JP2002028983A - Method and apparatus for detecting defective welding in welding by non-contact hot plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for detecting defective welding in welding by a non-contact hot plate in which a resin part, after being heated by the hot plate, is pressed and welded and a welding apparatus having a device for the detection. SOLUTION: In a welding method by the non-contact hot plate, in which the opposite contact surfaces 2a and 3a of resin parts 2 and 3 which are gripped by a pair of grippers 4 and 5 are made to face both surfaces of the hot plate through clearances and heat-melted without being contacted with the hot plate, and the melted contact surfaces 2a and 3a are pressed to each other by the relative movement of the grippers 4 and 5 to be welded, the relative approaching displacement quantity and/or approaching speed of the grippers 4 and 5 in the press process are measured, and the measured values are compared with an approaching displacement quantity and/or an approaching speed which are set in advance to judge whether the welding is acceptable or not.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-contact hot plate welding method in which a resin component is heated by a hot plate in a non-contact state and then welded by pressure welding, and a method for detecting a welding defect and an apparatus for performing the detection. The present invention relates to a welding device provided.

[0002]

2. Description of the Related Art A non-contact hot plate welding method and apparatus for heating a resin component by non-contact heating with a hot plate and then pressing and welding the melted joint surface portion are disclosed in, for example, JP-A-8-721.
No. 50 and Japanese Patent Application Laid-Open No. 9-104262.

However, in this welding method, changes in the ambient temperature (room temperature), fluctuations in the power supply voltage for heating the hot plate, an increase in the temperature of the hot plate due to the accumulation of the heating time of the hot plate, and a change in the hot plate due to the dimensional difference of the resin parts. Due to variations in the distance between the surface and the joining surface of the resin component, the amount of heat input to the joining surface, which is the heated surface of the resin component, fluctuates, resulting in incomplete welding due to insufficient heating of the joining surface or overheating of the joining surface. Due to heating, bubbles may be generated in the welded portion, and a defective bonding strength product may be generated.
In addition, at the time of the above-mentioned overheating, burrs greatly protrude from the welded portion to impair the appearance, and in the case of a pipe member, burrs greatly protrude toward the inner surface of the pipe to increase flow path resistance. Was.

[0004]

The inventors of the present invention have studied and studied in detail the relationship between the behavior of resin parts and the quality of welding during non-contact hot plate welding. After the contact by relative movement of the gripper and the start of pressure welding, the approaching displacement of the gripper over time, that is, the deformation of the welded portion, depends on the excess or insufficient heating of the joint surface, and from the approaching displacement curve with respect to the time axis. ,
It has been found that it is possible to judge whether the heating is excessive or insufficient, and thus whether the welding is good or bad.

Namely, as shown in FIG. 3 showing the example of the resin component (Example), the bonding surface of the resin component in close proximity at a high approach speed V ₀ are in contact, if the junction surface is insufficient heating, the gripper When the approaching displacement progresses along the underheating curve XL with a small amount of decrease in the distance (approaching displacement amount) and the joint surface is overheated, the lowering amount of the distance (approaching displacement amount) is large when the joint surface is overheated. It has been found that the approach displacement progresses along the excessive heating curve XH, and that between these two curves, there is a displacement allowable zone Z where the heating amount is appropriate and a predetermined bonding strength is obtained. In the figure, the interval A is the interval between the gripping tools (specifically, the interval measured by the displacement sensor).

[0006] The displacement allowable zone Z is obtained by performing a welding experiment on the target resin component by changing the degree of heating and melting of the bonding surface in various ways, and determining the approach displacement with respect to the time axis and the strength of the bonding portion at that time. Measurement and inspection of burr occurrence are performed, and an allowable minimum displacement amount curve AL for obtaining a predetermined bonding strength without insufficient heating and melting, and a predetermined bonding strength without excessive heating and melting, and a projection amount of burr By determining the allowable maximum displacement curve AH that does not become too large, the zone surrounded by both curves AL and AH becomes the above-mentioned displacement allowable zone Z, and the displacement curve at the time of actual welding is included in this zone. Depending on whether
It was confirmed that the quality of welding could be determined. In the figure,
A curve AS indicated by a broken line is a displacement amount curve of a sample product (non-defective product) of an embodiment described later.

The present invention has been completed on the basis of the findings of the inventor and provides a method and apparatus for detecting a welding defect in a non-contact hot plate welding capable of detecting a defective welding in a welding step. The purpose is to provide.

[0008]

According to the present invention, there is provided a method for detecting a welding defect in non-contact hot plate welding, in which a joining surface of a resin component held by a pair of holding tools is opposed to both surfaces of the hot plate with a gap. In a non-contact hot plate welding method of performing welding by heating and melting in a non-contact state and pressing the melted joint surfaces by relative movement of the gripping tool, the relative approach of the gripping tool in the pressing step The amount of displacement and / or approach speed is measured, and the measured value is compared with a preset reference value of the amount of approach displacement and / or approach speed to determine the quality of welding.

According to a second aspect of the present invention, there is provided a non-contact hot plate welding apparatus, comprising: a pair of grippers which grip a resin component and are driven to approach / separate by a driving device; and a joining surface of the resin component gripped by the gripper. In a non-contact hot plate welding apparatus having a hot plate that is inserted into and removed from the heating surface and heats and melts the bonding surface in a non-contact state, the approach displacement between the resin components in the pressing process of the molten bonding surface is measured. Sensor, a differential processing unit that differentiates the approach displacement measured by the displacement sensor and outputs the maximum value as an initial pressure contact speed, and a reference value that stores a reference value of the initial approach speed in the pressure contact process. A storage unit, and a judgment processing unit that compares the initial welding speed output from the differential processing unit with the reference value of the reference value storage unit to determine whether welding is good or not and outputs a good / bad judgment signal. To.

In the invention of claim 2, the reference value of the initial approach speed is a median of the allowable minimum speed and the allowable maximum speed,
When the difference between the initial pressing speed and the median exceeds an allowable amount, the determination processing unit may determine that the welding is defective. However, as in the invention according to claim 3, the reference of the initial approach speed is used. The value is an allowable minimum speed and an allowable maximum speed, and the determination processing unit is configured to determine that the welding is defective when the pressure welding initial speed output from the differential processing unit is out of the range of the allowable minimum speed to the allowable maximum speed. This is particularly preferable because the amount of storage in the reference value storage unit can be reduced and the calculation in the determination processing unit can be simplified.

According to a fourth aspect of the present invention, there is provided a non-contact hot plate welding apparatus, which comprises a pair of grippers which grip a resin component and are driven to approach and separate by a driving device, and a joining surface of the resin component gripped by the gripper. In a non-contact hot plate welding apparatus including a hot plate that is inserted and removed and heats and melts the joint surface in a non-contact state, the amount of approach displacement between the resin components in the pressure welding step of the melted joint surface is measured. Displacement sensor for
A reference value storage unit that stores a reference value of the amount of approach displacement at each predetermined time in the pressing step, and compares the amount of approach displacement measured by the displacement sensor with the reference value of the reference value storage unit to determine whether welding is good or bad. And a judgment processing section for outputting a pass / fail judgment signal.

In the invention of claim 4, the reference value of the initial approach displacement amount is a median value of the allowable minimum displacement amount and the allowable maximum displacement amount, and a difference between the approach displacement amount and the median value exceeds the allowable amount. At this time, the determination processing unit may determine that the welding is defective. However, as in the invention according to claim 5, the reference value of the approaching displacement amount is set to be equal to the allowable minimum displacement amount and the allowable maximum displacement amount at predetermined time intervals. When the approaching displacement measured by the displacement sensor is out of the range of the allowable minimum displacement to the allowable maximum displacement, the determination processing unit determines that the welding is defective. It is particularly preferable because the amount is small and the calculation in the judgment processing section is simple.

In the present invention, various types of non-contact or contact type sensors can be used as the displacement sensors by attaching them to a gripper or directly to a resin part, but they follow minute displacements. Eddy current type at the point,
It is preferable to use a non-contact type sensor such as an ultrasonic type or a laser type. In particular, as in the invention according to claim 6, wherein the displacement sensor is an eddy current type displacement sensor attached to the gripper. This is particularly preferable because a minute displacement measurement can be performed with high accuracy and the measurement range is suitable.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. FIG. 1 shows a main part of a non-contact hot plate welding apparatus 1 (hereinafter, simply referred to as a welding apparatus) and a heating and pressure welding process. Reference numerals 2 and 3 denote workpieces as resin parts, both of which are made of thermoplastic resin. In this example, the work 2 is composed of a pipe, and the work 3 is composed of a cap attached to the open end of the upper end. Reference numeral 4 denotes a gripper for gripping the work 2, and 5 denotes a gripper for gripping the work 3, each having a chucking mechanism (not shown) for gripping the work, and movable plates 6 and 7 driven up and down by an air cylinder (not shown). Mounted on Reference numeral 8 denotes an eddy current type displacement sensor, which is attached to the gripping tool 5 and is used to measure an interval A between the metal reference piece 9 and the metal piece 9 attached to the gripping tool 4 with time. Things.

Numeral 10 denotes a hot plate which is heated by a built-in heater or a heater arranged on the side and is driven forward and backward as indicated by an arrow Y. As shown in FIG. Is inserted between the workpieces 2 and 3 positioned at the separated position in a heated state, and the joining surfaces 2a and 3a of the workpieces opposed to each other with a clearance between the upper and lower surfaces are heated and melted by radiant heat. After the process, the welding surfaces 2a and 3a of the works 2 and 3 are pressed against each other by pressing down the gripper 5 against the gripper 4 supported at a fixed position as shown in FIG. However, in this pressing step, the relative approach displacement of the two grippers 4 and 5 is measured by the displacement sensor 8. 2A and 3A are annular portions for removing burrs generated at the time of welding and joining.

The construction of the apparatus for judging the quality of welding based on the output signal of the displacement sensor 8 is as shown in FIG.
Is an amplifier (sensor amplifier) that amplifies the electric signal of the displacement sensor 8, 12 is an A / D converter that converts this output signal into a digital signal, and 13 is a predetermined short time (1 in this example).
ms) A sampling timer for sampling the displacement amount at intervals. The displacement amount data An after A / D conversion is supplied to the computer C via the displacement amount input unit 14 together with the time data tn.
And the data file part (memory part) 15
(N, tn, An) are stored as data files. Here, n is a data number, and in this example, 1-1.
60 (= N).

Reference numeral 16 denotes a differentiation processing section for differentiating the displacement data in the data file section 15 to obtain a displacement speed, and outputting the maximum value as the initial pressure contact speed Vmax.

On the other hand, the reference value storage section 17 stores the allowable minimum speed and the allowable maximum speed of the initial pressing speed at the time of the pressing process for each resin component.
These allowable speeds are obtained as follows. That is, as described above, if the displacement curve of the gripper with respect to the time axis at the time of the pressing process is within the displacement allowable zone Z,
Although good welded products can be obtained, as shown in FIG. 3, each displacement amount curve shows the initial period T after the contact surfaces come into contact with each other.
₀ (for example, for 30 ms after the contact), it shows a substantially linear shape (that is, approaches at a substantially constant speed). In this example, the speed corresponding to the inclination angle of the linear portion of the allowable minimum displacement curve AL is set to the allowable minimum speed VL. The speed corresponding to the inclination angle of the linear portion of the allowable maximum displacement curve AH is defined as the allowable maximum speed VH,
Differential value (speed) of approach displacement during actual welding process
Is used as a reference value for comparison with the initial pressing speed Vmax which is the maximum value of FIG. 4 shows a velocity curve obtained by differentiating the allowable minimum displacement curve AL and the allowable maximum displacement curve AH of FIG. In the drawing, a curve VS indicated by a broken line is a displacement amount curve (curve A in FIG.
It is a speed curve obtained by differentiating S).

Then, the judgment processing unit 18
Is compared with the allowable minimum speed VL and the allowable maximum speed VH stored in the reference value storage unit 17 to output a pass / fail judgment signal.

The display unit 19 connected to the judgment processing unit 18 receives a pass / fail judgment signal from the judgment processing unit 18 and
The lighting of the K lamp and the NG lamp and the sounding of an alarm sound in the case of a failure are performed. In addition, a printer or another computer may be connected to the determination processing unit 18 to record the quality determination result.

Hereinafter, a procedure for detecting a welding defect using the above-described apparatus will be described mainly with reference to flowcharts shown in FIGS. First, the workpieces 2 and 3 are gripped by the grippers 4 and 5, and the movable plates 6 and 7 are moved up and down to be positioned at a predetermined heating position (step 21).
0 is advanced between the workpieces 2 and 3 as shown in FIG. 1A to start heating (steps 22 and 23). After a predetermined heating time (for example, 1 second), the joining surfaces 2a and 3
a is melted, the heating is terminated (steps 24 and 2).
5) Then, the heating plate 10 is retracted (Step 26), and the heating step is completed.

Next, as shown in FIG. 1B, the movable platen 7 is started to descend while the movable platen 6 is stopped on the stopper 6a (step 31), and the joining surfaces 2a and 3a come into contact with each other and press contact. When the process is started (step 32), the distance A (the distance from the reference piece 9) measured by the displacement sensor 8 is input to the computer C as the approach displacement amount, and the total N = 16 every 1 ms.
The zero displacement amount data An is input to the data file section 15 and stored as a displacement amount data file AN (steps 33 to 37).

In step 32, the contact of the joint surfaces is detected by, for example, directly detecting the contact of the joint surfaces 2a, 3a by a photoelectric sensor or the like, or by detecting the contact of the joint surface 3a or the gripper 4.
Alternatively, it can be performed by a method in which a point in time at which a predetermined time has passed from a point in time when the displacement sensor 8 or the like has passed a predetermined position is set as a contact point.

Next, the differentiating section 16 performs a differentiating process on the displacement data file AN of the data file section 15 (step 38), and uses the maximum value of the obtained speed data file VN as the initial pressing speed Vmax. (Steps 39 and 40), the determination processing unit 18 compares the speed Vmax with the allowable maximum speed VH and the allowable minimum speed VL of the reference value storage unit 17 (step 41), and outputs Vma
NG (bad) when x is out of the range of allowable speeds VL to VH
A signal is output (step 42). If Vmax is within the above range, an OK signal is output (step 43).

Based on these pass / fail signals, the display unit 19 indicates the occurrence of a defective product by turning on an NG lamp or an OK lamp, or sounding an alarm buzzer.

The above steps 31 to 42, 4
Steps up to 3 are completed in a short time (for example, 1 second),
After further progress of pressure welding and subsequent cooling and cooling (for example, after about 15 seconds), the pressing step is completed (step 44).
The gripping tools 4 and 5 are opened to take out the welded products of the works 2 and 3 (step 45). Since the welding of one set of works is completed, the same steps as described above are repeated for the next work. Detects welding defects.

As described above, in the welding step, specifically, in the pressure welding step, a welding defect can be reliably detected in a short time.

FIGS. 7 and 8 show a second embodiment of the present invention. In the first embodiment, the quality of welding is determined by comparing the approach speed with a reference value. Is to judge the quality of welding by comparing the approach displacement amount with a reference value.

The equipment used is as shown in FIG.
2 is omitted, and the differential processing unit 16 in FIG. 2 is omitted. The reference value storage unit 51 stores the allowable minimum displacement amount curve AL and the allowable maximum displacement amount curve AH in FIG. N (in this example, 160) data of the allowable minimum displacement ALn and data of the allowable maximum displacement AHn are stored as a data file for each predetermined short time (in this example, 1 ms). Further, the determination processing unit 52 calculates the displacement amount data An in the displacement amount data file AN of the data file unit 15 and the allowable minimum displacement ALn and the allowable maximum displacement A in the reference value storage unit 51.
Hn is compared one by one starting from n = 1, and a pass / fail judgment signal is output at most N times for one welding, but other configurations are the same as those in FIGS. 1 and 2.

The procedure for detecting a welding defect using the above-described apparatus will be described mainly with reference to the flowchart of FIG. After finishing the heating process according to the flow shown in FIG. 5, step 3 is the same as steps 31 to 37 in FIG.
According to 1 to 37, when the displacement data file AN is stored in the data file unit 15, the displacement data An is stored in the data file 15, and the minimum allowable displacement ALn and the maximum allowable displacement AHn are stored in the reference value storage unit 51. Each data is sequentially read from data number 1 and compared (steps 51 to 5).
3) If An is out of the range of each of the allowable displacement values ALn to AHn, an NG (defective) signal is output (step 56). If An is within the above range, the reading and comparison are performed for the next data number. If no defect is detected for all N data numbers, an OK signal is output (steps 52 to 55, 57). The display of the occurrence of defective products in the display unit 19 based on these pass / fail signals is performed in the same manner as in the first example.

The above steps 31 to 56, 5
7 are completed in a short time as in the first example.
In the same manner as in the example, the work is taken out after the completion of the pressurizing process (steps 58 and 59), and the same process is repeated for the next work to detect a welding defect.

In this example, it is the same as in the first example that the welding defect can be reliably detected in a short time in the welding step.
Further, since the differential processing unit 16 in the first example is unnecessary, the apparatus can be simplified.

In each of the above examples, the contact time is determined based on the elapsed time during the approach displacement without directly detecting the contact of the joining surface in step 32, and the thickness of the flange portion of the work 2 or the work 3 If the variation in the thickness cannot be ignored, the actual contact time is shifted due to the variation, and the measured displacement amount with respect to the time axis is on the small displacement side (when the thickness is large) or the large displacement side (when the thickness is large). Is thin), which affects the quality judgment. Therefore, in order to avoid this, for example, as shown in FIG.
In the example, a time point t that is later than the displacement width at the contact time point
_{If the} displacement A is measured from _0, it is possible to prevent the approach speed V ₀ before contact from being erroneously measured (as Vmax). Further, in the second example, the allowable minimum displacement curve AL is shifted toward the smaller displacement side from the case of FIG. 3 (shown by a dashed line in FIG. 9) by the above-described displacement at the time of contact (or variation in thickness). The permissible maximum displacement curve AH is shifted to the side where the displacement is smaller than the case of FIG. 3 (shown by a chain line in FIG. 9), and the displacement permissible zone Z is expanded up and down as compared with the case of FIG. The respective allowable displacement values ALn and AHn may be stored in the reference value storage unit 17 and used.

[0034]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an example will be described in which a weld defect of a work 2, 3 made of 66 nylon resin (the outer diameter of the work 2 and the outer diameter of the work 3 is 27 mm) is detected by using the apparatus having the above configuration. First, a hot plate 10 (standard heating temperature = about 6
A preliminary experiment was conducted by changing the heating conditions under the conditions of (00 ° C., heating time = approximately 20 seconds). As shown in FIG.
At a welding strength of 5 kN, the allowable maximum displacement curve AH for determining the amount of burr in the weld and the small amount of bubbles generated in the welded portion was determined, and the values of the minimum allowable speed VL and the maximum allowable speed VH were determined as shown in FIG.

Next, welding was performed by adjusting the heating conditions by the hot plate 10 to detect a defective product. As a result, the sample product having the displacement characteristic shown by the displacement curve AS in FIG. In both methods of the second example and the second example, the welded product was determined to be non-defective, the joint strength of the welded product was 4 kN, and the burrs at the welded portion were small. The curve VS in FIG. 4 is a speed curve representing the speed data file VN of this sample product.

On the other hand, the sample product having the displacement characteristic shown by the underheating curve XL in FIG. 3 was detected as a defective product by the above two methods, and the joint strength of the welded product was 2.5 kN. . FIG. 3 shows an overheating curve XH.
The sample product having the displacement characteristics indicated by was also detected as a defective product by the above two methods, the joint strength of the welded product was 5.5 kN, the burr was excessive, and a large amount of bubbles were observed in the welded portion. .

The present invention is not limited to the above examples. For example, a defective product may be detected by using the methods of the first and second examples in combination. In addition, the material and shape of the resin component, the specific configuration of the device, and the like may be other than those described above. The present invention is also applicable to butt welding of pipes having an axis extending in a horizontal direction.

[0038]

As explained above, according to the present invention,
In the welding step of non-contact hot plate welding, welding defects can be detected, and a welded product with stable quality can be provided.

[Brief description of the drawings]

FIG. 1 is an explanatory view of a main part structure and operation of a welding apparatus showing a first example of an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a signal processing unit of a displacement sensor in the welding device of FIG.

FIG. 3 is a diagram showing a state of displacement of a gripper at the time of welding by the apparatus of FIG. 1;

FIG. 4 is a velocity diagram showing a differential value of the displacement amount in FIG. 3;

FIG. 5 is a flowchart showing a heating step in the apparatus of FIG.

FIG. 6 is a flowchart showing a pressurizing step in the apparatus of FIG. 1 and the apparatus of FIG. 2;

FIG. 7 is a diagram corresponding to FIG. 2, showing a second example of the embodiment of the present invention.

FIG. 8 is a diagram corresponding to FIG. 6, showing a second example of the embodiment of the present invention.

FIG. 9 is a view corresponding to FIG. 3, showing still another example of the embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1: Non-contact hot plate welding device, 2: Workpiece (resin part), 2
a: joining surface, 3: work (resin part), 3a: joining surface,
4 gripping tool, 5 gripping tool, 8 displacement sensor, 9 reference piece, 10 hot plate, 15 data file section, 16 differential processing section, 17 reference value storage section, 18 determination processing section, 51
... Reference value storage unit, 52... Judgment processing unit.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4F211 AA29 AD05 AK07 AM32 AP06 AP07 AP20 TA01 TC08 TD11 TH01 TH02 TH06 TJ14 TJ30 TN07 TQ01 TQ10 TW39

Claims (6)

[Claims] 1. A heat-melting method in a non-contact state in which the opposing joint surfaces of resin parts gripped by a pair of grippers are opposed to both surfaces of a hot plate with a gap therebetween, and the melted joint surfaces are gripped with each other. In a non-contact hot plate welding method in which welding is performed by press-fitting by relative movement of a tool, a relative approach displacement and / or approach speed of the gripping tool in the pressing process is measured, and the measured value is set to a preset approach. A method for detecting poor welding in non-contact hot plate welding, comprising determining the quality of welding by comparing the displacement value and / or the reference value of the approach speed. 2. A state in which a pair of grippers gripping a resin component and driven to approach and separate by a driving device, and a joining surface of the resin component gripped by the gripper, which is inserted into and removed from the joint surface, is in a non-contact state. A non-contact hot plate welding apparatus having a hot plate to be heated and melted by a displacement sensor for measuring an approach displacement between the resin components in a pressure welding step of the melted joining surface, and measurement by the displacement sensor A differential processing unit that differentiates the approach displacement amount obtained and outputs the maximum value as a pressure welding initial speed, a reference value storage unit that stores a reference value of an initial approach speed in the pressure welding process, and a pressure welding output by the differentiation processing unit. A non-contact hot plate welding apparatus, comprising: a determination processing unit that compares the initial speed with a reference value in the reference value storage unit to determine whether the welding is good or not and outputs a good or bad judgment signal. 3. The reference value of the initial approach speed is a permissible minimum speed and a permissible maximum speed, and the determination processing unit determines that the initial pressing speed output from the differentiation processing unit is in a range from the permissible minimum speed to the permissible maximum speed. 3. The non-contact hot plate welding apparatus according to claim 2, wherein when it is outside, it is determined that the welding is defective. 4. A state in which a pair of grippers gripping the resin component and driven to approach and separate by a driving device, and a joining surface of the resin component gripped by the gripper is removed from the joint surface. A non-contact hot plate welding apparatus provided with a hot plate to be heated and melted by a displacement sensor for measuring an approach displacement between the resin components in the pressing process of the melted joining surface, and a predetermined sensor in the pressing process. A reference value storage unit that stores a reference value of the approach displacement amount for each time period, and compares the approach displacement amount measured by the displacement sensor with the reference value of the reference value storage unit to determine whether welding is good or not. A non-contact hot plate welding apparatus, comprising: a determination processing unit that outputs a signal. 5. The reference value of the approach displacement amount is a permissible minimum displacement amount and a permissible maximum displacement amount at predetermined time intervals, and the determination processing unit determines that the approach displacement amount measured by the displacement sensor is the allowable minimum displacement. Out of the range between
The non-contact hot plate welding apparatus according to claim 4, wherein it is determined that welding is defective. 6. The non-contact hot plate welding apparatus according to claim 2, wherein the displacement sensor is an eddy current type displacement sensor attached to the gripper.