Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
1998-12-29
2001-04-10
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S307000
Reexamination Certificate
active
06215252
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to a method and apparatus for operating lamps, and more particularly to lamp controllers for preserving the performance characteristics of discharge lamps.
2. Description of the Related Art
High performance lighting systems are known in the art which incorporate discharge lamps designed for producing bright light from a small arc, and precision reflectors. Such systems include HID lamps that are short-arc high pressure mercury lamps in the form of a spherical or ovoid quartz bulb with a very short gap between the electrodes, typically between 1 mm and 1.4 mm. As a result, arc luminances rank among the highest of all lamp types. The chief asset of the short-arc mercury lamp is that it forms a near point source of light of very high luminance which makes it ideal for systems that employ a critically focused optical light collection system which usually depends on the maintenance of a very small arc length and accurate arc position for optimum performance. This makes it ideally suited for projection purposes, for example, a TV projection system or an LCD display apparatus.
These lamps are used in connection with precision reflectors and optics to produce a uniform collimated output with maximum light collection. The imaging properties of these optics are not critical, but rather their function is to collect substantially most of the energy emitted from the arc lamp and direct it into a beam with minimum etendue or geometric optical size. It is advantageous for the optical collection system to produce a light beam with as small an extent as possible to reduce power consumption and to reduce manufacturing costs associated with projection systems of smaller size and/or aperture.
There are several advantages associated with this system. As described above, it is desirable to produce light from a small arc gap because, inter alia, the smaller the arc gap the smaller the power that is needed to produce a given light output. Likewise, the optical collection systems described above incorporate reflectors that require a stable arc, and whose collection efficiency is severely reduced when the arc moves about significantly. For example, in some systems, a shift in the arc of only 1 mm can result in loss of up to 50% of collected light.
Inherent characteristics of the above lamp are variations in the arc gap between the electrodes. During AC operation of the lamp, the discharge arc is not stable because the origin of the discharge arcs jumps among random locations at the tip of the electrode. Likewise, tungsten material (W) from the electrode itself evaporates during lamp operation and redeposits on the electrode tip.
It is known in the art to provide arc lamp power supplies which are configured to produce a constant power for the arc lamp. When such supplies are employed, power within the arc is measured, and the current is adjusted such that a desired power is achieved and maintained. Consequently, if the arc burns back the electrodes to produce a longer arc, or the arc for any reason becomes shorter, appropriate voltage and current changes are invoked to maintain the preset power desired. However, these power supplies have no mechanism for sensing or control of the lamp arc gap. As a result, the arc gap is left to find its own natural dimensional envelope while power is maintained. Thus the arc gap may fluctuate or reach unacceptable dimensions.
Several approaches have been tried to improve the consistency and reliability of lamp operation. For example, U.S. Pat. No. 5,608,294 to Derra et al, which is hereby incorporated by reference, discloses a method of operating a lamp to reduce flicker arising from the above-described random jumping of the discharge arc. The method disclosed therein comprises the addition of a stabilization current pulse which is generated in a predetermined fraction of the half periods of the AC square wave lamp current. The stabilization current pulse has the same polarity as the lamp current and is superimposed on the lamp current in the latter part of the half period in which it is generated. It was found that when operating a high pressure discharge lamp according to the Derra et al method and apparatus, flickering of the lamp can be substantially suppressed. However, this technique permits the arc gap to vary over an unacceptable range for some critical systems, such as the reflectors and optics described above.
SUMMARY
A method is disclosed for operating an electric discharge lamp having a pair of electrodes with an arc gap therebetween. The method includes the steps of applying a voltage and current to the electric lamp from a power supply, and providing a controller having means for sensing given parameters of lamp operation and means for generating a voltage and current to be applied by the power supply to the electric discharge lamp. The method further includes sensing the parameters of lamp operation such as lamp current and/or lamp voltage. The sensed parameters are compared with desired lamp parameters. The controller monitors the lamp parameters and in response commands the power supply to add a current pulse to a specific temporal portion of the lamp current waveform.
An apparatus for operating a discharge lamp having a pair of electrodes with an arc gap therebetween comprises a power supply for applying an operating voltage and current to the lamp electrodes and means for sensing the electric parameters of the discharge lamp. The apparatus also includes a controller responsive to the sensed electric parameters of the lamp for deriving an output control signal for the power supply. The controller includes means for comparing the sensed electric parameter with a corresponding desired parameter of the lamp. The power supply responds to the output control signal of the controller to adjust the lamp current so as to apply thereto a high-energy current pulse during a specific temporal portion of the lamp current waveform in a manner so as to compensate for any deviation of the lamp electric parameter from the corresponding desired parameter.
An important aspect of the invention is the recognition that the energy content of a narrow high-energy current pulse superimposed on the lamp current waveform exerts a significant influence over the length of the arc by control of the amplitude of the current pulse while monitoring the electrode voltage, which is a close analog of the length of the arc gap. The energy of the high-energy current pulse can be controlled by monitoring the length of the arc gap between the lamp electrodes by means of a microprocessor feedback loop, and in a manner so as to maintain substantially constant the length of the arc gap over a long time period. Another aspect of the invention is the discovery that the arc gap is very sensitive to the energy content of the high-energy current pulse, much more so than it is to the energy in the lamp current waveform.
As time goes on during the operation of the discharge lamp, it is usual for the length of the arc gap between the lamp electrodes to increase. The invention makes it possible it automatically adjust the high-energy current pulse so as to readjust the length of the arc gap to return it to its original length. If desired, the arc gap can, over time, be automatically adjusted to reduce the size of the arc gap below that of its original length. The opposite effect is also possible, i.e. to increase the length of the arc gap beyond its original length when the lamp was new.
Another feature of the invention is that one can describe the arc length as a function of the arc voltage and can store this function in the microprocessor controller.
This invention makes it possible to develop a discharge lamp ballast having a high narrow current pulse the amplitude of which can be adjusted automatically during operation of the discharge lamp. The pulse height (amplitude) of the narrow current pulse (high-energy pulse) can be adjusted according to an algorithm which keeps the arc voltage constant. For example, if the arc
Dinh Trinh Vo
Franzblau Bernard
Philips Electronics North America Corporation
Wong Don
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