Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-08-23
2001-06-12
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S323020
Reexamination Certificate
active
06246157
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to linear electric motors and more particularly to a high force, step and repeat piezoelectric linear motor.
2. Description of the Related Art
Many types of electrical motors have been developed that provide rotational force such as three-phase motors, induction motors, split-phase motors, etc. However, there is a need for motors that provide linear displacement for use in applications such as aircraft flaps, electrical powered sunroofs or electrically powered car seats. Some of the more heavy duty applications require a linear displacement force in excess of 220 Newtons.
Currently, ball and screw type motors are frequently used for producing linear displacement and comprise a threaded shaft and a metal ball with a threaded hole that mates with the threaded shaft. As the shaft turns, the metal ball rides up or down the shaft to linearly displace the metal ball and any attached mechanism. One problem with this motor is the high tolerance that is required between the ball and the shaft, particularly when used in high accuracy applications. Through use, the threads on the shaft and the ball tend to wear, resulting in a backlash when the motor is stopped or encounters a load, thus reducing positioning accuracy.
The ball and screw motor is powered by an electric motor which can overheat and be damaged or destroyed if the motor stalls under an excessive load. Also, electric motors generally work most efficiently at one speed; when the motor is slowed under load its efficiency drops.
Furthermore, the force produced by the shaft and ball displacement is directly related to the power of the electrical motor. For increased power, the size of the electrical motor must be increased. In applications requiring a great deal of linear displacement force, the size of the motor can become prohibitive. Ball and screw motors also tend to be relatively expensive.
Piezoelectric materials have been used for many different types of motors, primarily in motors that produce a rotation as opposed to displacement. As an example, see U.S. Pat. No. 5,780,956 to Oliver, Neurgaonkar, et.al. Certain piezoelectric materials are useful because of their ability to directly convert electrical energy into motion (mechanical energy). When a voltage is applied to the piezoelectric material, the material will experience a strain that causes it to expand. When the voltage is removed, the strain is removed and the material contracts.
Piezoelectric materials are generally formed from ceramics. One particularly valuable type of piezoelectric device has a plurality of laminated piezoelectric layers that can expand and contract quite rapidly, and combines the expansion of all the layers. The purpose of such layering is to keep the necessary drive voltage to a practical level, while obtaining significant expansion. The expansion can vary, but is generally on the order of 0.002 times the length of the layered piezoelectric material.
Linear piezoelectric motors have been developed using an “inchworm” piezoelectric mechanism to linearly translate a shaft. An example of this type of motor is the Burleigh PZ-577 Inchworm™ Translator System which comprises three piezoelectric cylinders coupled together on a shaft. One of the end cylinders is fixed to a support structure and the other cylinders are allowed to move linearly in relation to the fixed cylinder. The cylinders rely on an inchworm type motion to move the shaft. The first and third cylinders fit around the shaft with near zero clearance, while the middle cylinder has a clearance fit over the shaft. If the first cylinder were fixed, a voltage is applied to the first cylinder and it grips the shaft. A voltage is then applied to the middle cylinder causing it to expand longitudinally down the shaft, pushing the third cylinder ahead of it. A voltage is then applied to the third cylinder, causing it to grip the shaft. The voltage is next removed from the first cylinder causing it to release the shaft, and also from the middle cylinder, causing it to contract and pull the shaft with it. This inchworm cycle results in moving the shaft in the direction of the first cylinder and is repeated to move the shaft linearly.
The primary problem with inchworm type motors is that they typically provide linear push or pull forces in the range of 10 to 15 Newtons and cannot be used for heavy duty applications requiring a greater linear force. Such motors also require precise machining and are not easily adjusted for optimum performance
SUMMARY OF THE INVENTION
The present invention provides a improved linear pushpull motor that relies on the expansion characteristics of piezoelectric materials to produce linear displacement. The new motor employs a step and repeat action, using two clamps fixed on a motor base and a split motor shaft that travels within the clamps such that the clamps hold the shaft when closed.
The split motor shaft preferably comprises two shaft segments and a displacement actuator, with the two shaft segments coaxially aligned and attached to opposite ends of the displacement actuator. Each shaft segment travels within a respective clamp. The displacement actuator has a piezoelectric body that expands when a voltage is applied, with the body oriented such that the expansion is coincident with the axis of the two shaft segments. Expansion of the piezoelectric body results in an increase in the length of the split motor shaft.
Linear movement of the split motor shaft is produced by coordinating the opening and closing of the clamps with the expansion of the piezoelectric body. For example, the split motor shaft can be moved linearly by opening one of the clamps, activating the piezoelectric body to expand the split motor shaft in the direction of the open clamp, closing the open clamp, opening the other clamp, and deactivating the piezoelectric body. When the body is deactivated it contracts, pulling the shaft segment in the open clamp toward the clamped shaft. This cycle results in the split motor shaft moving in the direction of the originally opened clamp. It can be repeated to move the shaft in the same direction or reversed to reverse the shaft direction.
The new motor is less complicated than other linear motors while providing consistent force output over a wide range of speeds with precision and reliability. The new motor is relatively small, but can provide sufficient linear force to pull or push in excess of 220 Newtons. It does not have gears and does not experience gear related power loss, wear and backlash, nor is it overheated or damaged when slowed or stalled. Furthermore, a uniform force output is produced at any motor speed.
These and other further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which:
REFERENCES:
patent: 3138749 (1964-06-01), Stibitz
patent: 3217218 (1965-11-01), Steele
patent: 3292019 (1966-12-01), Hsu et al.
patent: 3377489 (1968-04-01), Brisbane
patent: 3684904 (1972-08-01), Galutva et al.
patent: 4219755 (1980-08-01), O'Neill et al.
patent: 4714855 (1987-12-01), Fujimoto
patent: 4736131 (1988-04-01), Fujimoto
patent: 5595677 (1997-01-01), Neurgaonkar et al.
patent: 5751090 (1998-05-01), Henderson
patent: 5780956 (1998-07-01), Oliver et al.
patent: 1933205 (1971-01-01), None
patent: 0352858 (1989-07-01), None
patent: 1261523 (1968-10-01), None
Data Sheet for Burleigh Inchworm™ Translator Systems, 2 pages.
Neurgaonkar Ratnakar R.
Oliver John R.
Budd Mark O.
Koppel & Jacobs
Rockwell Science Center LLC
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