Patent Publication Number: US-2022226925-A1

Title: Welding apparatus and plate thickness testing method for welding apparatus

Description:
BACKGROUND 
     A technique disclosed herein belongs to the technical field that relates to a welding apparatus and a plate thickness testing method for the welding apparatus. 
     Conventionally, a welding apparatus including a servo gun having a fixed electrode and a movable electrode has been known (for example, Patent Document 1). In this welding apparatus, a distance between the electrodes is one factor for, e.g., determining welding conditions. Thus, many of the welding apparatuses have a function of measuring the distance between the electrodes. 
     For example, in a welding apparatus described in Japanese Unexamined Patent Publication No. 2002-35951, it is disclosed that, in a spot welding apparatus including a spot welding gun having a first electrode (movable electrode) on a gun shaft side driven by a servo motor and a second electrode (fixed electrode) fixed to a tip portion of a gun arm, an amount of movement of the first electrode is detected according to a detection signal of a rotary encoder that detects an amount of rotation of the servo motor, and an interval between both electrodes is detected according to the amount of movement of the first electrode. 
     SUMMARY 
     If the distance between the electrodes can be measured as in the above-mentioned conventional example, a plate thickness of an object to be welded can be measured. For example, the plate thickness of the object to be welded can be measured by the amount of movement when the movable electrode is moved until the movable electrode comes into contact with the other surface of the object to be welded while the fixed electrode is in contact with one surface of the object to be welded. If the plate thickness of the object to be welded can be measured in this way, it is possible to detect whether or not the plate thickness of the object to be welded is correct, that is, whether or not a member having a plate thickness different from that of a desired member is to be erroneously welded. 
     However, when the electrode is worn off by abrasion, the amount of movement of the movable electrode until the movable electrode comes into contact with the object to be welded changes, so that the plate thickness test as described above may not be performed normally. 
     On the other hand, for example, before actual welding operation for the object to be welded, it is conceivable to confirm whether or not the plate thickness test is normally performed by using a dummy workpiece imitating the object to be welded. However, it is necessary to prepare a dummy workpiece for each type of the objects to be welded, which increases cost and requires expansion of the working space in order to secure a storage place. In addition, it is necessary to set a test standard for each dummy workpiece, and the dummy workpiece must be put on a production line for test, which may lead to deterioration of work efficiency. 
     The technique disclosed herein attempts to provide a welding apparatus that can test a plate thickness of a workpiece, and in this welding apparatus, whether or not a plate thickness test is normally performed can be confirmed efficiently at low cost while reducing a working space. 
     A first aspect of the technique disclosed herein is directed to a welding apparatus which includes a servo gun having a fixed electrode and a movable electrode, in which the fixed electrode and the movable electrode sandwich an object to be tested to perform a plate thickness test of the object to be tested. The welding apparatus includes a gun control section that controls operation of the servo gun, a detection section that calculates a thickness of the object to be tested from an amount of movement of the movable electrode with respect to the fixed electrode and that detects an abnormality when, as an obtained calculation result, the plate thickness is thinner than a desired plate thickness by a predetermined amount or more or thicker than the desired plate thickness by the predetermined amount or more, a determination section that determines whether or not a plate thickness test of the object to be tested using the detection section is performed normally, and a plate-shaped jig that is fixed to a jig fixing position in a space and has a first plate portion having a plate thickness thinner than a predetermined specific plate thickness by the predetermined amount or more, and a second plate portion having a plate thickness thicker than the specific plate thickness by the predetermined amount or more. The gun control section is configured to operate the servo gun when determination is performed by the determination section and measure the plate thickness of the first plate portion and the plate thickness of the second plate portion, if the detection section tests the plate thicknesses of the first and second plate portions by taking a desired plate thickness as the specific plate thickness and detects abnormalities in both tests, the determination section determines that the plate thickness test is performed normally, and if the detection section does not detect an abnormality in at least one of the tests, the determination section determines that the plate thickness test is defective. 
     A second aspect of the technique disclosed herein is the above-described first aspect in which the jig further has a reference plate portion whose plate thickness is set to the specific plate thickness. 
     A third aspect of the technique disclosed herein is the above-described first or second aspect in which the welding apparatus further includes a robot which has a plurality of arms and to which the servo gun is attached, and a robot control section that controls operation of the robot. In the jig, a surface on one side of two surfaces in a plate thickness direction is a flat surface, a step is formed on a surface on the other side, at least the first plate portion and the second plate portion are formed, the jig is fixed so that the plate thickness direction is a horizontal direction or a vertical direction. At least when a production line is not operated, the robot control section operates the robot so that the servo gun is positioned at a predetermined standby position, and when the plate thicknesses of the first plate portion and the second plate portion are measured, the robot control section operates the robot to move the servo gun from the standby position to the jig fixing position so that the fixed electrode is positioned on the one side in the plate thickness direction. 
     A fourth aspect of the technique disclosed herein is that the jig has a Rockwell hardness HRC of 60 or more. 
     The technique disclosed herein also covers a plate thickness testing method for a welding apparatus. Specifically, a fifth aspect of the technique disclosed herein is directed to a plate thickness testing method for a welding apparatus including a servo gun having a fixed electrode and a movable electrode which sandwich an object to be tested to perform a plate thickness test of the object to be tested. The welding apparatus further includes a plate-shaped jig that is fixed to a jig fixing position in a space and has a first plate portion having a plate thickness thinner than a predetermined specific plate thickness by a predetermined amount or more, and a second plate portion having a plate thickness thicker than the specific plate thickness by the predetermined amount or more. The plate thickness testing method for the welding apparatus includes a determination step of determining whether or not the plate thickness test using the welding apparatus is performed normally, the determination step including: a first measurement step of sandwiching the first plate portion between the fixed electrode and the movable electrode and measuring the plate thickness of the first plate portion, a first detection step of detecting an abnormality of the plate thickness based on a measurement result in the first measurement step, a second measurement step of sandwiching the second plate portion between the fixed electrode and the movable electrode and measuring the plate thickness of the second plate portion, and a second detection step of detecting an abnormality of the plate thickness based on a measurement result in the second measurement step. The first and second detection steps are steps of detecting an abnormality when the plate thickness is thinner than the specific plate thickness by a predetermined amount or more or thicker than the specific plate thickness by the predetermined amount or more, and in the determination step, if abnormalities are detected in both the first and second detection steps, it is determined that the plate thickness test is performed normally, and if an abnormality is not detected in at least one of the first and second detection steps, it is determined that the plate thickness test is defective. 
     According to the above-described first and fifth aspects, it is possible to confirm whether or not the plate thickness test using the detection section is normally performed only by testing the plate thickness of the plate-shaped jig fixed to the jig fixing position, so that deterioration of work efficiency can be reduced. In particular, since the same jig is used, it is not necessary to change the test standard, so that improvement in work efficiency can be expected. 
     Since whether or not the plate thickness test is normally performed is determined by measuring both the first plate portion having a plate thickness thinner than the specific plate thickness by the predetermined amount or more, and the second plate portion having a plate thickness thicker than the specific plate thickness by the predetermined amount or more, the plate thickness test can be confirmed both when the plate thickness is thin and when the plate thickness is thick. Thus, high determination accuracy can be obtained. 
     Unlike conventional cases, it is not necessary to prepare a large number of dummy workpieces, and it suffices to provide one plate-shaped jig, so that the working space can be reduced, and the cost can also be reduced. 
     According to the second aspect, when there is a problem in the detection section, calibration can be performed using the measurement result of the reference plate portion. If the plate thickness of the reference plate portion is also measured, the determination accuracy according to the determination section can be further improved. 
     According to the third aspect, if the jig is disposed so that the plate thickness direction is the horizontal direction, the horizontal coordinates of the fixed electrode are constant when both electrodes of the servo gun sandwich the jig. On the other hand, if the jig is disposed so that the plate thickness direction is the vertical direction, the vertical coordinates of the fixed electrode are constant when both electrodes of the servo gun sandwich the jig. Thus, operation of the robot can be simplified. Consequently, the work efficiency of test can be further improved. 
     In addition, if the coordinates of the fixed electrode are constant when both electrodes of the servo gun sandwich the jig, for example, when the coordinates of the servo gun are deviated due to rattling of the robot, the deviation of the coordinates can be confirmed by confirming the position of the fixed electrode when the jig is sandwiched. It is also possible to confirm whether or not a position of a workpiece W or the like is deviated on the basis of the position of the jig. 
     According to the fourth aspect, the jig is hardly deformed even if the jig is sandwiched between both electrodes of the servo gun a plurality of times. Thus, problems of the determination based on the deformation of the jig can be reduced, and the determination accuracy according to the determination section can be further improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view schematically showing a welding apparatus according to an exemplary embodiment. 
         FIG. 2  is a block diagram showing a control system of the welding apparatus. 
         FIG. 3  is an operation diagram showing operation of a servo gun when a plate thickness of a workpiece is measured, and shows a state in which the servo gun is disposed so that a test jig is located between electrodes. 
         FIG. 4  is a view showing a state in which a movable electrode is displaced from the state of  FIG. 3  and brought into contact with the test jig. 
         FIG. 5  is a view showing a state in which the test jig is sandwiched between a fixed electrode and the movable electrode from the state of  FIG. 4 . 
         FIG. 6  is a front view of the test jig. 
         FIG. 7  is a side view of the test jig. 
         FIG. 8  is a view showing a state in which the test jig is sandwiched between the fixed electrode and the movable electrode when a plate thickness of the test jig is measured. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention will now be described in detail with reference to the drawings. The following explanations of a preferred embodiment are substantially mere examples. 
     &lt;Overall Configuration&gt; 
       FIG. 1  schematically shows a welding apparatus  1  according to the embodiment of the present invention. The welding apparatus  1  is provided on a production line. The welding apparatus  1  is a spot welding apparatus configured to perform spot welding on a workpiece W (see  FIG. 3 , etc.) formed by stacking a plurality of plate-shaped members and join the plate-shaped members to each other. 
     As shown in  FIG. 1 , the welding apparatus  1  includes a robot  10  having a displaceable arm  11 , a servo gun  20 , and a controller  100  that controls operation of the robot  10  and the servo gun  20 . As will be described in detail later, the welding apparatus  1  of the present embodiment has a test jig  30  fixed at a predetermined jig fixing position. 
     The robot  10  is an articulated industrial robot having three arms  11  rotatably connected to each other and a base  12 . The three arms  11  include a base-side arm  11   a  whose base end is connected to the base  12 , a gun-side arm  11   b  in which the servo gun  20  is held at the tip, and an intermediate arm  11   c  that connects a tip portion of the base-side arm lla and a base end side of the gun-side arm  11   b . The base  12  is installed on a floor surface of a factory or the like. Consequently, the welding apparatus  1  is fixed to the floor surface of the factory or the like. 
     The base-side arm  11   a  is connected to the base  12  rotatably around a shaft extending in a vertical direction. When the base-side arm  11   a  rotates around the shaft, each of the arms  11   a  to  11   c  rotates in the horizontal direction together with the servo gun  20 . 
     The base  12  and the base end portion of the base-side arm  11   a , the tip portion of the base-side arm  11   a  and one longitudinal end portion of the intermediate arm  11   c , and a base end portion of the gun-side arm  11   b  and the other longitudinal end portion of the intermediate arm  11   c  are each connected rotatably around a shaft  13  extending in the horizontal direction. The servo gun  20  can be moved back and forth in a direction orthogonal to both a vertical axis and the shaft  13  by rotating each of the arms  11   a  to  11   c  around the shaft  13 . 
     Although not shown, the servo gun  20  is attached to a tip portion of the gun-side arm  11   b  so as to be rotatable around a shaft extending along the gun-side arm  11   b.    
     In the following description, the extending direction of the shaft  13  is referred to as a left-right direction, and the direction orthogonal to both the vertical direction and an axial direction of the shaft  13  is referred to as a front-rear direction. In the front-rear direction, a side where the servo gun  20  is away from the base  12  is referred to as a front side, and a side where the servo gun  20  approaches the base  12  is referred to as a rear side. 
     The servo gun  20  has a U-shaped gun arm  21 , a fixed electrode  22  provided at one of two tips of the gun arm  21 , a movable electrode  23  that is provided at the other tip of the gun arm  21  so as to face the fixed electrode  22  and can be connected to or disconnected from the fixed electrode  22  by a servo motor  25  (see  FIG. 2 ), and a voltage application circuit  26  (see  FIG. 2 ) that applies a voltage between the fixed electrode  22  and the movable electrode  23 . The movable electrode  23  is provided so as to project from a tip of an electrode case  23   a . The servo gun  20  also has a torque sensor  24  (see  FIG. 2 ). The torque sensor  24  detects a load (torque) applied to the servo motor  25  when the workpiece is sandwiched between the fixed electrode  22  and the movable electrode  23 . 
     When the workpiece W (see, e.g.,  FIG. 4 ) is joined, the welding apparatus  1  sandwiches the workpiece W between the fixed electrode  22  and the movable electrode  23 . After that, the welding apparatus  1  applies an electric current between both the electrodes  22  and  23  to join the plate-shaped members constituting the workpiece W. The operation of the servo gun  20  when the workpiece W is sandwiched will be described later. 
     As shown in  FIG. 2 , the controller  100  includes a CPU  101 , a memory  102 , an interface  103 , and a microprocessor having a path for connecting these units. The CPU  101  constitutes a gun control section  101   a  that controls the operation of the servo gun  20 . The CPU  101  constitutes a robot control section  101   b  that controls the operation of the robot  10 . When the fixed electrode  22  and the movable electrode  23  sandwich an object to be tested, the CPU  101  calculates the thickness (plate thickness) of the object to be tested from an amount of movement of the movable electrode  23  with respect to the fixed electrode  22 , and constitutes a detection section  101   c  that detects an abnormality when, as the calculation result, the plate thickness is thinner by a predetermined amount or more or thicker by the predetermined amount or more relative to a predetermined set plate thickness. The CPU  101  constitutes a determination section  101   d  that determines whether or not a plate thickness abnormality detection function of the detection section  101   c  functions normally. The gun control section  101   a  of the CPU  101  controls the operation of the servo motor  25  and the voltage application circuit  26 . The robot control section  101   b  of the CPU  101  controls the operation of each of the arms  11  of the robot  10 . 
     The memory  102  stores a position where the servo gun  20  should be set and welding conditions (voltage to be applied between the electrodes, etc.) of the workpiece W in the welding operation for the workpiece W. 
     In addition to the welding apparatus  1 , the controller  100  is connected to a display  104  showing a test result obtained when the thickness (plate thickness) of the object to be tested (workpiece W and the test jig  30  described later) is tested by the servo gun  20 . 
     &lt;Plate Thickness Test&gt;The welding apparatus  1  according to the present embodiment has a plate thickness test mechanism that tests the plate thickness of the workpiece W or the like. The plate-shaped members constituting the workpiece W have various plate thicknesses, and the plate thickness may differ depending on the type of the workpiece W. The plate thickness of the workpiece W as a whole may differ depending on the number of plate-shaped members stacked. Thus, by testing whether or not the plate thickness of the workpiece W to be welded has a desired plate thickness, it is possible to confirm whether or not the workpiece W different from the desired workpiece W is flowing on the production line or whether or not the plate-shaped member itself is defective. 
     The plate thickness test mechanism of the welding apparatus  1  according to the present embodiment measures the plate thickness of the object to be tested from the amount of movement of the movable electrode  23  when the object to be tested is sandwiched between the fixed electrode  22  and the movable electrode  23  and tests whether or not the plate thickness is a desired thickness. The operation of the servo gun  20  and the control of the controller  100  at this time will be described with reference to  FIGS. 3 to 5 . 
     First, as shown in  FIG. 3 , the robot control section  101   b  of the CPU  101  operates the robot  10  so that the workpiece W is located between the fixed electrode  22  and the movable electrode  23  of the servo gun  20  to move the servo gun  20 . At this time, neither the fixed electrode  22  nor the movable electrode  23  is in contact with the workpiece W. 
     Next, as shown in  FIG. 4 , the gun control section  101   a  of the CPU  101  drives the servo motor  25  to move the movable electrode  23  until the movable electrode  23  is abutted against the workpiece W. Since the workpiece W is usually positioned by a positioning device or the like, the position of the movable electrode  23  itself does not change after the abutment against the workpiece W. 
     From the state shown in  FIG. 4 , when the servo motor  25  is driven by the gun control section  101   a  of the CPU  101  so as to bring the movable electrode  23  closer to the fixed electrode  22 , the reaction force moves the gun arm  21  to which the electrode case  23   a  is attached as shown in  FIG. 5 . Consequently, the workpiece W is sandwiched between the fixed electrode  22  and the movable electrode  23 . At this time, although the position of the movable electrode  23  does not change, an amount of projection of the movable electrode  23  from the electrode case  23   a  increases, so that an amount of movement of the gun arm  21  can also be regarded as the amount of movement of the movable electrode  23 . Whether or not the workpiece W is sandwiched between the fixed electrode  22  and the movable electrode  23  is determined by whether or not the torque detected by the torque sensor  24  has reached a predetermined torque. Specifically, when the torque detected by the torque sensor  24  has reached the predetermined torque, the CPU  101  determines that the workpiece W is sandwiched between the fixed electrode  22  and the movable electrode  23 . 
     The detection section  101   c  of the CPU  101  calculates the amount of movement of the movable electrode  23  with respect to the fixed electrode  22 , and calculates a distance between the fixed electrode  22  and the movable electrode  23  when the workpiece W is sandwiched, thereby calculating the plate thickness of the workpiece W. The amount of movement of the movable electrode  23  with respect to the fixed electrode  22  can be calculated, for example, by obtaining the number of rotations of the servo motor  25  before the workpiece W is sandwiched. 
     After the calculation of the plate thickness of the workpiece W, the detection section  101   c  of the CPU  101  detects an abnormality based on the calculated plate thickness. That is, when the plate thickness is thinner by a predetermined amount or more or thicker by the predetermined amount or more relative to a predetermined set plate thickness, the detection section  101   c  detects an abnormality. When the detection section  101   c  detects an abnormality, the controller  100  stops the welding operation, for example, and displays on the display  104  that the abnormality has been detected. The set plate thickness and the predetermined amount can be different for each of the workpiece W, and can be arbitrarily set by an operator. 
     As described above, by providing the plate thickness test mechanism in the welding apparatus  1 , when plate-shaped members having an incorrect plate thickness are overlapped, or when the plate thickness is thin or thick due to a defect of the plate-shaped member itself, the operator can confirm this fact. Consequently, it is possible to prevent or reduce defective products from being manufactured in advance. 
     However, since both the electrodes  22  and  23  of the servo gun  20  are worn, the amount of movement of the movable electrode  23  cannot be calculated accurately due to the wear. Thus, for example, even when the plate thickness of the workpiece W is thicker than the set plate thickness by the predetermined amount or more, the detection section  101   c  does not detect an abnormality, and the plate thickness test using the detection section  101   c  is not normally performed. Even if the gun arm  21  deflects when the workpiece W is sandwiched between both the electrodes  22  and  23 , the amount of movement of the movable electrode  23  cannot be calculated accurately, so that there is a risk that the above-described plate thickness test may not be performed normally. 
     Although it is possible to measure and correct amounts of wear of both the electrodes  22  and  23 , if a correction value at this time is not accurate, the above-described plate thickness test is not performed normally after all. 
     Thus, in the welding apparatus  1  according to the present embodiment, before the workpiece W is put on the production line, whether or not the plate thickness test using the detection section  101   c  is performed normally is determined using the test jig  30  by the determination section  101   d  of the CPU  101 . 
     As shown in  FIGS. 6 and 7 , the test jig  30  is formed by processing a rectangular metal plate so that portions having different plate thicknesses are formed. In the metal plate constituting the test jig  30 , the material and thickness are set so that the metal plate is not displaced or deformed even if the metal plate is sandwiched between both the electrodes  22  and  23 . In the present embodiment, the test jig  30  is a metal plate having a Rockwell hardness HRC of 60 or more. Specifically, for example, a JIS standard high-speed tool steel (SKH) may be quenched so that the Rockwell hardness after quenching is HRC 60  to  66 . As a material of the test jig  30 , carbon tool steel (SK) or carbon steel for machine structure (SXXC) may be used. 
     In the test jig  30 , as shown in  FIG. 6 , a test portion  31  is formed in about half of one side in a short side direction, and a fixing portion  32  fixed at the jig fixing position is formed in half of the other side in the short side direction. The test portion  31  and the fixing portion  32  each extend in the long side direction without changing the width in the short side direction. 
     The test portion  31  has a reference plate portion  33  having a predetermined specific plate thickness, a first plate portion  34  having a plate thickness thinner than the specific plate thickness by the predetermined amount or more, and a second plate portion  35  having a plate thickness thicker than the specific plate thickness by the predetermined amount or more. The reference plate portion  33  is formed at the center in the long side direction, the first plate portion  34  is formed on one side in the long side direction with respect to the reference plate portion  33 , and the second plate portion  35  is formed on the other side in the long side direction with respect to the reference plate portion  33 . Consequently, as shown in  FIG. 7 , the test portion  31  has a shape in which the plate thickness gradually increases from the one side in the long side direction to the other side. In the test portion  31 , as shown in  FIG. 7 , a surface on one side (hereinafter referred to as a flat surface  31   a ) of two surfaces in a plate thickness direction is a flat surface, and, on the other hand, a step is formed on a surface on the other side (hereinafter referred to as a step surface  31   b ), so that the reference plate portion  33 , the first plate portion  34 , and the second plate portion  35  are formed. 
     A portion of the reference plate portion  33  on one side in the long side direction and on the other side in the short side direction is a curved portion  33   a  that is curved and inclined to the one side in the long side direction toward the other side in the short side direction. Also in the second plate portion  35 , similarly to the reference plate portion  33 , a portion on one side in the long side direction and on the other side in the short side direction is a curved portion  35   a  that is curved and inclined to the one side in the long side direction toward the other side in the short side direction. The curved portions  33   a  and  35   a  do not necessarily have to be provided. The fixing portion  32  is provided with two bolt holes  32   a  into which bolts are inserted. Although detailed illustration is omitted, the test jig  30  is fixed to the jig fixing position by fastening and fixing the fixing portion  32  to a support portion, provided at the jig fixing position in a working space of the welding apparatus  1 , with a fastener such as a bolt. In the present embodiment, as shown in  FIGS. 1 and 8 , the test jig  30  is fixed to the jig fixing position so that the plate thickness direction is the front-rear direction (horizontal direction) and the long side direction is the vertical direction. 
     Next, an operation of the welding apparatus  1  when the determination section  101   d  makes a determination using the test jig  30  will be described. When the determination section  101   d  makes a determination using the test jig  30 , the set plate thickness that serves as a detection reference for the detection section  101   c  is predetermined to the specific plate thickness. The determination by the determination section  101   d  using the test jig  30  is automatically executed at least immediately after the production line is put into operation and before the workpiece W is put on the production line. 
     First, when the production line is not operated immediately before the production line is operated, as shown in  FIG. 1 , the robot control section  101   b  operates the robot  10  so that the servo gun  20  is positioned at a predetermined standby position. When the production line is operated from this state, in order to measure the plate thickness of the test jig  30 , the robot control section  101   b  operates the robot  10  so that the servo gun  20  is moved from the standby position to the jig fixing position. At this time, as shown in  FIG. 8 , the robot control section  101   b  operates the robot  10  so that while the fixed electrode  22  of the servo gun  20  is located on the flat surface  31   a  side of the test jig  30 , the movable electrode  23  of the servo gun  20  is located on the step surface  31   b  side of the test jig  30 . 
     Next, the gun control section  101   a  operates the servo gun  20  to test the plate thickness of the test jig  30 . In the present embodiment, at least the plate thicknesses of the first plate portion  34  and the second plate portion  35  are tested in the determination by the determination section  101   d . The gun control section  101   a  operates the servo gun  20  in the same manner as when the workpiece W is sandwiched, and, as shown in  FIG. 8 , the servo gun  20  sequentially sandwiches the first plate portion  34  and the second plate portion  35 . The detection section  101   c  calculates each of the plate thicknesses of the first plate portion  34  and the second plate portion  35 . 
     As described above, in the determination by the determination section  101   d , the set plate thickness is set to the specific plate thickness. Thus, if the detection section  101   c  functions normally, the detection section  101   c  detects abnormalities in both the test of the first plate portion  34  thinner than the specific plate thickness by the predetermined amount or more and the test of the second plate portion  35  thicker than the specific plate thickness by the predetermined amount or more. Thus, when the plate thicknesses of the first plate portion  34  and the second plate portion  35  are tested, the determination section  101   d  determines that the plate thickness test using the detection section  101   c  is performed normally when the detection section  101   c  detects an abnormality in each test. On the other hand, the determination section  101   d  determines that the plate thickness test is defective when the detection section  101   c  does not detect an abnormality in at least one measurement. Consequently, the determination by the determination section  101   d  using the test jig  30  is completed. 
     When the determination section  101   d  determines that the plate thickness test is defective, the CPU  101  notifies the operator that the plate thickness test is defective by, e.g., an alarm or a display on the display  104 . After the defect is transmitted, the operator investigates causes such as whether the defect of the plate thickness test occurs due to wear of both the electrodes  22  and  23 , whether the defect occurs due to abnormal deflection of the gun arm  21 , or whether the correction value of wear correction is incorrect when the wear correction is performed, and operation (calibration) that eliminates the defect of the plate thickness test may be performed. At this time, the operator may cause, for example, the robot  10  and the servo gun  20  to measure the plate thickness of the reference plate portion  33  and may perform calibration on the basis of the measurement result. 
     Therefore, according to the present embodiment, the detection section  101   c  of the CPU  101  that calculates the thickness of the object to be tested from the amount of movement of the movable electrode  23  with respect to the fixed electrode  22  and detects an abnormality when, as the calculation result, the plate thickness is thinner by a predetermined amount or more or thicker by the predetermined amount or more relative to a desired plate thickness, the determination section  101   d  of the CPU  101  that determines whether or not the plate thickness test using the detection section  101   c  is performed normally, and the plate-shaped test jig  30  fixed to the jig fixing position in the space and having the first plate portion  34  having a plate thickness thinner than a predetermined specific plate thickness by the predetermined amount or more, and the second plate portion  35  having a plate thickness thicker than the specific plate thickness by the predetermined amount or more are provided. When the detection section  101   c  tests the plate thicknesses of the first and second plate portions  34  and  35  by taking a desired plate thickness as the specific plate thickness and detects abnormalities in both the tests, the determination section  101   d  determines that the plate thickness test is performed normally. On the other hand, when the detection section  101   c  does not detect an abnormality in at least one of the tests, the determination section  101   d  determines that the plate thickness test is defective. Consequently, it is possible to confirm whether or not the above-described plate thickness test is normally performed only by testing the plate thickness of the test jig  30  fixed to the jig fixing position, so that deterioration of work efficiency can be reduced. In particular, unlike conventional cases, since it is not necessary to change the test standard for each dummy workpiece, improvement in work efficiency can be expected. 
     Since whether or not the plate thickness test is normally performed is determined by measuring both the first plate portion  34  having a plate thickness thinner than the specific plate thickness by the predetermined amount or more, and the second plate portion  35  having a plate thickness thicker than the specific plate thickness by the predetermined amount or more, the plate thickness test can be confirmed both when the plate thickness is thin and when the plate thickness is thick. Thus, high determination accuracy can be obtained. 
     Unlike conventional cases, it is not necessary to prepare a large number of dummy workpieces, and it suffices to provide one test jig  30 , so that the working space can be reduced, and the cost can also be reduced. 
     In the present embodiment, the test jig  30  is fixed to the jig fixing position so that the plate thickness direction is the front-rear direction (horizontal direction) and the long side direction is the vertical direction, and the robot control section  101   b  of the CPU  101  moves the servo gun  20  from the standby position to the jig fixing position so that the fixed electrode  22  is located on the flat surface  31   a  side. Consequently, when both the electrodes  22  and  23  of the servo gun  20  sandwich the test jig  30 , the horizontal coordinates of the fixed electrode  22  are constant. Thus, operation of the robot  10  can be simplified. Consequently, it is possible to more efficiently determine whether or not the plate thickness test is performed normally. 
     In addition, suppose that the horizontal coordinates of the fixed electrode  22  are constant when both the electrodes  22  and  23  of the servo gun  20  sandwich the test jig  30 . In such a situation, for example, if the coordinates of the servo gun  20  are deviated due to rattling of the robot  10 , the deviation of the coordinates can be confirmed by confirming the position of the fixed electrode  22  when the test jig  30  is sandwiched. It is also possible to confirm whether or not the position of the workpiece W or the like is deviated on the basis of the position of the test jig  30 . 
     In the present embodiment, the test jig  30  has a Rockwell hardness HRC of 60 or more. Thus, the test jig  30  is hardly deformed even if the test jig  30  is sandwiched between both the electrodes  22  and  23  of the servo gun  20  a plurality of times. Thus, problems of the determination based on the deformation of the test jig  30  can be reduced, and the accuracy of the determination whether or not the plate thickness test is normally performed can be further improved. 
     The technique disclosed herein should not be limited to the foregoing embodiments, and various changes and modifications may be made without departing from the scope of the claims. 
     For example, in the above embodiment, in the determination by the determination section  101   d  using the test jig  30 , the first plate portion  34  and the second plate portion  35  are the test targets. However, the present invention is not limited to this embodiment, and the reference plate portion  33  may also be a test target. When the plate thickness of the reference plate portion  33  is tested, the determination section  101   d  determines that the plate thickness test using the detection section  101   c  is normally performed when the detection section  101   c  does not detect an abnormality. On the other hand, when the detection section  101   c  detects an abnormality, the determination section  101   d  determines that a defect has occurred in the plate thickness test. 
     Further, in the above embodiment, the test jig  30  is fixed to the jig fixing position so that the plate thickness direction is the front-rear direction and the long side direction is the vertical direction. However, the present invention is not limited to this embodiment, and the test jig  30  may be disposed so that the plate thickness direction is the vertical direction and the long side direction is the front-rear direction. 
     Furthermore, in the above embodiment, the determination by the determination section  101   d  using the test jig  30  is executed immediately after the production line is operated. However, in addition to this embodiment, the operator may execute the determination at any timing. 
     The embodiment described above is provided by way of illustration only and should not be construed to limit the present disclosure. The scope of the present disclosure should be measured solely by reference to the claims. All the modifications and changes within an equivalent scope of the claims fall within the scope of the present disclosure.