Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1997-05-12
2001-04-10
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C345S097000, C345S094000
Reexamination Certificate
active
06215533
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a ferroelectric liquid crystal device such as a large area flat panel display including a driving arrangement for reducing adverse effects caused by non-uniform heating of the device. The invention further relates to a driving arrangement for a ferroelectric liquid crystal array device and to a method of driving a ferroelectric liquid crystal array device.
BACKGROUND OF THE INVENTION
Ferroelectric liquid crystal materials are of important application to flat panel liquid crystal array devices because of their high switching speed and bistability. Unlike supertwist nematic liquid crystal devices, for example, the pixels of such a device will remain in a particular state without continued application of a particular drive voltage. In a large area panel display device which has to be addressed by multiplexing this is a significant advantage. Ferroelectric liquid crystal arrays and a driving scheme therefor are described in ‘The JOERS/Alvey Ferroelectric Multiplexing Scheme’ published in
Ferroelectrics,
1991, Vol.122 pages 63 to 79. In such driving schemes a liquid crystal array has a first and second set of driving electrodes arranged at right angles to each other defining a matrix. A plurality of pixels are defined at the intersection of an electrode from the first plurality and an electrode from the second plurality. However, by the very nature of this layout, it is not possible to address each pixel individually. The type of addressing scheme used most commonly applies a strobe signal in sequence to one of the sets of electrodes (referred to hereafter as the row electrodes) while applying the relevant data signals for the currently-strobed row to the second set of electrodes (hereafter referred to as the column electrodes).
One consequence of such a scheme is that the data signals applied to the column electrodes are applied to every pixel in the respective column, even though only one pixel in the column is actually being addressed at any one time. In a ferroelectric display it is not feasible to remove such signals (for example by open-circuiting the non-strobed row column electrodes) because they are required to apply an AC stabilisation signal to the pixels of the array. Such a signal prevents the liquid crystal molecules in the array relaxing to a position which has an unfavourable optical performance. These signals, however, are continually applied at a high frequency to every column electrode to drive a capacitive load including the pixels of the device. The column electrodes generally include transparent indium tin oxide (ITO) tracks which have a certain resistance so the charging and discharging of the pixels dissipates power in these tracks which heats the device.
The temperature of the device is particularly critical in a ferroelectric liquid crystal array device because of the large temperature sensitivity of ferroelectric materials themselves. To some extent effects of global temperature changes to the device can be compensated for in the addressing waveforms. For example changes in the switching speed (operating region) can be compensated for by changing the shape or amplitude of the strobe voltage, whilst changes in the angle of the director in an AC stabilised position can be compensated for by changing the amplitude of the column (data) waveforms. However, the prior art drive schemes such as the one described in the reference above, apply rectangular waves to the column electrodes to drive the device and these waveforms have a rich harmonic content including substantial frequency components at high multiples of the fundamental frequency. Since each column of the array appears as a distributed RC ladder to the driving circuitry, these higher harmonics of the driving waveform are attenuated heavily by the device and the highest attenuation occurs at the driven end of the column electrodes, in other words at the edge of the device. This causes non-uniform heating of the device that cannot be compensated by adjusting the row or column signals (since they clearly apply to all of the pixels in a column). The consequence of this is variations in contrast or colour over the array display device (or, in extreme cases failure to switch when addressed) which is unacceptable. Liquid crystal devices based on nematic liquid crystal phases do not suffer from these problems because of their higher tolerance of temperature variations.
SUMMARY OF THE INVENTION
It is an object of the present invention to ameliorate the above problem in ferroelectric liquid crystal devices.
It is a further object of the invention to provide a novel driving arrangement for a ferroelectric liquid crystal array device and to provide a novel method of driving such a device.
According to a first aspect of the present invention there is provided a ferroelectric liquid crystal device including a layer of ferroelectric liquid crystal material contained between a pair of substrates and a first plurality of electrodes and a second plurality of electrodes defining a plurality of addressable liquid crystal pixels and a driving arrangement for applying a first signal in succession to the first plurality of electrodes and for applying a plurality of second signals simultaneously to the second plurality of electrodes, wherein the plurality of second signals include non-rectangular wave signals which have a lower harmonic content than a rectangular wave.
According to a second aspect of the present invention there is provided a driving circuit for a ferroelectric liquid crystal device which device includes a matrix of liquid crystal cells addressable via a plurality of row electrodes and a plurality of column electrodes, the driving circuit including row driving means for applying a first signal in succession to the plurality of row electrodes and column driving means for simultaneously applying a plurality of second signals, which second signals each include one of at least two data signals, to the plurality of column electrodes, wherein at least the means for applying a plurality of second signals provides a signal, at least a portion of which signal has a substantially continuously varying level.
According to a third aspect of the present invention there is provided a method of driving a ferroelectric liquid crystal device which device includes a matrix of liquid crystal cells addressable via a plurality of row electrodes and a plurality of column electrodes, the method including driving the rows of the device by applying a first signal in succession to the plurality of row electrodes and driving the columns of the device by simultaneously applying a plurality of second signals to the plurality of column electrodes, which second signals each include one of at least two data signals, wherein at least a portion of the data signals has a substantially continuously varying level.
The present invention is based upon the realisation that the non-uniform heating of a ferroelectric liquid crystal device as described above can be reduced considerably by driving the column electrodes with a signal that is substantially lower in harmonic content than the rectangular wave type driving waveforms of the prior art technologies. Particular non-rectangular waveforms of interest are sinusoidal waveforms, triangular waveforms and trapezoidal waveforms. The sinusoidal waveform clearly has the lowest harmonic content of the three: ideally being zero above the fundamental frequency. However, the higher harmonic content of the other two waveforms is low and these waveforms have the advantage that they can generally be provided with simpler circuit arrangements than can a suitable sinusoidal waveform. If, for example, the waveforms are provided by a digital circuit connected to a digital to analogue converter (D/A), a triangular waveform can be generated by an up-down counter connected to the D/A. A sinusoidal waveform would generally require a memory containing a large number of sample values for feeding to the D/A. A trapezoidal signal could be provided using a smaller numb
Bonnett Paul
Hughes Jonathan Rennie
Raynes Edward Peter
Scattergood David
Shigeta Mitsuhiro
Parker Kenneth
Renner , Otto, Boisselle & Sklar, LLP
Sharp Kabushiki Kaisha
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