Electric heating – Metal heating – By arc
Reexamination Certificate
2001-06-19
2003-03-11
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S1370PS
Reexamination Certificate
active
06531684
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the art of welding power supplies. More specifically, it relates to the control and/or calibration of welding power supplies.
BACKGROUND OF THE INVENTION
There are many known welding power supplies used for a variety of welding processes. Welding power supply or system for welding, as used herein, includes one or more of the following components: a wire feeder, a power source or source of power, a torch or gun, a controller, including a wire feeder controller, and a power source controller to control the various components (it may also exclude some of these components). The components may share a housing, or be in separate housings.
Power source, or source of power, as used herein, includes the power circuitry such as rectifiers, switches, transformers, SCRs, etc that process and provide the output power. Controller, as used herein, includes digital and analog circuitry, discrete or integrated circuitry, microprocessors, DSPS, etc., software, hardware and firmware, located on one or more boards, and used to control a welding process, or a device such as a power source or wire feeder.
The components of a welding power supply cooperate to produce a welding output. Generally, the controller controls the other components such that the output parameters (welding current and/or voltage, wire feed speed, etc.) are at a desired level, either set by the user or set by the power supply for the type of process being used.
There are numerous control schemes currently being used. Typically, a control scheme includes receiving feedback, and controlling a command signal in response to that feedback. Feedback, as used herein, includes a signal indicative of or responsive to an output or intermediate signal, which is provided to the controller and control decisions are made in response thereto. Responsive to a parameter, as used herein, includes responding to changes in a value of the parameter or a function of that parameter, such as changing the value of a control signal or other parameter, opening or closing a switch, etc.
Prior art controllers use any number of well known control schemes, such as PID control, comparing a feedback signal to a threshold, open loop control, etc. An example of a prior art control scheme is the control scheme in the MM250®. That control is particularly well suited for MIG welding.
The MM250® controller receives two user-selectable inputs, one indicating desired welding voltage, and the other desired wire feed speed. User-selectable, as used herein, includes the user setting an operating parameter set point. The controller also receives feedback of these parameters, and compares the set points to the fedback back values. The difference between the set point and the fedback value, or difference error, is integrated over time, and used to change commands such that the output tends to the set point.
One welding process is a short-arc process (and is performed particularly well by the MM250® power supply). The process has an arc phase, in which the wire advances to the puddle faster than it is melted by the arc. Eventually it reaches the puddle, and the process enters the short phase. Current flow increases in this phase, until it causes a molten metal bridge between the weld puddle and the wire to be broken. This causes the short to be opened, and the process returns to the arc phase. The process alternates between the short and arc phases many times each second.
Prior art short arc-welding systems use voltage control in order to maintain a relatively constant average arc length during welding. This may consist of an open loop system in a constant voltage tapped transformer machine or a voltage control loop. Control loop, open or closed, as used herein, includes a portion of a controller that controls in response to the value of a particular variable.
A prior art voltage control loop filters voltage feedback and compares it to a user-selected voltage set point. The difference, or error, between the set point and actual voltage will result in an adjustment of the output of the welder in the appropriate direction to bring the actual arc voltage closer to the set point.
The amount of filtering of the voltage feedback signal, (or alternately, the error) affects response time and stability. Response time, as used herein, includes the time it takes for a control loop to change the control output in response to changes in a fed back variable. If the filtering is excessive, the response time will be slow, and the output of the machine will not be able to respond to changes in arc length quickly enough and the process may become unstable. If the response time is too short, the intrinsic stability of the periodic molten puddle oscillations may be perturbed and the characteristic regular audible feedback from the process (a.k.a. ‘the buzz’) can be compromised.
The prior art has suggested that the variable eta may be useful in controlling the welding process. Eta, as used herein, is Tsht/(Tsht+Tarc), where Tsht is the length of time of a short circuit and Tarc is the length of time of the successive arc. Some prior art literature suggests that the MIG welding process will be more stable when eta has a value between 0.2 and 0.3. However, prior art control schemes, particularly those used for CV output, do not generally monitor eta, much less control in response to it.
Accordingly, a welding power supply that provides a fast response, yet avoids instability, is desirable. Additionally, a welding power supply that determines eta, and controls in response to eta, is desirable.
Another welding process (which may be used with or without short arc welding) is a fast-tack process. Fast-tack process, as used herein, includes a welding process consisting of successive short-duration arcs or welds, typically separated by trigger releases and re-triggering at a new location, or at the same location, whereby the process is a start and stop welding process. Such a process is often used to tack weld two components prior to a more complete welding or bonding of them. Arc, as used herein, includes a single arc or a number of sequential arcs, such as those in a fast-tack process
MIG welding may be described as four fundamental sequential states: wait, run-in, weld, and burnback. During the wait state the controller is waiting for a gun or torch trigger, which signals the users intent to weld. The transition to run-in begins when the trigger signal is received. During the run-in state the wire begins to move toward the base metal and the power source produces open circuit voltage. The transition to the weld state occurs when current is detected (indicating an arc or short has been established). During the weld state the wire feeds at a constant speed, and the power source is regulated at a constant voltage in order to maintain a steady arc length. The transition to burnback begins when the trigger signal indicates the trigger has been released. During the burnback state the wire feed motor brakes to stop the wire as quickly as possible, and the power source maintains a constant voltage. As the wire feeder is braking, and the wire feed speed is decreasing, the output voltage ensures that the wire will not stick into the freezing weld pool on the base metal. The transition back to the wait state occurs when a burnback timer expires. These states repeat with the next weld.
Some prior art systems used for fast-tack welding allow the operator to set the desired voltage and wire feed speed for the weld state. However, other parameters such as: wire feed speed during run-in; ramp to run-in wire feed speed (an acceleration parameter which determines how quickly the run-in wire feed speed is achieved); ramp to weld wire feed speed (an acceleration parameter which determines how quickly the weld wire feed speed is achieved); open circuit voltage (the output voltage from the power source during run-in); and burnback voltage (the output voltage from-the power source during burnback) affect the welding process.
These parameters (called a
Hutchison Richard M.
Rice Jody K.
White Galen J.
Corrigan George R.
Illinois Tool Works Inc.
Shaw Clifford C.
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