Transducers

ME 3251 Thermal Fluid Systems

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Transducers

• Transform values of physical variables into equivalent electrical signals

• Converts a signal from one form to another form

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Types of Transducers• Variable-Resistance

Transducer• Differential Transformer

(LVDT)• Capacitive Transducer• Piezoelectric Transducer• Photoelectric Transducer• Photoconductive

Transducer

• Photovoltaic Transducer• Ionization Transducer• Magnetometer Search

Coil• Hall-Effect Transducer• Digital Displacement

Transducer

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Variable-Resistance Transducer• Resistance Potentiometer, rheostat• Converts linear or angular displacement into an electric

signal• Can be used to convert force and pressure to a

displacement through mechanical methods• Moving contact on a slide-wire or moving contact that

moves through an angular displacement on a solid conductor (e.g. graphite)

• Fairly cheap – some can be expensive, though

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Differential Transformer (LVDT)• Coverts displacement into electric signal• Three coils placed in a linear relationship with a magnetic core free

to move inside the coils• Alternating input voltage impressed in center coil, and output voltage

from two end coils depends on magnetic coupling between the coreand coils

• The coupling is dependent on the position of the core• If the core remains near the center of the coils the output is very

nearly linear. • Most differential transformers are only operated in this linear range,

and these are called linear variable differential transducers (LVDT)

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LVDT Continued

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Capacitive Transducer

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• Measures a change in capacitance through a change in distance between 2 plates, d• Measures capacitance change through a change in overlapping area, A, resulting

from plate movement or a change in dielectric constant, ε

• 0.225 is use 0.0885 for

ε = 1 for airε = 1 for plastics

dAC ⋅⋅= ε225.0 2in 2cm

Capacitive Transducer cont.• For liquid level measurements two electrodes are placed in the

liquid, and the dielectric constant varies between them according to the liquid level

• Thus, capacitance between electrodes is a direct indication of liquid level

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Piezoelectric Transducer• A force applied to a piezoelectric crystal

between two electrodes produces a stress and corresponding deformation in the crystal

• Deformation produces potential difference at crystal surface

• The stress on the crystal produces an output voltage:

E = gtp

where t is crystal thickness in m p is impressed force in

g is voltage sensitivity

• Piezoelectric crystals used as pressure transducers for dynamic measurements

2mN

εdg =

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Photoelectric Transducer

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• Converts a light beam into a usable electric signal• Light strikes cathode and releases electrons• Electrons are attracted toward anode, thereby producing an electric

current in external circuit• Cathode and anode enclosed in glass or quartz, which is evacuated

or filled with inert gas• Photoelectric tubes useful for measuring light intensity

Photoconductive Transducers• Voltage impressed on semiconductor material• Light strikes semiconductor material and a decrease in

resistance occurs, thereby producing an increase in current

• Useful for measurement of radiation at all wavelengths

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Photovoltaic Transducers• Sandwich design of a metal base plate and a thin

transparent metallic layer with a semiconductor material in between

• Light strikes barrier between transparent metal layer and semiconductor material, and a voltage is generated

• Most widely used application of photovoltaic cell is the light exposure meter in photographic work

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Ionization Transducer

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• Converts displacement to a voltage through a capacitance change

• As tube moves horizontally between the two plates, a dc potential difference is created between electrodes 1 and 2

Magnetometer Search Coil

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• Transforms a magnetic field signal into a voltage

• A flat coil with a specified number of turns is placed in a magnetic field, and a voltage is generated

• To measure a steady magnetic field, the search coil must be moving. Typically a rotating coil is used in this situation.

Hall-Effect Transducers• An external current passes

through a semiconductor plate.

• When a magnetic field is impressed on a plate in a direction perpendicular to the plate surface, a voltage is generated

• This voltage is the Hall voltage

•

where is the Hall coefficient

tIBKE HH ⋅=

HK

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Digital Displacement Transducers• Used to measure angular and linear displacements• For an angular measurement, a wheel rotates and light is alternately

transmitted and stopped through small cutouts• The signal is sent to the photodetector, amplified, then sent to a

counter.• The number of counts is proportional to angular displacement.• Sensitivity can be improved by increasing the number of cutouts

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Digital Displacement Transducers

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• For a linear measurement, small reflecting strips are installed on a motion device.

• Light is then alternately reflected and absorbed with linear motion presenting a signal to the photodetector

• Calibration with a known displacement standard must be performed