KR20020007319A - Active electrooptic filtering device and method for operating the same - Google Patents

Abstract

Especially suitable filtering devices for use in welding protective masks, helmets or goggles are connected with electronic circuits as well as electronic circuits for controlling active filter elements and light protective filters having at least one active optical filter element. It is equipped in this known manner with the optical sensors and the electrical power sources for the active circuit elements and the active filter elements, in particular solar cells. The drive circuit for the active filter element is carried out in such a way that the load capacitor is simply discharged in the range of a configuration frequency (1 / T) of 0.01 to 1 Hz, as a result of which the power demand is cut in half compared to the known method. At the same time, the operating voltage U falls within a range that is quantitatively defined and within a range where the scattered light ratio of the liquid crystal display used is minimal as a result of this limitation.

Description

ACTIVE ELECTROOPTIC FILTERING DEVICE AND METHOD FOR OPERATING THE SAME}

Filtering devices of this type are known, for example, from documents WO 97/15254, US-5,315,099 or EP-0 550 384. As an active filter element, this generally comprises at least one liquid crystal cell (liquid crystal cell, LC-cell), which liquid crystal cell has a larger range or smaller as soon as the optical sensor collides with a light intensity exceeding a predefined threshold level. Interferes with the transmission of light up to the range. The use of such filtering devices is numerous and varied, a common example being use as a viewing window for welding protective masks, helmets and goggles.

The filtering device described in the above document consists of an active filter element made of, for example, a 0 to 90 ° rotating nematic liquid crystal element, which is arranged between two cross polarizers. It is operated with an operating voltage that passes through several times on the Freedericksz limit. The driving voltage of the liquid crystal cell, in which the first optical activity of the cell can be observed, is represented by the Frederiks limit. The choice of high operating voltage is suitable for the above-mentioned document with regard to the reduction of the scattered light generated, the reduced dependency of the temperature on the electro-optic effect and the generation of light transmission of less than 1%.

The driving frequency of such an active filter element for reasons of low power demand is between 0 and 32 Hz. The operation of the filter element using electricity from the support / buffer cell and the solar cell is mentioned as the main reason for the limited availability of the electrical power source. Continuous direct current operation still damages the liquid crystal cell through electrolysis and ion migration today or otherwise significantly impairs its optical performance, while the continuous improvement of the insulating layer, the reduction of impurities and the high liquid crystal material used Significant progress is achieved with the achievement of the electrical conductance value. Since the driving frequency has a linear effect influencing the power demand of the liquid crystal cell, the selection of the driving frequency as small as possible is tried. However, it is desirable to further reduce power demand.

Two values characteristic of this electro-optical filtering device, transmission and scattering, are of particular importance in this regard. The requirements for these values are specified in various production standards, for example EN 166, EN 167, EN 169 or EN 379. The European standard EN 169, which is defined within the scope of classification of transmissions (T), can be found for various welding methods. In this way, the protection level number is introduced.

N = 1-(7/3) logT (1)

Acceptable scattered light for active filter elements is defined in European standard EN 379. In doing so, the reduced scattered light coefficient is defined as follows.

l ^* = (1 / ω) (φ _1R -φ _2R ) / φ _1L , (2)

By this,

ω is the spatial angle,

φ _1R -φ _2R is the scattered light flow of the test sample at the defined spatial angle (minus the scattered light ratio of the measuring instrument),

φ _1L is the unscattered light flow of the test sample (zero diffraction order).

In the case of known electro-optical filtering devices, the optical properties are very damaged by scattered light. Light scattering on an LC-cell has a variety of causes: inter alia, the particles wrapped in the LC-cell, spacers between different layer thicknesses, scratches, edges and / or glass plates surrounding the liquid crystal.

The present invention relates to an active electro-optical filtering device according to the preamble of the independent claim and a method of operating the same. The filtering device is particularly suitable as a dazzle protective device for use in welding protective masks, helmets or goggles.

1 is a filtering device according to the invention performed with a dazzle protection device;

2 is an equivalent circuit diagram of a control circuit according to the invention;

3 is an operating voltage as a function of time for a preferred embodiment of the method of operation;

4 is a reduced radiance luminance as a function of operating voltage.

It is an object of the present invention to devise an active electro-optic filtering device and to present a method for its operation, in which case an operating voltage as small as possible is necessary, but due to good optical properties, in particular light scattering, Small damage is achieved. This object is solved by the filtering device and by the method as defined in the independent claim.

In order to reduce the power demand of the liquid crystal cell, the electro-optical filtering device according to the present invention is preferably equipped with a special driving circuit. The drive circuit according to the invention comprises a switch, which shorts the liquid crystal cell for a certain period of time in every half cycle. Therefore, neither a continuous trigger circuit nor a continuously changing drive voltage is selected. The drive according to the invention differs from the state of the art by the method of inserting and driving an active flank, which corresponds more to pulse width modulation instead of continuous frequency. The configuration frequency of the drive pulse is in the range of 0.01 to 1 Hz. The energy requirements using this method are halved compared to the state of the art, which represents a major advance.

The present invention shown here uses a clearly defined operating voltage. On the one hand, in order to achieve the optical density specified in production standard EN 169, it is done several times on the Frederiks limit. In addition, the operating voltage is limited in this way, which is at the voltage, where light scattered to the LCD is minimal.

The limitation of the operating voltage according to the invention lies in the result achieved with the minimum of the scattered light when the molecule (original φ _1R ) in the scattered light equation (2) is less than or otherwise equal to the denominator φ _1L . In other words, this means that when the scattered light ratio φ _1R at the operating point of the liquid crystal display is adjusted to be less than or equal to the residual oil transfer T = 10 ^{(3/7) (1-N)} , then the operating voltage is scattered light. It means that it is optimally selected with respect to. Experience has shown that the operating frequency defined in this way is in the range of 10 to 50 volts. Adjustment of the residual oil transmission can be achieved, for example, by adaptation of the small offset or polarization efficiency of the polarizer. The scattered light effect of the measuring facility φ _2R is ignored in the above discussion.

Hereinafter, the present invention will be described in detail based on the drawings.

In Fig. 1 a filtering device according to the invention, which is designed as a dazzle protection device, is shown. It comprises at least one active light filter element 1 with liquid crystals. Liquid crystals are performed according to one of the following techniques. TN technology, STN technology, dichroic technology, ferroelectric technology or π mode LCD technology. Apart from this, the filtering device comprises electronic means 2 for driving the active filter element 1. At least one optical sensor 4 is operated in connection with the electronic means 2. For example, the output signal of the optical sensor 4 for control purposes is generated by the electronic means 2 respectively, and the closed circuit controls the operating voltage of the filter element. For the electronic means 2, an optical filter element 1 and / or an optical sensor 4, an electrical power supply means 5 is envisaged. This can be done for example with solar cells.

As schematically shown in FIG. 2, it is advantageous to be equipped with electronic means 2 having a drive circuit. In this, the power demand of the liquid crystal cell 1 can be significantly reduced. In the equivalent circuit diagram of FIG. 2 the liquid crystal is represented by a resistor R _LC and a capacitor C _LC . The other resistor in the circuit is connected to resistors R _S1 and R _S2 . The alternating current source 21 generally has an alternating current having a constituent frequency of 0.01 to 1 Hz. ). The drive circuit according to the invention comprises a switch S ₁ , which shorts the liquid crystal cell for any time period t _s . This achieves a complete discharge of the capacitor C _LC . Thus, in this drive circuit the energy required for the opposite charge of the capacitor C _LC is reduced by half compared to the state of the art.

3 shows the operating voltage U (t) supplied to the drive circuit according to FIG. 2 as a function of time t. In the time period T with a typical duration of 1 to 100 seconds, initially during the first time interval t ₊ , for example a positive voltage (+ | U _LC |) is applied to the liquid crystal cell 1. Thereafter, for example, by closing the switch S ₁ (see FIG. 2), the liquid crystal cell 1 is shorted during the second time interval t _S1 . During the third time interval t ₋ , for example, a negative voltage (-| U _LC |) is applied to the liquid crystal cell 1, so that a short circuit is caused once more during the fourth time interval t _S2 . Is generated. In this way, the active flanks 31, 32 are thus inserted in the path of the operating voltage U (t). This driving method according to the invention corresponds most similarly to pulse width modulation. The constituent frequency f = 1 / T of the drive pulse is in the range of 0.01 to 1 Hz. It is observed that the time interval in FIG. 3 is not drawn to scale for reasons of clarity. The first time interval t ₊ and the third time interval t ₋ have a typical length of 0.5 to 50 seconds, whereas the length of the second time interval t _S1 and the fourth time interval t _{S2 generally} It is in the range of microseconds to milliseconds. Therefore, the short circuit times t _S1 and t _S2 are shorter than the driving time t ₊ , t ₋ with a factor in the range of 10 ³ to 10 ⁷ .

4 shows a general dependency of the reduced luminance luminance factor l ^* (U) (see equation (2)) as a function of operating voltage U. Analysis of the scattering phenomenon in the liquid crystal cell 1 is important for understanding the present invention. The causes of light scattering are, for example, particles wrapped in the liquid crystal cell 1, different layer thicknesses, scratches, edges and / or spacers between two glass plates surrounding the liquid crystal. In the case of scattered light, this is distinguished between a fixed ratio l ^* _s and a dynamic ratio l ^* _d . By means of suitable technical measurements, the fixed scattered light ratio l ^* _s can be reduced to the extent that the user of the active dazzle protective filter does not impair the image properties (scattered light classification 1 according to European standard EN 379). In the case of the dynamic voltage dependent scattered light ratio l ^* _d , the situation is completely different. Around the scattered light center described above, when the operating voltage U is applied, local azimuth disruption occurs. The foreign material, or edges, that cause scattered light centers interfere with the uniform and chiral orientation of the liquid crystal molecules. This local orientation split is mostly due to the voltage dependent scattered light ratio (l ^* _d ). At high operating voltages U, the liquid crystal molecules are aligned more and more parallel to the field strength vector so that local azimuth cleavage disappears.

According to equation (2) the reduced radiant luminance coefficient l ^* shown in FIG. 4 is essentially the ratio of the scattered light flow φ _1R and the unscattered light flow φ _1L . In the case of the curve l ^* (U), it can be distinguished between the three ranges.

I. For low operating voltage U, φ _1R is less than φ _1L , so l ^* is less than one. In this first range I, φ _1L is reduced even more than φ _1R with increasing U, for which (l ^* (U)) increases monotonously.

Ⅱ. For the intermediate operating voltage U, φ _1R is almost φ _1L, so l ^* is almost one. In this second range (II), (l ^* (U)) is nearly constant.

III. For high operating voltage U, l ^* is smaller than 1 since once more φ _1R is smaller than φ _1L is applicable. In this third range (III), φ _1L is only slightly reduced or nearly constant with increasing U, while φ _1R for the reasons mentioned above is still reduced, for this reason (l ^* (U )) Is monotonically reduced.

In the present invention, the operating voltage U = U _LC is selected in such a way that the following conditions are performed.

a) the necessary transmission is achieved;

b) The reduced radiant luminance coefficient l ^* is minimal.

The operating voltage U _LC is determined by this as follows. Condition a) defines the band on the U-axis, where the operating voltage U _LC must be present to achieve the required transmission. As a result in this band, l ^* is minimum since the operating voltage U _LC is clearly determined according to condition b). In general, the operating point U _LC is arranged in the third range III of the curve l ^* (U).

If this is necessary, the transmission can be adjusted with slight rotation of the polarizers relative to each other or with adaptation of polarizer efficiency.

The active electro-optic filtering device and the method for operating the same according to the present invention as described above require a small operating voltage, there is an effect that the damage of the optical properties due to light scattering is less.

Claims (7)

A method for the operation of an active electro-optical filtering device having an active light filter element 1, The optical filter element (1) is driven with the opposite drive pulses, whereby the optical filter element (1) is shorted between two successive drive pulses. The method of claim 1, The short time (t _S1 and t _S2 ) is preferably shorter than the time duration (t ₊ , t ₋ ) of the drive pulse with a factor in the range of 10 ³ to 10 ⁷ . The method according to claim 1 or 2, And the constituent frequency f of the drive pulse is between 0.01 and 1 Hz. Preferably in a method for the operation of an active electro-optic filtering device having an active light filter element 1 according to claim 1, The operating voltage U _LC is applied to the optical filter element, and at the operating voltage U _LC , the scattered light ratio φ _1R of the optical filter element 1 is less than or equal to the transmission ratio of the optical filter element 1. How to feature. The method of claim 4, wherein The operating voltage U _LC is characterized in that it is carried out several times on the Frederiks limit of the optical filter element (1). At least one active optical filter element 1 having a liquid crystal, electronic means 2 for driving at least one active optical filter element 1, an optical sensor 4 operated in connection with the electronic means 2, and Active electric power which can be operated in a method according to any of the preceding claims, comprising an electric power source 5 for electronic means 2 and at least one active light filter element 1, in particular a solar cell. In the optical filtering device, And the liquid crystal is performed according to one of the following techniques: TN technique, STN technique, dichroic technique, ferroelectric technique or π mode LCD technique. In the drive circuit 2 for the active electro-optical filtering device according to claim 6, The drive circuit 2, characterized in that the active optical filter element 1 can be shorted together with the switch S ₁ .