2. Sensor classification and Thermal sensor

1.1 Sensor:

A device which senses the physical changes in the surrounding and converts into a reliable quantity or electrical signal.

Classification of Sensor (Based on Physical Parameter)

1.Temperature

2.Pressure

3.Flow

4.Level

5.Proximity

6.Bio parameter

7.Gas and chemical

8.Humidity ,speed.

Classification of Sensor (Based on application)

1.Industrial Process control

2.Medical Application

3.Aircraft control

Classification based on technology used:

 1.Thermal sensor

2.Optical sensor

3.Magnetic sensor

4.Electric sensor

1.2 Thermal sensor

Application :used for temp. measurement

Types of Thermal Sensor

A)   Contact type

1) Thermocouple

  2) RTD

  3) Thermistor

  4) IC/Semiconductor type (example LM35)

•Voltage output

•Digital output

•Analog Output

•Resistance Output

  5) liquid in glass

B)Non contact type

 Pyrometer 

Liquid in Glass Thermal sensor

Figure 1. Liquid in glass Thermal Sensor.

Working Principle:-

The liquid-in-glass (LiG) thermometer is one of the most common thermometers used now a day. A LiG thermometer, by definition,consist a liquid-filled bulb at bottom and a glass capillary tube at another side as temperature of liquid increases, it expands and rises into capillary tube.

The level marked on the outer side of the liquid in the column corresponds to a specific temperature.

Usually liquid used in thermometer is mercury, toluene and low-hazard biodegradable liquids.

Advantages:

cheap cost

simple to use

do not require external power for functioning

reasonably accurate

Disadvantages:

Delicate 

Not highly accurate

Human error are more.

Non contact type: Pyrometer

Optical sensor family.

Used in metallurgy to measure temp of hot molten metal.

The object at high temp radiates infrared energy at wavelength of above 0.78 micron.

This energy is captures and passed onto optical sensor.

Types of pyrometer

A) Radiation Pyrometer

B) Optical Pyrometer

C) Photoelectric Pyrometer 

A) Radiation Pyrometer:

Figure 2 : Radiating Pyrometer.

Temperature range 1200 -3500 degree Celsius.

Does not make direct contact with hot body but pick up radiation of hot body with a tube

aperture arrangement.

The radiation are applied on thermocouple or thermopile.

  The law of radiation is given by the expression:

W= K(T⁴  -T₀⁴)   ( 2.1)

W- energy per square meter per second received by cold body

K- constant (value is obtained through experimentation)

T- Absolute temperature of hot body

T₀ -Absolute temperature of surrounding

Radiation Pyrometer are of Two types :

Fixed Focus Type

Variable Focus Type

Optical Pyrometer:

Photoelectric Pyrometer:

Figure 03.Photoelectric Pyrometer

Operates on the principle of automatically matching the radiation from standard source and 

radiation from target surface.

Lens concentrate the radiation from hot body to sighting tube,the photo emissive vacuum 

tube produces an electron current which is proportional to radiation intensity.

the response from photo tube is amplified with an amplifier unit.

Optical Sensor

Light Signal -> Electrical Signal

 Optical sensor Classification Based On Output signal :

•Photoconductive- Change in light causes change in resistance can change in 

electrical output

•Photovoltaic- change in incident light causes change in output voltage.

•Photodiode- change in incident light causes change in output voltage.

 Optical sensor Classification Based On transmitter receiver used :

•Through beam sensor

•Retro reflective sensor (reflected from rotor/shaft)

•Diffuse reflection sensor (Led, Laser)

Advantages of optical sensor 

•Operation not affected by external magnetic field    

•Remote operation

•Can be used in hazardous or explosive environment

•Compact size

•Convenient arrangement    

Disadvantage:

 malfunction due to interference of light     

Magnetic Sensor

Magnetic Field à Electric signal

Types of Magnetic Sensor

•Low range magnetic field sensor

•Earth magnetic field sensor / medium range

•BIAS Magnetic field sensor

•

Low range magnetic field sensor

1) Super conducting Quantum interface Device (SQUID)

•Josephson effect

•Current is passed through super conducting loop

•When a magnetic field is not present around the loop current is distributed uniformly.

Figure 04: Josephson Junction.

Application:

•Used in biology field

•MEG (Magnetoencephalography) which maps the neural activity inside brain.

•Cardiology

2) Fiber Optic sensor –Polarization Processing—detect magnetic field

Earth magnetic field sensor / medium range

Uses Earth magnetic field for navigation and vehicle detection

Example: 1. Hall effect sensor    2. Magneto resistor 

Figure:05 Hall Sensor.

Figure 06:Hall Effect. 

When semiconductor material carrying current  kept perpendicular to magnetic field it produces emf at edges.