27-04-2011, 03:51 PM
Prepared by:
Jaime Fernandez
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Piezomotors “Ultrasonic Motors”
Piezoelectric Notions
“Piezoelectricity is the ability of certain crystals to generate a voltage in response to applied mechanical stress. The word is derived from the Greek 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.”
Piezoelectric Motor Notions
A piezoelectric motor or piezo motor is a type of electric motor based upon the change in shape of a piezoelectric material when an electric field is applied.
The ultrasonic motor ( USM ) is a new type of solid state motor, which is driven by the ultrasonic vibration of a piezoelectric transducer. The ultrasonic vibration is transformed into output torque (in rotary USM) or thrust (in linear USM) by the friction between the stator and the rotor (in rotary USM) or the moving part (in linear USM).
This new breed of motors has many excellent performances and features such as high torque, low speed, simple and compact structure with great variability in design, not interfering or being affected by magnetic field when in operation, power off self-brake, etc.
The chief drawback of USM lies in the fatigue wearing of the stator due to the frictional driving mechanism.
Major Applications
Camera auto focus lenses,
Watch motors and compact paper handling.
Medical application within micro-surgery and sensor scanning
Driving fluids
Optoelectronics area
Motion control
Piezo motors consists mainly of an electro-mechanical resonator and a rotor. The resonator transfer electric energy into small elliptical mechanical vibrations that forces the rotor to rotate due to friction between resonator tip and rotor. Piezoelectric motors have some very nice characteristics compared to traditional induction motors and in general they are characterized by a high torque at low angular velocity, silent operation, high positioning precision, simple mechanical construction and it induces no magnetic fields.
Modeling and Control of a Standing Wave Piezo Electric Motor, Brian Andersen.
Basics Principles
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.
To generate a traveling wave within the stator two orthogonal modes are activated simultaneously. These modes are induced by constructing the drive piezoelectric actuators in a pattern of two poling sections that are bonded to the stator. Geometrical examination of this pattern shows that driving the two sections using cos(w t) and sin(w t) signals, respectively, will produce a traveling wave with a frequency of w /2p . Also, by changing the sign on one of the drive signals, the traveling wave would reveres its direction.
The equation of motion of the ultrasonic motor can be derived from Hamilton’s principle. The analytical model has been derived by many authors (e.g. Hagood and A. McFarland [5], Kagawa et al [6]). The generalized equation of motion of the stator can be summarized as
USM Prototypes
DOF planar pin-type actuator
The objective of this project is to design and develop a piezoelectric actuator based on the fundamental operating mechanism of ultrasonic motors. Two pin-type prototypes with piezoelectric bimorph plate and a contact pin for generating driving force in the X-Y direction were designed and fabricated. A test rig was also constructed for the evaluation of the two prototypes and basic characteristics of the actuators were investigated. The working principle of the actuator was verified and proven during the experiment. Basically, the optimal driving speed of an actuator is dependent on the driving frequency, the input voltage, the contact surface and the friction coefficient between the stator and motor. An analytical study of the prototypes has been carried out by means of finite element analysis utilizing ANSYS5.4. With comparison to the experimental results, it was proven that the optimal driving condition occurred at the specific resonant mode depending on the pin vibration. Maximum unloaded driving speed was obtained to be approximately 0.68 cm/s at a frequency of 14.8 kHz and the optimum input voltage was found to be approximately 70 Vp-p.
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.
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.
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.
