Displacement Velocity Transducer

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Measurements and Instrumentation

ECCE3036

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Potentiometer Rotary Potentiometer Optical Potentiometer


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Rectilinear/Rotary Potentiometer:Thetransducer output signal is a dc voltagewhich is proportional to the potentiometerresistancethat is proportional to the sliderdisplacement.

Optical Potentiometer: It has a photo-resistive layerwhich acts as an electricalinsulator if no light is projected on it. The

displacement of moving object a movinglight beam which cause a change in voltageaccordingly.


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They are relatively inexpensive. Potentiometers provide high-voltage

(low-impedance) output signals, requiring

no amplification in most applications. They are simple voltage dividers and don't

need any special conditioning electronics

They monitor in real-time, without any

signal lag from electronics, to give anaccurate results


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The force needed to move the slider is providedby the displacement source. This mechanicalloading distorts the measured signal itself.

High-frequency (or highly transient)measurements are not feasible

Variations in the supply voltage cause error.

Resolution is limited by the number of turns inthe coil and by the coil uniformity.

Wear out and heating up in the coil or film, and

slider contact cause accelerated degradation.


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The majority of suspension positionmonitoring on saloon cars, single seat racecars and motorbikes use hybrid linearpotentiometers.

In servo technology applications. All applications that requires simple

displacement transducer For real time applications since there is no

signal lag.


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Linear-Variable Differential Transformer (LVDT)


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Motion of a magnetic core changesthe mutual inductance of twosecondary coils relative to a primarycoil.

Primary coil voltage: VS

sin wt)

Secondary coil induced emf:V1=k1sin wt)

andV2=k2sin wt)

Where k1 and k2 depend on theamount of coupling between theprimary and the secondary coils,which is proportional to the positionof the coil.

When the coil is in the centralposition,

k1=k2

VOUT

=V1-V2=0

When the coil is displaced xunits,k1 2

V

OUT

= k1-k2)sin wt)

Positive or negative displacementsare determined from the phase ofV

OUT

.


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It is essentially a non-contacting devicewith no frictional resistance. Near-idealelectromechanical energy conversion andlight-weight core will result in very small

resistive forces It has low output impedance, typically on

the order of 100 . (Signal amplification isusually not needed beyond what is

provided by the conditioning circuit.) Directional measurements


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Inductive sensors require sophisticated signalconditioning electronics to condition andlinearize the coil signal.


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In making tablets from medicinalpowder, dual LVDTs control pill weight &thickness.

Portable Friction Welder:LVDT measuresthe distance between the approachingmetals.

Manufacturing process controls, valveand flow controls, pneumatic cylindercontrols, head box (papers and pulp)


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Proximity sensors (either capacitive orinductive) can be used to sense distance.

Proximity sensors are usually used asswitches to provide a clear indication whena certain, preset distance is reached.

Inductive sensors can produce an electricoutput such as voltage based on the change

in their impedance


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An "E core" carries the primary windings in itsmiddle limb and the secondary windings onthe other two limbs. The two voltagesinduced in the secondary windings areadditive.


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The capacitance between two plates isdetermined by three things: Size of the plates: capacitance increases as the

plate size increases

Gap Size: capacitance decreases as the gapincreases Material between the plates (the dielectric):

Dielectric material will cause the capacitance toincrease or decrease depending on the material

Gap

DielectricAreaC

=


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Capacitive-Displacement Sensors

One of the capacitor plates is attached tothe moving object and the other is keptstationary. Therefore the capacitance is

proportional to the object displacement.


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Capacitive-Displacement SensorsOne plate of the capacitor rotates with a rotatingobject (shaft) and the other plate is keptstationary. Since the common area is proportionalto the angle of rotation then the capacitance is

proportional to the angle of rotation.


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Liquid level sensor


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Low cost and power usage, Good stability, resolution, and speed. They also are easy to be integrated into

ICs or onto printed-circuit boards (pc

boards). Capacitive sensors can detect motion,

acceleration, flow, and many othervariables, and are used in a wide range of

applications. Mechanical loading effects are negligible,

because they are non-contacting device


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They are affected by temperature andhumidity

Sensitivity to noise, Difficulties in designing,

Capacitive sensors do need somespecialized design know-how to avoidsome hazards,

Capacitance sensors need to be adjustedfor sensitivity. This adjustment also needsto include variations due to moisture orother environmental factors.


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Capacitive sensors can be used todetermine the presence or absence ofliquids or solids through non-conductivecontainers, tubes or pipes.

A capacitance type sensor can be an idealsolution where cost is a critical factor. Capacitive gauges and capacitive sensors

are used to measure structural vibration. Capacitance sensors are ideal for

providing servo system feedback in piezomotor driven nanopositioningapplications.


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DC TachometerSimilar to a dc generator. The rotor is directly connected tothe rotating object. The output signal that is

induced in the

rotating coil

is picked up as adc voltage

using a suitablecommutator device. According to Faraday's law,

the induced

voltage is proportional to the rate of change in magnetic flux

linkage.

The proportionality between the output voltage andthe angular velocity is used to measure the angular velocity,


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For a coil of height h and width2rthat has nturns, moving at an angular speed w

c

in auniform magnetic field of flux this is given by:

cco Kwwnhrv == )2(

This proportionality between vo

and wc

is usedto measure the angular speed w

c

. Theproportionality constant Kis known as back-emfconstant.


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Permanent Magnate AC Tachometer

One set of the windings is energized using an acreference voltage. When the rotor is stationary ,the output voltage is a constant amplitude signalmuch like the reference voltage. As the rotormoves in a finite speed an additional inducedvoltage in the other set of windings, is generatedin the secondary windings. This voltage isproportional to the rotor speed


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AC Induction Tachometer

Similar to a 2-phase induction motor. It is alsosame as the Permanent-Magnate AC Tachometerexcept that the rotor has windings, which areshorted and not energized by an external source.The induced voltage is proportional to the speedof the rotor rotation. The output voltage is aresult of both the stator (primary) windings andthe rotor winding.


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The absence of slip-ring and brush devices,since the output is obtained from the stator.

Relatively accurate speed readings


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The noise components will dominate at low levelsof output signal. In particular, since the output ofa tachometer is proportional to the measuredspeed, at low speeds, the level of noise, as afraction of the output signal, can be large.

Signal demodulation is necessary, particularly formeasuring transient speeds. The output signal level depends on the supply

voltage; hence, a stabilized voltage source, whichhas very small output impedance is necessary foraccurate measurements.

At high speeds the output from an ac tachometeris somewhat nonlinear (primarily due to thesaturation effect)


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AC and DC motors speed control. Automotive speed gauge

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Displacement Velocity Transducer