SLIDING CONTACT DEVICES

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Definition 

Sliding contact device is a resistive type of transducer, whichconverts a mechanical displacement input into an electricaloutput either in the form of voltage or current.

We know that

R=Resistance ρ=Resistivity of the material

L=Length of conductor A= Crossectional Area

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AρLR =

– Here ‘L’ is the mechanical displacement.– When ‘L’ changes resistance changes, which leads to change in voltage or current (V=IR).

– These devices are commonly called resistance potentiometer (POT)

– Generally a POT consists of a resistive element and a slider known as wiper.

– Depending upon motion of wiper POT are of 3 types:‐1. Translational POT2. Rotational POT3. Helipot (Both Translational & Rotational)

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Translational/Linear POT

+ ei -- eo +

Guide Rod

Slider or brush Resistance wire

Translational/Linear POT

• The effective length of the conductor is changed bya sliding contactor or brush mechanism, that slideon some form of electrical resistance element.

• The effective resistance exist between either end ofthe wire and the wiper/brush become a measure ofthe mechanical displacement.

• Devices of this type have been used for sensingrelatively large displacement.

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• There is a commonly used resistance elementformed by wrapping a resistance wire around a cardknown as mandrel.

• The turns are spaced to prevent to prevent shortingand wiper slides across the turns from one turns tonext.

• The translational resistive element are straightdevices and have a stroke of 2mm to 0.5m.

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Translational/Linear POT

Derivation for output voltage/emf

• ei=Input voltage (V)• eo=Output voltage (V)• xt=total length of translational POT (m)• xi=Displacement of wiper from zero position (m)• Rp=Total resistance of potentiometer

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• The resistance per unit length• Output voltage under ideal condition

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ageinput voltinalsinput term at the resistance

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×=⎥⎥⎦

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InputOutputySensitivit ===

Rotational POT

These are circular in shape and usedfor measurement of angulardisplacement.They may have full scale angulardisplacement as small as 10°.A full single turn POT may provideaccurate measurement up to 357°.Rotational POT are of two type:-

(a) Wire-wound type Rotational POT

(b) Carbon-composition film type Rotational POT

Rotational POT

In carbon-composition film typerotational POT, the film is supportedby a ceramic or plastic backing.These are less expensive than wire-wound type.These are of good wear characteristicand long life.

Helipot(Both Translational & Rotational)

Helipot(Both Translational & Rotational)

Hlipot have helical resistive elementsare multi turn rotational deviceswhich can be used for measurementof either translational or rotarymotion.It can measure up to 3500° ofrotation.

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Application

– Thermistor/Variable resistor– Volume changer– Fan switch– Pressure gauges– Strain gauges 

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Potentiometer Resolution

• It is the smallest increment up to which thewhole resistance can be divided. In a wirewound resistance, the limiting resolution is thereciprocal of number or turns.

• ExampleSuppose number of turns of wire used is 1200Resolution = 1/1200 = 0.09083 %.

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Potentiometer Linearity

• A linear potentiometer is normally used asmeasurement transducer. The term linearity isused assuming that the resistance measuredbetween one end of the element and thecontactor is a direct linear function of thecontactor position in relation to that end.

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Question

#1 A linear resistance potentiometer is 50 mm longand is uniformly wound with wire having aresistance of 10,000 Ω. Under normal conditions,the slider is at the centre of the potentiometer.Find the linear displacement when the resistanceof the potentiometer as measured by awhetstone’s bridge for two cases is (a) 3850 Ω and(b) 7560 Ω.

• Are the two displacements in same direction? If itis possible to measure a minimum value of 10 Ωresistance with the above arrangements, find theresolution of the potentiometer in mm.

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Variable-Inductance Transducer Elements

• Inductance is that property of a circuit by which a change incurrent in the circuit induces a voltage (emf) in both the circuititself (self-inductance) and any nearby circuits (mutualinductance). This effect derives from two fundamentalobservations of physics: First, that a steady current creates a steadymagnetic field (Oersted’s law); second, that a time-varyingmagnetic field induces voltage in a nearby conductor (faraday’slaw of induction). In the SI system the unit of inductance is Henry.

• The relationship between the self inductance L of an electricalcircuit, voltage, and current is

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• Variable-Inductance transducers are based on the principle ofvoltage output of an inductor or a coil due to change in itsinductance in response to change in a measurand, which maybe displacement, velocity or acceleration etc.

• Variable-Inductance transducers are broadly classified in to three categories:

A. Variable self inductance (i) Single coil (ii) Two coils (or single coil with center

tap)B. Variable mutual inductance (i) Two coils (ii) Three coils (using series opposition)C. Variable reluctance(i) Moving iron (ii) Moving coil(iii) Moving magnet

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.• Inductive reactance, which quantifies inductive effect may be expressed by,

XL = 2πfL -------(2)

where XL = Inductive reactance (Ω)f = Frequency of applied voltage (Hz)L = Inductance (H)

• Inductance depends upon number of turns of the coil, coil size and permeability of flux path.

• For a cylindrical air-core coil, inductanceL = (d2.n2) / (18.d + 40.a) -----------(3)

where d = diameter of coil, a = coil length, n= no. of turns

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• When the flux path includes both magnetic material (usually iron) and air gap(s), then inductance

L = (1.26 n2 x 10-8) / [(hi / µai) + (ha / aa)] -------(4)where hi = Length of iron circuit

ha = Length of air gapsai =c.s. area of iron

aa = c.s. area of air gapµ = Permeability of magnetic material at maximum flux

density• If permeability of magnetic material is very high, then only air gaps

are to be considered. So, above equation reduces to L = 1.26 n2 (aa / ha) x 10-8 --------------(5)

• Total impedance of a coil may be expressed by, Z = √ (XL2 + R2),

where R = dc resistance• Quality of a coil, Q = XL / R -----------(6)

Variable self inductance transducers

Fig-1 Single coil self inductance arrangement

A single coil variable selfinductance type transducer isshown in Fig-1, where themechanical input changes thequantity of the flux pathgenerated by the coil, therebychanging its inductance. Thechange in inductance is thenmeasured by a suitable circuit,indicating the input value.The path of flux may be changedby changing air gap or amountor type of core material.

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• A two coil self inductance type transducer element is shown inFig-2. Movement of the core or armature alter the relativeinductance of two coils. This transducer elements are incorporatedwith inductive bridge circuits, in which the variation of inductanceratio between the two coils provides the output.

Fig-2 Two coil induction ratio transducer

Variable self inductance transducers

Two coil variable mutual inductance transducers

• Fig-3 shows a two coil variable mutual inductance transducer.Coil A is energizing coil and B is the pick up coil. If thearmature moves, there by altering the air gap, the coupling oftwo coils changes and output of coil B is changed and thischange may be used as a measure of armature movement.

Fig-3 Mutual inductance Transducer

Linear Variable Differential Transformer (LVDT)

• It is an example of three coil variable mutual inductance transducer,which provides an ac voltage output proportional to displacement ofthe core passing through the windings.

• It consists a primary winding (P) and two secondary windings (S1 andS2), placed on either side of the primary, mounted on same movablemagnetic core.

Fig-4 Linear Variable Differential Transformer (LVDT)

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• The two secondaries S1 and S2 have equal number of turns andconnected in series, so that the e.m.f.s (E1 and E2) induced inthem are 1800 out of phase, hence by cancelling each other’seffect. The primary is energized by suitable A.C. source.

• When the core is at the centre (reference position), E1 and E2 areequal and opposite, hence by cancelling each other’s effect andnet output voltage V0 is zero.

• When the core moves towards S2, E2 increases and E1 decreases,thus net voltage available is (E2 - E1) and is in phase with E2.Similarly, if the core moves towards S1, E1 increases and E2decreases, thus net voltage available is (E1 – E2) and is in phasewith E1. So, it is found that the magnitude of V0 is a function ofdistance moved by the core and its polarity indicates to whichdirection it is moved.

#2 QuestionThe output of a LVDT is connected to a 4 V voltmeterthrough an amplifier whose amplification factor is 500.An output 1.8 mv appears across the terminals of LVDTwhen the core moves through a distance of 0.6 mm. Ifthe millivoltmeter scale has 100 divisions and the scalecan be read to ¼ of a division, calculate

(i) Sensitivity of LVDT(ii) Resolution of the instrument

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Variable Reluctance Transducer

• The term variable reluctance implies some form of inductance deviceincorporating a permanent magnet. Generally these devices areemployed to dynamic application, either periodic or transient.

Fig-5 Simple variable reluctance transducer

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• It involves relative motion between two elementsdue to which magnetic flux lines are cut by turns ofthe coil.

• The simplest form of a variable reluctance transduceris shown in Fig-5 , where a coil is wound on apermanent magnet core. Relative motion is providedby form shape shown in figure. Variation of quantityof magnetic field causes a change in flux. As the fluxfield expands or collapses, a voltage is induced in thecoil, according to Faraday’s law, i.e. V = -n (dΦ/dt)

Where V = Induced voltage (V)n = No. of turns in coilΦ = Magnetic flux through coil (Wb)