1. Field of the Invention
The present invention relates to an inter-welding electrode moving amount detecting method of detecting an amount of movement of pair of electrodes to a direction separating from/ approaching to each other due to expansion/contraction of a nugget during welding, and relates to an apparatus thereof.
2. Description of the Related Art
Conventionally, in the field of resistance welding represented by spot welding, as a method of judging good/inferior welding quality at real time (during welding), a method of detecting an expansion amount of a nugget formed on a member to be welded as an inter-welding electrode moving amount is known. This utilizes that there is a deep causal relationship between thermal expansion and welding quality of an object to be welded during welding.
A physical phenomenon which is thermal expansion of the member to be welded is considered as an amount of movement between welding electrodes during welding. As to how to detect an inter-welding electrode moving amount or as to whether the detected result is processed to be a substitute property of the welding quality, various methods are suggested.
In general, as a detector for detecting an inter-welding electrode moving amount of a welding gun, a linear scale (scale which is capable of detecting a position in a linear direction), a laser distance measuring sensor or the like is used. Moreover, in a welding gun which uses a servo motor as driving means, a position detector which is contained in or provided to a motor can be used.
However, the above detectors for an inter-welding electrode moving amount cannot directly measure the physical phenomenon of a member to be welded during welding.
Namely, since interference between a welding jig or a member to be welded on a circumference of welding-shot point easily occurs, a detector for an inter-welding electrode moving amount cannot but be mounted to a portion separated from electrodes, namely, in a vicinity of a driving section or the like such as a rotating shaft or a piston rod connected to electrodes and moved (rotated or linearly moved) in driving means such as an air cylinder or a servo motor.
For this reason, a force which pushes back electrodes of a welding gun due to thermal expansion of a nugget is absorbed by mechanical loss such as deflection of a welding gun or friction of an air cylinder or a servo motor. For this reason, an accurate inter-welding electrode moving amount cannot be detected, and as a result, accuracy of judgment as to good/inferior welding quality is deteriorated.
Namely, exactly an inter-welding electrode moving amount which is a changing amount of an inter-electrode dimension is not detected, but only a moving amount of a driving section in a driving apparatus for moving electrodes is detected substitutionally. A disadvantage due to this is remarkable particularly in the case of a large-sized welding gun in which deflection of the welding gun is great.
The present invention has been achieved with such points in mind.
Therefore, it is an object of the present invention to provide an inter-welding electrode moving amount detecting method and an apparatus thereof which detect an inter-welding electrode moving amount during welding accurately so as to be capable of improving accuracy of judgment as to good/inferior welding quality.
To achieve the object, according to one aspect of the present invention, there is provided an inter-welding electrode moving amount detecting method comprising the steps of: moving a driving section of electrode driving means having the driving section connected to at least one of a pair of electrodes provided to opposed end portions of a welding gun to move the pair of electrodes to a direction approaching to each other; detecting an inter-welding electrode moving amount that the pair of electrodes are moved to a direction separating from/ approaching to each other due to expansion/contraction of a nugget when a member to be welded is pressed, electrified and welded by the pair of electrodes; and obtaining the inter-welding electrode moving amounts by adding a moving amount of the driving section to an electrode moving direction due to the expansion/contraction of the nugget during welding and a deflection amount of the welding gun due to a pressing force applied from the electrodes to the member to be welded.
On the other hand, according to another aspect of the present invention, there is provided an inter-welding electrode moving amount detecting apparatus, comprising: a pair of electrodes mounted to opposed end portions of a welding gun; electrode driving means having a driving section connected to at least one of the pair of electrodes; driving section moving amount detecting means for detecting a moving amount of said driving section to an electrode moving direction due to expansion/contraction of a nugget during welding; and control means for adding the moving amount detected by said driving section moving amount detecting means and a deflection amount of said welding gun due to a pressing force applied from said electrodes to a member to be welded so as to obtain the inter-welding electrode moving amount. In the construction, the driving section is moved so that one of said electrodes is moved to a direction where the electrodes approach to each other, and when said member to be welded is pressed and electrified and welded by said pair of electrodes, the inter-welding electrode moving amount that said pair of electrodes are moved to a direction separating from/ approaching to each other due to the expansion/contraction of the nugget is detected.
According to the above aspects, an inter-welding electrode moving amount during welding is detected accurately so that the forming state of the nugget is obtained definitely and accuracy of the judgment of good/inferior welding quality can be improved.
In other wards, the pressing force applied from the electrodes to the member to be welded can be detected more accurately, and the stiffness factor of the gun arm can be calculated more accurately.
Furthermore, those manners according to the present invention have a great advantage that a stiffness factor related to pressing force and a stiffness factor related to strain amount can be obtained easily without adding a new structure. Further, those manners can be applied commonly to various kinds of welding guns, and the stiffness factor related to pressing force and the stiffness factor related to strain amount can be easily checked or corrected suitably.