WO2003048831A2

WO2003048831A2 - Mounting system particularly for lenses

Info

Publication number: WO2003048831A2
Application number: PCT/GB2002/005323
Authority: WO
Inventors: James Allan
Original assignee: 1... Limited
Priority date: 2001-11-29
Filing date: 2002-11-26
Publication date: 2003-06-12
Also published as: GB2398854A; WO2003048831A3; AU2002343097A1; GB0413692D0; GB0128591D0; GB2398854B; AU2002343097A8

Abstract

A mounting system is described having a frame structure and an elastic structure adapted to mount a suspended object with the elastic structure having a critical configuration point close to which its stiffness in a desired direction approaches zero. The mounting system is particular useful for suspending small objects, such as camera lenses actuated by a piezoelectric actuator.

Description

MOUNTING SYSTEM

FIELD OF THE INVENTION This invention pertains to mounting or suspension systems. In particular, it relates to mounting systems for use in combination with electro-active actuators systems. More specifically, it pertains to a mounting system for a camera system for mobile digital data processing and transmitting devices.

BACKGROUND OF THE INVENTION Mounting or suspension system for actuator-driven devices usually provide a rigid support structure and moving parts. The support structure constrains the motion of the moving parts in some manner to obtain the required function.

While different designs for mounting and suspension systems are found in abundance, it is usually desirable to reduce the force necessary to drive the moving part along the required path. Suspension systems or bearings often take the form of components that are translational or rotational moveable relatively to each other. They have a low stiffness in the direction of the desired motion, but are very stiff in other directions, thus preventing significant movement in these other directions. Examples of suspension systems or bearings are linear or rotary plain bearings, e.g., bushes, and linear or rotary ball or roller bearings.

Flexures operate by elastic deformation and have no static friction threshold. They can, when correctly designed, constrain the motion of an object by exhibiting a low stiffness in the required direction and higher stiffness in other directions.

In the field of vibration damping, it is known to employ damping systems with low stiffness to provide sufficient damping even at very low frequencies. Such vibration isolators are for example described in the United States Patents No. 4,778,037 and 5,370,352. In the known arrangements damping isolator are described that rely on a principle of loading a particular elastic structure which forms the isolator or a portion of it (the loading being applied by either the weight of the object, the combined weight of the object and another isolator, or by an external loading mechanism) to approach the elastic structure's point of elastic instability. This point of elastic instability, also called the "critical buckling load" of the structure causes a substantial reduction of either the axial or transverse stiffness of the isolator to create zero or near zero stiffness in the axial and any transverse direction. The isolators still retain sufficient axial stiffness to support the payload weight; however, there will be little or no stiffness in both the axial or transverse directions. As a result, the isolators suppress the transmission of vibratory motion between an object and a base. The magnitude of the stiffness of each particular isolator depends upon how closely the point of instability is approached. The stiffness, also referred to as spring rate, of an elastic structure is usually defined as ratio of the applied force or load over the resulting deflection.

If the load on the isolator's elastic structure is greater than the critical buckling load, the excessive load will tend to propel the structure towards its buckled shape, creating a "negative-stiffness" or "negative-spring-rate" mechanism. By combining a negative-stiffness mechanism with a positive spring, adjusted so that the positive stiffness of the isolator cancels or nearly cancels the negative stiffness, the resulting device can be placed at or near its point of elastic instability. The magnitude of the load causing the negative stiffness can be adjusted, creating an isolator that can be "fine tuned" to the particular stiffness desired. On the other hand, electro-active actuators are well known in the art. Electro- active materials can be defined as materials that deform or change their dimensions in response to applied electrical conditions or, vice versa, have electrical properties that change in response to applied mechanical forces. The best known and most used type of electro-active material is piezoelectric material, but other types of electro- active material include electrostrictive and piezoresistive material.

Many devices that make use of electro-active materials are known. The simplest piezoelectric device is a block of pre-poled, i.e., pre-oriented, piezoelectric material activated in an expansion-contraction mode by applying an activation voltage in direction of the poling. To increase the displacements, other electro-active actuator designs have been introduced such as stacks, unimorph or bimorph benders, recurved benders, corrugated benders, spiral or helical designs.

Comparably large translation displacements have been recently achieved by using a helical structure of coiled piezoelectric tape, also referred to as super-helix.

Such twice-coiled devices are found to easily exhibit displacement in the order of millimetres on an active length of the order of centimetres. Further details of such devices are described in the commonly-owned international patent application WO-

01/47041 and by D. H. Pearce et al. in: Sensors and Actuators A 100 (2002), 281-

286.

Benders, stacks, tubes and other electro-active actuators are employed in a wide array of engineering systems, ranging from micro-positioning applications and acoustic wave processing to printing applications. Generally, actuators are used in such applications to generate force and effect displacement, for example, to move levers or other force transmitting devices, pistons or diaphragms, to accurately position components such as small camera lenses, or to enable similar system functions. Actuators employed for such functions typically are designed to provide a desired actuation displacement or stroke over which a desired force is delivered to a given load.

Depending upon design, electro-active actuators can generate a rotational or translational displacement or combinations of both movements. It would be desirable to provide a mounting system for devices to be moved by small actuators. In particular, it is desirable to have suspensions systems for moving parts with low stiffness in direction of the intended motion while narrowly constraining motions in all other directions.

SUMMARY OF THE INVENTION In view of the above objects, the present invention provides apparatus as claimed in the independent claims.

In one aspect, this invention provides a mounting system with a frame structure and an elastic structure. The elastic structure can be made of a variety of materials, for example metallic or plastic material. Steel or beryllium copper or alloys thereof are particularly preferred material for manufacture of the elastic structures of this invention.

The elastic structure is adapted to hold a device to be moved, often by an actuator. Preferably the frame structure forces or preloads the elastic structure close to a critical point where its stiffness approaches zero in the desired direction of the motion of the mounted object, while constraining motion in other directions. In other words the stiffness in the desired direction is smaller than the stiffness in other directions so that the predominate motion allowed is in said desired direction. The device is to be directly or indirectly coupled to an actuator. As the mounting system is designed to be movable with a very low driving force, an actuator may have low energy consumption so as to be capable of being energized by an autonomous mobile energy supply unit. A particularly preferred type of actuator to be used in combination with the mounting system of the invention comprises electro-active material.

In a preferred embodiment, the elastic structure or flexures of this invention take up the form of a double curve with two portions of opposite curvature so that it is reminiscent of a sine wave. Although in general the curve may not be an exact sine curve, it is referred to herein as a sine spring or sine flexure. A sine spring is a variant of a leaf spring, i.e., essentially an elongate member of material supported at both ends and mounted to pre-stress the strip. The leaf, when mounted, forms a wave or sine shape. With the appropriate selection of the material, its parameters and dimensions, and the manner it is mounted onto the frame structure, the sine spring can be preformed and/or preloaded so as to maintain a configuration close to its "buckling" point but still within a domain of positive stiffness. Preferably the sine spring is mounted such that a moving part or device is suspended with a small positive stiffness in the direction of motion. The flexures can be brought closer to the critical point in cases where the actuatur itself provides the necessary small positive stiffness. Although this form of elastic structure is preferred, other elastic structures providing the same properties are possible and within the scope of the invention. Generally, it is sufficient for the working of the present invention that the stiffness of the combined system comprising elastic structure and actuator has at least a small positive stiffness. Hence it may be possible to design the elastic structure with a negative stiffness as long as the actuator stiffness compensates for it.

Although motion can be constrained to be only substantial in the desired direction by use of, for example, linear slides or rolling bearings, these introduce both static and dynamic friction which resist the motion imparted by the actuator. The use of the elastic structure or flexures as a bearing eliminates these friction related problems and "stick-slip", thereby allowing a smoother control of the motion, although the elastic structure introduces additional spring stiffness and hence elastic energy storage into the mounting system. In addition to offering suspension and low stiffness in one direction, the elastic structure of the invention is preferably formed or adapted to constrain motion or, in other words, provides high stiffness in all other directions.

In another preferred embodiment, the elastic structure comprises a plurality of sine springs disposed in an array around the object. More preferably, the sine springs are arranged in parallel oriented pairs with the object mounted within the space between the two springs. The object to be mounted is thus retained within a framework of sine springs. In a particularly preferred variant of this embodiment, the elastic structure comprises a stack of two pairs of parallel oriented sine springs with the spacing between the pairs and between each spring of a pair selected so as to form a central space or volume large enough to accommodate at least a main part of the object to be mounted.

The invention provides a mounting system useful for mounting small objects such as lens systems for microcameras. One particularly small object is the lens of a mobile telephone camera or video camera, the purpose of the mounting being to allow the lens to move back and forth along a line to and from the image plane of a image processing system, so enabling the camera to focus, while at the same time preventing the lens moving laterally or in rotation.

These and other aspects of inventions will be apparent from the following detailed description of non-limitative examples making reference to the following drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1A is a cross-sectional view of a schematic mounting system in accordance with the invention; FIG. IB shows the sine spring of FIG. 1 A additionally illustrating a configuration with its centre point displaced from a default position.

FIG. 2 is a perspective view illustrating another example of a sine spring; FIG. 3 A-C are different views depicting a mounting system in accordance with the present invention used in combination with an electro-active actuator and a micro camera as object mounted.

DETAILED DESCRIPTION FIG. 1A shows in elevation elements of an embodiment 10 of a mounting system. The mounting system includes an elastic structure 11 and a frame structure 12. The elastic structure is a sine spring 110. Linked to the elastic structure at about mid point 111 is a suspended part 13 that in operation is moved by an actuator 14. Arrow 15 indicates the direction in which the mid or centre point of the moving object travels.

In FIG. IB the elastic structure 11 of FIG. 1 A is shown in isolation to illustrate further aspects of its application. The sine spring 110 is formed of an elongated member of elastic material. In the example illustrated, the spring is made of a flat thin strip of stainless steel. The width of the strip exceeds its thickness by at least one order of magnitude, thus giving the suspension system improved resistance against lateral motion, i.e. motion perpendicular to the paper plane.

When bringing the ends 112, 113 of the strip from an original position in which the strip is fully stretched or extended closer together, the strip assumes one of essentially three possible configurations. The first of these configurations is the sine spring 110. In this configuration the strip bends to form two arcs 114, 115 of approximately equal length, one lying below and the other above a centre line (not shown) connecting the two end points 112, 113 in a wave-like shape. (It should be noted that a sine spring does not necessarily assumes the mathematically exact shape of the sine function. To the contrary, it is more likely to deviate from that shape.)

In an unloaded state, the midpoint 111 of the strip remains close to the centre line. The other two configurations are referred to as "buckling" state(s). The buckling state is characterized in that after passing through a critical point the spring is suddenly flexes with one arc growing much larger at the expense of the other. The buckling of the spring forces the midpoint 111 far off the centre line. The two buckling states differ mainly in that the larger arc may be either located above (as indicated by the dashed line 116) or below the centre line (not shown).

Before describing the parameters that can potentially used to control the sine spring, it should be noted that sine spring 110 responds with various degrees of stiffness to movements of the midpoint 111. Given the shape of the sine spring, the least resistance is met in the vertical direction (upwards and downwards as indicted by arrow 15), whereas movements in horizontal directions, i.e., in the paper plane or perpendicular to it, are significantly more constrained by the sine spring. In the present embodiment, it is desirable to render the sine spring 110 as weak as possible with respect to vertical motions. While an analytical mathematical model of the system is not known, it had been found by numerical analysis and experiments that certain parameters could be altered to tune the stiffness of the spring in a vertical direction. For any given material, these parameters include the thickness and width of the spring, the ratio of arc length to wavelength (or the distance between the end points) and the mounting angle. The mounting angle is defined as the angle between a vertical line and a tangent to the arcs at their respective starting points. It can be set by mounting the end parts of the springs with the appropriate inclination with respect to the frame or, where these end parts are preferred to mounted flat, by plastically deforming the spring close to these flat end parts. The former method is illustrated in FIG. IB with α and β denoting left and right mounting angle, respectively. The latter method is illustrated when referring to the sine spring of FIG. 2 below. To increase the stability, it is advantageous to maintain a shallow configuration of the sine spring, i.e., a configuration with a relatively small ratio of arc length over wavelength. In FIG. 2, there is shown a sine spring 20 consisting of two parallel arranged narrow sine springs 21, 22 linked by joint elements 23. The purpose of the elements 23 is to maintain the springs in parallel. As is apparent from FIG. 2, the parallel arrangement of sine spring can equally be described as one tape- or sheet-like sine spring having parts of its centre section removed. With respect to the method of setting an mounting angle as described above, the joint elements 23 and the end parts of the sine springs are plastically deformed along the dashed lines 24, 25 such that the springs automatically flexes into the desired shape when the joint elements and end parts are flat (horizontally) mounted in a frame structure (not shown). The sine spring is shown as if appropriately mounted.

The two sine springs 21, 22 of FIG. 2 support a central mounting ring 26 onto which movable parts can be mounted. Two link elements 27, 28 join the ring to the sine springs 21, 22 close to their respective mid points. In the example, the link elements 27, 28 are an integral part of the strip forming the spring, but a part which projects laterally from the (mid point of the) strip like a short stub.

The spring system of FIG. 2 has an overall length and width of less than 20 mm, here approximately 11 mm. Its thickness is less than 100 micron and in the present example close to 25 microns. It is made of hard rolled stainless steel (grade 302). Springs this small and this thin can be made in a number of ways, but most conveniently by cutting them out from a sheet of material. Thus, they can be stamped out using a die, or cut out using a singular cutting tool such as a laser, or - and most preferably - etched out photochemically.

Photochemical etching is particularly suited to making large numbers of devices all at once from a single large sheet of material; with one single printing the resist pattern defines a multiplicity of springs spaced across the surface of the sheet, and the etching realises them all at the same time.

While the mounting ring 26, itself, can easily be replaced by any other mounting structure suitably adapted to the dimensions of the object to be mounted, it is noteworthy that the link elements 27, 28 of this example are etched from the same sheet of thin metal as the sine springs 21, 22 and hence initially project into a plane defined by the remainder of the (flat) sine spring. However to prevent a tilting of the ring when the sine spring assumes its wave shape, the link elements 27, 28 are permanently twisted or bend with respect to the sine springs so as to compensate for the inclination of the sine springs at their mid point. The ring 26 is thus held approximately horizontally thereby facilitating the mounting and alignment of the moving object.

Whereas the previous figures illustrated embodiments of the invention in a schematic way, the following FIGs. 3 A to 3C show an example of the mounting system in accordance with the invention applied to suspend and move the lens barrel of a microcamera. Such microcameras are found in many mobile devices such as portable computers, personal digital assistance, mobile telephones etc., as well as camera devices such as digital still cameras and video cameras.

FIGs. 3 A and 3B show a perspective top and side view, respectively, on the mounting system with all of the housing and frame structure cut away. The mounting system uses two sets 31, 32 of the sine springs as detailed in

FIG.2. Between the two sets there is positioned a cylindrical camera lens housing 33. At its bottom end 331, the camera has a chamfered section of reduced outer diameter to match the inner diameter of mounting ring 326. At its top end, the camera has a similar section (not visible) capped by a flange 332. The flange 332 firmly secures the camera housing between the two mounting rings 316, 326. The rings and the stubs 317, 327 are twisted slightly by plastical deformation to ensure that they remain horizontally aligned and are not tilted by the same angle of inclination as the mid sections of the sine springs.

The volume between the two sine spring sets 31, 32 is sufficiently large to provide space for a piezoelectric actuator 34. The actuator is of the coiled helical type described in more detail in the commonly-owned international patent application WO-01/47041 which is incorporated herein by refernce. The major turn bends to approximately three quarters of a full circle. Its outer diameter and inner diameter are 10.6 mm and 7.4 mm, respectively. The minor helix has an outside diameter of 1.6 mm and an inside diameter of 0.96 mm. The actuator weighs 0.13 g. The actuator moves the camera in a vertical direction as indicated by the arrow 35 by +/- 75 micron.

Optionally, the actuator 34 could be replaced by any actuator disclosed in

WO-01/47041 which is incorporated herein by reference. A battery unit (not shown) supplies the actuator 34 with electrical power.

However, whereas the piezoelectric actuator offers the advantage of low power consumption and a large central opening and is therefore preferred for compact designs, it will be appreciated that the invention is not dependent on any specific actuator type to drive the suspended object. One end 341 of the actuator is fixed to a protrusion 333 of the camera housing

33. The other end is supported by the outer housing 36 (shown in FIG. 3C) of the mounting system.

In FIG. 3C the mounting system and the actuator are shown in a housing unit

36 (with its top cover with the aperture removed). The housing unit provides a frame structure to which the two sets of sine springs 31, 32 and one end of the actuator 34 are fixed. The housing 36 is made of acetal. It has a length and width of 12 mm and a height of 5.8 mm high. Its weight is 0.7 g.

Claims

CLALMS

1. A mounting system comprising a frame structure and an elastic structure adapted to mount a suspended object, wherein the elastic structure has a critical configuration point close to which its stiffness in a desired direction approaches zero.

2. The mounting system of claim 1 wherein the elastic structure has when mounted on the frame structure and loaded with the object a small positive stiffness in a desired direction of motion.

3. The mounting system of claim 1 or 2 wherein the elastic structure is adapted to constrain motion in directions others than the desired direction.

4. The mounting system of claim 3 wherein the elastic structure has high stiffness in directions other than the desired direction.

5. The mounting system of any one of the preceding claims wherein the elastic structure is a sine spring forced to bend elastically into a double wave shape by the frame structure.

6. The mounting system of any one of claims 1 to 4 wherein the elastic structure is a sine spring with link elements close to its mid point.

7. The mounting system of claim 5 or 6 wherein the elastic structure comprises a pair of sine spring arranged in parallel.

8. The mounting system of claim 5 or 6 wherein the elastic structure comprises two or more stacked pairs of sine springs.

9. The mounting system of claim 5 or 6 wherein the elastic structure comprises two stacked pairs of sine springs with the object suspended in a volume between said pairs.

10. The mounting system of any one of the preceding claims wherein the suspended object is a lens system.

11. The mounting system of any one of the preceding claims further comprising an actuator to drive the suspended object.

12. The mounting system of claim 11 wherein the actuator being comprises electro-active material.

13. The mounting system of claim 12 wherein the actuator comprises a curved structure of electro-active material.

14. The mounting system of claim 13 wherein the actuator is a twice-coiled or super-helical bender.

15. The mounting system of any one of claims 11 to 14 wherein the actuator is located between two or more stacked pairs of sine springs.

16. The mounting system of any one of claims 1 lto 14 wherein the elastic structure combined with the actuator has a small positive stiffness in a desired direction of motion.

17. A miniature camera system comprising a lens system actuated by at least one electro-active actuator wherein the lens system is mounted in a mounting system in accordance with any one of the preceding claims.

18. A miniature camera system comprising a lens system actuated by at least one electro-active actuator wherein the lens system is mounted in a mounting system comprising one or more parallel sine springs forced to bend elastically into a double wave shape by a frame structure.

19. The camera system of claim 18 wherein the mounting system comprises a stack of two or more pairs of parallel sine springs.