Unit 2 - Ultrasonics PDF

UNIT -2 ULTRASONICS INTRODUCTION Sound waves are mechanical vibrations of smaller amplitude. Human ear is capable of receiving the sound waves with frequency range of 20Hz to 20,000Hz. The frequency ranges below 20Hz and above 20,000Hz are inaudible t...

UNIT -2 ULTRASONICS INTRODUCTION Sound waves are mechanical vibrations of smaller amplitude. Human ear is capable of receiving the sound waves with frequency range of 20Hz to 20,000Hz. The frequency ranges below 20Hz and above 20,000Hz are inaudible to human being. Based on the frequency, sound waves are generally classified into three types: i) Infrasonic waves: The sound waves having frequency below 20Hz are called infrasonic sound. ii) Audible range of frequency: The sound waves having frequency 20Hz to 20000 Hz are said to be audible sound. iii) Ultrasonic waves: The sound waves having frequency more 20000 Hz are called ultrasonic waves. CLASSIFICATION OF ULTRASONIC WAVES Based on particle displacement of the media, ultrasonic waves are classified into two types or modes: (i) Transverse ultrasonic or shear waves – Particles of the medium vibrate perpendicular to the direction of propagation of the wave. It consists of alternate crest and trough. It can propagate only through solids. (ii) Longitudinal/compressional waves – These waves travel through medium as alternate compressions and rarefactions vibrating parallel to the direction of propagation of the wave. It can travel solid, liquid and gases. PROPERTIES OF ULTRASONIC WAVES They are highly energetic. Ultrasonic waves undergo reflection, refraction and diffraction like sound waves. When ultrasonic waves passed through the liquids, stationary wave patterns are produced and it behaves as acoustical grating element. When an object is exposed to ultrasonic for longer time it produces heating effect. By increasing the frequency of ultrasonic waves, energy can be increased. They produce cavitation effect in liquids. They can travel over long distances without any loss of energy PRODUCTION OF ULTRASONIC WAVES In general, there are three methods of producing the ultrasonic waves. 1. Mechanical generator 2. Magnetostriction generator 3. Piezo electric generator MAGNETOSTRICTION GENERATOR PRINCIPLE: When a rod of ferromagnetic material like nickel (cobalt, iron, etc.) is magnetized. Longitudinally, it undergoes a very small change in length. This is called Magnetostriction effect. CONSTRUCTION The circuit diagram of magnetostriction ultrasonic generator is as shown in the figure. A short permanently magnetized nickel rod is clamped in the middle between two knife edges. A coil L1 is wound on the right-hand portion of the rod. C is a variable capacitor. L1 and C1 form the resonant circuit of the collector-tuned oscillator. Coil L2 wound on the LHS of the rod is connected in the base circuit. The coil L2 is used as a feedback loop. WORKING When the battery is switched on, the resonant circuit L1C1 sets up an alternating current of frequency. 1 𝑓= 2𝜋√𝐿1 𝐶1 Where, L1 is the inductance of the coil and C1 is the capacitance of the coil This current flowing round the coil L1 produces an alternating magnetic field of frequency along the length of the nickel rod. The rod starts vibrating due to magnetostrictive effect. The vibrations of the rod create ultrasonic waves. 1 𝐸 Frequency of the vibrating rod n= √ 2𝑙 𝜌 Where, l is the length of the rod  is the Density of the rod E is the Young’s modulus of the rod This e.m.f is applied to the base of the transistor. Hence the amplitude of high frequency of high oscillations in coil L1 is increased due to positive feedback. The developed alternating current frequency can be turned with the natural frequency of the rod by adjusting the capacitor. Condition for resonance Frequency of the oscillator circuit = Frequency of the vibrating rod The resonance condition is indicated by the rise in the collector current shown in the milliammeter. ADVANTAGES: Magnetostriction Oscillators are mechanically rugged. The construction cost is low. They are capable of producing large acoustical power with fairly good efficiency. LIMITATIONS It can produce frequencies up to 3MHz only. They frequency of oscillation depends upon the temperature. Breadth of the resonance curve is large. It is due to vibrations of elastic constants of ferromagnetic material with the degree of magnetization. So, we cannot get a constant single frequency. PIEZO ELECTRIC CRYSTALS The crystals which produce piezo-electric effect and converse Piezo electric effect are termed as Piezo electric crystal. Example: Quartz, Tourmaline, Rochelle Salts etc. At typical example or a piezo electric crystal (Quartz) is as shown in the figure. It has a hexagonal shape with pyramids attached at both ends. It consists of 3 axes. Viz., I. Optic or Z - axis, which joins the edges of the pyramid II. Electrical axis or X - axis, which joins the corners of the hexagon and III. Mechanical axis or Y – axis, which joins the center or sides of the hexagon X-Cut crystal: When the crystal is cut perpendicular to the X-axis, as shown in the figure, then it is called X- crystal. Generally, X-cut crystals are used to produce longitudinal ultrasonic waves. Y-Cut Crystal: When the crystal is cut perpendicular to the Y-axis, as shown in the figure, then it is called Y-cut crystal. Generally, Y-Cut crystal produces transverse ultrasonic waves. PIEZOELECTRIC EFFECT: When a pressure or mechanical stress is applied to the one pair of opposite faces of a quartz crystal then equal and opposite charges are developed along the perpendicular axis. Piezoelectric Effect Inverse Piezoelectric Effect INVERSE PIEZOELECTRIC EFFECT: When an alternating electric field is applied to along certain axis then alternate expansion or contraction takes is developed across other pair of opposite faces of the crystal. PIEZO ELECTRIC GENERATOR PRINCIPLE: This is based on the Inverse piezoelectric effect. When a quartz crystal is subjected to an alternating potential difference along the electric axis, the crystal is set into elastic vibrations along its mechanical axis. If the frequency of electric oscillations coincides with the natural frequency of the crystal, the vibrations will be of large amplitude. If the frequency of the electric field is in the ultrasonic frequency range, the crystal produces ultrasonic waves. CONSTRUCTION: The circuit diagram is shown in the figure. It is base turned oscillator circuit. A slice of Quartz crystal is placed between the metal plates A and B so as to form a parallel plate capacitor with the crystal as the dielectric. This is coupled to the electronic oscillator through the primary coil L3 of the transformer. Coils L2 and L1 of oscillator circuit are taken for the primary of the transformer. The collector coil L2 is inductively coupled to base coil L1. The coil L1 and variable capacitor C form the tank circuit of the oscillator. WORKING: When the battery is switched on, the current is passed through the coil L1 and L2 and the oscillator produces high frequency oscillations. The frequency of the oscillator is given by 1 𝑓= 2𝜋√𝐿1 𝐶1 Where, L1 is the inductance of the coil and C1 is the capacitance of the coil An oscillatory e.m.f is induced in the coil L3 due to transformer action. So, the crystal is now under high frequency alternating voltage. Due to the principle of inverse piezo-electric effect the crystal starts vibrating along the mechanical axis of the crystal. 𝑃 𝐸 Frequency of the vibrating crystal n = 2𝑙 √𝜌 Where, l is the length of the rod  is the Density of the rod E is the Young’s modulus of the rod P = 1,2,3 ….etc. for fundamental, first overtone, second overtone, etc respectively. The capacitance of C1 is varied so that the frequency of oscillations produced is in resonance with the natural frequency of the crystal. Now the crystal vibrates with larger amplitude due to resonance. Thus, high power ultrasonic waves are produced. Condition for Resonance: Frequency of the oscillator circuit = Frequency of the vibrating crystal Where ‘l’ is the length of the rod ‘E’ is the Young’s modulus of the rod ‘ρ’ is the density of the material of the rod. ‘P’ = 1,2,3 …. etc for fundamental, first overtone, second overtone, etc respectively Advantages: 1. Ultrasonic frequencies as high as 500MHz can be generated. 2. The output power is very high. It is not affected by temperature humidity. 3. It is more efficient than the Magnetostriction oscillator. 4. The breadth of the resonance curve is very small. So, we can get a stable and constant frequency of ultrasonic waves. Disadvantages: 1. The cost of the quartz crystal is very high. 2. Cutting and shaping the crystal is quite complex. DETERMINATION OF ULTRASONIC VELOCITY IN LIQUID (ACOUSTICAL GRATING METHOD) PRINCIPLE: When ultrasonic waves travel through a transparent liquid, due to alternate compression and rarefaction, longitudinal stationary waves are produced. The regions of compression act as opaque medium and rarefaction act as transparent medium for light waves. If monochromatic light is passed through the liquid perpendicular to these waves, the liquid behaves as diffraction grating. Such a grating is known as Acoustic Grating. It is used to find wavelength and velocity(v) of ultrasonic waves in the liquid. CONSTRUCTION: It is consisting of a glass tank, filled with the liquid. A piezo-electric (Quartz) is fixed at the bottom of the glass tank and is connected with piezo-electric oscillatory circuit. The Laser is used as a monochromatic source (S) and a telescope arrangement is used to view the diffraction pattern. WORKING (i) When the piezo-electric crystal is kept at rest: Initially the piezo-electric crystal is kept at rest and the monochromatic light is switched ON. When the light is focused in the glass tank filled with the liquid, a single image, a vertical peak is observed in telescope. i.e., there is no diffraction. (ii) When the piezo-electric crystal is set into vibrations: Now the crystal is put into vibrations using piezo-electric oscillatory circuit. At Resonance, Ultrasonic waves are produced and are passed through the liquid. These Ultrasonic waves are reflected by the walls of the glass tank and form a stationery wave pattern with nodes  and antinodes in the liquid. At nodes the density of the liquid becomes more and at antinodes the density of the liquid becomes less. Thus, the liquid behaves as a diffraction element called acoustical grating element. Now when the monochromatic light is passed the light gets directed and a diffraction pattern consisting of central maxima and principal maxima on either side is viewed through the telescope. CALCULATION OF ULTRASONIC VELOCITY The velocity of Ultrasonic waves can be determined using the condition. 𝒅 𝐬𝐢𝐧 𝜽 = 𝒏𝝀 --------------(1) Where, d is the distance between successive node or antinodes.  is the angle of diffraction n is the order of the spectrum  is the wavelength of the monochromatic source of the light. If 𝜆𝑢 = 2𝑑 ----------------(2) Then equation (1) becomes 𝜆𝑢 sin 𝜃 = 𝑛𝜆 2 2𝑛𝜆 Wavelength of ultrasonic waves 𝜆𝑢 = ----------------(3) sin 𝜃 We know Ultrasonic velocity = Frequency of ultrasonic ( f ) x wavelength of ultrasonic (𝜆𝑢 ) 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑢𝑙𝑡𝑟𝑎𝑠𝑜𝑛𝑖𝑐𝑠 𝑤𝑎𝑣𝑒𝑠 𝐯 = 𝝂𝒖 𝐱 𝝀𝒖 ------------(4) Subsituting equation(3) in(4) 𝒇𝟐𝒏𝝀 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑢𝑙𝑡𝑟𝑎𝑠𝑜𝑛𝑖𝑐𝑠 𝑤𝑎𝑣𝑒𝑠 𝐯 = 𝐬𝐢𝐧 𝜽 Thus, this method is useful in measuring the wavelength and velocity of ultrasonic waves in liquids and gases at various temperatures. NON-DESTRUCTIVE TESTING Nondestructive testing (NDT) is a method by which materials are tested without destructing or damaging the material and by passing some radiations through the material. The goal of NDT is to detect defects and give information about their distribution. There are several non-destructive tests in use. The common methods are  Visual inspection Liquid penetrant method Magnetic particle testing Eddy current testing Ultrasonic flaw detection technique Radiography method ULTRASONIC FLAW DETECTION METHOD: PRINCIPLE: Whenever there is a change in the medium, then the Ultrasonic waves will be reflected. This is the principle used in Ultrasonic flaw detector. Thus, from the intensity of the reflected echoes, the flaws are detected without destroying the material and hence this method is known as a Non-Destructive method. Block diagram of the Ultrasonic Flaw detector Working: The pulse generator generates high frequency waves and is applied to the Piezo-electric transducer and the same is recorded in the CRO as pulse A. The piezo electric crystals are resonated to produce Ultrasonic waves. These ultrasonic waves are transmitted through the given specimen. These waves travel through the specimen and is reflected back by the other end. The reflected ultrasonic are received by the transducer and is converted into electric signals. These reflected signals are amplified and is recorded in the CRO as pulse B. If the reflected pulse (pulse B) is same as that of the transmitted pulse (Pulse A), then it indicates that there is no defect in the specimen. On the other hand, if there is any defect on the specimen like a small hole or pores, then the Ultrasonic will be reflected by the holes(i.e.) defects due to change in the medium. These defects give rise to another signal in between pulses A and B. Similarly, if we have many such holes, many Z-pulses will be seen over the screen of CRO. From the time delay between the transmitted and received pulses, the position of the hole can be found.  From the height of the pulse received the depth of the hole can also be determined. ADVANTAGES It can reveal internal defects. This method is sensitive to most of the cracks and flaws. It gives immediate results at very low cost and at a very high speed. It indicates the size and location of the flaws exactly. Since there is no radiation in this process, it is a safest method among the other methods. LIMITATIONS It is difficult to find the defects of the specimen which has complex shapes. Trained and motivated technicians alone can perform this testing. ULTRASONIC WELDING The properties of some materials change on heating. In such cases, the electric or gas welding is not advisable. Such materials can be welded at room temperature with the help of ultrasonic waves. Proper welding can be achieved by sending ultrasonic waves in between the surfaces of the weld during welding called cold welding. This effect is attributed to the momentary relaxation of the bonds. CONSTRUCTION: When materials are welded through ultrasonic waves, the energy required comes in the form of mechanical vibrations. The welding tool hammer is attached to a powerful ultrasonic generator. The part to be welded is placed on the anvil and located just below to tip of hammer as shown in Figure. WORKING: When ultrasonic waves are produced, the hammer is made to vibrate ultrasonically. As a result, the parts to be welded are pressed simultaneously. The ultrasonic vibrating force disrupts the oxide layer of both materials. The atom diffuses from one part to the other when the oxides are dispersed. Since no oxides are at the interface, a true metallurgical bond is achieved. Thus, the metals are joined by molecular transfer. ADVANTAGES 1. It does not require any additional welding material. 2. The process is very fast and safe. 3. No fumes and flames are produced. 4. The process is noiseless operation. 5. Its cost is very low. ULTRASONIC SOLDERING Metals like Aluminium and copper form an oxide layer when contact with air. This oxide layer prevent solder from making contact with the metal. CONSTRUCTION AND WORKING An ultrasonic soldering iron consist of an ultrasonic generator having a tip fixed at its lower and. The tip can be heated by an electrical heating element. When ultrasonic vibration is applied to the solder in the tank, many cavitation bubbles are created. The size of the bubbles increases until they become unstable and implode. When a bubble implodes, the solder around the bubble accelerates towards the bubble middle. This solder can then hit the surface of a part creating a strong impact which simultaneously cleans the surface and destroys the oxide film on the metal. This allows soldering without using flux. Thus, during the working process, the tip of the soldering iron melts the solder on Aluminium and the ultrasonic vibration removes the Aluminium oxide layer. ADVANTAGES 1. This process does not require any flux. 2. Since the flux is not used, the flame is not produced. 3. It is very safe and easy process. 4. Its cost is less. SONAR SONAR stands for Sound Navigation and Ranging. It is based on the principle of echo-sounding. When the Ultrasonic waves are transmitted through water, it is reflected by the objects in the water and will produce an echo signal. The change in frequency of the echo signal, due to Doppler Effect helps us in determining the velocity and direction of the object. Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects. Sonar consists of a transmitter and a detector and is installed at the bottom of boats and ships. The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the object on the seabed, get reflected back and are sensed by the detector. The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted. The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time interval between transmission and reception of the ultrasound. Let the time interval between transmission and reception of ultrasound signal be ‘t’ and the speed of sound through sea water be 2d = v × t. This method is called echo-ranging. Sonar technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine, icebergs etc. APPLICATIONS 1. It is used in location of shipwrecks and submarines on the bottom of the sea. 2. It is used for fish – finding applications. 3. It is used for seismic survey. SONOGRAM: FETAL HEART MOVEMENT PRINCIPLE: It works under the principle of Doppler Effect i.e., there is an apparent change in the frequency between the incident sound waves on the fetus and the reflected sound waves from the fetus. DESCRIPTION: It consists of a Radio Frequency Oscillator (RFO), for producing 2 MHz of frequency and RFA (Radio Frequency Amplifier) to amplify the receiver signal as shown in the figure. Mixer is used to mix the transmitted signals and the received signals. The loud speaker and the CRO helps to hear and view the output of the sound waves respectively. WORKING: The transducer is fixed over the mother’s abdominal wall, with the help of a gel or oil. RFO is switched ON to drive the pulses and hence the transducer produces Ultrasonic waves of 2 MHz These Ultrasonic waves are made to be incident on the fetus. The reflected Ultrasonic waves from the fetus are received by the transducer and are amplified by RFA. Both the incident and the received signals are mixed by the mixer and is filtered to distinguish the various types of sound and finally the Doppler shift or change in frequency is measured. The movement of the heart can be viewed visually by CRO or can be heard by the Loud Speaker, after necessary amplification by Audio Frequency amplifier. CARS’ AIR BAG SENSOR Airbag sensors are devices that typically detect when a collision has occurred in a vehicle and send a signal to deploy the airbags. External Airbag System has been introduced which works considering the response time of human brain and braking distance depending on the current speed of the vehicle, this ensures complete security of the driver as well as vehicle in an accident as it does not depend on the driver but works based on data given and also does not inflate the airbags after the accident but before the accident to avoid the damage and impact of the accident. BLOCK DIAGRAM OF THE EXTERNAL AIRBAG SYSTEM The system consists of total five blocks, the function of the five blocks are as follows: 1. ULTRASONIC SENSOR: An ultrasonic sensor is an electronic device that measures the distance of a target object by emitting ultrasonic sound waves, and converts the reflected sound into an electrical signal. Ultrasonic waves travel faster than the speed of audible sound (i.e., the sound that humans can hear). In this system the purpose of the sensor is to detect the presence of any object within 50 meters of range and send the corresponding signal to the microcontroller. Below shown is a working diagram of an ultrasonic sensor. 2. MICROCONTROLLER: A microcontroller is a compact integrated circuit designed to govern a specific operation in an embedded system. In this system we are using ARDUINO NANO as microcontroller. The microcontroller runs the software algorithms and controls the airbag inflators. 3. LED: LED (Light Emitting Diode) is basically a small light emitting device that comes under “active” semiconductor electronic components. Here, in this system the LED works as alert signal for the driver in the vehicle. 4. AIR PUMP: This is a DC motor driver air pump or air inflator. It can be controlled by any controller like Arduino, Raspberry Pi, AVR, PIC or any other controller. The pump is very easy to assemble, interface and a low power device. The pump used for inflating the airbag can be changed as per the requirement or the scale of requirements. 5. PRESENT VEHICLE SPEED: This block is given as input to the microcontroller. It consists of present speed of the vehicle. Continuous vehicle speed reading will be given as the Input. WORKING There are three stages, 1. In the First Stage, the Ultrasonic Sensor gives continuous readings of the front distance as input to the microcontroller. Along with the Ultrasonic Sensor readings, Present Vehicle Speed will be given as input to microcontroller.  2. In the Second Stage, the microcontroller will process both the front distance and the present vehicle speed and compare with the standard table. 3. During this process, the microcontroller will compare the distance readings. If the change in the distance readings is very less and the present vehicle speed is greater than zero, then this situation will occur when our vehicle and the front vehicles is at similar speed. In this situation, if the present vehicle speed is greater than required stopping distance’s speed, then the LED will turn on until the driver should reduce the speed to increase the distance between our vehicle and the front vehicle. This LED will be an alert system for the driver that if incase the front vehicle instantly stops in this situation; the driver won’t get enough distance to stop his vehicle from hitting. 4. In the Third Stage, if there is a situation where the driver ignores the alert system and the front vehicle stops suddenly and there is not enough stopping distance for our vehicle, then surely the accident is going to occur. During this accident, just seconds away the external airbags will be deflated and these airbags will prevent damage to the body and engine of the vehicle. The passengers and the driver’s safety will be taken care by the internal airbags and this method will help to avoid major damage to the vehicle. PROBE SONICATION FOR 2D MATERIAL FORMATION Sonication is defined as the process in which sound waves are used to agitate the particles in the solutions. These disruptions are used for mixing of the solutions, to increase the speed of dissolution of a solid into a liquid, and for the removal of dissolved gases from the liquids. SONICATOR The equipment used for sonication is known as a sonicator. The following are the three parts of the sonicator: A generator A transducer A probe The generator is used for transforming the input electrical power into an electrical signal that drives the transducer. The transducer is used for converting the electrical signal into vibration. This vibration is used in the probe tip by amplifying it into a longitudinal vibration causing a cavity in the sample. The ultrasound energy is the creation of cavitation which causes the disruption of the sample and makes it easy to break down the particles into smaller ones. PRINCIPLE OF ULTRASONICATION In the ultra-sonication process, cavitation leads to dispersion, homogenization, disintegration, emulsions, extraction, and sonochemical effects of the liquids. High power ultrasound is introduced to the liquid which creates regions of high pressure (known as compression) and low pressure (known as rarefaction). The creation of these regions is dependent on the rate of frequency at which the ultrasound is applied. When low pressure is applied to the liquid, high-intensity ultrasonic waves are produced, creating small vacuum bubbles in the liquid. As the bubbles reach their saturation level, they collapse and this happens in the high-pressure cycle. This process is termed cavitation. During cavitation, the bubbles in the liquid can jet up to 280 m/s velocities. SONOCHEMICAL METHOD Ultrasound Generation: The ultrasound generator produces high-frequency electrical signals that are transferred to the ultrasonic transducer. Transducer Conversion: The ultrasonic transducer converts the electrical signals into mechanical vibrations (ultrasound) that propagate through the liquid medium in the reaction vessel. Cavitation: As ultrasound waves pass through the liquid, they create alternating high- pressure and low-pressure regions. In regions of low pressure, bubbles form due to the expansion of dissolved gases. When the bubbles move to regions of high pressure, they  rapidly collapse, leading to localized high temperatures and pressures. This phenomenon is known as cavitation. Nucleation and Growth: During bubble collapse, the extreme conditions created by cavitation promote the formation of reactive species, nuclei, and subsequent nanoparticle growth. The high-energy environment facilitates the diffusion of precursor molecules to the nuclei, leading to accelerated particle growth. Stabilization: Stabilizing agents or capping agents present in the reaction mixture adsorb onto the nanoparticle surfaces, preventing their agglomeration and ensuring particle stability. Reaction Control: Reaction parameters such as ultrasound intensity, frequency, temperature, and precursor concentrations can be controlled to achieve the desired nanoparticle size, shape, and properties. Termination and Collection: Once the reaction is complete, the ultrasound is turned off, and the nanoparticles can be collected from the reaction mixture. Further purification steps may be required to remove excess reagents or byproducts. The sonochemical route for nanoparticle synthesis exploits the unique effects of cavitation induced by ultrasound to accelerate reaction kinetics and promote the formation of nanoparticles. Careful control and optimization of reaction conditions are essential to achieve the desired nanoparticle characteristics.