WO2003073597A1

WO2003073597A1 - Electrostatic micro actuator

Info

Publication number: WO2003073597A1
Application number: PCT/KR2002/001812
Authority: WO
Inventors: Kyu-Ho Hwang
Original assignee: M2N Inc.
Priority date: 2002-02-28
Filing date: 2002-09-26
Publication date: 2003-09-04
Also published as: AU2002334440A1; KR100439908B1; KR20030071297A

Abstract

An electrostatic micro actuator having a comb drive assembly, which is electrostatically movable in response to an applied voltage to thereby generate a drive force which can be coupled to a load, is enhanced by reconfiguring its spring assembly in such a manner that its spring stiffness behaves in a non-linear fashion, thus improving the precision of the control of the motion in a wide range of actuation voltages, which reduces limitations in operation.

Description

ELECTROSTATIC MICRO ACTUATOR

Technical Field

The present invention relates to an electrostatic micro actuator; and, more particularly, to a spring assembly thereof having a non-linear spring stiffness, and thus being capable of improving the precision of the control on its motion.

Background Art

Various types of micro electromechanical (MEM) devices employ electrostatic comb actuators as motive sources. Such devices use linear spring comb actuators. However, there exist many problems in such a linear electrostatic comb actuator in that the control of its motion within a given voltage range is not so precise due to a constant spring stiffness . There is shown a conventional electrostatic micro actuator 100 in Fig. 1. The micro actuator 100 formed on a substrate (not shown) includes a spring assembly 13, an actuating rod 9, and a comb drive assembly 10 having a fixed comb member 11 mounted on the substrate and a movable comb member 12 overlying the substrate. The fixed comb member 11 has a comb spine 7 and a plurality of fixed comb fingers 1 provided thereto. The movable comb member 12 has a comb spine 8 and a plurality of movable comb fingers 2 provided thereto . The plurality of movable comb fingers 2 interdigitate with the plurality of fixed comb fingers 1 to generate a drive force, which can be coupled to a load, and are moved toward the fixed comb spine 7, in response to a supplied voltage generating an attractive electrostatic force between the fixed and the movable fingers 1, 2.

The attractive electrostatic force F exerting on one movable comb finger 2 can be defined as:

F = ε₀-V² Eq. 1

8

wherein ε₀, h, g, and V are the permittivity in a vacuum; a height of the comb finger; a gap between two mating fingers; and an actuation voltage supplied to the fixed comb member 11 and the movable comb member 12.

The spring assembly 13, which is formed with, e.g., two beam springs 14b, 14f, is of a constant spring stiffness. The movable comb member 12 is connected to the spring assembly 13 by an actuating rod 9.

The attractive electrostatic force F due to a supplied actuation voltage is applied to move the movable comb member 12 toward the fixed comb spine 7 until a static equilibrium is reached between the attractive electrostatic force due to the supplied voltage and the spring force of the deflected beam springs 14b, 14f. Accordingly, as the actuation voltage increases, the displacement of the movable comb member 12 also increases. However, when the supplied voltage is removed, the movable comb member 12 is restored to a rest position.

The drawback of the conventional electrostatic micro actuator 100 is the lack of precise control over the motion of the movable comb member 12 in a given voltage range. As shown above in Eq. 1, the electrostatic force, which is proportional to the square of the actuation voltage, is exerted on the spring assembly 13 of constant spring stiffness. That is, if a small constant spring stiffness is chosen, the comb actuator may work appropriately at a low supplied voltage. However, at a high supplied voltage, the precise control over ^' the spring displacement is compromised due to the aforementioned proportionality between the attractive electrostatic force and the voltage, which yields large electrostatic force, thereby resulting in a correspondingly augmented spring displacement.

On the other hand, if a high constant spring stiffness is chosen, the comb actuator may work appropriately at a high voltage. However, at a low supplied voltage, the electrostatic force is mitigated. Thus, the spring will be unresponsive to such mitigated electrostatic force due to the low supplied voltage. Thus, with the large constant spring stiffness, the mitigated electrostatic force proves to be insufficient in providing precise control over a motion of the actuator in a lower range of the applied voltage.

In essence, with a conventional electrostatic micro actuator incorporating a spring assembly of constant spring stiffness, it fails to provide a precise control of the motion in a wide range of actuation voltages, limiting the operability of the actuator.

Disclosure of Invention

It is, therefore, an object of the present invention to provide an electrostatic micro actuator capable of having a precise control of the motion thereof in a wide range of actuation voltage.

In accordance with the present invention, there is provided a micro actuator including: a drive assembly disposed on a substrate, the drive assembly including a movable member and a fixed member; a spring assembly having a non-linear spring stiffness; and an elongated means for translating a motion of the movable drive member of the drive assembly to the spring assembly. The spring assembly includes two substantially identical spring groups disposed symmetrically with respect to the elongated means at two opposite lateral sides thereof, each spring group having more than one spring component, wherein the movable member has a predetermined maximum displacement, which is divided into a plural number of displacement segments, each displacement segment corresponding to a different spring stiffness determined by one of said one or more spring component .

Brief description of Drawings

The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:

Fig. 1 is a perspective view of a conventional electrostatic micro actuator;

Fig. 2 shows a top view of an electrostatic micro actuator in accordance with a first preferred embodiment of the present invention;

Fig. 3 describes a top view of an electrostatic micro actuator in accordance with a second preferred embodiment of the present invention;

Fig. 4 offers a top view of an electrostatic micro actuator in accordance with a third preferred embodiment of the present invention;

Fig. 5 sets forth a top view of an electrostatic micro actuator of the first preferred embodiment with springs deflected; Fig. 6 illustrates a top view of an electrostatic micro actuator of the second preferred embodiment with springs deflected; and

Fig. 7 presents a top view of an electrostatic micro actuator of the third preferred embodiment with springs deflected.

Best Mode for Carrying Out the Invention

The preferred embodiments of the present invention will now be described with reference to the accompanying drawings . Referring to Fig. 2, there is shown an electrostatic micro actuator 100a in accordance with a first preferred embodiment of the present invention. The micro actuator 100a disposed on a substrate 200 includes a comb actuation assembly 10 having a fixed comb member 11 and a movable comb member 12, a spring assembly 13 of a non-linear spring stiffness characteristic, and an actuating rod 16 having a joint member 18.

The configuration of the comb actuation assembly 10 is similar to the comb drive assembly 10 of the prior art shown in Fig. 1. The fixed comb member 11 having a fixed comb spine 112 and fixed comb fingers 111 extending from a side of the fixed comb spine 112 is rigidly attached to the substrate 200. The corresponding movable comb member 12 overlying the substrate 200 has a movable comb spine 122 and movable comb fingers 121 extending from a side of the movable comb spine 122. The comb spine 122 of the movable comb member 12 is connected to one end of the actuating rod 16, whereas the joint member 18 is connected to the other end portion thereof.

The actuating rod 16 bisects the spring assembly 13, which incorporates, e.g., two spring groups 14, 114. Each of the spring groups 14, 114 is joined to the actuating rod 16 at the joint member 18 and the other end of each of the spring groups 14, 114 is fixed at an anchor portion 17.

Further, each of the spring groups 14, 114, includes e.g., three sets of folded springs 14f, 14m, 14b, wherein each of the folded springs 14f, 14m, 14b is of an approximate U-shape, having two beam springs 14f-f/14f-b, 14m-f/14m-b, 14b-f/14b-b that are substantially parallel to each other and are interconnected at one of the ends by connecting beams 14f-c, 14m-c, 14b-c, respectively.

The folded springs 14f, 14m, 14b are connected in a series, by interconnecting the free ends of two neighboring beam springs 14f-b, 14m-f, and 14m-b, 14b-f, with connecting portions 6f, 6m, respectively. Further, the free end of the beam spring 14f-f is joined to the actuating rod 16 through the joint member 18 and the free end of the beam spring 14b- b is coupled to a supporting beam 19 via a connecting portion 6b, wherein the supporting beam 19 can vary in degree of stiffness depending on the application. Thus, one end of each of serially connected folded springs 14f, 14m, 14b in a zigzag shape is coupled to the actuating rod 16 and the other end thereof is fixed to the anchor portion 17 via the supporting beam 19. In this exemplary embodiment, a deflection of a pair of beam springs in each of the folded springs 14b, 14f, 14m is governed by the spring stiffness characteristic of a corresponding folded spring, wherein a spring stiffness characteristic of a folded spring is characterized by the stiffness of a corresponding pair of beam springs of the folded spring.

The deflection of each of the folded springs 14b, 14f, 14m propagates from a folded spring of lower spring stiffness to a folded spring of higher spring stiffness in sequence. Thus, it is after the folded spring of lower spring stiffness reaches its maximum deflection that the folded spring of higher spring stiffness starts to deflect. A small amount of deflection that the folded springs of high spring stiffness may experience prior to the maximum deflection in the folded spring of lower stiffness is considered negligible in the present embodiment.

Accordingly, due to the non-linear spring characteristic of the present embodiment, it is required that maximum deflection of the folded springs of lower spring stiffness must be reached first before the folded springs of higher spring stiffness starts to be deflected. Such condition gives rise to an overlapping of the free ends of the pair of beam springs in the folded spring, in which case sticking contact may occur between contacting surfaces in the folded spring, which in turn affects the responsiveness of the actuation device. In an effort to eliminate sticking contact between the contacting surfaces in a folded spring, a set 15 of protrusions 15b, 15f, 15m are employed, wherein each of the protrusions 15b, 15f, 15m can be placed on either surfaces in contact. Such enhancement transforms the surface contact to a line contact.

In this exemplary embodiment, each of the serially connected folded springs 14b, 14f, 14m has a spring stiffness determined by its corresponding beam springs. A deflection of a pair of beam springs will hereinafter be referred to as a deflection of a folded spring. Such being the case, the deflection that takes place in each of the folded springs 14b, 14f, 14m corresponds to its spring stiffness and is inclusive of that of a folded spring of lower spring stiffness.

Thus in this exemplary embodiment, in order to embody a non-linear spring stiffness characteristic of the spring assembly 13, each of the folded springs 14b, 14f, 14m must be deflected at a different applied actuation voltage, which necessitates each of the folded springs 14b, 14f, 14m to have a distinct spring stiffness. Such being the case, the spring stiffness of the folded springs 14b, 14f, 14m can be changed by varying the thickness of the pair of beam springs of each of the folded springs 14b, 14f, 14m. It should be noted however that, depending on the nature of the application, the arrangement of folded springs and an actuating rod need not conform to this exemplary embodiment. Thus, the number and the placement of the rods and folded springs could be tailored to meet the needs of the application.

Fig. 3 illustrates an electrostatic micro actuator 100b in accordance with a second preferred embodiment of the present invention. The present micro actuator 100b inherits the same configuration as the electrostatic micro actuator 100a in Fig. 2, in that it has a comb actuation assembly 10, an actuation rod 16, and a spring assembly 13' . Thus, an explanation of the redundant parts will be omitted for the sake of simplicity.

In this exemplary embodiment, in order to embody a non-linear spring stiffness characteristic of the spring assembly 13', the folded springs 14'b, 14' f, 14'm have different spring stiffness from one another, by varying the respective lengths of the pair of beam springs of the folded springs 14'b, 14' f, 14'm. It should be noted however that the number and the arrangement of actuating rods and folded springs can be modified, depending on the nature of the application, given that the modification does not undermine the non-linear characteristic of the spring assembly 13' .

Fig. 4 sets forth an electrostatic micro actuator 100c in accordance with a third preferred embodiment of the present invention. A comb actuation assembly 10 and a actuating rod 16 in the present micro actuator 100c inherits the same configuration and characteristics as the comb actuation assembly 10 and the actuating rod 16 of the previous illustrations in Figs. 2 and 3, respectively, and thus explanation thereof will be omitted for the sake of simplicity.

The spring assembly 23 includes two spring groups 24, 124. Each of the spring groups 24, 124 provides a nonlinear spring stiffness characteristic by employing a combination of, e.g., one U-shaped folded spring 24f, and two beam springs 24m, 24b. The U-shaped folded spring 24f has a pair of substantially parallel spring beams 24f-f, 24f-b, which are connected by a connecting beam 24f-c, in a similar manner as in the folded springs in Figs. 2 and 3. One end of the folded beam 24f is fixed to an anchor

27 and the other end to a joint member 18 on the actuating rod 16. In contrast, the spring beams 24m, 24b and a supporting beam 19 are fixed to an anchor portion 17 and are substantially parallel to each other. In a similar manner to the first and the second preferred embodiment, the deflection in each of the spring components 24b, 24f, 24m, 19 is governed by the spring stiffness of a sequentially preceding spring component, in addition to its spring stiffness.

The overlapping of the beam springs 24b, 24m on the supporting member 19 yields surface contact, which may cause them to make a sticky contact, thus protrusions 25b, 25f, 25m are employed on either surfaces that are in contact, in order to effectively reduce the stickiness of the surfaces, by transforming a surface contact to a line contact thereof, in a similar fashion as in the first and the second preferred embodiment. Furthermore, the supporting beam 19 may serve to provide specified spring stiffness, while providing a safeguard for an excessive loading or deflection of the folded spring 24f and the beam springs 24b, 24m. Although in the present exemplary embodiment, a nonlinear spring stiffness characteristic is obtained by providing distinct spring stiffness by varying the thickness of each of the beam springs 24b, 24m and the supporting beam 19, it should be noted however that the same can be done by varying lengths thereof. Furthermore, the number and the arrangement of the spring components can be modified as long as the spring assembly 23 retains the non-linear spring stiffness characteristic.

The operation of the electrostatic micro actuators 100a, 100b and 100c in accordance with the preferred embodiments of the present invention will now be described.

Referring to Fig. 5, there is shown the micro actuator 100a of the first preferred embodiment with its folded springs 14f, 14m fully deflected. An attractive electrostatic force due to a supplied actuation voltage is applied to the spring assembly 13. The electrostatic force deflects the folded spring of the smallest spring stiffness first. Such being the case, the electrostatic force is first exerted on the folded spring 14f yielding a deflection, until a static equilibrium is reached between the electrostatic force and the spring force in the folded spring 14f or until the protrusion 15f makes a contact with the beam spring 14f-b, at which point a predetermined maximum deflection of the folded spring 14f has been reached. Upon further applying higher voltage after the protrusion 15f makes a contact with the beam spring 14f- b, the electrostatic force further increased by the additional applied voltage is transmitted through the connecting portion 16 and exerted onto the folded spring 14m. The folded spring 14m starts to be deflected until a static equilibrium is reached between the electrostatic force and the spring force of folded springs 14f, 14m or until the protrusion 15m makes a contact with the beam spring 14m-b at which point a predetermined maximum deflection of the folded spring 14m has been reached. The folded spring 14b operates in the same fashion in response to a further increased electrostatic force generated by the comb drive assembly 10.

Each of the folded springs 14b, 14f, 14m has a predetermined spacing therein, which limits the amount of deflection that can take place at a specified spring stiffness. Thus, the predetermined maximum displacement of the movable comb drive 12, which is divided into a plural displacement segments, each displacement segment having a different spring stiffness, is determined by one or more folded springs. In the present exemplary embodiment, the folded springs 14b, 14f, 14m vary in spring stiffness which is achieved by employing beam springs of different thickness, whereas in Fig. 6, the spring stiffness of each of the folded springs 14'b, 14' f, 14'm is varied by employing spring beams of different lengths, thereby yielding a non^¬ linear spring stiffness in both cases. It should be noted that the operational and functional principles of these devices are equivalent, thus the operation of the micro actuator 100b in Fig. 6 will be omitted. Referring to Fig. 7, there is shown the micro actuator 24f and beam spring 24m fully deflected. The present exemplary embodiment provides same operational and functional principles as those in the first and the second preferred embodiment. In essence, by providing a spring assembly having a non-linear spring stiffness characteristic, the precision of the control of the motion of the actuator is greatly improved. In all of the exemplary embodiments, the nonlinear -spring stiffness characteristic is captured in a spring assembly. More specifically, in Figs. 5 and 6, nonlinear spring stiffness is provided by serially connected folded springs that are of different stiffness, whereas in Fig. 7, different spring stiffness is embodied in distinct spring components, thereby providing a non-linear spring stiffness characteristic thereof.

The exemplary embodiments have been described in detail for the case of employing electrostatic force as a drive mechanism. However, it should be noted that a spring assembly having a non-linear spring stiffness characteristic can be applied to devices employing electromagnetic, piezoelectric, or thermal force as a drive mechanism.

While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims .

Claims

1. A micro actuator comprising: a drive assembly disposed on a substrate, the drive assembly including a movable member and a fixed member; a spring assembly having a non-linear spring stiffness; and an elongated means for translating a motion of the movable drive member of the drive assembly to the spring assembly.

2. The micro actuator of claim 1, wherein the motion of the movable member is caused by an electrostatic force induced between the movable and the fixed member

3. The micro actuator of claim 1, wherein the spring assembly includes two substantially identical spring groups disposed symmetrically with respect to the elongated means at two opposite lateral sides thereof, each spring group having more than one spring component, wherein the movable member has a predetermined maximum displacement, which is divided into a plural number of displacement segments, each displacement segment corresponding to a different spring stiffness determined by one of said one or more spring component.

4. The micro actuator of claim 3, wherein each of said more than one spring component in each of the spring groups is of a folded spring having a pair of beam springs and a connecting beam, the pair of beam springs being coupled by the connecting beam.

5. The micro actuator of claim 4, wherein said more than one spring component in one of the spring groups are arranged in a series in order of magnitudes of their spring stiffnesses .

6. The micro actuator of claim 4, wherein beam springs of said more than one spring component in one of the spring groups have different thicknesses.

7. The micro actuator of claim 4, wherein beam springs of said more than one spring component in one of the spring groups have different lengths.

8. The micro actuator of claim 4, wherein one of the beam springs in each of said more than one spring component has a protrusion at a free end portion thereof.

9. The micro actuator of claim 3, wherein said more than one spring component in each of the spring groups include one or more beam springs and a folded spring, one end of each beam spring being fixed to the substrate, wherein the folded spring has a pair of beam springs and a connecting beam, the pair of beam springs being coupled by the connecting beam.

10. The micro actuator of claim 8, wherein each beam spring has a protrusion at a free end portion thereof.

11. The micro actuator of claim 9, wherein if said more than one spring component in one of the spring groups include a plurality of the beam springs, the plurality of the beam springs have different thicknesses.

12. The micro actuator of claim 9, wherein if said more than one spring component in one of the spring groups include a plurality of the beam springs, the plurality of the beam springs have different lengths.