Photocell operation controlling circuit

The invention relates to a control system for an optical cell (also called a light valve).

The optical cell can be made by placing a layer of a suitable modifiable material between two parallel plates of a rigid, substantially transparent, flat material, each plate having an electrically conductive surface layer facing the modifiable material. Examples of the components of a modified material are slurry particles, liquid crystals or electrochromic material.

By applying or not applying an electric potential difference between the plates, a transition state or a state in which light is absorbed, scattered or reflected can be obtained.

Optical cells, including optical cells having a control system according to the invention, may be used in mirrors or in vehicle or building windows, to be provided in mirrors or in vehicle or building windows for variable light transmission. For example, they can be used as windows with adjustable light transmission from opaque to transparent to reduce the intensity of sunlight transmitted, or to block the interior of a room or vehicle from being viewed by other people. They can be used in automotive sunshades or in a sliding roof or in aircraft portholes.

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The control system according to the invention is particularly well suited for use with an optical cell used in a car's rear-view mirror and is described here mainly with reference to this application.

The use of an optical cell in the rear view mirror is known, for example, from French patent FR 2 366 958 (Brisard Gerard), making it possible to provide a rearview device in which the reflectivity of the light varies as a function of the degree of glare. Traditional car rear view mirrors with anti-dazzle properties, often referred to as prism mirrors, have a day and night status with the mirror status being manually changed by the vehicle driver. In daylight conditions, a high degree of reflection from the rear view mirror is required, typically greater than 50%. In the night condition, the reflectivity is limited to 12% or less, often around 4%, to avoid dazzling the driver by e.g.

Optical cells make it possible to make a rear-view mirror with the ability to automatically adjust to the intensity of the incident light, changing the state from day to night and vice versa and setting intermediate states between these limits, increasing driving comfort and safety. The cell is placed next to and parallel to the reflecting surface of the mirror and in an optical line between the reflecting surface and the driver of the vehicle. The cell therefore delivers a different level of reflected light from the mirror to the driver. In one preferred embodiment, the reflecting layer is provided with one of the electrically conductive material layers which is also light-reflective.

The level of light transmittance or reflectivity of the optical cell is regulated by a control system that is regulated by external factors. The control system is preferably placed in a common housing with the optical cell, forming a single element with it. Traditional control circuits were bulky due to the difficulty of miniaturizing their components.

An object of the present invention is to provide a control system for an optical cell that can be conveniently placed next to the cell.

According to the invention, there is provided an optical cell operation control circuit which comprises a first circuit, powered by a non-voltage power source, and including an oscillator and an induction coil primary winding, and a second circuit comprising an optical cell and a secondary winding of said induction coil, characterized by that the second circuit is a resonant circuit which contains the inductance of the secondary winding and the optical cell, and the inductor provides weak coupling between the primary and secondary windings.

The control system according to the invention thus uses the inductance of the secondary coil winding as the inductance of the resonant (oscillating) circuit. The system according to the invention facilitates a significant reduction in the volume of the control system.

A particular advantage of the system according to the invention is that only the second system has high voltages, so that the system has fewer components exposed to high voltages. Since high voltage can pose safety and electromagnetic interference problems, limiting the occurrence of high voltage to the second system is beneficial to reduce the footprint of the high voltage components and to reduce the size of the protective enclosure that should surround them.

Several different types of optical cells are known. These include an electrochromic optical cell, a liquid crystal optical cell, or a galvanic optical cell. During electroplating, the passage of a current through a transparent liquid containing a metal salt causes the metal ions to migrate to the glass surface and the formation of a metal coating that absorbs the light. The electrodes in this case are SnO 2 coatings. Crystalline liquid optical cells, galvanic optical cells and electrochromic optical cells are generally transparent in a quiescent state, but under certain conditions, such as under overvoltage or prolonged excited state, return to the transparent state from the excited state

176 548 may take some time, even several hours, and thus the cell switching speed may be relatively slow.

A preferred type of optical cell for use with a control system according to the invention comprises a suspension of scattered, fine particles in a liquid that can change their spatial orientation depending on an applied electric field, changing the light transmission of the suspension as described, for example, in the US patent 3,655,267 (Research Frontiers). Such optical cells switch rapidly from transparent to opaque. Many intermediate permeability values ​​can also be obtained therein. A suspension of the herapatite in a suitable liquid, such as 3-methylbutyl acetate, is preferred, although other types of molecules may also be used, for example graphite, mica, red garnet, aluminum, and derivatives of alkaloid sulfate salts.

The plates of transparent material that make up the optical cell are usually spaced about 50 µm apart. If this distance is not kept within an appropriate tolerance, for example around 5 to 10 [mu] m, the cell transparency is not uniform or problems may arise as a result of shorting electric fields between the excessively similar points of the two plates. The requirement for a uniform distance imposes certain limitations on the material from which the tiles can be made. Thus, while plastics such as polyethylene terephthalate may be contemplated, there may be difficulties in keeping the distance between the plastic plates constant over the entire cell surface. It is therefore generally preferable to use glass plates.

The surfaces of the plates, facing each other in the cell, are covered with an electrically conductive material. A preferred coating is indium tin oxide (ITO), which both conducts electricity and transmits light. The mirror surface of the rear view mirror of which the cell is a part can advantageously be made in the form of a reflective layer on the surface of one of the plates of the cell opposite the surface covered with the electrically conductive layer. A commonly used material for such reflective coatings of plates is silver, chrome or aluminum.

When the cell is part of the rearview mirror of an automobile, the reflecting surface of the mirror and the cell are housed in a housing attached to the vehicle, such as a vehicle windshield or door. The optical cell operation control system is placed inside or on the housing. Also included are electrical connectors inside or on the housing for connecting the optical cell to the vehicle's electrical system.

In the control circuit of the invention, the optical cell is preferably connected directly to the secondary winding and the inductor provides substantially all of the inductance of the second (oscillating) circuit. A parallel resonant circuit provides a greater overvoltage factor and greater stability than a series circuit. The fact that the inductor supplies substantially all of the inductance of the resonant circuit ensures that the control circuit occupies a small volume.

The term weak coupling is used herein in reference to the magnetic coupling between the primary and secondary windings to denote a coupling similar to that present in a transformer but differing in greater stray inductance. The coupling coefficient K can be calculated from the expression:

K = M / (Lp * Ls)^1/2 where Lp is the inductance of the primary winding, Ls is the inductance of the secondary winding, and M is the mutual inductance. In the case of the invention, the coil should preferably have a coupling factor of less than 0.7, preferably less than 0.5.

The coupling should be weak to reduce the influence of the primary circuit on the impedance of the secondary circuit, while at the same time strong enough to transfer the energy necessary to initiate and maintain the resonance of the secondary circuit. Thus, energy is introduced in the form of a coil bias, while avoiding disturbing the characteristics of the secondary circuit.

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The coil according to the invention is therefore not designed like a real transformer, in which normally the strongest possible coupling is required, but acts like a weak transformer. The purpose of the low-coupling coil according to the invention is not to transfer energy with the minimum possible losses.

The magnetic core of the inductor is preferably designed to provide a magnetic resistance to the magnetic flux in the magnetic circuit formed by the core. This may conveniently be achieved by placing the slit in the path of the magnetic flux lines passing through the magnetic core. The gap is formed of a non-magnetic material, such as air or, more usually, resin or plastic. The size of the gap in the magnetic core is preferably at least 0.1 mm, more preferably at least 0.2 mm.

The second circuit provides the energy of the reaction to activate the optical cell. The operating frequency of the circuit can be produced by the second circuit so that the frequency of the pulses in the circuit is continuously and automatically tuned to the resonant frequency of the other circuit. This is preferably achieved by designing the electrical system in such a way that the second (resonant) circuit is an element which acts directly on the oscillating system and thus itself generates the operating frequency.

For a cell with an electrically adjustable particle suspension, the frequency typically used is in the order of 8 to 25 kHz, often in the order of 16 to 25 kHz. The use of such an alternating current prevents the migration of suspended particles between the closely spaced plates, which would negatively affect the uniformity of opacity or transparency required of the cell. Select a frequency value outside the acoustic frequency range.

In an arrangement where the frequency is produced by the oscillator in the first circuit, but not in the case of an auto-oscillator circuit, the frequency must initially be tuned for each cell to a resonant frequency as determined by the design of the cell circuit.

The control system of the invention offers the advantage that only a small amount of energy is required to maintain the required resonance. A particular further advantage of the system according to the invention is that if the cell breaks, the current can be sustained in the system, but at a much lower voltage.

The coil contains a traditional core, usually soft iron. The size of the core is preferably such as to fit inside the rearview mirror housing together with the optical cell, with the core positioned behind the mirror as viewed from the driver's side of the vehicle.

The primary and secondary windings of the coil preferably do not interrupt each other. In other words, the primary winding is preferably formed around one part of the core and the secondary winding around another part of the core. A configuration without breaking the windings also achieves poor coupling between the windings and has the added benefit of making an arrangement small enough to fit in the mirror housing.

The primary winding preferably comprises less than 100 turns around the core, more preferably 10 to 80 turns and most preferably 40 to 80 turns. The secondary winding usually contains 140 to 300 windings. The winding ratio (gear ratio) is therefore typically in the order of 3: 4: 1. The main factor in obtaining the desired cell voltage, however, is not the winding ratio, but rather the overvoltage in the second circuit. The overvoltage is a function of the capacitance, inductance and resistance of the elements that make up the second circuit. The possibility of using the overvoltage in the second circuit to obtain the desired voltage drop across the cell is a particular advantage of the invention.

The waveform of the second system is essentially sinusoidal, even though the waveform of the pulses generated by the oscillator is not sinusoidal. The conversion of a non-sinusoidal wave (e.g., square wave) produced by the oscillator into a sine wave in the second system is facilitated by weak magnetic coupling in the magnetic coil. The voltage applied to the optical cell can be regulated by selecting the amount of energy emitted by the oscillator by selecting the size of the pulses, for example by

176 548, the choice of their length or, alternatively, may be adjusted by the choice of the voltage of the pulses in the primary circuit.

The circuit preferably includes one or more feedback lines from the second circuit to the first circuit. This gives the advantageous possibility of adjusting the oscillator depending on the electrical parameters of the optical cell.

A feedback loop may be arranged to regulate the voltage of the second circuit and thus provide the desired voltage value on the optical cell for the desired level of cell darkening. Alternatively or additionally, the feedback loop may also adjust the frequency acting on the frequency of the oscillator in the first circuit to ensure that the operating frequency of the second circuit always matches its resonant frequency.

A detection loop may be implemented that will control that the operating frequency of the second circuit is actually the resonant frequency of the second circuit and will send an appropriate correction signal to the oscillator of the first circuit to tune its frequency so as to resonate it to the second circuit.

If the operating frequency is not the same as the resonant frequency of the second circuit, the overvoltage is lower and active energy consumption increases. It is therefore advantageous to ensure that the operating frequency is equal to the resonant frequency for good performance, although adjusting the difference between the operating frequency and the resonant frequency may also, to some extent, affect the voltage applied to the cell by adjusting the overvoltage factor.

In one embodiment of the invention, the second circuit comprises at least one capacitor connected in parallel to the optical cell. It is generally preferable to use two or more such capacitors in series. The use of series-connected capacitors has the advantage that the voltage across the individual capacitors is lower.

The control system according to the invention can be applied to different types of optical cells. The control system regulates the value of the alternating current energy supplied to the optical cell. For a cell with an electrically adjustable particle suspension, a voltage up to 125 V may be required with the voltage being applied between the conductive surfaces of the cell to create an electric field that orients the molecules to allow light transmission. In order to change the reflectivity or light transmission of an optical cell, it is sufficient to change the voltage applied to the optical cell. You can also change the frequency, but it's less effective. Changes in the intensity of the transmitted light are mainly proportional to the applied voltage, up to the saturation value. In the absence of an electric field, molecules follow Brownian motion, obstructing the passage of light through the cell. In the presence of a weak field, the particles tend to orientate in the field, but still oscillate within certain limits, as a result of which some light absorption occurs. A certain limit of the electric field, for example corresponding to a voltage of 100V, is necessary for the molecules to align substantially fully along the field line with minimal light absorption.

The control system is preferably controlled by at least one photosensitive optical device which measures the intensity of the light incident on the optical cell. Preferably, at least two such light intensity measuring devices are used, the first one controls the potential glare from the rear of the vehicle and the second controls the intensity of ambient light, for example light passing through car windows, reflected by the roof, or light diffused by a transparent roof. vehicle.

The control is based on the principle that a signal proportional to the light level detected by a photosensitive optical device or proportional to the difference in light intensity measured by two such devices is used to influence the primary oscillator so as to adjust the voltage applied to the cell terminals optical, and thus - select the value of the cell opacity.

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If there are one or more exterior mirrors in addition to the interior rearview mirror, the transmittance and / or reflectivity of the exterior mirrors may be adjusted by the same electronic system made to control the interior mirror to provide for the simultaneous adjustment of transmittance and / or reflectivity. However, due to the miniaturization and low energy consumption of the adjusting system according to the invention, it is possible to have separate control systems in each mirror. With these separate systems, each mirror is adjusted according to the surrounding light conditions.

The invention is further described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a system in one embodiment of a control system according to the invention for use in an automobile; figure 2 shows a cross-section of the inductor used in the system and figure 3 shows another cross-section of the induction coil shown in figure 2 taken along line AA 'in figure 2.

The illustrated control circuit comprises a first circuit 1 comprising a 12VDC battery 3, an oscillator 4, and a primary winding 11 of an inductor having a magnetic core 10. The circuit also includes a second circuit 2 which includes a secondary winding 12 of an inductor, an optical cell 14, and a capacitor 16. connected in parallel to the optical cell 14. The induction coil is shown schematically in figure 1 and more specifically in figures 2 and 3.

The battery 3, which powers the oscillator 4 with low voltage, is the power source for the entire electronic circuit. In addition to the battery 3, there are elements in the circuit which generate negative or positive reference voltages at certain points in the circuit. The first of these is a 3 'DC-DC converter, attached to the front between battery 3 and oscillator 4.

Associated with the oscillator 4 are damping means 5, in this case simply a potentiometer, which serve to adjust the frequency of the oscillator 4. In an alternative configuration, damping means 5 are replaced by frequency feedback loop elements 5 '(shown in figure 1 by dashed lines). which measure the frequency in the secondary circuit and regulate the oscillator 4 to this frequency. The alternative circuit has the advantage that the oscillator frequency is automatically tuned to the frequency of the other circuit.

The optical cell 14 is of the type that contains a suspension of fine, solid particles in a liquid, capable of orienting itself spatially under the influence of an electric field. The capacitor 16 is preferably formed by four capacitors connected in series.

The system also includes an adjuster, generally designated 30, which includes two photoelectric light intensity measuring devices 32 and 33 connected to the adjuster 34 of the detector. A reference voltage is provided to circuit 34. Detector signal line 35 leads from circuit 34 to comparator 40. Circuit 30 also includes an operational amplifier 36 with a feedback loop 39. The control circuit line 37 leads from the second circuit 2 to the comparer 40, from which the signal line 38 leads to the amplifier 36.

In the illustrated arrangement, the comparison circuit 40 is also provided with damping means 41 (in this case, a potentiometer) to determine the threshold voltage. The non-essential component 41 serves to limit the voltage to a level sufficient for proper mirror operation while preventing unnecessarily high voltage from being applied to the cell.

The configuration of the inductor used in this embodiment of the invention is shown in more detail in the cross-sections in figures 2 and 3. The magnetic core 10 is formed of two E-shaped ferrites facing each other with a plastic spacer 13 between them. The spacer 13 provides resistance to the flow of the magnetic flux in the core 10. The secondary winding 12 is wound around a central arm formed by the center arms of the opposing ferrites E, and the primary winding 11 is wound around one of the end arms of each ferrite.

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The control system as a whole is mounted in a housing (not shown) and can be connected via vehicle wiring to a 312V supply.

In use, a light detector is placed to measure the intensity of the surrounding light, for example by capturing light from the front of the vehicle and / or light reflected from the roof, and the light detector 33 is positioned to measure the intensity of the light incident from the rear of the vehicle. Detector control system 34 includes a differential detector that compares the signals from the light detectors 32 and 33 and injects a signal into the conduit 35 proportional to the ratio of the intensity of the light incident from the back (33) to the intensity of the ambient light (32). Light shining on the detector 33 which blinding the driver produces a corresponding signal at the differential detector. Line 37 carries a signal proportional to the voltage of the second circuit 2. The signal sent by the control system 34 through the line 35 is compared in the comparator 40 with the signal from the line 37, which indicates the secondary voltage, and the comparator 40 transmits the control signal to the operational amplifier 36 on the line 38. The control signal takes into account the value of the intensity ratio of the light sources and the voltage. currently applied to the optical cell. Similarly, further feedback elements may be provided for the transmission from the second control loop to the fixed frequency oscillator 4 of the cell 14 (the feedback elements 5 'are shown in dashed lines).

The signal from line 38 is amplified by amplifier 36 to force the oscillator 4 to generate a low AC voltage pulse wave (this may be a sine or square wave) in the first circuit 1. This wave through the coil induces a higher voltage in the second circuit 2, which is applied. on cell 14. The voltage on cell 14 is further boosted by resonance in the second circuit, and is typically up to 120 VAC, allowing a voltage to be applied to cell 14 that will partially or fully orient the suspension particles. Full orientation gives the maximum light transmission through the cell 14 and thus the maximum reflectivity of the rear view mirror of which the cell 14 is part.

If the ambient light detector 32 sees good daylight or a high intensity of artificial light, and the rear-facing detector 33 sees similar conditions, then the difference between the respective photoelectric signals is small and circuit 34 sends a signal through wires 35 and 38 and amplifier 36 to activate the oscillator. 4 and create a voltage in the second circuit 2 that activates the cell. The comparator 40 is informed on line 37 of the voltage actually produced by the second circuit and adjusts the command signal sent via line 38 to obtain the maximum voltage on cell 14.

The oscillator 4 converts the voltage of 12 V from the supply battery to an AC voltage, and a voltage of 120 V AC is produced in the second circuit 2. The frequency of the signal produced by the oscillator 4 is matched to the resonant frequency of the second circuit 2 by external influence through the elements 5 on the oscillator 4, and is typically in the order of 20 kHz. Regulation can easily be obtained by measuring the amperage of the second circuit. When the frequency changes, the current follows a curve that goes through the minimum. The resonant frequency is obtained when the current reaches a minimum. The maximum voltage of 120V in the second circuit 2 produces full orientation of the suspended particles in cell 14.

Alternatively, the frequency feedback element 5 'provides the frequency control of the oscillator 4.

If the ambient light detector 32 detects muffled or nighttime conditions and the rearward-facing detector 33 perceives similar conditions, then the difference between the respective photoelectric signals is again small. Arrangement 34 again transmits a signal via wires 35 and 38 and, as described above, again the highest level of cell transparency and mirror reflectivity is achieved.

However, if the ambient light detector 32 detects dim or night conditions and the rear-facing detector 33 picks up dazzling, intense light from the car's headlights, then the difference between the respective signals is

176 548 is large and circuit 34 sends the appropriate signal to amplifier 36. In this situation, no voltage is produced in the inductor 11/12 and no voltage is applied to cell 14. In the absence of voltage in circuit 2, the cell molecules assume random settings, causing dullness of the cell and reducing the reflectivity of the mirror to the lowest level.

If the difference between the signals from the detector 32 measuring the intensity of the ambient light and from the detector 33 towards the rear is between the extreme values ​​described above, e.g. through the amplifier 36 to the oscillator 4, which produces some stimulation for the oscillator 4, but the width of the pulses produced by the circuit 1 is relatively small, hence the voltage of the second circuit 2 is correspondingly low. Under these conditions, the voltage of the second system gives only a partial orientation of the particles suspended in cell 14, producing an intermediate level of opacity of cell 14 and an intermediate level of reflectivity of the mirror as a whole.

If desired, the ambient light detector 32 may be equipped with a time delay generating element (not shown in the figures) such that the cell 14 does not revert to full transparency prematurely as a result of the light from a passing vehicle briefly increasing the intensity of the ambient light reaching the detector. 32.

In a typical example of the control circuit of the invention, the cell 14 has a capacity of 11 nF and the capacitor 16, formed by four 22 nF capacitors connected in series, has a capacity of 5.5 nF. Each E-shaped ferrite is 25mm high, 13mm wide and 8mm thick and is made of 3C8 material. The inductor has 66 turns in the primary winding (1) on one part of the core, 240 turns in the secondary winding (2) on another part of the core, and a gap of 2.5mm in the magnetic circuit. The primary winding 11 has an inductance Lp of 0.318 mH, the secondary winding 12 has an inductance Ls of 6.31 mH and the mutual inductance M is 0.6 mH. The inductor coupling factor K, calculated according to the above relation, is equal to 0.423.

Alternatively, the second system may be a self-oscillating system. In this case, the second (resonant) circuit forces the operating frequency on the oscillator.