Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
1999-07-12
2001-01-30
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S101000
Reexamination Certificate
active
06180910
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 controlling the force on a resistance welding machine.
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 between the two pieces causing the material to become soft. At that 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 certain welding applications, such as railroad rails, it is desirable to have welds forged to a specific force. By forging to a specific force instead of a distance, each weld produced will forge to the correct distance according to the amount of heat that is in the part. Since the actual amount of heat introduced into the part tends to vary, this forging technique leads to more constant welds.
In other welding applications, such as flash welding parts with large cross sections, 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 accomplished 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 their resistance. By holding the two pieces together at a constant force, the resistance between the parts can be made more consistent, which allows for the heat produced in the pieces to be controlled more precisely.
In other welding applications, such as butt welding, it is 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 its electrical resistance. It is critical to the welding process that this resistance not be excessive so as to cause overheating of the parts. If the parts overheat, there is a risk ejecting material from the joint in the form of sparks. These sparks are known as flashing. Flashing unevenly removes heat and material from the joint, lowering its quality, or even preventing proper forging. In most cases, the parts will have to be scrapped. To avoid these undesirable results, butt welding equipment must incorporate a method for controlling the force between the pieces. Further this force may be modified during the course of the weld depending on the requirements of the weld. It is also important that the force be achieved quickly with minimal overshoot to prevent flashing.
In all these welding applications, the welding machine typically utilizes a hydraulic servo valve that controls the movement of one of the parts being welded. By varying the flow of oil, the servo valve also applies a force to the movable part. 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.
Resistance welding machinery incorporate various techniques to control the force between the pieces. The simplest technique is to use compressed air instead of hydraulic oil. This technique has limitations in that it is only suitable for small welding applications due to the relatively low pressure of compressed air. Also this technique is generally stable only at a specific pressure which makes it difficult to quickly change pressure to a changing force setpoint. Another technique uses a standard Proportional Integral Derivative (PID) -type controller to control the servo valve. This technique has limitations in that it responds quickly to a change in force but overshoots the desired target in an oscillatory manner. This characteristic overshooting can be minimized at the expense of the quickness of response. Other limitations to this technique are that it only uses knowledge of the overall force and a history of previous force. This would be sufficient if there were a constant force to be acted upon. In resistance welding machinery, the force is being applied to an object whose characteristics are being changed over time due to heating. Since the PID control has no knowledge on the changes in the parts, and is tuned for a specific situation, it will not behave optimally in all situations.
It is therefore an object of the present invention to provide an improved welding technique for controlling the force supplied by the servo valve.
SUMMARY OF THE INVENTION
The present invention is an improved welding technique that provides stable real-time control of the force produced by the servo valve regardless of 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. The present invention includes means for making instantaneous pressure measurements within the first and second chambers of the hydraulic cylinder. Also included is a force control program that reacts to pressures within the first and second chambers to control the servo valve. The force control software receives the instantaneous pressure information from the first and second chambers of the hydraulic cylinder and determines whether the platen is moving and how far the servo valve is from its target force. The force control software implements an adaptive proportional/integral (PI) controller. Based on the instantaneous pressure information, the force control software adjusts the proportional constant of the PI controller.
In accordance with the preferred embodiments, the servo valve may be operated to supply a controlled force to the parts being welded. Advantageously, by periodically adjusting the contributions of the proportional portion of the PI controller, the servo valve may chase the desired force when movement is possible and may scale back the output when the close to the desired force. Further, this periodic adjusting of the PI controller reduces the risk of the servo valve overshooting the desired force.
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 herein below, which are useful in explaining the invention.
REFERENCES:
patent: 4408114 (1983-10-01), Nakata et al.
patent: 4634828 (1987-01-01), Okabe et al.
patent: 5021625 (1991-06-01), Destefan et al.
patent: 5124521 (1992-06-01), Boyer et al.
patent: 5367138 (1994-11-01), Moss et al.
patent: 5436422 (1995-07-01), Nishiwaki et al.
patent: 5440092 (1995-08-01), Kawai
patent: 5483035 (1996-01-01), Kawai et al.
patent: 5523541 (1996-06-01), Ishikawa
patent: 5545872 (1996-08-01), Nakajima et al.
patent: 5582747 (1996-12-01), Sakai et al.
patent: 5587091 (1996-12-01), Kawagoe et al.
patent: 5591355 (1997-01-01), Ishikawa
patent: 5652488 (1997-07-01), Rennau
Automation International Inc.
Banner & Witcoff , Ltd.
Shaw Clifford C.
LandOfFree
Force control techniques on a resistance welding machine does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Force control techniques on a resistance welding machine, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Force control techniques on a resistance welding machine will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2505707