JP2012088581A - Liquid lens and control method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology of controlling optical power of a liquid lens with higher accuracy while decreasing influences of hysteresis when controlling a voltage to obtain target optical power.SOLUTION: The liquid lens 209 includes a non-conductive liquid 202 and a conductive liquid 201 in a casing 203, and electrodes 206 to apply a voltage to the conductive liquid. The lens also includes control means to control a voltage between the electrodes and the conductive liquid, and changes optical power by changing a shape of an interface 208 between two liquids. The control means controls in such a manner that upon rendering the interfacial shape into a shape giving the target optical power in a region with hysteresis, a contact angle of the aimed liquid always reaches a target contact angle in terms of a receding angle. When a contact angle of one liquid is an advancing angle during changing the voltage in a direction to obtain the target optical power, the voltage is controlled to exceed one voltage of two voltages giving the target optical power and further to exceed the other voltage of the two voltages, and then reversed in the opposite direction to reach the one voltage.

Description

The present invention relates to a power variable element, and more particularly to a liquid lens capable of adjusting a refractive power (optical power) using a liquid and a control method thereof.

Conventionally, many power variable elements that change optical power with one element have been proposed. Among them, a liquid lens using the electrowetting (hereinafter also referred to as EW) phenomenon that changes the contact angle of a droplet by applying a voltage has no dependence on polarized light and its optical characteristics. Is attracting attention. As shown in FIG. 1, an EW element using the EW phenomenon usually changes the optical power by changing the interface 101 between two liquids 102 and 103 having different refractive indexes. At this time, the density of the two liquids 102 and 103 is generally made uniform in order to eliminate the influence of gravity.

Patent Document 1 describes such a liquid lens driving method. Patent Documents 2 and 3 describe the configuration of an EW liquid lens. Here, the contact angle of the two-liquid interface with respect to the solid part is uniquely determined from the surface energy of two kinds of liquids and the surface free energy of the solid part. However, in actuality, it varies depending on the surface state and surface roughness of the solid part, the viscosity of the liquid, the work of bonding, and the like. In particular, the contact angle is known to show different angles called advancing and receding angles depending on the direction in which the contact point of the two-liquid interface moves. Here, the interface shape of the two liquids is fixed because the two liquids are sealed, and is uniquely determined if the surrounding contact angle is determined. If the two liquids have the same density, the interface shape is theoretically a spherical surface. Furthermore, when a voltage is applied here, the contact angle changes, and the spherical shape of the two-liquid interface is uniquely determined accordingly. That is, the applied voltage and the interface shape have a one-to-one relationship.

Japanese Patent Laid-Open No. 2001-249203 Japanese Patent No. 4414641 Special table 2008-518906 gazette

However, when the contact angle difference between the advance angle and the receding angle described above occurs, two types of contact angles exist at the target voltage. The different contact angles are generated when the voltage increases and decreases, respectively, and as a result, this is recognized as hysteresis of the optical power of the liquid lens. That is, in a region having hysteresis, the same optical power is realized by two different voltages from the viewpoint of optical power, and two optical powers by the same voltage are realized from the viewpoint of voltage. In order to reduce this hysteresis, it is necessary to select a liquid material, the material of the surface of the solid part, or the like, or to correct the hysteresis by the driving method (that is, reduce the influence of the hysteresis). Patent Document 1 describes a driving method for performing an overshoot in which the voltage is changed once exceeding the target voltage and then returned to the target voltage in order to reduce the influence of hysteresis. However, there is no description as to which overshoot should be performed when the voltage changes in the upward or downward direction and the excess amount. In Patent Documents 2 and 3, a material that hardly generates hysteresis is selected by providing a lubricating layer between a liquid and a solid or defining a surface state. However, when such a material is selected, the material is limited and it becomes more difficult to achieve compatibility with the optical characteristics of the lens.

In view of the above problems, an electrowetting type liquid lens of the present invention includes a non-conductive liquid and a conductive liquid sealed inside a housing, and an electrode for applying a voltage to the conductive liquid. Have. In addition, the optical power is changed by changing a shape of an interface formed by the two kinds of liquids. Further, when the interface means brings the interface shape to a target optical power shape in a region where there is optical power hysteresis, the interface energy with the inner wall of the housing that is in contact with the end of the interface is small. The contact angle of one of the two liquids is always a receding angle so as to reach the target contact angle. That is, if the contact angle of one liquid becomes a forward angle when the voltage is changed in the direction to achieve the target optical power, the voltage exceeds one of the two voltages that achieve the target optical power. Further, after the other voltage of the two voltages is changed, it is returned to the opposite direction and brought to the one voltage.

In view of the above problems, the control method according to the present invention of an electrowetting type liquid lens that changes the optical power by changing the shape of the interface between the two types of liquids is as follows. The voltage applied between the liquids is controlled. That is, when the interface shape formed by the two liquids is brought to the target optical power shape in a region where there is optical power hysteresis, the interface energy with the inner wall of the casing that is in contact with the end of the interface is small. The voltage is controlled so that the contact angle of one of the two liquids always becomes the receding angle and reaches the target contact angle. For this reason, when the contact angle of one liquid becomes a forward angle when the voltage is changed in the direction for realizing the target optical power, the voltage is set to one of the two voltages for realizing the target optical power. Is exceeded and further changed over the other of the two voltages, and then reversed to bring it to the one voltage.

According to the present invention, since the overshoot as described above is performed, when the voltage is controlled to achieve the target optical power, the influence of the hysteresis generated from the difference in the contact angle between the forward angle and the backward angle is reduced. The optical power of the liquid lens can be controlled with higher accuracy.

It is a schematic diagram explaining the 2 liquid interface change of a liquid lens. It is a figure showing the structure of EW system liquid lens. It is a figure explaining the advancing angle and receding angle in a contact angle. 6 is a diagram illustrating liquid lens control in Embodiment 1. FIG. FIG. 6 is a diagram for explaining an interface change of the liquid lens in the first embodiment. FIG. 6 is a diagram illustrating liquid lens control in Embodiment 2. FIG. 10 is a diagram illustrating a time change of a liquid lens in Example 2. It is a figure explaining the effect of control in Example 2. FIG. It is a schematic diagram explaining the structure which controls a liquid lens. FIG. 6 is a diagram for explaining liquid lens control in Embodiment 3.

The features of the present invention are as follows. In the hysteresis region, if the voltage is changed in one direction as it is so that the contact angle of the liquid of interest always reaches the target contact angle with a receding angle, Like that. That is, the voltage is changed in one direction to exceed one of the two voltages for realizing the target optical power, and the voltage is changed to exceed the other of the two voltages and then returned to the opposite direction. To one of the voltages. At this time, the amount of excess from the one voltage to the other voltage is usually different depending on the target optical power because the shape of the hysteresis is not symmetrical. Based on this concept, the electrowetting type liquid lens and the control method thereof according to the present invention have the basic configuration as described in the section for solving the above-mentioned problems. Hereinafter, an embodiment embodying this basic configuration will be described.

Example 1
Embodiment 1 of the present invention will be described with reference to the drawings. FIG. 2 is a cross-sectional view of the liquid lens 209 according to the first embodiment. Two kinds of liquids 201 and 202 are sealed in a housing 203. The liquid 201 is a conductive liquid such as an electrolytic solution, and the liquid 202 is a nonconductive liquid such as oil. The housing 203 is sealed by a window member 204 and a window member 205 that are arranged to face each other. An electrode 206 and an insulator 207 are formed on the inner wall of the side surface of the housing 203. The electrode 206 is formed on the surface of the insulator 207 opposite to the side where the two liquids are sealed. The liquid 201 and the liquid 202 are not mixed with each other, and an interface 208 is generated between the two kinds of liquids. Here, if the refractive indexes of the two kinds of liquids 201 and 202 sealed inside the casing are different, refractive power is generated by the interface 208. The interface shape is a spherical surface if the densities of the two types of liquids 201 and 202 are substantially the same, so that the interface 208 acts as a lens refracting surface. At this time, the radius of curvature of the interface 208 is uniquely determined by the interface energy acting on each of the two types of liquids 201 and 202 and the insulator 207.

The radius of curvature of the interface 208 is obtained as follows. When the interfacial energies acting between the liquid 201 and the insulator 207, between the liquid 202 and the insulator 207, and between the two kinds of liquids 201 and 202 are γ ₁ , γ ₂ , and γ ₃ , respectively, the conductive liquid 201 The initial contact angle θ ₀ can be expressed by the following equation, which determines the radius of curvature of the interface 208. This state of curvature radius is a state where the energy of the entire liquid lens 209 is minimized.
cosθ ₀ = γ ₁ / γ ₃ −γ ₂ / γ ₃

Here, when a voltage is applied between the electrode 206 and the conductive liquid 201 (the electrode for applying a voltage to the liquid 201 is not shown in FIG. 2, but is a rod-shaped electrode inserted into a hole formed in the housing 203. The energy Q of the electric field is generated between the liquid 201 and the electrode 206. As a result, the energy of the entire liquid lens 209 changes. In response to this energy change, the contact angle θ of the liquid 201 changes, and the total system energy is minimized when the contact angle θ is expressed by
_{cosθ = γ 1 / γ 3 -γ} 2 / γ 3 -Q
This shows that the contact angle of the liquid 201 can be changed by applying a voltage. Since the volumes of the liquids 201 and 202 are constant, the radius of curvature of the interface 208 changes as the contact angle of the liquid 201 changes. Therefore, by applying a voltage, the radius of curvature of the two-liquid interface 208 can be changed, and as a result, the refractive power generated at the interface 208 can be changed.

At this time, there are advancing angle and receding angle in the contact angle of the liquid. The receding angle is a contact angle generated by moving the end of the contact surface between the liquid and the solid in the direction of the liquid of interest, and the advancing angle is a contact angle generated by moving in the opposite direction. Assuming that the liquid of interest is the liquid 301 and the applied voltage is brought to the same voltage, FIG. 3A shows the contact angle reached with the receding angle, and FIG. 3B shows the contact angle reached with the advancing angle. . In the case of an interface between two liquids, paying attention to one of the liquids, the receding angle is when the contact surface edge moves in the direction in which the liquid exists, and the advancing angle is when the direction is opposite. For example, in the case of FIGS. 3C and 3D, the liquid of interest is the liquid 301, the direction of (c) is the receding angle, and the direction of (d) is the advancing angle.

The advancing and receding angles vary depending on various parameters such as the bonding work between the liquid and the solid, the interfacial energy between the solid and the liquid, and the surface roughness of the solid surface. Further, the following relationship is established between the forward angle θ _a and the backward angle θ _b realized by applying the same voltage (see FIGS. 3A and 3B).
θ _a > or = θ _b
With regard to the advance angle and the receding angle, the receding angle tends to be a more stable angle especially considering the wetness of the liquid. As the contact angle realized with the same voltage, there are such advancing angle and receding angle, and there is a difference between them, so the optical power of the liquid lens is realized by increasing the voltage and decreasing it There is a difference between That is, hysteresis occurs. In order to reduce this hysteresis (that is, the difference in optical power), there is a method of selecting a liquid material or an insulator material. However, in the case of a liquid lens, since a high voltage needs to be applied, a high withstand voltage is required for the insulator material. In addition, since the two types of liquids also require a desired refractive index difference, it may not be possible to select a material that can reduce this hysteresis.

Therefore, in this embodiment, the influence of the hysteresis is reduced by controlling the two-liquid interface so that the liquid of interest always becomes one of the advance angle and the receding angle and reaches the target contact angle. At this time, by controlling the receding angle to be more stable, the influence of hysteresis is reduced, and the shape of the two-liquid interface is stabilized.

This control will be described with reference to FIGS. FIG. 4 is a schematic diagram showing hysteresis in a curve with the applied voltage on the horizontal axis and the optical power of the liquid lens on the vertical axis. FIG. 5 is a diagram showing the state of the liquid lens 501 corresponding to each point in FIG. In FIG. 5, 502 is a conductive liquid, 503 is a non-conductive liquid, 504 is an insulator, and 505 is an electrode. In this embodiment, the liquid of interest is a non-conductive liquid 503. 5A corresponds to the lens interface shape at the point 401 in FIG. Similarly, FIG. 5B corresponds to the point 402, FIG. 5C corresponds to the point 403, and FIG. 5D corresponds to the point 404.

Here, the hysteresis in the liquid lens has an asymmetric shape with respect to the optical power as shown in FIG. As shown in FIG. 4, it is assumed that the power is reduced from a certain optical power φ1 to φ2. At this time, the target values of the voltages of the liquid lens 501 with the optical power of φ1 and φ2 are V1 and V2, respectively (refractive index of the nonconductive liquid 503> refractive index of the conductive liquid 502). When attention is paid to the non-conductive liquid 503, the direction in which the optical power is reduced is the direction in which the contact angle becomes the advance angle. For this reason, as shown in FIG. 4, the optical power when the voltage decreases from V1 to V2 is larger than the optical power when the voltage increases from V0 to V2. In other words, when the optical power is reduced, hysteresis that is difficult to reduce the optical power occurs.

In order to reduce the influence of this hysteresis, when controlling the voltage in the direction of decreasing optical power, the voltage is lower than V2 instead of lowering to the higher voltage corresponding to the target optical power (V1⇒V2). Reduce to V3 (ie, overshoot). V3 is a voltage lower than the lower voltage V4 corresponding to the target optical power. Thereafter, by increasing the voltage from V3 to V2, the voltage is always controlled in the direction in which the contact angle is the receding angle. At this time, in order to set the contact angle as the receding angle, the magnitude of V3 needs to be at least smaller than the other low voltage V4 at which the optical power becomes φ2. In the above, the example of changing the voltage in the region without hysteresis to the voltage in the region with hysteresis has been described. However, as described above, the overshoot also occurs when the voltage is decreased toward the target voltage in the region with hysteresis. The voltage may be increased after the operation.

On the other hand, when the voltage is controlled in the direction in which the variation direction of the contact angle originally becomes the receding angle, for example, in the direction of V4 → V2, there is no need to overshoot. In this embodiment, the liquid of interest is the liquid 502 and 503 having the smaller interface energy with the insulator 504, and the liquid 503, which is a non-conductive liquid, is focused. By selecting the one having a smaller interface energy in this way, it becomes possible to increase the stability of the receding angle, and it is possible to reduce the influence of hysteresis more effectively. Further, even when the target power is realized by changing the voltage toward a voltage in a region where there is no hysteresis, the target voltage may be simply changed without overshooting.

As described above, in this embodiment, the control means for controlling the voltage applied between the electrode and the conductive liquid 502 controls the voltage as follows. When the interface shape of the two liquids is brought to the target optical power shape in the region where there is optical power hysteresis, the contact angle of the non-conductive liquid 503 is always set as the receding angle so as to reach the target contact angle. To do. Therefore, when the contact angle of the liquid 503 becomes a forward angle when the voltage is changed in the direction for realizing the target optical power, the voltage exceeds one of the two voltages for realizing the target optical power. After changing to the other voltage, it returns to the opposite direction and brings it to one voltage.

Note that the insulator 504 is not necessarily formed of one substance. For example, the insulator 504 may be covered with a thin film (not shown in FIG. 5) whose surface in contact with the two liquids 502 and 503 has a smaller interface energy with the non-conductive liquid 503. In this case, since the thin film material covering the insulating film 504 can be made different from the insulating film 504, the interface energy with the non-conductive liquid 503 can be made smaller, and the stability of the receding angle can be further increased. Further, the effect of hysteresis can be reduced.

(Example 2)
A second embodiment of the present invention will be described with reference to FIGS. In the second embodiment, a control method adapted to the asymmetry of hysteresis and a voltage excess amount will be described. FIG. 6 shows the hysteresis in the voltage-optical power curve of the liquid lens, as in FIG. As shown in FIG. 6, the hysteresis amount of the liquid lens varies depending on the target optical power. That is, the difference between two different voltages that achieve the same optical power in the optical power hysteresis depends on the optical power. For this reason, if the overshoot amount, which is an excess amount of voltage for reducing the influence of hysteresis, is not set appropriately, a problem may occur. Specifically, when the overshoot amount is too large, the response speed may be decreased. On the other hand, if the overshoot amount is too small, there is a concern that the effect of hysteresis is insufficiently reduced.

In order to avoid this, as shown in FIG. 7, the amount of overshoot is changed for each target optical power, and the influence of hysteresis is reduced efficiently and effectively. Specifically, for example, when the power is changed to an optical power having a relatively small hysteresis amount, such as point O to point A in FIG. 6, the overshoot amount (as shown in FIGS. 7A and 7B). _Reduce the excess amount from V _A1 to V _A2 . On the other hand, when changing the optical power to a relatively large amount of hysteresis, such as point O to point B, the overshoot amount (excess amount from V _B1 to V _{B2 as} shown in FIGS. 7C and 7D). ). In a place where the hysteresis amount is not measured like the optical power at point C in FIG. 6, the overshoot amount is set to 0 as shown in FIGS. 7 (e) and 7 (f).

Thus, in the present embodiment, the voltage excess OS is expressed as OS = V _Hys + ΔV. However, _let V _{Hys be the} amount of hysteresis that is the difference between one voltage and the other voltage that achieves the target optical power in a region with optical power hysteresis, and ΔV be 0 or more. The amount of ΔV may be constant, or may be varied according to the target optical power as in Example 3 described later.

FIG. 8 shows the time change of the optical power when the overshoot amount is too large and the time change when set appropriately. FIGS. 8A and 8B show time changes when the same optical power changes as in FIGS. 7C and 7D are given. FIGS. 8C and 8D show a case where the amount of overshoot is too large when the same optical power change as in FIGS. 8A and 8B is applied. If the overshoot amount as shown in FIG. 8 (c) is not appropriate, time to reach the target optical power t ₂ becomes long, the response speed is seen to decrease as shown in FIG. 8 (d) . On the other hand, when the overshoot amount is appropriate as shown in FIG. 8A, the time t ₁ until reaching the target optical power is appropriate as shown in FIG. 8B.

In this embodiment, the hysteresis amount is stored in a look-up table (LUT) 904 as shown in FIG. The liquid lens control unit 903 determines an overshoot amount and a target voltage with reference to the LUT 904 based on the current optical power and the target optical power in response to the input signal 905 for instructing the target optical power. Thereafter, the control unit 903 sends a signal of the overshoot amount and the target voltage to the voltage source 902, and the voltage source 902 applies a necessary voltage to the liquid lens 901 according to the signal. The configuration of FIG. 9 can also be used in other embodiments.

As described above, in this embodiment, by appropriately setting the overshoot amount in accordance with the magnitude of the hysteresis amount (that is, according to the target optical power), the influence of the hysteresis is reduced without impairing the response speed. Is possible.

(Example 3)
A third embodiment of the present invention will be described with reference to FIG. FIG. 10 shows the hysteresis in the voltage-optical power curve of the liquid lens, as in FIGS. 4 and 6. Here, as the target voltage V _T corresponding to the target optical power phi ₁ of FIG. 10, by decreasing the voltage from V _O considered to control the phi ₁ power. At this time, since the voltage decreasing direction is the direction in which the contact angle becomes the advance angle, the optical power decreases along the curve (1) in FIG. Here, since the target optical power is phi _1, voltage, further other voltage V _TA voltage lower than V _OS beyond one of the voltage V _T corresponding to the optical power phi ₁ on the descending curve of (1) And then up to the target voltage V _T. At this time, it is necessary to lower the voltage to V _TA as a minimum, but in actuality, V _{OS that is} lower than V _TA by ΔV in consideration of changes in hysteresis due to aging of the insulator surface in contact with the two liquids It is desirable to reduce the voltage to By doing so, it becomes possible to reduce the influence of hysteresis more stably.

At this time, if ΔV is larger than necessary, the response speed is lowered. Therefore, ΔV is preferably in the following range.
0.01V _diff <ΔV <0.3V _diff
Here, V _diff = V _S + V _Hys . V _S is the difference between the voltages V _O and V _T before the change, and V _Hys is the difference between V _T and V _TA . Below the lower limit of this equation, it becomes difficult to reduce the influence of hysteresis when the amount of hysteresis changes due to changes over time. If the upper limit is exceeded, the effect of hysteresis can be reduced, but the response speed is reduced.

It is further desirable that the above equation satisfies 0.01 V _diff <ΔV <0.1 V _diff . By doing so, it is possible to further reduce the decrease in response speed. As described above, in this embodiment, the amount of ΔV is also changed according to the target optical power, and the overshoot amount is set more appropriately, so that the effect of hysteresis is more effectively reduced without impairing the response speed. Will be possible. In the above formula, a numerical value such as 0.01 is experimentally confirmed to be a practically appropriate range within this range. Further, in the present embodiment adopts the concept of changing linearly proportional to ΔV to V _diff, in some cases, or varied linearly proportional to ΔV like V _S and V _Hys, V _diff It is also possible to change in proportion to 1/2 power, square power, etc.

By the way, in the above embodiment, the voltage decreasing direction (optical power decreasing direction) is the direction in which the contact angle becomes the advance angle, but the present invention is not limited to this. Depending on the relationship between the refractive indexes of the non-conductive liquid such as the non-electrolytic solution and the conductive liquid such as the electrolytic solution, the direction in which the contact angle becomes the advance angle may be the voltage increase direction. Also, depending on the relationship between the non-conductive liquid and the interfacial energy between the conductive liquid and the insulator, etc., the combination of the direction in which the contact angle becomes the advancing / retreating angle and the direction in which the voltage increases / decreases differs. It may be the opposite. In the above embodiments, the voltage applied to the conductive liquid is handled as a DC voltage, but an AC voltage may be used. In the case of alternating current, the effective voltage of alternating current may be handled as being equivalent to the direct current voltage value.

201 ... conductive liquid, 202 ... non-conductive liquid, 203 ... housing, 206 ... electrode, 207 ... insulator, 208 ... liquid interface, 209 ... liquid lens 902 ... Voltage source 903 ... Control means

Claims (7)

A housing, a non-conductive liquid and a conductive liquid sealed inside the housing, an electrode for applying a voltage to the conductive liquid, and a control means for controlling the applied voltage. , An electrowetting type liquid lens that changes the optical power by changing the shape of the interface between the two liquids by controlling the control means,
The control means includes
When the interface shape is brought to the target optical power shape in a region where there is optical power hysteresis, one of the two kinds of liquids having a small interface energy with the inner wall of the housing that is in contact with the end of the interface. When the contact angle of one of the liquids becomes a forward angle when the voltage is changed in a direction to achieve the target optical power so that the contact angle of the liquid always becomes a receding angle and reaches the target contact angle The voltage is changed to exceed the voltage of one of the two voltages that realizes the target optical power, and further exceeds the voltage of the other of the two voltages, and then is reversed to bring the voltage to the one of the two voltages. ,
A liquid lens characterized by that. The liquid lens according to claim 1, wherein an excess amount from the one voltage to the voltage exceeding the other voltage is varied according to the target optical power. When the excess amount OS is V _Hys is the difference between the one voltage and the other voltage that achieves the target optical power in a region where the optical power has hysteresis, and ΔV is an amount of 0 or more. OS = V _Hys + ΔV, The liquid lens according to claim 2. Said [Delta] V is a V _diff and _{V diff = V S + V Hys} , when the difference between the V _S voltage before the change and the other voltage is 0.01V _diff <ΔV <0.3V _diff The liquid lens according to claim 3. The inner wall of the housing is made of an insulator,
5. The liquid lens according to claim 1, wherein the insulator has a lower interface energy with the non-conductive liquid than the conductive liquid. 6. The inner wall of the housing is made of an insulator, and the insulator is covered with a thin film whose surface in contact with the two liquids has a smaller interface energy with the non-conductive liquid than the conductive liquid. The liquid lens according to claim 1, wherein the liquid lens is a liquid lens. A housing, a non-conductive liquid and a conductive liquid sealed inside the housing, and an electrode for applying a voltage to the conductive liquid, and having an interface shape formed by the two liquids An electrowetting type liquid lens control method that changes optical power by changing,
When the interface shape is brought to the target optical power shape in a region where there is optical power hysteresis, one of the two kinds of liquids having a small interface energy with the inner wall of the housing that is in contact with the end of the interface. When the voltage applied between the electrode and the conductive liquid is changed in a direction to achieve the target optical power so that the contact angle of the liquid always becomes a receding angle and reaches the target contact angle, If the contact angle of the liquid is an advancing angle, the voltage is changed so as to exceed one of the two voltages that achieve the target optical power and further exceed the other of the two voltages. And then reverse to bring it to the one voltage,
A control method for a liquid lens.

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