Patent Publication Number: US-2002012367-A1

Title: Laser processing apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a waveform-controlled laser processing apparatus.  
       [0003] 2. Description of the Related Art  
       [0004] Up until now, in a laser processing apparatus effecting laser processing such as welding, cutting, etc., by irradiation of pulsed laser beams onto workpieces, use is made of technique for variably controlling waveforms of the laser output of the pulsed laser light so as to meet a diversity of processing demands.  
       [0005]FIG. 6 shows a major configuration of a conventional waveform-controlled laser processing apparatus. The laser processing apparatus comprises a laser oscillation unit  100 , a laser power supply unit  102  and a control unit  104 . The laser power supply unit  102  is electrically connected via a switching element  106  typically formed of a transistor to an excitation light source not shown of a laser oscillation unit  100  so that a DC power or current supplied from the laser power supply unit  102  to the excitation light source is waveform controlled by providing a feedback switching control of the switching element  106  by the control unit  104 . The laser oscillation unit  100  excites a solid-state laser medium not shown by energy of a light emitted from the excitation light source, to thereby oscillatorily output a laser beam LB having a controlled laser output (light intensity) waveform.  
       [0006] In order to effect the above laser output waveform control, the control unit  104  includes a laser output measurement unit  108  for measuring an output of the laser beam LB, and a switching control unit  112  for providing a switching control of the switching element  106  on the basis of an error between a feedback signal which is a measured-value-of-laser-output signal ML from the laser output measurement unit  108  and a reference signal M ref  from a reference signal generation unit  110 .  
       [0007] In the switching control unit  112 , an error amplifier  122  consisting of an operational amplifier  114 , input resistors  116  and  118  and a feedback resistor  120  compares levels of the two signals ML and M ref  with each other to generate an error signal er indicative of the error or difference therebetween, and a PWM (pulse-width modulation) circuit  124  generates a PWM signal MW of a predetermined frequency having a pulse width in conformity with the error signal. The PWM signal MW is fed as a switching control signal via a drive circuit  126  to the switching element  106 .  
       [0008] In the above laser processing apparatus, the output of the laser beam LB may have a phase lag to a large extent, e.g., as much as about 180 degrees relative to the switching control signal MW fed for waveform control from the PWM circuit  124  to the switching element  106 .  
       [0009] For this reason, to stabilize the waveform control feedback loop, a phase compensation circuit, consisting of a capacitor  128  and a resistor  130  in series, is disposed in parallel with the resistor  120  in a feedback circuit of the operational amplifier  114  so as to strengthen the negative feedback of, esp., high frequency components to thereby compensate a large phase lag almost likely to reverse the phase as described above.  
       [0010] However, this may result in lowering of the gain of the error amplifier  122 , esp., gain of high frequency components. For this reason, upon the rise of the laser beam LB, both the measured-value-of-laser-output signal ML and the reference signal M ref  tend to contain a plenty of high frequency components, so that the gain or sensitivity of the error signal er for the error (difference) between the respective high frequency components may become lower with a reduced response speed of the PWM circuit  124 . This may often bring about a slow rise of the laser output as shown in FIGS. 7A to  7 C and an overshoot as its reaction with impaired accuracy and reliability in the waveform control. Also from the viewpoint of laser processing quality, it was undesirable.  
       SUMMARY OF THE INVENTION  
       [0011] The present invention was conceived in view of such problems involved in the prior art. It is therefore an object of the present invention to provide a laser processing apparatus having enhanced accuracy or reliability in the laser output waveform control.  
       [0012] It is another object of the present invention to provide a laser processing apparatus having improved laser output rise characteristics.  
       [0013] In order to attain the above objects, according to one aspect of the present invention there is provided a laser processing apparatus having a laser oscillation unit exciting a solid-state laser medium by energy of an excitation light emitted from an excitation light source to thereby oscillatorily output a laser beam, a laser power supply unit supplying a power to the excitation light source, and laser output measurement means measuring an output of the laser light, the output of the laser light being controlled on the basis of an error between a feedback signal and a preset reference value, the feedback signal being a laser power measured value from the laser output measurement means, the laser processing apparatus comprising power measuring means arranged to measure a power supplied to the excitation light source, wherein the feedback signal is corrected by an AC component of a signal indicative of a power measured value from the power measuring means so as to control the output of the laser light.  
       [0014] In the above configuration, the measured-value-of-laser-output signal or the feedback signal whose phase significantly lags relative to the control signal is corrected by the AC component of the measured-value-of-power signal substantially in phase with the control signal whereby the feedback signal can have a compensated phase to stabilize the power feedback loop.  
       [0015] In another aspect, the reference value may be corrected by an AC component of the measured-value-of-power signal to control the output of the laser light. Alternatively, arrangement may be such that in place of the power measuring means there are provided current measuring means for measuring a current supplied to the excitation light source so that the feedback signal can be corrected by an AC component of a signal indicative of a power measured value from the power measuring means to thereby control the laser light output. As an alternative, the reference value may be corrected by the AC component of the measured-value-of-power signal to thereby control the laser light output.  
       [0016] Preferably, the laser processing apparatus of the present invention further comprises switching means connected between the laser power supply unit and the excitation light source; and switching control means arranged to provide a switching control of the switching means at a predetermined frequency by pulse-width modulation.  
       [0017] In such a configuration, preferably, the switching means include an adder which adds an AC component of the measured value signal to the feedback signal; an operational amplifier which compares an output signal from the adder with the reference value to amplify an error therebetween; and a capacitor for phase compensation disposed in a feedback circuit of the operational amplifier. Alternatively, the switching means may include a subtractor which subtracts an AC component of the measured value signal from the reference value; an operational amplifier which compares an output signal from the subtractor with the feedback signal to amplify an error therebetween; and a capacitor for phase compensation disposed in a feedback circuit of the operational amplifier. In the present invention, the phase of the feedback signal is compensated by the AC component of the measured-value-of-power signal or the measured-value-of-current signal so as to stabilize the power feedback loop, with the result that it is possible to render rapidly responsive the frequency characteristics of the operational amplifier constituting the error amplifier to thereby provide a stable and rapid-response laser output waveform control. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0018] The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:  
     [0019]FIG. 1 is a block diagram showing a principal configuration of a laser processing apparatus in accordance with an embodiment of the present invention;  
     [0020]FIG. 2 is a circuit diagram showing an exemplary configuration of a switching control unit included in the laser processing apparatus of the embodiment;  
     [0021]FIG. 3 is a diagram showing, in comparison with a conventional example, frequency characteristics of an error amplifier included in the switching control unit of the embodiment;  
     [0022]FIGS. 4A to  4 C are diagrams showing waveforms at respective parts of the laser processing apparatus of the embodiment;  
     [0023]FIG. 5 is a circuit diagram showing a variant of the switching control unit of the embodiment;  
     [0024]FIG. 6 is a block diagram showing a principal configuration of a conventional laser processing apparatus; and  
     [0025]FIGS. 7A to  7 C are diagrams showing waveforms at respective parts of the conventional laser processing apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0026] The present invention will now be described with reference to FIGS.  1  to  5  which illustrate a presently preferred embodiment thereof in a non-limitative manner.  
     [0027]FIG. 1 depicts the configuration of a principal part of a laser processing apparatus in accordance with the embodiment of the present invention. The laser processing apparatus comprises a laser oscillation unit  10 , a laser power supply unit  12 , a laser cooling unit  14  and a control unit  16 .  
     [0028] The laser oscillation unit  10  includes an excitation lamp  20  acting as an excitation light source and an YAG rod  22  acting as a laser medium which are both arranged within a chamber  18 , and a pair of mirrors  24  and  26  disposed in an optical axis of the YAG rod  22  outside the chamber  18 .  
     [0029] When the excitation lamp  20  lights and emits an excitation light, the YAG rod  22  is excited by energy of the excitation light. The light emanating onto the optical axis from the opposite ends of the YAG rod  22  is iteratively reflected for amplification between the optical resonator mirrors  24  and  26  and then passes through the output mirror  24  in the form of a pulsed laser beam LB. After passing through the output mirror  24 , the pulsed laser beam LB is sent via an optical transmission system not shown consisting of e.g., a reflection mirror and an optical fiber to a laser output unit not shown at a site to be laser processed and is irradiated from the laser output unit onto a workpiece not shown.  
     [0030] The laser power supply unit  12  includes a capacitor  28  for storing laser oscillation powers to be supplied to the laser oscillation unit  10 , and a charging circuit  30  for converting a commercial AC, e.g., three-phase AC power supply voltage (U, V, W) into a direct current to thereby charge the capacitor  28  up to a predetermined DC voltage. An output terminal of the laser power supply unit  12 , i.e., one terminal of the capacitor  28  is electrically connected via a switching element, e.g., an IGBT (insulated gate bipolar transistor)  32  to the excitation lamp  20 .  
     [0031] The laser cooling unit  14  serves to discharge heat generated by the excitation lamp  20  and the YAG rod  22  of the laser oscillation unit  10  to the exterior and is arranged to supply a coolant, e.g., a cooling water CW having a controlled temperature to the laser oscillation unit  10 .  
     [0032] The control unit  16  includes a CPU (microprocessor)  34  for providing a control of operations of the entire apparatus and of each unit, a memory  36  for storing various programs, various set values and other data required to cause the CPU  34  to effect predetermined processings, and various measuring means  38  to  48  and a switching control unit  50  for waveform control.  
     [0033] Of the various measuring means, a laser output measurement unit  38  has a photosensor receiving a laser beam LB′ leaking backward from the optical resonator mirror  26 , and a measuring circuit determining the laser output of the pulsed laser beam LB on the basis of an electric signal output from the photosensor. The laser output measurement unit  38  feeds a measured-value-of-laser-output signal SL acquired from the measuring circuit to both the switching control unit  50  and the CPU  34 .  
     [0034] A voltage measuring circuit  40  is electrically connected via voltage sensing leads  42  to opposite ends of the excitation lamp  20  and measures, e.g., an RMS voltage (lamp voltage) applied from the power supply unit  12  to the excitation lamp  20  to feed a signal SV indicative of a measured value of lamp voltage to a power computing circuit  44 .  
     [0035] A current measuring circuit  46  receives a current detection signal from, e.g., a Hall CT  48  which is a current sensor fitted to a lamp-current-supplying circuit of the power supply unit  12  and measures an RMS current (lamp current) I supplied to the excitation lamp  20  to feed a signal SI indicative of a measured value of lamp current to both the power computing circuit  44  and the switching control unit  50 .  
     [0036] The power computing circuit  44  finds a measured value of lamp power SP by computation on the basis of both the measured-value-of-lamp-voltage signal SV from the voltage measuring circuit  40  and the measured-value-of-lamp-current signal SI from the current measuring circuit  46 , to feed a signal SP indicative of a measured value of lamp power to the switching control unit  50 .  
     [0037] The switching control unit  50  feeds a switching control signal SW for waveform control via a drive circuit  52  to the IGBT  32 . In the laser output waveform control of this embodiment, the switching control unit  50  uses as a main feedback signal the measured-value-of-laser-output signal SL from the laser output measurement unit  38  and as a sub-feedback signal for correction an AC component SPc of the measured-value-of-lamp-power signal SP from the power computing circuit  44  or of the measured-value-of-lamp-current signal SI from the current measuring circuit  46 . The switching control unit  50  then compares the feedback signals SL, SPc (SIc) with a reference signal S ref  having a desired pulse waveform from the CPU  34  to obtain a comparison error, and generates a switching control signal SW for e.g., PWM so as to nullify the comparison error. The IGBT  32  is thus switching controlled via the drive circuit  52  by the switching control signal SW.  
     [0038] Such a feedback control system provides a control so that the output waveform of the pulsed laser beam LB oscillatorily output from the laser oscillation unit  10  follows the waveform of the reference signal S ref .  
     [0039] The CPU  34  is further associated with a communication interface unit  54 , an input unit  56 , a display unit  58 , etc. The communication interface unit  54  is used for interchanging data or signals with an external device not shown. The input unit  56  includes e.g., key switches arranged on a console panel of the apparatus and is used for e.g., entries of various set values. The display unit  58  includes a display fitted to the console panel and displays various entered set values, various measured values, etc. In FIG. 1, only the measured-value-of-laser-output signal SL from the laser output measurement unit  38  is entered as a measured value into the CPU  34  and is displayable. However, measured value signals obtained by the other measurement units  40 ,  44  and  46  may be entered into the CPU  34  for display or desired data processing.  
     [0040]FIG. 2 depicts by way of example the configuration of the switching control unit  50  included in this embodiment. This switching control unit  50  is formed from an analog circuit and includes an adder  60 , an inverting circuit  62 , and an error amplifier  64  and a PWM circuit  66 .  
     [0041] In the adder  60 , the measured-value-of-laser-output signal SL from the laser output measurement unit  38  (FIG. 1) is fed as the main feedback signal via an input resistor  70  to an inverting input terminal (−) of an operational amplifier  68 . A capacitor  72  accepts the measured-value-of-power signal SP from the power computing circuit  44  (FIG. 1) or the measured-value-of-current signal SI from the current measuring circuit  46  (FIG. 1). The AC component SPc (SIc) of signal SP (SI) passing through the capacitor  72  is fed as the sub-feedback signal via an input resistor  74  to the inverting input terminal (−) of the operational amplifier  68 . A non-inverting input terminal (+) of the operational amplifier  68  is connected to the ground potential, with a feedback resistor  76  intervening between the output terminal and the non-inverting input terminal (−).  
     [0042] The thus constituted adder  60  adds the AC component SPc of the measured-value-of-power signal SP or the AC component Sic of the measured-value-of-current signal SI to the measured-value-of-laser-power signal SL and provides a polarity-inverted signal as its output in the form of a corrected feedback signal −SF having an inverted polarity.  
     [0043] The inverting circuit  62  includes an operational amplifier  78 , an input resistor  80  and a feedback resistor  82 . The inverted polarity corrected feedback signal −SF is polarity inverted by this inverting circuit  62  to obtain a positive polarity corrected feedback signal SF.  
     [0044] The error amplifier  64  includes an operational amplifier  84 , a pair of input resistors  86  and  88 , and a feedback resistor  90 . The operational amplifier  84  has a non-inverting input terminal (+) which receives the corrected feedback signal SF from the inverting circuit  62  through the input resistor  86  and has an inverting input terminal (−) which receives the reference signal S ref  from the CPU  34  through the input resistor  88 . It is to be noted that the reference signal S ref  from the CPU  34  is fed thereto after the conversion into an analog signal by a digital-to-analog converting circuit not shown.  
     [0045] In this error amplifier  64 , the operational amplifier  84  has an output terminal which outputs an error signal ER indicative of a difference or error (SF−S ref ) between the two input signals SF and S ref . The signal amplification factor is determined by the ratio of the resistance value of the input resistors  86 ,  88  to the resistance value of the feedback resistor  90 .  
     [0046] In this error amplifier  64  as well, to provide a stability to the feedback loop for the waveform control, the feedback circuit of the operational amplifier  84  is provided with a phase compensation circuit consisting of a resistor  90 , and a capacitor  92  and a resistor  94  in series which are connected in parallel with the resistor  90 . It is to be noted however in this apparatus that a weak negative feedback of, esp., a high frequency component may be provided to the phase compensation circuit due to a small phase shifting (lag) of the corrected feedback signal SF relative to the switching control signal SW output from the PWM circuit  66  as will be described later.  
     [0047] By way of example, the capacitor  128  has had a capacitance of 47,000 picofarads with the resistance value of the resistor  130  set to 500 ohms in the phase compensation circuit of the conventional apparatus (FIG. 6) whereas the capacitor  92  can have a capacitance of 4,700 picofarads with the resistance value of the resistor  130  set to 20 kiloohms in the phase compensation circuit of this apparatus. As a result of such a weakened negative feedback of the high-frequency component in the phase compensation circuit, the error amplifier  64  can have remarkably improved frequency characteristics.  
     [0048]FIG. 3 depicts an example of the frequency characteristics of the error amplifier  64  of this embodiment in comparison with the conventional example (FIG. 1). The resistance values of the input resistors  86 ,  88  ( 116 ,  118 ) and of the feedback resistor  90  ( 120 ) were set to 2 kiloohms and 100 kiloohms, respectively.  
     [0049] The PWM circuit  66  includes a circuit for generating a comparison reference signal e.g., a sawtooth signal of a constant frequency, and a comparator for comparing the error signal ER from the error amplifier  64  with the sawtooth signal to generate a PWM signal, i.e., the switching control signal SW. The pulse width of the switching control signal SW in each switching cycle defines ON time of the IGBT  32  and can increase accordingly as the comparison error ER becomes larger but decrease accordingly as the comparison error ER becomes smaller.  
     [0050] The switching control unit  50  operates as follows in the laser output waveform control of this embodiment.  
     [0051] In each switching cycle, during the ON time of the switching control signal SW or of the IGBT  32 , a direct current I is supplied from the laser power supply unit  12  via the switching element  32  to the excitation lamp  20  of the laser oscillation unit  10 , to light the excitation lamp  20 . In the laser oscillation unit  10 , as described above, the YAG rod  22  is excited by an excitation light from the excitation lamp  20  to generate a light for laser, the light being subjected to an optical resonance or amplification by the optical resonator mirrors  24  and  26  and resulting in a laser beam LB. Herein, the variation, i.e., the AC component of the current or power supplied from the laser power supply unit  12  to the excitation lamp  20  is in synchronism with the switching control signal SW or the ON time (pulse duration) which is a control variable of the IGBT  32 . On the contrary, the variation in the output of the laser beam LB is not in synchronism therewith, with a possible substantial lag, e.g., of the order of 180 degrees.  
     [0052] The switching control unit  50  of this embodiment uses as the main feedback signal the measured-value-of-laser-power signal SL from the laser output measurement unit  38  and uses as the sub-feedback signal the AC component SPc (SIc) of the measured-value-of-power signal SP from the power computing circuit  44  or of the measured-value-of-current signal SI from the current measuring circuit  46 . In the switching control unit  50 , the adder  60  adds the two feedback signals SL and SPc (SIc) together and the error amplifier  64  compares the corrected feedback signal SF obtained as a result of the addition with the reference signal S ref  to obtain an comparison error (SF−S ref ). The PWM circuit  66  then determines the ON time of the IGBT  32  in the next switching cycle depending on the comparison error (SF−S ref ).  
     [0053] The corrected feedback signal SF fed to the error amplifier  64  herein is one obtained by correcting (phase compensating) the main feedback signal SL by the sub-feedback signal SPc (SIc) nearly in phase with the switching control signal SW, the main feedback signal SL lagging to a larger extent accordingly as it becomes reversed in phase relative to the switching control signal SW. This assures a high stability in the waveform control feedback loop. For this reason, improved frequency characteristics and improved response speed are achieved with negative feedback of, esp., high-frequency component weakened by the phase compensation circuit ( 92 ,  94 ) of the error amplifier  64  as described above.  
     [0054] Thus, as depicted in FIGS. 4A to  4 C, this embodiment is capable of effecting a stable and rapid rise in the output of the pulsed laser beam LB and eliminating any overshoot. Such enhanced accuracy and stability of the laser output waveform control can lead to improved laser processing quality.  
     [0055]FIG. 5 depicts a variant of the switching control unit  50  included in this embodiment. In the diagram, identical reference numerals are given to elements having similar configurations to those of FIG. 2.  
     [0056] In this variant, the reference signal S ref  is passed through the inverting circuit  62  and thereafter added to sub-feedback signal SPc (SIc) by the adder  60  to obtain a first comparison error {S ref −SPC (SIc)}. That is, the inverting circuit  62  and the adder  60  make up a subtractor for subtracting the sub-feedback signal SPc (SIc) from the reference signal S ref . Then, in the error amplifier  64 , the first comparison error {S ref −SPc (SIc)} is fed to the inverting input terminal (−) of the operational amplifier  84  whilst the main feedback signal SL is fed to the non-inverting input terminal (+) to obtain a second comparison error {SL+SPc (SIc)−S ref }. The second comparison error is the same result as in the above embodiment, allowing the same error signal ER to be fed to the PWM circuit  66 .  
     [0057] Although the preferred embodiment of the present invention has been set forth hereinabove, the present invention may variously be changed or modified based on the technical idea thereof. For example, the configurations and operative functions of the laser oscillation unit  10 , laser power supply unit  12  and control unit  16  are not limited to those in the above embodiments, but instead may variously be altered. Although the above embodiment has employed the PWM as the switching control system, any other switching control system suited to the feedback loop is also available.  
     [0058] According to the laser processing apparatus of the present invention, as set forth hereinabove, the feedback signal (laser output measured value) or the reference value is corrected by the AC component of the power or the current supplied from the laser power supply unit to the excitation light source of the laser oscillation unit to provide a power feedback control, whereby it is possible to effect a stable and rapid-response laser power waveform control and especially to improve the rise characteristics of the laser output.