Electric heating – Metal heating – By arc
Reexamination Certificate
2000-04-12
2002-01-01
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S124030, C219S130310
Reexamination Certificate
active
06335511
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus and method for controlling a welding parameter in an arc welding system. More particularly, the invention relates to an apparatus and method for adjusting the frequency or some other parameter of pulses applied to an electrode of an arc welding system in response to changes in the arc voltage, and for returning the pulse frequency or other pulsation parameter to a desired value by varying a further parameter. In an illustrated example of the invention, the arc welding system is a consumable electrode arc welding system, the pulsation parameter may be the pulse frequency, and the further parameter that is varied to return the pulsation parameter to the desired value is the distance between a workpiece and the consumable electrode or an electrical connection element for the consumable electrode, such as a contact tip. The invention may also have applicability to pulsed arc welding systems which use non-consumable electrodes.
2. Description of Related Art
It is well-known to control the arc in an arc welding system by varying the power supplied to an electrode to compensate for changes in the arc voltage as a result of changes in the distance between the workpiece and the electrode or the contact tip that supplies power to the electrode. Such changes occur because of variations in workpiece contour as the torch moves along a weld path, or misalignment of torch travel relative to the workpiece, and must be rapidly compensated in order to ensure good weld quality.
One way to compensate for arc voltage deviations caused by changes in the distance between workpiece and the electrode or contact tip is to mechanically move the electrode or contact tip in response to changes in the arc voltage. In the case of a consumable electrode system, it is the contact tip that is moved, the electrode being arranged in sliding engagement with the contact tip and fed at a constant rate. An example of such a proximity controller is described in U.S. Pat. No. 4,631,385 (Rothermel)
Another way to compensate for variations in the distance between the contact tip and the workpiece is to adjust an electrical parameter of the power supplied to the electrode, such as the average current, or in the case of pulsed currents, the peak and/or base amplitudes, width, or shape of waveforms supplied to the contact tip in response to changes in the arc voltage. Examples of current control based on the arc voltage are found in U.S. Pat. No. 3,614,377 Stearns), U.S. Pat. No. 3,896,287 (Cook), and U.S. Pat. No. 3,904,846 (Risberg), with the Stearns and Cook patents being directed to pulse width modulation control of constant current arc welding power supplies, and the Risberg patent disclosing adaptive control of both the pulse width and interval of the arc welding current. Such systems are effective in controlling the overall burn off rate, but only so long as the adjusted parameter is kept within relatively narrow limits since the weld quality, particularly in pulsed arc welding systems, is highly sensitive to the current amplitude and waveform. For example, too low a current or too short a pulse width will cause globular and intermittent metal transfer with attendant spatter. Detailed descriptions of the considerations involved in selecting a suitable waveform are described in a number of the U.S. patents cited herein.
The present invention is particularly directed to an arc welding control system of the type in which power to the contact tip is in the form of pulses and the pulse frequency is controlled based on changes in voltage between the contact tip and the workpiece. Such systems are known, for example, from U.S. Pat. No. 4,409,465 (Yamamoto), U.S. Pat. No. 4,427,874 (Tabata et al.), U.S. Pat. No. 4,620,082 (Graville), and U.S. Pat. No. 4,758,707 (Ogilvie). However, as in other conventional current or pulse control systems, the conventional pulse frequency controls are limited so a relative narrow adjustment range and cannot be used where the variations in arc voltage are likely to cause the adjusted parameter to deviate beyond an acceptable range. Since the pulse frequency is a critical parameter in pulsed consumable electrode arc welding systems, variation in the pulse frequency based on arc voltage has not been widely utilized, despite the simplicity and accuracy of frequency controllers in general.
The present invention solves this problem by adding a control loop which returns the adjusted parameter to a desired value by adjusting the contact tip-to-workpiece distance. This action also restores the contact tip-to-workpiece distance back to its original or intended value, which further guarantees that gas coverage and electrode stick-out (i.e., the distance between the end of the contact tip and the arc) remains constant. As will be discussed below, the use of an additional, proximity based control loop to return a deviating parameter to its original or desired value is known from U.S. Pat. No. 4,794,232 (Kimbrough et al.).
Before discussing the Kimbrough et al. patent, the conventional pulse frequency control system will be described.
FIG. 1
, shows the basic elements of a conventional arc welding system of the type which uses a torch to which is supplied a consumable electrode as the source of welding material. During welding, a voltage is applied between the contact tip
1
and a workpiece
2
, and the consumable electrode
3
is continuously fed from a spool
4
towards the workpiece by rollers
5
driven by a motor
6
at a rate corresponding to the rate that material is burned off the tip of the electrode
3
by the applied voltage, so that a constantly controlled amount of material is deposited as the torch is moved along the weld. A feed rate controller
7
is used to regulate the feed rate in accordance with commands entered manually or by computer via input
8
. Power to the electrode is provided by a weld power source
9
that delivers electrical power to the electrode via the sliding contact tip
1
.
In this type of consumable electrode arc welding system, the arc voltage is a function of the distance between the contact tip and the workpiece, while the overall rate of burn off of the consumable electrode is determined by the applied voltage and current, i.e., by the power applied to the electrode. In order to compensate for increases or decreases in the torch-to-work distance and/or arc length, and thereby maintain a constant burn off rate, some conventional pulsed arc welding systems proportionally increase or decrease the power supplied to the contact tip by varying a nominal pulse frequency input
10
based on feedback from a voltage sensor
11
. Adder
12
compares the voltage sensor feedback with the desired arc voltage input
13
and the resulting difference signal is processed by processor
14
to obtain a compensation signal. The feedback signal may be representative of a peak voltage during the pulse, a background voltage between pulses, an average voltage, or an instantaneous voltage, taken at any point in the pulse cycle or over multiple cycles. The resulting frequency command is then supplied to a waveform generator
15
which generates the desired waveform, which may optionally be based on a stored or preset pulse profile data input
16
, at the commanded frequency to obtain a control signal for the weld power source. If the arc voltage is less than the desired voltage, the feedback loop automatically increases the pulsation frequency, causing the average current and electrode consumption rate to increase, until the arc voltage equals the commanded value. Conversely, if the arc voltage is greater than the desired voltage, the loop automatically decreases the pulsation frequency, causing the consumption rate to decrease, until the arc voltage again equals the commanded value.
The purpose of pulsing the power supply to the electrode is to facilitate control over the size of the molten puddle by controlling the average weld power via pulse frequency and feed rate adjustments. In addition,
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