Abstract:
A optical scanner control method and a optical scanner capable of positioning a mirror at a high speed independently of a rocking angle, and a laser machining apparatus for irradiating a printed circuit board with a laser beam by use of the optical scanner to thereby perforate the printed circuit board. In order to operate an actuator for rocking the mirror based on a deviation of a current position from an commanded value, a change in gain of the actuator is measured in accordance with each rocking angle in advance, and the manipulated variable of the actuator is corrected to cancel the change in gain. Thus, the influence of the alteration of a torque constant in accordance with the rocking angle can be suppressed so that the response characteristic becomes uniform all over a scanning region, and the positioning speed can be improved.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an optical scanner control method and an optical scanner for scanning lights such as laser beams, and a laser machining apparatus for irradiating a printed circuit board with laser beams by use of such an optical scanner so as to perforate the printed circuit board. 
   BACKGROUND ART OF THE INVENTION 
   A printed circuit board perforating laser machining apparatus as an example of a laser apparatus having a function of scanning with laser beams is an apparatus for irradiating a printed circuit board with pulsed laser beams so as to make holes for connecting conductor layers of the board with each other. For example, a background-art printed circuit board perforating laser machining apparatus has an XY table servo mechanism and a pair of optical scanner servo mechanisms (for example, see JP-A-2002-137074, page 2 and FIG. 6). A printed circuit board is mounted on the XY table servo mechanism and moved thereby in X)- and Y-directions within a horizontal plane. The pair of optical scanner servo mechanisms are provided for scanning the printed circuit board with laser beams in the X- and Y-directions. 
   As for the structure of an optical scanner, for example, a coil is fixed to a central section of a single-piece penetrating rocking shaft, and a pair of bearings are disposed adjacently to the opposite ends of the coil. An angle detector is disposed outside one of the bearings, while a mirror mounting portion is disposed outside the other bearing (for example, see JP-A-2002-6255, page 2 and FIG. 6). 
   Description will be made below more in detail with reference to the drawings. 
     FIG. 5  is a block diagram showing an example of the configuration of a mirror servo mechanism (optical scanner servo mechanism) in a background-art laser machining apparatus. 
   An electromagnetic rocking actuator  110  of a optical scanner  100  rocks a rocking shaft  111 . A mirror  130  is attached to one end portion of the rocking shaft  111  with a mirror mount  131 , while an angle detector  120  is attached to the other end portion. 
   Due to the aforementioned configuration, the direction of the mirror  130  is changed in accordance with the rocking of the rocking shaft  111  so that the outgoing direction of a laser beam  30  incident on the mirror  130  is changed. The rocking angle of the rocking shaft  111 , that is, the mirror  130  is detected by the angle detector  120 . 
   An upper control unit  10  compiles an NC program and gives an command to a optical scanner control unit  20  as to a target positioning angle  11  of the mirror  130  in accordance with the position on a to-be-machined piece on which the laser beam  30  should be positioned. 
     FIG. 6  is a block diagram of a scanner servo mechanism constituting the background-art optical scanner control unit  20 . A portion to be executed by software with a servo processor is illustrated on the left of the broken line, and the connection relationship of hardware and the flow of signals are illustrated on the right. 
   A target trajectory generating unit  210  calculates a target value  215  of the rocking angle of the optical scanner every moment based on the target positioning angle  11 , and generates a target trajectory of the scanner servo mechanism. A detected rocking angle  255  of the scanner is subtracted from the target value  215  by a subtracting unit  222  so as to obtain a deviation  225 . The deviation  225  is subjected to control and processing in a compensating element  220  so that a manipulated variable signal  226  is calculated. The manipulated variable signal  226  is converted into an analog signal in a D/A converter  230 . Thus, an commanded value (driving signal)  21  of a current control system  240  is obtained. An armature of the actuator  110  is connected to the output side of the current control system  240 , and a current detection resistor  241  is connected in series with the armature. The terminal voltage of the current detection resistor  241  is detected by a differential amplifier  242 , and fed back to the current control system  240  as a current signal. An encoder  120  linked with the rocking shaft  111  generates pulses (position signal)  22  in accordance with the rocking quantity. The pulses are counted in a pulse counter  250  and fed back as a rocking angle  255 . 
   When the aforementioned processes are repeated, the mirror  130  approaches the target position gradually. When the mirror  130  has been positioned, a positioning completion signal  12  is sent from the optical scanner control unit  20  to the upper control unit  10 . 
   The laser beam  30  outputted from a not-shown laser oscillator is reflected by the mirror  130 . Thus, a machining position of the piece to be machined is irradiated with the reflected laser beam  30  through an Fθ lens  140 . In  FIG. 5 , three machining positions A, B and C corresponding to three rocking angles of the mirror  130  are illustrated. 
     FIG. 7  is a sectional view of the actuator. 
   A cylindrical inner yoke  112  is disposed to surround the rocking shaft  111 . Outside the inner yoke  112 , circumferentially divided four permanent magnets  113   a ,  113   b ,  113   c  and  113   d  are disposed to be separated from the inner yoke  112  through a cylindrical air gap G. The permanent magnets  113   a  to  113   d  have been magnetized to be polarized radially. The permanent magnets  113   a  and  113   c  are magnetized in one direction, while the permanent magnets  113   b  and  113   d  are magnetized in the opposite direction. 
   An outer yoke  114  is disposed outside the permanent magnets  113   a  to  113   d , and these parts form a magnetic circuit. Due to a magnetic field formed by the permanent magnets  113   a  to  113   d  and the inner yoke  112 , a magnetic flux M is generated substantially radially in the air gap G. In addition, strand sets  115   a ,  115   b ,  115   c  and  115   d  forming the coil of the armature are disposed in the air gap G as illustrated. 
   With the configuration described above, when a current is applied to the coil, a current flows in the illustrated direction in the strand sets  115   a ,  115   b ,  115   c  and  115   d . Due to the interaction between the magnetic flux and the current, a force (Lorentz force) acts on the strand sets  115   a  to  115   d  circumferentially. Since the coil, that is, the strand sets  115   a  to  115   d  are fixed to the rocking shaft  111 , the force serves as a torque for driving the rocking shaft  111 . The torque is proportional to the current flowing through the coil, and the proportionality factor is a torque constant. 
   In recent years, there has increased a requirement to improve the efficiency in machining with a optical scanner application product such as a laser machining apparatus. High speed response is also required to a optical scanner servo mechanism. A optical scanner operates to rock a mirror within a limited angle range, but the characteristic of the optical scanner is not always uniform in this angle range. 
   That is, in the optical actuator  110  shown in  FIG. 7 , the magnetic flux M near the center of each permanent magnet  113   a - 113   d  trends radially, but tilts with respect to the radial direction and is low in density as approaches an end portion of the permanent magnet  113   a - 113   d . Therefore, the torque constant is lowered when the strand sets  115   a - 115   d  approach the end portions of the permanent magnets  113   a - 113   d.    
     FIG. 8  is a graph showing a relationship between the rocking angle and the torque constant. The rocking angle θ B  designates the center of rocking, the rocking angle θ A  designates a negative-side angle, and the rocking angle θ C  designates a positive-side angle. The rocking angles θ A , θ B  and θ C  correspond to the machining positions A, B and C in  FIG. 5  respectively. 
     FIG. 9  is a graph showing deviation signals of the servo mechanism when positioning was performed in identical strokes in the machining positions with the rocking angles θ A , θ B  and θ C . 
   As is apparent from  FIG. 9 , a suitable response is shown at the rocking angle θ B , but overshoot appears at the rocking angle θ A  or θ C . That is, due to the torque constant changing in accordance with the rocking angle, there occurs a problem that the servo mechanism having good positioning responsiveness at one angle produces overshoot or overdamp at another angle. 
   However, it is difficult to uniformalize the intensity of the magnetic field acting on the coil independently of the rocking angle. Accordingly, the magnitude of the torque acting on the rotor alters in accordance with the rocking angle in spite of the same current applied thereto. In the same manner, even in an optical scanner generating torque by use of another structure, it is difficult to prevent the torque constant from having any alteration depending on the rocking angle. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an optical scanner control method and an optical scanner in which a mirror can be positioned at a high speed independently of a rocking angle, and a laser machining apparatus for radiating a printed circuit board with a laser beam using the optical scanner so as to perforate the printed circuit board. 
   According to a first configuration of the present invention, there is provided an optical scanner control method for operating an actuator for rocking a mirror based on a deviation of a current position from an commanded signal so as to control an outgoing angle of light incident on the mirror, the optical scanner control method including the steps of: measuring a change in gain of the actuator in accordance with a rocking angle in advance, and correcting a manipulated variable of the actuator so as to cancel the change in gain. 
   According to a second configuration of the present invention, there is provided an optical scanner for operating an actuator for rocking a mirror based on a deviation of a current position from an commanded signal so as to control an outgoing angle of light incident on the mirror, the optical scanner including a unit for measuring a change in gain of the actuator in accordance with a rocking angle, so that a change in gain is measured by the measuring unit prior to real operation, the measuring result is stored, and a manipulated variable of the actuator is corrected in real operation so as to cancel the change in gain. 
   According to a third configuration of the present invention, there is provided a laser machining apparatus including a optical scanner defined in the second configuration, by which a work is scanned and machined with a laser beam. 
   The mirror can be positioned at a high speed independently of the rocking angle so that the machining speed can be improved. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a scanner servo mechanism constituting a optical scanner control unit having a torque constant measuring function according to the present invention; 
       FIG. 2  is a block diagram about processing a measured gain signal according to the present invention; 
       FIG. 3  is a graph showing a relationship between a rocking angle and a torque constant; 
       FIG. 4  is a configuration diagram of a printed circuit board perforating laser machining apparatus to which the present invention is applied; 
       FIG. 5  is a block diagram showing a configuration of a mirror servo mechanism in a background-art laser machining apparatus; 
       FIG. 6  is a block diagram of a scanner servo mechanism constituting a background-art optical scanner control unit; 
       FIG. 7  is a sectional view of an actuator; 
       FIG. 8  is a graph showing a relationship between a rocking angle and a torque constant; and 
       FIG. 9  is a graph showing deviation signals of the servo mechanism when positioning was performed in identical strokes in machining positions with rocking angles θ A , θ B  and θ C . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a block diagram of a scanner servo mechanism constituting a optical scanner control unit  50  having a torque constant measuring function according to the present invention. A portion to be executed by software with a servo processor is illustrated on the left of the broken line, and the connection relationship of hardware and the flow of signals are illustrated on the right. Incidentally, functions equivalent to those in  FIG. 6  are referenced correspondingly, and redundant description thereof will be omitted. Though not shown, the optical scanner control unit  50  has a LAN interface such that the optical scanner control unit  50  can communicate with a remote host computer through a LAN. 
   An excitation signal generating unit  260  generates a sine-wave signal having a small amplitude. Here, the frequency of the sine wave is set to be away from resonance frequencies of a scanner actuator to be controlled, frequencies of various noises from amplifiers, a low frequency band conspicuously affected by friction, or the like, and to be a frequency in which the transfer function shows a double integral characteristic as an inertial body. 
   A signal connection/disconnection processing unit  261  connects or disconnects the signal outputted from the excitation signal generating unit  260  to or from an adding unit  262 . The adding unit  262  adds the signal outputted from the excitation signal generating unit  260  to the target value  215  outputted from the target value generating unit  210 , and outputs the result of the addition to a subtracter  222 . 
   The output of a compensating element  220  is supplied to a terminal a of a signal connection changeover processing unit  227  and a torque constant alteration compensating unit  300 . The output of the torque constant alteration compensating unit  300  is supplied to a terminal b of the signal connection changeover processing unit  227 . The output of the signal connection changeover processing unit  227  is supplied to the D/A converter  230  through a terminal c. 
   A rocking angle  255  is inputted into the measured gain signal processing unit  270  through a signal connection/disconnection processing unit  271 , while a manipulated variable signal  226  is inputted thereto through a signal connection/disconnection processing unit  272 . A processing result  284  is stored in an address assigned in a memory  290 . 
   Next, description will be made about the measured gain signal processing unit  270 . 
     FIG. 2  is a block diagram showing the configuration of the measured gain signal processing unit according to the present invention. 
   The input side of a first digital filter  273  is connected to the signal connection/disconnection processing unit  271 , while an output signal is inputted into a dividing unit  279  through a first peak hold processing unit  277 . The input side of a second digital filter  274  is connected to the signal connection/disconnection processing unit  272 , while an output signal is inputted into the dividing unit  279  through a second peak hold processing unit  278 . The first and second digital filters  273  and  274  have a band pass characteristic passing a frequency component identical to the frequency of the sine wave generated by the excitation signal generating unit  260 , but cutting off the other components. Incidentally, the reference numeral  283  in  FIG. 2  represents a timing signal. 
   Next, the operation of the present invention will be described. 
   As described above, the torque constant changes in accordance with the rocking angle. Accordingly, in order to suppress the fluctuation of the response waveform, it is necessary to measure the alteration of the torque constant shown in  FIG. 8  in accordance with an individual scanner and settings thereof. 
   Therefore, the torque constant corresponding to each rocking angle is measured in the following procedure prior to positioning. 
   First, the signal connection/disconnection processing units  261 ,  271  and  272  are set on the connection side, while the terminal c of the signal connection changeover processing unit  227  is connected to the terminal a so that the output signal of the compensating element  220  is supplied directly to the D/A converter  230 . In this state, the target value generating unit  210  sets the target value  215  corresponding to a rocking angle to be measured, while the excitation signal generating unit  260  generates a sine wave signal having a small amplitude. The sine wave signal outputted from the excitation signal generating unit  260  is added to the target value  215  by the adding unit  262 . Thus, the rocking shaft  111  is driven to draw a sine wave around the rocking angle to be measured. 
   The rocking angle  255  and the manipulated variable signal  226  are imported in every sampling period, and processed by the first and second digital filters  273  and  274  respectively. The first and second digital filters  273  and  274  have a band pass characteristic passing a frequency component identical to the frequency of the sine wave generated by the excitation signal generating unit  260 , but cutting off the other components. Therefore, when a brief time has passed since the beginning of the excitation, an input signal component  275  to the device to be controlled corresponding to the excitation signal and an output signal component  276  from the device to be controlled corresponding to the excitation signal appear on the outputs of the filters  273  and  274  respectively. The first and second digital filters  273  and  274  perform identical processing. Accordingly, influences of the filters cancel each other when the transfer characteristic between the input signal component  275  and the output signal component  276  is examined. 
   The input signal component  275  and the output signal component  276  are inputted into the first and second peak hold processing units  277  and  278  respectively. Thus, peak values of the sine wave signal are held, and an input signal amplitude  281  and an output signal amplitude  282  are obtained as outputs. 
   After a sufficient time has passed, division processing is performed in the dividing unit  279  in response to the timing signal  283 , so that the input signal amplitude  281  is divided by the output signal amplitude  282 . Thus, a reciprocal  284  of the gain of the device to be controlled is calculated and stored in an address of the memory  290  specified by the address  291  corresponding to the measured rocking angle. 
   A series of processes described above with reference to  FIGS. 1 and 2  are repeated while changing the target value of the rocking angle step by step within a movable range. Thus, changes in torque constant are measured all over the movable range, and stored in the memory  290 . 
   At the time of machining, the signal connection/disconnection processing units  261 ,  271  and  272  are set on the disconnection side, while the terminal c of the signal connection changeover processing unit  227  is connected to the terminal b so that the output signal of the compensating element  220  is supplied to the D/A converter  230  through the torque constant alteration compensating unit  300 . Then, the torque constant alteration compensating unit  300  amplifies the manipulated variable signal  226  in accordance with the rocking angle  255 , that is, the rocking angle θ with reference to the memory  290  so as to cancel the change in torque constant (for example, so as to make the torque constant equal to the largest one of the torque constants obtained by measuring). The torque constant alteration compensating unit  300  outputs the amplified manipulated variable signal  226  to the D/A converter  230 . As a result, the torque constant is fixed independently of the rocking angle θ. Thus, the mirror can be positioned at a high speed so that the machining speed can be improved. 
   Incidentally, even when the target value  215  is used instead of the rocking angle  255  with reference to the memory  290 , considerable compensating effect can be obtained. 
   When it is intended to improve the positioning accuracy in the aforementioned method, the number of measurements of the rocking angle θ have to be increased. Thus, the time required for measuring changes in torque constant is prolonged. When the following method is used, the positioning accuracy can be improved while the measuring time can be shortened. 
   That is, most of changes in torque constant exhibit the characteristic shown in  FIG. 8 . Normalized with respect to a torque constant at the origin which is the center of the movable range, the torque constants draw a curve illustrated by the solid line in  FIG. 3 . Then, the value at the origin is 1. Reciprocals of the normalized torque constants draw a curve illustrated by the broken line in  FIG. 3 , which curve can be approximated by a biquadratic function of the rocking angle. This biquadratic function is expressed by:
 
 K   c   =a   1 θ 4   +a   2 θ 3   +a   3 θ 2   +a   4   θ+a   5   Expression 1
 
wherein K c  designates the reciprocal of the gain of the device to be controlled, θ designates the rocking angle, and a 1 , a 2 , a 3 , a 4  and a 5  designate coefficients. Assume that in rocking angles
 
θ=[θ 1 , θ 2 , . . . , θ n ]  Expression 2
 
gains
 
K c =[K c1 , K c2 , . . . , K cn ]  Expression 3
 
are obtained by a series of measurements. When n=5, the coefficients a 1 , a 2 , a 3 , a 4  and a 5  are determined uniquely. When n is larger than 5, the coefficients can be determined in a least square method. When A designates an estimated value of each coefficient obtained thus, correction coefficients can be calculated by the following expression.
 
 K   c   =A   1 θ 4   +A   2 θ 3   +A   3 θ 2   +A   4   θ+A   5   Expression 4
 
   In such a configuration, the torque constant alteration compensating unit  300  calculates a correction coefficient from the rocking angle and the coefficients, multiplies the manipulated variable signal  226  by the calculated correction coefficient, and delivers the obtained manipulated variable signal  226  to the D/A converter  230 . Thus, the change in torque constant at each rocking angle with respect to the torque constant at the origin is compensated so that the dynamic characteristic of the servo mechanism can be kept constant. 
   The change in torque constant typically differs from one optical scanner to another, and there is also a secular change. In addition, the torque constant changes in accordance with how to attach the optical scanner to the laser machining apparatus or how to attach the mirror to the optical scanner. Therefore, when the scanner or the mirror is removed and attached again due to maintenance or the like, initial response may be not reproduced even in one and the same machining position. According to the present invention, however, the machining speed can be always improved. 
   Although it is assumed in Expression 1 that the biquadratic curve is symmetric with respect to the center (that is, the rocking angle θ=0), the maximum torque constant may be out of the center of the movable range due to a variation in characteristic among the permanent magnets  113   a - 113   d , a variation among the strand sets  115   a - 115   d , a variation in assembling, or the like. Even in such a case, a displacement from the center of the biquadratic curve can be obtained when the number n of samples is set to be 5 or more. Thus, accurate compensation can be attained. 
   Although the torque constant alteration compensating unit  300  is disposed between the compensating element  220  and the D/A converter  230  in the aforementioned embodiment, a forward portion in the servo control loop, that is, a not-shown gain in the compensating element  220  may be corrected. 
     FIG. 4  is a configuration diagram of a printed circuit board perforating laser machining apparatus to which the present invention is applied. Units which are not essentially concerned with the invention are not shown in  FIG. 4 . In FIG.  4 , a laser beam  30  outputted from a laser oscillator  310  enters an optical beam processing system constituted by a collimator  312 , an aperture  314 , etc., with a mirror  313   a  and a mirror  313   b , so as to be shaped thereby. Further, the laser beam  30  is incident on a mirror of a first optical scanner  100   a  with mirrors  313   c ,  313   d ,  313   e  and  313   f . The mirror of the first optical scanner  100   a  reflects an incident beam from the illustrated right side toward the illustrated front side when the mirror is in a neutral position. However, when the angle of the mirror is changed, the traveling path of the reflected beam can be changed within the illustrated horizontal plane, that is, in the illustrated left/right direction (Y-axis direction) in a spot position on an XY table. The beam reflected from the first optical scanner  100   a  is incident on a mirror of a second optical scanner  100   b . The mirror of the second optical scanner  100   b  reflects an incident beam from the illustrated deeper side toward the illustrated lower side when the mirror is in a neutral position. However, when the angle of the mirror is changed, the traveling path of the reflected beam can be changed within the vertical plane in the illustrated front/rear direction, that is, in the illustrated front/rear direction (X-axis direction) in a spot position on the XY table. The beam reflected from the mirror of the second optical scanner  100   b  strikes a printed circuit board  352  mounted on an XY table  353 , through an Fθ lens  140 . The XY table  353  is driven in the Y-axis direction by a Y-axis drive mechanism  354 , while the Y-axis drive mechanism  354  is driven in the X-axis direction by an X-axis drive mechanism  355 . Thus, the XY table  353  can be positioned so as to be mounted in the X- and Y-directions. The X-axis drive mechanism  355  is fixed onto a bed  356 . Each optical scanner  330   a ,  330   b  has a torque constant alteration compensating function as described above. 
   The torque constant alteration compensating function described above is provided as one of the functions of the optical scanner control unit  50 . The torque constant alteration compensating function is set to be able to be used in initial setting for connecting a optical scanner to this control unit or in adjustment for replacing the optical scanner. In addition, this function can be operated from a remote site via a LAN and/or via the Internet, or a function of monitoring a measuring result can be also provided.