What is a Sensor ?
In simple, sensor is a device that measures a physical quantity and converts it into a signal which can be used for further processing.
CONTENTS:
* Classification of Sensors
* Types of Sensors
* Advanced Sensor Technologies
* Applications
* Classification Of Sensors:
1). Primary Input quantity
2). Transduction principles
3). Material and Technology
4). Property
5). Applications
Classification based on property is as given below:
(i) Temperature - Thermistors, Thermocouples, RTD’s, IC and many more.
(ii) Pressure - Fiber optic, Vacuum, Elastic liquid based manometers, LVDT etc.
(iii) Flow - Electromagnetic, Differential pressure, Positional displacement, Thermal mass, etc.
(iv) Level - Differential pressure, Ultrasonic radio frequency, radar, Thermal displacement, etc.
(v) Proximity and displacement - LVDT, Photoelectric, Capacitive, Magnetic, Ultrasonic.
(vi) Bio sensors - Resonant mirror, Electrochemical, Surface plasmon resonance, Light addressable potentio-metric.
(vii) Image - Charge coupled devices, CMOS.
(viii) Gas and chemical - Semiconductor, Infrared, Conductance, Electrochemical.
(ix) Acceleration - Gyroscopes, Accelerometers.
(x) Others - Moisture, Humidity, Mass, Force, Viscosity.
Classification based on Application is as given below:
(i) Industrial process control, measurement and automation .
(ii) Non-industrial use – Aircraft, Medical products, Automobiles, Consumer electronics, other type of sensors.
Classification based on power or energy supply requirement of the sensors:
(i) Active Sensor - Sensors that require power supply.
Example: LiDAR (Light detection and ranging), photo-conductive cell.
(ii) Passive Sensor - Sensors that do not require power supply.
Example: Radiometers, film photography.
Classification based on applications, sensors can be classified into groups as follows:
(i) Accelerometers - These are based on the Micro Electromechanical sensor technology. They are used for patient monitoring which includes pace makers and vehicle dynamic systems.
(ii) Bio sensors - These are based on the electrochemical technology. They are used for food testing, medical care device, water testing, and biological warfare agent detection.
(iii) Image Sensors - These are based on the CMOS technology. They are used in consumer electronics, bio-metrics, traffic and security surveillance and PC imaging.
(iv) Motion Detectors - These are based on the Infra Red, Ultrasonic, and Microwave/radar technology. They are used in video-games and simulations, light activation and security detection.
* Types of Sensors:
Some commonly used sensors are,
1. Temperature Sensors:
This device collects information about temperature from a source and converts into a form that is understandable by other device or person.
(i) Thermocouple –
They are made of two wires (each of different homogeneous alloy or metal) which form a measuring junction by joining at one end. This measuring junction is open to the elements being measured. The other end of the wire is terminated to a measuring device where a reference junction is formed. The current flows through the circuit since the temperature of the two junctions are different. The resulted milli-voltage is measured to determine the temperature at the junction.
(ii) Resistance Temperature Detectors (RTD) –
These are types of thermal resistors that are fabricated to alter the electrical resistance with the alteration in temperature. They are very expensive than any other temperature detection devices.
(iii) Thermistors –
They are another kind of thermal resistor where a large change in resistance is proportional to small change in temperature.
2. IR Sensor:
This device emits and/or detects infrared radiation to sense a particular phase in the environment. Generally, thermal radiation is emitted by all the objects in the infrared spectrum. The infrared sensor detects this type of radiation which is not visible to human eye.
3. UV Sensor:
These sensors measure the intensity or power of the incident ultraviolet radiation. This form of electromagnetic radiation has wavelengths longer than x-rays but is still shorter than visible radiation. An active material known as poly-crystalline diamond is being used for reliable ultraviolet sensing. UV sensors can discover the exposure of environment to ultraviolet radiation.
4. Touch Sensor:
A touch sensor acts as a variable resistor as per the location where it is touched.
A touch sensor is made of:
(i) Fully conductive substance such as copper,
(ii) Insulated spacing material such as foam or plastic,
(iii) Partially conductive material.
5. Proximity Sensor:
A proximity sensor detects the presence of objects that are nearly placed without any point of contact. Since there is no contact between the sensors and sensed object and lack of mechanical parts, these sensors have long functional life and high reliability. The different types of proximity sensors are Inductive Proximity sensors, Capacitive Proximity sensors, Ultrasonic proximity sensors, photoelectric sensors, Hall-effect sensors, etc.
* Advanced Sensor Technologies:
1. Bar-code Identification -
The products sold in the markets has a Universal Product Code (UPC) which is a 12 digit code. Five of the numbers signify the manufacturer and other five signify the product. The first six digits are represented by code as light and dark bars. The first digit signifies the type of number system and the second digit which is parity signifies the accuracy of the reading. The remaining six digits are represented by code as dark and light bars reversing the order of the first six digits. The bar code reader can manage different bar code standards even without having the knowledge of the standard code. The disadvantage with bar coding is that the bar scanner is unable to read if the bar code is concealed with grease or dirt.
2. Transponders -
In the automobile section, Radio frequency device is used in many cases. The transponders are hidden inside the plastic head of the key which is not visible to anyone. The key is inserted in the ignition lock cylinder. As you turn the key, the computer transmits a radio signal to the transponder. The computer will not let the engine to ignite until the transponder responds to the signal. These transponders are energized by the radio signals.
3. Electromagnetic Identification of Manufactured Components -
This is similar to the bar code technology where the data can be coded on magnetic stripe. With magnetic striping, the data can be read even if the code is concealed with grease or dirt.
4. Surface Acoustic Waves -
This process is similar to the RF identification. Here, the part identification gets triggered by the radar type signals and is transmitted over long distances as compared to the RF systems.
5. Optical Character Recognition -
This is a type of automatic identification technique which uses alphanumeric characters as the source of information. In United States, Optical character recognition is used in mail processing centers. They are also used in vision systems and voice recognition systems.
* Applications:
Sensors are used in many kinds of applications such as:
1). Shock Detection
2). Machine monitoring applications
3). Vehicle dynamics
4). Low power applications
5). Structural Dynamics
6). Medical Aerospace
7). Nuclear Instrumentation
8). As pressure sensor in Mobiles ‘touch key pad’
9). Lamps which brighten or dim on touching its base
10). Touch sensitive buttons in elevators
