatic sensors are used for the measurement of the displacements of fractions of millimeters in ranges which typically are about 3 to 12mm.

3. Light sensors:-

• photo diodes
• photo transistors
•  photo resistors

• Photodiodes:-

Photodiodes

Photodiodes are semiconductor junction diodes which are connected into a circuit in reverse bias, so giving a very high resistance, so that when light falls on the junction , the diode resistance drops and the current rises appreciably. these are used as a variable resistance device controlled by the light incident on it.These are used as a fastest photo detector, both for visible light and invisible light.

• Phototransistors:- 

The phototransistors have a light sensitive collector-base P-N junction. When there is no incident light there is a very small collector-to-emitter current. When light is incident , a base current is produced that is directly proportional to the light intensity. This leads to the production of a collector current which is then a measure of the light intensity. 

• Photoresistors:-

It has a resistance which depends upon the intensity of the light falling on it, decreasing linearly as the intensity increases. The cadmium sulphide photo resistor is most responsive to light having wavelengths shorter than about 515nm and cadmium selenide photoresistor for wavelength is less than about 700nm.
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