WO2007006299A1

WO2007006299A1 - Microactuator

Info

Publication number: WO2007006299A1
Application number: PCT/DE2006/001235
Authority: WO
Inventors: Peter Dürr; Jan Schmidt; Andreas Gehner; Detlef Kunze
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2005-07-11
Filing date: 2006-07-11
Publication date: 2007-01-18
Also published as: DE102005032863A1

Abstract

The invention relates to microactuators which can be used for many applications and comprise an element that can be deflected under the effect of force. The aim of the invention is to obtain a greater degree of deflection within an available gap and/or reduce the power required for the deflection while being able to follow desired deflecting movements and positions. Said aim is achieved by providing the inventive microactuator with one or several restoring elements which have an at least sectionally progressive, preferably disproportionately rising characteristic curve of the restoring force.

Description

microactuator

The invention relates to microactuators which are versatile, e.g. as a surface light modulator, scanner mirrors, optical cross connectors, microvalves, microswitches, micropumps and the like. may be formed and then also a small space required making can be arranged in large numbers in array form. A single microactuator has dimensions that can be smaller than 1 mm.

On microactuators usually a deflectable element is present, which can be deflected electrostatically, magnetically, electromagnetically, utilizing piezoelectric effects but also defined by thermal expansion.

The element can be moved translationally and / or pivoted about one or more axis (s). The deflectable elements of microactuators are usually held by means of elastic return elements, preferably springs. For the deflection, a gap is provided within which the deflection can take place. The available

However, in a large class of known solutions, gap size can only be utilized to a small extent, generally about 25%. This is always the case when a deflection is to take place at least approximately or completely parallel to a field.

This also leads to the fact that increased power, in particular higher electrical voltage and / or currents are required for the control of the deflection.

Conventional microactuators are often operated electrostatically or electromagnetically. The deflection takes place in one direction, against the force of elastic return elements, which act on deflectable elements. Often these are feathers. The achievable deflection can be limited or adjusted by means of mechanical stops or by setting a meta-stable balance of forces in one position.

Following the action of force exerted on the deflection, the deflectable element reaches its initial position. Limiting the return movement can be achieved with mechanical stops.

However, certain deflection positions can also be maintained by setting a meta-stable equilibrium of forces in a predefinable deflection position. This equilibrium of forces is, however, deflections increasingly less stable and the resulting deflection position increasingly inaccurate.

For frequently used parallel-plate-capacitor actuators, for example, the stability of the equilibrium of forces disappears already at 33% of the available gap. The part of the available gap that can be used for the deflection lies correspondingly below 25%, taking into account the safety aspect. This leads to an increased size and also to the fact that due to the at least four times too large gap size higher electrical voltages, possibly electrical currents to control the deflection are required lent.

To comply with the meta-stable balance of forces must act on the element to be deflected according to the respective deflection position forces. In this case, the electrostatic force is proportional to the square of the applied electrical voltage divided by the respective instantaneous plate distance to E- electrodes.

At constant electrical voltage and increased

Deflection and reduced plate spacing increases the force required for the deflection inversely proportional to the gap in a deflection inversely proportional unlimited.

By contrast, the restoring force of the suspension of the deflectable element, taking into account Hooke's law, only increases linearly with the respective deflection and thus no balance of forces can be achieved with large deflections of the element. This effect is commonly referred to as "pull-in" records.

In Tanaka, H. K .; et al "Large displacement control system beyond pull-in limitation"; Electrostatic Micro-cantilever "; IEEE 2002 will help avoid the

"pull-in" an electronic control proposed. This requires the use of a position sensor and faster control of the actuator drive signal than its mechanical response time. Such a solution is very complex and not suitable for larger arrays of actuators.

In another form, this problem has so far been counteracted by so-called "comb drives", which act electro-statically, whereby the electric field of such a "comb drive" is oriented essentially perpendicular to the deflection movement. In contrast, the alignment of the field in parallel-plate actuators is parallel to the Auslenkbewegungsrichtung.

The force component in the direction of deflection of a "comb drive" actuator is generated by the stray fields which are unaffected by the respective deflection, thereby significantly increasing the force required for deflection just before the tips of the comb fingers meet the base of the opposing comb The deflection range can be greater than half the distance between the tips of comb elements and the opposing comb base.The electrostatic forces are proportional to the square of the applied voltage, but the deflection is not linear when using linear characteristic springs.

From WO 03/073597 Al the use of folded Fe- Several sections are known which each achieve a fixed spring constant for small deflections and incrementally increased spring constant for greater deflections.

This also does not lead to a complete linearity of deflection and respective electrical voltage or effective force. The stable usable range for the deflection is thus only marginally enlarged. The gradual increase in the spring constant results in a complicated restoring force curve, which depends sensitively on production parameters. In particular, when a high-precision deflection is desired, these jumps have a negative effect, since in certain deflection ranges with rapidly changing spring constant no precise deflection can be achieved.

It is therefore an object of the invention to provide microactuators which allow increased deflection within an available gap and / or reduced power for the deflection, with good ease of each desired deflection movements and positions.

According to the invention, this object is achieved by a microactuator comprising the features of claim 1.

Advantageous embodiments and further developments of the invention can be achieved with the features described in the subordinate claims.

The microactuator according to the invention can be designed in many ways like conventional microactuators and can also be driven in this way. The can For the deflection of a deflectable element required forces are preferably applied electrostatically or electromagnetically, but possibly also magnetically, in the latter case, a regulation of the magnetic field strength should be feasible.

The deflection of the respective element can take place translationally or rotationally or a combination of these movements. But it can also be a tilt, especially if only a restoring element engages a deflectable element. In this case, deflectable elements which have a high rigidity and strength, especially in comparison to the respective restoring element, are to be preferred.

The deflectable element is held by means of at least one elastic return element, preferably a spring. Essential to the invention is at least partially the progressive, preferably disproportionately increasing with the deflection restoring force characteristic (spring characteristic) with increasing deflection, so that act at higher deflections also disproportionately higher restoring forces. For smaller deflections, a linear increase may be allowed.

Advantageously, two such return elements engage a member to be deflected, which preferably engage diametrically opposite to the respective element.

Restoring elements can be bendable, compressible, expandable and / or compressible, wherein a selection is possible taking into account the respectively desired deflection movement. One or more return Control element (s) may be deformable in different forms, for example in two different modes.

If more than one such restoring element are used on a deflectable element, they should be as identical as possible, which applies in particular to the dimensioning and the mechanical properties.

A microactuator according to the invention can be designed, for example, as a plano-plate capacitor actuator, and the electric field can be aligned substantially parallel to the direction of the deflection movement.

As restoring elements, bar or leaf springs can be used which, by virtue of their design, have the spring force course desired according to the invention as a function of the respective deflection.

Such a spring force curve can be achieved by more complex geometries, e.g. kinked or curved rods or with T-shaped springs the Erforderungsrnissen be fitted.

The microactuators according to the invention can be designed so that a deflection over at least 1/3, preferably 40% and particularly preferably over 50% of an available gap dimension is possible. If an enlarged deflection is not required, by reducing the gap dimension, the electrical voltage required for the deflection (possibly electric current) can be reduced or, at a constant electrical voltage, the achievable force for the

Deflection can be increased. Last mentioned effect can reduce the risk of breakage on reset elements with higher spring constants.

The pull-in ^Xλ effect either does not occur or only at a larger deflection.

The restoring elements (springs) can be varied and adapted in their dimensioning and design as well as the attachment to the element to be deflected.

The invention will be explained in more detail by way of example in the following.

Showing:

FIG. 1 shows an exploded perspective view of an example of a microactuator according to the invention;

FIG. 2 shows, in a schematic form, a clamped bending beam which is bent by a force according to Hooke's Law;

Figure 3 shows in schematic form a clamped string, which is deflected by a force and unlike Figure 2 has no bending stiffness;

Figure 4 is a graph of restoring force curves as a function of the deflection for different springs;

Figure 5 is a diagram of achievable electrostatic forces at different electrical voltages as a function of the deflection;

FIG. 6 shows a diagram with electrostatic and restoring forces at different electrical voltages. conditions with different springs depending on the deflection;

7 shows a diagram with electrostatic and restoring forces at different electrical voltages and different springs as a function of the deflection, with a small gap at the beginning of a deflection of a deflectable element;

Figure 8 is another diagram for return elements with a smaller width;

Figure 9 is another diagram for return elements with a smaller thickness;

Figure 10 is a diagram of the achievable deflection as a function of the electrical voltage at different springs;

Figure 11 is a perspective view of a deflectable element with two springs acting thereon;

FIG. 12 shows a diagram of the relative stiffness as a function of buckling angles on an example according to FIGS. 11 and

FIG. 13 shows an exploded perspective view of elements of an example of a microactuator according to the invention.

In Figure 1, elements for an example of a microactuator according to the invention in a perspective exploded view are shown, which is designed as an electrostatic parallel plate actuator with clamped bending beam, as return elements 2. The deflectable element 1 is here plate-shaped. The two springs are not only bent during operation, but also subjected to tensile forces, so also pulled when they have been dimensioned with respect to the respective desired deflection movement.

The deflectable element 1 is here at least on its upwardly facing surface reflective of electromagnetic radiation and thus forms a mirror.

The two outwardly facing parts of a clamped bending beam, as restoring elements 2 are wider here in the horizontal direction, as the inwardly facing and acting on the deflectable element 1 parts. The outwardly facing parts rest on spacers 3, so that between deflectable element 1 and an electrode plate 4, a defined gap of less than 1 micron to a few microns gap is formed.

If an electrical voltage is now applied between the deflectable element 1 and the electrode plate 3, a controllable electrostatic force acts between these two elements 1 and 3, which in turn leads to a deflection of the deflectable element 1, which counteracts the restoring forces of the restoring elements 2.

A stable equilibrium of forces can be maintained by influencing the electrostatic forces as a function of the respective deflection, in which electrical voltage is controlled as a function of the desired deflection. Thus, the respective deflection of the element 1 can be set exactly and, if desired, at least temporarily maintained become.

FIG. 2 shows a simplified side view of the spring elements 2 without deflectable element 1. For small deflections, the bending stiffness of the restoring elements 2 is at least approximately proportional to the deflection. With known modulus of elasticity, the spring force can be calculated taking into account the dimensioning of the restoring elements 2 according to FIG. 2 with formula (1) as a function of the respective deflection d. The width b of the return elements 2 is directed into the plane of the drawing and consequently not shown in FIG.

For larger deflections d, ie those greater than the thickness a of the return elements 2, the flexural rigidity can be neglected. The system then behaves like a string. With a residual stress without deflection σo, the spring force for the residual stress of the undeflected string according to formula (2) from FIG. 3 can be calculated to a good approximation.

2σoαZ, _ _{J +} Eab

(2:

Thus, the progressive, here disproportionately increasing force curve at larger deflections, in this case achieved with the third power.

In reality, the thickness a of the return elements 2 can not be neglected, taking into account the deflection, so that both approaches overlap and lead to progressively increasing restoring forces.

For example, with a thickness a = 1 .mu.m, a width b = 4 .mu.m, a length L = 80 .mu.m, modulus E = 70 GN / m ² and a residual stress of "zero" Figure 4 shows the two limiting approaches for the restoring forces depending on the deflection and also their combination.

It becomes clear that for large deflections the total force as well as the increase are clearly larger than with the linear approximation.

The diagram shown in FIG. 5 gives an overview of electrostatic forces as a function of the deflection at different constant applied electrical voltages in the range from 3 V to 24 V. The gap between deflectable element 1 and electrode plate 3 had a maximum gap h = 3 μm , The rising force curve reflects the preceding versions again.

The positions of the force equilibrium of the microactuator are where the electrostatic forces in their absolute value coincide with the respective restoring force. These positions can be seen from FIG. The force equilibrium positions are the points at which the electrostatic drive force curve intersects the restoring force curve with their respective absolute values.

The increase in the force-deflection curve is the essential parameter determining the stability of the respective operating point of the microactuator, ie the desired deflection position. The position of the meta-stable balance of forces is determined by the fact that the increase in the restoring force curve is greater than the increase in the driving force curve. Areas in which the increase in the restoring force curve is smaller than the increase in the driving force ^curve are unstable and the pull-in ^λλ effect can occur there.

The stable region can be used up to 1.5 μm for a microactuator according to the invention, which accounts for 50% of the available gap dimension.

A microactuator with restoring elements 2 in the form of clamped bending beams, which fulfill Hooke's law, already reaches this unstable range at 1 μm, ie one third of the available gap.

The decisive improvement that can be achieved with the invention is achievable by the combination of linear and nonlinear restoring force profile, which is influenced by the balance between restoring elements 2 and deflection force. Dimensioning or design and deflection and the strength of the attachment of the return elements 2 have an influence on the non-linear course.

In the known solutions, however, non-linearity should be avoided, since the required drive forces are greater for a certain deflection, when all other parameters of reset elements 2 remain constant.

This is made clear with FIG. Reset elements 2 with a linear restoring force curve are available at a voltage of 18 V for a selection kung to 1 μm, with non-linear reset elements requiring 21 V.

A microactuator according to the invention can utilize a larger range of an available gap dimension. In this example, the output gap is reduced from 3 μm to 2.6 μm, so that the required electrical voltage for stable operation is also 18 V, ie the same voltage for return elements 2 with a linear restoring force curve with a gap of 3 μm (see FIG.

However, it can also be seen from FIG. 7 that the deflection at this electrical voltage or the stability limit is 1.3 μm in contrast to 1 μm. It is thus possible to realize a greater deflection even with limited electrical voltage.

When comparing the formulas (1) and (2), it becomes clear that the non-linearity increases as the width-to-thickness ratio (a / d) becomes smaller. The above-mentioned dimensioning parameters were selected so that the difference for linear and non-linear restoring elements 2 can be clearly shown in equally dimensioned diagrams.

If, for example, one chooses a thickness a = 0.5 μm and a width d = 7 μm, a return element 2 still has almost the same cross section. The point of intersection of the force curves is reached with 18 V and the balance of power is at a deflection of 1.5 microns. The stability limit is approximately 1.7 μm, which corresponds to more than 50% of the available gap. It is also noteworthy, however, that with increased deflection of a deflected element 1, a reduction of the distance and consequently also of the still existing gap dimension occurs. As a result, the deflecting force increases with increasing deflection. This effect increases with increasing deflection, so that the course of a deflection and electrical voltage characteristic of a microactuator also increases steeply. This can then lead to an unstable operating state (pull-in). The deflection is then strongly dependent on the currently applied electrical voltage, so that a precise control of the electrical voltage is required, which is not necessarily required for small deflections.

With the invention, the adjustment behavior of the deflection improves, since this is closer to a linear relationship, as with springs with linear characteristic line.

With the diagram shown in Figure 10, the improvement of the adjustment behavior of a clamped bending beam of Figure 4 in comparison with a non-linear spring of Figure 9 can be illustrated.

For some applications, however, the available space may not be sufficient to allow a sufficient length L of return elements 2 with the greatest possible deflections and maximum allowable tensile stress (see formula (3)). In this case, the rigidity of restoring elements 2 would increase too much, so that a deflection is not possible or counterproductive only with very high driving forces and correspondingly increased performance.

But this can be counteracted by the usable length of reset elements 2 increases and the effective tensile stiffness is reduced. This can be achieved by not just forming restoring elements 2. Thus, at least one kink or bend can be formed on a return element 2.

FIG. 11 shows an example with twofold kinking with blunt kink angles. Alternatively, however, a sinusoidal shape or slight deviations from a straight shape can increase the usable length without additional space / space being required. With such a configuration, the tensile stiffness can be reduced, as this allows an additional bending mode.

FIG. 12 shows that the bending angle only very little influences the linear component of the spring force curve for bending angles of less than 15 °, while the proportion which increases with the third power of the deflection is adjustable over a wider range.

With this design and dimensioning possibility of restoring elements 2, possibilities for the use of the invention are opened up, with very small available dimensions or for a further miniaturization.

But there are also other geometries of return elements 2 possible with which the non-linear component of the spring stiffness can be reduced. For example, return elements 2 in T-shape be used.

With bent or bent return elements 2, the spring constant (linear portion) is not so sensitive to the residual stress of the spring layer.

Thus, for example, arrays with sinking mirrors for wave front correction of electromagnetic radiation with a stroke of 2 μm and a pixel size of 40 μm can be produced.

In the case of microactuators with pivotable, deflectable elements 1, it is known, for example, that a stable deflection (measured at the edge of such an element) over the entire available gap is possible. In these solutions, however, large electrodes and unfavorable leverage ratios are required, so that the effectively utilized for the deflection area of such a deflected element can still use only a small limited range of the available gap dimension

With the invention, however, a larger deflection range can be achieved with greater deflection force and with the same electrical voltage (possibly electrical current) or the same deflection force with lower electrical voltage (possibly electrical current).

An example is shown in FIG. 13. Again, the non-linearity of the restoring elements 2 used improves the properties. The return elements 2 do not engage close to the axis of rotation, so that an elongation again leads to a progressively increasing restoring force course. The invention improves the properties of microactuators with high positive force feedback that limits the range for stable use. With a positive force feedback increases for the deflecting force at constant voltage with increasing deflection.

The invention makes it possible to produce improved microactuators which are operated electrostatically or electromagnetically. The deflection can take place essentially parallel to the respective electrical or electromagnetic field.

It is a much larger area for the deflection of the available gap available.

On the other hand, a required deflection range with a smaller available gap dimension can be covered with increased stability and with microactuators manufactured in a simple form.

By possibly reduced electrical voltage, if necessary electrical current, the electronic control for the drive can be simplified.

The microactuators according to the invention can be produced by means of conventional technologies. The improved properties can be optimized by the design and dimensioning of the individual elements, in particular the return elements 2.

Claims

claims

1. microactuator with a deflectable by force element, which is held with at least one elastic return element, characterized in that the one or more restoring element (s) (2) has at least partially a progressively increasing restoring force characteristic.

2. microactuator according to claim 1, characterized in that the one or more restoring element (2) is one or more spring (s) is / are.

3. microactuator according to claim 1 or 2, characterized in that the / the restoring element (s) (2) is elastically deformable in different form.

4. Microactuator according to one of the preceding claims, characterized in that the one or more restoring element (s) (2) at least partially a disproportionately rising

Has restoring force characteristic.

5. Microactuator according to one of the preceding claims, characterized in that the / the return element (s) (2) in at least one direction bendable bendable, rotatable, compressible and / or stretchable is / are.

β. Microactuator according to one of the preceding claims, characterized in that the deflection of the deflectable element (1) takes place electrostatically, electromagnetically and / or magnetically.

7. Microactuator according to one of the preceding claims, characterized in that the deflection of the deflectable element (1) takes place parallel to an electric, magnetic or electromagnetic field.

8. microactuator according to one of the preceding claims, characterized in that the deflection takes place in translation.

9. Microactuator according to one of the preceding claims, characterized in that the deflection takes place rotationally or the deflectable element (1) can be tilted.

10. Microactuator according to one of the preceding claims, characterized in that the / the restoring element (s) (2) is designed as a rod or leaf spring / are.

11. Microactuator according to one of the preceding claims, characterized in that rod or leaf spring (s) kinked along its longitudinal axis and / or is bent / are.

12. Microactuator according to one of the preceding claims, characterized in that the / the restoring element (s) (2) is designed as a T-shaped spring / are.

13. Microactuator according to one of the preceding claims, characterized in that the deflectable element (2) over at least 1/3 of an available gap dimension is metastable deflected.

14. microactuator according to one of the preceding claims, characterized in that the on steerable element (1) at least one reflective surface is present.