Ultrasonic Motor.pdf


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A general and brief description about Ultrasonic Motors (USMs)

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Ultrasonic Motor.pdf

  1. 1. Seminar Report, 2013 SSET, Karukutty 1 Dept. of EEE Chapter 1 INTRODUCTION Air, cheer, water, shelter.....motor, yes; a motor is something that we can’t live without. Motor is a device that converts electrical energy to mechanical energy. If you are lazy to move, if you aren’t close enough to move, if you aren’t bold enough to make the move, well, this is a machine that you ought to have. An electric motor is a machine that converts electrical energy into mechanical energy for obtaining useful work. In normal motoring mode, most electric motors operate through the interaction between an electric motor's magnetic field and winding currents to generate force within the motor. i.e. Almost all the motor that dominates the present world such as DC motors, induction motors, synchronous motors etc. work on the principle of Faraday’s Law of Electromagnetic Induction – “when a current carrying conductor is kept in a magnetic field, it experiences a force or a torque”. The energy conversion in such motors involves two stages, where initially electrical energy is converted to magnetic energy and further, the magnetic energy is converted to mechanical energy. Because of this two-stage energy conversion, the electromagnetic motor suffers from several losses that lead to a rigorous energy wastage. The ultrasonic motors belong to the class of piezoelectric motors. In this prose, the term USM will be used for denoting the machine. The working principles have been well known for at least 50 years. However they gained widespread interest with the influential work of T Sashida in 1982. Before that piezo-ceramic materials with high conversion efficiency and fast electronic power control of ultrasonic vibrations were not available. Because of their specific advantages compared to conventional electromagnetic motors, USMs fill a gap in certain motor & actuator applications. Besides that, USMs also offer a high potential for miniaturization. The study deals with the fundamentals of this modern machine such as, principles, parts, working and eventually mentions the pros and cons. The study wraps up with the major applications of this quick response device and the scope for further development.
  2. 2. Seminar Report, 2013 SSET, Karukutty 2 Dept. of EEE Chapter 2 NEED FOR A BETTER MACHINE The electromagnetic motors that are widely used in home, industries and other sectors suffer from several severe disadvantages. Viz. • Noisy operation. • Surge currents and spikes. • Electromagnetic interference. • Magnetic losses (Eddy currents & Hysteresis). • High power consumption & high temperature. • Low power factor. • Comparatively lesser efficiency. Here emerged the requirement of a better machine. After years of studies and researches the engineers developed an entirely different kind of motor that directly converted electrical energy to mechanical energy. One among such motors was the Ultrasonic Piezoelectric Motor or simply, the Ultrasonic Motor (USM). USMs play an important role in a few niche markets, where the size, torque, speed or other requirements couldn’t be satisfied by the traditional electromagnetic motor. As technologies improved in course of time, USMs replaced not only the EM motors, but also the servomotors, stepper motors and sycnhros.
  3. 3. Seminar Report, 2013 SSET, Karukutty 3 Dept. of EEE Chapter 3 ULTRASONICS A HEALTHIER OPTION Ultrasonics are vibrations of frequencies greater than the upper limit of the audible range for humans i.e., greater than about 20 kHz. The term sonic is applied to ultrasound waves of very high amplitudes. Hyper sound, sometimes called praetor sounds or micro sounds are sound waves of frequencies greater than 1013 hertz. At such high frequencies it is very difficult for a sound wave to propagate efficiently; indeed, above a frequency of about 1.25 × 1013 hertz it is impossible for longitudinal waves to propagate at all, even in a liquid or a solid, because the molecules of the material in which the waves are traveling cannot pass the vibration along rapidly enough. An ultrasonic transducer is a device used to convert some other type of energy into an ultrasonic vibration. There are several basic types, classified by the energy source and by the medium into which the waves are being generated. Mechanical devices include gas-driven, or pneumatic, transducers such as whistles as well as liquid-driven transducers such as hydrodynamic oscillators and vibrating blades. These devices, limited to low ultrasonic frequencies, have a number of industrial applications, including drying, ultrasonic cleaning, and injection of fuel oil into burners. Electromechanical transducers are far more versatile and include piezoelectric and magnetostrictive devices. By far, the most popular and versatile type of ultrasonic transducer is the piezoelectric crystal, which converts an oscillating electric field applied to the crystal into a mechanical vibration. Piezoelectric crystals include quartz, Rochelle salt and certain types of ceramics. Piezoelectric transducers are readily employed over the entire frequency range and at all output levels. Particular shapes can be chosen for particular applications. For example, a disc shape provides a plane ultrasonic wave, while curving the radiating surface in a slightly concave or bowl shape creates an ultrasonic wave that will focus at a specific point.
  4. 4. Seminar Report, 2013 SSET, Karukutty 4 Dept. of EEE Chapter 4 PRINCIPLE OF OPERATION Before learning about USMs it is required to know about piezoelectric effect. Piezoelectric effect was discovered by Jacques Curie & Pierre Curie [France-1880]. It was found that in certain types of crystals when a pressure is applied across a pair of opposite faces, an equivalent potential difference is developed across the other pair of opposite faces. Further, if we reverse the direction of application of force, the polarity of the potential difference developed also reverses. In fig 4.1, a compressive force is applied across the crystal and we obtain a potential difference as shown. If the compressive force is replaced by an elongation force, the polarity reverses (i.e. the top face becomes negative and the bottom face become positive). Fig 4.1 It was later discovered that the converse of this phenomenon is also possible. When a potential difference is applied across the pair of opposite faces, compressions or elongations are obtained across the other pair of opposite faces depending upon the polarity of the applied PD.
  5. 5. Seminar Report, 2013 SSET, Karukutty 5 Dept. of EEE Further, the application of AC voltage across these faces resulted in alternating compressions and elongations (mechanical vibrations) across the other pair of faces. This is the driving force behind the USMs. Crystals that exhibit the above phenomenon are called piezoelectric materials. The interesting fact is that converse piezoelectric effect was discovered on a later occasion, 2 years after the discovery by the Curie brothers. Almost 80 years, the immense potential of motoring produced by this discovery was unknown to the scientific world, until the V V Lavrinenko modeled the 1st ever USM in the year 1946. In short, the Ultrasonic Motors works on the principle of converse piezoelectric effect.
  6. 6. Seminar Report, 2013 SSET, Karukutty 6 Dept. of EEE Chapter 5 CONSTRUCTION The Ultrasonic Motors constitutes mainly 4 parts, viz. 1. Actuator 2. Stator 3. Rotor 4. Casing 5.1 ACTUATOR Actuator is the driving unit of the USM. It is made up of a piezoelectric material such as Quartz, Barium Titanate, Tourmaline, Rochelle salt etc. Actuator is directly connected to the supply. It is fixed on the stator using thin metal sheets and bearings. 5.2 STATOR Stator is the stationary but vibrating part. It is constructed using a malleable material, usually, steel. It can be of ring, cylindrical or rod shaped. 5.3 ROTOR Rotor is the rotating part, which acquires the energy conversion and produces the desired torque (work) on the shaft. Rotor is made of the same material as that of the stator and does have the same shape. Rotor and stator are coupled by a certain method called the frictional coupling which is much effective and simpler. The idea of frictional coupling is discussed in Chapter 6. 5.4 CASING Casing is used to protect the USM from external interferences and abrasive forces and extreme environmental conditions. They are made of non- corrosive alloys or fiber. They too can be constructed in any of the desired shape as per requirement.
  7. 7. Seminar Report, 2013 SSET, Karukutty 7 Dept. of EEE 5.5 ASSEMBLY Fig 5.1 Comb tooth grooves are created on the stator surface that adheres to the rotor. These are devised to make the amplitude of elliptic motion large and to reduce abrasions. Further feature of comb tooth is to amplify vibrations. The grooves also allow the dust created by friction to escape and thus keep the contact surface dust free. So the USMs are well known for their low speed operation. Torque ratings averaging 10 times greater than a comparably sized electromagnetic motor can be achieved. Voltage inputs vary piezo-crystal assembly. That is, if the piezo-ring assembly is of a thinner design, the voltage requirements will be less than that of a thicker type piezo-ring assembly. Power requirements for the USM usually rate in the low range.
  8. 8. Seminar Report, 2013 SSET, Karukutty 8 Dept. of EEE Chapter 6 WORKING Ultrasonic motor involves direct conversion of electrical energy to mechanical energy. We have seen in the previous chapter that actuator is directly connected to the electrical supply mains. When the supply is switched ON, the actuator starts vibrating owing to converse piezoelectric effect. The stator, upon which the actuator is fixed, also receives the vibration. The particles of the stator receive energy from the actuator and starts vibrating in the plane. This results in the formation of a surface wave. The stator and rotor are placed so close to each other that their surfaces almost grazes upon each other. The surface waves so produced have a frequency in the ultrasonic range and are not visible by our bare eyes. These waves cause the stator to slide against the rotor. As the wave propagates, the rotor is pulled back in the opposite direction of movement of the wave. In the fig 6.1 the surface wave produced is propagating in the anti-clockwise direction, whereby, the rotor is pulled to rotate in the clockwise direction. The shaft, upon which the rotor is mounted, now rotates and the output torque is thus obtained. Fig 6.1
  9. 9. Seminar Report, 2013 SSET, Karukutty 9 Dept. of EEE From the figure, it is evident that the stator and rotor always possess the same shape. The figure depicts a rotational USM. Fig 6.2 is the schematic diagram of a linear USM driven by dual actuators. Fig 6.2
  10. 10. Seminar Report, 2013 SSET, Karukutty 10 Dept. of EEE Chapter 7 FRICTIONAL COUPLING We have already discussed in the working of USM that a surface wave is generated in the stator, by the converse piezoelectric effect occurring in the actuator. As the wave propagates through the stator, the rotor is pulled back in the opposite direction. Refer fig 6.1. Here the surface wave generated in the stator propagates in the anti-clockwise direction, whereby, the rotor is pulled in the reverse direction. Now, there is a chance that the rotor may get slipped off from the stator. To avoid this hitch, the surface of contact between rotor and stator is made rough or rather uneven. This method of coupling of rotor and stator is known as frictional coupling. There are other methods of coupling involved in the modern USMs, but frictional coupling is found to be the simplest and rugged. In course of time, the qualm arises whether the friction between the rotor-stator combo lead to wear and tear, and the counter was found to be NO, because, during its operation, as the motor gathers speed, the rotor is slightly levitated in the perpendicular direction owing to centrifugal force. Concurrently, the real area of contact of the stator and rotor decreases. Under the assumption that friction force is proportional to real area of contact, frictional coefficient of stator/rotor decreases under ultrasonic dynamic contact. But still, the slightest of contacts produces wear on the interface. The grooves are designed in such a shape as to toss away the dust produced by this process. During the design of frictional grooves, care is to be taken that during slow speeds, interlocking of rotor and stator doesn’t occur.
  11. 11. Seminar Report, 2013 SSET, Karukutty 11 Dept. of EEE Chapter 8 TYPES OF ULTRASONIC MOTORS There are several classifications for ultrasonic motors depending upon their shape, size, type of motion, application etc. The USMs are also classified according to the type of wave that is being generated in the stator. If the wave produced in the stator has its particles vibrating only in the vertical direction, i.e. if the wave produced is a standing wave (static wave), such type of motors are called Standing Wave type USMs. And, if the wave produced in the stator has its particles vibrating both in the vertical direction as well as horizontal direction (travelling wave), such type of motors are called Travelling Wave type USMs. Even though the type of wave produced is different, both these types of USMs has got the same construction and working principles. However, the Travelling Wave-type USMs are found to have better operating characteristics. The mechanics of standing and travelling waves are beyond the scope of this paper. The other methods of classifications of ultrasonic motors are, 1. Mode of operation: a. Static b. Resonant 2. Type of motion: a. Rotary b. Linear c. Spherical 3. Shape of implementation: a. Beam b. Rod c. Disk
  12. 12. Seminar Report, 2013 SSET, Karukutty 12 Dept. of EEE Chapter 9 ADVANTAGES & DISADVANTAGES 9.1 ADVANTAGES OF USMs Since the ultrasonic motors are designed after much of studies and researches, so as to eliminate the drawbacks of electromagnetic motors, they ought to have new features and advantages. The foremost advantage of USMs is that they have got high o/p torque & efficiency. Compared to the conventional electromagnetic motors they exhibit high power to weight ratio. A small sized USM is capable of producing the same amount of output power as that of an EM motor which is 14 times its size. Owing to its good positioning accuracy, the USMs in the modern times are used as a substitute for synchros, servomotors and stepper motors. The rugged structure and construction makes it capable of working in extreme environmental conditions. The major advantage of using ultrasonics as the principle is that no magnetic interference occurs. The simplest of constructions makes it economical to the layman. The device is well known for its miniaturization and simplicity, yet boasts for it immense output power and can withstand extreme environmental conditions. No magnets, no losses, no interference, as clean as a whistle! 9.2 DISADVANTAGES OF USMs However, no machine is efficient to a cent per cent ceiling as is the case of USMs also. These motors do eliminate the major drawbacks of the conventional motors but still lags behind in certain constraints. Even being the most modern of all the technologies, the ultrasonic motor has got a major disadvantage of requiring a high frequency power supply in the ultrasonic range. The normal 50Hz supply has to be stepped up to the kHz range by making use of cycloconverters, inverters
  13. 13. Seminar Report, 2013 SSET, Karukutty 13 Dept. of EEE and other mandatory devices. By the advent of power electronics, SCRs and other lossless switching devices, the construction of the so called frequency boosters has neither elevated the capital costs, nor diminished the efficiency. The piezoelectric material which has been the driving unit of the USM is quite expensive by which, the construction of larger USMs has been hampered. Other than these, the USMs suffers from the trouble of ultrasonic noise, but has not curtailed the operating characteristics to a great extent. The ultrasonic noise, since not captured by the bare ears, makes us feel better but not the structure. The drooping torque-speed characteristics are an area where further researches and explanations are yet to be made. It is also necessary to increase the frictional force to an extreme degree, but this inevitably put excessive stress on the bearings which causes the bearings to wear out and shorten the life of the motor. It is difficult to obtain high torque form an ultrasonic motor if it is miniaturized beyond a certain size.
  14. 14. Seminar Report, 2013 SSET, Karukutty 14 Dept. of EEE Chapter 10 APPLICATIONS Though not much used for heavy duty applications, USMs are best known for their miniaturization of equipments. The 90% of the USMs constructed in the era are used in SLR, DSLR cameras for the auto focusing & optical zooming. The positioning of surveillance cameras are also carried out using these motors. In the automobile industry, the USMs are used for the operation of power steering, power windows, tilt steering, ORVMs, car seat adjustment and almost all motoring devices etc. In the computer hardware industry the USMs has got wide applications. The disk heads of hard disks, floppies & CD drives can be controlled by the USM. Ever wondered the name “quartz” written on wrist watches & clocks??? It is the quartz USM that drives the motor inside it. The ultrasonic motor has got applications also in robotics, aerospace and medicine.
  15. 15. Seminar Report, 2013 SSET, Karukutty 15 Dept. of EEE Chapter 11 HISTORY OF USM 2009 Invented spherical USM & 2 DOF optical sensing system for MRI scanning. 2005 Draft works of multiple DOF cameras was commenced. 2003 A reduced size micro USM was put forward by Cannon. 1990 1st micro USM was developed by Cannon. 1986 Cannon uses the ring type USM in the SLR camera autofocus. 1980 1st practical model of USM was developed by Sashida. 1965 1st Ultrasonic Motor was developed by V V Lavrinenko. 1880 Piezoelectric effect was discovered by Curie brothers. Fig11.1
  16. 16. Seminar Report, 2013 SSET, Karukutty 16 Dept. of EEE Chapter 12 CONCLUSION The above prose has gone through almost all the fundamental details of this modern technology machine. Though USMs are not widely used for heavy motoring activities, it still has got its unique stand in the miniature applications as mentioned above. The main reason for USMs not being used in the heavy duty activities is because of the requirement of a larger & expensive piezoelectric material that causes the production cost to shoot up double fold. However, researches are in progress to improve the technology to meet the heavy industrial requirements and also to minimize the problems caused by ultrasonic noise. A world might be expected in the near future that replaces the futile electromagnetic motors by the proficient ultrasonic motors.
  17. 17. Seminar Report, 2013 SSET, Karukutty 17 Dept. of EEE REFERENCES 1. Published in: Ultrasonics Symposium, 1988. Proceedings., IEEE 1988. Date of Conference: 2-5 Oct 1988, Page(s): 519 - 522 vol.1, INSPEC Accession Number: 3439385, Conference Location: Chicago, IL, Product Type: Conference Publications. Meeting Date : 02 Oct 1988-05 Oct 1988. 2. http://en.wikipedia.org/wiki/Ultrasonic_motor 3. http://www.britannica.com/EBchecked/topic/613488/ultrasonics 4. http://www.seminarsonly.com/electrical%20&%20electronics/Ultrasonic%20Motor.php