SENSORS

  Sensors

 

 

 

 

 

What is sensor? This device is very common used in manufacturing industry,office,house hold,school,highway and every placed around us.It very important to sensing and send a signal to control panel for further application. Basically we can elaborate meaning of sensor in easy understanding are device to detected physical condition such as temperature,pressure,motion,metal and non-metal and many.......

          

 

 

 

 Sensors are sophisticated devices that are frequently used to detect and respond to electrical or optical signals. A Sensor converts the physical parameter (for example:  temperature, blood pressure, humidity, speed, etc.) into a signal which can be measured electrically. 

Let’s explain the example of temperature. The mercury in the glass thermometer expands and contracts the liquid to convert the measured temperature which can be read by a viewer on the calibrated glass tube.

 

Criteria to choose a Sensor

There are certain features which have to be considered when we choose a sensor. They are as given below:

1.     Accuracy

2.     Environmental condition - usually has limits for temperature/ humidity

3.     Range - Measurement limit of sensor

4.   Calibration - Essential for most of the measuring devices as the readings changes with time

5.     Resolution - Smallest increment detected by the sensor

6.     Cost

7.     Repeatability - The reading that varies is repeatedly measured under the same environment 

Classification based on property is as given below:

·        Temperature - Thermistors, thermocouples, RTD’s, IC and many more.

·        Pressure - Fibre optic, vacuum, elastic liquid based manometers, LVDT, electronic.

·        Flow - Electromagnetic, differential pressure, positional displacement, thermal mass, etc.

·        Level - Differential pressure, ultrasonic radio frequency, radar, thermal displacement, etc.

·        Proximity and displacement - LVDT, photoelectric, capacitive, magnetic, ultrasonic.

·    Biosensors - Resonant mirror, electrochemical, surface Plasmon resonance, Light addressable              -            potentio metric.            

·        Image - Charge coupled devices, CMOS

·        Gas and chemical - Semiconductor, Infrared, Conductance, Electrochemical.

·        Acceleration - Gyroscopes,     Accelerometers.

·        Others - Moisture, humidity, mass, force, viscosity.

 

Surface Plasmon resonance and Light addressable potentio-metric from the Bio-sensors group are the new optical technology based sensors. CMOS Image sensors have low resolution as compared to charge coupled devices. CMOS has the advantages of small size, cheap, less power consumption and hence are better substitutes for Charge coupled devices. Accelerometers are independently grouped because of their vital role in future applications like aircraft, automobiles, etc and in fields of videogames, toys, etc. 

 

 

 

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. The infrared 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.

 

 

 

 

 

ELECTRICAL TRANSFORMERS

ELECTRICAL TRANSFORMERS

Most of the electronic circuits used in Circuitstoday.com have different applications of the transformer. Therefore, it is important to know the working principle, construction and types of transformers used in different analog circuits.

Transformer – Working Principle

A transformer can be defined as a static device which helps in the transformation of electric power in one circuit to electric power of the same frequency in another circuit. The voltage can be raised or lowered in a circuit, but with a proportional increase or decrease in the current ratings.

The main principle of operation of a transformer is mutual inductance between two circuits which is linked by a common magnetic flux. A basic transformer consists of two coils that are electrically separate and inductive, but are magnetically linked through a path of reluctance. The working principle of the transformer can be understood from the figure below.

Transformer Working

 As shown above the transformer has primary and secondary windings. The core laminations are joined in the form of strips in between the strips you can see that there are some narrow gaps right through the cross-section of the core. These staggered joints are said to be ‘imbricated’. Both the coils have high mutual inductance. A mutual electro-motive force is induced in the transformer from the alternating flux that is set up in the laminated core, due to the coil that is connected to a source of alternating voltage. Most of the alternating flux developed by this coil is linked with the other coil and thus produces the mutual induced electro-motive force. The so produced electro-motive force can be explained with the help of Faraday’s laws of Electromagnetic Induction as

e=M*dI/dt

If the second coil circuit is closed, a current flows in it and thus electrical energy is transferred magnetically from the first to the second coil.

The alternating current supply is given to the first coil and hence it can be called as the primary winding. The energy is drawn out from the second coil and thus can be called as the secondary winding.

In short, a transformer carries the operations shown below:

1. Transfer of electric power from one circuit to another.
2.  Transfer of electric power without any change in frequency.
3. Transfer with the principle of electromagnetic induction.
4. The two electrical circuits are linked by mutual induction.

 Transformer Construction

For the simple construction of a transformer, you must need two coils having mutual inductance and a laminated steel core. The two coils are insulated from each other and from the steel core. The device will also need some suitable container for the assembled core and windings, a medium with which the core and its windings from its container can be insulated.

In order to insulate and to bring out the terminals of the winding from the tank, apt bushings that are made from either porcelain or capacitor type must be used.

In all transformers that are used commercially, the core is made out of transformer sheet steel laminations assembled to provide a continuous magnetic path with minimum of air-gap included. The steel should have high permeability and low hysteresis loss. For this to happen, the steel should be made of high silicon content and must also be heat treated. By effectively laminating the core, the eddy-current losses can be reduced. The lamination can be done with the help of a light coat of core plate varnish or lay an oxide layer on the surface. For a frequency of 50 Hertz, the thickness of the lamination varies from 0.35mm to 0.5mm for a frequency of 25 Hertz.

Types of Transformers

The types of transformers differ in the manner in which the primary and secondary coils are provided around the laminated steel core. According to the design, transformers can be classified into two:

1.      Core- Type Transformer

In core-type transformer, the windings are given to a considerable part of the core. The coils used for this transformer are form-wound and are of cylindrical type. Such a type of transformer can be applicable for small sized and large sized transformers. In the small sized type, the core will be rectangular in shape and the coils used are cylindrical. The figure below shows the large sized type. You can see that the round or cylindrical coils are wound in such a way as to fit over a cruciform core section. In the case of circular cylindrical coils, they have a fair advantage of having good mechanical strength. The cylindrical coils will have different layers and each layer will be insulated from the other with the help of materials like paper, cloth, micarta board and so on. The general arrangement of the core-type transformer with respect to the core is shown below. Both low-voltage (LV) and high voltage (HV) windings are shown.

Core Type Transformer Cruciform Section

Core Type Transformers

The low voltage windings are placed nearer to the core as it is the easiest to insulate. The effective core area of the transformer can be reduced with the use of laminations and insulation.

2.      Shell-Type Transformer

In shell-type transformers the core surrounds a considerable portion of the windings. The comparison is shown in the figure below.

Core Type and Shell Type Transformer Winding

The coils are form-wound but are multi layer disc type usually wound in the form of pancakes. Paper is used to insulate the different layers of the multi-layer discs. The whole winding consists of discs stacked with insulation spaces between the coils. These insulation spaces form the horizontal cooling and insulating ducts. Such a transformer may have the shape of a simple rectangle or may also have a distributed form. Both designs are shown in the figure below:

Shell Type Transformers Rectangular Form

Shell Type Transformers Distributed Form

 A strong rigid mechanical bracing must be given to the cores and coils of the transformers. This will help in minimizing the movement of the device and also prevents the device from getting any insulation damage. A transformer with good bracing will not produce any humming noise during its working and will also reduce vibration.

A special housing platform must be provided for transformers. Usually, the device is placed in tightly-fitted sheet-metal tanks filled with special insulating oil. This oil is needed to circulate through the device and cool the coils. It is also responsible for providing the additional insulation for the device when it is left in the air.

There may be cases when the smooth tank surface will not be able to provide the needed cooling area. In such cases, the sides of the tank are corrugated or assembled with radiators on the sides of the device. The oil used for cooling purpose must be absolutely free from alkalis, sulphur and most importantly moisture. Even a small amount of moistures in the oil will cause a significant change in the insulating property of the device, as it lessens the dielectric strength of the oil to a great extent. Mathematically speaking,  the presence of about 8 parts of water in 1 million reduces the insulating quality of the oil to a value that is not considered standard for use. Thus, the tanks are protected by sealing them air-tight in smaller units. When large transformers are used, the air tight method is practically difficult to implement. In such cases, chambers are provided for the oil to expand and contract as its temperature increases and decreases. These breathers form a barrier and resists the atmospheric moisture from contact with oil. Special care must also be taken to avoid sledging. Sledging occurs when oil decomposes due to over exposure to oxygen during heating. It results in the formation of large deposits of dark and heavy matter that clogs the cooling ducts in the transformer.

The quality, durability and handling of these insulating materials decide the life of the transformer. All the transformer leads are brought out of their cases through suitable bushings. There are many designs of these, their size and construction depending on the voltage of the leads. Porcelain bushings may be used to insulate the leads, for transformers that are used in moderate voltages. Oil-filled or capacitive-type bushings are used for high voltage transformers.

The selection between the core and shell type is made by comparing the cost because similar characteristics can be obtained from both types. Most manufacturers prefer to use shell-type transformers for high-voltage applications or for multi-winding design. When compared to a core type, the shell type has a longer mean length of coil turn. Other parameters that are compared for the selection of transformer type are voltage rating, kilo-volt ampere rating, weight, insulation stress, heat distribution and so on.

Transformers can also be classified according to the type of cooling employed. The different types according to these classifications are:

1.      Oil Filled Self-Cooled Type

Oil filled self cooled type uses small and medium-sized distribution transformers. The assembled windings and core of such transformers are mounted in a welded, oil-tight steel tanks provided with a steel cover. The tank is filled with purified, high quality insulating oil as soon as the core is put back at its proper place. The oil helps in transferring the heat from the core and the windings to the case from where it is radiated out to the surroundings. For smaller sized transformers the tanks are usually smooth surfaced, but for large size transformers a greater heat radiation area is needed, and that too without disturbing the cubical capacity of the tank. This is achieved by frequently corrugating the cases. Still larger sizes are provided with radiation or pipes.

2.      Oil Filled Water Cooled Type

This type is used for much more economic construction of large transformers, as the above told self cooled method is very expensive. The same method is used here as well- the windings and the core are immersed in the oil. The only difference is that a cooling coil is mounted near the surface of the oil, through which cold water keeps circulating. This water carries the heat from the device. This design is usually implemented on transformers that are used in high voltage transmission lines. The biggest advantage of such a design is that such transformers do not require housing other than their own. This reduces the costs by a huge amount. Another advantage is that the maintenance and inspection of this type is only needed once or twice in a year.

3.      Air Blast Type

This type is used for transformers that use voltages below 25,000 volts. The transformer is housed in a thin sheet metal box open at both ends through which air is blown from the bottom to the top.

E.M.F Equation of a Transformer

Transformer EMF Equation

Let,

 N_A =  Number of turns in primary

 N_B = Number of turns in secondary

Ø_max  = Maximum flux in the core in webers = B_max X A

f   = Frequency of alternating current input in hertz (H_Z)

As shown in figure above, the core flux increases from its zero value to maximum value Ø_max  in one quarter of the cycle , that is in ¼ frequency second.

Therefore, average rate of change of flux = Ø_max/ ¼ f = 4f Ø_maxWb/s

Now, rate of change of flux per turn means induced electro motive force in volts.

Therefore, average electro-motive force induced/turn = 4f Ø_maxvolt

If flux Ø varies sinusoidally, then r.m.s value of induced e.m.f is obtained by multiplying the average value with form factor.

Form Factor = r.m.s. value/average value = 1.11

Therefore, r.m.s value of e.m.f/turn = 1.11 X 4f Ø_max = 4.44f Ø_max

Now, r.m.s value of induced e.m.f in the whole of primary winding

= (induced e.m.f./turn) X Number of primary turns

Therefore,

 E­_A = 4.44f N_AØ_max = 4.44fN_AB_mA

Similarly, r.m.s value of induced e.m.f  in secondary is

E­_B = 4.44f N_B Ø_max = 4.44fN_BB_mA

In an ideal transformer on no load,

V_A = E_A  and V_B = E_B  , where V_B is the terminal voltage

Voltage Transformation Ratio (K)

From the above equations we get

E_B/ E_A = V_B/ V_A = N_B/N_A = K

This constant K is known as voltage transformation ratio.

(1)   If N_B>N_A , that is K>1 , then transformer is called step-up transformer.

(2)   If N_B<1, that is K<1 , then transformer is known as step-down transformer.

Again for an ideal transformer,

Input V_A = output V_A

V_AI_A = V_BI_B

Or, I_B/I_A = V_A/V_B = 1/K

Hence, currents are in the inverse ratio of the (voltage) transformation ratio.