CA1038021A

CA1038021A - Laser power supply

Info

Publication number: CA1038021A
Application number: CA196,947A
Authority: CA
Inventors: David R. Whitehouse; David W. Hartshorn
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1973-05-14
Filing date: 1974-04-05
Publication date: 1978-09-05
Also published as: GB1432920A; JPS5017784A; JPS5512753B2; US3896396A; CH582434A5; DE2422678C2; DE2422678A1

Abstract

A B S T R A C T
A method of energizing a laser source stimulating flash lamp directly from an AC power line. Uncontrolled diodes couple the anode and cathode of the flash lamp directly to the AC line. The lamp is triggered by a separate triggering circuit which produces its trigger pulse at a predeter-mined phase of the AC power source. The use of high current carrying con-trolled rectifiers and large energy storage devices is thereby eliminated.

Description

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Producing sufficient light to exci~e a laser source used in welding and drilling applications has long be~n a problem.
Initial attempts included those where large encrgy storage dovices such as capacikors and inductors were charged or periods of time long in comparison to typical AC line frequencies then, when fully charged, were switched across the flash lamp. These energy stor-age devices tended to be both bulky and expensive and moreover the maximum rate at which the laser could be fired was low in that the time required to bring them to their peak storage capabilities ; 10 from a rectified power line source is long compared with desirable welding pulse rates. Later attempts included coupling the flash lamp to an AC line source through controlled rectifiers.
These la*er attempts required large, expensive, controlled recti-fiers to implement any scheme in which enough power was trans-ferred from the power line to the 1ash lamp. The prior art has not shown methods of coupling a ~lash lamp to an AC power source through simple uncontrolled rectifier means.
It is thus an object of the present invention to power a flash lamp without the use of large energy storage devices.

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It is also an object of the present invention to fire a flash lamp from an AC power source without the use of controlled rectifiers.
Furthermore, it is an object of the present invention to fire a flash lamp from an AC power source at an arbitrary phase angle of the power source without the use of controlled rectifiers in the high current carrying portions of the circuitry.
Moreover, it is an object of the present invention to fire a flash lamp from an AC power source for a longer period of time than a single half cycle of the AC power source.
These objectives, as well as others of the present invention, may be met by providing the combination of uncontrolled rect~ ing mean~ for coupling one or more means for producing radiat.ion to a sourcc of sinuso:idal alternating power, power for operating ~a:Ld noncoherent light producing means flowing directly from said source of alternating power through said uncontrolled rectifying means with no substantial energy storage means therebetween, said radiation producing means operating upon application thereto of a trigger signal; and means for producing said trigger signal at a predetermined phase angle of said alternating power source, said means for producing said trigger signal comprising means for sensing the phase of said alternating power source.
In the preferred embodiment, the radiation producing means comprises means for producing light from the ionization of the gas, such as may be done with a xenon flash lamp. Also, in the preferred embodiment, the rectifying means comprises semiconductor diodes~ In the preferred embodiment the means for sensing the phase of the alternating power source generates the trigger signal at that predetermined phase.
Brief Description of the Drawings Figure 1 is a simplified circuit diagram showing therein a flash lamp excited from an ~C power line, in accordance with the present invention;

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Figure 2 is a simplified circuit diagram showing an alternative method of exciting the flash lamp in accordance with the present invention;
Figure 3 is a circuit showing a flash lamp being excited from two phases of a three-phase Y-connected alternating current source, Figure 4 is a circuit diagram of the trigger circuitr~

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2~l shown in the ci~cuit of ~igure 3;
Figures 5~ through 5E are a series of waveforms useful in explain-ing the operation of the present invention as described in the embodiment of Figure 3;
Figure 6 is a circuit showing a flash lamp being excited from a three-phase delta-connected alternating power source;
Figure 7 is a waveform showing the rectified voltage in Figure 6;
and Figure 8, on the same sheet as Figure 4, shows a laser welding system in which the pre~ent invention is used to advantage.
Description of the_Preferred ~mbodiment In the circuit o~ Figure 1 the anode 107 o~ th~ la9h :Lamp 106, which is preferably a xenon flash :Lamp, is coupled to one terminal of the single phase AC power line 100 through diode 101. The frequency of the AC
line may be the conventional ~0 or 60 cycles per second. The cathode 108 of the flash lamp 106 is coupled directly to the other terminal of the AC power line 100. With this connectlon, the flash lamp, once excited, will emit light and conduct only during the positive half cycles of the single phase AC power source. During the negative half cycles, no light will be emitted and no current will exist in the lamp. The input of the trigger circuit 102 is connected to the same AC power source 100 so that the circuit may sense the phase of the source. The trigger circuit 102, one embodiment of which will be discussed in conjunction with Figure 4, provides a triggering pulse to the flash lamp 106 at a predetermined phase angle of the AC power source 100 which thereby triggers the flash tube 106 through an external trigger electrode 111.
Since the trigger electrode 111 is controllably switched rather than the power input to the cathode and anode, it becomes lmnecessary to use controlled .~., .

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rec~ifier means in the high power carrying anode and cathode leads of the flash lamp 105. The output trigger pulse from the trigger circuit 102 is coupled on lines 110 to the primary 104 of trigger-ing transformer 103. The secondary 105 o this trans:former is connected between the cathode 108 and the trigger electrode 111.
The trans~ormer 103 is used to increase the triggering pulse voltage to a sufficient level so as to initiate the gas breakdown in the flash lamp 106 between the anode 107 and cathode 108.
After the triggering pulse has been applied, while the voltage applied between the cathode 108 and anode 107 :Erom the power l:ine 100 through diode 101 is su:E:E:iciently high, the gas breakclown ancl light output will continue without the pres~ncc oE the tr.igger:ing pulse. The light output will be extinguished when the voltage applied between anode 107 and cathode 108 falls below that voltage needed to sustain the gas breakdown. O:f course, light output will not be produced during the negative hal:E cycle of the AC power source 100.
In Figure 2 there is illustrated an alternative method for triggering the flash lamp. This circuit is similar to Figure 1 except that it utilizes series injection triggering through transformer 203 to initiate ionization of the flash lamp 207. As in the previous circuit, a diode 201 couples the anode 208 of the flash lamp 207 to one terminal of the AC power source on lines 200. Here the discharge or gas breakdown is ;.nitiated by impress-ing a high voltage trigger pulse as an addition to the power line voltage rather than by coupling the pulse through a separate triggering electrode as was shown in Figure 1. In this case, the gas breakdown path initiated by the triggering pulse lies directly between anode 208 and cathode 209 rather than through the inter-mediate triggering electro(le lll as in Figure l. The lamp ,~

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triggering pulse produced by a triggering CiTCUit 206 is coupled through lines 210 to the primary 205 of transformer 203. At the secondary 204 of the transformer 203, the transformer stepped-up trigger voltage is added to the AC line voltage. When the sum of the two reaches the breakdown voltage of the flash lamp 207, the gas discharge and light output will be initiated. Once the discharge begins, it will be sustained in the absence o~ the trigger pulse once the rectified AC line voltage is sufficiently high. This sustaining AC line voltage is less than the voltage necessary to initiate the discharge. The inpllt o~ the triggering circuit 206 is coupled to the AC power sourco Z00 so that it may sense the phase of the source and produce the triggering pulse at the desired phase angle.
In Figure 3 there is shown the circuit diagram of a flash lamp circuit operating from a three-phase Y-connected AC power source in accordance with the teachings of the present invention using a half-wave rectifier. This figure will be discussed in conjunction with the waveforms shown in Figures 5A-5E. In Figure 5A two of the phases of the three-phase AC power source are shown by curves 501 and 502, which represent ~A and ~B respectively.
In the circuit of Figure 3, curve 501, ~A, is the voltage waveform between points 302 and 301 while the curve 502, ~B, is the wave-form between points 303 and 301, (~C, not shown, lags ~B by 120.
Point 301 is the neutral or grouncl of the power source. Points 302 and 303 are coupled through diode 305 and 325 to common point 320. The voltage wave~orm between points 320 and ground as it would appear with no load is shown as the heavy black line in Figure 5A. The voltage at point 32n is coupled through smoothing choke 307 to the anode 314 of the flash lamp 313. The center troughs of the rectified and summed portions of the waveform at , - ,7-;

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point 320 are suf~iciently high-after being smoothed by choke 307 that the gas breakdown in the flash lamp 313 will not be ceased when the waveform passes through these points of low voltage.
The neutral point 301 is coupled directly to the cathode 315 of the flash lamp 313. This method of using two o-f -the three phases of the ~C power source permits lengthening of the flash lamp pulses over that possible with a single phase system thereby producing a more desirable lengthened laser pulse waveform for laser welding applications. Furthermore, since the recovery time o~ most flash lamps, particularly xenon flash lamps, is typically no more than several hundred microseconds, the time between pulses using either a 50 or 60 cycle sinusoida:l three-phase source will be more than su:~.icient ~or complet~ lamp r~covcry between firings.
Included in the circuit shown in Figure 3 are the prepulse

3~7 capacitor 308, aging capacitor ~ and the capacitor charger 309.
While the flash lamp is in the recovery period, shown at 512 in Figure 5A, the capacitors are charged to a DC ~oltage preferably in the range of 1.2 to 2.4 kilovolts from the capacitor charger 309.
Charge will flow during this time into the capacitors 308 and 326 rather than the flash lamp 313 or power source 300 since the flash lamp 313 is at a high impedance when it is not being fired and the 1.2 to 2.4 kilovolts back biases diodes 305 and 325. When the flash lamp 313 is initially fired, the charge on the prepulse 3 ~f capacitor 308 and aging capacitor ~ will add to the initial current surge through the ~lash lamp 313 and w:ill produce a high narrow current peak at the beginning of the firing of the lamp.
The current flow from the prepulse capacitor 308 primarily ~
initiates the arc inside the flash lamp 313 while aging capacitor is primarily responsible for the spreading or "aging" of the arc throughout the ~lash lamp 313. These peaks 51~ in Figure 5D
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~3 appear at the leading edges of the current pulses 505 which represent the current through the flash lamp 313~ ensuring that there is an initial high peak of light out of the flash lamp 313 and into the laser. This high initial peak is desirable to ensure a fast rise time of the laser light output and that the lamp fires dependably. If this fast rise time o the laser light output were not present, the laser light would tend to be reflected off the surface being welded during the initial portions of the laser light pulses thereby reducing the ef-Eiciency of the welding opera-tion. A fast rise time on the laser light output ensures that the surface oE the material being wolded w:ill bc initially brokon by the laser and that the rem~l:inder o~ the pulse will be used to effectuate the weld rather than being reflected off the surface.
In the preferred embodiment, the capacitor charger 309 is con-structed using a transformer with a rectiying diode in the trans-former secondary resistively coupled to the capacitor 308.
The flash tube triggering circuit 310 also may be used for the triggering circuits shown in Figures 1 and 2. The trigger logic 311 senses the alternating ~A voltage between points 302 and 301 and produces a relatively low voltage narrow pulse, for example, 12 volts with a width of one microsecond, at the desired phase angle. The lamp trigger circuit 312 converts the relatively low voltage input pulse from line ~20 to a relatively high voltage pulse, such as two kilo~olts, on lines 326 which :is coupled to the primary 317 of triggering transformer 316. The secondary 318 of triggering transformer 316 is connected to the cathode 315 and triggering electrode 319. I`he trigger logic 311 also contains binary logic pulse counting circuitry which counts out the desired number of pulses -firing the flash lamp with each pulse, then counts out a pause between bursts of pulses. In this ,,~

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particular illustration, the flash lamp 313 is fired at the beginning of the cycle of ~A although any other point of any phase could be used as well. Here, three pulses are counted out each burst. Then, after counting out a pause, the same burst of three pulses will be repeated.
In the circuitry shown in Figure 4, the trigger logic 311 employs a full wave diode bridge 415, the inputs 416 and 417 of which are connected to ~A on points 302 and 301 respectively, as in Figure 3. The output of the ~ull wave bridge 415 on point 421 is connected to the input of monostable multivibrator 414. ~`his multivibrator 414 produces a pulse on line 418 at thc phase angle of each cycle o~ (p~ selected ~y the Eiring angle adjust~n~nt.
These pulses on line ~18 are shown as pulses 503 in ~igure 5B.
The pulses on line 418 are coupled both to the input of digital counter 413 and to the input of gate circuit ~12. The number of pulses in the burst of pulses is set on lines 425 from external switches. The digital counter 413 produces a logical 0 state out-put on line 419 whenever the desired number of pulses has been counted out by the counter 413. The gate circuit 412 comprises a binary logic AND gate which produces an output only when both of its inputs are in the logical 1 state. Since the peaks 503 represent the logical l state, the pulses will be allowed to pass through the AND gate 412 only when the output from digital counter 413 on line 419 is in the logical 1 state, ind;cating that the end of the count has not been reached. The pulses on line 420, which will be of the form as shown as pulses 504 in Figure 5C, are coupled to the primary 411 oE pulse transEormer 410. The secondary winding 408 o:E transEormer 410 is connected between the gate 409 and the cathode of the silicon controlled rectifier 404 which causes the rectifier to assume the ON state whenever pulses Erom ,~

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line 420 are present. A potential of 200 VDC is connected through resistor 401 to the anode of silicon controlled rectifier 404 and one terminal of capacitor 403. When silicon controlled rectifier 404 is switched on, the capacitor 403 is coupled on lines 326 across the primary 317 of transformer 316 of Figure 3. The current through that loop will be a half sine wave pulse since the capacitor 403 and transformer inductance form a resonant circuit and the current cannot reverse through the silicon controlled rectifiers 404. The pulse is then coupled through triggering transformer 316 to the triggering electrode oE the Elash lamp 313.
Pigure 6 shows a circuit similar to the circu.i~ o~ Pigure 3 where a delta-connected three-phase power source> S]loWrl generally at 700, is used to excite the 1ash lamp. With the diodes 707-710 connected as shown in Figure 6, the voltage between point 701 and ground is shown by the heavy lines in the waveform shown in ~igure 7. The dotted lines are used for clarity to indicate partially the individual phases of the three-phase source 700. The ripple in each pulse 720 is less than for the circuit shown in Figure 3 since three overlapping half-sine waves contribute to the pulse rather than two, thus smoothing the center troughs of the pulses.
The remainder of the circuit is the same as in Figure 3.
Figure 8 illustrates the use of the present invention in a laser welding application. Here, the flash lamp 605 and laser rod 606 are located each at one of the two Foci of the cavity 603 which is elliptical in cross sectlon. The inside surface 604 of cavity 603 is mirrored so that all light emanating from the flash lamp 605 will be focussed onto laser rod 606. The trigger circuit 602 is coupled on line 609 to the cavity 603 which also serves as the external triggering electrode slnce it is a conducting surface, The power supply 601, the input f which is furnished from the ,~

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three-phase AC source, is coupled to the cathode of flash tube 605 on line 611 and to the anode on line 610. The pulsed laser beam 612 is focussed through lens 613 onto the material to be welded 614. Many other arrangements could be used for the flash tube and laser rod. For example, the flash tube could be in the form of a helix wound around the laser rod, the combination of which is located at the center of a mirrored cylindrical cavity. Also~ two or more such flash tubes could be used. The invention may be used to advantage in laser drilling operations as well as in laser welding applications.
Although preferred embodiments o the invention have been described, numerous modi~ications and alterations thereto would be apparent to one skilled in the art without departing from the spirit and scope of the present invention.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In combination: uncontrolled rectifying means for coupling one or more means for producing radiation to a source of sinusoidal alternating power, power for operating said noncoherent light producing means flowing directly from said source of alternating power through said uncontrolled rectifying means with no substantial energy storage means therebetween, said radiation producing means operating upon application thereto of a trigger signal; and means for producing said trigger signal at a predetermined phase angle of said alternating power source, said means for producing said trigger signal comprising means for sensing the phase of said alternating power source.

2. The combination of claim 1 wherein said rectifying means comprises semiconductor diodes.

3. The combination of claim 1 wherein said radiation producing means comprises means for ionizing a gas.

4. The combination of claim 3 wherein said means comprises a flash lamp.

5. The combination of claim 4 wherein said flash lamp comprises a xenon flash lamp, said xenon flash lamp being located adjacent to laser light producing means.