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Transducers

Introduction • A transducer is defined as a device that receives energy from one system and transmits it to another, often in a different form (electrical, mechanical or acoustical). • Basically, there are two types of transducers : 1) Electrical 2) Mechanical • The electrical output of a transducer depends on the basic principle involved in the design. • The output may be analog, digital, or frequency modulated. • An electrical transducer is a device that converts a physical, mechanical or optical quantity into a proportional voltage or current quantity.

• Electrical transducers can be classified into two major categories : 1) Active transducers Generates an electrical signal directly in response to the physical parameter (does not require external power to operate). Example : piezo-electric sensor and photo cells. 2) ive transducers Requires external power to operate. Example : Strain gauges and thermistors.

CAPACITIVE TRANSDUCER

• Change in the capacitance of a capacitor due to the variation in A, d, or ϵ can be utilized for the measurement of physical variables like displacement, force, pressure, etc.

• The liquid level in a container can also be measured by measuring the change in capacitance due to change in dielectric constant of the liquid poured into the container having two plates inserted inside.

• Another example of capacitive transducer is the capacitive pressure transducer as shown in the figure above. • This sensor is designed to measure pressure (in vacuum). • A metallic diaphragm will move to the right when pressure is applied to the chamber and to the left when vacuum is applied. • This diaphragm is used as one plate of a variable capacitor. • The capacitive transducer is simple to construct, and inexpensive.

Inductive Transducer

• Inductive transducers operate on the principle of variation of self inductance of a coil or on the principle of variation of mutual inductance. • There are two common type inductive transducers  Simple inductance type  Two-coil mutual inductance type

• Figure (a) and (b) are transducers used for the measurement of displacement of linear and angular Inductive Transducer movement respectively. • In both cases, as the number of turns are changed, the self inductance and the output also changes. • Where N = # turns,

Therefore

R = Reluctance of the materials/path where the flux is going through μ = permeability A = cross-sectional area of the coil winding (round loop) in m2 l = length of the winding (magnetic flux path)

• Fig shows an inductive transducer which works on the principle of the variation of permeability causing a change in self inductance. • If a permeable core is inserted into an inductor as shown in Fig, the net inductance is increased. • When the iron core is inside the winding, its permeability is increased, and so is the inductance. • Every new position of the core produces a different inductance. • In this fashion, the inductor and movable core assembly may be used as a displacement sensor. An ac bridge or other active electronic circuit sensitive to inductance then may be employed for signal conditioning.

• Figure shows the variable reluctance transducer. • The transducer consists of a coil wound on a ferromagnetic core. • The displacement which is to be measured is applied to a ferromagnetic target. • The core and the target are separated by an air gap. • The self inductance of the coil is inversely proportional to the length of the air gap. • When the target is near the core, the length ( of the air gape) is small, thus increases the self inductance.

Inductive Transducer (variable reluctance) • The inductance of the variable reluctance transducer is given by:

• The reluctance of the iron target (ferromagnetic) part is negligible, thus:

• Rg is proportional to lg, thus L is inversely proportional to lg.

Linear Variable Differential Transformers • The LVDT is an important and common sensor for displacement in the industrial environment. • An LVDT Displacement Transducer comprises 3 coils; a primary and two secondaries. • The primary coil is excited by some ac source as shown. • Flux formed by the primary is linked to the two secondary coils, inducing an ac voltage in each coil. • When the position of the core is at null position, the induced EMF in the two secondary windings, E1 and E2 are equal and opposite. The transducer output voltage is zero. • When the core is displaced by a force towards the left as shown in Fig, E1 will be greater than E2 due to the difference in flux linkage created by the primary winding ampere turns. • When the core is moved towards the right as shown in fig (c), the induced EMF E2 will be greater than E1. • The magnitude of the differential output voltage E0 will vary with the change in core position.

• The output voltage, E0 at null position is ideally zero and will increase when the core is made to move either towards the left or towards the right. • It is observed that the output voltage changes linearly with the displacement of the core. The output voltage is a function of the displacement of the core.

• A remarkable result, shown in Fig below, is that the differential amplitude is found to increase linearly as the core is moved to one side or the other. • In addition, as noted, there is a phase change as the core moves through the central location. • Thus, by measurement of the voltage amplitude and phase, one can determine the direction and extent of the core motion, that is, the displacement.

POTENTIOMETRIC TRANSDUCER • A potentiometric transducer is basically an electrical resistive transducer which converts linear or angular motion into a changing resistance which may be converted directly to voltage and or current. • A potentiometer is an electromechanical device having a resistance element with a sliding facility. • Motion of the slider results in a resistance change that may be linear , logarithmic, exponential and so on, depending on the manner in which the resistance wire is wound. • In some cases deposited carbon, platinum film, and other techniques are used to provide the resistance element. • The sliding is known as wiper, which may be translatory or rotary, according to the design. • Some potentiometers, also called ‘pots’, are designed combining these two types of motions. • The output voltage is • and in the case of translational and rotational potentiometer, respectively, as shown in Fig. 12.8.

• The resistance element of a potentiometer is excited either by ac or dc voltage. • The motion of the wiper or slider makes a resistance change that may be linear, logarithmic, or exponential. • In Fig. 12.9 are shown applications of potentiometric transducers in the measurement of linear and angular displacement.

Piezoelectric Transducer • Piezoelectric transducer consists of a crystal material such as Quartz, Rochelle salt and Barium titanite which produces an emf when they are placed under stress. • This property is used in piezoelectric transducers, where a crystal is placed between a slid base and the force-summing member, as shown in fig . • An externally applied force, entering the transducer through its pressure port, applies pressure to the top of a crystal. This produces an emf across the crystal, proportional to the magnitude of the applied pressure to the top of a crystal.

• This produces an emf across the crystal , proportional to the magnitude of the applied pressure.

• Advantage: • The device needs no external power source and is therefore selfgeneration. • Disadvantages: • The principal disadvantage of this transducer is that it cannot measure static conditions. • The output voltage is also affected by temperature variations of the crystal. Application: • Since the Piezoelectric transducer has a very good high-frequency response, its principal use is in high-frequency accelerometer. • In this application its output voltage is typically on the order of 1 to 30mV per g of acceleration.

Temperature Transducers

• There are two temperature sensing methods 1) 2) Non- • sensing brings the sensor in physical with a substance or object. It can be used with solids, liquids or gases. • Non- (infrared) temperature sensing reads temperature by intercepting a portion of the infrared energy emitted by an object or substance, and detecting its intensity. Non- is used to sense the temperature of solids and liquids. Non- cannot be used on gases due to their transparent nature.

Non- Temperature Sensors • • • • • • • • • •

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Advantages Relatively rugged Remote mounting away from heat source Ideal for measuring objects in motion Does not interfere with process. Faster response (milliseconds compared to seconds for sensing) Can sense temperature of irregular shaped objects Will not deface, mar or contaminate (To make impure or unclean by ) Will not act as a heat sink. Measurements of high temperatures (greater than 1300°C) present no problems. In similar cases, thermometers cannot be used, or have a limited life. Disadvantages Will not measure gas temperatures Emissivity variations

• Ambient temperature (surrounding temp) restrictions • Indicated temperature affected by environmental conditions (dust, smoke, etc.)

• Summary: The main advantages of non IR thermometry are speed, lack of interference, and the ability to measure in high temperature ranges to 3000°C. • Keep in mind that only the surface temperature can be measured.

Temperature Sensors • • • • • • • •

Advantages Relatively rugged Economical Wide application range Relatively accurate Simple to apply Disadvantages Requires physical , may damage, mar or contaminate • Can cause wear on rotary components (slip rings) • Slow to respond relative to non- sensing • Acts as a heat sink, alters readings on small objects

• An IR thermometer can be compared to the human eye. The lens of the eye represents the optics through which the radiation (flow of photons) from the object reaches the photosensitive layer (retina) via the atmosphere. This is converted into a signal that is sent to the brain. Fig. above shows an infrared measuring system process flow.

• TARGET: • Every form of matter with a temperature (T) above absolute zero emits infrared radiation according to its temperature. This is called characteristic radiation. • The cause of this is the internal mechanical movement of molecules. • The intensity of this movement depends on the temperature of the object. Since the molecule movement represents charge displacement, electromagnetic radiation (photon particles) is emitted. These photons move at the speed of light and behave according to the known optical principles. • They can be deflected, focused with a lens, or reflected from reflective surfaces. • The spectrum of this radiation ranges from 0.7 to 1000 µm wavelength. For this reason, this radiation cannot normally be seen with the naked eye. • This area lies within the red area of visible light (having a wavelength just greater than that of the red end of the visible light spectrum)and has therefore been called "infra"-red after the Latin.

Bodies at high temperatures still emit a small amount of visible radiation. This is why everyone can see objects at very high temperatures (above 600°C) glowing somewhere from red to white. Experienced steelworkers can even estimate temperature quite accurately from the color.

Temperature Sensors • Types: 1) Expansion Thermometer • Bimetallic Thermometer (Solid expansion). • Liquid in Glass thermometer (Mercury or Ethyl alcohol in glass) • Gas / Vapor Thermometer. 2) RTD (Resistance Temperature Detector) • Its metallic resistance based. 3) Thermister • Its semiconductor resistance based. 4) Thermocouple • Its based on Thermoelectric Effect.

RTD • Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material, typically platinum, nickel or copper. The material has a predictable change in resistance as the temperature changes and it is this predictable change that is used to determine temperature. • They are slowly replacing the use of thermocouples in many industrial applications below 600 °C, due to higher accuracy and repeatability.

• The RTD resistance at temperature T ̊C is given by :

• Resistive temperature detectors have positive temperature coefficients (PTC) but unlike the thermistor their output is extremely linear producing very accurate measurements of temperature. • The more common types of RTD's are made from platinum and are called Platinum Resistance Thermometer or PRT's with the most commonly available of them all the Pt100 sensor, which has a standard resistance value of 100Ω at 0oC. The downside is that Platinum is expensive and one of the main disadvantages of this type of device is its cost. • Platinum resistance thermometers (PRTs) offer excellent accuracy over a wide temperature range (from –200 to +850 °C).

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Advantages: Accurate Provide good stability and repeatability RTD's are also relatively immune to electrical noise and therefore well suited for temperature measurement in industrial environments, especially around motors, generators and other high voltage equipments. The resistance changes linearly with temperature so the measurement is quite simple. Suitable for precision applications Disadvantages: Lower temperature measuring range High cost The RTD's are not well suited to use in harsh environments compared to thermocouple. Have poor sensitivity, that is a change in temperature only produces a very small output change for example, 1Ω/oC.

Bi-metallic Thermostat • The thermostat consists of two thermally different metals stuck together back to back. • When it is cold the s are closed and current es through the thermostat. • When it gets hot, one metal expands more than the other and the bonded bi-metallic strip bends up (or down) opening the s preventing the current from flowing. • On/Off Thermostat • There are two main types of bi-metallic strips based mainly upon their movement when subjected to temperature changes. There are the "snap-action" (Switch) types that produce an instantaneous "ON/OFF" or "OFF/ON" type action on the electrical s at a set temperature point, and the slower "creep-action" types that gradually change their position as the temperature changes.

Thermistor • A thermistor is a temperature sensitive semiconductor resistor whose resistance varies significantly with temperature, more so than in standard resistors. • Thermistor are generally made from ceramic materials such as oxides of nickel, manganese and sulphides of iron , cooper or aluminum. • Most types of thermistor's have a Negative Temperature Coefficient of resistance or (NTC), that is their resistance value decreases with an increase in the temperature but some with a Positive Temperature Coefficient, (PTC), their resistance value goes UP with an increase in temperature are also available but they are less in use. • Thermistors typically have range −90 °C to 130 °C.

• Advantages: • Thermistors offer better accuracy in comparison to RTDs and thermocouples. • Unlike RTDs and thermocouples, they are highly sensitive.. • They are smaller in size as compared to thermocouples. • Thermistors provide faster response than RTDs. • They offer high stability and brilliant repeatability.. • Unlike thermocouples which provide millivolt outputs, use of thermistors result in reasonable output voltages. • Thermistors are particularly low cost temperature sensors. Hence, they are widely employed for simple temperature measurements. • Disadvantages: • Since thermistors are semiconductor devices, their operation is highly non linear. This effect of nonlinearity needs to be compensated before applying them in measurement circuits.

• Another disadvantage of thermistors is their limited temperature range due to which they are rendered unsuitable for use at higher temperatures. • Some of the applications of thermistors are • Thermistors are widely used in applications as temperature sensors and as current limiters. • A PTC is used as a current limiter. As the current increases the temperature increases. hence the resistance increases limiting the current.

Thermocouples • Thermocouples are thermoelectric sensors that basically consists of two junctions of dissimilar metals, such as copper and constantan that are welded or crimped together. • One junction is kept at a constant temperature called the reference (Cold) junction, while the other the measuring (Hot) junction. • When the two junctions are at different temperatures, a voltage is developed across the junction which is used to measure the temperature sensor.

• Advantage: • Capable of being used to directly measure temperatures up to 2600°C. • The thermocouple junction may be grounded and brought into direct with the material being measured. • High resolution • Disadvantage: • Temperature measurement with a thermocouple requires two temperatures be measured, the junction at the work end (the hot junction) and the junction where wires meet the instrumentation copper wires (cold junction). • Thermocouples operation are relatively complex with potential sources of error. The materials of which thermocouple wires are made are not inert and the thermoelectric voltage developed along the length of the thermocouple wire may be influenced by corrosion etc. • The relationship between the process temperature and the thermocouple signal (millivolt) is not linear.

Piezoelectric Transducers • The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element is the heart of the transducer as it converts the electrical energy to acoustic energy, and vice versa. • The active element is basically a piece of polarized material (i.e. some parts of the molecule are positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces. • When an electric field is applied across the material, the polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. This alignment of molecules will cause the material to change dimensions. This phenomenon is known as electrostriction.

• In addition, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. This phenomenon is known as the piezoelectric effect. • The active element of most acoustic transducers used today is a piezoelectric ceramic, which can be cut in various ways to produce different wave modes. • The thickness of the active element is determined by the desired frequency of the transducer. • A thin wafer element vibrates with a wavelength that is twice its thickness. Therefore, piezoelectric crystals are cut to a thickness that is 1/2 the desired radiated wavelength. The higher the frequency of the transducer, the thinner the active element. The primary reason that high frequency transducers are not produced is because the element is very thin and too fragile.