Introduction to sensors

The transfer function shows the functional relationship between physical input signal or stimulus (s) and electrical output signal (S), as $S=f(s)$, where 'S' is response to the stimuli. This function can be linear or non-linear depending on the relation between input and output and nonlinearity may be in different forms such as logarithmic, exponential, or power function [1–3]. In most of the cases, the relationships are defined by unidimensional function which means that the relation between the output and input is associated with one stimulus. This linear relationship is described by:

$$\begin{eqnarray}&&S=a+b(s)\end{eqnarray} \tag{ 1.1 }$$

where $a$ is the intercept used by the output signal at zero input signal and $b$ is the slope, also called sensitivity $S$. It is also known as the sensor's output used by devices to acquire data and depending on the property of sensors, this can be amplitude, frequency, or phase. Other non-linear functions are given as:

$$\begin{eqnarray}&&\text{Logarithmic function}:\,S=a+b\,{\rm{ln}}\,s\end{eqnarray} \tag{ 1.2 }$$

$$\begin{eqnarray}&&\text{Exponential function}:\,S=a{e}^{{ks}}\end{eqnarray} \tag{ 1.3 }$$

$$\begin{eqnarray}&&S=\frac{1}{k}{\rm{ln}}\,\frac{s}{a}\end{eqnarray} \tag{ 1.4 }$$

$$\begin{eqnarray}&&\text{Power function}:\,S={a}_{0}+{a}_{1}{s}^{k}\end{eqnarray} \tag{ 1.5 }$$

$$\begin{eqnarray}&&S=\sqrt[k]{\frac{s-a}{b}}\end{eqnarray} \tag{ 1.6 }$$

where constant numbers are given by k.

There are some sensors that may not fulfil the above properties and in such cases, higher-order polynomial approximation is required.

This is defined as the difference between the maximum and minimum values of input stimulus which can be represented in decibels (dB). This is also a logarithmic measurement in terms of ratios of power or force and voltage. Decibels are calculated as equal to 20 times the log of the force, current, or voltage:

$$\begin{eqnarray}&&1\,{\rm{dB}}=20\,{\rm{log}}\frac{{s}_{2}}{{s}_{1}}\end{eqnarray} \tag{ 1.7 }$$

where ${s}_{2}$ and ${s}_{1}$ are the maximum and minimum values of input, respectively.

Full-scale output indicates the changes between the maximum and minimum values of electrical output signals when maximum and minimum input stimulus is applied. The FSO also includes all the deviations from the ideal transfer function.

Accuracy is an important characteristic in sensors which is calculated in terms of error in measurement and defined as the difference between measured value and true value. It is represented in terms of % of full scale or % of reading.

$$\begin{eqnarray}&&{\rm{Absoulute}}\,{\rm{error}}=\left|{\rm{Measured}}\,{\rm{value}}-{\rm{True}}\,{\rm{value}}\right|{A}_{e}=\left|{M}_{v}-{T}_{v}\right|\end{eqnarray} \tag{ 1.8 }$$

where ${T}_{v}$ is calculated by taking the mean of an infinite number of measurements and relative error can also be calculated as

$$\begin{eqnarray}&&{\rm{Relative}}\,{\rm{error}}=\frac{{\rm{Absolute}}\,{\rm{error}}}{{\rm{True}}\,{\rm{value}}}{R}_{e}=\frac{\left|{M}_{v}-{T}_{v}\right|}{{T}_{v}}\end{eqnarray} \tag{ 1.9 }$$

The accuracy rating indicates a collective effect of variation, linearity, calibration, repeatability errors, dead band etc, in measurements used in sensors.

There are many sensors available but to get the best possible sensor with optimal value of accuracy, the sensor needs to be calibrated in the device where it will be used. It is an adjustment or set of adjustments made on a sensor or device to make that device function accurately and error free. For instance, we have to measure the pressure with an accuracy $\pm$ 5 pa, and a given sensor is rated with an accuracy of $\pm$ 10 pa. Can we use this pressure sensor? Yes, we can, but the given sensor needs to be calibrated and we have to find out its initial transfer function during calibration.

In the calibration method, we have to find out its particular variables. These variables describe the complete transfer function and should be identified before calibration. Calibration of linear devices is calculated by equation (1.1) and variable 'a' and 'b' should be determined accurately.

In order to get constant values in the equation with good accuracy, the linear transfer function is calculated as $v=a+b(\,p\,)$. To find constants 'a' and 'b', a sensor can be exposed with two pressure values (${p}_{1}$ and ${p}_{2}$) with respect to their corresponding output voltages (${v}_{1}$ and ${v}_{2}$), then we get ${v}_{1}=a+b({p}_{1})$ and ${v}_{2}=a+b({p}_{2})$, and the constants are calculated as

$$\begin{eqnarray}&&b=\frac{{v}_{1}-{v}_{2}}{{p}_{1}-{p}_{2}}\,{\rm{and}}\,a={v}_{1}-b{p}_{1}\end{eqnarray} \tag{ 1.10 }$$

and pressure of calibration can be computed as

$$\begin{eqnarray}&&p=\frac{v-a}{b}\end{eqnarray} \tag{ 1.11 }$$

The calibration error is actually a type of inaccuracy which is accepted by manufacturers during the time when the devices or sensors are calibrated in the factory. This obtained error is not uniform and can change during the process of calibration.

Hysteresis is a common phenomenon or characteristic which is caused by changing properties of a material such as frictional and structural changes. Hysteresis error is the difference between two output values that correspond to the same input depending on direction followed by the sensor, as shown in figure 1.1.

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The value of hysteresis error is represented by a positive or negative percentage of the given pressure range. We can identify this hysteresis error for half of the given range of pressure reference point.

The linearity function in a sensor is calculated by the maximum deviation by straight line given in equation (1.1), over the specified dynamic range. The non-linearity of the sensor is calculated by the measurement of the difference in Y-axis of two lines of equal slope, one passes through the minimum points and the other passes through the maximum points of the output curve. The total non-linearity is the difference between intercept value of Y-axis and a parallel line which goes through the maximum deviation point. The total non-linearity is approximated by determining the maximum deviation at the midpoint of the X-axis and the largest error is observed between the actual and calculated values.

The midpoint (X_m ) is calculated as:

$$\begin{eqnarray}&&{X}_{m}=\left(\frac{{X}_{{\rm{ma}}{\rm{x}}}-{X}_{{\rm{\min }}}}{2}\right)+{X}_{{\rm{\min }}}\end{eqnarray} \tag{ 1.12 }$$

and the line connecting end points represents the total amount of non-linearity, which is calculated by % of linearity as:

$$\begin{eqnarray} &&\% \,{\rm{Linearity}}=\left|\frac{\left[\left(\frac{{Y}_{{\max }}-{Y}_{{\rm{\min }}}}{2}\right)+{Y}_{{\rm{\min }}}\right]-{Y}_{s}}{\left[\left(\frac{{Y}_{{\max }}-{Y}_{{\rm{\min }}}}{2}\right)+{Y}_{{\rm{\min }}}\right]}\right|\times 100 \% \end{eqnarray} \tag{ 1.13 }$$

where ${Y}_{s}$ is the actual Y value at this X value and if the actual measured Y values pass through zero, then it is simplified to be:

$$\begin{eqnarray} &&\% \,{\rm{Linearity}}=\left|\frac{\left(\frac{{Y}_{{\rm{\max }}}}{2}\right)-{Y}_{s}}{\left(\frac{{Y}_{{\rm{\max }}}}{2}\right)}\right|\times 100 \% \end{eqnarray} \tag{ 1.14 }$$

The resolution of a sensor is defined as minimum detectable signal fluctuation while reading or measuring some quantity using a suitable sensor. This is also an ability of the measurement to obtain and notice minor changes in the characteristic of the measurement result.

Every sensor has its certain operation limit, which is the state where the output signal of the sensor will no longer to respond at some level despite increasing the input stimuli values. This characteristic is referred as saturation of the sensor. There is a point where output of the sensor does not respond as required to an increase in the input stimuli, and that particular point is known as saturation point or threshold point.

Repeatability means occurrence of a value again and again when the system returns to the same position multiple times to measure the range of output signals. Direction of approach is an important issue to measure the position in repeatability, which can be represented as the maximum change among readings of output as notified by two repeated cycles unless otherwise specified. It is generally denoted as % of full scale (FS):

$$\begin{eqnarray}&&{\delta }_{r}=\frac{{\rm{\Delta }}}{{\rm{FS}}}\times 100 \% \end{eqnarray} \tag{ 1.15 }$$

Dead band is a region where the sensitivity of a sensor does not have any effect. In this range, the output can remain nearly zero over the entire dead band ranges without any change in measurement.

Reliability of the sensor node is the ability to perform the desired function under any given circumstances a for definite period. This can be represented in terms of statistics as a probability that the sensor will operate without failure in the stated time interval. When the performance of the sensor is exceeded under given circumstances, it causes a failure of the sensor and it can be temporary or permanent. So before manufacturing or designing the sensor, it should be properly checked, considering the various worst circumstances and conditions.

Just like the input characteristics, this depends upon the type of input which the sensor is measuring, output characteristics depend upon the type of electrical output we are getting from the sensors. The output of the sensor can be voltage, current, impedance or it can be a function of any other quantities. So, the output quantity should be acceptable by the other stages of the instrumentation system, because as mentioned for measurement or in an instrumentation system, we not only have a sensor but there are also other components present in the overall system. These components should be compatible with the output of the sensor. The electrical output of the sensor should be acceptable by the other stages of the measurement system, so that the components and their evaluations can work over it.

Impedance of a circuit is the total effective resistance or the measure of opposition to the passes of current when an AC voltage source is applied. It is represented as Z and its unit is ohm (Ω). This is calculated by applying a voltage source to the sensor with a resistor and measuring the changed voltage across the resistor and sensor device. Input impedance is the impedance that is observed by the voltage source between the two terminals of the circuit. Now, we are assuming that the applied voltage source is the idle source but in an actual case this voltage source will also have some finite resistance. In the case of output impedance, in the circuit we have connected some load to the output terminal and the circuit is giving some voltage; ideally this output voltage should also appear across the two terminals of this load. From the load perspective, it will also have some finite resistance with this output voltage and this finite resistance are known as the output impedance of the circuit or device.

Excitation is the appropriate electrical signal required to operate the active sensor. It is given by the range of voltage and/or current. The frequency of the excitation signal must also be specified in some sensors. Transfer function of the sensor may be altered by changing this excitation signal or it would cause output error.

Dynamic characteristics of a sensor are determined by analyzing the response of transfer function, span, calibration and input (steps, impulse, ramp, sinusoidal) of the sensor. This is when input response varies and the response does not follow it due to coupling characteristics and this is determined with a time dependent behavior, also known as dynamic characteristics. When a sensor does not respond immediately, the given value of input is not a properly received value and is different from the real time value, then dynamic error will occur.

It can be described as the nearest among a set of values and it is dissimilar from accuracy. Let ${p}_{t}$ be the true value of the variable $p$ and a random experiment measure ${p}_{1}$, ${p}_{2}$, ..., ${p}_{i}$ as the value of $p$. Then our measurements ${p}_{1}$, ${p}_{2}$, ..., ${p}_{i}$ are precise when they are very close to each other but not necessarily close to true value ${p}_{t}$. However, if we say ${p}_{1}$, ${p}_{2}$, ..., ${p}_{i}$ are accurate, it means that they are close to true value ${p}_{t}$ and hence they are also close to each other. Hence accurate measurements are always precise.

Environmental conditions are major factors which affect input and output stimuli of the sensor. There are mainly three types of factors namely air, soil and water. Each sensor is designed to work only with certain conditions. Noise can also affect the output signal of sensor and aging can degrade the performance of the sensor. There are some other factors that reduce the performance of sensors, such as electrical, mechanical, chemical and thermal etc. Therefore, performance and long-term stability of the sensor can be improved by designing the components for extreme conditions, so that all these factors do not create an adverse effect during the operation of the sensor.

Every measurement has some uncertainty. Uncertainty in the data contains some variable values that make them deviate from the correct or original values and is measured by the amount of error as mean or average value of a data set. Error is the difference between the true value and the measured value. Uncertainty is the range of values within which the true value lies in some levels of confidence.

Sensors are not generally designed for general purpose and are application oriented. Sensors are required according to applications of different types of sensors such as: speed sensor for synchronizing the speed of multiple motors; temperature sensor used for controlling the temperature; ultrasonic sensor for measuring the distance, etc.

A temperature sensor is used to measure the amount of energy in the form of heat and cold produced by an object and system. It allows one to sense or detect any physical change to that energy and gives the output as analog or digital. Temperature sensors are used in various applications such as notification of environmental temperature, medical instruments, automobiles etc. According to application and its characteristics, many different types of temperature sensors are available. There are basically two types of temperature sensors, contact temperature sensor and non-contact temperature sensor. In contact temperature sensor, there is physical contact with the object being sensed and to monitor the change in temperature, conduction is used. It is used to sense solids, liquids or gases over a wide range of temperatures. In a non-contact temperature sensor, we use convection and radiation properties to measure the changes in temperature. It uses radiant energy in the form of heat and cold.

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  Thermostat: The thermostat is a kind of contact temperature sensor employing an electro-mechanical component and using two thermally different kinds of metals, nickel, copper, tungsten or aluminium etc, which are stuck together to form a Bi-metallic strip. When it is cold, one of the strips is contracted and its contacts are closed and current passes through the thermostat. When it is hot, one metal strip is expanded and opens the contacts to stop the flow of current.
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  Thermistor: The thermistor is another type of temperature sensitive device or resistance whose electrical resistance changes as the object temperature changes. This is made up of semiconductor materials. When temperature of the object or surroundings increases or decreases, resistance will also increase or decrease. How much the resistance will increase or decrease depends on the properties of the semiconductor material. The thermistor is of two types: positive temperature coefficient, (PTC) and negative temperature coefficient, (NTC). In PTC, resistance value increases with an increase in the temperature and in NTC, its resistance value goes down with an increase in the temperature. Thermistors are used for precise temperature measurement, control and compensation. Thermistors are highly sensitive and exhibit non-linear characteristics of resistance versus temperature. Generally, these are made up of manganese, nickel, cobalt, copper and iron.
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  Resistive temperature detector: The resistive temperature detector (RTD) is also known as resistance thermometer, and used for measurement of temperature. It is based on the temperature coefficient of sensors and generally composed of high-purity conducting metals like platinum, copper or nickel. These materials are looped into a coil whose changes of electrical resistance depend on a temperature function. The working principle of an RTD is very similar to that of the thermistor.
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  Thermocouple: The thermocouple is a device which is used for the measurement of the temperature variation in a measurement of sensors. The thermocouples are coupled with two metals joined together forming a junction. Thus, there are two junctions in the metals, one is called hot junction and other is called cold junction, also referred as measuring junction and reference junction, respectively. These junctions are kept at different temperatures due to the change of EMF (electromotive force) induced in a thermocouple and output voltage obtained with the help of the relationship between the voltage and temperature. When the two junctions are at different temperatures, a voltage is developed across the junction which is used to measure the temperature sensor. The thermocouple is based on three main effects: Thomson effect, Seebeck and Peltier effect. It has broadest range of temperatures of all the temperature sensors, covering from −200 °C to 2000 °C.

The position sensor detects the position of an object either linearly or in rotation with respect to some fixed point or position. Position can be determined by the distance between two points moving away from some fixed points. We can measure the displacement of position in a straight line by linear sensor and angular displacement using rotational sensors. Position sensors are also known as potentiometers and used to measure the displacement of the object. A potentiometer can be an electrical or resistive type of sensor, because its working principle is based on change in resistance of wire with its length. This converts rotary or linear displacement to electrical voltage. The resistance of wire is directly proportional to length of wire. If the length of wire changes then the resistance of wire also changes. Potentiometers are available as rotary and linear potentiometers in the market, and can be used to measure the angular position and linear position, respectively; through voltage division the changes in resistance can be used to create an output voltage that is directly proportional to the input displacement.

The sensors have three terminals, where the one in the middle is known as the wiper, and the other two are known as the ends. The wiper is a movable contact where resistance is measured with respect to it and either one of the end terminals. The displacement of the moving object is measured with the help of the sliding element of the potentiometer. When position of the moving body changes then its resistance between two fixed points also changes. The result is obtained in the form of differential output voltage which varies linearly with the movement of core position. The resulting output signal has both the amplitude and polarity. Amplitude is calculated as linear function of the displacement and polarity gives the direction of movement. Major advantages of the potentiometer include user friendly operation, low cost, high amplitude output and the sensors are used for measuring even large displacement, but its operating cycles are limited.

A light sensor is a photoelectric passive sensor which changes the light energy into an electrical signal output. It measures the ambient light which is surrounding light, room light and reflected light. The major component of a light sensor is the light dependent resister (LDR) or photoresistor. It is a resistor that depends on the light which changes its resistance depending on the amount of light incident on it. The sensors are made up of semiconductor materials and therefore when light is incident on semiconductor material it becomes low conductive and therefore has less resistance. When we increase the light intensity, its resistance decreases and vice versa which is shown in figure 1.2. Intensity of light falling on an LDR is measured in lux.

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There are different kinds of light sensors such as photoresistors, photodiodes, photovoltaic cells, phototubes, photomultiplier tubes, phototransistors, charge coupled devices (CCDs) etc.

A sound sensor is also known as auditory and used to detect the intensity of sound. It converts the acoustic wave into an electrical signal output. These sensors can also detect sound pressure waves which are not within the audible range, making them suitable for a wide range of tasks. Sound sensors are mostly used for security purposes.

A proximity sensor can be used for detecting the presence of a nearby object without any physical contact. It emits an electromagnetic field for a beam of electromagnetic radiation as infrared instances and changes in the field returning a signal. The object being sensed is often referred to as the proximity sensor's target. Depending on different types of proximity sensors, different targets are used. For example, an inductive proximity sensor needs a metal object, whereas a capacitive photoelectric sensor is suitable for a plastic target. A proximity sensor has high reliability due to the absence of mechanical parts and lack of physical contact between the sensor and target. It has very short range when used as a touch switch. It is commonly used in industrial applications, manufacturing of food production, mobile phones etc.

This sensor is used to detect the acceleration of an object, and operates by sensing the acceleration of gravity, and the direction of the object is calculated. This sensor is a kind of microelectromechanical system (MEMS), which uses a silicon integrated circuit. These sensors convert the mechanical motion caused in an accelerometer into an electrical signal by using the piezoelectric, piezo-resistive and capacitive components.

An infrared (IR) sensor consists of two packs, one is Rx (receiver) and the other is Tx (transmitter). Transmitters are used in transmitting the rays in the infrared spectrum and the receiver receives the IR spectrum range. In the IR spectrum, the voltage is given between its terminals and then it emits rays. The main principle of working of an IR sensor is reflectivity by an object. When an object is placed in front of the transmitter it tends to reflect the rays that are coming from the IR sensor back to the IR sensor. Whenever a ray that is reflected by an object is received by the receiver it generates a voltage level across the terminal. This voltage level depends upon the intensity of light that is reflected by the object. Transmitter and receiver are placed side by side, and the IR transmitter transmits a signal within a limited range and going to a certain distance. When IR rays hit the surface, some rays are reflected depending upon the colour of the surface. The brighter the colour the more IR rays are reflected; similarly, the darker the surface the more IR rays are absorbed by the surface and fewer IR rays are reflected back.

Pressure is an external force exerted on a surface in unidirectional areas. We commonly measure the pressure of liquid, air and other gases. A pressure sensor monitors this pressure and is sometimes called a pressure transmitter as it converts pressure into an electrical signal. The most common type of pressure sensor is the strain gauge-based pressure sensor. Conversion of pressure into electrical signal is achieved through the physical deformation of strain gauge which is bound into the diaphragm of the pressure sensor. The strain will produce a change in electrical resistance which is proportional to the pressure. Change in voltage is the result of ambient pressure. A pressure sensor can also be used to measure other variables such as fluid or gas flow, speed, water level, and altitude.

An ultrasonic sensor uses ultrasonic waves for the purpose of sensing and measuring the distance of a particular object. Ultrasonic waves are very high frequency waves. The sensors have two main transducers, namely transmitter and receiver. A transmitter uses 40 KHz of frequency wave transmitted in the air and when it is blocked by an object then its gets reflected and bounced back to the sensor. These reflected waves are absorbed by the receiver of the sensor. So, the total time taken by the ultrasonic waves to travel from the transmitter to the object and again from the object to the receiver of the sensor is given by the output of the sensor. Ultrasonic sensors are used in many applications such as robotics, driverless cars, for measuring distance, and also in radar systems etc.

Touch sensors are sensitive to touch, pressure and force. The sensors operate as switches and when the surface of the sensor is touched the current starts to flow in the circuit just like current flowing in a closed circuit. When there is no contact, it performs like an open circuit and no flow of current is reported. There are two types of touch sensors, capacitive and resistive. The touch sensors are used popularly in modern gadgets such as smartphones, and other handy devices.

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  Capacitive sensor: The capacitive sensor has an important element as a capacitor. Parallel capacitors are generally placed like top and bottom plates at some certain distance and between these parallel capacitor plates there is a dielectric medium. The main principle of change in capacitance is used such that it may be caused by change in overlapping area, change in distance between two plates and change in dielectric constant. Changes of these parameters can be made by the physical variables like displacement, force, pressure and flow of liquid. Capacitance and output impedance are measured with a bridge circuit. An extremely small force is needed to operate them and hence they are very useful for a small system. The sensors are highly sensitive with good frequency response and high output. As force requirement is small, thus the power requirement is also less to operate the sensors. The metallic parts of the sensor must be insulated from each other in order to reduce the effect of stray capacitance.
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  Resistive sensor: The resistive sensor is based on the change in resistance of the material and is used to measure temperature, displacement, moisture etc. A slider is free to move between two points and at a certain point we get the zero output and at some other point we get the maximum output. The output voltage is obtained between these two points and it is directly proportional to displacement. So, the change in length of the wire causes the change in the value of the resistance. This property is utilized to measure the changes in displacement using resistivity and resistance. When the fixed voltage is applied across end terminals of the sensor, a proportional voltage is generated across the slider and this voltage can be calculated using voltage divider rule. As distance increases, the output voltage will also increase. The resistive technique used in this sensor can be used to sense or measure linear displacement.

Humidity is the amount of water present in the surrounding air and a hygrometer is the device which measures humidity directly. Humidity is a non-electrical quantity that is converted into electrical quantity by using resistance, capacitance and impedance properties. There are various parameters that change due to humidity. There are five basic types of humidity sensor: resistive hygrometer, capacitive hygrometer, microwave refractometer, aluminium oxide hygrometer and crystal hygrometer.

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  Resistive hygrometer: In a resistive hygrometer, the main element is a material whose resistance changes with the change in humidity or relative humidity. A wire or electrode coated with hydroscopic salt (lithium chloride) can be used for measurement of the humidity. Resistance of salt changes with humidity because hydroscopic salt absorbs moisture and its resistance decreases.
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  Capacitive hygrometer: In a capacitive hygrometer, the changes in humidity are caused by the changes in the capacitance. Dielectric medium is used in the capacitor and the capacitor consists of two electrodes or plates and a dielectric medium is there between the plates. There is also some hydroscopic material which exhibits the change in dielectric constant with the change in the humidity. Therefore, such hydroscopic material or salt can also be used for construction of a capacitive hygrometer. If the change is very small, then the capacitor includes a frequency determining element in the oscillator and another frequency is produced by the beat frequency oscillator. This frequency is heterodyned and the difference in frequency is a measure of relative humidity.
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  Microwave refractometer: A microwave refractometer consists of two cavities, each coupled with Klystron. Klystron is a material which produces microwaves in which one cavity is filled with dry air and another cavity is filled with a mixture whose humidity is measured. In the mixture, water vapour will be present and due to the presence of water vapour, there will be a change in dielectric constant, and frequency of one of the oscillators changes consequently. If there is no change in dielectric constant, its frequency is going to be constant, whereas in the mixture water vapours are present, and there is change in dielectric constant which results in change in its frequency. Frequency changes are measured as the measure of humidity.
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  Aluminium oxide hygrometer: In an aluminium oxide hygrometer, aluminium oxide is coated on anodized aluminium and this aluminium oxide exhibits a change in the dielectric constant with respect to changes in humidity. There are two electrodes in which one is the inner electrode and the other is the outer electrode made from a very thin layer of material like gold. Some pores are presents in the inner layer. Due to the change in humidity, dielectric constant changes and this change can be measured to measure the humidity by bridge or electric method. The errors are much reduced and the response time is small and therefore the response is very fast.
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  Crystal Hygrometer: In a crystal hygrometer, crystals are coated with hydroscopic materials (hydroscopic polymers). These crystals are used as frequency determination elements in the oscillator, and therefore just like with the capacitive hygrometer, if there is change in humidity then frequency also changes. Frequency changes due to the humidity as the mass of the crystal changes with amount of water absorbed by the coating. This change in frequency is measured. Humidity sensors are used in industry, agriculture, the medical field, environment monitoring etc.

A colour sensor is used to detect and identify various colour patterns and convert them into desired frequency as output. It consists of four photodiodes of red, green, blue and clear (no colour). All these photodiodes are connected in parallel and work as filters. For example, if we have to detect red colour, we use red colour filter for this purpose. Colour light signals are sensed by the photodiodes and we get the square wave signals with the frequency directly proportional to light intensity and that is transferred to the microcontroller and we get the result of colour.

A chemical sensor is a device which transmits chemical information from a chemical reaction. The chemical information may be of composition, concentration and chemical activity which originates from a chemical reaction or from physical activities. It has different applications such as for home appliances and the chemical industries. The chemical sensor usually contains two basic components, which are a chemical resonance system known as the receptor and a physical chemical transducer. The receptor interacts with analytic molecules and the transducer sends the electric signal. A test sample is given to the receptor which checks composition connected with the transducer. The transducer collects the information from the receptor and sends it to the signal amplifier. This amplifies the signal from the transducer and sends it as output signals. There are two types of chemical sensors used to detect the composition: optical sensor and electro chemical sensor.

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  Optical sensor: In the optical sensor, there are an emitter and a detector as the main elements. The emitter senses the light to the optical sensor and the light rays fall on the analyte and these rays may be reflected or refracted. These reflected or refracted lights are passed through the detector. Now the detector receives these lights and according to their intensity, the chemical compound present is analysed. Operation of an optical sensor is very simple and it uses absorption coefficient characteristics of the medium and path length travelled by the rays.
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  Electrochemical sensor: The electrochemical sensor operates by acting on gas molecules of interest and produces an electric signal proportional to the compound present in the gas. It consists of sensing modules and electrodes, separated by a thin layer of electrolyte. There are two plates and the centre is filled by electrolytes. One plate is the cathode and the other plate is the anode. An external membrane is introduced in solution and it is absorbed by certain ions from the solution. Therefore, chemical properties of the solution change and the electromagnetic field will also change; and consequently change in the electromagnetic field ensures that the chemical composition is present in the gas.

A seismic sensor measures small movements of the ground and also amplifies and records these small movements. It is also known as a seismometer, and is mostly used in measuring the details of earthquakes, volcanic eruptions and other vibrations. There are two types of seismic sensor, inertial seismometer and strain meter or extensometer seismic sensor.

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  Inertial Seismometer: The inertial seismometer consists of a weight suspended from a frame by a spring. The frame moves due to the vibration being measured but the mass is held stationary due to the spring. It is used to measure a large-scale vibration such as an earthquake. Now the movement of mass is converted for output as a digital electric signal. Since both types of seismic sensors most commonly output an electric signal, calibration is necessary to derive a relationship between the input and output.
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  Strain meter or extensometer seismic sensor: In a strain meter seismic sensor, a strain gauge is used to measure the motion relative to the various points. It generally is used for smaller scale measurement and movement of mass is converted for output as a digital electric signal.

Magnetics sensors respond to the presence or interruption of a magnetic field like flux, strength and direction by producing a proportional output. It converts magnetic information into an electrical signal for processing by the electronic circuit. A magnetic sensor is used in different types of application such as sensing position, velocity and movement of an object. There are different kinds of technology used to design a magnetic sensor. Fluxgate, Hall effect, resistive, inductive, proton processing etc, have a dissimilar approach of using magnetic sensors. A resistive magnetic sensor keeps the electrical resistance of the magnetic field and an inductive magnetic sensor uses coils surrounding its magnetic material, which have the ability to detect changes within the Earth's magnetic field. A fluxgate magnetic sensor uses the approach of changing flux parameters. Each type of technology focuses on a specific area for identifying measurements to be detected. Sensitivity of the magnet is increased by combining layers of magnetic alloys and the magnetic field is surrounded by an electric current, and variation within the field is detected. The output of a magnetic sensor increases with a strong magnetic field and decreases with a weak magnetic field.