Bài giảng Sensors and analytical devices - Part C: Some Basic Measurement Methods (Phần 1) - Nguyễn Công Phương

Nguyễn Công Phương Sensors and Analytical Devices Some Basic Measurement Methods, Sensors Technologies Contents A. Introduction B. Sensors Characteristics C. Some Basic Measurement Methods D. Measurement Systems sites.google.com/site/ncpdhbkhn 2 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 sites.google.com/site/ncpdhbkhn 3 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 sites.google.com/site/ncpdhbkhn 4 Capacitive Sensors S C  r 0 d S d • d is variable, or • εr is variable, or • S is variable. sites.google.com/site/ncpdhbkhn 5 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. sites.google.com/site/ncpdhbkhn 6 Sensor Technologies 1. Capacitive Sensors 2. Resistive Sensors 3. Magnetic Sensors a) Inductive Sensors 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 sites.google.com/site/ncpdhbkhn 7 Inductive Sensors i V I  2 fL ( d ) + – v V L() d  2 fI d  f(,,) V I f Displacement d • To measure translational & rotational displacement. sites.google.com/site/ncpdhbkhn 8 Variable Reluctance Sensors  d()  v  dt   f() V N • To measure rotational V velocities. S sites.google.com/site/ncpdhbkhn 9 Eddy Current Sensors Thin metal sheet Sensor • To measure the displacement sites.google.com/site/ncpdhbkhn 10 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 sites.google.com/site/ncpdhbkhn 11 Hall – Effect Sensors Current I V KIB + Output voltage V V – B  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 sites.google.com/site/ncpdhbkhn 12 Piezoelectric Transducers (1) kFd V  A • 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. sites.google.com/site/ncpdhbkhn 13 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). sites.google.com/site/ncpdhbkhn 14 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 sites.google.com/site/ncpdhbkhn 15 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. sites.google.com/site/ncpdhbkhn 16 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. sites.google.com/site/ncpdhbkhn 17 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 sites.google.com/site/ncpdhbkhn 18 Optical Sensors Light Light source 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. sites.google.com/site/ncpdhbkhn 19 Optical Sensors (Air Path) Light Light source 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. sites.google.com/site/ncpdhbkhn 20 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. sites.google.com/site/ncpdhbkhn 21 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. sites.google.com/site/ncpdhbkhn 22 Photodiode • The output current is a function of the amount of incident light. • Made from various types of semiconductor material. sites.google.com/site/ncpdhbkhn 23 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. sites.google.com/site/ncpdhbkhn 24 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 sites.google.com/site/ncpdhbkhn 25 Optical Sensors (Fiber Optic) Light Light source 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. sites.google.com/site/ncpdhbkhn 26 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. sites.google.com/site/ncpdhbkhn 27 Intrinsic Sensors (2) Shutter switch Light Light in out Optical microswitch Reflective Light switch Light Light out in out Light source sites.google.com/site/ncpdhbkhn 28 Intrinsic Sensors (3) Light Light in out Light Light in out sites.google.com/site/ncpdhbkhn 29 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. sites.google.com/site/ncpdhbkhn 30 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 sites.google.com/site/ncpdhbkhn 31 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. sites.google.com/site/ncpdhbkhn 32 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. sites.google.com/site/ncpdhbkhn 33 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 a) Transmission Speed b) Directionality of Ultrasonic Waves c) Wavelength, Frequency, and Directionality d) Attenuation of Ultrasonic Waves e) Ultrasound as a Range Sensor f) Effect of Noise in Ultrasonic Measurement Systems g) Exploiting Doppler Shift in Ultrasound Transmission 10. Nuclear Sensors 11. Microsensors sites.google.com/site/ncpdhbkhn 34 Transmission Speed VTair 331.6  0.6 (m/s) Medium Velocity (m/s) Air 331.6 Water 1440 Wood (pine) 3320 Iron 5130 Rock (granite) 6000 sites.google.com/site/ncpdhbkhn 35 Directionality of Ultrasonic Waves sites.google.com/site/ncpdhbkhn 36 Wavelength, Frequency, and Directionality v   f Nominal frequency (kHz) 23 40 400 Wavelength (in air at 0oC) 14.4 8.3 0.83 Cone angle of transmission (–6dB limits) ±80o ±50o ±3o sites.google.com/site/ncpdhbkhn 37 Attenuation of Ultrasound Waves X ed d  X0 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. sites.google.com/site/ncpdhbkhn 38 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 a) Transmission Speed b) Directionality of Ultrasonic Waves c) Wavelength, Frequency, and Directionality d) Attenuation of Ultrasonic Waves e) Ultrasound as a Range Sensor f) Effect of Noise in Ultrasonic Measurement Systems g) Exploiting Doppler Shift in Ultrasound Transmission 10. Nuclear Sensors 11. Microsensors sites.google.com/site/ncpdhbkhn 39 Ultrasound as a Range Sensor Transmitter Receiver d d vt d  g() environment v f() environment Transmitter Receiver d, t Transmitter Receiver dref, t ref dref vref  d  vref t tref sites.google.com/site/ncpdhbkhn 40 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. sites.google.com/site/ncpdhbkhn 41 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 a) Transmission Speed b) Directionality of Ultrasonic Waves c) Wavelength, Frequency, and Directionality d) Attenuation of Ultrasonic Waves e) Ultrasound as a Range Sensor f) Effect of Noise in Ultrasonic Measurement Systems g) Exploiting Doppler Shift in Ultrasound Transmission 10. Nuclear Sensors 11. Microsensors sites.google.com/site/ncpdhbkhn 42 Exploiting Doppler Shift in Ultrasonic Transmission (1) v vre f  ftr v vtr • 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 sites.google.com/site/ncpdhbkhn 43 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 sites.google.com/site/ncpdhbkhn 44 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. sites.google.com/site/ncpdhbkhn 45 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. sites.google.com/site/ncpdhbkhn 46