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Showing posts with label Measurement. Show all posts
Showing posts with label Measurement. Show all posts

Saturday, December 28, 2019

December 28, 2019

Energy Meter with Lag Adjustment Devices

Energy Meter with Lag Adjustment Devices


We know that in induction type energy meters, in order to maintain speed of rotation proportional to power “The phase angle between supply voltage and pressure coil flux should be equal to 90o“. However in actual practice, the angle between supply voltage and pressure coil flux is exactly not 90o but few degrees less. Therefore, some lag adjustment devices are used for adjustment of lag angle. Let us consider the figure given beside:
In the beside figure we have introduced another coil which is located on the central limb with number of turns equal to N. This coil is called lag coil. When we give supply voltage to the pressure coil it produces flux F. Now this flux is divided into two parts Fp and Fg, Fp flux cuts the moving disc and also links with the lag coil. Due to lag coil there induces an emf El which lags behind the flux Fp by angle of 90o, also Il is lagging behind El by an angle of 90o. The lagging coil produces a flux Fl. The resultant flux so obtained that cuts the moving disc is the combination of Fl and Fp. Now the resultant value of this flux is in phase with resultant mmf of lag or shading coil and the resultant value of mmf of shading coil can adjusted by using two methods
  1. By adjusting electrical resistance.
  2. By adjusting shading bands.
Let us discuss these points in more detail:
(1) Adjustment of coil resistance:
If electrical resistance in the coil is high the current will be low and hence mmf of the coil decreases therefore the lag angle also decreases. So we have to decrease the resistance,and the resistance can be decreased by using thick wire in coils. So by adjusting electrical resistance we can indirectly adjust lag angle.
(2) By adjusting the shading bands up and down on the central limb we can adjust lag angle because when we move shading bands upward, then they embrace more flux hence the induced emf increases therefore mmf increases with increase in the value of lag angle. When we move shading bands downwards then it will embrace less flux hence the induced emf will decrease therefore mmf decreases with decrease in the value of lag angle. So by adjusting the position of shading bands we can adjust the lag angle.

Friction Compensation

In order to compensate friction forces we have to apply small force in the direction of rotation of disc. This applied force should be independent of load, so that the meter can read correctly at light load also. But over compensation of friction leads to creeping. Creeping may be defined as the continuous rotation of disc only by energizing the pressure coil while there is no current flowing through the current coil. In order to avoid creeping two holes are drilled, which are diametrically opposite to each other on the disc. Due to this, the effective circular eddy current path of the disc is distorted as shown in the figure. Also the center of effective eddy current paths is shifted to C1 from C. Now C1 becomes the equivalent magnetic pole as produced by these eddy currents so the net force on the rotating disc, will tend to move C1 further away from the pole axis C. Thus disc will creep until the drilled hole reaches near the edge of pole, however further rotation of disc is opposed by opposite torque which is produced by above mechanism.

Overload Compensation

Under load conditions the disc continuously moves. Therefore there induced an emf which is due to rotation called dynamically induced emf. Due to this emf the eddy currents are produced which interacts with the series magnetic field to produce breaking torque. Now this breaking torque is directly proportional to square of current hence it continuously increases and opposes the rotation of disc. In order to avoid the production of this self breaking torque, the full load speed of disc is kept as low as possible so that self breaking torque can be reduced. Errors in single phase energy meters: The errors caused by both the system (i.e. driving and braking) are separating written as follows:

Error caused by Driving System

  1. Error Due to Non SymmetricalMagnetic Circuit
    If the magnetic circuit is not symmetrical there produce a driving torque, due to which meter creep.
  2. Error Due to Wrong Phase Angle
    If there is not a proper phase difference between the various phasors then it results in improper rotation of disc. Improper phase angle is due to improper lag adjustment, variation of resistance with temperature or it may be due to abnormal frequency of supply voltage.
  3. Error Due to Wrong Magnitude of Fluxes
    There are several reasons for wrong magnitude of fluxes out of these main reasons are abnormal values of current and voltage.
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December 28, 2019

Watt Hour Meter

Watt Hour Meter


Watt-hour meter is in fact a measuring device which can evaluate and records the electrical power passing through a circuit in a certain time. By implementing the Watt-hour meter, we can know how much amount of electrical energy is used by a consumer or a residence or an electrically powered device or a business. Electrical utilities install these meters at their consumer’s place to evaluate the electrical usage for the purpose of billing. The reading is taken in each one billing period. Usually, the billing unit is Kilowatt-hour (kWh). This is equal to the total usage of electrical energy by a consumer of one kilowatt during a period of one hour and it is also equal to 3600000 joules. The Watt-Hour Meter is often referred as energy meter or electric meter or electricity meter or electrical meter.
Mainly the watt-hour meter comprises of a tiny motor and a counter. The motor will operate by diverting exact fraction of current which is flowing in the circuit to be measured.
The running or turning speed of this motor is directly proportional to the amount of current flow through the circuit. Thus, every revolution of the rotor of the motor is analogous to the given quantity of current flow in the circuit. A counter is attached to the rotor to add and the usage of electrical energy is displayed from the total number of rotor revolutions.

Tampering and Security

Attaching a magnet outside of the old energy meter is the common tampering method seen. The use of the combination of some capacitance and inductive load also result in reduction in rotor speed. The most modern meter can store up the previous value with time and date. So the tampering is avoided. Utilities install remote reporting meters to detect tampering.

Types of Watt Hour Meter

Basically, the watt-hour meter is classified into three different types as follows:
  • Electromechanical type induction meter
  • Electronic energy meter
  • Smart energy meters

Electromechanical Type Induction Meter

In this type of meter, a non-magnetic and electrically conductive aluminium metal disc is made to revolve in a magnetic field. The rotation is made possible with the power passing through it. The rotation speed is proportional to the power flow through the meter. Gear trains and counter mechanisms are incorporated to integrate this power. This meter works by counting the total number of revolutions and it is relative to the usage of energy.
A series magnet is connected in series with the line and that comprises of a coil of few turns with thick wire. A shunt magnet is connected in shunt with the supply and comprises of a coil of large number of turns with thin wire. A braking magnet which is a permanent magnet is included for stopping the disc at the time of power failure and to place the disc in position. This is done by applying a force opposite to the rotation of the disc.
A flux is produced by the series magnet that is directly proportional to the current flow and another flux is produced by the shunt magnet corresponding to the voltage. Because of the inductive nature, these two fluxes lag each other by 90o. An eddy current is developed in the disc which is the interface of the two fields. This current is produced by a force that is corresponding to the product of instantaneous current, voltage and the phase angle among them. A break torque is developed on the disc by the braking magnet positioned over one side of the disc. The speed of the disc becomes constant when the following condition is achieved, Braking torque = Driving torque. The gear arrangement linked with the shaft of the disc is implemented for recording the number of revolution. This is for single phase AC measurement. Additional number of coils can be implemented for different phase configuration.

Electronic Energy Meter


The major feature of the electronic meter other than power usage measurement is that it can display the energy usage on a LED or LCD display. In some advanced meter, the readings can be transmitted to remote areas. It can also record the amount of usage energy in on-peak hours and off-peak hours. In addition, this meter can record the parameters of supply and load like voltages, reactive power used, instantaneous rate of usage demand, power factor, maximum demand etc.

Smart Energy Meter

In this type of meter communication in both directions (Utility to the customer and customer to the utility) is possible. Customer to the utility communication include parameter values, consumption of energy, alarms etc and utility to consumer communication include disconnect/reconnect instructions, automatic meter reading system, upgrading of the software of the meter etc. Modems are implemented in this meter to make communications easy. Communication system includes fiber cable, power line communication, wireless, telephone etc.

Advantages of Different Types of Watt Hour Meter

Electromechanical Energy MeterElectronic Energy MeterSmart Energy Meter
Simple construction.Non-linear loads.Reduction in the need of visit for reading/taking monthly bills.
Accurate and precise measurement.More robust.Tampering of meter can be avoided.
Reliable.Improvement in quality of electrical distribution.Better accuracy.
Cost is low.Highly accurateEfficiency is high.
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December 28, 2019

Measurement of Electrical Energy

Measurement of Electrical Energy


Electrical energy is the product of electrical power and time, and it is measured in joules. It is defined as “1 joule of energy is equal to 1 watt of power is consumed for 1 second’’.
i.e.
Energy and power are closely related. Electrical energy can be measured only when electrical power is known. So first, we understand electrical power. Electrical power is the amount of electrical current that results from a certain amount of voltage or we can say that power is the rate at which energy is delivered. It is measured in watts. Mathematically it is 
written as
The measurement of electrical energy is completely dependent on power which is measured in watt, kilowatts, Megawatts, gigawatts, and time which is measured in an hour. Joule is the smallest unit of energy. But for some bigger calculation, some better unit is required. So, the unit used for electrical energy is watt-hour.
Watts are the basic unit of power in which electrical power is measured or we can say that rate at which electric current is being used at a particular moment.

Units of Electrical Energy

  • Watt-hour is the standard used for measurement of energy, describing the amount of watts used over time. It shows how fast the power is consumed in the period of time.
  • Kilowatt-hour is simply a bigger unit of energy when large appliances drawn power in kilowatts. It can be described as one kilowatt hour is the amount of energy drawn by the 1000 watts appliance when used for an hour.
    Where, One kilowatt = 1000 watts
  • Megawatt-hour is the unit of energy which normally used when output of power generation is very large. In power plant,’ Megawatt’ term is used to show the power generation capacity of power plant.
    • Solved Example for Showing Calculation
    Problem – A consumer uses a 8 KW geezer, a 5 KW electric press and four 100 watt bulbs for 10 hours. How many units (KWh) of electrical energy have been used.
    Solution –

    Time taken = 10 hours
    Therefore



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Energy Meter Testing
December 28, 2019

Energy Meter Testing

Energy Meter Testing


We cannot think of life without electricity and when there is electricity consumption there is a need to measure its consumption. Here energy meter comes into picture. In every residence, malls, industry, everywhere energy meters are used to measure the electrical energy consumed. Those consumers which consume large energy needs better technology to manage their energy consumption and need more data to improve their services. Improvement in energy meter technology has increased the value-added features such as remote sensing, LCD display, recording of tempering events, and many more quality monitoring features in it, along with compactness of size. But it has raised the problem of electromagnetic interference which affects the performance of the equipment. So for better reliability, energy meters have to pass through various electromagnetic compatibility (EMC) tests where meters are compared under various normal and abnormal conditions with a laboratory to ensure its accuracy in the field.

Standard Tests for Energy Meters

The performance tests of an energy meter as per IEC standards are divided mainly in three segments which include its mechanical aspects, electrical circuiting, and climatic conditions.
  1. Mechanical component tests.
  2. Climatic conditions test include those limits which influence the performance of the meter externally.
  3. Electrical requirements covered many tests before giving accuracy certificate. Under this segment, energy meter is tested for:
  • Heating effect
  • Proper insulation
  • Supply of voltage
  • Protection to earth fault
  • Electromagnetic compatibility

Electromagnetic Compatibility Test

An electromagnetic compatible test is the most important test which finally ensures the accuracy of the energy meter. This test is fragmented in two parts- one is Emission tests, and the other is Immunity test. The electromagnetic interference problem is very common today.
Those circuits in use today, can emit electromagnetic energy which can affect the performance and reliability of both its inner circuitry and the nearby equipment. EMI can travel through conduction or by radiation. When EMI goes through the wire or through cables, it is called conduction. When it travels through free space, it is called radiation.

Emission Test

In an electronic system, there are many components like switching elements, chokes, circuit layout, rectifying diodes and much more which produce EMI. This test ensures that the energy meter does not affect the performance of the nearby instruments or we can say that it ensures that it does not conduct or radiate EMI beyond a definite limit. There are two types of emission test based on the EMI escapes from the system.
Conducted emission test-
In this test, power lead and cables are checked to measure the EMI escape, and it covers small meter of the frequency range from 150 kHz to 30 MHz.
Radiated emission test-
This test measures the EMI escape through free space, and it covers large meters of the frequency range from 31 MHz to 1000MHz.

Immunity Test

The emission test ensures that meter does not work as the source for EMI for other nearby equipment; similarly immunity test ensures that meter does not work as a receptor and properly function in the presence of EMI. Again, immunity tests are of two types based on radiation and conduction.
Conducted immunity test-
These tests ensure that meter’s functioning do not get disturbed if it is in the blanket of EMI. The electromagnetic interference source either in contact through data, interface lines, power lines, or by contact.
Radiated immunity test-
During this test, meter functioning is monitored and if it gets affected by EMI present in the surrounding area, that fault is recognized and corrected their only. It also is known as the electromagnetic high-frequency field test. Radiations generated by sources like small handheld radio transceivers, transmitters, switches, welders, fluorescent lights, switches, operating inductive loads etc.
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December 28, 2019

Construction of AC Energy Meter

Construction of AC Energy Meter


Energy meters are the basic part to measure the power consumption. It is used everywhere, no matter how big or small consumption it is. It is also known as watt-hour meter. Here we discuss the construction and working principle of induction type energy meter.
To understand the structure of watt-hour meter, we must understand the four essential components of the meter. These components are as follows:
  1. Driving system
  2. Moving system
  3. Braking system
  4. Registering system

Driving System

The components of this system are two silicon steel laminated electromagnets. The upper electromagnet is called shunt magnet and it carries a voltage coil consisting of many turns of thin wire. The lower electromagnet is called series magnet and it carries the two current coils consisting of a few turns of thick wire. Current coils are connected in series with the circuit and load current passes through it.
Where as voltage coil is connected to the supply mains and produce a high ratio of inductance to resistance. There is copper bands in the lower part of shunt magnet which provides frictional compensation so that the phase angle between shunt magnet flux and the supply voltage is exactly 90o.

Moving System

As you can see in the figure, there is a thin aluminum disk placed in the gap between the two electromagnets and mounted on a vertical shaft. The eddy currents are induced in the aluminum disk when it cuts the flux produced by both the magnets. As a result of interference of eddy currents and two magnetic fields constitute a deflecting torque in the disk. As you start consuming power the disk slowly starts rotating and the several rotation of the disk displays the power consumption, in the particular interval of time. Normally it is measured in kilowatt-hours.

Braking System

The main part of this system is a permanent magnet called brake magnet. It is located near the disk so that eddy currents are induced in it due to movement of rotating disk through the magnetic field. This eddy current reacts with the flux and exerts a braking torque which opposes the motion of the disk. The speed of the disk can be controlled by changing flux.

Registering System

As its name suggest, it registers the number of rotation of the disk which is proportional to the energy consumed directly in kilowatt-hour. There is a disk spindle which is driven by a gear on the disk shaft and indicates the number of times the disk has turned.

Working Principle of Energy Meter

The working of single phase induction type energy meters are based on two main fundamentals:
  1. Rotation of aluminum disk.
  2. Arrangement of counting and displaying the amount of energy consumed.

Rotation of an Aluminum Disk

The rotation of metallic disk is operated by two coils. Both the coils are arranged in such way that one coil produces a magnetic field in proportion to voltage and the other coil creates a magnetic field proportion to current. The field produced by voltage coil is delayed by 90o so that eddy current is induced in the disk. The force exerted on the disk by the two fields is proportional to the product of the immediate current and voltage in the coils.
As a result of it, a lite weight aluminum disk rotates in an air gap. But there is a need to stop a disk when there is no power supply. A permanent magnet works as a brake which opposes the rotation of the disk and balances the speed of rotation with respect to power consumption.

Arrangement of Counting and Displaying the Energy Consumed

In this system, the rotation of the floating disk has been counted and then displayed on the meter window. The aluminum disk is connected to a spindle which has a gear. This gear drives the register and the revolution of the disk has been counted and displayed on the register which has series of dials and each dial represent a single digit. There is a small display window in the front of the meter which displays the reading of energy consumed with the help of dials. There is a copper shading ring at the central limb of the shunt magnet. To make the phase angle between flux produced by shunt magnet and supply voltage about 900, small adjustments in the place of the ring is required.
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Friday, December 27, 2019

December 27, 2019

Thermistor: Definition, Uses & How They Work

Thermistor: Definition, Uses & How They Work


What is a Thermistor?

A thermistor (or thermal resistor) is defined as a type of resistor whose electrical resistance varies with changes in temperature. Although all resistors’ resistance will fluctuate slightly with temperature, a thermistor is particularly sensitive to temperature changes.
Thermistors act as a passive component in a circuit. They are an accurate, cheap, and robust way to measure temperature. While they do not work well in extremely hot or cold temperatures, they are the sensor of choice for many different applications. They are ideal when a precise temperature reading is required. The circuit symbol for a thermistor is shown below:

Uses of Thermistors

Thermistors have a variety of applications. They are widely used as a way to measure temperature as a thermistor thermometer in many different liquid and ambient air environments. Some of the most common uses of thermistors include:
  • Digital thermometers (thermostats)
  • Automotive applications (to measure oil and coolant temperatures in cars & trucks)
  • Household appliances (like microwaves, fridges, and ovens)
  • Circuit protection (i.e. surge protection)
  • Rechargeable batteries (ensure the correct battery temperature is maintained)
  • To measure the thermal conductivity of electrical materials
  • Temperature compensation (i.e. maintain resistance to compensate for effects caused by changes in temperature in another part of the circuit)
  • Used in wheatstone bridge circuits

How Does a Thermistor Work

The working principle of a thermistor is that its resistance is dependent on its temperature. We can measure the resistance of a thermistor using an ohmmeter. If we know the exact relationship between how changes in the temperature will affect the resistance of the thermistor – then by measuring the thermistor’s resistance we can derive its temperature.
How much the resistance changes depends on the type of material used in the thermistor. The relationship between a thermistor’s temperature and resistance is non-linear. A typical thermistor graph is shown below:

If we had a thermistor with the above temperature graph, we could simply line up the resistance measured by the ohmmeter with the temperature indicated on the graph. By drawing a horizontal line across from the resistance on the y-axis, and drawing a vertical line down from where this horizontal line intersects with the graph, we can hence derive the temperature of the thermistor.

Thermistor Types

There are two types of thermistors:
  • Negative Temperature Coefficient (NTC) Thermistor
  • Positive Temperature Coefficient (PTC) Thermistor

NTC Thermistor

In an NTC thermistor, when the temperature increases, resistance decreases. And when temperature decreases, resistance increases. Hence in an NTC thermistor temperature and resistance are inversely proportional. These are the most common type of themistor.
The relationship between resistance and temperature in an NTC thermistor is governed by the following expression:

Where:
  • RT is the resistance at temperature T (K)
  • R0 is the resistance at temperature T0 (K)
  • T0 is the reference temperature (normally 25oC)
  • β is a constant, its value is dependant on the characteristics of the material. The nominal value is taken as 4000.
If the value of β is high, then the resistor–temperature relationship will be very good. A higher value of β means a higher variation in resistance for the same rise in temperature – hence you have increased the sensitivity (and hence accuracy) of the thermistor.
From the expression (1), we can obtain the resistance temperature co-efficient. This is nothing but the expression for the sensitivity of the thermistor.

Above we can clearly see that the αT has a negative sign. This negative sign indicates the negative resistance-temperature characteristics of the NTC thermistor.
If β = 4000 K and T = 298 K, then the αT = –0.0045/oK. This is much higher than the sensitivity of platinum RTD. This would be able to measure the very small changes in the temperature.
However, alternative forms of heavily doped thermistors are now available (at high cost) that have a positive temperature co-efficient. The expression (1) is such that it is not possible to make a linear approximation to the curve over even a small temperature range, and hence the thermistors is very definitely a non-linear sensor.

PTC Thermistor

A PTC thermistor has the reverse relationship between temperature and resistance. When temperature increases, the resistance increases. And when temperature decreases, resistance decreases. Hence in a PTC thermistor temperature and resistance are inversely proportional.
Although PTC thermistors are not as common as NTC thermistors, they are frequently used as a form of circuit protection. Similar to the function of fuses, PTC thermistors can act as current-limiting device.
When current passes through a device it will cause a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings then the device heats up. In a PTC thermistor, this heating up will also cause its resistance will increase. This creates a self-reinforcing effect that drives the resistance upwards, therefore limiting the current. In this way, it acts as a current limiting device – protecting the circuit.

Thermistor Characteristics

The relationship governing the characteristics of a thermistor is given below as:

Where:
  • R1 = resistance of the thermistor at absolute temperature T1[oK]
  • R2 = resistance of the thermistor at temperature T2 [oK]
  • β = constant depending upon the material of the transducer
We can see in the equation above that the relationship between temperature and resistance is highly nonlinear. A standard NTC thermistor usually exhibits a negative thermal resistance temperature coefficient of about 0.05/oC.

Thermistor Construction

To make a thermistor, two or more semiconductor powders made of metallic oxides are mixed with a binder to form a slurry. Small drops of this slurry are formed over the lead wires. For drying purpose, we have to put it into a sintering furnace. During this process, that slurry will shrink onto the lead wires to make an electrical connection. This processed metallic oxide is sealed by putting a glass coating on it. This glass coating gives a waterproof property to the thermistors – helping to improve their stability.

There are different shapes and sizes of thermistors available in the market. Smaller thermistors are in the form of beads of diameter from 0.15 millimeters to 1.5 millimeters. Thermistors may also be in the form of disks and washers made by pressing the thermistor material under high pressure into flat cylindrical shapes with diameter from 3 millimeters to 25 millimeters.
Types of Temperature Sensors
The typical size of a thermistor is 0.125mm to 1.5 mm. Commercially available thermistors have nominal values of 1K, 2K, 10K, 20K, 100K, etc. This value indicates the resistance value at a temperature of 25oC.
Thermistors are available in different models: bead type, rod type, disc type, etc. The major advantages of thermistors are their small size and relatively low cost.
This size advantage means that the time constant of thermistors operated in sheaths is small, although the size reduction also decreases its heat dissipation capability and so makes the self-heating effect greater. This effect can permanently damage the thermistor.
To prevent this, thermistors have to be operated at low levels of electric current compared to resistance thermometer – resulting in lower measurement sensitivity.

Thermistor vs Thermocouple

The main differences between a thermistor and a thermocouple are:
Thermistors:
  • A more narrow range of sensing (55 to +150oC – although this varies depending on the brand)
  • Sensing parameter = Resistance
  • Nonlinear relationship between the sensing parameter (resistance) and temperature
  • NTC thermistors have a roughly exponential decrease in resistance with increasing temperature
  • Good for sensing small changes in temperature (it’s hard to use a thermistor accurately and with high resolution over more than a 50oC range).
  • The sensing circuit is simple and doesn’t need amplification & is very simple
  • Accuracy is usually hard to get better than 1oC without calibration
Thermocouples:
  • Have a wide range of temperature sensing (Type T = -200-350oC; Type J = 95-760°C; Type K = 95-1260°C; other types go to even higher temperatures)
  • Can be very accurate
  • Sensing parameter = voltage generated by junctions at different temperatures
  • Thermocouple voltage is relatively low
  • Linear relationship between the sensing parameter (voltage) and temperature

Thermistor vs RTD

Resistance Temperature Detectors (also known as RTD sensors) are very similar to thermistors. Both RTDs and thermistors have varying resistance dependent on the temperature.
The main difference between the two is the type of material that they are made of. Thermistors are commonly made with ceramic or polymer materials while RTDs are made of pure metals. In terms of performance, thermistors win in almost all aspects.
Thermistors are more accurate, cheaper, and have faster response times than RTDs. The only real disadvantage of a thermistor vs an RTD is when it comes to temperature range. RTDs can measure temperature over a wider range than a thermistor.
Aside from this, there is no reason to use a thermistor over an RTD.
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December 27, 2019

Sensor | Types of Sensor

Sensor | Types of Sensor


Let us consider a measurement system. It is composed of an input device which senses the environment or surrounding to generate an output and, a signal processing block which processes the signal from input device and an output device which presents the signal to human or machine operator in a more readable and usable form.
The initial stage is input device which is mainly what we are going to discuss in this chapter.

Sensor

A sensor is a device that responds to any change in physical phenomena or environmental variables like heat, pressure, humidity, movement etc. This change affects the physical, chemical or electromagnetic properties of the sensors which is further processed to a more usable and readable form. Sensor is the heart of a measurement system. It is the first element that comes in contact with environmental variables to generate an output.
The signal produced by the sensor is equivalent to the quantity to be measured. Sensors are used to measure a particular characteristic of any object or device. For example a thermocouple, a thermocouple will sense heat energy (temperature) at one of its junction and produce equivalent output voltage which can be measured by a voltage read by the voltmeter.
All sensors need to be calibrated with respect to some reference value or standard for accurate measurement. Below is the figure of a thermocouple.

Note that a transducer and a sensor are not the same. In the above given example of thermocouple. The thermocouple acts as a transducer but the additional circuits or components needed like the voltmeter, a display etc together from a temperature sensor. Hence the transducer will just convert the energy from one form to another and all the remaining work is done by the additional circuits connected. This whole device forms a sensor. Sensors and transducers are closely related to each other.

Characteristics of Sensors

A good sensor should have the following characteristics
  1. High Sensitivity: Sensitivity indicates how much the output of the device changes with unit change in input (quantity to be measured). For example the voltage of a temperature sensor changes by 1mV for every 1oC change in temperature than the sensitivity of the sensor is said to be 1mV/oC.
  2. Linearity: The output should change linearly with the input.
  3. High Resolution: Resolution is the smallest change in the input that the device can detect.
  4. Less Noise and Disturbance.
  5. Less power consumption.

Types of Sensors

Sensors are classified based on the nature of quantity they measure. Following are the types of sensors with few examples.

Sensor classification

Based on the quantity being measured
  • Temperature: Resistance Temperature Detector (RTD), Thermistor, Thermocouple
  • Pressure: Bourdon tube, manometer, diaphragms, pressure gauge
  • Force/ torque: Strain gauge, load cell
  • Speed/ position: Tachometer, encoder, LVDT
  • Light: Photo-diode, Light dependent resistor
And so on.
(2) Active and passive sensors: Based on power requirement sensors can be classified as active and passive. Active sensors are those which do not require external power source for their functioning. They generate power within themselves to operate and hence called as self-generating type. The energy for functioning is derived from the quantity being measured. For example piezoelectric crystal generate electrical output (charge) when subjected to acceleration.
Passive sensors require external power source for their functioning. Most of the resistive, inductive and capacitive sensors are passive (just as resistors, inductors and capacitors are called passive devices).
(3) Analog and digital sensor: An analog sensor converts the physical quantity being measured to analog form (continuous in time). Thermocouple, RTD, Strain gauge are called analog sensors. A digital sensor produces output in the form of pulse. Encoders are example of digital sensors.
(4) Inverse sensors: There are some sensors which are capable of sensing a physical quantity to convert it to other form and also sense the output signal form to get back the quantity in original form. For example a piezoelectric crystal when subjected to vibration generates voltage. At the same time when a piezo crystal is subjected to varying voltage they begin to vibrate. This property make them suitable to use in microphone and speakers.
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