I. Field of the Invention
The present invention relates to a circuit for conditioning an AC arc voltage to provide a DC voltage to an arc voltage controller. Such a device provides a stable signal to allow the arc voltage controller to maintain a preselected arc length, and is particularly suitable for use in automatic gas tungsten arc welding to improve the resulting weld.
II. Description of the Prior Art
Arc voltage control units (AVC's) are commonly used in the welding industry for gas tungsten arc welding (GTAW). The purpose of an AVC is to maintain a constant arc length as a GTAW torch with a tungsten electrode is moved along a weld seam, regardless of any variation in the height of the workpiece relative to the line of travel. This purpose may be accomplished, for example, by a mechanical drive system capable of moving the tungsten electrode closer to or further from the workpiece. A proportional relationship exists between arc length and arc voltage, thus, maintaining the welding arc voltage at a preselected reference arc voltage maintains the desired arc length.
There are three general methods of current delivery to the arc in GTAW. The most prevalent are DC straight polarity (DCSP) and DC reverse polarity (DCRP). Most GTAW is done with DCSP (electrode negative with respect to the workpiece). However, with some metals such as aluminum and magnesium which develop refractory oxides, DCSP is inadequate. DCRP may be used to remove the oxide while welding, however, in this mode heat transfer to the workpiece is poor thus limiting DCRP to thin gauge material.
AC welding achieves the welding penetration of DCSP on the negative half-cycle and the cleaning action of DCRP (electrode positive with respect to the workpiece) on the positive half cycle and is, therefore, a superior mode of welding such metals as aluminum and magnesium.
AVC's are specifically designed for DC welding, either DCSP or DCRP, but do not adapt readily to AC welding. To allow an AVC to operate with AC GTAW requires a signal conditioner which converts AC arc voltage information to a DC level which can be used effectively by an AVC.
U.S. Pat. No. 2,671,843 to Steele recognizes that the positive half cycle of AC arc voltage is not steady or uniform and, hence, in order to obtain a useful signal from AC arc voltage, only the negative half cycle should be used. As explained in Steele, as current changes from electrode negative to electrode positive, there is an interruption in current flow causing the arc voltage to have a spike in the positive half cycle, whereas there is no such interruption when the current changes from electrode positive to electrode negative providing a clean signal in the negative half cycle. The result is that, for reliability, only the negative half cycle of an AC arc voltage should be used.
Accordingly, Steele turns to the use of a half-wave rectifier to provide a DC voltage corresponding to the negative half cycle of the AC arc voltage. In order to provide a steady DC signal which is required to avoid oscillations in the AVC and subsequent hunting of the electrode, Steele also includes a filter to smooth out the half-wave voltage and attain a smoother, steady DC voltage.
However, the use of a filter introduces a delay in moving the electrode in response to a change in the arc voltage. Such a delay can cause instability in the overall control system. It can also cause the electrode to dive into the workpiece at the end of the weld as the filter would still present a control voltage to the AVC when the arc voltage is removed.