Types of Sensors

Some commonly used sensors alongwith their principle and applications are explained as follows:

        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. The best illustration of a temperature sensor is mercury in glass thermometer. The mercury in the glass expands and contracts depending on the alterations in temperature. The outside temperature is the source element for the temperature measurement. The position of the mercury is observed by the viewer to measure the temperature. There are two basic types of temperature sensors:

·        Contact Sensors – This type of sensor requires direct physical contact with the object or media that is being sensed. They supervise the temperature of solids, liquids and gases over a wide range of temperatures.

·        Non contact Sensors – This type of sensor does not require any physical contact with the object or media that is being sensed. They supervise non-reflective solids and liquids but are not useful for gases due to natural transparency. These sensors use Plank’s Law to measure temperature. This law deals with the heat radiated from the source of heat to measure the temperature.

Working of different types of Temperature Sensors along with examples

(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. The diagram of thermocouple is shown below.(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. The diagram of Resistance Temperature Detectors is shown below.  

(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. Theinfrared sensor detects this type of radiation which is not visible to human eye.

Advantages

·        Easy for interfacing

·        Readily available in market

Disadvantages

·        Disturbed by noises in the surrounding such as radiations, ambient light etc.

Working

The basic idea is to make use of IR LEDs to send the infrared waves to the object. Another IR diode of the same type is to be used to detect the reflected wave from the object. The diagram is shown below.

When IR receiver is subjected to infrared light, a voltage difference is produced across the leads. Less voltage which is produced can be hardly detected and hence operational amplifiers (Op-amps) are used to detect the low voltages accurately.

Measuring the distance of the object from the receiver sensor: The electrical property of IR sensor components can be used to measure the distance of an object. The fact when IR receiver is subjected to light, a potential difference is produced across the leads.

Applications

·        Thermography – According to the black body radiation law, it is possible to view the environment with or without visible illumination using thermography

·        Heating – Infrared can be used to cook and heat food items. They can take away ice from the wings of an aircraft. They are popular in industrial field such as, print dying, forming plastics, and plastic welding.

·       Spectroscopy – This technique is used to identify the molecules by analysing the constituent bonds. This technique uses light radiation to study organic compounds.

·        Meteorology – Cloud heights, calculate land and surface temperature is possible when weather satellites are equipped with scanning radiometers.

·        Photobiomodulation – This is used for chemotherapy in cancer patients. This is used to treat anti herpes virus.

·        Climatology – Monitoring the energy exchange between the atmosphere and earth.

·       Communications – Infra red laser provide light for optical fibre communication. These radiations are also used for short range communications among mobiles and computer peripherals.

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 polycrystalline diamond is being used for reliable ultraviolet sensing. UV sensors can discover the exposure of environment to ultraviolet radiation.

Criteria to select a UV Sensor

·        Wavelength ranges in nanometres (nm) that can be detected by the UV sensors.

·        Operating temperature

·        Accuracy

·        Weight

·        Power range

Working

The UV sensor accepts one type of energy signal and transmits different type of energy signals.

To observe and record these output signals they are directed to an electrical meter. To create graphs and reports, the output signals are transmitted to an analog-to-digital converter (ADC), and then to a computer with software.

Examples include:

·        UV phototubes are radiation-sensitive sensors supervise UV air treatments, UV water treatments, and solar irradiance.

·        Light sensors measure the intensity of incident light.

·        UV spectrum sensors are charged coupled devices (CCD) utilized in scientific photography.

·        Ultraviolet light detectors.

·        Germicidal UV detectors.

·        Photo stability sensors.

Applications

·        Measures the portion of the UV spectrum which sunburns human skin

·        Pharmacy

·        Automobiles

·        Robotics

·        Printing industry for solvent handling and dyeing processes

·        Chemical industry for the production, storage, and transportation of chemicals

 4.   Touch Sensor

A touch sensor acts as a variable resistor as per the location where it is touched. The figure is as shown below.

A touch sensor is made of:

·        Fully conductive substance such as copper

·        Insulated spacing material such as foam or plastic

·        Partially conductive material

Principle and Working

The partially conductive material opposes the flow of current. The main principle of the linear position sensor is that the current flow is more opposed when the length of this material that must be travelled by the current is more. As a result, the resistance of the material is varied by changing the position at which it makes contact with the fully conductive material.

Generally, softwares are interfaced to the touch sensors. In such a case, a memory is being offered by the software. They can memorize the ‘last touched position’ when the sensor is deactivated. They can memorize the ‘first touched position’ once the sensor gets activated and understand all the values related to it. This act is similar to how one moves the mouse and locates it at the other end of mouse pad in order to move the cursor to the far side of the screen.

Applications

The touch sensors being cost effective and durable are used in many applications such as

·        Commercial – Medical, vending, Fitness and gaming

·        Appliances – Oven, Washing machine/dryers, dishwashers, refrigerators

·        Transportation – Cockpit fabrication and streamlining control among the vehicle manufacturers

·        Fluid level sensors

·        Industrial Automation – Position and liquid level sensing, human touch control in automation applications

·        Consumer Electronics – Provides a new feel and level of control in various consumer products

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.

Working 

A proximity sensor emits an electromagnetic or electrostatic field or a beam of electromagnetic radiation (such as infrared), and waits for the return signal or changes in the field. The object which is being sensed is known as the proximity sensor's target.

Inductive Proximity sensors – They have an oscillator as input to change the loss resistance by the proximity of an electrically conductive medium. These sensors are preferred for metal targets.

Capacitive Proximity sensors – They convert the electrostatic capacitance variation flanked by the detecting electrode and the ground electrode. This occurs by approaching the nearby object with a variation in an oscillation frequency. To detect the nearby object, the oscillation frequency is transformed into a direct current voltage which is compared with a predetermined threshold value. These sensors are preferred for plastic targets.

Applications

·        Used in automation engineering to define operating states in process engineering plants, production systems and automating plants

·        Used in windows, and the alarm is activated when the window opens

·        Used in machine vibration monitoring to calculate the difference in distance between a shaft and its support bearing

Principle

Different definitions are approved to distinguish sensors and transducers. Sensors can be defined as an element that senses in one form of energy to produce a variant in same or another form of energy. Transducer converts the measurand into the desired output using the transduction principle.

Based on the signals that are obtained and created, the principle can be categorized into following groups namely, Electrical, Mechanical, Thermal, Chemical, Radiant, and Magnetic.

Let’s take the example of an ultrasonic sensor.

An ultrasonic sensor is used to detect the presence of an object. It achieves this by emitting ultrasonic waves from the device head and then receiving the reflected ultrasonic signal from the concerned object. This helps in detecting the position, presence and movement of objects.

Since ultrasonic sensors rely on sound rather than light for detection, it is widely used to measure water-levels, medical scanning procedures and in the automobile industry. Ultrasonic waves can detect transparent objects such as transparent films, glass bottles, plastic bottles, and plate glass, using its Reflective Sensors.

Working

The movement of ultrasonic waves differ due to shape and type of media. For example, ultrasonic waves move straight in a uniform medium, and are reflected and transmitted back at the boundary between differing media. A human body in air causes considerable reflection and can be easily detected.

The travelling of ultrasonic waves can be best explained by understanding the following:

1.     Multi-reflection 

Multi-reflection takes place when waves are reflected more than once between the sensor and the detection object.

2.      Limit zone
The minimum sensing distance and maximum sensing distance can be adjusted. This is called the limit zone.

3.     Undetection zone

The undetected zone is the interval between the surface of the sensor head and the minimum detection distance resulting from detection distance adjustment. The figure is shown below. 

The Undetection zone is the area close to the sensor where detection is not possible due to the sensor head configuration and reverberations. Detection may occur in the uncertainty zone due to multi-reflection between the sensor and the object.

Applications

Sensors are used in many kinds of applications such as:

·        Shock Detection

·        Machine monitoring applications

·        Vehicle dynamics

·        Low power applications

·        Structural Dynamics

·        Medical Aerospace

·        Nuclear Instrumentation

·        As pressure sensor in Mobiles ‘touch key pad’

·        Lamps which brighten or dim on touching its base

·        Touch sensitive buttons in elevators

Advanced Sensor Technology

Sensor technology is used in wide range in the field of Manufacturing. The advanced technologies are as follows:

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. Bar code is shown in the figure given below.

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. The figure of a transponder is as shown below:

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 centres. They are also used in vision systems and voice recognition systems.