Abstract:
A flashlamp circuit includes a charge reservoir that receives a first voltage from an external source. The charge reservoir is coupled to a resonator and a plurality of discharge capacitors to provide a second voltage to the plurality of discharge capacitors that is greater than the first voltage. A switch is disposed between at least one of the discharge capacitors and ground to selectively charge the at least one discharge capacitor based upon an input to the switch. Discharge energy is passed from the discharge capacitor(s) to a flashlamp through a discharge bank without passing through any inductive elements. A bleeder circuit can be interposed between the power supply and the reservoir to discharge the reservoir upon shutdown.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application has related patented applications all owned by the same assignee as the present invention identified as follows: Ser. No. 09/522,519 filed Mar. 10, 2000 entitled “INSPECTION SYSTEM WITH VIBRATION RESISTANT VIDEO CAPTURE”; Ser. No. 09/754,991 filed Jan. 5, 2001 entitled “PHASE PROFILOMETRY SYSTEM WITH TELECENTRIC PROJECTOR”; Ser. No. 09/524,133 filed Mar. 10, 2000 entitled “SOLDER PASTE INSPECTION SYSTEM”, which all claim priority to provisional application Serial No. 60/175,049, filed Jan. 7, 2000. 
    
    
     COPYRIGHT RESERVATION 
     A portion of the disclosure of this patent document contains material, which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     FIELD OF THE INVENTION 
     The present invention relates to optical image processing, and in particular to flashlamp circuits for stroboscopic illumination of objects. 
     BACKGROUND OF THE INVENTION 
     Modern digital computing technology is being called upon to perform an ever-increasing variety of tasks. Machines, which once responded purely to manual direction, are now being equipped with computer processors, enabling them to assist a human operator. Manufacturing lines, which produce volumes of standardized assemblies, are being equipped with computer-controlled process machinery. Industrial robots have the capability of being re-programmed to perform many different tasks within the mechanical limits of motion of the device. 
     The automated analysis of captured optical images has great utility for digital control systems. For example, optical imaging may be used to great advantage in automated manufacturing environments, although this is not necessarily the only possible application. Usually, the acquisition of optical images does not interfere with sensitive parts or manufacturing processes, as other forms of measurement might. Optical images of manufactured articles may be captured and analyzed for purposes of inspection, or for guiding the motion of process machinery, such as an industrial robot, relative to a workpiece. 
     In many applications, it is desirable to create a height image or profile of a target object, in order to produce a 2-dimensional map of surface heights. One particular example of this is the inspection of solder deposits on electronic printed circuit cards. As well appreciated by those knowledgeable in the industry, at an intermediate stage of manufacture, these cards may have hundreds or thousands of small solder deposits, which are electrically coupled to circuit paths printed within the card. When electrical components are later mounted on the card, the solder is melted to form electrical connections between the circuit paths in the card and pins, wires, or other conductors from the components. The increasing complexity of the information age demands that these components have larger and larger numbers of connections, usually within smaller and smaller areas. An insufficient amount of solder at a connection site may result in a failure to make the connection, or a connection that intermittently fails or fails after some time in the field. Excess solder or misplaced solder can similarly wreak havoc with the resulting product. The size and number of such connections places great demands on the consistency of the manufacturing process. It also makes it difficult to inspect a card for defects. At the same time, the cost of an undetected defect can be large. Accordingly, there is substantial potential benefit in an automated process, which can accurately inspect solder deposits quickly and without damage to the card. A height profile of a circuit card with solder deposits, taken from optical measurements, can be used to determine the volume of solder at each connection site. 
     One technique for generating a height profile of a target object from optical measurements is known as phase profilometry. In this technique, light illuminates the target object and at least two images of the target object are acquired, each image acquired either at different phases of light, or at differing positions of the target. In either event, a phase shift is introduced between any two of the images. The images are then combined by image processing techniques to reconstruct a height image. Various methods for phase profilometry are disclosed in U.S. Pat. Nos. 4,657,394, 4,641,972, 5,636,025, 5,646,733 and 6,049,384. 
     The technical problem of capturing at least two images of a target object is non-trivial. It is desirable to capture the images in rapid succession, in order to reduce mis-registration caused by undesired motion between the different exposures, and support a high throughput of image capture and analysis. In particular, it is desirable to wait no more than 1 millisecond between any two successive image acquisitions to be combined. While it may be possible to generate successive images within approximately 1 millisecond or less using existing techniques, such techniques involve excessive power consumption and/or excessive hardware, or involve other undesirable side effects. For example, in the case of three-phase profilometry, it is possible to replicate three separate lamps, circuits, and associated hardware for acquiring three separate images, but this would involve considerable hardware expense, and would introduce additional variables if the illumination from different sources were not identical. Additionally, the peak power consumption for known circuits that discharge a single flashlamp with approximately 1 millisecond spacing is typically on the order of 200 watts, which is beyond the capabilities of known small high-voltage (HV) supplies. 
     Techniques have been proposed that reduce power consumption and/or excessive hardware by providing a resonant charging circuit that charges a discharge capacitor from a large reservoir capacitor. An example of such teaching is set forth in U.S. Pat. No. 3,953,763 to Herrick. The inherent dynamics of the circuit of Herrick allow the discharge capacitor to be charged to roughly twice the voltage of the reservoir capacitor. Such resonant charging is accomplished with low dissipation. While the circuit of Herrick provides a number of advantages, it is not without need for improvement. For example, aspects of the Herrick circuit are believed to have unduly shortened the lifetime of a tested flashlamp. The circuit of Herrick cannot be used without an inductor, because without adequate inductance in the circuit, the di/dt of the circuit would exceed the maximum allowable for most commercially available SCRs, causing SCR failure from internal hotspots. Here i denotes current and t denotes time. Addition of an inductor can relieve this problem, since the di/dt is limited to approximately v/L, where v is the discharge potential and L is the inductance. For typical SCRs, the di/dt limit of 200 A/μs, together with the 450-V discharge potential, indicates that an inductor of at least 2 μH is needed. This value of inductance significantly lengthens the tail of the discharge, which has the disadvantage of shortening lamp life. Although SCR devices are available with higher di/dt ratings than the usual 200 A/μs, they are expensive and prohibitively bulky. 
     Further, the circuit does not provide for a fast, convenient discharge of the reservoir capacitor for safety in handling and repairing the circuit. Finally, the circuit of Herrick does not provide selectable discharge energies. A rapid firing flashlamp discharge circuit providing resonant charging and addressing the limitations above thus provides a significant improvement. 
     SUMMARY OF THE INVENTION 
     A flashlamp circuit includes a charge reservoir that receives a first voltage from an external source. The charge reservoir is coupled to a resonator and a plurality of discharge capacitors to provide a second voltage to the plurality of discharge capacitors that is greater than the first voltage. A switch is disposed between at least one of the discharge capacitors and ground to selectively charge the at least one discharge capacitor based upon an input to the switch. Discharge energy is passed from the discharge capacitor(s) to a flashlamp through a discharge bank without passing through any inductive elements. A bleeder circuit can be interposed between the power supply and the reservoir to discharge the reservoir upon shutdown. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of an environment for utilizing a flashlamp apparatus in accordance with embodiments of the present invention. 
     FIG. 2 is a system block diagram of a flashlamp discharge circuit in accordance with an embodiment of the present invention. 
     FIG. 3 is a simplified schematic diagram of a flashlamp discharge circuit in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an environment for utilizing the flashlamp apparatus in accordance with the preferred embodiment of the present invention. In this embodiment, images of printed circuit cards are captured and analyzed as part of a printed circuit card inspection step in an electronic assembly manufacturing process, the analysis for evaluating the adequacy of solder paste deposits on the card by determining the height and volume of paste deposits. As shown in FIG. 1, a printed circuit card  101  to be inspected is mounted on a moveable carriage  102 , the carriage being driven by electric motor  103 . A pulse of light is generated by flashlamp  110 , and directed through reticle  111  and source lens  112  to strike the surface of printed circuit card  101  at an angle. The reflected striped light is imaged by camera lens  113  on charge coupled device (CCD) array camera  114 , which digitizes the reflected image. Digital images from camera  114  are transmitted to data analyzer  121  for analysis, specifically, for determination of the volume of solder paste deposits. Controller  120  controls the simultaneous operation of motor  103 , flashlamp  110 , camera  114 , and data analyzer  121 . 
     Reticle  111  structures the light passing through it into a sinusoidally varying intensity pattern. Reticle  111  has alternating areas of relatively dark and relatively clear stripes, which vary sinusoidally in opacity. The structured light is projected on card  101  at least 2 different times. 
     Carriage  103  moves in the plane of printed circuit card  101 , i.e., perpendicular to the path of light into the camera. Since the height of circuit card features is being measured from the reflected light, it is important that carriage  102  maintain card  101  at a constant distance from camera  114  and lamp  110  although other systems where the invention finds use may operate otherwise, and are found outside the printed circuit card inspection business. Carriage  102  is capable of x-y motion, which permits inspection of any arbitrary region of card  101 . However, for purposes of providing height information over a portion of the board in a single scan of the board, only one direction of motion is required; this should have a component perpendicular to the illumination stripes. For simplicity, FIG. 1 shows only a single motor  103 , while in fact two motors may be used to move the card in 2 dimensions. 
     While in the preferred embodiment it is the card (i.e., the target object) which is moving, it will be appreciated that the same effect could be produced by moving the flashlamp, reticle, lenses and camera together (as by mounting these devices on a common moving carriage), while card  101  remains still. It is also possible to produce out-of-phase images by moving reticle  111 . 
     Controller  120  controls the operation of the various devices as follows. Controller  120  causes motor  103  to position carriage  102  so that a region of interest on card  101  is within the field of view of lens  113  and camera  114 . At a given position, controller  120  generates a series of at least two (preferably three) flashlamp discharge signals to flashlamp  110 , each discharge associated with a distinct phase of the light, the signals being approximately 1 msec apart. Concurrently with the discharges of flashlamp  110 , controller  120  causes camera  114  to capture three separate images, any one image corresponding to each discharge of the flashlamp. Camera  114  transmits these three separate images to data analyzer  121 . Controller  120  causes analyzer  121  to store the images as received, and to analyze the height (and ultimately volume) of solder deposits in the region of interest on card  101 . Typically, controller  120  will cause the apparatus to capture and analyze images in several different regions of interest on a single card. 
     While controller  120  and data analyzer  121  are illustrated as separate blocks in FIG. 1 for conceptual purposes, in fact these may be implemented as software functions executing on a programmable processor of a single general purpose digital computer system. 
     Further background information concerning the analysis of feature height in a target object from multiple out-of-phase images can be found in commonly assigned U.S. Pat. No. 6,049,384 filed Feb. 27, 1996, entitled “Method and Apparatus for Three Dimensional Imaging Using Multi-Phased Structured Light”, which is herein incorporated by reference (using two separate lamp sources to produce two-phase images, which can be analyzed using certain simplifying assumptions). 
     FIG. 2 is a system block diagram of a flashlamp discharge circuit in accordance with an embodiment of the present invention. Circuit  130  is preferably disposed within controller  120  illustrated in FIG. 1, but may reside in any suitable location. Circuit  130  includes bleeder circuit  134 , reservoir  136 , resonant charging circuit  138 , discharge capacitors  140 ,  142 , switch  144 , discharge bank  146  and flashlamp  148 . Bleeder circuit  134  is interposed between the input to reservoir  136  and ground  150 . During normal operation bleeder circuit  134  simply allows charge to flow therethrough and accumulate within reservoir  136 . However, when circuit  130  is shut down, or otherwise disabled, bleeder circuit  134  creates a current path from reservoir  136  to ground  150  thereby discharging reservoir  136 . 
     Resonant charging circuit  138  is disposed between reservoir  136  and discharge capacitors  140 ,  142 . Circuit  138  resonates between about 0 volts and about 500 volts for one-half cycle, at which time SCR opens and maintains the potential across the capacitor(s)  140 ,  142 . As illustrated, circuit  130  preferably includes a plurality of discharge capacitors, such as capacitors  140 ,  142 . When such a plurality of discharge capacitors are used, all but one of the capacitors have a switch, such as switch  144 , disposed between it and ground  150  to thereby selectively determine whether the given discharge capacitor will be charged in a given charge cycle. While FIG. 2 illustrates a pair of discharge capacitors  140 ,  142 , those skilled in the art will recognize that additional discharge capacitors could be provided to provide additional discharge energies. As illustrated, discharge capacitor  142  will always be charged during the charging cycle. However, discharge capacitor  140  will only be charged if switch  144  couples capacitor  140  to ground  150 . Thus, the LEVEL SELECT  152  allows switching between energy levels. 
     When capacitor(s)  140 ,  142  are suitably charged, a TRIGGER signal provided to discharge bank  146  will provide the discharge energy from the discharge capacitor(s) to flashlamp  148 . This will pulse flashlamp  148 , which is preferably a model EG&amp;G FX-1160 available from Perkins Elmer Optoelectronics, 44370 Christy St., Fremont, Calif. 94538. Those skilled in the art will notice that the discharge energies are conveyed from discharge capacitor(s)  140 ,  142  to lamp  148  without passing through an inductor. By not passing the discharge current through an inductor, the lifetime of flashlamp  148  is increased. 
     A flashlamp discharge circuit in accordance with an embodiment of the invention is illustrated in FIG.  3 . An external power supply provides 250 V to charge a reservoir capacitor  136 . Components Q 7  and R 13 -R 15  form a fast bleeder circuit  134 . This bleeder circuit is used to discharge the +250 VDC to facilitate repair and handling of the circuit by reducing the possibility of electric shock. 
     In contrast, typical discharge circuits use an unswitched parallel resistor to discharge the reservoir capacitor. If τ is the time constant of the discharge, C is the capacitance, V is the voltage, and E=CV 2 /2 is the energy stored by the capacitor, then the power dissipation in the bleeder resistor is 2E/τ. If the capacitor in this prior art circuit were 220 μF, then E=6.9 joules. If τ is set at five seconds, for a reasonably fast discharge, then the dissipation in the bleeder circuit is 2.75 watts, which is an excessive amount of waste heat. On the other hand, discharge circuits in accordance with embodiments of the present invention overcome this problem by monitoring the power-supply current through resistor R 15 . If the external supply is delivering power to the circuit, transistor Q 7  is reverse-biased, and the dissipation is minimal. If the external supply is turned off, resistor R 13  turns on Q 7 , which then discharges C 1  quickly through resistor R 14 . In the preferred embodiment, τ is less than five seconds. Resistor R 14  must still be rated for a peak dissipation of 3 watts, for survival under gross faults. However, during normal operation, R 14  dissipates such a power level only during the brief discharge period when the power supply is turned off. 
     The reservoir circuit includes inductor L 1 , SCR Q 2  and opto-coupled SCR U 4 . When U 4  receives a pulse from the Trigger Input, its SCR is triggered, which in turn triggers SCR Q 2 . Current flows through L 1  and Q 2 , charging discharge capacitor C 10  (and possibly C 9 , as will be described later). During this time, L 1  and C 10  form a high-Q resonant circuit, which produces a lightly-damped sinusoid. This sinusoid has negative peaks at 0 V and positive peaks at +500 V. However, the circuit rings for only half a cycle, since when the voltage across C 10  begins its downswing, the current across Q 2  is reversed and it goes out of conduction, leaving approximately +450 volts across C 10 . 
     After C 10  is charged, a pulse is applied on the “Discharge Trigger” line to the gate of insulated gate bipolar transistor (IGBT) device  146 , which in turn discharges C 10  and possibly C 9  as well, through flashlamp  148 . The use of IGBT devices in the discharge path increases the operating lifetime of the flashlamp circuit, since undesirable inductors are obviated. Suitable IGBT devices are available from International Rectifier, of El Segundo Calif., as part number IRG4PC50F. 
     Depending on the application, IGBT  146  may have to be implemented using a plurality of these devices in parallel. However, even such a parallel combination is practical because the low duty cycle of the discharge makes heat sinking unnecessary. Even when there is a plurality of IGBTs, the discharge circuit Q 9 -D 12  is able to satisfactorily handle the current with a single diode. 
     It is important to note that the Trigger Input signal and Discharge Trigger signal must not overlap in time or else the capacitor C 1  will charge through flashlamp  184 , damaging the entire circuit. (Interlock circuitry to prevent this problem is described by Herrick.) 
     Discharge circuits in accordance with some embodiments of the present invention also allow a plurality of discharge energies by using IGBT Q 9  and diode D 12 . For a low-energy discharge, the gate of Q 9  is held low, which turns it off. Thus, the node shared between C 9  and D 12  closely follows the voltage on C 10 . Since this voltage remains positive throughout the discharge cycle, D 12  never becomes forward biased, and C 9  is effectively out of the circuit. However, if the gate of Q 9  is held high, Q 9  turns on and thus holds the bottom of C 9  at ground during the charging cycle. During the discharge, the current levels in the preferred embodiment are several hundred amperes, which is beyond the current handling capability of Q 9 . However, the discharge turns D 12  on, and D 12  is rated for the discharge current. Thus, during the entire cycle, the bottom of C 9  remains near ground and C 9  is effectively in the discharge circuit. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, it is feasible to use the invention in areas other than in phase profilometry, such as the area of high speed photography or the like.