An ultrasonic motor is a type of electric motor formed from the ultrasonic vibration of a component, the stator, placed against another, the rotor or slider depending on the scheme of operation (rotation or linear translation). Piezoelectric ultrasonic motors are a new type of actuator. They are characterized by high torque at low rotational speed,simple mechanical design and good controllability. They also provide a high holding torque even if no power is applied. Compared to electromagnetic actuators the torque per volume ratio of piezoelectric ultrasonic motors can be higher by an order of magnitude. Many different types of ultrasonic motors have been proposed up to date. Ultrasonic motors are of great interest due to the flexibility of miniaturization in comparison with conventional electromagnetic motors whose efficiency decreases significantly. Especially in information systems and medical industry, compact size of these motors makes them find wider applications. This paper describes the advantages of ultrasonic motors over the conventional electromagnetic motors, several types of piezoelectric ultrasonic motors. The basic working principle; the mechanism ofthe ultrasonic motor are also explained. Finally, its applications and the only disadvantage are discussed.
An ultrasonic motor is a type of electric motor formed from the ultrasonic vibration of a component, the stator, placed against another, the rotor or slider depending on the scheme of operation (rotation or linear translation). Ultrasonic motors differ from piezoelectric actuators in several ways, though both typically use some form of piezoelectric material, most often lead zirconate titanate and occasionally lithium niobate or other single-crystal materials. The most obvious difference is the use of resonance to amplify the vibration of the stator in contact with the rotor in ultrasonic motors. Ultrasonic motors also offer arbitrarily large rotation or sliding distances, while piezoelectric actuators are limited by the static strain that may well be induced in the piezoelectric element.
Piezoelectric ultrasonic motors are a new type of actuator. They are characterized by high torque at low rotational speed, simple mechanical design and good controllability. They also provide a high holding torque even if no power is applied. Compared to electromagnetic actuators the torque per volume ratio of piezoelectric ultrasonic motors can be higher by an order of magnitude.
The ultrasonic motor is characterized by a “low speed and high torque”, contrary to the “high speed and low torque” of the electromagnetic motors. Two categories of ultrasonic motors are developed at our laboratory: the standing wave type and the traveling wave type.
The standing wave type is sometimes referred to as a vibratory-coupler type, where a vibratory piece is connected to a piezoelectric driver and the tip portion generates flat-elliptical movement. Attached to a rotor or a slider, the vibratory piece provides intermittent rotational torque or thrust. The travelling-wave type combines two standing waves with a 90¡-phase difference both in time and space. By means of the traveling elastic wave induced by the thin piezoelectric ring, a ring-type slider in contact with the surface of the elastic body can be driven.
Advantages of ultrasonic motor over electromagnetic motor:
1. Little influence by magnetic field:
The greatest advantage of ultrasonic motor is that it is neither affected by nor creates a magnetic field. Regular motors which utilize electromagnetic induction will not perform normally when subjected to strong external magnetic fields. Since a fluctuation in the magnetic field will always create an electric field (following the principle of electromagnetic induction), one might think that ultrasonic motors will b affected as well. In practice, however, the effects are negligible. For example, consider a fluctuation in the flux density by, say, 1T (which is a considerable amount), at a frequency of 50 Hz , will create an electric field of 100 volts per meter. This magnitude is below the field strength in the piezoelectric ceramic and hence can be ignored.
2. Low speed, high torque characteristics, compact size and quiet operation:
Ultrasonic motors can be made very compact in size. The motor generates high torques at low speeds and no reduction gears are needed unlike the electromagnetic motors. The motor is also very quiet, since its drive is created by ultrasonic vibrations that are inaudible to humans.
3. Compact-sized actuators:
The ultrasonic motors small size and large torque are utilized in several applications. The ultrasonic motors hollow structure is necessary for an application in several fields such a robotics etc where it would be very difficult to design a device with an electromagnetic motor and satisfy the required specifications.
Piezoelectricity is the ability of certain crystals to generate a voltage in response to applied mechanical stress. The word is derived from the Greek word piezein, which means to squeeze or press. The piezoelectric effect is reversible in that piezoelectric crystals, when subjected to an externally applied voltage, can change shape by a small amount. The deformation, about 0.1% of the original dimension in PZT, is of the order of nanometers, but nevertheless finds useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, and ultra fine focusing of optical assemblies.
In a piezoelectric transducer, the acceleration acts on the seismic mass that develops a force on piezoelectric quartz, or ceramic crystal, or on several crystals. The force causes charges on the crystals proportional to the acceleration.
The general principle of the operation of ultrasonic motors is to generate gross mechanical motion through the amplification and repetition of micro-deformations of active material. The active material induces an orbital motion of the stator at the rotor contact points and frictional interface between the rotor and stator rectifies the micro-motion to produce macro-motion of the stator.
The active material, which is a piezoelectric material excites a traveling flexural wave within the stator that leads to elliptical motion of the surface particles. Teeth are used to enhance the speed that is associated with the propelling effect of these particles. The rectification of the micro-motion an interface is provided by pressing the rotor on top of the stator and the frictional force between the two causes the rotor to spin. This motion transfer operates as a gear leads to a much lower rotation speed than the wave frequency.
A stator substrate is assumed to have a thickness, tS, with a set of piezoelectric crystals that are bonded to the back surface of the stator in a given pattern of poling sequence and location. The thickness of the piezoelectric crystals is tp. The total height, h, is the sum of the thickness of the crystals and the stators (bonding layer is neglected). The overall height of the stator is also allowed to vary with radial position. The outer radius of the disk is b and the inner hole radius is a. To generate traveling wave, the piezoelectric crystals poling direction is structured such that quarter wavelength out-of-phase is formed. This poling pattern is also intended to eliminate extension in the stator and maximize bending. The teeth on the stator are arranged in a ring at the radial position.
Dry friction is often used in contact, and the ultrasonic vibration induced in the stator is used both to impart motion to the rotor and to modulate the frictional forces present at the interface. The friction modulation allows bulk motion of the rotor (i.e., for farther than one vibration cycle); without this modulation, ultrasonic motors would fail to operate.
Two different ways are generally available to control the friction along the stator-rotor contact interface, traveling-wave vibration and standing-wave vibration
A key observation in the study of ultrasonic motors is that the peak vibration that may be induced in structures occurs at a relatively constant vibration velocity regardless of frequency. The vibration velocity is simply the time derivative of the vibration displacement in a structure, and is not (directly) related to the speed of the wave propagation within a structure. Many engineering materials suitable for vibration permit a peak vibration velocity of around 1 m/s. At low frequencies 50 Hz, say a vibration velocity of 1 m/s in a woofer would give displacements of about 10 mm, which is visible to the eye. As the frequency is increased, the displacement decreases, and the acceleration increases. As the vibration becomes inaudible at 20 kHz or so, the vibration displacements are in the tens of micrometers, and motors have been built that operate using 50 MHz surface acoustic wave (SAW) that have vibrations of only a few nanometers in magnitude. Such devices require care in construction to meet the necessary precision to make use of these motions within the stator.
More generally, there are two types of motors, contact and non-contact, the latter of which is rare and requires a working fluid to transmit the ultrasonic vibrations of the stator toward the rotor. Most versions use air.
Types of ultrasonic motors
1.Bi-directional linear standing wave USM:
A standing wave bi-directional linear ultrasonic motor has been fabricated. This linear USM has very simple structure and can be easily mounted onto any commercially available linear guide. A high precision positioning x-y table was built by mounting these individual movable linear guides together. The basic parameters of our linear USM are: moving range 220mm(variable depending on the linear guide), no-load speed 80mms/s, ratings 23mm/s at 300gf, stall force 700gf, starting thrust 500gf, resolution<50nm, response time of 12ms from stationary status to constant velocity(80mm/s) with a initial mass of 260g.
2. Rotary Ultrasonic Motor:
The characteristics of the rotary disc type motor will be investigated and theoretical model will be formed to relate the important components on the power of the motor. The scope includes designing different motor with various dimensions, form calculation of the analytical model, experimental testing and ultimately, setting a standard for practical application of this particular type of USM. This project will lay the foundation of the characteristics and performance of the rotary disc type USMs for future application.
3. Spherical ultrasonic motor
Presently a new type of spherical USM is under investigation. This particular USM consists of a thin square plate, 30x30mm in area. It can rotate in more than 4 individual directions. Now we are trying to compile rotation in any direction by using a computer to control the 4 individual directions properly.
The development and the study of both linear and rotational ultrasonic motors open new ways to the future for more applications in the medical micro-surgery or for miniature space robotics. Indeed, the ongoing miniaturization of systems and drives confines the electromagnetic motors to their limits and thus opens the way to the ultrasonic motors in the industrial world. Some of the major applications are explained below:-
- Camera auto focus lenses
Electromagnetic motors contain hundreds of parts, including iron cores, copper windings and permanent magnets. Miniature stepper and DC motors are marvels of precision engineering, and millions are produced at small size and low cost. Unfortunately, they have reached their limits in terms of miniaturization. Phone camera designers need motors that are four times smaller than traditional technology can deliver.
Another limitation of electromagnetic motors is that they become less efficient at smaller sizes. This is because more and more of the electrical drive power is converted to heat rather than to mechanical motion. For mobile phone cameras, this means greater drain on the batteries. It also results in lower reliability, because less torque is available to overcome friction in micro-gear mechanisms. Smaller motors must also operate at higher speeds to produce significant mechanical power. Therefore even greater gear ratio reduction is needed, which increases system complexity, adds parts, further reduces efficiency and degrades precision Ultrasonic motor serves all these purposes.
- Medical application
Because traditional electromagnetic motors contain ferrous metals, they represent a safety hazard in applications with strong magnetic fields, such as MRI technology. Electromagnetic motors also generate their own magnetic and radio-frequency (RF) fields, which can result in RF arcing and cause hardware damage and image artifacts. In addition, conventional motor operation can be influenced by the static and gradient magnetic fields used during MRI data acquisition, causing unpredictable motor function or damage.
Because standard motion technologies are based on electromagnetic motors, they generate an electromagnetic field, which can interfere with imaging while endangering people within the MRI field itself. To avoid the problems associated with conventional piezoelectric ultrasonic motor is used that is constructed from nonferrous materials.
- Motion control
- Watch motors and compact paper handling
- Driving fluids
Like any other technology, ultrasonic motor also has its shortcomings as well as advantages and there is ample room for further research to overcome these shortcomings and make the motor better.
The ultrasonic motor is subjected to the limitations of the piezoelectric ceramics.The motors vibrations create alternating stresses in the ceramic elements which can result in fatigue failure. The ceramic is strong in compression but weak in tension, with respective failure limits in the ratio 30:1.
The chief drawback of ultrasonic motors lies in the fatigue wearing of the stator due to the frictional driving mechanism.