Cảm biến và thiết bị phân tích: Công nghệ cảm biến

Nguyễn Công Phương

Sensors and Analytical Devices

Some Basic Measurement Methods, Sensors Technologies

A. Introduction B. Sensors Characteristics C. Some Basic Measurement Methods D. Measurement Systems

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Some Basic Measurement Methods

I. Sensor Technologies II. Temperature Measurement III. Pressure Measurement IV.Flow Measurement V. Level Measurement VI.Mass, Force, and Torque Measurement VII.Translational Motion, Vibration, and Shock

Measurement

VIII.Rotational Motion Transducers

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Sensor Technologies

1. Capacitive Sensors 2. Resistive Sensors 3. Magnetic Sensors 4. Hall – Effect Sensors 5. Piezoelectric Transducers 6. Strain Gauges 7. Piezoresistive Sensors 8. Optical Sensors 9. Ultrasonic Transducers 10. Nuclear Sensors 11. Microsensors

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Capacitive Sensors

  0r

S d

• d is variable, or • εr is variable, or • S is variable.

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Resistive Sensors

• They rely on variation of the resistance of a

material when the measured variable is applied to it.

• This principle is applied most commonly in temperature measurement using resistance thermometers or thermistors.

• It is also used in displacement measurement using strain gauges or piezoresistive sensors. • Some moisture meters work on the resistance –

variation principle.

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Sensor Technologies

1. Capacitive Sensors 2. Resistive Sensors 3. Magnetic Sensors

Inductive Sensors

a) b) Variable Reluctance Sensors c) Eddy Current Sensors

4. Hall – Effect Sensors 5. Piezoelectric Transducers 6. Strain Gauges 7. Piezoresistive Sensors 8. Optical Sensors 9. Ultrasonic Transducers 10. Nuclear Sensors 11. Microsensors

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Inductive Sensors



V fL d ( )

2 

+ –

L d ( )





V fI 2

f V I , (

)

d  

Displacement d

• To measure translational & rotational

displacement.

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Variable Reluctance Sensors



)



(   dt

f V (

)

 

• To measure rotational

velocities.

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Eddy Current Sensors

Thin metal sheet

Sensor

• To measure the displacement

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Sensor Technologies

1. Capacitive Sensors 2. Resistive Sensors 3. Magnetic Sensors 4. Hall – Effect Sensors 5. Piezoelectric Transducers 6. Strain Gauges 7. Piezoresistive Sensors 8. Optical Sensors 9. Ultrasonic Transducers 10. Nuclear Sensors 11. Microsensors

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Hall – Effect Sensors

Current I

V KIB 

Output voltage V

+ –

B  

V KI

Applied magnetic field B • Basically to measure the magnitude of a

magnetic field.

• Can be a proximity sensor • Used in computer keyboard push buttons

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Piezoelectric Transducers (1)



kFd A

http://www.ask.com/wiki/Piezoelectricity

• V: the induced voltage (its polarity depends on whether the material

is compressed or stretched) • k: the piezoelectric constant • F: the applied force • d: the thickness of the material • A: the area of the material

• They produce an output voltage when a forcce is applied to them, • Or being applied a voltage, they produce an output force.

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Piezoelectric Transducers (2)

• Used frequently as ultrasonic transmitters &

receivers. – Transmitters: application of a sinusoidal voltage at a frequency in the ultrasound range causes sinusoidal variations in the thickness of the material & results in a sound wave being emitted at the chosen frequency.

– Receivers: sinusoidal amplitude variations in the

ultrasound wave received are translated into sinusoidal changes in the amplitude of the force applied to the piezoelectric transducer.

• Also used as displacement transducers

(particularly as part of devices measuring acceleration, force, & pressure).

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Sensor Technologies

1. Capacitive Sensors 2. Resistive Sensors 3. Magnetic Sensors 4. Hall – Effect Sensors 5. Piezoelectric Transducers 6. Strain Gauges 7. Piezoresistive Sensors 8. Optical Sensors 9. Ultrasonic Transducers 10. Nuclear Sensors 11. Microsensors

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Strain Gauges

• Experience a change in resistance

when they are stretched or strained.

• Able to detect very small

displacement, usually 0 – 50μm. • Manufactured to various nominal values of resistance, e.g. 120, 350, & 1000Ω.

• The typical maximum change of resistance in a 120-Ω device would be 5Ω at maximum deflection.

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Piezoresistive Sensors

• Made from semiconductor material in which a p-type region has been diffused into an n-type base.

• Its resistance varies greatly when the sensor is

compressed or stretched.

• Used in semiconductor – diaphragm pressure sensors & in semiconductor accelorometers.

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Sensor Technologies

1. Capacitive Sensors 2. Resistive Sensors 3. Magnetic Sensors 4. Hall – Effect Sensors 5. Piezoelectric Transducers 6. Strain Gauges 7. Piezoresistive Sensors 8. Optical Sensors a) Air Path b) Fiber Optic

9. Ultrasonic Transducers 10. Nuclear Sensors 11. Microsensors

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Optical Sensors

Light source

Light detector

• Based on transmission of light between a light

source & a light detector.

• The path can be air or fiber – optic. • They give immunity to electromagnetically

induced noise.

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Optical Sensors (Air Path)

Light source

Light detector

• Light sources: tungsten – filement lamps, laser

diodes, light – emitting diodes (LED).

• Visible light is rather easy to be interfered with sun & other sources, so infrared laser diodes & LED are preferred.

• Light detectors: photoconductors, photovoltaic

devices, photodiodes, phototransistor.

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Photoconductors

• Also photoresistors. • Convert changes in incident light into changes

in resistance.

• This resistance is reduced according to the

intensity of light to which photoconductors are exposed.

• Made from cadmium sulfide, lead sulfide, etc.

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Photovoltaic Devices

• Also photocells or solar cells. • Their basic mode of operation is to generate an output voltage whose magnitude is a function of the magnitude of the incident light that they are exposed to.

• Made from various types of semiconductor

material.

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Photodiode

• The output current is a function of the amount

of incident light.

• Made from various types of semiconductor

material.

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Phototransistor

• Effectively a standard bipolar transistor with a transparent case that allows light to reach its base – collector junction.

• Has an output in the form of an electrical

current.

• Can be regarded as a photodiode with an

internal gain.

• More sensitive to light than a photodiode. • Has a slower response time.

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Sensor Technologies

1. Capacitive Sensors 2. Resistive Sensors 3. Magnetic Sensors 4. Hall – Effect Sensors 5. Piezoelectric Transducers 6. Strain Gauges 7. Piezoresistive Sensors 8. Optical Sensors a) Air Path b) Fiber Optic

9. Ultrasonic Transducers 10. Nuclear Sensors 11. Microsensors

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Optical Sensors (Fiber Optic)

Light source

Light detector

• Use fiber – optic cable to transmit light between a source & a

detector.

• Fiber – optic cables can be made from plastic fiber, glass fiber, or a

combination of the two.

• Advantages: long life, high accuracy, simplicity, low cost, small

size, high reliability, capability of working in hostile environments.

• Difficulty: the proportion of light entering the cable must be

maximized.

• Two major classes:

– Intrinsic sensors: the fiber – optic cable itself is the sensor. – Extrinsic sensors: the fiber – optic cable is only used to guide light

to/from a conventional sensor.

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Intrinsic Sensors (1)

• In intrinsic sensors, the physical quantity being measured causes some measurable change in characteristics of the light transmitted by the cable.

• The modulated light parameters consist of intensity, phase, polarization, wavelength, transit time.

• A very useful feature: they can provide

distributed sensing over distances of up to 1 meter.

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Intrinsic Sensors (2)

Shutter switch

Light in

Light out

Optical microswitch

Reflective switch

Light in

Light out

Light source

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Intrinsic Sensors (3)

Light in

Light out

Light in

Light out

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Extrinsic Sensors

• Use a fiber – optic cable to transmit modulated

light from a conventional sensor.

• Most important advantage: excellent protection against electromagnetic noise (e.g. temperature measurement in electrical transformers). • Disadvantage: the output of many sensors is

not in a form that can be transmitted by a fiber – optic cable.

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Sensor Technologies

1. Capacitive Sensors 2. Resistive Sensors 3. Magnetic Sensors 4. Hall – Effect Sensors 5. Piezoelectric Transducers 6. Strain Gauges 7. Piezoresistive Sensors 8. Optical Sensors 9. Ultrasonic Transducers 10. Nuclear Sensors 11. Microsensors

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Ultrasonic Transducers (1)

• Used in many fields of measurement, such as fluid flow rates, liquid levels, translational displacement. • Ultrasound: frequencies in the range above 20kHz (above the sonic range that humans can hear). • Ultrasonic devices consist of one device that

transmits an ultrasound, & another one that receives the wave.

• Changes in the measured variable are determined by: – Measuring the change in time taken for the ultrasound wave to travel between the transmitter & receiver, or

– Measuring the change in phase or frequency of the

transmitted wave.

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Ultrasonic Transducers (2)

• The most common form of ultrasonic element (transmitter/receiver) is a piezoelectric crystal. • Those elements can operate interchangeably as

either a transmitter or a receiver.

• Operating frequencies: 20kHz – 15MHz. • Principles of operation: an alternating voltage generates an ultrasonic wave, & vice versa.

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Sensor Technologies

1. 2. 3. 4. 5. 6. 7. 8. 9.

Capacitive Sensors Resistive Sensors Magnetic Sensors Hall – Effect Sensors Piezoelectric Transducers Strain Gauges Piezoresistive Sensors Optical Sensors Ultrasonic Transducers

a) b) c) d) e) f) g)

Transmission Speed Directionality of Ultrasonic Waves Wavelength, Frequency, and Directionality Attenuation of Ultrasonic Waves Ultrasound as a Range Sensor Effect of Noise in Ultrasonic Measurement Systems Exploiting Doppler Shift in Ultrasound Transmission

Nuclear Sensors

10. 11. Microsensors

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Transmission Speed

331.6 0.6 (m/s)





airV

Medium

Velocity (m/s)

Air

331.6

Water

1440

Wood (pine)

3320

Iron

5130

Rock (granite)

6000

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Directionality of Ultrasonic Waves

http://www.robot-electronics.co.uk/htm/srf235tech.htm

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 

Wavelength, Frequency, and Directionality v f

Nominal frequency (kHz)

400

Wavelength (in air at 0oC)

14.4

8.3

0.83

Cone angle of transmission (–6dB limits)

±80o

±50o

±3o

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Attenuation of Ultrasound Waves





dX X

e fd

• Xd: the amplitude of the ultrasound wave at a

distance d from the emission point

• X0: the amplitude of the ultrasound at the

emission point

• f: the nominal frequency of the ultrasound • α: the attenuation constant • The attenuation depends also on type of

transmission medium, level of humidity, & dust.

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Sensor Technologies

1. 2. 3. 4. 5. 6. 7. 8. 9.

Capacitive Sensors Resistive Sensors Magnetic Sensors Hall – Effect Sensors Piezoelectric Transducers Strain Gauges Piezoresistive Sensors Optical Sensors Ultrasonic Transducers

a) b) c) d) e) f) g)

Transmission Speed Directionality of Ultrasonic Waves Wavelength, Frequency, and Directionality Attenuation of Ultrasonic Waves Ultrasound as a Range Sensor Effect of Noise in Ultrasonic Measurement Systems Exploiting Doppler Shift in Ultrasound Transmission

Nuclear Sensors

10. 11. Microsensors

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Ultrasound as a Range Sensor

Transmitter

Receiver

d vt

g environment

(

)

d  

f environment

(

)



Transmitter

Receiver

,d t

Transmitter

Receiver

ref

d  



v t ref

ref

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Effect of Noise in Ultrasonic Measurement Systems • Signal levels at the output of ultrasonic measurement

systems are usually of low amplitude,

•  prone to contamination by electromagnetic noise. • Solutions:

– To make ground line thick. – To use shielded cables. – To locate the signal amplifier as close to the receiver as

possible.

• Another form of noise: background ultrasound (up to 200kHz) produced by manufactoring operations in industrial environments.

• Solution: set nominal frequencies > 200kHz.

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Sensor Technologies

1. 2. 3. 4. 5. 6. 7. 8. 9.

Capacitive Sensors Resistive Sensors Magnetic Sensors Hall – Effect Sensors Piezoelectric Transducers Strain Gauges Piezoresistive Sensors Optical Sensors Ultrasonic Transducers

a) b) c) d) e) f) g)

Transmission Speed Directionality of Ultrasonic Waves Wavelength, Frequency, and Directionality Attenuation of Ultrasonic Waves Ultrasound as a Range Sensor Effect of Noise in Ultrasonic Measurement Systems Exploiting Doppler Shift in Ultrasound Transmission

Nuclear Sensors

10. 11. Microsensors

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Exploiting Doppler Shift in Ultrasonic Transmission (1)

 

v v

v re v

 

• f ' : the apparent frequency • f: the frequency of the transmitter as measured

with no relative motion

• v: the speed of sound • vre: the speed of the receiver • vtr: the speed of the transmitter

http://rlsoundlab.wikispaces.com/Doppler+Effect

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Sensor Technologies

1. Capacitive Sensors 2. Resistive Sensors 3. Magnetic Sensors 4. Hall – Effect Sensors 5. Piezoelectric Transducers 6. Strain Gauges 7. Piezoresistive Sensors 8. Optical Sensors 9. Ultrasonic Transducers 10. Nuclear Sensors 11. Microsensors

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Nuclear Sensors

• Uncommon measurement devices, because of the strict safety regulations that govern their use, & because they are usually expensive.

• The principle of operation of nuclear sensors is very similar to optical sensors: radiation is transmitted between a source & a detector • Caesium – 137 is used commonly as a γ-ray

source.

• Sodium iodide is used commonly as a γ-ray

detector.

• Applications: liquid level measurement, mass

flow rate measurement, & medical applications.

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Microsensors

• Milimeter-sized two- & three-dimensional

micromachined structures that have smaller size, improved performance, better reliability, & lower production costs than many alternative forms of sensors.

• Used to measure temperature, pressure, force,

acceleration, humidity, magnetic fields, radiation, chemical parameters.

• Usually constructed from a silicon semiconductor material, sometimes from other materials (metals, plastics, polymers, glasses, & ceramics deposited on a silicon base).

• Problems:

– Typically have very low capacitance  the output signals are very prone to

noise contamination.

– Generally produce output signals of very low amplitude.

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