Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Patent
1995-08-29
1997-07-29
Wysocki, Jonathan
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
318645, 219 8651, B25J 910
Patent
active
056524889
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention pertains to a system of components for the automatic control of an electrode holder, referred to in abbreviated form below as an automatic control system, which is especially suitable for use on a welding robot, and which includes a servo valve-controlled drive for the welding electrodes and an electronic control system for the servo valves.
Electrode holders of this type are used especially in automated resistance welding operations, where they serve primarily to join pieces of sheet metal. When spot welding is to be carried out, the surfaces resting on each other are usually welded with pin-shaped electrodes; when continuous seams are to be produced, electrode wheels are used.
The electrode holders according to the state of the art have welding electrodes which can be driven pneumatically between their extreme positions. The electrode pressure can be adjusted by means of a pressure-actuated servo valve acting on the air cylinder. The pressure-actuated servo valve reduces the external supply pressure present in the main compressed-air supply line to the desired variable final pressure. So that the air cylinder can drive the electrodes into their extreme positions, one of the two sides of the cylinder is completely evacuated, while the opposite side is completely filled. After the air cylinder has been driven into the extreme closed position, the welding process is initiated after a preliminary contact time, the duration of which can be programmed in advance. The preliminary contact time allows the pres- sure to be built up. It is assumed that, by the end of this time, a minimum required percentage of the final welding pressure has been reached. This time is always made as short as possible, because it prolongs the overall time required to produce a spot weld. If, however, the welding occurs before the minimum required final pressure is reached, that is, if the spot is welded too soon, melted material is ejected at high speed from the workpiece being welded (so-called spatter formation), which negatively affects the quality of the spot weld and possibly the surface of the workpiece.
The known automatic control system for electrode holders, furthermore, suffers from the disadvantage that it generates a significant amount of noise when the electrodes make contact. The high speed at which the electrodes make contact also leads to a considerable amount of wear on the electrodes and to significant stress on the mechanical parts of the holder. The known control systems for electrode holders, furthermore, consume a large amount of compressed air, which is extremely disadvantageous when the compressed air supply system is underpowered and also results in the consumption of a great deal of energy.
To eliminate these problems, the attempt has been made to use water-cooled electric motors as drives for the welding electrodes, but this approach has not been successful because of the heat problems from which the electric drives suffer in conjunction with the high pressures used and because of the cost of the electronic control systems.
In addition, programmable, open-loop control systems have also been developed, but primarily with the goal of simplifying the control activities.
German Offenlegungsschrift No. 3500806 describes a programmable, open- loop control system for an industrial welding robot, especially a resistance welding machine with an integrated open-loop control system. The welding parameters themselves are stored in the program memory of the actual robot control system. These parameters can be called up at the proper time and can be converted by a stored-program control system into firing commands for the welding current regulator. This control system is intended primarily to simplify the control of the resistance welding current.
The standard process variables of resistance welding, specifically the force, acceleration, and path of the electrodes, the welding current, and the electrode voltage as well as the measurement sensors suitable for tracking them are de
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Lehmkuhl et al., "Fortschwitte in der Processdotenerfassung bein Widerstordspress schweissen", Schweissen und Schneider 42 (1990), Heft 1, Seiten 26-29.
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