FIELD OF THE INVENTION

The present application relates to a power-efficient and low-cost method and device for controlling an AC voltage applied to a suspended particle device (SPD). The present application also relates to methods and devices for shock prevention, detecting forced entry, and reducing the manufacturing costs of SPD film.

BACKGROUND OF THE INVENTION

Light valves have been in use for more than sixty years for the modulation of light. As used herein, a light valve is defined as a cell formed of two walls that are spaced apart by a small distance, at least one wall being transparent, the walls having electrodes thereon, usually in the form of transparent, electrically conductive coatings. The cell contains a light-modulating element (sometimes herein referred to as an activatable material), which may be either a liquid suspension of particles, or a plastic film in which droplets of a liquid suspension of particles are distributed.

The liquid suspension (sometimes herein referred to as a liquid light valve suspension or a light valve suspension) comprises small, anisometrically shaped particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension assume random positions due to Brownian movement, and hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles, and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state. Light valves of the type described herein are also known as suspended particle devices or SPDs. More generally, the term suspended particle device, as used herein, refers to any device in which suspended particles align to allow light to pass through the device when an electric field is applied.

Light valves have been proposed for use in numerous applications including, e.g., alphanumeric and graphic displays; television displays; filters for lamps, cameras, optical fibers, and windows, sunroofs, sunvisors, eyeglasses, goggles and mirrors and the like, to control the amount of light passing therethrough or reflected therefrom as the case may be. As used herein the term light generally refers to visible electromagnetic radiation, but where applicable, light can also comprise other types of electromagnetic radiation such as, but not limited to, infrared radiation and ultraviolet radiation.

For many applications, as would be well understood in the art it is preferable for the activatable material, i.e., the light modulating element, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film, in which droplets of liquid suspension are distributed, is preferable to a liquid suspension alone because hydrostatic pressure effects, e.g., bulging, associated with a high column of liquid suspension, can be avoided through use of a film, and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that in a plastic film, the particles are generally present only within very small droplets, and hence do not noticeably agglomerate when the film is repeatedly activated with a voltage.

As used herein, the terms SPD film or light valve film mean at least one film or sheet comprising a suspension of particles used or intended for use by itself or as part of a light valve. The light valve film or SPD film includes either: (a) a suspension of particles dispersed throughout a continuous liquid phase enclosed within one or more rigid or flexible solid films or sheets, or (b) a discontinuous phase of a liquid comprising dispersed particles, the discontinuous phase being dispersed throughout a continuous phase of a rigid or flexible solid film or sheet. The light valve film or SPD film may also include one or more other layers such as, without limitation, a film, coating or sheet, or combination thereof, which may provide the light valve film or SPD film with (1) scratch resistance (2) protection from ultraviolet radiation (3) reflection of infrared energy, and/or (4) electrical conductivity for transmitting an applied electric or magnetic field to the activatable material.

U.S. Pat. No. 5,409,734 illustrates an example of a type of light valve film that is formed by phase separation from a homogeneous solution. Light valve films may be made by cross-linking emulsions such as those described in U.S. Pat. Nos. 5,463,491 and 5,463,492, both of which are assigned to the assignee of the present invention.

The following is a brief description of liquid light valve suspensions known in the art, although the invention is not limited to the use of only such suspensions.

1. Liquid Suspending Media and Stabilizers

A liquid light valve suspension for use with the invention may be any liquid light valve suspension known in the art and may be formulated according to techniques well known to one skilled in the art. The term liquid light valve suspension, as used herein, means a liquid suspending medium in which a plurality of small particles is dispersed. The liquid suspending medium includes one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer, which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.

Liquid light valve suspensions useful in the present invention may include any of the liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media known in the art which are useful herein include, but are not limited to the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175 and 4,407,565. In general, at least one of the liquid suspending medium and the polymeric stabilizer.dissolved therein is chosen in a manner known in the art so as to maintain the suspended particles in gravitational equilibrium.

The polymeric stabilizer, when employed, can be a single solid polymer that bonds to the surface of the particles, but which also dissolves in the non-aqueous liquid or liquids of the liquid suspending medium. Alternatively, two or more solid polymeric stabilizers may serve as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, provides a plain surface coating for the particles, after which they are re-coated with one or more additional types of solid polymeric stabilizer that bond to or associate with the first type of solid polymeric stabilizer and which also dissolve in the liquid suspending medium to provide dispersion and steric protection for the particles. also, liquid polymeric stabilizers may be used to advantage, especially in SPD light valve films, as described in U.S. Pat. No. 5,463,492.

2. Particles

Inorganic and organic particles may be incorporated into a light valve suspension useful in forming a switchable suspended particle device. Such particles may be either light-absorbing or light-reflecting in the visible portion of the electromagnetic spectrum. For some particular applications the particles can be reflective at infrared wavelengths.

Conventional SPD light valves have generally employed polyhalide particles of colloidal size, that is the particles generally have a largest dimension averaging about 1 micron or less. As used herein, the term colloidal, when referring to particle size, shall have the meaning given in the preceding sentence. Preferably, most polyhalide or other particles used or intended for use in an SPD light valve suspension used in accordance with the invention will have a largest dimension which averages less than one-half of the wavelength of blue light, i.e., less than 2000 Angstroms, to keep light scatter extremely low. As used herein, the term anisometric, which refers to particle shape, means that at least one dimension is larger than another. Typically, anisometric particles (sometimes referred to as particles which are anisometrically shaped), are desirable in an SPD light valve suspension so that the particles will block less light when the suspension is activated than when it is unactivated. For some suspensions the reverse is true, however. Desirable anisometric shapes for the particles include, without limitation thereto, particles shaped like rods, cylinders, plates, flakes, needles, blades, prisms, and other shapes known in the art.

A detailed review of prior art polyhalide particles is found in The Optical Properties and Structure of Polyiodides by D. A. Godina and G. P. Faerman, published in The Journal of General Chemistry, U.S.S.R. Vol. 20, pp. 1005-1016 (1950).

Herapathite, for example, is defined as a quinine bisulfate polyiodide, and its formula is given under the heading quinine iodsulfate as 4C20H24N2O2.3H2SO4.2HI.I4.6H2O in The Merck Index, 10th Ed. (Merck & Co., Inc., Rahway, N.J.). In polyiodide compounds, the iodide anion is thought to form chains and the compounds are strong light polarizers. See U.S. Pat. No. 4,877,313 and Teitelbaum et al. JACS 100 (1978), pp. 3215-3217. The term polyhalide is used herein to mean a compound such as a polyiodide, but wherein at least some of the iodide anion may be replaced by another halide anion. More recently, improved polyhalide particles for use in light valves have been proposed in U.S. Pat. Nos. 4,877,313, 5,002,701, 5,093,041 and 5,516,463. These polyhalide particles are formed by reacting organic compounds, usually containing nitrogen, with elemental iodine and a hydrohalide acid or an ammonium halide, alkali metal halide or alkaline earth metal halide.

For some applications, however, it may be desirable to use non-polyhalide particles in light valve suspensions and films, especially where the stability of the material composing the particles is known to be excellent.

Regardless of the type of suspended particle device used, it is necessary to have a method and/or means of producing and varying the AC voltage applied to the suspended particle device, which may be referred to as an SPD load, from 0V to a maximum voltage that is acceptable for the specific SPD application. For the purposes of the present disclosure the term SPD load includes SPD films, SPD light valves, and all other SPD products that rely on the application of an electric field to control the orientation of suspended particles. Where the SPD load utilizes an SPD film, the voltage that produces maximum light transmission in the SPD load is a function of SPD film thickness and other properties. Since the light transmission of the SPD load is a nonlinear function of voltage, i.e., increasing rapidly at lower voltages and slowly at high voltages, a design compromise can be made by defining a maximum acceptable voltage which provides a sufficiently clear state of the SPD load, currently in the 30 to 60 V rms region. In this discussion, 60 V rms will be used as the AC voltage that produces an acceptable clear state with the understanding the newer SPD films may be developed that produce an almost clear state with less than 30 V rms. Although providing a maximum voltage of 0 to 60 V rms is suitable for most SPD loads, the SPD load current shows a large variation because of all the possible configurations and sizes of SPD loads. For instance, a single SPD window can vary in size from as little as 1 square foot to as much as 32 square feet or more. In addition, multiple panels of 8 ft4 ft windows or larger can aggregate hundreds or even thousands of square feet. For these larger SPD loads, there are advantages in generating the AC voltage for the SPD loads, which will be discussed in further detail below. Furthermore, the busses (also known as bus bars) through which electricity is supplied to the SPD loads may be optimized to reduce their manufacturing costs. All of these improvements contribute to a highly efficient and minimum cost system for controlling voltages across SPD loads.

Unless otherwise indicated, the following will be assumed throughout this discussion:

Voltage for almost clear state60 V rms at 60 Hz

Capacitance per square foot40 nF (of the SPD film)

Resistance per square350 ohms (of the SPD film)

Based on the foregoing assumptions, a voltage controller for an SPD load preferably delivers a load current of 0.905 mA for an SPD load of 1 square foot up to 28.8 mA for an SPD load of 32 square feet. As a conservative approximation, 1 mA per square foot will be used as a guideline. For instance, an office building with 40 panels of 8 ft by 4 ft windows has a film area of 1280 square feet. In such a case, the current demand is approximately 1.28 A at 60 V and 60 Hz to attain an almost clear state for all the windows. Although future developments in SPD film may alter the voltage-current-power requirements of SPD film, the voltage controlling device of the present application will accommodate a wide range of film characteristics.

Currently existing voltage controlling devices commonly use a transformer and/or potentiometer to provide and vary the AC voltage provided to the SPD load. Transformers can be used to step down voltages if desired, while potentiometers allow for variations of voltages through a range of values. Transformers, however, tend to be rather expensive and also reduce efficiency of the voltage controlling device due to coil losses and core losses inherent in the transformer.

One example of a currently existing voltage control device is described in U.S. Pat. No. 5,764,402 which relates to an optical cell control system that includes a first oscillator circuit supplied by a low voltage power source and including a primary winding of an induction coil and a secondary resonant circuit that includes the optical cell and a secondary winding of the induction coil. The second circuit includes the inductance of the secondary winding and the optical cell. The induction coil provides a weak coupling between the primary and secondary windings. The resonant circuit provides a large over-voltage coefficient and good stability.

One problem encountered in traditional voltage controlling devices is that potentiometers provide a continuous range of voltage values between a minimum value and a maximum value such that a slight adjustment to the potentiometer results in a slight change in voltage applied to the SPD load and a corresponding slight increase in the clarity of the SPD load. Since potentiometers are resistive circuit elements, power losses in potentiometers tend to be rather high. In addition, the fine control provided by the potentiometer is unnecessary in an SPD application. The human eye is not able to detect slight variations in clarity of the SPD load, thus the continuous range of voltages provided by the potentiometer which provide for minute increases in clarity of the SPD load are unnecessary. Thus, traditional voltage controlling devices are rather inefficient and provide little observable benefit in controlling clarity of the SPD load.

Safety is also a concern in the voltage controlling device. While SPD loads commonly use relatively small currents, even these small currents could be hazardous to a user who is exposed to them. For example, if an SPD window were to crack, the current conducting layer may be exposed. If one were to contact the exposed current conducting layer and inadvertently provide a path to ground, the individual may receive a shock. Traditional current controlling devices typically utilize a ground fault circuit interrupt (GFCI) which cuts off current to the SPD load if an unintended ground path develops. GFCI's, however tend to be somewhat expensive and may not guard against another shock risk in SPD loads. For example, where an SPD window is pierced by a sharp object, a user may inadvertently provide a path between the two conducting layers directly which may result in a shock to the user. Thus, it would be advantageous to provide a voltage controlling device which avoids these problems at a low cost.

Therefore, it is desirable to provide a voltage controlling method and device that provide efficient and low cost voltage control while avoiding the problems discussed above.

SUMMARY OF THE INVENTION

A voltage controlling device for controlling voltage provided to at least one suspended particle device (SPD) which includes an AC terminal adapted to receive an AC voltage at a specific frequency, a voltage dividing device adapted to divide the AC voltage into a plurality of distinct voltage values within a predetermine range a controller adapted to control the voltage dividing device to provide a selected voltage value based on voltage level information and an SPD terminal adapted to provide the selected voltage value to the SPD device.

The voltage dividing device may include a capacitor array including a plurality of capacitors where each capacitor in the capacitor array has a predetermined capacitance and a switch array, connecting each capacitor of the capacitor array to the SPD terminal such that each switch of the switch array connects one capacitor of the capacitor array to the SPD terminal, wherein the controller controls the switches of the switch array to connect at least one of the capacitors of the capacitor array to the SPD terminal based on the voltage level information. The capacitor array may include 8 capacitors and the switch array may include 8 switches, respectively connecting each of the eight capacitors to the SPD terminal, such that the voltage dividing device is capable of providing a total of 256 different voltages to the SPD terminal.

The predetermined range of distinct voltages provided by the voltage dividing device may be sufficiently large to provide a maximum voltage to the SPD terminal sufficient to operate a suspended particle device of a corresponding size.

The voltage controlling device may include an input device adapted to provide the voltage level information from a user of the voltage controlling device.

The controller may monitor an SPD voltage provided to the SPD terminal and control the voltage dividing device to provide zero voltage to the SPD terminal after the SPD voltage drops a predetermined amount below a predetermined level. The voltage controlling device may include an alarm, wherein the controller may provide an alarm signal that activates the alarm after the SPD voltage exceeds a predetermined level by a predetermined amount. The predetermined level may be based on an average value of the SPD voltage over a predetermined period of time. The predetermined level may be stored in a memory of the controller.

The voltage controlling device may include a photo detector adapted to monitor a level of light at the suspended particle device, wherein the level of light at the suspended particle device is used as the voltage level information utilized by the controller.

The voltage controlling device may include an AC power supply providing an AC voltage at a low frequency to the AC terminal. The AC power supply may provide an AC voltage with a frequency of at least 15 hertz. The AC power supply may include an AC/DC converter converting an AC voltage signal of a predetermine frequency into a DC voltage signal, a DC motor operated by the DC voltage signal and a generator connected to the DC motor to provide the AC voltage signal at the low frequency.

The AC power supply may further include a plurality of solar cells connected in parallel, at least one rechargeable battery adapted to provide a DC voltage signal and a converter, adapted to convert a DC voltage signal of the at least one rechargeable battery into the AC voltage signal with the low frequency; wherein the at least one rechargeable battery is recharged by a recharging voltage signal supplied by the plurality of solar cells.

The AC power supply may be mounted in a movable support in which the suspended particle device is mounted, such that the AC power supply moves with the suspended particle device.

The suspended particle device may include a first conducting layer, a second conducting layer and an emulsion including a plurality of suspended particles which align in a predetermined pattern when exposed to an electric field, where the emulsion is positioned between the first conducting layer and the second conducting layer, and wherein a first conducting bus connecting the first conducting layer to the SPD terminal and a second conducting bus connecting the second conducting layer to the SPD terminal are positioned on one edge of the suspended particle device.

The length of the one edge of the suspended particle device may be less than a length of another edge of the suspended particle device such that the suspended particle device is substantially rectangular in shape.

The bus length of the first conducting bus and the second conducting bus may be less than a length of the one edge of the suspended particle device.

A voltage controlling device for controlling voltage provided to at least one suspended particle device includes an AC terminal adapted to receive an AC voltage at a specific frequency, an SPD terminal adapted to provide a selected AC voltage value to the suspended particle device, a capacitor array including a plurality of capacitors where each capacitor in the capacitor array has a predetermined capacitance, a switch array, connecting each capacitor of the capacitor array to the SPD terminal such that each switch of the switch array connects one capacitor of the capacitor array to the SPD terminal and a controller adapted to control the switches of the switch array to connect at least one of the capacitors of the capacitor array to the SPD terminal based on voltage level information, such that a plurality of distinct voltage values within a predetermined range are selectively provided to the SPD terminal.

A method of controlling voltage provided to a suspended particle device includes receiving an AC voltage signal at a specific frequency, dividing the AC voltage signal into a plurality of distinct voltage levels within a predetermined range and controlling the dividing step to provide a selected voltage level of the plurality of distinct voltage levels to an SPD terminal connected to the suspended particle device based on voltage level information.

The dividing step may include providing a capacitor array including a plurality of capacitors, each said capacitor having a predetermined capacitance, connecting each capacitor of the capacitor array to the SPD terminal via a switch of a switch array, wherein a number of switches in the switch array is the same as a number of capacitors in the capacitor array and controlling the switch array and the capacitor array such that at least one capacitor of the capacitor array is connected to the suspended particle device to provide the selected voltage level.

The connection step may include connecting eight capacitors in the capacitor array via eight switches in the switch array to the SPD terminal, such that a total of 256 selected voltage levels are provided to the SPD terminal.

The predetermined range of distinct voltages may be sufficiently large to provide a maximum voltage to the SPD terminal sufficient to operate a suspended particle device of a corresponding size.

The method of controlling voltage provided to a suspended particle device may further include inputting the voltage level information via an input device for use in the lo controlling step. The method may further include monitoring an SPD voltage level provided to the SPD terminal, determining a normal SPD voltage level based on an average SPD voltage level over a predetermined period of time and comparing the SPD voltage level to the normal SPD voltage level, wherein the controlling step may include providing zero voltage to the SPD terminal after the SPD voltage level drops a predetermined amount below the normal SPD voltage level. The method may further include generating an alarm signal to activate an alarm after the SPD voltage exceeds the normal SPD voltage by a predetermined amount. The normal SPD voltage level is stored in a memory.

The method may include detecting a light level at the suspended particle device and generating a light level signal wherein the light level signal is used as the voltage level information in the controlling step.

The method may include generating the AC voltage signal at a low frequency. The AC voltage signal preferably has a frequency of at least 15 hertz.

The generating step may further include converting an AC voltage signal of a predetermined frequency into a DC voltage signal, driving a DC motor with the DC voltage signal and generating an AC voltage signal having the low frequency via a generator powered by the DC motor.

The generating step may include connecting a plurality of solar cells in series to at least one rechargeable battery, converting a DC voltage signal from the at least one rechargeable battery into an AC voltage signal with the low frequency with a DC/AC converter and recharging the at least one rechargeable battery with a recharging voltage signal supplied by the plurality of solar cells.

The method may also include mounting the solar cells, the DC/AC converter and the at least one rechargeable battery in a movable support in which the suspended particle device is mounted.

Construction of the suspended particle device may include providing a first conducting layer, providing a second conducting layer, positioning an emulsion including a plurality of suspended particles, which align in a predetermined pattern when exposed to an electric field between the first conducting layer and the second conducting layer, positioning a first conducting bus on the first connecting layer to connect the first conducting layer to the SPD terminal and positioning a second conducting bus on the second conducting layer to connect the second conducting layer to the SPD terminal, such that the first conducting bus and the second conducting bus are positioned on one edge of the suspended particle device.

The bus length of the first conducting bus and the second conducting bus may be less than a length of the one edge of the suspended particle device.

A system of controlling voltage includes a voltage controlling device, wherein the voltage controlling device includes an AC terminal adapted to receive an AC voltage at a specific frequency, a voltage dividing device adapted to divide the AC voltage into a plurality of distinct voltage values within a predetermine range a controller adapted to control the voltage dividing device to provide a selected voltage value based on voltage level information, an SPD terminal device adapted to receive the selected voltage value and a suspended particle device, wherein the suspended particle device includes a first conducting layer, a second conducting layer, an emulsion including a plurality of suspended particles, which align in a predetermined pattern when exposed to an electric field, where the emulsion positioned between the first conducting layer and the second conducting layer, a first connecting bus connecting the first conducting layer to the SPD terminal and a second conducting bus connecting the second conducting layer to the SPD terminal.

The voltage dividing device may further include a capacitor array including a plurality of capacitors where each capacitor in the capacitor array has a predetermined capacitance and a switch array, connecting the capacitor array to the SPD terminal such that each switch of the switch array connects one capacitor of the capacitor array to the SPD terminal, wherein the controller controls the switches of the switch array to connect at least one of the capacitors of the capacitor array to the SPD terminal based on the voltage level information.

The capacitor array may include 8 capacitors, and the switch array may include 8 switches, respectively connecting each of the eight capacitors to the SPD terminal, such that the voltage dividing device is capable of providing a total of 256 different voltages to the SPD terminal.

The voltage controlling device may further include an input device adapted to provide the voltage level information from a user of the system.

The controller may monitor an SPD voltage provided to the SPD terminal and controls the voltage dividing device to provide zero voltage to the SPD terminal after the SPD voltage drops a predetermined amount below a predetermined level.

The voltage controlling device may further include an alarm, wherein the controller provides an alarm signal that activates the alarm after the SPD voltage exceeds the predetermined level by a predetermined amount.

The predetermined level may be based on an average value of the SPD voltage over a predetermined period of time. The predetermined level may be stored in a memory of the controller.

The voltage controlling device may further include a photo detector adapted to monitor a level of light at the suspended particle device, wherein the level of light at the suspended particle is used as the voltage level information utilized by the controller to control the voltage provided to the SPD terminal by the voltage dividing device.

The voltage controlling device may further include an AC power supply providing an AC voltage signal at a low frequency to the AC terminal. The AC power supply may provide an AC voltage signal with a frequency of at least 15 hertz.

The AC power supply may include an AC/DC converter converting an AC voltage signal of a predetermine frequency into a DC voltage signal, a DC motor operated,by the DC voltage signal and a generator connected to the DC motor to provide an AC voltage signal at the low frequency.

The AC power supply may further include a plurality of solar cells connected in parallel, at least one rechargeable battery adapted to provide a DC voltage signal, and a converter, adapted to convert the DC voltage signal of the battery into an AC voltage signal with the low frequency; wherein the at least one rechargeable battery is recharged by a recharging voltage signal supplied by the plurality of solar cells.

The AC power supply may be in a movable support in which the suspended particle device is mounted, such that the AC power source moves with the suspended particle device.

The first connecting bus and the second connecting bus of the suspended particle device may be connected to the first conducting layer and the second conducting layer, respectively, on one edge of the suspended particle device.

The bus length of the first connecting bus and the second connecting bus, respectively, may be less than a length of the one edge of the suspended particle device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of an SPD film;

FIG. 2 is the series equivalent circuit of the SPD film of FIG. 1;

FIG. 3 is an example of a square piece of SPD film;

FIG. 4 is an example of a rectangular piece of SPD film;

FIG. 5 is a block diagram of a voltage controlling device according to an embodiment of the present application;

FIG. 6 is a circuit diagram of a voltage dividing device according to an embodiment of the present application;

FIG. 7 is a simplified AC equivalent circuit of the total capacitance of the circuit of FIG. 6;

FIG. 8 is a graph of the light transmission through an SPD load versus the AC voltage across the SPD load;

FIG. 9 is a table showing the physiological effects of shock currents upon the human body;

FIG. 10 is a simplified AC equivalent circuit of a voltage controlling device driving an SPD load under shock conditions;

FIG. 11 is a modified equivalent circuit of FIG. 10 after the SPD load has been transformed from a series equivalent circuit to a parallel equivalent circuit;

FIG. 12 is a simplified equivalent circuit of FIG. 11 where the shock resistance has been combined with the equivalent parallel resistance of the SPD load,

FIG. 13 is a final series equivalent circuit of FIG. 12 providing for a comparison of SPD load voltage with shock to normal SPD load voltage;

FIG. 14 is a block diagram of a voltage controlling device according to an embodiment of the present application;

FIG. 15 is a block diagram of a voltage controlling device according to an embodiment of the present application;

FIG. 16 is a circuit diagram of a voltage dividing device according to an embodiment of the present application;

FIG. 17 is a circuit diagram of a voltage controlling device and an SPD load according to an embodiment of the present invention;

FIG. 18 is a diagram illustrating a conventional position of connecting busses in an SPD load;

FIG. 19 is an RC equivalent circuit for the SPD of FIG. 18;

FIG. 20 is a diagram illustrating positioning of conducting busses on an SPD load according to an embodiment of the present application;

FIG. 21 is an RC equivalent circuit for the SPD load of FIG. 20;

FIG. 22 is a diagram illustrating the placement of conducting busses in an SPD load according to an embodiment of the present application; and

FIG. 23 is a block diagram of an AC power supply according to an embodiment of the present application.

FIG. 24 is a flow chart illustrating a method of controlling voltage provided to a suspended particle device according to an embodiment of the present application.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an example of a typical SPD film. The two conducting layers 10 act like the two plates of a parallel-plate capacitor and the emulsion 12 acts like its dielectric. The small dots 14 represent cells (droplets) enclosing anisometrically shaped particles such as rod-shaped particles that change their orientation in the presence of an electric field. The capacitance of SPD film is given by Equation 1;

Equation 1: $C_{spd} = \frac{A}{d}$

Where is the permittivity of the emulsion 12, A is the area of one conducting layer 10 and d is the distance between the two conducting layers 10.

A voltage controlling device in accordance with the present invention enables one to control AC voltage applied to SPD loads in a novel, cost-effective, and safe manner. As used herein the term SPD load includes SPD films, SPD light valves, and all other SPD products that rely on the application of an electric field to control the orientation of suspended particles. When the electric field is zero, the suspended particles become randomly oriented because of Brownian movement, and this randomness has the effect of reducing or blocking the passage of light through the SPD load. Upon application of an electric field, the particles align, usually with their long axes parallel to the electric field, which allows light to pass through the SPD load.

Currently available SPD films utilize AC voltages up to 200 V rms to create a maximum clear state in the SPD load. However, much lower voltages can create an almost clear state. The value of acceptable voltage required for a clear state depends on the thickness of the dielectric layer, such as emulsion 12 of FIG. 1, between conducting layers, the dielectric constant of the SPD emulsion, and the nature of the SPD particles in the SPD emulsion. The function of an SPD voltage controller, therefore, is to produce an AC voltage between 0 and Vmax, the voltage level that produces an acceptable level of clarity in the SPD load, for a given application. Unless otherwise indicated, this discussion uses 60 V rms as the acceptable value of Vmax. It is noted that the value of 60 V rms is selected merely for convenience and that the voltage controlling device and methods of the present application are not limited to use with SPD devices in which 60 V rms provides an acceptable clear state in the SPD load. The voltage controlling method and device invention are further defined below with particular reference to the figures submitted herewith.

Typical SPD film has a capacitance of approximately 40 nF per square foot, although smaller and larger values may occur with future SPD films. This capacitance is one of the most important parameters of SPD film because it determines how much AC current is required by a given SPD load to produce an acceptably clear state.

Another important parameter of SPD film is the resistance of its conducting layers 10. These conducting layers 10 usually have a resistance between 200 and 500 ohms per square, but the resistance of conduction layer may vary. This resistance is the main cause of I2R power dissipation in the SPD load. It can be shown that the power losses of an SPD load are

Directly proportional to the square of the frequency of the AC voltage applied to the SPD load;

Directly proportional to the square of SPD load area;

Directly proportional to the square of SPD load voltage; and

Directly proportional to the resistance of the conducting layers.

FIG. 2 shows an equivalent electrical circuit for SPD film. A window including the SPD film may be of any practical size. However, the size of individual SPD windows typically varies from as little as 1 square foot to as many as 32 square feet, and thus, the capacitance of the SPD load varies over a 32-to-1 range. The resistance of the SPD load, on the other hand, has a much smaller variation, because its value depends at least in part on which edges of the conducting layers 10 are used for conducting (see FIG. 3). If the SPD load is square, the conducting busses 30 appear as shown in FIG. 3, for example. In this case, the equivalent resistance to use in FIG. 1 is given by Equation 2:

Equation 2:

RspdRsq

Where Rsq is the resistance per square of the conducting layers 10 and typically ranges from 200 to 500 ohms.

When the SPD load is rectangular, the conducting busses 30 may run along either the longer or the shorter sides of the SPD load. FIG. 4 illustrates an SPD load in which the busses 30 run along the longer sides, which is the preferred location of the busses if the main consideration in bus placement is to minimize power losses. However, as will be discussed in further detail below, manufacturing costs and aesthetic considerations may also be considered in bus attachment, location, and size. For long busses 30, the resistance in the equivalent circuit of FIG. 2 is illustrated in Equation 3:

Equation 3: $R_{spd} = R_{sq} \frac{L_{short}}{L_{long}}$

Running the conducting busses 30 along the longer sides of the SPD load has been preferred in the past because it results in a more energy-efficient window since the SPD resistance in the charging path of each active cell is decreased. That is, the path between the respective busses 30 to the opposite edge of the SPD load is minimized. Since the charging current for the SPD windows must pass through this resistance, placement of the conducting busses 30 along the longer side of a rectangular SPD load results in smaller I2R power losses.

FIG. 5 is a block diagram of a voltage controlling device 50 according to an embodiment of the present application. The voltage controlling device 50 of FIG. 5 is more specifically a block diagram of a local controller for use preferably with a single window (SPD load), or at most for use with a few SPD loads.

The voltage controlling device 50 includes an AC terminal 51 adapted to receive an AC voltage signal, a voltage dividing device 52 that divides the AC voltage signal into a plurality of distinct voltage levels within a predetermined range. Controller 56 controls the voltage dividing device 52 to provide a selected distinct voltage level to the SPD terminal 54 based on voltage level information. The SPD terminal 54 provides the selected distinct voltage level to the SPD load 55. An input device 57 may be provided to allow a user to input the voltage level information. A photocell 58 may be provided to monitor a light level at the SPD load 55 and the light level may be used as the voltage level information.

The AC terminal 51 provides an AC voltage signal to the voltage controlling device 50. In a simple case, AC voltage is supplied by the AC line voltage, 120 V/60 Hz in the United States and 240 V/50 Hz in other parts of the world. However, any AC power source may be utilized, such as a dc-to-ac converter, commonly referred to as an inverter, a transformer working off the mains, a capacitive-voltage divider working off the mains, or any circuit or device capable of delivering sufficient AC voltage of any frequency to satisfy the SPD load requirements. In certain situations it may be preferable to provide a separate AC power supply which will be discussed in further detail below.

The voltage dividing device 52 may contain any electrical device that produces a voltage drop when AC current flows through it. Preferably, the voltage dividing device 52 divides the AC voltage signal into a plurality of distinct voltage levels in a predetermined range. Preferably, the voltage dividing device divides the AC voltage into a plurality of non-continuous distinct voltage levels in a predetermined range. In a preferred embodiment of the present application, the voltage dividing device 52 includes a capacitor array 60 and a switch array 62 (see FIG. 6). The capacitor array 60 preferably includes n capacitors which will provide 2n voltage levels to the SPD load. It is preferable to provide an array of capacitors capable of providing a large range of distinct voltages. For instance, a large capacitor array 60 of 8 capacitors can be properly switched via the switch array 62 to produce 256 distinct voltage levels. A large array such as described above, referred to as a byte array, would be desirable in a universal controller, that is, a controller that is capable of controlling an SPD load of any size. For instance, if a structure has many windows of different sizes between 1 and 32 square feet, a byte array is preferable in that it has enough range to provide a range of voltages which would be applicable to all SPD loads in the structure regardless of the size of any specific SPD load. In other words, a voltage controlling device 50 like this can be used as a local controller for any SPD windows without regard to the area of the window and thus is referred to as a universal controller. In fact, a byte array has enough inherent range that it can simultaneously control a bank of SPD windows whose aggregate area may be hundreds of square feet.

The switch array 62 preferably includes n switches. Each capacitor (C0 to C7) of the capacitor array 60 is connected to one of the switches of the switch array 62. If one switch is activated, the corresponding capacitor is connected to the SPD load 55, preferably via the SPD terminal 54. If two switches are activated, two capacitors in parallel are connected to the SPD load and will drive the SPD load. In general, if n switches are activated, there are n parallel capacitors driving the SPD load. In the byte-array embodiment of this invention, the voltage dividing device 52 includes 8 capacitor-switch combinations as shown in FIG. 6. In this example, optocoupled triacs are used as bilateral switches, however, solid-state relays, mechanical relays, and other types of electronic or even ordinary switches may also be used for the bilateral switches.

In a voltage dividing device 52 of the present invention, using the capacitor array 60 and the switch array 62 allows the voltage controlling device 50 to provide a wide range of distinct voltage levels to the SPD load. In addition, since the voltage is divided using a primarily capacitive device, the voltage controlling device 50 of the present invention minimizes losses which are common in traditional voltage controlling devices since capacitive devices are largely lossless when used in AC circuits. As noted above, a continuous range of voltage levels is unnecessary for most SPD loads, thus, the capacitor array 60 which provides an excellent means for dividing the AC voltage into a plurality of distinct voltage levels is preferable to the continuous range commonly provided by potentiometers of conventional voltage controlling devices. The byte array embodiment discussed above, provides an additional advantage in that the range of distinct voltage levels available is large enough to be used by an SPD load of almost any practical size.

A smaller capacitor array, however, may be preferable for other applications. For example, a capacitor array 60 of 4 capacitors can produce 16 distinct voltage levels. Such a small capacitor array, referred to as a nibble array, would be suitable for an SPD window of a specific size. For instance, if every window in a building is exactly 16 square feet in size, then a voltage controlling device 50 utilizing a small 4-capacitor array designed for 16 square-foot windows would be preferable. It is important to note that the reduced number of capacitors in the nibble array does not in any way limit the advantages in efficiency provided by use of a capacitor array in general discussed above. In general, byte arrays are used in SPD applications in which SPD load size varies widely and nibble arrays are used for SPD applications involving the SPD windows of the same size. Although the discussion emphasizes byte and nibble arrays, it is understood that the voltage controlling device and methods of the present invention can be implemented with fewer than four bits, more than eight bits, or any number of bits in between.

The SPD terminal 54 connects the SPD load 55 to the voltage dividing device 52. In a simple embodiment, the SPD terminal may simply include the wires and connecting busses 30 which connect the voltage dividing device 52 to the SPD load. More simply, the SPD terminal may simply be embodied by the connecting busses 30 discussed above.

The controller 56 controls the voltage dividing device 52 such that a plurality of distinct voltage levels with a predetermined range are provided to the SPD load 55 based on the voltage level information. More specifically, in a preferred embodiment of the present invention, outputs of the controller 56 control the on-off action of diodes D0 to D7 of the bilateral switch array 62. When activated, these diodes close the bilateral switches and create a parallel connection of the activated capacitors. By combining the effects of different capacitors (C0 to C7) a wide range of voltages are provided to the SPD load 55 such that any desired light transmission level can be produced in an SPD load of any size.

Generally, the controller 56 determines which bilateral switches of the bilateral switch array 62 are active. When 8 capacitors and 8 bilateral switches are used in the voltage dividing device 52, the total parallel capacitance of the capacitor array 60 is given by Equation 4:

Equation 4

CrC7*Bit 7C0*Bit 6C5*Bit 5C4*Bit 4C3*Bit 3C2*Bit 2C1*Bit 4C0*Bit 0

where bit 7 through bit 0 represent the outputs of controller 56. These bits may be either high or low, resulting in a minimum nonzero capacitance of: