Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-05-01
2004-04-20
Budd, Mark (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S323040, C310S323020
Reexamination Certificate
active
06724128
ABSTRACT:
BACKGROUND—FIELD OF THE INVENTION
This invention relates to plain bearing-like devices and more particularly to precision, ultrastiff plain bearing-like structures both macroscopic and microscopic with frictional and structural properties that can be electronically controlled and which contain a built-in ultrastiff force sensing mechanism.
Precision bearings, specifically designed to translate or rotate a mechanical load with submicron accuracy have become increasingly important in the optics, semiconductor manufacturing, and micromachining industries. Unlike regular bearings that are used, for example in automobiles, these precision bearings have unique primary and secondary characteristics. Among the most important primary characteristics are (1) extremely high stiffness, (2) controllable frictional characteristics, (3) enhanced smoothness-of-travel, and (4) superior accuracy-of-travel. Some of the secondary features are (a) high damage threshold, (b) high reliability, (c) low maintenance, (d) low external support requirements, and of course (e) low cost.
A common bearing, found in many semi-precision, mechanisms, is the sliding bearing or plain bearing which coincidentally also exhibits many of the primary and secondary characteristics of a precision bearing. As used herein, the term “plain bearing” simply means that the bearing's load is supported through sliding motion between two solid surfaces.
FIG. 1A
illustrates a lubricated planar configuration of a simple plain bearing, which consists of a flat bar-shaped upper slide-plate
50
and a similar lower slide-plate
54
sliding against each other with a lubricant
52
, such as oil, between the sliding surfaces.
FIG. 1B
shows two forces acting on the upper slide-plate of the simple plain bearing of FIG.
1
A. The force, F
APP
45
, is an applied force sufficient to maintain contact along an interface between the surfaces of the two slide-plates. The other force is an applied sliding force
46
, which, along with the component of the force
45
parallel to the interface, can be used to translate the two slide-plates relative to each other.
A plain bearing is characterized by (1) its inherent simplicity, having a minimum number of moving parts; (2) its superior runout or off-axis error characteristic which is obtained by averaging any local smoothness imperfections over the entire sliding surface area; (3) its high shock-loading or damage threshold which is the result of spreading the shock force on the bearing over a large surface area, and (4) its extremely high compressive stiffness since the bearing has large direct material to material sliding contacts.
However, despite all of these advantages, a lubricated plain bearing is not generally used in precision applications for several reasons. The first reason is because of the bearing's relatively high frictional forces generated by the component [F
APP
]
Z
of the force
45
which is perpendicular to the interface. These frictional forces are a direct result of its high coefficient of friction, which can be ten to one hundred times larger than an equivalent ball or air bearing. The second reason is because of stiction, an extra frictional holding phenomenon, above the “normal” static friction, that occurs when two extremely smooth and lubricated contacting surfaces that were stationary, start to slide. The third reason is because of a phenomenon known as stick-slip which results in fluctuations in the frictional forces while the bearing is in motion. The cumulative effects from all three phenomena associated with a plain bearing, will usually render an instrument equipped with this type of bearing unable to perform precise and microscopic movements.
In prior art, there are three general methods used to change the frictional forces between two sliding surfaces. First, the actual coefficient of friction between the sliding surfaces can be modified by very thin films or coatings with good tribological properties. This is generally accomplished by using some combination of a solid, liquid or gas as reviewed in U.S. Pat. No. 4,944,606 of Lindsey et al., (1990) and U.S. Pat. No. 5,911,514 of Davies et al., (1999). Second, if one or more operating parameters between the two sliding surfaces can be altered, the frictional characteristics can be changed. Some common operating parameters that can be readily controlled to produce a relatively small variation in frictional behavior are surface temperature, as described in U.S. Pat. No. 5,441,305 of Tabar (1995) and sliding speed, as shown in U.S. Pat. No. 5,043,621 issued to Culp (1991). Finally, if the compressive force between the two sliding surfaces is minimized, or time-modulated as revealed in U.S. Pat. No. 3,774,923 of Suroff (1973), U.S. Pat. No. 3,756,105 of Balamuth et al., (1973), and U.S. Pat. No. 4,334,602 of Armour et al., (1982), or even totally eliminated by, for example, magnetic levitation as shown in U.S. Pat. No. 3,937,148 issued to Simpson (1976), then the frictional force generated between these two surfaces is correspondingly minimized.
All these techniques of friction reduction can be used individually or possibly, in some cases, in combination. Examples are air bearings, like those described in U.S. Pat. No. 3,683,476 of Lea et al., (1972), used in precision translational stages where a combination of an air film and levitation techniques are employed to reduce friction.
Solutions to the stiction phenomenon were revealed in applications related to magnetic disk storage. U.S. Pat. No. 4,530,021 issued to Cameron (1985) and U.S. Pat. No. 6,002,549 issued to Berman et al., (1999), teach that vibrational techniques, which dither the slider head also free it from the force of stiction. Further solutions to the stiction problem involve surface texturing techniques, as illustrated by U.S. Pat. No. 5,079,657 of Aronoff et al., (1991), which allow a slider head to leave a surface smoothly.
Similarly, the stick-slip problem has also been successfully addressed in diverse fields, such as those including focusing mechanisms used in optical disk storage and wiping mechanisms for automotive windshields. Their solutions, like the stiction case, are also based upon oscillation methods as revealed in U.S. Pat. No. 4,866,690 issued to Tamura et al., (1989) and U.S. Pat. No. 5,070,571 issued to Masuru (1991).
However, all these prior art techniques suffer from one or more drawbacks which render their benefits, in a plain bearing-like device application, individually insufficient and in some cases, incompatible with precision. For example, simply adding a lubricant between the two surfaces of a plain bearing does not minimize either stiction or stick-slip behavior which are required for smooth microscopic motion as well as positioning accuracy. Controlling the aforementioned operating parameters does not reduce the coefficient of friction enough for most applications. Changing the compressive force by periodic or continuous levitation compromises bearing compressive stiffness, produces changes in the bearing position depending upon whether the bearing is levitated or not, and may require substantial external facility support such as, in the case of air bearings, a continuous supply of clean, dry air delivered at a regulated pressure. Furthermore, U.S. Pat. No. 4,648,725 of Takahashi (1987), teaches that positioning devices employing bearings with only a fixed, very low coefficient of friction, such as an air bearing, tend to have an extended settling time even after moving at relatively low velocity to a particular position of interest.
Also, prior art examples using vibrational or oscillation techniques to minimize stiction or stick-slip make no effort to separate the vibrational motion from the ideally desired motion of the load. Separation of the two motions is paramount in precision bearings where maintaining the integrity of the slidable path, continuously contacting surfaces, and vibrational insensitivity are all desired properties. Furthermore, prior techniques that use the vibrational motion only
Carruno Christopher Damien
Cheng Jack
Budd Mark
GRQ Instruments, Inc.
Haverstock & Owens LLP
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