Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1998-09-16
2001-02-06
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S367000
Reexamination Certificate
active
06184609
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention relates to small motors and actuators comprising electro-mechanical materials which alters its shape under the influence of an electric field. In particular the invention relates to motors and actuators in which the motion relative another body is created by repetition of small steps. The invention also relates to a method of driving such motors and to a method for manufacturing the motors.
RELATED ART
There is a great need for high performance motors in the size range below a few millimeters, motors of the kind which should be able to create linear and/or rotating motion. It is often desirable that this kind of motors both have a high precision and can exert large forces. One may realize that since reliable and cheap motors of this kind is requested for e.g. driving cameras, hard disks, CD-players etc the potential market is huge. Also in the area of medical instrumentation, e.g. pumps, such motors are of great interest.
In the state of the art, various devices based on electromechanical materials exist. Electromechanical materials have the interesting property of changing its shape when they are influenced by an electric field. Pieces of electromechanical materials, fixed to a base plate will therefore move their non fixed surfaces when electric fields are applied to them. Such motions, contractions or expansions, may be used for constructing different types of motors or actuators.
Techniques often used for motors in the size range one centimeter and above are referred to as ultrasonic motor techiques. Other terms often used for the same kind of devices are resonance, vibration, travelling wave or impact motor devices. Typically, in such motors, electromechanical materials impose a resonance vibration into itself and a solid piece material, normally a metal block. In e.g. a travelling wave motor, protruding portions of the metal block are forced into an elliptical movement, and another object in contact with these protruding portions is forced to move in accordance with these movements . When going to miniature motors, this technique will be disadvantageous, since the movements become too small and limited by a non-controllable surface topography etc.
A more appropriate approach to miniature motors based on electromechanical materials is to use devices which operate off the inherent resonance. One particular actuation principle which has a great potential to fulfil the demands for such motors is an Inchworm® type of motor (M. Bexell, A.-L. Tiensuu, J-A. Schweitz, J. Söderkvist, and S. Johansson, Sensors and Actuators A, 43 (1994) 322-329). The motion is created by repetition of small steps in a similar way to the insect inchworm, hence the name (The micropositioning book. Fishers, N.Y.: Burleigh Instruments, Inc. (1990)). This motion principle will in the remaining part of this appplication be referred to as a “non-resonance step” technique, to be distinguished from the above described ultrasonic techniques. Portions of electromechanical material may also be referred to as PZT.
The principle for this motion is simple. A moving body is held between two claws, one on each side of the moving body. Each claw consists of a longitudinal piece of PZT, substantially parallel to the moving body, and at each end a transversal PZT is present. The PZT:s are assembled onto metal bodies. Assuming all of the transversal PZT:s are energized and expanded in the start position, gripping the moving body, the two opposite front transversal PZT:s are recontracted, loosing the grip of the moving body. An electrical field is applied to the longitudinal PZT:s, expanding their lenghts, and the front transversal PZT:s are subsequently forced to expand again, gripping the moving body at a new position. The rear transversal PZT:s loose their grip of the moving body and the longitudinal PZT:s are allowed to contract again, whereafter the rear PZT:s again grip the body. The result of such a cycle is that the moving body has moved relative to the two claws.
An electronic control device is needed for a controlled operation of the above actuator. The electronics should supply the different PZT:s with appropriate voltages in an appropriate order. Since such a sequence of voltages can be repeated very fast, a relatively fast movement is possible to obtain despite the small step size.
There are some crucial factors limiting the development of existing products based on the non-resonance step principle. Among the limits there is the difficulty of achieving a sufficient stroke of the individual actuating elements and the need for a costly high precision assembly of the elements and other parts in the system. Some solutions to these problems have been presented in the Swedish Patent Application No. 9300305-1 by Johansson. Using actuating elements with at least a two-axial motion capacity, the number of elements has been reduced. At the same time motion magnification by internal levers (e.g. bimorphs) in the elements can be included which gives a large freedom in design. According to these ideas a miniature motor has been built and has proven to present the desired high torque and motion capacity as predicted (M. Bexell and S. Johansson, Transducers, Stockholm, Sweden (1995) 528-News).
By the above mentioned solution, a motion relative to another body may be acheived in the following way. Four active elements of electromechanical material are mounted on a passive base plate, normally made of silicon, and the moving body is held against the protruding active elements. All elements consist of two vertically divided controlled portions of PZT, both extending between the base plate and the moving body. By applying a voltage resulting in an electrical field in the horisontal direction to the first portion of the PZT but not to the other portion, one part is tending to contract in the vertical direction, while the other is unaltered. Since the two portions are mechanically integrated into one piece, the active element will subsequently bend towards the side of the live portion. If both portions are energized, the whole element will contract, and if only the second PZT is imposed by a voltage, the element will bend in the other direction. By varying the voltage in the different portions, a contact point on the top of the active element can travel along any path within a rhombic area. A “contact point” is of course not a point in a mathematical sense, but rather a small “contact area” depending on the actual geometries and normal forces, and these expressions are in the present description used in a synonomous manner.
By using four active elements arranged after each other, in the direction of the sidewise movement, a moving action on the body can be acheived. By letting the first and third elements move in phase, and moving the second and fourth elements out of phase, a non-resonance step motion similar to the above described, is acheived.
At present, this motor gives the highest torque per volume of all presently known miniature motors. There are some disadvantages even with this construction, which is the origin of the present invention. In the previous patent application the motor, for instance, consisted of active elements mounted on a substrate, and typically soldering has been used as the assembling method. This is a fairly time consuming operation and therefore costly. Most applications demand however that the price of each motor should be very low.
The above patent application, bimorph or multimorph elements were used to obtain two-axial motion and at the same time possibility for stroke magnification. The disadvantage with a single clamped bimorph is that the force capacity is greatly reduced in comparison with an ideal lever, which is the reason why these types of elements normally are used for positioning when there is no considerable need for forces. A double clamped bimorph, a curved membrane or an arch-shaped structure have a better force capacity for a given stroke magnification, as disclosed in the U.S. Pat. No. 5,589,725. However, no completely satisfying d
Bexell Mats
Johansson Stefan
Budd Mark O.
Piezomotors Uppsala AB
Young & Thompson
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