Electric heating – Metal heating – For bonding with pressure
Utility Patent
1999-08-02
2001-01-02
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S101000
Utility Patent
active
06169263
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
In general, the present invention relates to welding techniques, and more particularly to techniques for adaptively adjusting the force exerted by a platen on a resistance welding machine to achieve a controlled resistance across the welding interface.
2. Statement of Related Art
Resistance welding machines are well known in the art. In a resistance welding process, two pieces of material are held by clamps so they are touching or are nearly touching. An electric current is then delivered through these pieces. The current heats up the interface or the joint between the two pieces causing the material to become soft. At this point, the machine rapidly brings the two pieces together which forges the weld. Flash welds and butt welds are two common types of resistance welding techniques.
In flash welding applications, particularly where parts having large cross sections are to be flash welded, it is desirable to preheat the parts prior to flashing in order to minimize the amount of material consumed in the flash and to decrease welding time. Preheating is generally performed by forcing the two pieces together for some time and then separating them. When the pieces are in contact, the electrical current will cause the pieces to heat according to the joint resistance or the resistance at the interface between the two parts. By holding the two pieces together at a controlled force, the joint resistance between the parts can be made more consistent, which allows for the heat produced in the pieces to be precisely controlled.
In butt welding applications, it is also desirable to control the force with which the parts are held together. The butt weld requires that the two parts be in solid contact with each other. When current is delivered through the parts, the interface between the parts heats up due to the joint resistance at the interface between the parts. The heat thereby allows the parts to be forged or upset together. By forging to a specific force instead of a distance, the weld will forge to the correct distance according to the amount of heat that is in the part. By precisely controlling the joint resistance we can control the heat introduced into the part and thereby provide a consistent upset distance.
In either flash weld preheating or butt welding, it is important to the welding process to control the joint resistance or the resistance at the interface of the part. The higher the joint resistance, the less energy that is dissipated at the joint, thereby slowing the rate at which the joint is heated. As shown in
FIG. 6
, a small decrease in joint resistance (Segment C) results in an exponential increase in power dissipation at the joint. The joint resistance is generally controlled by the force with which on the part to be welded are held together. A greater force generally reduces joint resistance. Since thermodynamic forces tend to equalize the heat across the part, it is desirable that the weld be made quickly while the heat is still concentrated at the interface. Accordingly, a lower joint resistance causes greater power to be passed through the part, thereby resulting in faster heating.
A lower joint resistance, however, produces significant heating throughout the part itself which undermines the desire to localize the heat effect. Further, achieving a lower joint resistance requires a greater force which may deform the part to be welded. A lower joint resistance may also overheat the part. If the parts overheat, they will become softer and deform or melt. This will preventing proper forging, and result in parts having to be scrapped.
A higher joint resistance avoids some of these problems, however, it increases the heating time for the interface. An increased heating time, allows thermodynamic forces to equalize the heat across the part. A high joint resistance also introduces the risk of a flashing where material from the joint is ejected in the form of sparks. Flashing unevenly removes heat and material from the joint, thereby reducing its quality, or even preventing proper forging. In most cases, flashing will result in the part having to be scrapped. To avoid these undesirable results, welding equipment incorporate various techniques to achieve rapid heating of the interface while ensuring that the interface does not overheat. Welding equipment therefore typically incorporate a method for controlling the force between the pieces. For example, a constant force may be supplied to the part as it is being welded. The welding machine typically utilizes a servo valve that controls the movement of one of the parts being welded. The servo valve also supplies a force so that the parts may be welded. By maintaining a constant force as applied by the servo valve; the two pieces are maintained in contact with each other thereby reducing the risk of overheating of the parts at the interface.
However, such known techniques are of limited value since the constant force technique does not always ensure against overheating. Further, the welding operator must by trial and error determine the best approximate servo valve force that will adequately avoid the risk of overheating. This technique generally requires much experimentation and prior knowledge to produce a good weld. Further, the control of force of the servo valve is subject to great deviation in the resulting force on the parts, due in large part to the frictional forces on the platens, platen mass and inertia. Since the current practice relies on absolute force parameters, the effects of these forces must be compensated or reduced. This requires more expensive, low mass, low friction machinery and/or the need for sophisticated control algorithms to compensate for these forces. Another problem with providing a constant force is that the system is incapable of adjusting the force in response to the changing conditions of the parts being welded. For example, in the case where a flash weld is about to occur, the present system is incapable of adjusting the force applied by the servo valve. Accordingly, even with control of force, there remains substantial risk of overheating and poorer quality welds.
It is therefore an object of the present invention to provide an improved welding technique for adaptively controlling the force applied to the part being welded.
It is another object of the invention to provide an improved welding technique to maintain a desired joint resistance during a welding cycle.
It is yet another object of the invention to provide an improved welding technique to maintain a desired voltage drop across the joint to be welded during a welding cycle.
SUMMARY OF THE INVENTION
The present invention is an improved butt welding technique that provides real-time control of the servo valve to account for the changing conditions of the parts being welded. The present invention may be implemented within any electrical welder having at least one movable platen for carrying a part to be welded, at least one hydraulic cylinder coupled to the movable platen, the hydraulic cylinder having a first chamber and a second chamber, and a servo valve coupled to the hydraulic cylinder. In one embodiment, the present invention includes means for measuring the voltage differential across the joint. Based on this information, the present invention may adaptively adjust the target force of the weld to maintain a constant voltage drop across the joint. The present invention may include force controller having one or more pressure transducers coupled to the servo valve and a proportional/integral (PI) controller to accurately deliver the desired force on the platen.
These as well as other novel advantages, details, embodiments, features and objects of the present invention will be apparent to those skilled in the art from following the detailed description of the invention, the attached claims and accompanying drawings, listed hereinbelow, which are useful in explaining the invention.
REFERENCES:
patent: 3542994 (1970-11-01), Deffenbaugh et al.
patent: 3681563 (1972-08-01), Li
Derby, Jr. William M.
Formanek Randolph P.
Automation International Inc.
Banner & Witcoff , Ltd.
Shaw Clifford C.
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