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8/3/2019 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
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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.htm8/3/2019 Input Transducer
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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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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
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