Method of controlling an LC smart glazing

Abstract

Method of controlling a driver device for driving an energy efficient liquid crystal (LC) smart glazing (10) comprising an optical cell (12) containing an LC material and having a pair of terminals (22, 24) for connecting the optical cell (12), comprising Applying a sequence of driving steps carried out over a repeating period (T), wherein each step is carried out during at least one interval of the period (T), wherein the sequence comprises at least the steps: a) Connecting the terminals to a voltage source generating a control voltage; b) Connecting the terminals to a first driving circuit; c) Connecting the terminals to said voltage source inversely.

Description

[0001] Foreignfiling text P24-199

[0002] 1

[0003] Method of controlling an LC smart glazing

[0004] The invention relates to a method of controlling a driver device for driving a liquid crystal (LC) smart glazing having a pair of terminals for connecting said liquid crystal (LC) smart window, comprising applying a sequence of driving steps carried out over a

[0005] 5 repeating period, wherein each step is carried out during at least one interval of the period. The invention further relates to a driver device for carrying out said method.

[0006] Smart glazing is, in particular, used to build smart windows that have electrically switchable functions, e.g. sustainable smart windows used in buildings and vehicles. Smart glazing in general is available for two main functions, privacy and transparency control. Privacy type smart windows may be switched between a transparent (active) state, in which objects viewed through the window are clearly visible, and a translucent state, in which light passing through the window is scattered, creating an effect like milk glass.

[0007] Transparency control windows may be switched in a similar way between a clear and an opaque (active) state, wherein states of partial transparency (e.g. 60% transparency) are reachable. Transparency control windows allow objects to be seen through them in their clear and partially transparent state and do not generally scatter light passing through them.

[0008] To achieve these effects, smart glazing may, for example, comprise a layer of liquid crystals (LC) that may e.g. be reoriented by an external stimulus, like an electric field. Such LC smart glazing can transition quickly between their clear and active states.

[0009] LC glazing devices comprising smart glazing, for example LC privacy windows, have two electric terminals that connect to flat electrodes of the smart glazing between which the LC is arranged. They are generally operated by providing an AC voltage to the terminals, in particular a square wave. The frequency of the AC voltage can result in interference patterns with electric lighting exhibiting high-speed flicker based on the frequency of the public network if it is powered by, e.g. 50 Hz in most of Europe, or 60 Hz in the USA. When the properties of the control voltage and / or current do not align with the properties of their LC layer, LC privacy windows may further show artifacts0 such as residual haze or flicker. Foreignfiling text P24-199

[0010] 2

[0011] Based on the above, the invention seeks to reduce an operating power consumption and extend a useful lifetime of LC glazing while keeping or improving the switching speed and visual quality of LC windows.

[0012] This problem is solved by a method according to claim 1 and a driver device according to claim 8.

[0013] Further embodiments are the subject of the dependent claims.

[0014] To solve the problem, a method of controlling a driver device for driving a liquid crystal (LC) smart glazing having a pair of terminals for connecting said liquid crystal (LC) smart glazing is proposed, comprising Applying a sequence of driving steps carried out over a repeating period, wherein each step is carried out during at least one interval of the period, wherein the sequence comprises at least the steps: a) Connecting the terminals to a voltage source generating a control voltage; b) Connecting the terminals to a first driving circuit; c) Connecting the terminals to said voltage source inversely.

[0015] By having at least one first driving circuit to drive the smart glazing within at least an interval of each period of the AC waveform of the control voltage, the current peaks usually caused by rapid polarity reversals may be reduced so that the absolute values of the maximum and minimum control voltage may be set to higher values. For example, instead of ± 30V, as before, the maximum and minimum control voltage may be set to for example ± 50V or ±60V. This allows for a higher RMS value of the voltage to be reached within a shorter interval, namely within the first and second intervals of each period. Reaching a higher RMS value faster results in faster switching of the LC smart glazing.

[0016] In some embodiments, the first driving circuit is implemented as a short-circuit of the terminals.

[0017] This can be easily done, e.g. if an output of the driving device is implemented as an Flbridge, its switches can be controlled to create a short-circuit. Also, the short circuit avoids actively driving the LC smart window which would require energy. In this way, some of the charge of the LC smart window can be removed at no energetic cost. Foreignfiling text P24-199

[0018] 3

[0019] In some embodiments, the sequence comprises the following steps, in order: step a), step b), step c), step b).

[0020] This results in a signal at the terminals wherein the LC smart glazing is driven to a maximum and minimum voltage in different intervals, wherein in an interval between these intervals, during the transition between the minimum and maximum voltage so to speak, the terminals are short-circuited for a short time.

[0021] In some embodiments, the sequence further comprises at least the steps: d) Connecting the terminals to a storage capacitance to transfer a charge from a connected LC smart glazing to the capacitance; e) Connecting the terminals to the storage capacitance to transfer a charge from the storage capacitance to the connected LC smart glazing.

[0022] In this way, charge that was collected from discharging the LC smart glazing may be used to re-charge the LC smart glazing in another interval, thus saving energy.

[0023] In some embodiments, the sequence comprises the following steps, in order: step a), step d), step b), step e), step c), step d), step b), step e).

[0024] In this way, other than in the previous sequence, an additional interval is introduced between each transition from maximum voltage (steps a) and c)) to the short circuit (step b)). Each time, before discharging the LC smart glazing during the short-circuit period, part of the charge contained in the LC smart glazing is stored in the storage capacitor. Then, after the short-circuit period, the stored charge is provided to the LC smart glazing, e.g. by swapping the leads of the storage capacitor to convert a positive charge stored to a negative one and, half a period later, vice versa. This contributes to saving energy and further relaxes the current regime of the LC smart glazing.

[0025] In some embodiments, a second LC smart glazing is used as the storage capacitance.

[0026] In this way, energy can be saved since at least a part of the charge is swapped to and between the LC smart glazings and does not need to be generated again every period.

[0027] In some embodiments, the storage capacitance is an adjustable capacitor, and the method comprises the step f) Adjusting the adjustable capacitor. Foreignfiling text P24-199

[0028] 4

[0029] The capacitor may, for example, be adjusted during setup of the LC smart glazing to account for installation parameters, for example a size of the smart glazing.

[0030] The problem is further solved by a Driver device for driving an LC smart glazing having an optical cell (12), comprising: a pair of terminals for connecting the LC smart glazing; a controllable voltage source for providing a control voltage; an output circuit, comprising an H-bridge, midpoints of which are connected to the terminals; a control device arranged and configured to control devices of the driver device to carry out the method according to any of the methods above.

[0031] Such a driver device allows for reduced wear on the LC smart glazing and reduced energy use.

[0032] In some embodiments, the driver device comprises a storage capacitor for temporary charge storage.

[0033] Using such a capacitor to store charge from discharging the LC smart glazing for use in a later interval allows for energy savings.

[0034] In some embodiments the capacitor comprises a multitude of switchable capacitors to form a controllable capacity capacitor.

[0035] Such switchable capacitors may be tailored exactly to the LC smart glazing the driver device is used for.

[0036] In some embodiments, the capacitor is a capacitance of a second LC smart glazing having a second driver device and the driver device comprises an interconnect device for connecting to the second driver device.

[0037] Connecting two LC smart glazings in this way allows swapping a certain quantity of charge between the LC smart glazings.

[0038] Embodiments and advantages of the invention will be apparent from the following description of an embodiment and the enclosed figures, of which

[0039] Fig. 1 shows a schematic drawing of an LC smart glazing with a control circuit; Foreignfiling text P24-199

[0040] 5

[0041] Fig. 2 shows a driver output voltage and voltage obtained at the LC smart glazing resulting from a method according to the prior art;

[0042] Fig. 3 shows a driver output voltage and voltage obtained at the LC smart glazing resulting from a method according to an embodiment of the present

[0043] 5 invention;

[0044] Fig. 4 shows a schematic circuit diagram of an output circuit of a driver device according to an embodiment of the present invention;

[0045] Fig. 5 shows a driver output voltage and voltage obtained at the LC smart glazing resulting from a method according to an embodiment of the present invention using a storage capacitor;

[0046] Fig. 6 shows a schematic circuit diagram of an output circuit of a driver device according to an embodiment of the present invention comprising a storage capacitor;

[0047] Fig. 7 shows a schematic circuit diagram of an output circuit of a driver device according to an embodiment of the present invention comprising an adjustable capacitor;

[0048] Fig. 8 shows a schematic circuit diagram of an output circuit of a driver device connected to a second driver device according to an embodiment of the present invention and

[0049] Fig.9 shows voltage levels produced by the driver devices according to Fig. 8.

[0050] A smart glazing 10 as shown in Fig. 1 has an optical cell 12 arranged between a first electrode 14 and a second electrode 16. A driver device 18 comprises terminals 22, 24 connected by means of wires to the electrodes 14, 16.

[0051] The optical cell 12 comprises, for example, an LC privacy layer and can be controlled to transition between a first state, for example a clear state, and a second state, for example a scattering state. When an alternating voltage of appropriate frequency and amplitude is applied to the electrodes 14, 16, the LC layer 12 becomes clear. When no alternating voltage is applied to the electrodes 14, 16, the LC layer 12 becomes returns to the scattering state but is translucent. 0 The kind of alternating voltage to apply to the electrodes 14, 16 depends on electrical characteristics of the optical cell 12. The relevant electrical characteristics of the optical Foreignfiling text P24-199

[0052] 6 cell 12 may comprise a series resistance, a capacitance, a quality factor, a reactive capacitance, a reactance, an equivalent series resistance and / or an equivalent series inductance. Electrical characteristics my change over the lifetime of the optical cell 12 and may thus be indicative of the age and / or quality of the optical cell 12.

[0053] Fig. 2 shows a diagram 50 of a well-known driving method according to the prior art, wherein the top diagram shows a voltage graph 52 applied to the optical cell 12 and a bottom diagram shows a current graph 54, both graph 52, 54 showing their respective values over time.

[0054] The voltage 52 alternates between a positive control voltage +Vappiied and a negative control voltage -Vappiied. At each transition, a current peak occurs. Dimming of the optical cell 12 is implemented by lowering the control voltage +Vappiied, -Vappiied. This driving method results in very high current peaks, requiring a lot of energy to continuously reverse charge of the optical cell 12.

[0055] Fig. 3 shows a diagram 60, wherein, again the top part of the diagram shows a voltage graph 62 and the bottom part of the diagram shows a current graph 64. The drilling method applied comprises a sequence of four phases I, II, III, IV that together form a period. The phases I, II, III, IV are continuously repeated in that order.

[0056] During phase I, the voltage applied to the optical cell 12 is the positive control voltage +Vappiied. During phase II, the terminals 22, 24 are connected to a first driving circuit which, in this case, is a short-circuit, causing the optical cell 12 to discharge. Then, in phase III, the negative control voltage is applied to the optical cell 12. In phase IV, the first driving circuit is again connected to the terminals 22, 24 as a short-circuit. After phase IV, the sequence starts again with phase I.

[0057] The current 64, in this driving method, does not reach a peak as high as in the prior art.

[0058] Fig. 4 shows an example output circuit 100 comprising an H-bridge comprising electrical switches Sa, Sb, Sc, Sd. The output circuit 100 may apply a sequence of driving steps carried out over repeating., Wherein each step is carried out during at least one interval of the period. In particular, the output circuit 100 may carry out the step a) connecting the terminals 22, 24 to a voltage source generating control voltage, the step Foreignfiling text P24-199

[0059] 7 b) connecting the terminals to a first driving circuit which, in this embodiment, is the short-circuit, and the step c) connecting the terminals to said voltage source inversely.

[0060] The first switch Sa and the second switch Sb are connected in series between the control voltage and ground, wherein the terminal 22 is connected between the two

[0061] 5 switches Sa, Sb. The third switch Sc and the fourth switch Sd are connected in series between the control voltage and ground, wherein the terminal 24 is connected between the two switches Sc, Sd. The switches Sa, Sb, Sc, Sd may now be controlled to implement the phases I, II, III, IV by opening and closing during the phases I, II, III, IV as follows: 0 Dimming of the optical cell 12 may be implemented by extending phases II and IV and shortening phases I and III. The control voltage may, in this case, remain constant.

[0062] Charging and discharging the optical cell 12 requires energy. The energy from the charge is, in the embodiment of Figs. 3 and 4, thrown away entirely, e.g. via the short- circuit. 5 In some embodiments, such as the embodiment shown in Figs. 5 and 6, the H bridge of the driver device 200 may be extended to comprise a storage capacitor 26 connected to ground at one terminal and, at its other terminal, via a first storage switch S1 to terminal 22 and via a second storage switch S2 to terminal 24.

[0063] This allows for the sequence to comprise the steps Connecting the terminals 22, 24 to a0 storage capacitance 26 to transfer charge from connected optical cell 12 to the capacitance 26 and Connecting the terminals 22, 24 to the storage capacitance 26 to transfer charge from the storage capacitance 26 to the connected optical cell 12. Foreignfiling_text P24-199

[0064] 8

[0065] Thus, the repeating sequence in this case comprises eight intervals or phases, wherein during phase I, the optical cell 12 is charged from the capacitor 26. During phase II, the control voltage is applied to the optical cell 12. During phase III, the charge from the optical cell 12 is discharged into capacitor 26. During phase IV, a short-circuit is applied 5 to the optical cell 12 to discharge it completely. Then, during phase V, the capacitor 26 is reconnected to the optical cell 12 with its polarity reversed so that it will provide a negative charge to the optical cell 12. During phase VI, the negative control voltage is applied to the optical celll 2. During phase VII, the charge from the optical cell 12 is discharged into capacitor 26. Then, during phase VIII, the short-circuit is applied to the0 optical cell 12 again to discharge it completely. After phase VIII, the sequence begins again with phase I.

[0066] To implement the eight phases, the switches of the output circuit 200 are switched as follows: 5 In this way, in every period, the energy for charging the optical celll 2 does not need to be taken from the voltage source any longer in its entirety. Rather, part of the charges stored in the storage capacitor 26 which is then connected to the optical cell 12 again with its terminals reversed to support the transition from positive control voltage to negative control voltage or vice versa. Foreignfiling text P24-199

[0067] 9

[0068] Further, as can be seen from fig. 5, the current is 74 is much lower than in the prior art as shown in fig. 2. A relevant factor in keeping the peak current low is the size of the capacitor 26. If it is too large, then it will cause a very high current peak again, counteracting part of the positive influence of using this circuit. If it is too small, a lot of

[0069] 5 energy will be wasted again in each period. The ideal size of the capacitor 26 depends on the capacitance of the optical celU 2 which is, of course, not known at the time the driver device is built.

[0070] To allow the driver device to be adjusted to a particular optical cell 12, some embodiments, as the example shown in Fig. 7, of a driver device 300 comprise a multitude of switchable capacitors Ci to Cn forming an adjustable capacitor 302. It is particularly useful for the values of the capacitors to follow the formula Cn-1 = Cn / 2 since in this way, the switches may be activated in a binary fashion. In that way, if 8 capacitors following said formula are present, the switchable capacitor 302 may be switched in intervals of Cn / 128 from 0 to 255 / 128 * Cn.

[0071] In further embodiments, such as shown in Fig. 8, the storage capacitor 26 is formed by a second optical cell28. The second optical cell28 comprises its own proper second driver device 402. The first driver device 401 and the second driver device 402 are connected by means of an interconnect device allowing each of the terminals of each of the optical cells 12, 28 to be connected to each other. In this way, there are in for connections in total each having a switch S3a, S3b, S3c, S3d.

[0072] The resulting control algorithm would result in the voltage waveforms 82, 84 applied to each of the optical cellsl 2, 28 during each period T, resulting in the same eight phases as for the embodiments displayed in fig. 5, 6 and 7.

[0073] All of the above-mentioned embodiments of the driver device 100, 200, 300, 401 , 402 lead to a step-up-step-down-shape of the repeating AC voltage waveform used to drive the optical celU 2. Relative to the known square waveforms for controlling optical cell 12, such step-up-step-down sequences allow for using a higher maximum voltage and lower minimum voltage which are being applied for a shorter time.

[0074] This kind of waveform has a variety of advantages over square and / or sine waveforms. 0 Firstly, the total energy consumption is reduced due to the shorter time spent at Foreignfiling text P24-199

[0075] 10 maximum and minimum voltage. Second, due to the higher maximum and lower minimum voltages, in general due to the higher absolute values of the maximum and minimum voltages, switching of the optical celU 2 to active state occurs much faster. Thirdly, higher frequencies may be used to reduce flicker without the previously known

[0076] 5 disadvantages. Lastly, larger smart windows may be controlled without flicker since the increased voltage allows for electric charge to be distributed faster over the optical cell 12.

[0077] The intervals 26, 30, 34, 38, during which intermediary voltages are applied, each have a duration of between 1 % and 20% of the period duration, for example 3%.

[0078] The intermediate voltages may, for example, in some embodiments be comprised between 50% and 1 %, 20% and 1 % or 30% and 10% of the maximum or minimum voltage. In some embodiments, the intermediate voltage may be 0 V. In some embodiments, a protective device may be connected in series or in parallel to the optical cell 12, for example a resistor, a capacitor or a current limiting device. 5 In some embodiments, some voltages and / or periods may be omitted. For example, it is possible to omit the zero voltage and / or one of the intermediary voltages. In some embodiments, more intermediary voltages may be used. It may also be possible to use voltage transitions instead of fixed levels during at least one period, for example piecewise defined curves, in which, for example, a section of a sine curve or a gradient,0 e.g. a linear gradient, defines the voltage.

[0079] The waveform provided by the methods according to the invention may be repeated at a frequency between for example 25 Hz and 250 Hz, resulting in period durations between 40ms and 4ms. Higher frequencies are generally advantageous to avoid flicker but require more energy to run. In general, a higher frequency requires more frequent charge alternation leading to higher RMS current passing through resistive elements of the optical cell 12 which leads to more energy dissipated, for example as heat.

[0080] The maximum voltage may be selected according to the electrical characteristics of the optical cell 12 to have, for example, a value between 30 V and 120 V. The minimum voltage may be selected according to the electrical characteristics of the optical cell 120 to have, for example, a value between -30 V and -120 V. In some embodiments, the Foreignfiling_text P24-199

[0081] 11 values of the maximum voltage and the minimum voltage will be chosen to be the negative of each other, for example +50 V and -50 V.

[0082] In some embodiments, the intermediate voltages may be chosen to have values between 1% and 50% or 1 % and 20% of the maximum voltage or the minimum voltage.

[0083] 5 If the maximum voltage and / or minimum voltage have absolute values higher than 50 V, the control circuitry carrying out the method may fall under a less constraining safety directive.

[0084] In some embodiments, the minimum and maximum voltage as well as the frequency are chosen depending on the optical cell 12, which, depending on its size and shape,0 requires different voltages and frequencies to reduce flicker and residual haze. In some embodiments, when a resistance of the optical cell 12 is high, further intermediate voltages applied in periods to be allocated may be used to decrease peak currents and thus decrease resistive losses.

[0085] The invention presented herein allows to conserve power during operation of the optical5 cell 12, in particular by recycling the charge used to activate the optical cell 12.

[0086] Furthermore, the method presented herein reduces current peaks during operation and thus extends the useful life and durability of the optical cell 12.

[0087] Foreignfiling text P24-199

[0088] 12

[0089] REFERENCES

[0090] 10 (liquid crystal) smart glazing

[0091] 12 optical cell

[0092] 14 first electrode

[0093] 5 16 second electrode

[0094] 18 control circuit

[0095] 22 terminal

[0096] 24 terminal

[0097] 26 storage capacitor

[0098] 28 second optical cell

[0099] 50 diagram

[0100] 52 voltage

[0101] 54 current

[0102] 60 diagram

[0103] 62 voltage

[0104] 64 current

[0105] 70 diagram

[0106] 72 voltage

[0107] 74 current

[0108] 80 diagram

[0109] 82 voltage

[0110] 84 voltage

[0111] 100 output circuit

[0112] 200 output circuit

[0113] 300 output circuit

[0114] 302 adjustable capacitor

[0115] 401 first output circuit

[0116] 402 second output circuit

[0117] 403 interconnect circuit 0 Sa, Sb, Sc, Sd, S1 , S2 switch

[0118] S1a, S1b, S1c, S1d, S2a, S2b, S2c, S2d, S3a, S3b, S3c, S3d switch

Claims

Foreignfiling text P24-19913CLAIMS1. Method of controlling a driver device for driving a liquid crystal (LC) smart glazing (10) comprising an optical cell (12) and having a pair of terminals (22, 24) for connecting the optical cell (12), comprising Applying a sequence of driving steps carried out over a repeating period (T), wherein each step is carried out during at least one interval of the period (T), wherein the sequence comprises at least the steps: a) Connecting the terminals to a voltage source generating a control voltage; b) Connecting the terminals to a first driving circuit; c) Connecting the terminals to said voltage source inversely.

2. Method according to claim 1 , characterized in that the first driving circuit is implemented as a short-circuit of the terminals (22, 24).

3. Method according to claim 1 or 2, wherein the sequence comprises the following steps, in order: step a), step b), step c), step b).

4. Method according to claim 1 or 2, characterized in that the sequence further comprises at least the steps: d) Connecting the terminals (22, 24) to a storage capacitance (26) to transfer a charge from a connected LC smart glazing (10) to the capacitance (26); e) Connecting the terminals (22, 24) to the storage capacitance (26) to transfer a charge from the storage capacitance (26) to the connected LC smart glazing (10).

5. Method according to claim 4, wherein the sequence comprises the following steps, in order: step a), step d), step b), step e), step c), step d), step b), step e).

6. Method according to claim 4 or 5, characterized in that a second LC smart glazing (28) is used as the storage capacitance (26).Foreignfiling text P24-199147. Method according to any of the claims 4 to 6, characterized by the step f) Adjusting an adjustable capacitor (302) comprised by the storage capacitance (26).

8. Driver device for driving an LC smart glazing having an optical cell (12),5 comprising: a pair of terminals (22, 24) for connecting the LC smart glazing (10); a controllable voltage source for providing a control voltage (+Vappiied); an output circuit (100, 200, 300, 401 , 402), comprising an H bridge, midpoints of which are connected to the terminals; 0 a control device arranged and configured to control the driver device to carry out the method according to any of claims 1 to 7.

9. Driver device according to claim 8, characterized in that the driver device comprises a storage capacitor (26) for temporary charge storage.

10. Driver device according to claim 9, characterized in that the storage capacitor (26) comprises a multitude of switchable capacitors to form an adjustable capacitor (302).11 . Driver device according to claim 9 or 10, characterized in that the storage capacitor (26) is a capacitance of a second LC smart glazing (28) having a second driver device and in that the driver device comprises an interconnect0 device (403) for connecting to the second driver device.

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