Method of determining operating parameters for driving a liquid crystal glazing device

Abstract

A method (200) of determining a transmittance-capacitance function of a liquid crystal glazing (12) containing an LC material, a Method (300) of calibrating a transmittance- capacitance function, a Method (400) of driving an energy efficient liquid crystal glazing (12) device and a Method (500) of measuring a capacitance of a liquid crystal glazing (12) are disclosed.

Description

[0001] Foreignfiling text P24-200

[0002] 1

[0003] Method of determining operating parameters for driving a liquid crystal glazing device

[0004] The invention relates to a method of determining operating parameters of an LC smart glazing.

[0005] 5 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.

[0006] Privacy type smart glazing may be switched between a transparent (active) state, in0 which objects viewed through the glazing are clearly visible, and a translucent state, in which light passing through the glazing is scattered, creating an effect like milk glass.

[0007] Transparency control glazing 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 glazing allows 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 its clear and active states. 0 Liquid crystal 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 driven by providing an AC voltage waveform to the terminals which results in a changing electrical field being applied to the LC. The LC basically orients itself under the influence of the electric field5 and thus produces the desired effect on transmittance or privacy.

[0009] A widely used waveform for driving liquid crystal glazing devices is the square wave as its fast polarity change ensures a maximum current flow. Since the electrodes are arranged flat on the surface of the liquid crystal or liquid crystal mixture and have a non- Foreignfiling text P24-200

[0010] 2 negligible resistance, the electrodes themselves form an RC network with the space between the electrodes acting as a capacitor with the LC as a dielectric. The control voltage is generally applied at terminals in comers or on edges of the electrodes. A control voltage change thus does not propagate to the entire electrodes immediately but takes time to take effect over their entire surface. This effect is more noticeable the larger the surface of the liquid crystal glazing device is.

[0011] A particular voltage difference or rather a particular electric field strength needs to be attained between the electrodes to obtain a desired transmittance. Providing a higher voltage difference at the terminals increases non-uniform ity of the transmittance across the smart glazing. However, using a higher voltage to drive the liquid crystal glazing device requires more energy than driving the liquid crystal glazing device with a lower voltage. Also, the voltage required at the terminals will vary with a surface area of the liquid crystal glazing device and has to be manually configured in the driver upon installation.

[0012] Liquid crystal glazing devices may also be controlled to have a transmittance between a dark state and a bright state, the transmittance of which each is dependent on properties of the glazing used. However, the relation between the control voltage and the transmittance is highly nonlinear and hard to approximate.

[0013] Other than with its surface area, the parameters for driving the liquid crystal glazing device may also vary with its age. Thus, any configuration of a driver circuit carried out upon installation of the liquid crystal glazing device will no longer be optimal after a given amount of time has passed.

[0014] The current invention thus aims to overcome the shortcomings of the prior art, in particular to simplify the driver configuration, to improve fidelity of the configuration as the liquid crystal glazing device ages and to determine when a liquid crystal glazing device is failing.

[0015] This problem is solved by methods according to claims 1 , 5, 7, 9 and 10.

[0016] Further embodiments are described in the dependent claims. Foreignfiling text P24-200

[0017] 3

[0018] The problem is solved by a Method of determining a transmittance-capacitance function of a switchable optical cell, comprising two substrates arranged at a distance and coated with at least one electrode on one side with a switchable optical material sandwiched between said substrates, wherein the electrodes are arranged to supply an

[0019] 5 electric field to form within the switchable material, wherein the switchable optical cell is built according to a construction template, wherein the transmittance-capacitance function provides a means to determine a capacitance of optical cells built according to the same construction template for a desired transmittance value, comprising: Determining the transmittance-capacitance function representing a relation between a transmittance of the switchable optical cell and a capacitance of the switchable optical cell.

[0020] As outlined above, optical cells form a capacitor between two electrodes, wherein the space between the electrodes is filled with a liquid crystal as a dielectric. The liquid crystal is anisotropic in at least two ways: As an optical agent and as a dielectric. Thus, when a voltage is applied to the liquid crystal glazing, the liquid crystal is reoriented, which changes both the transmittance and the capacitance of the liquid crystal glazing. The relation between transmittance and capacitance is much more linear than the relation between transmittance and voltage applied. Furthermore, the transmittancecapacitance function may be scaled to conform to any particular liquid crystal glazing device without requiring to acquire data for each individual liquid crystal glazing device once the transmittance-capacitance function is determined.

[0021] In some embodiments, Determining the function comprises the steps: Arranging a sample of the optical cell in a testing environment for transmittance measurement; Driving the optical cell sample by means of a voltage source, an output voltage of which is controllable; Setting the output voltage to each of a set of measuring points; Measuring, at each of the measuring points, the capacitance and the transmittance of the optical cell sample and Fitting parameters of a function to the measured pairs of capacitance and transmittance to obtain the transmittance-capacitance function.

[0022] The resulting function describes the transmittance-capacitance function which is0 congruent for all liquid crystal glazing devices made from the same construction template. Foreignfiling text P24-200

[0023] 4

[0024] In some embodiments, Determining the function comprises: Normalizing the function to a maximum measured capacitance or a minimum measured capacitance to obtain a normalized transmittance-capacitance function.

[0025] By using a reference capacity, the function is normalized to be used with different liquid crystal glazing devices. Since the relation between transmittance and capacitance is the same, only linearly scaled, for every liquid crystal glazing device built with the same construction template, it becomes comparatively easy to calibrate the transmittancecapacitance function for use with a particular liquid crystal glazing device.

[0026] In some embodiments, the method comprises Determining an accuracy parameter for defining a capacitance interval for which the resulting difference in transmittance is within a predetermined accuracy.

[0027] Said accuracy parameter may be used e.g. to determine control errors during operation.

[0028] To allow calibration and thus also further solving the above-mentioned problems, a Method of calibrating a transmittance-capacitance function determined as mentioned above to a liquid crystal glazing device comprising an optical cell built according to the same construction template comprises: Measuring a maximum capacitance of the liquid crystal glazing device and / or a minimum capacitance of the liquid crystal glazing device; Normalizing a function of the transmittance-capacitance function to the measured capacitance to obtain a calibrated transmittance-capacitance function.

[0029] As the transmittance-capacitance function was scaled with either the highest or lowest measured capacitance during its determination, its function may be scaled again to fit the individual liquid crystal glazing device. Thus, the required configuration is limited to providing the transmittance-capacitance function for the liquid crystal material used to produce the liquid crystal glazing device. After that, the driver will measure the maximum and / or minimum capacitance of the layer and scale the transmittancecapacitance function accordingly.

[0030] In some embodiments, the method comprises Verifying the calibrated transmittancecapacitance function by comparing a ratio of the measured maximum and minimum capacitance to a reference ratio associated with the transmittance-capacitance function. Foreignfiling text P24-200

[0031] 5

[0032] If the ratio of the measured maximum and minimum capacitance deviates from the ratio measured during determination of the transmittance-capacitance function by more than a predetermined amount, the configuration may be determined to be for a different construction template, for example one comprising a different cell gap, and thus a faulty

[0033] 5 configuration may be detected.

[0034] Once the calibration is over, a method of driving a liquid crystal glazing device according to claim 7 may be employed to solve the above-mentioned problems, comprising Determining, from a calibrated transmittance-capacitance function as obtained above and a desired transmittance, a target capacitance of the liquid crystal0 glazing device; Controlling a driving voltage of the liquid crystal glazing device to obtain the target capacitance on the liquid crystal glazing device.

[0035] By controlling the capacitance of the liquid crystal glazing device, the transmittance may be controlled in a much more precise manner.

[0036] In some embodiments, the method of driving the liquid crystal glazing device comprises Measuring a capacitance of the liquid crystal glazing device to obtain a measured cap; Determining the difference between the measured capacitance and the target capacitance; Adjust the driving voltage according to the difference; Repeating these three steps until the difference is within a predetermined accuracy.

[0037] This implements a closed control loop, approximating the desired capacitance step by0 step.

[0038] In all of the above-mentioned methods, a Method of measuring a capacitance of a liquid crystal glazing according to claim 9 may be employed, comprising the steps: Driving the liquid crystal glazing with an AC voltage waveform having a controllable peak voltage and a frequency; Measuring a current through the liquid crystal glazing at least once after a polarity change of the AC waveform and Determining, from the current measurements and their timing, an RC time of the liquid crystal glazing; Calculating, from the RC time and the current measured during the first measurement after the polarity change, a serial resistance of the liquid crystal glazing and Calculating, from the RC time and the serial resistance of the liquid crystal glazing device, the capacitance of0 the liquid crystal glazing device. Foreignfiling text P24-200

[0039] 6

[0040] In some embodiments, a method of determining a vitality indicator of a liquid crystal glazing device, comprises: Measuring a maximum and minimum capacitance of the liquid crystal glazing device, Calculating a capacitance ratio of the measured maximum and minimum capacitance, Determining the vitality indicator to indicate non-vitality when

[0041] 5 the capacitance ratio is not comprised within a predefined good capacitance ratio interval and / or Measuring a parallel resistance of the liquid crystal glazing device, Determining the vitality indicator to indicate non-vitality when the parallel resistance is not comprised within a predefined good parallel resistance interval.

[0042] In this way, a determination whether the liquid crystal glazing device is broken can be easily made.

[0043] Further embodiments and advantages of the invention will be apparent from the following description of an embodiment and the enclosed figures, of which

[0044] Fig. 1 shows a schematic drawing of a liquid crystal glazing device with a control circuit for carrying out the method according to the invention;

[0045] Fig. 2 shows a schematic equivalent circuit of a liquid crystal glazing device;

[0046] Fig. 3 shows a schematic flow diagram of a method of driving a liquid crystal glazing device;

[0047] Fig. 4 shows a schematic diagram of a method of determining a transmittancecapacitance function;

[0048] Fig. 5 shows a schematic diagram of a method of normalizing a transmittancecapacitance function;

[0049] Fig. 6 shows a schematic diagram of a closed control loop for driving a liquid crystal glazing device;

[0050] Fig. 7 shows a schematic flow diagram of a method of driving the liquid crystal glazing device in a closed control loop and

[0051] Fig. 8 shows a diagram to visualize an effect used by a method for measuring a capacitance of the liquid crystal glazing device.

[0052] A liquid crystal glazing control circuit 10 for controlling a liquid crystal glazing device 12 is shown in Fig. 1 The liquid crystal glazing device 12 comprises a switchable optical0 cell 13 as well as devices for mounting and connecting the optical cell 13 such as, in Foreignfiling text P24-200

[0053] 7 some embodiments, a window frame, cabling terminals, contacting devices for carrying electrical voltage and current to the optical cell 13. The optical cell 13 comprises at least two substrates 15, 17 arranged at a distance and each coated with at least one electrode on one side or alternatively with one substrate coated with two electrodes

[0054] 5 while the other substrate has no electrode layer, with a switchable optical material 19 sandwiched between said substrates 15, 17. Switchable optical cells 13 are usually mass-produced such that a construction template is used to construct many and various optical cells 13, for example of varying sizes. However, parameters from the construction template, among which may be, for example, the distance between the substrates, a thickness of the electrodes, an arrangement of the electrodes, a material of the electrodes and / or which switchable optical material is to be used, remain the same.

[0055] Accordingly, optical cells 13 built according to one particular construction template share certain properties, which shall be used according to the invention to provide calibrated driving of liquid crystal glazing devices 12 as well as, in some embodiments, automatic configuration of the driving method.

[0056] The control circuit 10 comprises a driver device 14 connected to terminals of the liquid crystal glazing device 12. The control circuit 10 further comprises a voltage measurement device 16 connected to the terminals of the liquid crystal glazing device 12 and a current measurement device 18 connected in series with the liquid crystal glazing device 12.

[0057] The liquid crystal glazing device 12 can be represented as an equivalent circuit, which is an electrical representation of the liquid crystal glazing device 12 that retains the relevant electrical characteristics of the liquid crystal glazing device 12. The equivalent circuit comprises electrical components that, when connected to each other, have the same or at least approximately the same electrical characteristics as the liquid crystal glazing device 12.

[0058] In some embodiments, the equivalent circuit 20 may be represented as shown in Fig. 2, for example, comprising a first resistor device Ri , a second resistor device R2 and a0 capacitor device Ci , wherein the first resistor device R1 and the capacitor device Ci are Foreignfiling text P24-200

[0059] 8 connected to each other in parallel to form an RC circuit 22. The RC circuit 22 is connected in series with the second resistor device R2 to form the equivalent circuit 20.

[0060] The RC circuit 22 represents the electrical characteristics of the liquid crystal of the liquid crystal glazing device 12. The second resistor device R2 represents the collective resistance of the electrodes and the connecting driver lines. The first resistor R1 may represent, in addition to losses of the switchable optical material 19, resistances of parts of the optical cell 13 and / or the liquid crystal glazing device 12, such as, for example, resistances introduced by glue lines. The equivalent circuit 20 is, as a representation, sufficient to both more precisely control the liquid crystal glazing device 12 and to detect various categories of malfunction of the liquid crystal glazing device 12.

[0061] As noted above, when a certain model of switchable optical cell 13 is constructed, there will usually be a multitude of devices produced that share, as parameters of the construction template, certain basic construction details such as the particular kind of liquid crystal used, the size of the cell gap between the electrodes into which the liquid crystal is inserted or the material and thickness of the electrodes. All liquid crystal glazing devices 12 that are constructed using optical cells 13 with the same basic construction details share certain electric properties which allows for a simplified configuration of the driver device 14.

[0062] Providing an advanced driving method 100 for a range of switchable optical cells 13 based on the same basic construction comprises three major steps as shown in Fig. 3:

[0063] In a first step 200, the basic construction of the switchable optical cell 13 is characterized to determine a transmittance-capacitance function. The first step 200 is executed once for a given basic construction or construction template.

[0064] A second step 300 comprises calibrating the transmittance-capacitance function to the individual liquid crystal glazing device 12 and its driver unit 14. The second step 300 is executed once for a given combination of a liquid crystal glazing device 12 and its driver unit 14 upon its installation. It may, in some instances, be necessary to repeat second step 300 at a later point, for example if the calibration data obtained in second step 300 is lost for some reason or the driver unit 14 is exchanged. Foreignfiling text P24-200

[0065] 9

[0066] A third step 400 comprises driving the liquid crystal glazing device 12 according to the transmittance-capacitance function and the calibration from steps 200 and 300 and to a desired transmittance of the liquid crystal glazing device 12. The third step 400 is executed repeatedly while the liquid crystal glazing device 12 is operated, in particular

[0067] 5 when the desired transmittance is changed, for example by user input.

[0068] The transmittance-capacitance function represents a relation between a controllable property of the liquid crystal glazing device 12, namely its capacitance, and a measurable transmittance value of the liquid crystal glazing device 12 that is influenced by said controllable property. Since liquid crystal changes its orientation relative to an electrical field in which it is arranged and is measurably slow in doing so, an exact transmittance-capacitance function would generally comprise a time component such that when the controllable property is changed, the transmittance value of the liquid crystal glazing device 12 will slowly change afterwards. However, at frequencies generally used for controlling the liquid crystal glazing device 12, such as, for example, 50Hz to 300Hz, said time component is insignificant.

[0069] However, for the purpose of precisely controlling the liquid crystal glazing device 12, such a complicated relation is not necessary. In the present embodiments, the transmittance-capacitance function is characterized by providing a relation between the capacitance Ci and a transmittance value measured on the optical cell 13. Such a function allows to determine the required capacitance Ci from a desired transmittance value.

[0070] To determine the transmittance-capacitance function, step 200 thus comprises multiple sub steps as outlined in Fig. 4. Also, step 200 may be executed on its own as a method 200 of determining a transmittance-capacitance function of a switchable optical cell 13. As outlined above, method 200 comprises Determining a function representing a relation between the transmittance of the switchable optical cell 13and the capacitance Ci of the switchable optical cell 13.

[0071] To determine set function, a sample of the switchable optical cell 1 Sis, in a first arranging step 202, arranged in a testing environment for light transmittance 0 measurement. Such a testing environment may, for example, be configured according Foreignfiling text P24-200

[0072] 10 to norm EN 410. The testing environment provides a measure of the light transmittance of the switchable optical cell 13, for example a value between 0% and 100%.

[0073] In a second step 204, the sample is driven with an AC source having a controllable output voltage. Since the sample is driven by a repeating voltage waveform of the AC source, the term “output voltage” is to be understood as an appropriate voltage measure such as, for example, a peak-to-peak voltage or an RMS voltage. Furthermore, the AC source may have, in some embodiments, a controllable frequency.

[0074] In a third step 206, the output voltage is set to one of a set of measuring points. The set of measuring points may be defined, in this case, as a multitude of output voltages for each of which a transmittance of the switchable optical cell 13 is to be measured.

[0075] In a fourth step 208, the transmittance of the switchable optical cell 1 Sis measured as well as the capacitance Ci of the switchable optical cell 13. The fourth step 208 may in some embodiments comprise a waiting time to allow the liquid crystals of the liquid crystal glazing material 12 to settle after the output voltage is changed in step 206.

[0076] In a repeating step 212, the fourth step 208, the second step 204 and the third step 206 are repeated, until a transmittance has been measured for each of the measuring points.

[0077] In a fitting step 210, the measured pairs of capacitance Ci and transmittance are fitted to a function or to parameters of a function. In some embodiments, interpolation between the measured pairs may be used to obtain the transmittance-capacitance function.

[0078] Thus, after fitting step 210, a transmittance-capacitance function of the optical cell 13 of the form T(C) is provided. So for any given capacitance C of the measured optical cell 13, the transmittance may be calculated. Vice versa, an inverse function C(T) = T’1(C) may be provided to determine a capacitance C required to reach a desired transmittance T.

[0079] In some embodiments, the transmission function may be, in a normalising step, normalised to a particular capacitance Cn. Cn may, for example, be chosen to be the Foreignfiling text P24-200

[0080] 11 highest measured capacitance Cmax or lowest measured capacitance Cmin measured in the fourth step 208. In the current embodiment, the lowest measured capacitance Cmin is selected. The resulting normalised transmission function is then defined as, for example:

[0081] Tnorm(Cnorm ) =T(C / C min) or, for the inverse function:

[0082] Cnorm (T) = C(T) / Cmin

[0083] For any optical cell 13 produced with the same construction template, e.g. materials and basic construction parameters, a maximum change in capacitance Ci will be approximately the same ratio Cmax / Cmin. This results from the change in permittivity £ of the liquid crystal layer as it transitions from dark to light or vice versa, depending on the dielectric used. The ratio Cmax / Cmin will be very close if not virtually equal to £max / £min. In some embodiments, Emax / smin may be influenced by limitations of the driver which may influence the ratio to a negligible extent. Having a normalised transmittance-capacitance function allows for a comparatively easy auto configuration or calibration of the driving device 14 to be used in step 300.

[0084] To calibrate the transmittance-capacitance function to one individual liquid crystal glazing device 12, step 300 comprises multiple sub steps as outlined in Fig. 5. Also, step 300 may be executed on its own as a method 300 of calibrating a transmittancecapacitance function determined for a switchable optical cell 13 to an individual liquid crystal glazing device 12 comprising an optical cell 13 produced with the same construction template.

[0085] To carry out method 300 comprises, in a first step 302, measuring a maximum capacitance Cmax of the liquid crystal glazing device 12 and / or a minimum capacitance Cmin of the liquid crystal glazing device 12.

[0086] In a second step 304, the transmittance-capacitance function of the transmittancecapacitance function is normalised to the measured minimum or maximum capacitance. For example, if the minimum capacitance Cmin has been measured and a normalised Foreignfiling text P24-200

[0087] 12 transmittance-capacitance function based on the minimum capacitance Cmin has previously been determined in step or method 200, the resulting individual transmittance-capacitance function may be calculated as follows:

[0088] Tindiv(Cl )=Tnorm(Cnorm * Cmin)

[0089] 5 or, for the inverse function:

[0090] Cindiv(T)=C(T) * Cmin

[0091] In an optional verifying step 306, the measured ratio Cmax / Cmin may be compared to a reference ratio previously determined for liquid crystal glazing produced using the same construction template. If the ratios do not match, i.e. are more different than an 0 expected variance, a failure condition may be indicated. To match the ratio, in some embodiments, a good capacitance ratio interval may be defined. If the ratio falls into the interval, the liquid crystal glazing device 12 is in good order. Reasons for the ratios not matching may be, for example, a damaged liquid crystal glazing device 12 or a misconfiguration such that the wrong transmittance-capacitance function for this kind of liquid crystal glazing was configured.

[0092] The sub steps of step 400 as shown in Fig. 7 implement a closed control loop 30 as shown in Fig. 6 Also, step 400 may be executed on its own as a method 400 of driving a liquid crystal glazing device.

[0093] In the closed control loop 30, the target capacitance Ct is provided as an input and is0 compared to a feedback Ct to obtain a control difference de. The control difference DC is provided to the driving device 14 as a signal to adjust the driving voltage Vcprovided to the liquid crystal glazing device 12. In the control loop 30, the resulting capacitance Ci of the liquid crystal glazing device 12 is then measured and provided to a feedback controller 32 which applies a control algorithm to the measured capacitance Ci to obtain5 the feedback Cf. The control algorithm may simply be a linear control algorithm (P only), but may comprise other components to account for the time required for the liquid crystals to reorient themselves after a voltage change. I- and D-components may additionally be used to accelerate transitions. Foreignfiling text P24-200

[0094] 13

[0095] To implement this closed control loop 30, the following steps of method 400 may be carried out:

[0096] In a first step 402, a target capacitance Ct is determined from a target transmittance Tt. To this end, the individual transmittance-capacitance function may be used as follows every time the target transmittance Tt is changed, for example by user input:

[0097] Ct=Cindiv(Tt)

[0098] In a second step 404, the capacitance Ci of the liquid crystal glazing device 12 is measured.

[0099] In a third step 406, the measured capacitance Ci of the liquid crystal glazing device 12 is compared to the target capacitance Ct to determine the difference between the two capacitances.

[0100] In a fourth step 408, the driving voltage is adjusted according to the difference. This step 408 may take into account the control algorithm chosen for the controller 32.

[0101] The second step 404, the third step 406 and the fourth step 408 may be executed repeatedly in regular or non-regular intervals continuously or until a break condition is reached, for example the measured capacitance Ci is within an accuracy threshold of the target capacitance Ct.

[0102] Because the capacitance and transmittance of the liquid crystal glazing device 12 are physically linked, this closed control loop over the capacitance Ci allows for very precise control of the liquid crystal glazing device 12.

[0103] To carry out the above-mentioned methods, methods for measuring the capacitance Ci are required. A variety of such methods exist. However, it would be preferred to measure the capacitance Ci without interruption to or degradation of operation of the liquid crystal glazing 12. To this end, a method 500 of measuring a capacitance of a liquid crystal glazing 12 is proposed comprising the following steps:

[0104] In a driving step 502, the liquid crystal glazing 12 is driven with an AC voltage waveform having a controllable peak-to-peak voltage and frequency. Foreignfiling text P24-200

[0105] 14

[0106] In a measuring step 504, a current iw as shown in Fig. 2 is measured at least once after a polarity change of the AC waveform. AC voltage waveforms used for controlling liquid crystal glazing 12 include, for example, variants of the square wave. Such waveforms comprise very rapid polarity changes. Such rapid polarity changes bring with them large, well-measurable currents iw flowing into or out of the liquid crystal glazing 12 as shown in Fig. 8

[0107] Fig. 8 comprises two graphs. The upper part of Fig. 8 shows a part of the square wave used to drive the liquid crystal glazing 12. The lower part of Fig. 8 shows the current charging the capacitance Ci as time evolves. The polarity change occurs at time to. At that time, a current Io is measured. After a predetermined period of time ti , a second measurement of the current is carried out, yielding the measured current h. Further measurements may be carried out, yielding further pairs of tn and In.

[0108] In a determining step 506, an RC time of the liquid crystal glazing 12 is determined from two pairs of time elapsed tn and current measured In according to the following formula:

[0109] In a calculating step 508, the series resistance R2 of the liquid crystal glazing 12 is calculated from the voltage +Vonapplied after the polarity change and the current Io measured during the first measurement after the polarity change, wherein: 2 = Von I Io

[0110] The closer the current Io is measured to the polarity change, the more precise the value of the series resistance R2 can be calculated.

[0111] In some embodiments, a known series resistance may be added to the circuit. In such embodiments, said known series resistance will need to be subtracted from the R2 value calculated above to determine the actual R2.

[0112] In a further calculating step 510, the capacitance Ci is calculated from the previous results: Foreignfiling text P24-200

[0113] 15

[0114] Ci = RC / R2

[0115] In some embodiments, a calculating step 512 is provided for calculating the parallel resistance Ri. Ri is calculated from the last current measurement before a polarity change, yielding a measured current IN at a time TN. In some embodiments, a number of n last measurements before a polarity change may be used instead to reduce the influence of noise on the result. At this point, the capacitance Ci is considered to be completely charged so that the remaining current IN is the current flowing through Ri and R2. The total resistance R = Ri + R2 may then be calculated as

[0116] R=Von I IN

[0117] Since R2 is known from the previous calculating step 508, Ri is calculated as Ri = R — R2.

[0118] These methods and calculations disclosed above may be effected by means of a computer, e.g. a microprocessor, microcontroller or other computing device, integrated or other.

[0119] The properties measured may, in some embodiments, be used to calculate parameters of the liquid crystal glazing 12, such as a size of the liquid crystal glazing 12, a shape of the liquid crystal glazing 12, an indication of product quality of the liquid crystal glazing 12, an age of the liquid crystal glazing 12, an optimal control frequency for controlling the liquid crystal glazing 12, and / or an optimal control amplitude for controlling the liquid crystal glazing 12.

[0120] For example, the parallel resistance R1 may diminish as the liquid crystal glazing 12 ages. When the parallel resistance R1 falls under a certain threshold, the liquid crystal glazing 12 is considered defective. Such a measurement may also be used to warn before failure of a liquid crystal glazing 12. Foreignfiling_text P24-200

[0121] REFERENCES

[0122] 10 control circuit

[0123] 12 liquid crystal glazing device

[0124] 5 13 (switchable) optical cell

[0125] 14 driver device

[0126] 15 substrate

[0127] 16 voltage measurement device

[0128] 17 substrate 0 18 current measurement device

[0129] 19 (switchable) optical material

[0130] 20 equivalent circuit

[0131] 22 equivalent circuit of the liquid crystal glazing

[0132] 32 controller 5

Claims

Foreignfiling text P24-20017CLAIMS1 . Method (200) of determining a transmittance-capacitance function of a switchable optical cell (13), comprising two substrates (15, 17) arranged at a distance and coated with at least one electrode on one side with a switchable optical material (19) sandwiched between said substrates (15, 17), wherein the electrodes are arranged to supply an electric field to form within the switchable material, wherein the switchable optical cell is built according to a construction template, wherein the transmittance-capacitance function provides a means to determine a capacitance of optical cells built according to the same construction template for a desired transmittance value, comprisingDetermining the transmittance-capacitance function representing a relation between a transmittance of the switchable optical cell and a capacitance of the switchable optical cell.

2. The method according to claim 1 , wherein Determining the function comprises the steps:Arranging (202) a sample of the optical cell (13) in a testing environment for transmittance measurement;Driving (204) the optical cell (13) sample by means of a voltage source, an output voltage of which is controllable;Setting (206) the output voltage to each of a set of measuring points; Measuring (208), at each of the measuring points, the capacitance and the transmittance of the optical cell (13) sample;Fitting (210) parameters of a function to the measured pairs of capacitance and transmittance to obtain the transmittance-capacitance function.

3. The method according to claim 2, wherein Determining the function comprises: Normalizing the function to a maximum measured capacitance or a minimum measured capacitance to obtain a normalized transmittance-capacitance function.Foreignfiling text P24-200184. The method according to any of the previous claims, comprising Determining an accuracy parameter for defining a capacitance interval for which the resulting difference in transmittance is within a predetermined accuracy.

5. Method (300) of calibrating a transmittance-capacitance function determined5 according to any of the previous claims to a liquid crystal glazing device comprising an optical cell built according to the same construction template, comprising:Measuring a maximum capacitance of the liquid crystal glazing device and / or a minimum capacitance of the liquid crystal glazing device;Normalizing the transmittance function to the measured capacitance to obtain a calibrated transmittance function.

6. The method according to claim 5, comprisingVerifying the calibrated transmittance-capacitance function by comparing a ratio of the measured maximum and minimum capacitance of the liquid crystal glazing device to a reference ratio associated with the transmittancecapacitance function.

7. Method (400) of driving a liquid crystal glazing (12) device comprising Determining (402), from a transmittance-capacitance function acquired according to any of the previous claims and a desired transmittance, a target capacitance of the liquid crystal glazing device;Controlling a driving voltage of the liquid crystal glazing device to obtain the target capacitance in the liquid crystal glazing device.

8. The method according to claim 7, comprisingMeasuring (404) a capacitance of the liquid crystal glazing device to obtain a measured capacitance;Determining (406) a difference between the measured capacitance and the target capacitance;Adjusting (408) the driving voltage according to the difference;Repeating these three steps until the difference is within a predetermined0 accuracy.Foreignfiling text P24-200199. Method (500) of measuring a capacitance of a liquid crystal glazing device, comprising the steps:Driving (502) the liquid crystal glazing device with an AC voltage waveform having a controllable peak voltage and a frequency;5 Measuring (504) a current through the liquid crystal glazing device at least once after a polarity change of the AC waveform andDetermining (506), from the current measurements and their timing, an RC time of the liquid crystal glazing (12);Calculating (508), from the RC time and the current measured during the first0 measurement after the polarity change, a serial resistance (R2) of the liquid crystal glazing (12) andCalculating (510), from the RC time and the serial resistance (R2) of the liquid crystal glazing device (12), the capacitance (Ci) of the liquid crystal glazing device (12).

10. A method of determining a vitality indicator of a liquid crystal glazing device, comprising:Measuring a maximum and minimum capacitance of the liquid crystal glazing device, Calculating a capacitance ratio of the measured maximum and minimum capacitance, Determining the vitality indicator to indicate non-vitality when the0 capacitance ratio is not comprised within a predefined good capacitance ratio interval and / orMeasuring a parallel resistance of the liquid crystal glazing device, Determining the vitality indicator to indicate non-vitality when the parallel resistance is not comprised within a predefined good parallel resistance interval.

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