Abstract:
The first embodiment of the invention is a spot welding method using a welding gun including a pair of electrodes holding and welding a welding point of a workpiece. The method including: transmitting an ultrasonic from one of the electrode pair to the workpiece with the electrode pair holding the workpiece; receiving the ultrasonic at the other electrode passed through the electrode pair and the workpiece; and detecting a holding state of the workpiece on the basis of an amplitude of the received ultrasonic. According to the embodiments of the invention, the welding gun can hold the workpiece at the proper position and with the correct angle, thereby reducing the welding flaws.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a technique of spot welding. 
         [0003]    2. Description of Related Art 
         [0004]    Spot welding is one of a welding process for joining metals, applying an electrical current to a workpiece to melt the metals by the heat obtained from resistance. The conventional spot welding apparatus includes a welding gun for holding the workpiece and applying current thereto, and a transformer for applying a high electrical current to the welding gun. In the case that spot welding is performed to a large workpiece such as a vehicle body, the welding gun is mounted on an industrial robot. 
         [0005]    For example, JP 2006-088160 A discloses a spot welding apparatus enabled to weld at an accurate welding position. The spot welding apparatus of JP 2006-088160 A determines whether the welding position is in the predetermined range by comparing a precise measurement data of a CAD data with an actual measurement data of the welding position obtained by a coordinate measuring machine. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    In the spot welding apparatus of JP 2006-088160 A, even if the actual measurement data is within the predetermined range of the precise measurement data, there may occur flaws such as an edge shot, which means that the electrical current partially runs off the edge, when the welding gun does not hold the workpiece at proper position or with proper angle. Accordingly, it is required for the welding gun to hold the workpiece at correct position and with proper posture in order to reduce welding defects. 
         [0007]    The objective of the present invention is described above. 
         [0008]    The first embodiment of the invention is a spot welding method using a welding gun including a pair of electrodes holding and welding a welding point of a workpiece. The method including: transmitting an ultrasonic from one of the electrode pair to the workpiece with the electrode pair holding the workpiece; receiving the ultrasonic at the other electrode passed through the electrode pair and the workpiece; and detecting a holding state of the workpiece on the basis of an amplitude of the received ultrasonic. 
         [0009]    The preferable embodiment includes calculating a lap amount, which is a distance from the center of the electrodes to a first end of the workpiece located nearest to the center capable of being welded, on the basis of the amplitude of the received ultrasonic, while the welding gun holding the workpiece, and detecting the calculated lap amount as the holding state of the workpiece; and performing the spot welding in the case that the calculated lap amount is not smaller than a minimum lap amount required to perform the spot welding at the first end of the workpiece. 
         [0010]    More advantageous embodiment includes correcting the holding position of the workpiece with the electrode pair on the basis of the difference between the calculated lap amount and the minimum lap amount, and performing the spot welding. 
         [0011]    Furthermore, the embodiment includes correcting the holding position of the workpiece and performing the spot welding, in the case that a distance from the first end of the workpiece capable of being welded to a second end of the workpiece capable of being welded located on the extended line connecting the first end and the center of the electrode is not smaller than a sum of the calculated lap amount, the correction amount for correcting the holding position, a diameter of the electrode and an allowance set as a dimension to avoid the electrode contacting the second end of the workpiece. 
         [0012]    The alternative embodiment includes calculating an inclination angle of the workpiece inclining against the plane perpendicular to the axis of the electrode on the basis of the amplitude of the ultrasonic, and detecting the inclination angle as the holding state of the workpiece, and performing the spot welding in the case that the calculated inclination angle is not larger than a tolerable angle required to perform the spot welding at the first end of the workpiece. 
         [0013]    In the above embodiments, it is preferable to output an alert in the case that the spot welding is not performed. 
         [0014]    The second embodiment of the invention is a spot welding apparatus which includes a welding gun with a pair of electrodes for holding and welding a welding point of a workpiece; an ultrasonic transmitter located at one of the electrode pair for transmitting an ultrasonic to the workpiece; an ultrasonic receiver located at the other of the electrode pair for receiving the ultrasonic passed through the electrode pair and the workpiece; and a controller for detecting a holding state of the workpiece on the basis of an amplitude of the received ultrasonic and controlling the welding gun on the basis of the holding state. 
         [0015]    Preferably, the controller, while the welding gun holding the workpiece, calculates a lap amount, which is a distance from the center of the electrodes to a first end of the workpiece located nearest to the center capable of being welded, on the basis of the amplitude of the received ultrasonic, and detects the calculated lap amount as the holding state of the workpiece, and the controller performs the spot welding in the case that the calculated lap amount is not smaller than a minimum lap amount required to perform the spot welding at the first end of the workpiece. 
         [0016]    More advantageously, the controller corrects the holding position of the workpiece with the electrode pair on the basis of the difference between the calculated lap amount and the minimum lap amount, and performs the spot welding. 
         [0017]    Furthermore, the controller preferably corrects the holding position of the workpiece and performs the spot welding, in the case that a distance from the first end of the workpiece capable of being welded to a second end of the workpiece capable of being welded located on the extended line connecting the first end and the center of the electrode is not smaller than a sum of the calculated lap amount, the correction amount for correcting the holding position, a diameter of the electrode and an allowance set as a dimension to avoid the electrode contacting the second end of the workpiece. 
         [0018]    In the alternative embodiment, the controller calculates an inclination angle of the workpiece inclining against the plane perpendicular to the axis of the electrode on the basis of the amplitude of the ultrasonic, and detects the inclination angle as the holding state of the workpiece, and the controller performs the spot welding in the case that the calculated inclination angle is not larger than a tolerable angle required to perform the spot welding at the first end of the workpiece. 
         [0019]    In the above embodiments, it is preferable to output an alert in the case that the spot welding is not performed. 
         [0020]    According to the embodiments of the invention, the welding gun can hold the workpiece at the proper position and with the correct angle, thereby reducing the welding flaws. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  depicts a structure of a spot welding apparatus. 
           [0022]      FIG. 2  shows a structure of electrical devices around a controller. 
           [0023]      FIG. 3  shows a correlation between a lap amount R and an ultrasonic amplitude D. 
           [0024]      FIG. 4  is a flowchart of controlling the spot welding according to the first embodiment. 
           [0025]      FIG. 5  shows a correlation between an inclination angle θ and an ultrasonic amplitude D. 
           [0026]      FIG. 6  is a flowchart of controlling the spot welding according to the second embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    FIG  1  shows a spot welding apparatus  100 . 
         [0028]    At the left side of  FIG. 1 , a welding gun  10  holding a workpiece W is depicted as enlarged in the drawing. The broken lines in FIG  1  show electrical cables. 
         [0029]    The spot welding apparatus  100  is a welding device which holds the workpiece W and applies the electrical current to the workpiece to melt the metals by the heat obtained from the resistance. The workpiece W, in the embodiment, is a body of vehicle (the joining portion of the door open). 
         [0030]    The spot welding apparatus  100  includes the welding gun  10 , a power feeder  20 , a robot hand  30  and a controller  50  for detecting the holding state of the workpiece W. 
         [0031]    The welding gun  10  is used to weld workpiece W by applying an electrical current to the workpiece held therewith. The welding gun  10  is attached to the robot hand  30  being capable of changing the position and posture thereof. The welding gun  10  includes a pair of electrodes  11  and  12 , a pair of shanks  21  and  22 , a pair of wedges  31  and  32 , and an ultrasonic transmitter  41  and an ultrasonic receiver  42 . 
         [0032]    The electrodes  11  and  12  hold the workpiece W, and the electrical current flows therebetween. The electrodes  11  and  12  are disposed coaxially along the axis perpendicular to the point on the workpiece W to be welded. The shanks  21  and  22  support the electrodes  11  and  12 , respectively. The wedges  31  and  32  are located at the centers of the shanks  21  and  22 , respectively, and they transmit the ultrasonic. 
         [0033]    The ultrasonic transmitter  41  is mounted on the wedge  31 . The ultrasonic transmitter  41  transmits the ultrasonic wave toward the workpiece W, holding the workpiece W with the electrodes  11  and  12 . 
         [0034]    The ultrasonic receiver  42  is mounted on the wedge  32 . The ultrasonic receiver  42  receives the ultrasonic wave transmitted from the ultrasonic transmitter  41  passing through the wedge  31 , the shank  21 , the electrode  11 , the electrode  12 , the shank  22 , and the shank  32  in order. 
         [0035]    The power feeder  20  feeds a high electrical current to the electrodes  11  and  12  of the welding gun  10 . The power feeder  20  has a transformer for raising the voltage of commercial power supply, and supplies the welding gun  10  with the high electrical current. The power feeder  20  is located near the robot hand  30 . The power feeder  20  is connected to a welding controller  60 . 
         [0036]    The robot hand  30  is an industrial robot with multiple joints that controls the position and the posture of the welding gun  10 . The robot hand  30  is connected to a robot controller  70 . 
         [0037]    The controller  50  controls the ultrasonic transmitter  41  to transmit the ultrasonic wave toward the electrodes  11 ,  12  of the welding gun  10  and the workpiece W, and controls the ultrasonic receiver  42  to receive the ultrasonic wave passing through them, with holding the workpiece W with the electrodes  11  and  12 . The controller detects the holding state of the workpiece W by the electrodes  11  and  12  of the welding gun  10  on the basis of the amplitude of the ultrasonic wave received by the ultrasonic receiver  42 . The controller  50  transmits the information to the robot controller  70  regarding whether to weld or not, whether the welding gun shots the edge or not, or whether the angle is right or not. 
         [0038]    The welding controller  60  controls the power feeder  20  to feed the high electrical current to the welding gun  10 . The robot controller  70  controls the robot hand  30  to change the position and the angle of the welding gun  10 . The robot controller  70  transmits the information regarding whether to operate the welding or not to the welding controller  60 . 
         [0039]      FIG. 2  depicts the electrical structure around the controller  50 . 
         [0040]    In  FIG. 2 , the welding gun  10  and the workpiece W are shown in broken lines. In  FIG. 2 , the transmission route of the ultrasonic wave is shown in two-dot chain line. 
         [0041]    The ultrasonic transmitter  41  is connected to the controller  50  via an ultrasonic generator  51 . The ultrasonic generator  51  produces the ultrasonic wave. 
         [0042]    The ultrasonic receiver  42  is connected to the controller  50  via a preamplifier  52  and an A/D converter  53 . The preamplifier  52  adjusts the signal gain of the ultrasonic signal obtained by the ultrasonic receiver  42 . The A/D converter  53  converts the ultrasonic signal with the signal gain being adjusted by the preamplifier  52  to the digital signal. 
         [0043]    As described above, the ultrasonic wave transmitted from the ultrasonic transmitter  41  passes through the pair of electrodes  11 ,  12  and the workpiece W to the ultrasonic receiver  42 . Depending on the positional relationship between the electrode pair  11  and  12  and the workpiece W, the amplitude of the received ultrasonic may be varied. That is, detecting the amplitude of the ultrasonic as the holding state of the workpiece W, the controller  50  can control the movement of the welding gun  10  so as to reduce the welding flaws. 
       First Embodiment  
       [0044]    Referring to  FIG. 3 , the correlation between a lap amount R and an ultrasonic amplitude D is described. 
         [0045]    The lower side of  FIG. 3  shows a graph of the ultrasonic amplitude D versus the lap amount R. The upper side of  FIG. 3  shows the sectional view illustrating some states of lap amount R. 
         [0046]    The lap amount R is an index of the holding state of workpiece W with the electrodes  11  and  12 . The lap amount R means the distance from the center (axis) of the electrode pair  11  and  12  to the end of the workpiece W located nearest to the center, which is capable of being welded, while the electrodes  11  and  12  of the welding gun  10  hold the workpiece W The end of the workpiece W capable of being welded means the portion such as the end face of the workpiece or the vicinity of the flange, and where the shape of the portion to be welded is changed from flat. 
         [0047]    Total lap amount R 2 , shown in  FIG. 3 , is the distance between the first end of the workpiece W capable of being welded nearest to the center (axis) of the electrode pair  11 ,  12  and the second end of the workpiece W capable of being welded located on the extended line connecting the first end with the center, namely the total lap amount R 2  means the weldable length of the workpiece W. 
         [0048]    The ultrasonic amplitude D is the amplitude of the ultrasonic received by the ultrasonic receiver  42 . 
         [0049]    Regarding the correlation between the lap amount R and the ultrasonic amplitude D, as the lap amount R becomes larger, the ultrasonic amplitude D becomes larger, and if the lap amount R is larger than a predetermined value, the ultrasonic amplitude D is constant. 
         [0050]    Generally, the transmitting speed of the ultrasonic wave in air is much slower than in the rigid body (e.g. the electrodes  11  and  12 , the workpiece W). The acoustic impedance of the rigid body is quite different from that of air. So, the ultrasonic transmitted in the rigid body hardly travels into air. 
         [0051]    Small lap amount R indicates that the electrodes  11  and  12  hold the workpiece W at the vicinity of the end of the workpiece. So, as the lap amount R becomes smaller, the amount of ultrasonic transmitted to the workpiece W becomes smaller. The strength of the ultrasonic oscillated from the ultrasonic transmitter  41  becomes weak when the ultrasonic passes through the electrode pair  11 ,  12  and the workpiece W and reaches the ultrasonic receiver  42 , whereby the ultrasonic amplitude becomes small. As described above, the correlation between the lap amount R and the ultrasonic, amplitude D is obtained. 
         [0052]    Referring to FIG,  4 , a spot welding step S 100  for controlling the spot welding is described below. 
         [0053]    In the step S 101 , the operator set the welding point in the workpiece W in the robot controller  70 . In the step S 102 , the robot controller  70  controls the robot hand  30  to move the welding gun  10  to the welding point. In the step S 103 , the robot controller  70  controls the welding gun  10  to hold the workpiece W. 
         [0054]    In the step S 104 , the controller  50  controls the ultrasonic generator  51  to generate the ultrasonic and the ultrasonic transmitter  41  to transmit the ultrasonic. 
         [0055]    In the step S 105 , the ultrasonic receiver  42  receives the ultrasonic transmitted from the ultrasonic transmitter  41  and passed through the electrode pair  11 ,  12  and the workpiece W. The preamplifier  52  adjusts the signal gain that is received at the ultrasonic receiver  42 , and the A/D converter  53  converts the ultrasonic signal to the digital signal, That digital signal is transmitted to the controller  50 . 
         [0056]    In the step S 106 , the controller  50  calculates the lap amount R on the basis of the ultrasonic amplitude D of the received signal by using the correlation between the lap amount R and the ultrasonic amplitude D. Note that the correlation of the lap amount R and the ultrasonic amplitude D is stored in the controller  50  in advance. 
         [0057]    In the step S 107 , the controller  50  determines whether the lap amount R calculated in the step S 106  is larger than a predetermined lap amount R 1  or not. The predetermined lap amount R 1  is a minimum lap amount required to perform the spot welding without occurring joint defects after welding the end of the workpiece W, that is an allowable distance between the center of the electrode pair  11 ,  12  and the end of the workpiece W. The lap amount R 1  is stored in the controller  50  in advance. 
         [0058]    If the lap amount R is not smaller than the predetermined lap amount R 1 , the step S 111  is performed. If the lap amount R is smaller the predetermined lap amount R 1 , the step S 108  is performed. 
         [0059]    In the step S 108 , the controller  50  calculates a correction mount a according to the formula (1) below, 
         [0000]      [Correction Amount α]=[Predetermined Lap Amount  R 1]−[Lap Amount  R]   [Formula (1)]
 
         [0060]    In the step S 109 , the controller  50  checks whether the formula (2) is satisfied or not. If the formula (2) is satisfied, the welding gun  10  cannot be moved by the correction amount α, and the step S 112  is performed. If the formula (2) is not satisfied, the step S 110  is performed. 
         [0000]      [Total Lap Amount  R 2]&lt;[Lap Amount  R ]+[Correction Amount α]+[Diameter of Electrode  d ]+[Allowance β 3 ]  [Formula (2)]
 
         [0061]    The allowance β in the formula (2) is set as a dimension to avoid the electrodes  11 . and  12  contacting the second end of the workpiece W. If the sum of the lap amount R, the correction amount α and the diameter d is not larger than the total lap amount R 2 , the electrodes  11  and  12  can he prevented from contacting the second end of the workpiece W, however, there may he a dimensional tolerance in the electrode diameter d. Therefore, the allowance β is defined, and if the formula (2) is not satisfied, the welding gun  10 , namely the electrodes  11  and  12 , can be moved without contact with the second end of the workpiece W. 
         [0062]    In the step S 110 , the robot controller  70  controls the robot hand  30  to move the welding gun  10  by the correction amount α. In the step S 111 , the welding controller  60  controls the power feeder  20  to feed the high electrical current to the electrodes  11  and  12 , thereby operating the spot welding. 
         [0063]    In the step S 112 , the controller  50  outputs the alarm on the monitor or the like, and alerts the operator. 
         [0064]    According to the spot welding step S 100 , the welding gun  10  can hold the workpiece W at the proper position, thereby reducing the welding defect such as the edge shot; the welding current runs off the flange end of the workpiece W. 
       Second Embodiment  
       [0065]    Referring to  FIG. 5 , the correlation between an inclination angle θ and an ultrasonic amplitude D is described. 
         [0066]    The lower side of  FIG. 5  shows a graph of the ultrasonic amplitude D versus the inclination angle θ. The upper side of  FIG. 5  shows the sectional view illustrating the inclination angle θ, where the workpiece W is held with the electrodes  11  and  12  of the welding gun  10 . 
         [0067]    The inclination angle θ is an index of the holding state of workpiece W with the electrodes  11  and  12 . The inclination angle θ means the angle between the workpiece W and the plane perpendicular to axes of the electrodes  11  and  12 , while the electrodes  11  and  12  of the welding gun  10  hold the workpiece W. 
         [0068]    Generally, the transmitting speed of the ultrasonic wave in air is much slower than in the rigid body (e.g. the electrodes  11  and  12 , the workpiece W). The acoustic impedance of the rigid body is quite different from that of air. So, the ultrasonic transmitted in the rigid body hardly travels into air. 
         [0069]    As the inclination angle θ becomes larger, the contact area between the electrodes  11  and  12  and the workpiece W becomes smaller and the amount of ultrasonic transmitted to the workpiece W becomes smaller. The strength of the ultrasonic oscillated from the ultrasonic transmitter  41  becomes weak when the ultrasonic passes through the electrode pair  11 ,  12  and the workpiece W and reaches the ultrasonic receiver  42 , whereby the ultrasonic amplitude becomes small. As described above, the correlation between the inclination angle θ and the ultrasonic amplitude D is obtained. 
         [0070]    Referring to  FIG. 6 , a spot welding step S 200  for controlling the spot welding is described below. 
         [0071]    In the step S 201 , the operator set the welding point of the workpiece W in the robot controller  70 . In the step S 202 , the robot controller  70  controls the robot hand  30  to move the welding gun  10  to the welding point. In the step S 203 , the robot controller  70  controls the welding gun  10  to hold the workpiece W. 
         [0072]    In the step S 204 , the controller  50  controls the ultrasonic generator  51  to generate the ultrasonic and the ultrasonic transmitter  41  to transmit the ultrasonic. 
         [0073]    In the step S 205 , the ultrasonic receiver  42  receives the ultrasonic transmitted from the ultrasonic transmitter  41  and passed through the electrodes  11 ,  12  and the workpiece W. The preamplifier  52  adjusts the signal gain that is received at the ultrasonic receiver  42 , and the A/D converter  53  converts the ultrasonic signal to the digital signal. That digital signal is transmitted to the controller  50 . 
         [0074]    In the step S 206 , the controller  50  calculates the inclination angle θ on the basis of the ultrasonic amplitude D of the received signal by using the correlation between the inclination angle θ and the ultrasonic amplitude D. Note that the correlation of the inclination angle θ and the ultrasonic amplitude D is stored in the controller  50  in advance. 
         [0075]    In the step S 207 , the controller  50  determines whether the inclination angle θ calculated in the step S 206  is larger than a predetermined inclination angle θ 1  or not. The predetermined inclination angle θ 1  is a tolerable levelness required to perform the spot welding without occurring joint defects after the spot welding, that is an uprightness of the workpiece W against the center of the electrodes  11  and  12 . The predetermined inclination angle θ 1  is stored in the controller  50  in advance. 
         [0076]    If the inclination angle θ is not larger than the predetermined inclination angle θ 1 , the step S 208  is performed. If the inclination angle θ is larger than the predetermined inclination angle θ 1 , the step S 209  is performed. 
         [0077]    In the step S 208 , the welding controller  60  controls the power feeder  20  to feed the high electrical current to the electrodes  11  and  12 , thereby operating the spot welding. 
         [0078]    In the step S 209 , the controller  50  outputs the alarm on the monitor or the like, and alerts the operator. 
         [0079]    According to the spot welding step S 200 , the welding gun  10  can hold the workpiece W with the proper angle, thereby reducing the welding defects and securing the quality of spot welding.