Welding gun and methods conducted using the same

Details Welding gun and methods conducted using the same Welding gun and methods conducted using the same

2001-08-03

2003-03-11

Dunn, Tom (Department: 1725)

Electric heating

Metal heating

For bonding with pressure

C219S109000, C073S001010

Reexamination Certificate

active

06531674

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an intelligent welding gun provided with a sensor, and various kinds of methods conducted by using the welding gun including a calibration method of a sensor, a control method of welding (which includes, for example, a control method of a pressing force, a feedback control method of re-welding, a control method of welding strength, a control method of suppressing the generation of a weld expulsion, and a control method of a track of a welding robot), and a managing method of a change in a positional accuracy of a welding point.
2. Description of Related Art
Japanese Patent Publication No. 10-94882 discloses a method for controlling a pressing force of a welding gun. In the method, a small quantity of an elastic displacement, generated in a fixed side electrode tip when a moving side electrode tip is driven to contact the fixed side electrode tip and further driven to press the fixed side electrode tip, is detected by an encoder of a servo motor for driving the moving side electrode tip. More particularly, the elastic displacement is determined based on the rotation increment of the servo motor generated from the time when the moving side electrode tip begins to contact the fixed side electrode tip to when the electric current suddenly increases when the moving side electrode tip is further driven to press the fixed side electrode tip. A real pressing force between the electrode tips is calculated based on the measured elastic displacement of the fixed side electrode tip. Then, a setting pressing force between the electrode tips is modified to be equal to the calculated real pressing force.
However, there are the following problems with the above-described conventional method for controlling a welding gun.
First, since a moving side portion of the welding gun including the moving side electrode tip and the servo motor for driving the moving side electrode tip is provided with a speed reducer, a mechanical impedance of the moving side portion is greater and dynamically harder than that of a fixed side portion of the welding gun. In this instance, the mechanical impedance is defined as an impedance expressed by a vector of |m, c, k|, when a movement of the electrode tip is expressed by an equation:
m·d
2
x/dt
2
+c·dx/dt+kx=F
(
t
).
In a case where the vector only includes k, the mechanical impedance is a spring constant. The encoder is located on the opposite side of the moving side electrode tip with respect to the speed reducer of the servo motor, so that a pressing quantity of the moving side electrode tip, transmission of a change in the pressing displacement and the pressing force to the encoder through the speed reducer is a small amount and is delayed. As a result, the responsibility is lowered, so that it is difficult to obtain an accurate response with the conventional method, in which the welding gun is controlled based on an output of the encoder.
Second, since the rigidity of an arm supporting the fixed side electrode tip is increased so that the mechanical impedance of the fixed side portion including the fixed side electrode tip and the arm is nearly equal to the mechanical impedance of the moving side portion, the size and the weight of the arm is large, which causes the welding gun to be increased both in size and weight.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a welding gun which enables control with a high response and accuracy, and to provide various kinds of control methods conducted using the welding gun.
Another object of the present invention is to provide a welding gun which allows an arm supporting a fixed side welding tip to be decreased in size and rigidity, and to provide various kinds of control methods conducted using the welding gun.
The present invention for achieving the above objects are, as follows:
A welding gun includes a moving side portion including a moving side welding tip and a driving device for driving the moving side electrode tip, and a fixed side portion including a fixed side welding tip and an arm supporting the fixed side welding tip. A fixed side sensor for detecting at least one of a position of the fixed side welding tip and a pressing force imposed on the fixed side welding tip is provided in the fixed side portion.
In the welding gun, a mechanical impedance of the fixed side portion is smaller than that of the moving side portion. The mechanical impedance of the fixed side portion is set in a range where the fixed side sensor can effectively detect the at least one of the position of the fixed side welding tip and the pressing force imposed on the fixed side welding tip.
The fixed side sensor is any one of a force sensor, an optical distance sensor and a sensor using an optical fiber.
A moving side sensor for detecting at least one of a position of the moving side welding tip and a pressing force caused in the moving side welding tip may be provided in the moving side portion. In that case, the fixed side sensor and the moving side sensor constitute a redundant sensor measurement system.
A first method, which is a method of calibrating a sensor conducted using the welding gun, includes: (a) releasing the moving side welding tip from a position of contact with the fixed side welding tip; and (b) calibrating a reference point of at least one of a pressing force information and a positional information of the fixed side sensor.
A second method, which is a method of calibrating a sensor conducted using the welding gun, includes: (a) increasing pressure of the moving side welding tip against the fixed side welding tip and plotting a pressing force and/or a positional information to obtain characteristic curves of the moving side sensor and the fixed side sensor using a method of least squares; and (b) determining gains of the sensors such that the gains of the moving side sensor and the fixed side sensor are equal to each other.
A third method, which is a control method of welding a workpiece conducted using the welding gun, includes: (a) determining whether an expansion quantity of a welding portion of the workpiece detected by the fixed side sensor is equal to or greater than a predetermined value, and ending welding of the current welding point when the expansion quantity is equal to or greater than the predetermined value; (b) increasing the welding electric current when the expansion quantity is smaller than the predetermined value; (c) counting the number of times the welding electric current is increased and determining whether a re-welding program should be conducted when the counted number of times exceeds a predetermined number, and ending welding at the current welding point when it is determined that the re-welding program should not be conducted; (d) conducting re-welding when it is determined that the re-welding program should be conducted; and (e) determining, when re-welding is conducted, whether the expansion quantity of the welding portion of the workpiece during re-welding is equal to or greater than the predetermined value, ending welding of the current welding point when the expansion quantity is equal to or greater than the predetermined value, while issuing a warning when the expansion quantity does not reach the predetermined value.
A fourth method, which is a control method of welding a workpiece conducted using the welding gun, includes: (a) obtaining an expansion quantity of a welding portion of the workpiece, a position and a pressing force of the fixed side welding tip, from detected values detected at every moment by the fixed side sensor; (b) determining whether an expulsion is generated in the welding portion by comparing at least one of a value of the pressing force and the position of the fixed side welding tip at a point when the expansion quantity begins to decrease with at least one of a value of the pressing force and the position of the fixed side welding tip after a predetermined period of time has passed from the beginning of the decrease

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