X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-04-21
2001-02-27
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S019000, C378S193000, C378S196000
Reexamination Certificate
active
06195407
ABSTRACT:
TECHNICAL FIELD
The present invention relates to high-precision positioner assemblies, and more particularly to a high-precision follower assembly for use with a helical cam, such as a leadscrew.
BACKGROUND OF THE INVENTION
It is frequently necessary to position objects with high precision and accuracy, and with a high reliability so that the objects can be placed in the same location repeatedly and without variation. Although there are many applications for such high-precision positioners, one in particular is the positioning of an aperture for defining an x-ray beam in, for example, a computed tomography (CT) scanner. More specifically, it is critical for diagnostic accuracy and reliability that the x-ray beam of a CT scanner be positioned precisely and reproducibly on a receiving surface of an x-ray detector. However, the position of the x-ray source may change over time due to thermal and gravitational forces on it. Accordingly, the position of the beam should be monitored and the aperture moved to compensate for changes in the position of the x-ray source so that the x-ray beam remains properly positioned. Other applications for such positioners include, for example, coordinate measuring machines, high-precision laser-cutting and laser-printing machines, and other applications in which high-precision placement of relatively modest loads is required.
A known technique for controlling the position of an x-ray beam aperture is to use a rotatable leadscrew coupled to a rotating shaft of a motor. A nut mates with the leadscrew and is attached to a slide (which includes a structure configured so as to define the aperture) that moves on a rail rigidly connected to the motor. When the leadscrew is rotated within the nut in either a clockwise or counterclockwise direction, the slide moves along the leadscrew by a corresponding amount. Such a mechanism may be used, for example, in the focal spot movement compensation system disclosed in U.S. Pat. No. 5,550,886 to Dobbs et al. and assigned to the assignee of the present invention.
There are several disadvantages to this type of mechanism. First, the movement of a nut on a leadscrew is likely to result in backlash caused by the presence of small but necessary clearances between the threads of the nut and the leadscrew. This makes it difficult to smoothly and rapidly adjust the position of the nut in both directions. Another problem is that the precise repositioning of the slide at any one position is usually not reproducible. Many potential contact points exist between the threads of the leadscrew and of the nut. As the nut moves around and along the leadscrew, the points of contact between the nut and the leadscrew are constantly changing. The repositioning of the nut at any one position on the leadscrew, and thus of the slide on the rail, is usually not reproducible. Further, additional torque is required to overcome the binding that results from the nut traveling on the leadscrew. As a result, the driver for the leadscrew must be sufficiently oversized and/or more powerful than would otherwise be necessary, and this in turn causes other components to be oversized or more powerful than would otherwise be necessary.
In addition, unless the leadscrew and the nut are always in perfect alignment, the nut will tend to bind on the leadscrew as the nut approaches the fixed end of the leadscrew. In general, the rail on which the slide moves is not exactly parallel to the leadscrew, and the leadscrew itself is generally not exactly parallel to the shaft of the motor which drives it. Therefore, the distance between the center of the leadscrew and the center of the nut is not constant as the nut moves along the leadscrew, and this can result in binding of the nut on the leadscrew.
It would thus be advantageous to provide a positioner assembly which overcomes the deficiencies of prior art leadscrew and nut mechanisms and which can provide high-precision movement of modest loads with reproducible positioning of the loads.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided a determinate positioner assembly, comprising:
a helical cam extending in the direction of a first (z) axis;
a follower element extending partially around a portion of the outside surface of the helical cam and contacting a bearing surface of the cam in a predetermined number of locations;
a driven element connected to the follower element by a set of pivot elements; and
an elastic member elastically connecting the follower element to the helical cam. The follower element is urged toward the cam to ensure substantially continuous contact between it and the bearing surface of the cam. The elastic member and follower element exert substantially zero net force on the cam in the x and y directions orthogonal to the first (z) axis.
According to the invention, a given rotation of the helical cam produces a corresponding and reproducible linear translation of the follower element and the driven element relative to the cam in the direction of the first (z) axis.
The pivot elements extend, respectively, in the direction of the first (z) axis and a second, orthogonal (y) axis and fix the follower element with respect to the driven element to prevent relative movement of the follower element and the driven element in the direction of the first (z) axis. In addition, the pivot elements permit translation of the follower element in the direction of the second (y) axis as well as in the direction of a third, mutually orthogonal (x) axis while substantially prohibiting rotation of the follower element about the x, y and z axes.
In a preferred embodiment, the helical cam is a threaded leadscrew, and the follower element contacts adjoining flanks of the leadscrew in two places on each flank. The pairs of contact locations are preferably spaced apart by less than 180 degrees, and are more preferably spaced apart by approximately 90 degrees.
The assembly may further include a mechanism for continuously lubricating the bearing surfaces of the helical cam during operation of the positioner assembly.
In one preferred embodiment, the pivot elements comprise flexures which may be, for example, a pair of wires extending in the direction of the first (z) axis and a pair of wires extending in the direction of the second (y) axis. Alternatively, they may comprise a four-legged object extending in the direction of the second (y) axis, and a pair of wires extending in the direction of the first (z) axis. In still other embodiments, the pivot elements may comprise linkages with ball and socket joints, or trunnions, or other relatively heavy duty pivot structures.
According to another aspect of the invention, there is provided an x-ray scanning system of the type including a gantry including a disk for supporting at least an x-ray source, and a frame for rotatably supporting the disk for rotation about a rotation axis, an x-ray detector assembly including a plurality of x-ray detectors cooperative with said x-ray source, and a data acquisition system for processing signals received from said detectors. The x-ray scanning system further comprises a determinate positioner assembly as described above to control the positioning of the x-ray beam.
In accordance with another aspect of the present invention, a determinate positioner assembly is provided for reliably moving, and precisely and reproducibly positioning, a driven device constrained to move in a longitudinal direction. The positioner assembly comprises:
a helical cam extending substantially along a longitudinal axis; and
a follower coupling the driven device to the helical cam so that the follower moves the driven device in the direction of the longitudinal axis in response to movement of the helical cam, while allowing the follower and helical cam to move together relative to the driven device in directions orthogonal to the longitudinal axis for at least a limited range of motion.
In one preferred embodiment the follower contacts the cam at a predetermined number of points so as to properly constrain the follower wit
Bowers, Jr. James Albert
Dobbs John McGregor
McDermott Brian Michael
Analogic Corporation
Bruce David V.
Hobden Pamela R.
McDermott & Will & Emery
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