Patent Grant
7323824
This invention relates to arc discharge lamps and, more particularly, to methods and apparatus for operating very high pressure short arc mercury discharge lamps primarily used for projection applications.
Very high pressure discharge lamps include an arc tube containing an inert gas, mercury vapor, and two electrodes positioned at opposite ends of the arc tube. An arc discharge is established in the arc tube by supplying an electrical current to the electrodes. The very high pressure discharge lamp is typically utilized for projection applications, where the optical system requires point-like light sources. To achieve such optical performance, the arc length must be on the order of 1.0-1.5 millimeters. The lamps typically include an arc tube constructed of heat resistant and optical transparent material such as quartz, tungsten electrodes, mercury vapor and an inert starting gas. The electrodes are constructed of a tungsten rod with a tungsten coil attached to one end.
Typically, the electrode tip in very high pressure discharge lamps reaches temperatures close to or even above the melting point of tungsten. This is necessary to prevent movement of the point of arc attachment to the electrode, also called the arc root. However, if the electrodes become too cold, the molten tip solidifies and collapses to a relatively flat surface. The arc attachment becomes unstable, leading to sudden arc movement or jumping. Moreover, the distance between electrodes increases, thereby reducing the performance of the light collecting optics. On the other hand, if the electrodes become too hot, the molten region increases, leading to a meltback condition where the distance between electrodes increases, thereby reducing the performance of the light collecting optics. Moreover, during the meltback, increased amounts of tungsten are evaporated from the electrodes and deposited on the arc tube walls, leading to poor lamp maintenance. Therefore, the operation of very high pressure discharge lamps needs to be optimized in order to achieve the most beneficial electrode temperature.
A problem in establishing the optimal temperature for the two electrodes of a discharge lamp may arise when the lamp current is varied. Such a situation occurs as the lamp voltage increases. To maintain constant power conditions, prior arc ballast circuitry reduces the lamp current. As a result, the tip temperature of one electrode is reduced such that the tip starts to solidify. Upon solidification, the tip of the electrode diminishes, leading to a larger arc gap and therefore, a larger lamp voltage. The current supply to the lamp decreases and the electrode tip temperature is further reduced. The process continues until the electrode surface becomes flat. Arc instability may then occur.
Furthermore, a very high pressure discharge lamp is usually mounted in a reflector, which changes the thermal environment of the arc tube. One end of the lamp, and thus one electrode, may be hotter than the other end. In typical applications, very high pressure discharge lamps are operated with forced air cooling, which is usually directed to both sides of the lamp or to the upper side of the lamp. Depending on the configuration of the cooling airflow, different lamp performance is achieved.
During the lifetime of a discharge lamp, the structure of the electrodes may change due to tungsten transport from the tip of the electrode. In cases where one of the electrodes has started to melt back, its ability to conduct heat from the electrode tip changes, and the flattening process may accelerate, leading to early lamp failure. Using preshaped electrodes cannot compensate for most of these asymmetries, because they are unpredictable. Different electrode shapes require additional devices to permit proper mounting of the lamp in the system in which it is employed.
Techniques and circuits for operating high pressure discharge lamps are disclosed in U.S. Pat. No. 5,608,294, issued Mar. 4, 1997 to Derra et al.; U.S. Pat. No. 6,232,725, issued May 15, 2001 to Derra et al.; U.S. Pat. No. 6,239,556, issued May 29, 2001 to Derra et al.; and International Publication No. WO 2004/002200, published Dec. 31, 2003. All of the known prior art techniques have had one or more drawbacks and disadvantages.
Accordingly, there is a need for methods and apparatus for operating very high pressure discharge lamps with improved performance and lifetime.
According to a first aspect of the invention, a method is provided for operating a high pressure discharge lamp. The method comprises controlling an alternating lamp current supplied to the lamp at a constant RMS value, and adjusting the lamp current to a new RMS value to prevent power supplied to the lamp from exceeding an upper power limit. The lamp current is maintained constant at the new RMS value.
According to a second aspect of the invention, a lamp system comprises a high pressure discharge lamp, a power circuit for supplying alternating current to the lamp, and a controller configured to control the lamp current at a constant RMS value and to adjust the lamp current to a new RMS value to prevent power supplied to the lamp from exceeding an upper power limit. The lamp current is maintained constant at the new RMS value.
According to a third aspect of the invention, a method is provided for operating a high pressure discharge lamp including an arc tube. The method comprises controlling an alternating current supplied to the lamp at a constant RMS value, and adjusting lamp cooling to maintain a wall temperature of the arc tube below a softening temperature.
According to a fourth aspect of the invention, a lamp system comprises a high pressure discharge lamp including an arc tube, a power circuit for supplying alternating current to the lamp, a cooling source directed at the lamp, and a controller configured to control the lamp current at a constant RMS value and to adjust the cooling source to maintain a wall temperature of the arc tube below a softening temperature.
According to a fifth aspect of the invention, a method is provided for operating a high pressure discharge lamp. The method comprises controlling an alternating lamp current supplied to the lamp at a constant RMS value, and adjusting the lamp current to a new RMS value to maintain lamp power between an upper power limit and a lower power limit. The lamp current is maintained constant at the new RMS value.
According to a sixth aspect of the invention, a lamp system comprises a high pressure discharge lamp, a power circuit for supplying alternating current to the lamp and a controller configured to control the lamp current at a constant RMS value and to adjust the lamp current to a new RMS value to maintain lamp power between an upper power limit and a lower power limit. The lamp current is maintained constant at the new RMS value.
According to a seventh aspect of the invention, a method is provided for operating a high pressure discharge lamp. The method comprises controlling an alternating lamp current supplied to the lamp at a constant RMS value; and increasing the lamp current to a new RMS value in response to a prescribed value of lamp voltage and maintaining the lamp current constant at the new RMS value.
According to an eighth aspect of the invention, a lamp system comprises a high pressure discharge lamp; a power circuit for supplying alternating lamp current to the lamp; and a controller configured to control the lamp current at a constant RMS value, to increase the lamp current to a new RMS value in response to a prescribed value of lamp voltage and to maintain the lamp current constant at the new RMS value.
For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:
A schematic block diagram of a lamp system in accordance with a first embodiment of the invention is shown in
Very high pressure discharge lamps typically include an arc tube 30 constructed of a heat resistant and optically transparent material, such as quartz. Tungsten electrodes 32 and 34 are mounted at opposite ends of arc tube 30, and the interior volume of arc tube 30 contains mercury vapor and an inert starting gas. Each of electrodes 32 and 34 includes a tungsten rod 40 having a tungsten coil 42 attached to one end, as shown schematically in
Electronic power supply 20 includes a power circuit 50, an ignition circuit 52 and a controller 54. When input terminals I1 and I2 of electronic power supply 20 are connected to an AC voltage supply, power circuit 50 generates an alternating current having successive periods of alternate polarity and of predetermined shape. By way of example, the alternating current may be a square wave. However, the alternating current is not limited as to wave shape. Ignition circuit 52 ensures lamp starting.
Controller 54 senses the operation of discharge lamp 10 and controls lamp current in accordance with embodiments of the invention. The controller 54 includes a lamp current sensor 60, a lamp voltage sensor 62 and a control circuit 64. Lamp current sensor 60 senses a lamp current value and provides outputs to power circuit 50 and control circuit 64. Lamp voltage sensor 62 senses a lamp voltage value at the output of power circuit 50 and provides a voltage value to control circuit 64. Control circuit 64 controls power circuit 50 in this embodiment. Control circuit 64 computes lamp power and maintains lamp power within specified limits as described below. Power circuit 50 maintains a constant value of RMS lamp current, with changes to new constant values of RMS lamp current in response to changes in lamp operation. The lamp current is an alternating current, having successive periods of alternate polarity and of predetermined shape. The RMS (root mean square) value of the alternating current is maintained constant over extended periods of time, its value depending on lamp operation as described below.
When discharge lamp 10 is energized, the tips of electrodes 32 and 34 typically reach temperatures close to or even above the melting point of tungsten. Such operating temperatures are necessary to prevent movement of the point of arc attachment to the electrode, also called the arc root. As shown in
Vlmp2≅Vlmp nom·½·[Plmp hi/Plmp nom] (1)
where Vlmp nom=nominal lamp voltage,
Plmp hi=upper power limit, typically 120% of rated lamp power, and
Plmp nom=nominal lamp power.
When the discharge lamp is operated with a conventional constant power ballast, the electrical power supplied to the lamp is substantially the product of the RMS values of lamp voltage and lamp current. As the lamp RMS voltage increases, the lamp RMS current is decreased proportionally to maintain the lamp power constant. Consequently, the electrode tip temperature decreases and the molten tip region starts to solidify and contract. The electrode surface becomes flat and the arc root attachment becomes unstable, leading to lamp flicker. The flicker and flicker-free modes of lamp operation may be assessed by measuring lamp relative light output (RLO). A photodiode positioned in front of the lamp reflector was utilized to quantify the light output. Typical signal traces are illustrated in
In another embodiment of the lamp control routine shown in
In a further embodiment of the lamp control routine as shown in
The reaction of the discharge lamp to changes in RMS current is typically relatively slow. Thus, control circuit 64 preferably implements a delay following a change in RMS current before another change in RMS current can occur. The delay permits the discharge lamp to stabilize in response to the new operating conditions. The delay may be on the order of 25 to 50 hours, for example.
A flow chart of a control process implemented by controller 54 in the embodiments of
In step 88, the lamp power Plmp is compared to the upper power limit Plmp-hi. If the lamp power Plmp is equal to or greater than the upper power limit, the lamp current Ilmp is decreased to lamp current Ilmp3 in step 90. The process then returns to step 82 and the lamp current is maintained constant at the new RMS value Ilmp3. In step 92, the lamp power Plmp is compared with the lower power limit Plmp-lo. If the lamp power Plmp is less than or equal to the lower power limit, the lamp current Ilmp is increased to a new RMS value Ilmp4 in step 94. The process then returns to step 82 and the lamp current is maintained constant at the new RMS value Ilmp4. In the case where the lamp voltage does not exceed the prescribed value Vlmp2 and the lamp power remains between the upper and lower power limits, the lamp current is maintained constant at the present RMS value in step 82.
A further embodiment of the lamp control routine is illustrated in
The lamp current may be adjusted (increased or decreased) from one constant RMS value to a new constant RMS value continuously or in one or more steps. In one example, the lamp current is adjusted in increments of 1-2% of the lamp current to avoid an abrupt change in light output. Preferably, the lamp current is adjusted, based on known characteristics of the lamp, to bring the lamp power from the upper or lower power limit to or near the rated lamp power.
Depending on operating power and projector system fixturing, very high pressure discharge lamps operate at high temperatures. Accordingly, cooling techniques are employed to extend the useful life of the lamp system. A typical prior art arrangement is shown in
A block diagram of a lamp system incorporating lamp cooling control in accordance with a further embodiment of the invention is shown in
A lamp control routine in accordance with a further embodiment of the invention is illustrated in
When the material used for arc tube 30 is quartz, the critical temperature is not to exceed the devitrification point. This temperature is lower than the softening temperature and is the temperature at which quartz starts becoming opaque and loses optical transmission, typically around 1000° C.
A flow chart of a lamp control process implemented by controller 54 in the embodiments of
Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5572094 | Yamashita et al. | Nov 1996 | A |
| 5608294 | Derra et al. | Mar 1997 | A |
| 6153987 | Toda et al. | Nov 2000 | A |
| 6232725 | Derra et al. | May 2001 | B1 |
| 6239556 | Derra et al. | May 2001 | B1 |
| Number | Date | Country |
|---|---|---|
| WO 2004002200 | Dec 2003 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20060028149 A1 | Feb 2006 | US |
