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TRANSDUCERSTransducer a device that converts one form of energy into another form of energy

Sensor a device that detects/measures changes in physical/chemical quantity and gives

proportional electrical output

Detector a device used to sense physical quantity or phenomenon

Transmitter a device that converts output of the primary element into usable signal (4-20ma, 0-10v,etc)

TRANSDUCER = Sensing element + Signal conditioning circuits

amplifiers, bridge circuits, ADC, DAC, voltageconverters, current

converters, frequency to voltage converter, voltage to

frequency converter

ACCDG. TO FUNCTION

1. Input Transducer (Sensor) - converts physical/chemical quantity into electrical

signal (voltage, current)Ex. Microphone (acoustic/sound pressure into electrical signal)

2. Output Transducer (Actuator) converts signal (usually electrical) into action

(usually mechanical)

Ex. Loudspeaker (electrical signal into acoustic/sound pressure)

SENSORS ACTUATORS

LDR LampPhotodiode

PhotovoltaicPhotoconductive cell

Thermocouple HeaterRTD

ThermistorBimetallic

Liquid-filled glass thermometerPyrometers

Strain gauge SolenoidLoad cells Relay

Potentiometer Motor (stepper, servo)Capacitive sensorInductive sensor

LVDT

Piezoelectric sensorAccelerometer

EncodersTachometersBourdon tubes

bellowsDiaphragm

Loudspeaker

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microphone HeadphoneBuzzer

Hall-effect sensorsFlux-gate compassMagneto resistor

Gas sensor

ACCDG. TO POWER REQUIREMENT

1. Passive (self-generating) generates electrical output without power source

Ex. Thermocouple, photovoltaic cells

2. Active (modulating) requires power source to generate electrical output

Ex. Thermistor, potentiometer

ACCDG. TO PHYSICAL/CHEMICAL QUANTITY

1. Thermal temperature, heat, heat flow

2. Mechanical force, pressure, velocity, acceleration, position

3. Chemical concentration or composition

4. Magnetic field intensity and flux density

5. Radiant electro-magnetic wave intensity, wavelength, polarization and phase

6. Electrical voltage, current, charge

THERMAL SENSORS

1. Thermocouple temperature transducer consisting of two wires made of differentmetalssoldered or welded together.

- principle: Seebeck Effect (opposite of Peltier Effect)

- Seebeck Effect - a phenomenon by which a soldered or welded junction of two

dissimilar metals

generates a voltage that is proportional to the temperature of the junction.

- Thermopile group of thermocouples connected in series

Temp = Voltage Output

Types: Type T > copper(+) and constantan(-) /( for general purpose and low

temp)

Type E > chromel(+) and constantan(-)

Type J > iron(+) and constantan(-)

Type K > chromel(+) and alumel(-)

Constantan = copper and nickel

Chromel = nickel + chromium

Alumel= nickel + aluminium + Mn + Si

2. RTD Resistance Temperature Detector or resistive temperature detector

- Metal or alloys

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- Most stable and linear but expensive and with small resistance change with

temp

Temp = Resistance (PTC)

3. Thermistor - Thermal Resistor

- Semiconductor type, most sensitive but non-linear

- Self-heating is a disadvantage for several temperature transducers, including

thermistors. Selfheating is a device's tendency to heat up beyond its

surrounding (ambient) temperature due to its own power dissipation.

- Most thermistors have Temp = Resistance (NTC thermistor)

- But there are thermistors with Temp = Resistance (PTC thermistor)

4. Bimetallic (or thermostat) constructed from two strips of different metals bonded

together with different coefficient of thermal expansion. When the strip is heated

one metal expands lesser while the other metal expands more leading to the

deflection of the bimetallic strip, which is converted into the rotary motion of the

pointer that indicates the temperature.

types: snap-action(fast) and creep-action(slow)

converts temperature into physical displacement

5. Optical and Radiation Pyrometers used where direct contact is not possible

Principle: A body above 0K emits electromagneticradiation. Intensity of the radiation is a measure of the

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temperature of the body. Intensity ranges from invisibleinfra-red rays to visible light range.

6. Liquid Filled thermal systems ex. Glass Thermometers

MECHANICAL SENSORS

1. Capacitive sensor = principle:

2. Piezo electric sensor principle: piezo electric effect - when mechanical force is

applied to crystal, proportional voltage is produced, vice versa

3. Accelerometer converts acceleration into variable electric currentusing spring-

mass system

- Newtons 2nd law (F=ma) and Hookes Law (F=kDx)

4. Inductive sensor when a conductor is passed with current, electro-magnetic field iscreated around theconductor, and when disturbed by nearby ferrous material,

eddy current is produced

- Eddy current sensor

Eddy-Current sensors operate with magnetic fields. The driver creates an alternating

current in the sensing coil in the end of the probe. This creates an alternating

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magnetic field with induces small currents in the target material; these currents are

called eddy currents. The eddy currents create an opposing magnetic field which

resists the field being generated by the probe coil. The interaction of the magnetic

fields is dependent on the distance between the probe and the target. As the

distance changes, the electronics sense the change in the field interaction and

produce a voltage output which is proportional to the

change in distance between the probe and target.

5. LVDT (linear variable differential transformer)

6. Strain gauge

strain gauge - a transducer whose resistance varies as a function of strain.strain - the amount of deformation of a solid resulting from stress; expressedmathematically as the ratio of a change in an object's length to its initial unstressedreference length.stress - a force acting on a solid's unit area.tensile strain - strain that increases the length of a solid (also called positive strain).compressive strain - strain that reduces the length of a solid (also called negativestrain).

load cell - a device with one or more strain gauges mounted to a solid for the purposeof measuring strain in one or more directions

Strain gauges may be purchased as complete units, with both strain gauge elementsand bridge resistors in one housing, sealed and encapsulated for protection from the

elements, and equipped with mechanical fastening points for attachment to a

machine or structure. Such a package is typically called a load cell.

The schematic symbol for the strain gauge is similar to that of a variable resistor. Thesymbol (the lower-case Greek letter epsilon) indicates that the resistance is a function ofstrain.

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7. Potentiometer types: linear, rotary

wiper, slider or tap

8. Encoder- non-contact optical devices used for converting the angular position of a

rotating shaft into an analogue or digital data code. All optical encoders work on the

same basic principle. Light from an LED or Infrared light source is passed through a

rotating high-resolution encoded disk that contains the required code patterns, either

binary, grey code or BCD

Tachometer - The simplest incremental encoder is called a tachometer. It has one

single square wave output and is often used in unidirectional applications where

basic position or speed information only is required.

9. Bourdon tubes consist of a formed tube with one end fixed and the other end freeto deflect under pressure.

Bourdon tube gauge contains a curved tube that is open to external pressure input

on one end and is coupled mechanically to an indicating needle on the other end, as

shown schematically below.

10.Diaphragm - mechanical transducers capable of transforming pressure into a

mechanical movement proportional to the applied pressure

11.Bellows - These are the elastic elements that convert the air pressure into

displacement, and it is commonly used for the measurement of pressure.It is made of

a sealed chamber that has multiple ridges like the pleats of an accordion that are

compressed slightly when the sensor is manufactured. When pressure is applied to

the chamber, the chamber will try to expand and open the pleats.

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Movement rotates a pointer or actuates a controller or transmitter by mechanical linkage

CHEMICAL SENSORS

1. Gas sensor (CO, CH4) - is a device which detects the presence of

various gases within an area, usually as part of a system to warn about gases

which might be harmful to humans or animals.

MAGNETIC FIELD SENSORS

1. Flux-gate compasses electronic compass that measures the relative strength of the

earths magnetic passing through the coils of wires

2. Hall-effect sensor - When a current-carrying conductor is placed into a magnetic field,a voltage will be generated perpendicular to both the current and the field. Thisprinciple is known as the Hall effect.

3. Magneto resistor - magnetic field = Resistance

LIGHT TRANSDUCERS

Photo-electric or photo sensors - detect visible or infrared energy and convert into electrical

signal1. Photo conductive cell/photo resistor/LDR(light dependent resistor) - Light intensity

= Resistance

2. Photo voltaic/ solar cell - Light intensity = voltage

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3. Photodiode a photo-junction device, Light intensity = current

If any semiconductor diode is reverse biased and the junction illuminated, thereverse current flow will vary in proportion to the amount of light. This effect isutilized in the photodiode, which has a clear window through which light can fall onone side of the crystal and across the junction of the p and n zones.

In effect, such a diode will work in a circuit as a variable resistance, the amount of

resistance offered by the diode being dependent on the amount of light falling on the

diode. In the dark the photodiode will have normal reverse working characteristics;

that is, it will provide almost infinitely high resistance with no current flow. At

increasing levels of illumination, resistance will become proportionately reduced, thus

allowing increasing current to flow through the diode.

4. Photo transistor also a photo-junction device

5. Optoisolators/ optocouplers - A phototransistor and a light-emitting diode (LED) may

be combined in a single envelope, such a device being known as an optoisolator.

6. LED output transducer

7. LAMPS output transducer

Scanning techniques

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a. Through-beam/direct scan - In through (direct) scan the light source and

photoreceiver are positioned opposite each other, so light from the source

shines directly at the sensor. The object to be detected passes between the

two.

b. Retro-reflective scan - light source and photosensor occupy a common

housing. The light beam is directed at a reflector which returns the light along

the same path it was sent.

c. Diffuse scan the same with retro-reflective scan, only that the target object

serves as the reflector.

SOUND TRANSDCUCERS

infrasonic - a sound frequency below the audible range (less than about 20 Hz).ultrasonic - a sound frequency above the audible range (greater than 20 kHz).

1. Microphone

2. Loudspeaker/ Headphone3. Bell