CA1189557A - Light control device - Google Patents

Abstract

Abstract:

An electrically operated light control device includes an electrostatically actuated element that comprises, in superposition, a stationary electrode, an electrode move-able between a position overlying the stationary electrode and a position removed from the stationary electrode, and a non-conductive element between the electrodes for keeping the electrodes electrically separated. The moveable electrode is in the form of a sheet of flexible material having one end fixed with respect to the stationary electrode and the opposite end free with respect to the stationary electrode. This sheet of flexible material has a permanent stress which biases the sheet into a curl away from the stationary electrode. The element is characterized by the fact that the non-conductive element comprises a sheet of electret material that is capable of retaining an electrostatic charge to provide an electrostatic force to act on the moveable electrode. The element is actuated by the resultant sum of the electrostatic force provided by the electret material and the electrostatic force created when an electrical potential is applied to the electrodes.

Description

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Liqht control device Technical Field This invention relates to an electrically operated light control device, the application being a division o~ appli-cation Serial No. 330,030 filed June 18, 1979.
Back~round Art The background art contains various examples of electro-static display elements. One type of device, such as is sllown in U.S. 1,9~4,683 and 3,553,364, includes light valves having flaps extending parallel with the approaching light, with each flap electrostatically divertable to an oblique angle across the light path ~or either a transmissive or re1ective display. U.S. 3,397,997 discloses an electrode which is electrostatically wrapped about a curved fixed electrode to affect the light reflective character of the fixed electrode.
Further prior art such as is described in ELECTRONICS, 7 December 1970, pp. 78-83 and I.B.M. Technical Disclosure Bulletin, Vol. 13, No. 3, August 1970, uses an electron gun to electrostatically charge selected portions of a deformable material and thereby alter its light transmissive or reflective properties.
Summary of the Invention In one aspect the invention consists of an electrically operated light control device including an electrostatically actuated element, said element comprising, in superposition, a stationary electrode, an electrode moveable between a position overlying the stationary electrode and a position removed from the stationary electrode, and non-conductive means between the electrodes for keeping the electrodes electrically separated; the moveable electrode being in the q~

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form of a sheet of flexible material having one end fixed with respect to the stationary electrode and the opposite end free with respect to the stationary electrode, the sheet of flexible material having a permanent stress which biases the sheet into a curl away from the stationary electrode; the element being characterized by the non-conductive means com-prising a sheet of electret material which is capable of retaining an electrostatic charge to provide an electrostatic force to act upon the moveable electrode, the element being actuated by the resultant sum of the electrostatic force provided by the electret material and the electrostatic force created when an electrical potential is applied to the electrodes.
In another aspect the invention consists of an electrically operated light control device including an electrostatically actuated element, said element comprising, in superposition, a stationary electrode, an electrode move-able between a position overlying the stationary electrode and a position removed from the stationary electrode, and non-conductive means between the electrodes for keeping the electrodes electrically separated; the moveable electrode being in the form of a sheet of flexible material having one end fixed with respect to the stationary electrode and the opposite end free with respect to the stationary electrode, the sheet of flexible material having a permanent stress which biases the sheet into a curl away from the stationary electrode; the element being characterized by a layer of liquid between the electrodes providing an attractive force when the moveable electrode overlies the stationary electrode which force opposes a portion of the curl bias of the move-able electrode, and the element being actuated by the resultant sum of the attractive force provided by the liquid and the electrostatic force created when an electrical potential is applied to the electrodes.
Brief Description of Drawings Figure 1 is a perspective view of an embodiment of a display element.

~ ~9~'7 Figure 2 is a perspective view of another embodiment of a display element.
Figure 3 is a perspective view of a light reflective embodiment.
Figure 4 is a perspective view of a light transmissive embodiment.
Figure 5 is a schematic view of another embodiment.
Figure 6 is a perspective, exploded view illustrating another embodiment of a stationary electrode in a display element.
Figure 7 is a perspective, exploded view illustrating another embodiment of a stationary electrode in a display element.
Figure 8 is a perspective, exploded view illustrating 15 another embodiment of a stationary electrode in a display element.
Figure 10 is a schematic view of an embodiment of a display comprising an array of display elements.
Figure 11 is a plan view of various embodiments of stationary electrodes.
Figure 12 is a perspective view of an embodiment used to create grey scales and primary color scales.

Best Mode for Carrying Out the Invention As shown in the drawings, the display elements of the invention can be of several configurations which can be incorporated into varied display arrays.
Figure 1 depicts a display element 10 of the invention having a stationary electrode 12, to which is attached a layer of insulative material l4. A moveable ~(~ electrode 16 has a portion 18 adjacent to one end fixed with respect to the stationary electrode 12 and a free end 2n controllable between a curled position removed from the stationary electrode 12 and an uncurled position adjacent to the stationary electrode 12. The moveable electrode l6 is ~P~955~7 electcostatically controlled by means of a source of electrical potential V and a control switch 24. When the potential V is connected across the electrodes 12 and 16, the resulting electrostatic forces cause the moveable electrode 16 to uncucl into a position oveelying the stationary electrode 12, as shown by dotted lines 26. When the potential V is dlsconnected and the electrodes connected together, the electrostatic forces decrease and the restitution force of the moveable electrode 16 causes the body portion 20 to curl to its relaxed, curled position removed from the stationary electrode 12.
Figure 2 shows an embodiment in which the insulative layer 14 is attached to the inner surface of the moveable electrode.
The display element 10 of Figure 1 can be used for either a light reflective or light transmissive display device. Use in a reflective device is illustrated in Figure 3. As seen in Figuee 3, when the moveable electrode 16 is curled away ~rom the stationary electrode 12, the viewer sees the light reflected from the area 32, consisting of reflections off the exposed stationary electrode 12 and insulative layer 14, as well as off the exposed portion of inner sur~ace 34 of the moveable electrode 16. When the moveable electrode 16 is flattened to a position overlying the stationary electrode, as shown by dotted lines 6, the viewer sees only the light reflected from outer surface 36 of the moveable electrode.
As a light reflective device, thc element can be used in a variety of displays such as in a black and white or a multicolor array. For example, in a black and white display the insulative material layer 14 can be black, the inner surface 34 of the moveable electrode can be black, and the outer surface 36 of tlle moveable electrode white. In the curled state, no light is re1ected and area 32 appearS to be black. When the moveable electrode is uncurled or flattened, ~95~7 light is reflected ~rom the white sur~ace. Similarly, in a colored display the exposed surfaces in one state o~ the device ean be of one color with the exposed surfaees in the other state of another color.
The element ean also be part of a light transmissive deviee. Use as sueh a device is shown in Figure 4 ~ith the light souree 40 on the opposite side of the deviee from the viewer who sees the transmitted light emanating ~rom area 44.
~s a light gate device, light is transmitted through a translueent stationacy eleetcode 12 and tcanslueent insulative layer 14. In the flattened condition, an opaque moveable eleetcode 16 bloeks the light. In a multieolor display, the eurled condition reveals a color of light transmitted through either a clear or colored stationary eleetrode ~2 and insulative layer 14. The moveable eleetrode 16 ean be opaque, to eonstitute a eolor light gate deviee, or translucent and eolored to effeet a ehange of eolor of the teansmitted light.
In addition, other embodiments of deviees ean be eonstrueted for other light eonditions or display effeets.
For example, a eombination refleetive and transmissive display ean be eonstrueted for use in varying light eonditions by use of a translucent reflective eoating on the surfaees of the electrodes 12 and 16 wheceby the deviee ean be used in a refleetive mode when the light souree 40 is off, or in a transmissive mode when the light source is on.
In eonstcucting operating embodiments of the invention, several operating variables are to be considered in seleeting the materials for use in the electrodes, the insulative layer, and the further eomponents of a display device, such as the substrate. With respect to the moveable electrode, the material used must be capable of being eurled to the correct eurl size for the particular use. Other considerations include the mass since a lower mass moveable electrode will have a lower inertia and respond more quickly g5~i~

to a given electrostatic force. A further consideration is the stiffness of the material which affects the force needed to bend the material to effect flattening.
In general, a moveable electrode can be formed either of a metal or of a plastic laminate containing a conductive material. In one embodiment, beryllium copper 25 (BeCu 25) foil, 0.0001 inches thick, is curled by wrapping it about a 0.25 inch mandrel and heat treating it to set the curl. The resulting curled sheet is chemically etched into an array of 10 0.5 inch by 0.5 inch moveable electrodes. Other materials for use in opaque moveable electrodes include tin-alloys and aluminum. Materials for use in translucent electrodes include a translucent base material with a translucent deposited thin conductive layer such as deposited gold, 15 indium oxide, or tin oxide. The materials for moveable electrodes can be provided with the curl by heat forming or can be a laminate of two or more plies bonded together while stressed to form a curl.
Stationary electrodes can be formed of a conductive 20 material such as metal foil for a reflective display, or of a translucent layer of indium oxide or tin oxide on a translucent substrate in a transmissive display.
The insulative layer 14 can also be chosen from many materials. Materials having high dielectric constants are 25 preferred. A polymeric film may be used. One problem encountered in the use of certain materials arises in the temporary retention of a residual electrical charge or polarization after an electric potential has been removed.
For example, it has been found that in the embodiment of ~0 Figure 1, the application of sufficient potential to cause the moveable electrode to flatten to a position adjacent to the stationary electrode, may induce a temporary residual polarization in the dielectric insulative layer su~ficient to maintain the moveable electrode flattened for a time after 95 the electric potential has been removed or decreased.
Certain materials do not exhibit this e~ect or the effect is ~89S57 small. Cellulose, polypropylene and plYethylene are examples of such materials. ~nother solution is the use of dielectcics which allow the residual charge to leak off. As another solution to this residual polarization problem, a preferred embodiment o~ this invesltion uses an electret formed of material such as polyethylene terephthalate (MYL~R)*
as the insulative layer. ~n electret material maintains a celatively constant degree of residual polarization unaffectd by the further application of an electric potential across it. Since the residual charge is a constant, it can be accurately accounted for in the design of the element. As an illustration of the use of an electret in an element as shown in Figure 1, the insulative layer 14 is the electret. Since the electret provides a portion of the attractive force to flatten the moveable electrode, the electric potential V can be of a lower potential to add a further electrostatic force sufficient to cause the moveable electrode 16 to uncurl to a position adjacent to the stationary electrode 12. The removal of the electric potential V results in the recurling ' return of the moveable electrode to its original curled position since the force provided by the electret is less than the restorative force of the curl bias.
A further embodiment of the invention is illustrated in Figure 5 where a biasing power source 54 and an incremental drive power source 56 are used to control the moveable electrode 16. The biasing power source 54, set at V
volts, is at a voltage potential just below that needed to effect the uncurling of the moveable eletrode 16. The incremental drive source 56, set at ~V volts, adds sufficient further voltage potential when added to the bias potential to causc the moveable electrode to uncurl and overlie the Stationary electrode 12. The use of a bias voltage continually applicd across the electrode, requiring only tl-e switching of the ~v incremcntal voltage to effect a change o~
position of the moveable elec~rode, can be highly advantageous in a display system. For example, a high * Trade ~1ark 55~

voltage power supply can provide the bias voltage for all elements in the array. Only a small incremental potential is necessary to control the elements which the attendant cost savings resulting from the ability to use low vo]tage switching hardware.
This biasing effect and results are also obtained by the use of an electret as the insulative layer since the charge of the electret serves the same biasing function as bias power source 54. Therefore, only the incremental drive 10 voltage ~V is needed to actuate the moveable electeode.
The advantages of this biasing effect are also realizable when a liquid layer is present between the moveable and stationary electrodes. Surface tension forces of the liquid provide a portion of the attractive force 15 acting on the moveable electrode. The liquid thus acts in a manner similar to a bias voltage. Suitable liquids include silicone oil and petroleum oils and derivatives.
The embodiment of Figure 5 can also be operated with an excess of bias voltage sufficient by itself to maintain g0 the moveable electrode in a flattened position adjacent to the stationary electrode. In this embodiment, the inceemental drive voltage 56 is of opposite polarity, sufficient to decrease the electrostatic charge to a level allowing the moveable electrode to recurl to a position 25 removed from the stationary electrode. This embodiment can also take the form of a sufficiently charged electret insulative layer with the incremental drive source 56 of reverse polarity. This embodiment is advantageous in that in the quiescent state with no ~V potential applied, the ~0 moveable electrode is adjacent to the stationary electrode, rendering the moveable electrode less subject to accidental physical damage.
Figure 6 illustrates a display element 60 having a stationary electrode h2 with a plurality of discrete ~5 conductive regions 66-68, insulative layer 6~, and moveable electrode 65. This embodiment provides independently 55~
_9_ addressable conductive portions of the stationary electrode 62 to facilitate particular controi of the display element 60 for use in a display array. In the illustrated embodiment of a three region stationary electrode, for example, an 6 electrical potential can be applied indepelldently to the X
electrode region 66, to the Y electrode region 67, or to the hold-down electrode region 68. Only when the X, Y, and hold-down regions are energized, will the moveable electrode 65 fully flatten. Once fully flattened, the hold-down 10 electrode region 68, when energized, provides sufficient electrostatic force to latch the moveable electrode 65 in its flattened state regardless of whether the X or Y electrode regions are energized. To release the electrode 65 from its flattened state, all of the hold-down regions 68 and the X
15 and Y electrode regions must be de-energized.
~ When only the X electrode region is energized, that is the conductive region 66 proximate the fixed edge portion 61 of the moveable electrode 65, the moveable electrode will : partially uncurl. If, in addition to energization of the X
electrode region 66, the Y electrode region 67 is also energized, the moveable electrode 65 will further uncurl.
Energization of hold-down electrode region 68, the conductive region most remote from the fixed edge portion 61, will complete the uncurling of moveable electrode 65 to a fully ~5 flattened condition.
It should be noted that uncurling can not be effected by any conductive segment which is not immediately adjacent to the curled end portion of the moveable electrode.
Therefore, the Y electrode region 67 cannot cause uncurling S0 until the X electrode region 66 has been energized to cause partial uncurling.
In order that the moveable electrode be attracted by the electrostatic field of a particular stationary electrode region, the moveable electrode must sufficiently proximate to S5 that region. This proximity can be achieved by causing the ~8~7 moveable electrode to partially overlie the particular region. One manner of achieving the condition o partial overlying is to shape the stationary regions such that the demarcations between regions are not parallel to the curl axis of the moveable electrode. A chevron shape of the regions provides demarcations which are not parallel to the curl axis such that the moveable electrode partially overlies the adjacent electrode region and thereby is located within the domain of the electrostatic field of that adjacent region 1~ when it is subsequently energized.
The operation of the X, Y, hold-down configuration of Figure 6 is illustrated in Figure 7 where drive voltage V can be applied between the moveable electrode 65 and any or all of the regions of the stationary electrode, X region 66, Y
15 region 67, or hold--down region 68, by means of switches 70, 71 or 72 respectively. When switch 70 activates the X region 66, the moveable electrode 65 uncurls partially; activation of the Y region 67 provides further uncurling of the moveable electrode 65. Switch 72 activates the hold-down region 68 to ~ fùlly flatten and latch the moveable electrode 65 even if the switches 70 and 71 subsequently deactivate the X and Y
regions 66 and 67.
Control of display elements such as are illustrated in Figures 6 and 7 having segmented stationary electrodes ~5 provides for use of the elements in a display array in which each element of the array can be selectively actuated without affecting the state of the remainder of the elements in the array. Such a display array is illustrated in Figure 8 in which a plurality of display elements 81, 82, 83 and 84 are assembled in columns and rows to form a display array 80.
The moveable electrodes (not shown) are connected via a common lead 90 to one side of a source of electrical poten-tial 110. Each stationary electrode has an X region, a Y
region, and a hold-down region ~ 11 X regions in the first ~r~ column are connected via a common lead to switch Xl, and all X regions in the second column are connected to switch X2.
Similarly, all Y regions in the ~irst row are connected ~39557 to switch Yl and all Y regions in the second row are connected to switch Y2. All hold-down regions are connected in common to switch H. Thereby, each element 81-84 can be selectively actuated by selection of the appropriate switches, and latched down by the closure of hold-down switch ~1 .
As an example of the operation of the array in Figure 8, in order to actuate element 83, hold-down switch H and switch Xl are closed to connect the hold-down and the X
lU electrode regions in the first column to the potential 110, and switch Y2 is closed to connect the Y electrode regions in the second row to the potential 110. Since the element 83 i5 the only element in the array with both its X and Y electrode regions energized, it alone is caused to fully uncurl.
Hold-down switch H will latch element ~33 in the flattened state when the X and Y electrode regions are subsequently deactivated. The fact that a moveable electrode can be affected only by a stationary electrode region immediately adjacent the curled portion is of great value in simplifying the circuitry required to control an array of elements.
The display elements illustrated in Figures 6 and 7 have two independently controllable stationary electrode conductive regions in addition to the hold-down region.
Increasing the number of independently controllable con-25 ductive regions in each element permits a significantincrease in the number of elements in an array without a concomitant increase in the number of switch devices required. Specifically, in order to independently address an element in an array having a number of elements N, each ~ element having a number of independently controllable conductive regions d, the number of switch elements .equired is S = d ~

For example, ~or an array of N = 390,625 individu~lly controlled picture elements, a single conductive region per s~

element would require 390,625 switches, or one switch per element. I~ each element has two conductive regions, such as in Figure 8, 1250 switches are needed to individually control and address each element. If the elements have four regions, on]y 100 switches are required. The switch devices and all other switch devices referred to in this specification can be mechanical or electronic switches including semiconductor elements which apply one of two potentials to the element to be controlled.
Figure 9 illustrates an embodiment of an element wherein moveable electrode 120 can be selectively controlled - to change its state from either a flattened to a curled position, or from a curled to a flattened position when in a display array. The Figure 9 element has a stationary electrode formed of an X region 124, Y region 126 and two hold-down regions, 122 and 128. Hold-down region 122 (proximate the fixed edge of the moveable electrode~ is partially beneath the moveable electrode 120 when it is fully curled. The X and Y regions, 124 and 126 respectively, are positioned between the hold-down regions. In other words, the conductive regions are in a series progressing li~early from the fixed edge.
In operation, in order to selectively cause the moveable electrode 120 to change its state from a curled to a

2~ fully flattened condition, nold-down regions 122 and 128 are energized, as well as X regions 124 and Y regions 126, in the manner explained in reference to Figure 8. In this configuration, the hold-down region 122 lying underneath the moveable electrode 120 in its fully curled state, must be activated to partially uncurl the moveable electrode 120 to a position partially overlying X region 124 to enable the X
rcgion to cause ~urther uncurling upon ~ctivation. When all regions 122, 124, 126 and 128 are activated, the electrode 120 will ~ully ~latten.
In order to selcctively cause the moveable electrode 120 to qo ~rom a ~ully ~lattencd condition to a ~ully curled ~L~8955~7 condition without affecting other display elements in an array, the following operation is performed. At the start, only those moveable electrodes which have their hold-down portions energized are in a fully flattened condition. To 5 selectively release a moveable electrode first all Y regions in the array are deactivated. All X regions are then activated. The moveable electrodes thereby partially curl to a position above the Y Eegion. Deactivation of the X and Y
regions in the column and eow of the desired element will 10 thereby release that specific moveable electrode and cause that electrode to fully curl. The hold-down regions can then be reactivated to secure the remaining flattened electrodes.
The response speed of an element is related to the size of the element. Sub-dividing an element into a 15 plurality will promote increased response speed. Therefore, the element at a particular address in an array advantage-ously may be subdivided into two or more elements electrically connected in common.
Figure 10 illustrates the further use of a biasing ~ power source such as described with reference to Figure 5.
In the display array 240 of Figure 10, four display elements comprise moveable electrodes 242, 243, 244 and 245 and corresponding stationary electrodes having hold-down region 246, Xl row region 248, X2 row regions 250, Yl column regions ~S 252, and Y2 column regions 254. Bias voltage Vl is con-tinually applied to the electrodes of all elements. Further bias voltage V2 can be selectively applied in series with V
via switch 247. Incremental drive voltage V3 can be selectively applied in series with Vl and V2. In order to ~ cause a curled moveable electrode to change state, all three potentials Vl, V2 and V3 must be applied. To release a flattened electrode, the V2 and V3 potentials must be removed. The Vl potential therefore represents a relatively large bias voltage which can be applied across all elements.
35 The V2 potential reflects the residual polarization of the insulative layer in each element. The V3 potential is of an incremental level to drive an element already biased by V
and V2. The level of V3 potential is set to allow for the inherent deviations in the amoullt of potential required to cause a change in state in various individual display elements stemming from manufacturing variations in such element parameters as insulative layer thickness, dielectric characteristics and curl diameter. It has been found that the V3 potential may be in the order of ten percent of the V
+ V2 level. In the biasing configuration o Figure 10, the curls can be controlled to selectively cause their change of state from a curled to an uncurled position by control of V3 alone, once the biasing voltages Vl and V2 have been applied.
The control switches required in a display array can be operated at the lower V3 voltage, with fewer switches needed ~5 at the higher Vl or V2 voltages, with attendant savings in manufacturing cost.
The present invention can be used to create a digitally controlled two color, or black and white, display with desired gray scales, or a color display with desired ~ intensities of the three primary colors. Various procedures for creating the gray scale and color shades are discussed here. Figure 11 shows a plan view of element arrangements to create gray scales. Figure lla shows the use of curls 148 which have square or rectangular shapes in the plan view.
~5 Figures llb and llc, respectively, show the use of triangular shapes. To create an 80% black qray scale, 20% of the elements are curled. When the curled position represents white, 60~ of the elements are curled to create ao% black gray scale. In all these examples, the dotted lines, 144 3n represent the curl axes of the elements and the straight solid lines represent the element perimeters. The arrows 146 show the curl direction.
Various shade scales can be accomplished by grouping plural elements. The number of shade combinations available 95 in a qroup is S = 2N where N is the number of differently shaded elements. riluS, four elements will provide l6 shade 395~7 - 1 s -combinations, ranging from no actuation to all elements fully actuated.
Another procedure Eor the creation of different two color scales and primary color shades is through the control of the duty (up and down) cycles of elements. Therefore, a black and white e]ement, (where white is the curled position) when cycled faster than the ability of the eye to perceive the movement, would appear to be the percentage of the duty cycle devoted to the coiled up state vs. the flat (black) state. Where S is the number of different shade combinations achieved from N different discrete and additive duty cycles, then S = 2. Therefore, for four different discrete and additive duty cycles 16 different shades can be created.
Figure 12 shows another way to make use of the peesent invention to create gray scales and primary color scales shade. Separately driven X and Y electrode regions 150, 152 pull the selected moveable electrode 158 to the first hold-down electrode 154 representing a gray or shade scale. Additional separately driven regions X2 and X3, 156 and 157 are used to pull the selected electrode to the second hold-down electrode region 154 to create another gray or shade scale. Additional X, Y and hold-down electrode regions to create additional selectable shades or gray scales can be provided.

Claims (4)

Claims:

1. An electrically operated light control device including an electrostatically actuated element, said element comprising, in superposition, a stationary electrode, an electrode moveable between a position overlying the stationary electrode and a position removed from the stationary electrode, and non-conductive means between the electrodes for keeping the electrodes electrically separated;
the moveable electrode being in the form of a sheet of flexible material having one end fixed with respect to the stationary electrode and the opposite end free with respect to the stationary electrode, the sheet of flexible material having a permanent stress which biases the sheet into a curl away from the stationary electrode;
the element being characterized by the non-conductive means comprising a sheet of electret material which is capable of retaining an electrostatic charge to provide an electrostatic force to act upon the moveable electrode, the element being actuated by the resultant sum of the electrostatic force provided by the electret material and the electrostatic force created when an electrical potential is applied to the electrodes.

2. The device of claim 1 wherein the electrostatic force provided by the electret material is sufficient to overcome the permanent stress to cause the moveable electrode to overlie the stationary electrode in the absence of an electrical potential applied to said electrodes, and the potential, when applied, reduces the electrostatic force provided by the electret material.

3. The device of claim 1 wherein the electrostatic force provided by the electret material is insufficient to overcome the permanent stress, and when the electrical potential is applied to the electrodes the potential creates an electrostatic force acting in the same direction to that provided by the electret material, the resultant sum of the electrostatic forces being sufficient to overcome the permanent stress to cause the moveable electrode to overlie the stationary electrode.

4. An electrically operated light control device including an electrostatically actuated element, said element comprising, in superposition, a stationary electrode, an electrode moveable between a position overlying the stationary electrode and a position removed from the stationary electrode, and non-conductive means between the electrodes for keeping the electrodes electrically separated;
the moveable electrode being in the form of a sheet of flexible material having one end fixed with respect to the stationary electrode and the opposite end free with respect to the stationary electrode, the sheet of flexible material having a permanent stress which biases the sheet into a curl away from the stationary electrode;
the element being characterized by a layer of liquid between the electrodes providing an attractive force when the moveable electrode overlies the stationary electrode which force opposes a portion of the curl bias of the moveable electrode, and the element being actuated by the resultant sum of the attractive force provided by the liquid and the electrostatic force created when an electrical potential is applied to the electrodes.