STRAIN GAUGES →
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The strain gauge is an example of a passive transducer that uses the variation in electrical resistance in wires to sense the strain produced by a force on the the wires. It is well known that stress (force/unit area) and strain (elongation or compression/unit length. Pressure is directly related to the modulus of elasticity. ince strain can be measured more easily by using variable resistance transducers! It is a common practice to measure strain instead of stress! to serve as an index of pressure such transducers are popularly known as strain gauges. If a metal conductor is stretched or compressed! its resistance changes on account of the fact that both the length and diameter of the conductor changes. "lso! there is a change in the value of the resistivity of the conductor when sub#ected to strain! a property called the pie$o% resistive effect. Therefore! resistance strain gauges are also known as pie$o resistive gauges. &hen a gauge is sub#ected to a positive stress! its length increases while its area of cross%section decreases. ince the resistance of a conductor is directly proportional to its length and inversely proportional to its area of cross%section! the resistance of the gauge increases with positive strain. The change in resistance value of a conductor under strain is more than for an increase in resistance due to its dimensional changes. This property is called the pie$o%resistive effect.
The following types of strain gauges are the most important. 1. 2. !.
Wire strain gauges oil strain gauges Semi"on#u"tor strain gauges
1. Resi Resist stan an"e "e Wire Wire Gau Gauge ge →
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esistance wire gauges are used in two basic forms! the unbonded type! and the bonded type. Un$on#e# Resistan"e Wire Strain Gauge
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"n unbonded strain gauge consists of a wire stretched between two points in an insulating medium! such as air. The wires are kept under tension so that there is no sag and no free vibration. nbonded strain gauges are usually connected in a bridge circuit. The bridge is balanced with no load applied.
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%on#e# Resistan"e Wire Strain Gauge
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This is usually bonded to the member undergoing stress.
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The grid of fine wire is bonded on a carrier which may be a thin sheet of paper! +akelite! or Teflon. The wire is covered on the top with a thin material! so that it is not damaged mechanically. The spreading of the wire permits uniform distribution of stress. The carrier is then bonded or cemented to the member being studied. This permits a good transfer of strain from carrier to wire. " tensile stress tends to elongate the wire and thereby increase its length and decrease its cross%sectional area. The combined effect is an increase in resistance! as seen from the following e,uation
Types of Strain Gauges &Wire' 1. 2. !. ).
Gri# type Rossette type Tor(ue type *eli"al type
To o$tain goo# results+ it is #esira$le that a resistan"e wire strain gauge ha,e the following "hara"teristi"s.
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The strain gauge should have a high value of gauge factor (a high value of gauge factor indicates a large change in resistance for particular strain! implying high sensitivity). *. The resistance of the strain gauge should be as high as possible! since this minimi$es the effects of undesirable variations of resistance in the measurement circuit -. The strain gauge should have a low resistance temperature coefficient. This is necessary to minimi$e errors on account of temperature variation! which affects the accuracy of measurements.
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The strain gauge should not have hysteresis effects in its response. . In order to maintain constancy of calibration over the entire range of the strain gauge! it should have linear characteristics! i.e. the variation in resistance should be a linear function of the strain.
oil Strain Gauge →
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This class of strain gauges is an extension of the resistance wire strain gauge. The strain is sensed with the help of a metal foil. The metals and alloys used for the foil and wire are nichrome! constantan (0i 1 2u)! isoelastic (0i 1 2r 1 3o)! nickel and platinum. 4oil gauges have a much greater dissipation capacity than wire wound gauges! on account of their larger surface area for the same volume. 4or this reason! they can be used for a higher operating temperature range. "lso! the large surface area of foil gauges leads to better bonding. 4oil type strain gauges
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have similar characteristics to wire strain gauges. Their gauge factors are typically the same. The advantage of foil type strain gauges is that they can be fabricated on! a large scale! and in any shape. 5tched foil gauge construction consists of first bonding a layer of strain sensitive material to a thin sheet of paper or +akelite. The portion of the metal to be used as the wire element is covered with appropriate masking material. This method of construction enables etched foil strain gauges to be made thinner than comparable wire units.
Semi"on#u"tor strain gauges
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To have a high sensitivity! a high value of gauge factor is desirable. " high gauge factor means relatively higher change in resistance! which can be easily measured with a good degree of accuracy. emiconductor strain gauges are used when a very high gauge factor is re,uired. They have a gauge factor 7 times as high as wire strain gauges. The resistance of the semiconductor changes with change in applied strain. emiconductor strain gauges depend for their action upon the pie$o resistive effect! i.e. change in value of the resistance due to change in resistivity! unlike metallic gauges where change in resistance is mainly due to the change in dimension when strained. emiconductor materials such as germanium and silicon are used as resistive materials. " typical strain gauge consists of a strain material and leads that are placed in a protective box. emiconductor wafer or
filaments which have a thickness of 7.7 mm are used. They are bonded on suitable insulating substrates! such as Teflon. → 9old leads are generally used for making contacts.
A#,antages of Semi"on#u"tor Strain Gauge
'. emiconductor strain gauges have a high gauge factor. *. This allows measurement of very small strains! of the order of 7.7' micro strains. -. :ysteresis characteristics of semiconductor strain gauges are excellent! i.e. less than 7.7;. .
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T*ER IST/R →
The electrical resistance of most materials changes with temperature.
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+y selecting materials that are very temperature sensitive! devices that are useful in temperature control circuits and for temperature measurements can be made. Thermistor (T:53ally sensitive resIT>) are non%metallic resistors (semiconductor material)! made by sintering mixtures of metallic oxides such as manganese! nickel! cobalt! copper and uranium. Thermistors have a
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0egative Temperature 2oefficient (0T2)! i.e. resistance decreases as temperature rises. →
The resistance at room temperature (*@2) for typical commercial units ranges from '77 to '7 mega ohm.
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They are suitable for use only up to about =77@2.
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In some cases! the resistance of Thermistor at room temperature may decrease by 5% for each '@2 rise in temperature. This high sensitivity to temperature changes makes the Thermistor extremely useful for precision measurements! control and compensation.
A#,antages of Thermistor
'. mall si$e and low cost. *. 4ast response over narrow temperature range. -. 9ood sensitivity in the 0egative Temperature 2oefficient region.
0imitations of Thermistor
'. 0on%linearity in resistance vs temperature characteristics. *. nsuitable for wide temperature range. -. Aery low excitation current to avoid self% heating. . 0eed of shielded power lines! filters! etc. due to high resistance. 0INEAR ARIA%0E -IERENTIA0 TRANS-UER &0-T' →
The differential transformer is a passive inductive transformer.
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It is also known as a
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The the transformer consists of a single primary winding P l and two secondary windings S l and S 2 wound on a hollow cylindrical former.
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The secondary windings have an e,ual number of turns and are identically placed on either side of the primary windings. The primary winding is connected to an ac source. " movable soft iron core slides within the hollow former and therefore affects the magnetic coupling between the primary and the two secondary. The displacement to be measured is applied to an arm attached to the soft iron core. In practice! the core is made up of a nickel%iron alloy which is slotted longitudinally to reduce eddy current losses. &hen the core is in its normal (null) position! e,ual voltages are induced in the two secondary windings. The fre,uency of the ac applied to the primary winding ranges from 7 :$ to *7 k:$. The output voltage of the primary windings S 1 is E s1 and that of secondary winding * is E s2 . In order to convert the output from S l to S 2 into a single voltage signal! the two secondaries S 1 and S 2 are connected in series opposition! as shown in 4ig.
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:ence the output voltage of the transducer is the difference of the two voltages.
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Therefore the differential output voltage 5 o C E s1 ~
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&hen the core is at its normal position! the flux linking with both secondary windings is e,ual! and hence e,ual emfs are induced in them. :ence! at null position E s1 C E s2 . ince the output voltage of the transducer is the difference of the two voltages! the output voltage E o is $ero at null position.
A#,antages of 0-T
'. 0inearity .The output voltage of this transducer is practically linear for displacements upto mm (a linearity of 7.7; is available in commercial
. *igh sensiti,ity. The transducer possesses a sensitivity as high as 7 A/mm. . Rugge#ness. These transducers can usually tolerate a high degree of vibration and shock. 6. 0ess fri"tion. There are no sliding contacts. 8. 0ow hysteresis. This transducer has a low hysteresis hence repeatability is excellent under all conditions. =. 0ow power "onsumption .3ost
-isa#,antages
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. Temperature also affects the transducer.
Resistan"e Temperature -ete"tor &RT-' →
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esistance temperature detector commonly uses platinum! nickel or any resistance wire whose resistance varies with temperature and which has a high intrinsic accuracy. They are available in many configuration and si$esD as shielded or open units for both immersion and surface applications. The relationship between temperature and resistance of conductors in the temperature range near 7@2 can be calculated using the e,uation
"lmost all metals have a positive temperature coefficient (PT2) of
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resistance. o that their resistances increase with increase in temperature. ome materials! such as 2arbon and 9ermanium have a negative temperature coefficient (0T2) of resistance. " high value of E temperature coefficient of resistance E is desired in a temperature sensing element! so that sufficient change in resistance occurs for a relatively small change in temperature. This change in resistance (∆) can be measured with a &heatstoneEs bridge which can be calibrated to indicate the temperature that caused the resistance change rather than the resistance itself. TBEs are wire%wound resistance with moderate resistance and a PT2 of resistance. Platinum is the most widely used resistance wire type because of its high stability and large operating range. :owever! 0ickel and 2opper are also used in TBs. Platinum TBs provide high accuracy and
stability.
They ha,e the following a#,antages3
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Therm o"ouple
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>ne of the most commonly used methods of measurement of moderately high temperature is the thermocouple effect. &hen a pair of wires made up of different metals is #oined together at one end! a temperature difference between the two ends of the wire produces a voltage between the two wires. Temperature measurement with Thermocouple is based on the eebeck ef% fect. " current will circulate around a loop made up of two dissimilar metals when the two #unctions are at different temperatures
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&hen this circuit is opened! a voltage appears that is proportional to the observed seebeck current. There are four voltage sourcesF their sum is the observed seebeck voltage. 5ach #unction is a voltage source! known as Peltier emf. 4urthermore! each homogenous conductor has a self induced voltage or Thomson emf. The Thomson and Peltier emfs originate from the fact that! within conductors! the density of free charge carriers (electrons and holes) increases with temperature. If the temperature of one end of a conductor is raised above that of the other end! excess electrons from the hot end will diffuse to the cold end. This results in an induced voltage! the Thomson effect! which makes the hot end positive with respect to the cold end. 2onductors made up of
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different materials have different free%carriers densities even when at the same temperature. &hen two dissimilar conductors are #oined! electrons will diffuse across the #unction from the conductor with higher electron density. &hen this happens the conductor losing electrons ac,uire a positive voltage with respect to the other conductor. This voltage is called the Peltier emf. &hen the #unction is heated a voltage is generated! this is known as seebeck effect. The seebeck voltage is linearly proportional for small changes in temperature. Aarious combinations of metals are used in ThermocoupleEs. The magnitude of this voltage depends on the material used for the wires and the amount of temperature difference between the #oined ends and the other ends. The #unction of the wires of the thermocouple is called the sensing #unction.
A#,antages of Thermo"ouple
'. It has rugged construction. *. It has a temperature range from %*87 @2%*877 @2. -. sing extension leads and compensating cables! long distances transmission for temperature measurement is possible. . +ridge circuits are not re,uired for temperature measurement. . 2omparatively cheaper in cost. 6. 2alibration checks can be easily performed. 8. Thermocouples offer good reproducibility. =. peed of response is high compared to the filled system thermometer. ?. 3easurement accuracy is ,uite good. -isa#,antages of Thermo"ouple '.
2old #unction and other compensation are essential for accurate measurements. *. They exhibit non% linearity in the emf versus temperature characteristics. -. To avoid stray electrical signal pickup! proper separation of extension leads from thermocouple wire is essential. . In many applications! the signals need to be amplified.
