[0001] AMPLITUDE CONTROL OF GAS DISCHARGE LAMPS [0002] BACKGROUND [0003] This invention relates generally to a ballast control for a gas discharge lamp. More particularly, the invention relates to a method and apparatus for starting and operating a lamp for illumination of a target for a high speed camera. A preferred lamp for producing the requisite illumination is a gas discharge lamp, such as the high pressure sodium type.

[0004] Operation of a lamp directly from a standard 60 Hz power source, as is done with a standard flourescent lamp, is not practical because a charge-coupled device (CCD) camera has the ability to follow high illumination frequencies (up to 40Khz). Accordingly, using a 60Hz power source would result in the camera detecting a flicker in the lamp output, thereby degrading camera performance. Although DC power can be used to operate a lamp, it is typically avoided as it is known to cause deterioration in lamps and shortening of bulb life, along with some discoloration in phosphor coated lamps. Efficient switching means, such as introducing a switching transistor, can be used to produce the required AC power to drive the lamp.

[0005] Ideal conditions for gas discharge lamps are high voltage, high frequency current for starting the lamp, and lower voltage and frequency during the regular duty operation of the lamp. Inductor and capacitor tank circuits can be used to provide simple control of circuit frequency. However, when using L-C circuits, resonant frequencies can reduce the ballast control circuit impedance to the point where ballast control circuit elements can be exposed to dangerously excessive current levels. Therefore, the ballast control circuit must be designed to allow lamp starting or running at frequencies near resonance, but never equal to resonance.

[0006] Control of lamp intensity is also a preferable feature for ballast control circuits, especially in response to the negative effects of lamp aging. As gas discharge lamps age, the amount of current required to drive the lamp while maintaining the original illumination level increases. Therefore, there exists a need for lamp illumination to be variably controlled by the system, and that the amount of illumination be repeatable on command, regardless of bulb aging. Since relying solely on monitoring lamp current can be inaccurate due to the variable effect of lamp current with lamp aging, it is also preferable to include more accurate means for monitoring the lamp output to allow for appropriate compensation and adjustments to lamp input.

[0007] Variable control of lamp illumination intensity is also useful with high speed cameras where electronic adjustment of the lamp intensity is simpler and faster than the mechanical aperture adjustment of the camera.

[0008] SUMMARY [0009] A ballast control circuit for a gas discharge lamp applies variable voltage amplitude or pulse width modulation to provide variable lamp illumination intensity control. Power factor correction is employed in a manner that can also serve as a variable control of lamp input power. A power switch provides a switched square wave, which is a preferred power input for a gas discharge lamp. A closed loop controller monitors lamp illumination intensity, lamp current and lamp voltage, processes the lamp output signals and sends a control signal to the amplitude controller to make any necessary adjustments to either attenuate or intensify the lamp power input.

[0010] BRIEF DESCRIPTION OF THE DRAWING [0011] FIG. 1 shows a block diagram for a ballast control system for a gas discharge lamp.

[0012] FIG. 2 shows a variable voltage or current signal for input to a gas discharge lamp generated by variable control of system voltage amplitude.

[0013] FIG. 3 shows a variable voltage or current signal for input to a gas discharge lamp generated by a variable amplitude controller having a pulse width modulator.

[0014] FIG. 4 shows a block diagram for a ballast control system for a gas discharge lamp with variable amplitude power factor correction.

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0016] Referring to FIG. 1, a block diagram is shown for a ballast control system 10 of a gas discharge lamp H1. Ballast control system 10 comprises power generator 130, power switch 103, lamp circuit 104, and feedback circuit 131. Power generator 130 comprises power conversion unit 101, amplitude control unit 102, and frequency oscillator 107, which combine to generate a constant frequency power signal. Power switch 103 produces a square wave power signal to lamp circuit 104.

Lamp circuit 104 contains lamp H1, socket 108, transformer T1 and impedances C1, C2 and Ll. Feedback circuit 131 includes illumination control unit 105, which processes various inputs from a plurality of sensors, S1-S3, SN and input SX, to produce a control signal to amplitude control unit 102.

[0017] The power conversion unit 101 conditions the power supply to the remainder of the ballast control system 10. Conversion unit 101 converts AC voltage power source input to DC voltage, and boosts DC voltage power source input to an acceptable level. The input to the power conversion unit 101 can be any one of the following, but not necessarily limited to: 110 VAC, 220 AC, low voltage AC, universal line, low voltage DC, or high voltage DC. Only high voltage power is capable of directly driving the lamp H1 without conditioning. A typical range for drive voltage is between 230V and 400V ; the lower end of the range being for a new high pressure sodium (HPS) lamp, and the upper end for an aged HPS lamp.

[0018] Various options for power conversion in power conversion unit 101 are available, partially depending on whether an optional output transformer Tlis included in circuit 10. Transformer T1 is shown in FIG. 1 as part of lamp circuit 104, which is described hereafter in greater detail. If transformer T1 is employed, the power conversion in unit 101 can be achieved with a full bridge rectifier and filter for a 220 VAC power source input, or a voltage doubler and filter for 110 VAC power source input.

[0019] For a low voltage DC power source, power conversion 101 comprises a step-up DC/DC converter. Alternatively, a high turns ratio on the output power transformer T1 can be relied upon to provide adequate voltage to drive lamp H1.

[0020] Power conversion unit 10 1 produces a DC voltage that is converted back to AC at a frequency suitable to the camera specifications. Fixed frequency oscillator 107 provides the basic operating frequency for system 10. When lamp H1 is used for illumination in high speed photography, a suitable frequency is selected such that the camera can not detect the flicker in lamp output resulting from the AC mode of operation. For example, for a CCD camera sensitive to illumination up to 40kHz, a typical operational frequency for a lamp ballast is between 60 to 65kHz. Fixed frequency oscillator 107 can be selected to produce any suitable frequency higher than the standard line power source and even substantially higher than the sensitivity of the intended purpose for illumination.

[0021] The amplitude control unit 102 provides control of the brightness of the lamp H1 by modulating the amplitude of the AC power drive signal, as opposed to modulating the frequency of the AC power drive signal. In a first embodiment of the present invention, amplitude control unit varyies the voltage amplitude applied to the power switch 103 to produce variable signals 110,111 using any one of a"D"class amplifier, a linear amplifier or a variable DC/DC converter. Under this mode of operation, oscillator 107 directly produces an AC signal 112 at a fixed frequency for alternately operating switches Q1, Q2 in power switch 103. Except when a linear amplifier is utilized in the amplitude control unit 102, some sort of switching conversion on the incoming high DC voltage may be used to make it variable for the power switch 103, such as through a DC-DC converter or some equivalent voltage boosting device.

[0022] In a second embodiment ofthe present invention, amplitude control unit 102 includes a pulse width modulator (PWM) 140 to vary the duty cycle of the fixed frequency signal from oscillator 107, producing signal 112. Switches Q1, Q2 in the power switch 103 are then responsive to the pulse width modulated signal 112.

[0023] The power switch 103 operates on the output power from the amplitude control unit 102. Although FIG. 1 depicts switches Q1 and Q2 as MOSFET transistors, there are various suitable transistors (including but not limited to BJT and IGBT) that can be selected for the purpose of switching positive and negative voltage for a square wave input to transformer T1. In a first embodiment, variable voltage 110, 111 is sent from control unit 102 and power switch 103 is operated at a 50% duty cycle signal 112 to produce a variable amplitude square wave to be applied to the lamp circuit 104. FIG. 2 graphically shows an output signal 113 from power switch 103 where the voltage and/or current to lamp H1 decreases in magnitude in response to a control signal 114 demanding decreased power input to lamp H1.

[0024] In a second embodiment, the amplitude modulation is accomplished by pulse width modulation 112 at the power switch, in which the on time of each switch Q1, Q2 of the power switch 103 is a portion of the total time allowed by the basic operating frequency of the system 10, and each switch Q1, Q2 is on for a equal amount of time. FIG. 3 graphically shows an output signal 113 from power switch 103 which is pulse width modulated such that the voltage and/or current to lamp H1 decreases in magnitude in response to a control signal 114 demanding decreased power input to lamp H1.

[0025] The lamp circuit 104 comprises capacitors Cl, C2, inductor L1, lamp H, lamp socket 103 and optional transformer T1. Capacitor C 1 is sized to a minimum threshold impedance for passing AC, while providing DC isolation forpreventing DC bias. Capacitor C1 is not used to form a tank circuit with inductor L1 at the system operating frequency. Therefore, the basic impedance is provided by inductor L1, which does not operate in resonance with capacitor C 1 during normal operation of the lamp.

[0026] Capacitor C2 is used to resonate with inductor Ll only when the lamp Hl is not lit. A high starting voltage is required to start gas discharge lamps.

Capacitor C2 is selected such that it forms a resonant system with inductor LI at a third harmonic frequency, which is three times the normal operating frequency. The third harmonic frequency is preferable to take advantage of the large third harmonic component available in the square wave that is applied to lamp circuit 104.

[0027] When the lamp H1 is required to start, the normal operating frequency is applied to lamp circuit 104. This causes the tank circuit formed by inductor L 1 and capacitor C2 to resonate at the third harmonic of the operating frequency. Lamp H1 then receives the third harmonic frequency which is at a relatively higher frequency favorable for the starting characteristics of lamp H 1. The ballast control system 10 is designed to provide either a gentle start by ramping up the amplitude of the applied voltage until lamp H 1 current is sensed, or a sudden start by providing the bulb with a high voltage third harmonic pulse, which ever is more appropriate for the particular lamp type installed in the lamp circuit 104. Either of these controlled starts can be preprogrammed into illumination control unit 105. Transformer T1 produces a high voltage adequate for starting the lamp. Once lamp HI starts, it becomes a parallel low resistance path relative to capacitor C2, thereby rendering capacitor C2 ineffective.

The resonance of inductor LI and capacitor C2 is eliminated once capacitor C2 is essentially short circuited by the conducting lamp H 1 during post startup operation.

[0028] As shown in FIG. 1, feedback circuit 131 includes illumination control unit 105 ; a plurality of sensors: light sensor S 1, current sensor S2, voltage sensor S3, remote sensor SN; and external computer control input SX. Illumination control unit 105 comprises a processor with an associated memory (not shown). The illumination control unit 105 receives system parameter inputs, such as lamp H 1 current 120, lamp H 1 voltage 121, light intensity 122 sensed by light sensor S 1, remote sensor SN signal 123 and external computer input SX control signal 124. From these inputs, a processor in the illumination control unit 105 provides closed loop illumination control by sending DC voltage control signal 114 to the amplitude controller 102 to either vary the voltage at an amplifier or a variable PFC, or vary the duty cycle applied to PWM 140, depending on which embodiment of amplitude control unit 102 is utilized.

[0029] Operation of control unit 105 serves various control functions of lamp H1 including startup, lamp aging compensation, and diagnostics. With respect to lamp starting, control unit 105 uses input signals 120,121 and 122 from sensors S 1, SZ and S3 in order to efficiently control lamp Hl input power during the initial startup and initial running mode. For approximately the first four minutes following initial startup, illumination unit 105 predominantly uses current control since a cold lamp will not initially produce a readable voltage. As shown in FIG. 1, sensor S2 is a current transformer coupled near lamp H1 to sense current flow through lamp H1. After the four minute warmup period, lamp output power predominantly provides input to illumination unit 105 for producing control signal 114 during the next minute or so until luminous intensity level is at an adequate level for monitoring. Lamp output power is calculated by the processor of illumination controller 105 as a product of lamp current 120 and sensed lamp voltage 121. At approximately the fifth minute, luminous intensity input 122 from sensor S 1 is sufficiently reliable for providing input to control unit 105.

[0030] Control unit 105 and lamp current sensor S2 also provide a diagnostic capability for monitoring useable lamp life. As lamp H1 ages, the required current to maintain the same lamp intensity increases. Once the lamp current reaches a predetermined level, alarm 150 is triggered indicating that the useable life of lamp H1 has expired and replacement of lamp H1 is necessary [0031] Sensor SN is used to monitor system voltage prior to lamp start to determine any system faults or opens. Signal 123 is monitored by the illumination control unit 105 to detect the presence fault conditions sensed by sensor SN. Input SX represents an external computer input that communicates by signal 124 with the illumination unit 105 the belt speed of a conveyor that holds targets for the high speed camera, allowing variable control of illumination based on target speed, rather than adjustment of the camera aperture.

[0032] FIG. 4 shows an alternative embodiment where power factor correction (PFC) unit 200 may be included in system 10, replacing power conversion unit 101 and amplitude control unit 102. PFC unit 200 is similar to conversion unit 101 in that it rectifies an AC voltage input, but the voltage is left unfiltered. PFC unit 200 comprises a power correction circuit which operates to improve power factor in the power supply signal. Like amplitude control unit 102, PFC unit 200 produces a variably controlled voltage. The output boost of PFC unit 200 has a variable output which behaves similar to a D-class amplifier. Control signal 114 from illumination control unit 105 can variably adjust the setting of the output power boost. Since amplitude modulation is achieved with variable voltage amplitude, PFC unit 200 eliminates the need for PWM 140 as used in amplitude control unit 102. PFC unit 200 eliminates the need for transformer T1.

[0033] Alternatively, PFC unit 200 may employ a fixed output power boost.

With the output voltage fixed, a PWM (not shown) is combined with PFC unit 200 for providing amplitude modulation.

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