Input Transducer

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    INPUT TRANSDUCERSINDUSTRIAL MEASURING

    DEVICES

    ELECTRICAL TRANSDUCERS

    Devices which convert the value of acontrolled variable into an electrical signalare called electrical transducers.

    Physical variables include: position,speed, acceleration, force, power,pressure, flow rate, temperature, lightintensity, and humidity.

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    Advantages of Electrical SignalsOver Mechanical Signals

    can be easily transmitted from place toplace

    easier to amplify and filter

    easy to manipulate to find out such thingsas the rate of change of variable, the timeintegral of the variable, whether the

    variable exceeded some limit, etc.

    POTENTIOMETER

    an electrical transducer that is attached to

    mechanical sensor to convert a mechanical

    motion into an electrical variation

    parts include: resistive element and a movable

    contact called tap, wiper, or slider positioned

    anywhere along the length of the element

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    POTENTIOMETER LINEARITY AND

    RESOLUTION

    Linear means that a given mechanicalmovement of the wiper produces a given

    change in resistance, no matter where the

    wiper happens to be on the element.

    Potentiometers used as industrial transducers

    have linearities of less than 1% or as low as

    0.1%.

    POTENTIOMETER RESOLUTION

    Resolution: the minimum possibleresistance variation, expressed as apercentage of the total resistance.

    Potentiometer used for industrialmeasuring purposes have good resolution,good linearity, and good environmentcharacteristics (temperature and humidity);but expensive.

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    POTENTIOMETER

    POTENTIOMETER

    -Conductive plastic- Infinite

    resolution - Servo mount-3600 degree rotation - 3600degree electrical angle-Power rating : 2 watt-Options : Taps, bushing mount,gears, flatted and slotted shafts

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    POTENTIOMETER

    -7813/14 -7/8" diameter-Conductive plastic- Infiniteresolution - Servo mount-3600 degree rotation - 3600degree electrical angle-Power rating : 2 watt-Options : Taps, bushing mount,gears, flatted and slotted shafts

    POTENTIOMETER

    - Rocker pots - looks likean on/off switch operateslike a single axis joystick- Foot Controllers- Cobra head joysticks

    http://www.precisionsales.com/PageMill_Images/FCW
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    POTENTIOMETER

    - M2261 - Single or Ten Turn

    potentiometer- Up to 6 single turn pots or

    switches can be added

    - MOTOR 115VAC : Reversible -

    RPM 1, 2, 4, 6, 10, 12, 30, 60

    - M3279 - Ten Turn potentiometer

    - MOTOR 115VAC : Reversible -

    RPM 1, 2, 4, 6, 10, 12, 30, 60

    - M3248 - Single Turn

    potentiometer- MOTOR 115VAC : Reversible -

    RPM 1, 2, 4, 6, 10, 12, 30, 60

    LINEAR VARIABLE DIFFERENTIALTRANSFORMER (LVDT)

    consists of one primary winding and twosecondary winding all wrapped on the sameform and contains a magnetic core which isfree to slide inside the form

    when the magnetic core is perfectly centered,the magnetic field linkage will be the same forsecondary winding 1 and 2

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    LINEAR VARIABLE DIFFERENTIALTRANSFORMER (LVDT)

    the difference between the two secondary

    winding voltages is proportional to the core

    displacement

    LVDTs are used to measure mechanical

    displacement

    LINEAR VARIABLE DIFFERENTIALTRANSFORMER (LVDT)

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    LINEAR VARIABLE DIFFERENTIALTRANSFORMER (LVDT)

    LINEAR VARIABLE DIFFERENTIALTRANSFORMER (LVDT)

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    LINEAR VARIABLE DIFFERENTIALTRANSFORMER (LVDT)

    Spring Loaded LVDTs Harsh Environment LVDTs

    PRESSURE TRANSDUCERS

    PRESSURE SENSING DEVICES:devices that detect the measured pressureand convert it into a mechanicalmovement which is then converted into anelectrical signal by either a pot or an

    LVDT. Types:

    1. BOURDON TUBES

    2. BELLOWS

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    BOURDON TUBES

    a deformed metal elastic tube with anoval cross section open at one end andsealed at the other end

    Types:

    1. C shaped 2. spiral

    3. Helical shaped 4. Twisted

    most often used to measure pressuresin the range from 10 to 300 psi

    C SHAPED BOURDON TUBE

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    SPIRAL BOURDON TUBE

    HELICAL BOURDON TUBE

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    BOURDON TUBES

    Liquid filled pressure gauge

    BELLOWS

    a series of metal diaphragms connectedtogether that when subjected to fluidpressure will distort slightly

    metal diaphragms movement will actuatesa mechanical linkage which moves thewiper of a pot or the core of an LVDT tofurnish an electrical output signal

    Bellows measure pressures in the rangefrom 0.5 to 20 psi

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    BELLOWS

    DUAL BELLOWS INDICATORDifferential pressure sensor dual bellows.

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    Dual bellows assembly

    BELLOWS

    http://www.bdengineers.com/htmls/pro_4_hinge_exp.htmhttp://www.bdengineers.com/htmls/pro_3_tide_universal_exp.htmhttp://www.bdengineers.com/htmls/pro_2_universal_exp.htmhttp://www.bdengineers.com/htmls/pro_1_single_exp.htm
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    THERMOCOUPLES

    an industrial temperature measurement

    transducer consist of a pair of dissimilar

    metal wires joined together in a complete

    loop forming two junction

    Cold junction: subjected to a low temperature

    Hot junction: subjected to a high temperature

    THERMOCOUPLES

    a small net voltage is created in the loopproportional to the difference between thetwo junction temperature.

    the higher the temperature at the junction,

    the greater the voltage produced by thatjunction.

    relationship between voltage andtemperature is approximately linear.

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    THERMOCOUPLES

    THERMOCOUPLES

    Thermocouples will cause an electriccurrent to flow in circuit when subjected tochanges in temperature.

    The amount of current that will be

    produced is dependent on the temperaturedifference between the measurement andreference junction; the characteristics of thetwo metals used; and thecharacteristics of the attached circuit.

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    THERMISTOR

    a temperature measurement devicethat uses metallic oxides that is formedinto shapes which resemble small bulbsor small capacitors

    utilize the resistance change as theirtemperature changes

    with large negative temperature

    coefficients and are non constant

    THERMISTOR

    non linear response: poorly suited formeasuring temperatures over wideranges

    great response for small temperature

    change preferable when the expected

    temperature band is narrow (-150oF to+800oF)

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    THERMISTOR

    Thermistor Composite Network: connecting the

    thermistors in a series parallel combination

    to compensate its non linearity response.

    used over a wide temperature range (about

    200oF)

    more expensive

    Thermistor Self Heating (I2R): used for time

    delays, delicate components protection fromsurge current, detecting the presence or

    absence of a thermally conductive material

    RESISTIVE TEMPERATUREDETECTORS(RTDs)

    a temperature measuring device that uses a

    pure metal wire (nickel, copper, platinum)

    utilize the resistance change as their

    temperature changes

    with positive temperature coefficient orresistance

    preferable when the expected temperature

    band is wide (-400oF to +2000oF)

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    OPTICAL PYROMETERS

    a temperature transducer used in hightemperatures and when contact typetemperature sensors are impractical

    measures temperature from a distance bysensing the visible (light) and/ or invisibleelectromagnetic radiation emitted by the

    hot body

    PHOTOCELLS

    small devices which produce anelectrical variation in response to achange in light intensity (illumination)

    Types:

    1. photovoltaic

    2. photoconductive

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    PHOTOVOLTAIC CELL

    an energy source whose output voltagevaries in relation to the light intensity(illumination) at its surface

    fast response

    can be switched at higher frequencies

    PHOTOCONDUCTIVE CELL

    a passive device, not capable ofproducing energy; its resistance variesin relation to the light intensity(illumination) at its surface

    as the illumination goes up, resistancegoes down

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    2

    R2

    10k

    5V

    R1

    R2

    10k

    - Resistance Varies with Incident Light- More Light = Lower Resistance

    PHOTOCONDUCTIVE CELL

    PHOTOCONDUCTIVE CELL

    cells resistance change over 1 million ohms toless than 1 kilo ohms as the illuminationchanges from darkness (less than 0.01fc) toaverage room brightness (10 100fc)

    preferred when very sensitive response tochanging light condition is needed

    used in chopping a dc voltage signal insertioninto an ac amplifier (photo chopper)

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    END

    END