Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1997-08-25
2001-01-16
Saras, Steven J. (Department: 2775)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S098000, C345S097000, C345S209000
Reexamination Certificate
active
06175350
ABSTRACT:
This invention relates to a circuit for driving a ferroelectric liquid crystal device, preferably a shutter.
Such shutters have a bistable characteristic, being either open or closed, and are switched from one state to the other by an applied voltage pulse, whose polarity determines the final state of the shutter. A typical voltage pulse might be 40 volts in amplitude, and 10 microseconds in duration. At low ambient temperatures, a longer pulse may be beneficial.
It is known that, to maximise the shutter life, the applied voltage should have zero d.c. component. Even a small fraction of a volt can be detrimental. A known difficulty lies in devising a drive circuit without any d.c. component. For instance, any circuit using a series coupling capacitor is likely to have problems with the initial capacitor charging current. GB-A-2208741 and GB-A-2262830 have addressed this problem.
A second known difficulty lies in opening and closing the shutter under control of an arbitrary waveform; if the opening and closing pulses come sufficiently close in time as to overlap, and the driving waveform is not a square wave with a mark/space ratio of 1:1, logic gating circuitry will truncate one polarity of pulse more than the other, resulting in a d.c. component at the output. Known drive circuits for ferroelectric liquid crystals have included bulky arrangements of driver transistors operating on high voltage supplies. The timing of input control signals to the drive circuit has to be carefully controlled, since closely spaced input drive signals may create the above problems in the output drive waveform.
The object of the invention is to provide a driving circuit whose output is substantially free from d.c. offset, and which makes it simple to control a shutter from an input control signal with arbitrary timing.
The present invention provides in one aspect, a drive circuit for a ferroelectric liquid crystal device, the circuit including a count means arranged for counting between first and second predetermined values in response to an input signal, and being such as to count in a first direction in response to a first parameter of the input signal and in a second direction in response to a second parameter of the input signal, and output means responsive to the counting of the counter means and the direction of count such as to provide appropriate positive or negative output voltage pulses for driving a ferroelectric liquid crystal device.
Thus in a preferred embodiment, the circuit includes a counter with end stops for maximum and minimum counts. The counter, which is clocked by a reference frequency, counts up when the device (shutter) is to be switched to one state, and down when it is to be switched to the other state. While it is counting up, the output voltage pulse is of one polarity; when counting down, of the other polarity. When the counter reaches either of the end stops, the shutter drive voltage returns to zero.
The invention ensures that the d.c. component across the shutter is zero under all conditions, independent of the timing of the waveform of the input signal, since over a long period the number of up counts must equal the number of down counts.
A preferred form of count means comprises an up-down counter driven by a clock signal, for example a 5 MHz oscillator driving an 8 bit counter, which has control inputs for determining the direction of count. The output of the count means is coupled through logic circuitry for providing output signals indicating that the counter is in a count condition and the direction of count, to output drive circuitry. Other count means may be employed, for example a processor unit or a general purpose micro processor, a circulating shift register, or more than one shift register, having one or more stages containing a bit signal of one type, and the remaining stages with a bit signals of the other type. It would be possible to have a single bi-directional shift register.
In a further form, the count means may include a dual slope capacitive integration arrangement for providing a highly linear voltage slope with time.
As preferred, the output drive circuitry responsive to the output of the count means includes a bridge arrangement of MOSFETs operating at a relatively low voltage with an output drive signal being provided through a step up voltage transformer.
In addition, a biasing drive signal of say 100 Hz may be provided from the clock signal through output drive circuitry and coupled to the liquid crystal shutter through a further transformer arrangement. The advantage of using transformer couplings is that of lack of any d.c. component in the output signal which might tend to degrade the liquid crystal. Although other forms of isolation may be envisaged, the advantage of using a transformer is that the output drive signal can be stepped up in voltage magnitude.
Accordingly, in a second aspect, the present invention provides a electrical drive circuit for a ferroelectric liquid crystal device, including timing means responsive to an input signal for producing drive control pulses to an output circuit, the output circuit producing rectangular output pulses in response to the control pulses and including transformer means for coupling the rectangular drive pulses to the liquid crystal display.
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patent: 4065916 (1978-01-01), Leuschner
patent: 4904064 (1990-02-01), Lagerwall et al.
patent: 5227900 (1993-07-01), Inaba et al.
patent: 5233446 (1993-08-01), Inoue et al.
patent: 5345146 (1994-09-01), Koenck et al.
patent: 5379051 (1995-01-01), Suga et al.
patent: 5900849 (1999-05-01), Gallery
patent: 2208741 (1989-04-01), None
patent: 2262830 (1993-06-01), None
Alphonse Fritz
Central Research Laboratories Limited
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Saras Steven J.
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