1. Field of the Invention
In assembling micro-optic structures and in particular those utilizing fiber optic waveguides, it is often necessary to control relative spatial alignments with extremely fine precision. Some optical components use optical fibers having a core region that carries a beam of light that is of the order of 10 microns in diameter (and even smaller in some cases). In order to assemble such a component the optical fibers have to be manipulated with a precision level on the order of {fraction (1/10)}th of a micron.
2. Prior Art
In the past, optical fiber components have been assembled using known 3-axis ball-bearing positioners. Anyone who has attempted to achieve stable alignment using single-mode optical fiber with such known kinds of positioners equipped with micrometer actuators will attest to the lack of required precision. Crossed roller and ball type stages inherently require preload which generates motional friction and have a resolution limit set by the randomness of the required drive force due to dust and surface variations associated with the frictional interfaces and the limited stiffness of actuator mechanisms.
Other designs of positioners offer frictionless movement with the use of flexure-based designs but often at the expense of overall mechanical stiffness. A single parallel cantilever pair will generate an arc-error in its trajectory. What is commonly done is to combine two cantilever pairs into a compound cantilever stage so as to have one compensate the other and provide perfect linear motion. Compound cantilever stages are very large for their available travel as conventional designs consist of two separate compound stages that are effectively joined at a centerline to maintain high off-axis stiffness. An example of such a positioner mechanism is shown in U.S. Pat. No. 4,686,440, issued Aug. 11, 1987 to Hatamura et al., in FIGS. 12 and 13. One of the objects of this invention is to provide a compound cantilever stage that is much smaller than the conventional design.
The majority of 3-axis positioning equipment is made up by staging single axis units one on top of each other using angle plates. This results in a structure that has diminished resolution and stiffness as one moves progressively further from the mounting frame of reference. In many designs the stiffness of the overall unit is inadequate to resist the forces required to operate the actuators. In these cases the operator must use a touch and release method where the signal is adjusted and the operator then has to release the actuator to witness the result. Another downfall of a 3-axis positioner made up of three individual orthogonally arranged stages is the fact that the actuators are also arranged in an XYZ configuration, i.e. each has an axis perpendicular to the other two actuators, making prolonged use strenuous due to the required hand movements.
An inherent limitation to the resolution of nearly all positioning systems lies in the use of what can be termed simple axial actuators. A micrometer head or a complex piezo micrometer head are examples of simple actuators as they produce a displacement that is used to control the movement of a translation stage in a direct ratio. For example, a 1 micron movement of the shaft of said actuator is used to produce a 1 micron movement of the corresponding translation stage. As a result all motional errors such as hysteresis or randomness of movement inherent in the actuator itself are passed on directly to the translation stage. The requirement for sub-micron resolution also necessitates the requirement for differential micrometer stages and stepper-motor driven lead screw drives in order to achieve the necessary resolution since a single thread micrometer under hand control cannot be easily adjusted at such fine a resolution. The general trend towards increased resolution and stability in positioning equipment has been driven by the increased use of small-core single-mode optical fibers over the larger core multimode optical fibers which require less precision in alignment manipulation. In order to remove the effect of operator induced forces a number of sub-micron resolution remote driven motor driven stages have emerged on the market. Even with fully automated positioning systems where a scan routine is carried out under computer control, some level of operator intervention is required for handling and loading the individual elements to be assembled. In many labor intensive assembly applications the cost of an automated system cannot be justified and would not be considered if an appropriate mechanical positioner were available.
Copending U.S. patent application No.671,143 as aforesaid, which is hereby incorporated by reference, describes and claims a 3-axis positioning device ideally suited for, but not limited to, the assembly of single-mode fiber optic components. The invention of this earlier application allows for all of the actuators to be placed in a common orientation to reduce hand fatigue and improve adjustment efficiency. That invention teaches a structure that provides for both coarse and fine movement while using a simple adjustment screw and a single micrometer head for each of at least two axes of movement. The structure also allows for the fine movement control to be a fraction of the travel of the micrometer head while increasing movement resolution. Another aspect of the invention of this earlier application includes means of translating motion from one axis of movement to another. In addition that invention teaches a linear compound flexure stage that provides for large travel and perfect linear motion with high stiffniess. In its ideal form that invention can provide for operator insensitive adjustment when aligning single-mode optical fibers with a resolution limit that is comparable to a closed-loop piezo driven translation stage.
The invention of the aforesaid copending application is a significant improvement over a conventional stacked 3-axis unit as the operator adjustment forces act only on a single stiff linear translation stage instead of the sum of the total of all stages. It is then possible to realize a positioning device that can operate under hand control at resolution and stiffness levels required for single-mode fiber optic alignments wherein the operator does not influence the measured optical signal level during adjustment of the unit. It is also possible with that invention to implement linear motor drive on the second and third axes without affecting the overall sensitivity to hand adjustment forces.
Another aspect of the aforesaid copending application is an orthogonal drive conversion system which serves to isolate the holding means of a positioner from manual forces applied to it. Another advantage of this orthogonal drive conversion is that it allows micrometer type actuators to be all aligned in one direction, thus decreasing operator strain.
Yet another aspect of the positioner of the aforesaid copending application is a compound cantilever stage that is one-half of the conventional design; the latter being a design which uses two separate compound stages that are effectively joined at a centerline to maintain high off-axis stiffness. Traditionally the one half arrangement is not used as the intermediate frame of reference would move in response to external loads placed on the system and limit off-axis stiffness. It can be shown however, that if the intermediate frame in a compound cantilever stage were to be forced to move one-half of the overall displacement, then high stiffness can be achieved while requiring only one half of the conventional compound cantilever design. An aspect of this earlier invention was to provide a forcing or control means to set the displacement of the intermediate frame of reference of a compound cantilever stage to one half of the output displacement. In its preferred form said control means is a beam connected to the parts by frictionless elastic elements.
In the copending application the compound cantilever stage was claimed in relation to rectilinear movement; since that application was concerned with a 3-axis positioner, only rectilinear motion was required.
However, the present invention is concerned with a positioner having a mechanism capable of angular movements, and which mechanism can be combined with the mechanism of the copending application to make a positioner which moves an object such as an optical fiber with four, five or six degrees of freedom. The preferred form of the positioner has mechanism similar to that of the copending application for providing rectilinear motion in X, Y and Z axes; and in addition provides structure mounted on the movable support of the earlier positioner and which provides angular movement about one or more of the same three axes. Where angular motion is about two or three axes, these are preferably coincident. These angular adjustments are achieved, as are the rectilinear movements of the earlier stages, by use of flexures which eliminate friction and play or looseness of joints. It has been found that a compound cantilever stage somewhat similar in principle to that used for rectilinear movement in the three stage positioner can also be used to connect stages which move angularly relative to each other.
In accordance with this aspect of the invention, a positioning device includes:
a first support;
a movable support restrained for movement relative to said first support with one degree of freedom and carrying holding means for an object to be positioned;
an intermediate support which is connected to said first support by a pair of first spaced flexible elements of substantially equal length and which is connected to the movable support by a pair of second spaced flexible elements also of substantially equal length, said first and second flexible elements being oppositely arranged and being dimensioned so that arc errors in the movement of the intermediate support caused by bending of said first flexible elements are substantially compensated by arc movements of said second flexible elements to produce a desired movement of the movable support,
an actuator connected to cause movement of the movable support relative to the first support with bending of said flexible elements; and
control means connecting said first support, said intermediate support and the movable support so that the degree of movement of the intermediate support is one half that of the movable support.
The reference to the movable support xe2x80x9ccarrying holding meansxe2x80x9d is not limited to the case where holding means are directly mounted on the movable support; it includes an indirect mounting on the movable support via further adjustment stages.
Although in the preferred embodiments the flexible elements of each first or second pair are equal in length to each other, reasonable accuracy is possible with differences in these lengths of 5 or 10%.
Where, as in the copending application, the desired movement of the movable support is rectilinear, then all of the first and second flexible elements are of the same length, and preferably of the same stiffness. Slight differences in stiffness are however not of concern, due to the presence of the control means.
Where the desired movement of the movable support is angular motion about a fixed axis, then the first and second flexible elements may be radial elements all having the same radial dimensions, i.e. having the same inner and outer radii, and again preferably having the same stiffness. However, in the case of angular motion, other arrangements are possible, and for example the second flexible elements may be displaced radially outwardly relative to the first such elements, with the second elements also having greater radial length than the first elements.
As in the copending application, the control means may be a rigid member connected by further flexible elements to the first support, the intermediate support, and the movable support. Also, as before, in the case of rectilinear movement of the movable support and intermediate support, the control means is preferably a rigid member connected to the intermediate support at a position midway between its connections to the first support and the movable support, so as to ensure that the movement of the intermediate support relative to said first support is one-half that of the movable support.
As indicated, the movable support may have angular motion about an axis, in which case the xe2x80x9cdegree of movementxe2x80x9d of the intermediate support will be one-half the angle of movement of the movable support. In this case the flexible elements extend radially with respect to said axis when the movable support is in one position within its range of movement, and the control means is a rigid member connected to the intermediate support at a position chosen such that the angular movement of the intermediate support relative to the first support is the desired one half that of the movable support. The rigid member is preferably connected to the first support, the intermediate support, and the movable support by flexible elements connected to these parts at points which lie on a line extending radially with respect to the said stationary axis.
The aforesaid copending U.S. application described, with reference to FIGS. 5 to 11, control mechanisms for controlling movements in the X and Y directions by means of actuators mounted on a mounting plate held by the first, or Z axis, rectilinear stage and which were both parallel to the Z direction actuator. These mechanisms, which are also used in the preferred embodiment of the present positioner for the X and Y stages, are convenient for the operator and prevent hand forces undesirably affecting the X and Y adjustments. In a 6 axis positioner in accordance with of the present invention it is desirable that the three angular position actuators also be mounted on the first or Z axis stage and be parallel to the other actuators. However this requires somewhat complex linkage to ensure that there is little or no cross-coupling between the either the X and Y translational movements and the angular adjustments, nor between the first or second stage angular adjustments and the later stage angular adjustments. In other words it is required that later angular adjustment stages can be controlled by mechanisms isolated from movements in the earlier stages.
To take a simple case, it is required that where a positioning device has a first support and a second support connected to the first support by means allowing lateral movement of the second support relative to the first support in the X axis direction, for example, and where a third support is mounted for angular movement on the second support about a pivot axis, the actuating means for causing angular movement of the third support relative to the second support should operate while being substantially unaffected by movement of the second support at least in the X direction.
In accordance with this further feature of the present invention, the actuating means for this situation includes an axially movable control link extending in a direction generally perpendicular to the X axis and having an inner end arranged to be moved axially relative to the first support by an actuator mounted on the first support, the control link being flexibly connected at its inner and outer ends respectively to the first and second supports, the control link having its outer end connected to a control mechanism mounted on the second support, and in which the control mechanism includes:
a) restraining means for causing the outer end of the control link to move laterally with the second support in the direction of the X axis while allowing said outer end to move axially relative to said second support, and
b) motion conversion means for converting axial movement of the control link outer end relative to said second support into movement of a moment arm part of the third support so as to cause angular movement of said third support relative to the second support about the pivot axis.
The control mechanism is such that if lateral movement of the second support relative to the first support occurs in the X axis direction without axial movement of the inner end of the control link relative to the first support, the moment arm part remains substantially stationary relative to the second support.
The reference above to the X direction is of course arbitrary; the same considerations apply if motion of a second support relative to a first support occurs in the Y direction.
Where this mechanism is applied to the first angularly movable stage which is mounted on the third or last stage of the linear 3 axis positioner, the control mechanism has to take into account that the first and second supports as defined in the preceding paragraphs, which here would be the first and third stages of the 3 axis positioner, are connected primarily by flexible elements providing parallel linkages so that when the second support moves in either the X or Y direction, movements of points on the second support relative to the first support occur in arcs having equal radii determined by the effective lengths of the flexible elements. In this case the control link preferably has an effective length equal to the effective lengths of these flexible elements, which are themselves of equal length for the X and Y movements, so that the control link swings in unison with the flexible elements and so that movements of the second support in the X or Y directions do not change the relationship between the outer end of the control link and the second support.
The motion conversion means may include a bell crank having an inner end connected to the outer end of the control link, with an outer end of the bell crank being arranged to transmit force to said moment arm part. Pivotal movement of the bell crank is isolated from movement of the second support relative to the first support in the X direction, or in the Y direction if the latter movement is present.
Where a bell crank is used, this may be pivotally mounted on a fulcrum fixed to the second support.
Alternatively, the bell crank may be indirectly mounted on the second support, and may have a first arm connected both to the outer end of the control link and to a second link also connected to the first support, with the control and second links forming a first parallelogram linkage, a second arm of the bell crank being connected to the second support by third and fourth links forming a second parallelogram linkage operating perpendicularly to the first. With this arrangement, the rotational position of the bell crank relative to the second support is isolated from movements of said second support relative to the first support both in the X direction and in directions off-set or perpendicular to the X direction. This is particularly useful for the second or yaw stage of angular movement, which is mounted on the roll stage, since the roll stage moves not only with the swinging movements provided by the flexible elements connecting the Y and X movement stages, but also has circumferential movement about the axis of the roll stage which is slightly off-set from the swinging motion which characterizes the usual X direction.
The manner in which movement is transmitted from the actuators to the inner end of the control link may be similar to the mechanism used in the three axis positioner of the aforesaid copending application, in that the inner end of the control link may be held by a lever member located at one end by fulcrum means and at its other end by the respective actuator. Also, the fulcrum means may include an adjustment screw arranged to provide movement of the lever independent of the movement of the actuator.
A further feature of this invention relates to magnetic means used to reduce the forces needed to move the positioner parts against the restraining forces of the various flexible elements.
In positioning devices using flexible elements in place of bearings or sliding joints, there are conventionally trade-offs between having sufficient ease of movement, on the one hand, and having, on the other hand, sufficient rigidity to ensure accuracy of movement. Another aspect of the present invention is the provision of magnetic means to counteract stiffniess of flexible elements without reducing accuracy of movement.
In accordance with this aspect of the invention, in a positioning device including:
a first support;
a second support movably mounted on said first support and constrained to move relative thereto with one degree of freedom, said second support carrying holding means for an object to be positioned;
and at least one pair of resilient, flexible elements connecting said first and second supports and capable of flexing in response to movements of the second support relative to the first support to provide a restoring force when displaced from an initial position at which the restoring force is a minimum,
there are provided magnetic means acting between said supports, said magnetic means being positioned to counteract said restoring force when the second support is moved from said initial position, whereby the force required to move the second support against said restoring force is reduced by magnetic forces produced by said magnetic means.
The magnetic means may only partially counteract the restoring spring forces of the flexure elements, so that the second support would still tend to move to a stable central position in the absence of any actuator, as it would without the magnetic means. While the magnets may counteract all of the spring forces, the arrangements of this invention differ from known bistable mechanisms using magnets in which the magnets overcome the restoring forces and cause the mechanism to move between opposed end positions, in that here the magnetic surfaces do not contact each other and do not normally apply very strong forces which would hold the parts in extreme positions.
The magnetic means may include magnets having axes aligned obliquely to a direction of relative movement between the one support and the second support, these magnets being arranged so that the amount of relative movement between said latter parts may be greater than the maximum gap between said magnets.
Where the magnets are aligned obliquely in this manner, they may be provided symmetrically about an axis which is parallel to the direction of movement, so that the net forces provided by the magnets are solely in the direction of said movement. Additionally, the magnets may also be arranged symmetrically about an axis which is perpendicular to the direction of movement, the magnets being all aligned with sides of a rhombus.
Alternatively, the magnetic means may include magnets located respectively on said one support and on the second support and positioned so that like poles of said magnets lie directly opposite to each other when the movable support is in its initial position.
The magnetic means may include two or more pairs of magnets. Where two pairs of magnets are used, one pair may have opposed north poles and one pair may have opposed south poles, the two pairs being close enough that when the second support has been displaced from its initial position the north pole of one magnet on the second support may be attracted to the south pole of a magnet on the first support so that there are attractive as well as repulsive forces assisting in counteracting the forces in the flexible elements. Where three pairs of magnets are used, one magnet of each pair being on the first support and the other magnet of each pair being on the second support, the pairs of magnets may include a central pair having opposed poles of a first polarity and which lie directly opposite each other when the second support is in its initial position, and two outer pairs of magnets having opposed poles of a second polarity which also lie directly opposite each other when the second support is in said initial position, said magnets being positioned so that a magnet of the central pair is attracted to a magnet of an outer pair on movement of the second support from its initial position.