Basics of sensors.

In today's post we will be discussing about the working of different types of sensors.
To start with let us discuss the definition of a simple sensor.

  

"A sensor is a converter that measures a physical quantity and converts it into a signal which can be read by an observer or by an (today mostly electronic) instrument."

Its not necessary that a sensor will always be an electronic component. Sensors can be of natural type too. Our body too exhibit different sensing capabilities.

Electronically speaking a sensor is a device, which responds to an input quantity by generating a functionally related output usually in the form of an electrical or optical signal. A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes.

The different types of sensors that we use for robotic purposes are:-

1. Infrared sensors
2. Thermal sensors
3. Sound sensors
4. Pressure sensors
5. Ultrasonic sensors etc

For basic purposes like creating grid solvers , obstacle avoiders etc the IR sensors are commonly used . The other named sensors are used for advanced purposes.

Now we will study the working of an IR sensor with its circuitory.         

Working  principle of an IR sensor :

Infra red circuit diagram

A typical system for detecting infrared radiation using infrared sensors includes the infrared source such as blackbody radiators, tungsten lamps, and silicon carbide. In case of active IR sensors, the sources are infrared lasers and LEDs of specific IR wavelengths. Next is the transmission medium used for infrared transmission, which includes vacuum, the atmosphere, and optical fibers.

Thirdly, optical components such as optical lenses made from quartz, CaF₂, Ge and Si, polyethylene Fresnel lenses, and Al or Au mirrors, are used to converge or focus infrared radiation. Likewise, to limit spectral response, band-pass filters are ideal.

Finally, the infrared detector completes the system for detecting infrared radiation. The output from the detector is usually very small, and hence pre-amplifiers coupled with circuitry are added to further process the received signals.

 There are different types of IR sensors working in various regions of the IR spectrum              but the physics behind "IR sensors" is governed by three laws:

1.         Planck’s radiation law:

Every object at a temperature T not equal to 0 K emits radiation. Infrared radiant energy is determined by the temperature and surface condition of an object. Human eyes cannot detect differences in infrared energy because they are primarily sensitive to visible light energy from 400 to 700 nm. Our eyes are not sensitive to the infrared energy.

2.         Stephan Boltzmann Law

The total energy emitted at all wavelengths by a black body is related to the absolute temperature as

3.         Wein’s Displacement Law

Wein’s Law tells that objects of different temperature emit spectra that peak at different wavelengths. It provides the wavelength for maximum spectral radiant emittance for a given temperature.

The relationship between the true temperature of the black body and its peak spectral exitance or dominant wavelength is described by this law

The world is not full of black bodies; rather it comprises of selectively radiating bodies like rocks, water, etc. and the relationship between the two is given by emissivity (E).

Emissivity depends on object color, surface roughness, moisture content, degree of compaction, field of view, viewing angle & wavelength.

ELEMENTS OF INFRARED DETECTION SYSTEM

A typical system for detecting infrared radiation is given in the following block diagram :
1. Infrared Source: All objects above 0 K radiate infrared energy and hence are infrared sources. Infrared sources also include blackbody radiators, tungsten lamps, silicon carbide, and various others. For active IR sensors, infrared Lasers and LEDs of specific IR wavelengths are used as IR sources. 

2. Transmission Medium: Three main types of transmission medium used for Infrared transmission are vacuum, the atmosphere, and optical fibers.The transmission of IR – radiation is affected by presence of CO2, water vapour and other elements in the atmosphere. Due to absorption by molecules of water carbon dioxide, ozone, etc. the atmosphere highly attenuates most IR wavelengths leaving some important IR windows in the electromagnetic spectrum; these are primarily utilized by thermal imaging/ remote sensing applications.
• Medium wave IR (MWIR:3-5 µm)     •  Long wave IR (LWIR:8-14 µm)Choice of IR band or a specific wavelength is dictated by the technical requirements of a specific application. 
3. Optical Components: Often optical components are required to converge or focus infrared radiations, to limit spectral response, etc. To converge/focus radiations, optical lenses made of quartz, CaF2, Ge and Si, polyethylene Fresnel lenses, and mirrors made of Al, Au or a similar material are used. For limiting spectral responses, bandpass filters are used. Choppers are used to pass/ interrupt the IR beams. 

4. Infrared detectors: Various types of detectors are used in IR sensors. Important specifications of detectors are:
• Photosensitivity or ResponsivityResponsivity is the Output Voltage/Current per watt of incident energy. Higher the better. 

• Noise Equivalent Power (NEP)NEP represents detection ability of a detector and is the amount of incident light equal to intrinsic noise level of a detector.

• Detectivity(D*: D-star)D* is the photosensitivity per unit area of a detector. It is a measure of S/N ratio of a detector. D* is inversely proportional to NEP. Larger D* indicates better sensing element. In addition, wavelength region or temperature to be measured, response time, cooling mechanism, active area, no of elements, package, linearity, stability, temperature characteristics, etc. are important parameters which need attention while selecting IR detectors.

5.Signal Processing: Since detector outputs are typically very small, preamplifiers with associated circuitry are used to further process the received signals.

Application of IR sensors :-

In robtics the IR sensors are used for various detecting purposes , for e.g to detect nereby objects , in construction of edge avoiders and also in making grid solving intelligent robots.

In our next post we will be discussing about the working of DC geared motors and also the working of the H bridge circuit.

Comments and views will be appreciated.