Present methods for consumable electrode arc welding processes usually involve various means for regulating arc voltage indirectly control arc length. While the use of an automatic feedback control to vary the welding arc voltage also may control the arc length to same degree, there is no known practical method for sensing the arc voltage between the point at which the electrode material becomes molten or the point from which the arc emanates and the base metal or part to be welded. The closest practical measurement of arc voltage is usually made between the contact tip and the work piece. Controlling this measured voltage provides limited control over the actual arc voltage and an indirect control of the arc length, provided that the distance between the contact tip and the base metal does not vary over too wide a range.
Assuming the contact potential (i.e.--the potential across the point of current transfer between the electrode and the contact tip) to be negligible, the voltage being regulated between the contact tip and the base metal is equal to the voltage drop across the electrode stick-out (i.e.--current (I) times electrode stick-out resistance (Re)) plus the voltage drop across the actual arc. Both of these voltage drops are functions of length: increased arc length and/or stick-out yield higher voltage. Stick-out is defined as the length of electrode material or the distance from the end of the contact tip to the point on the electrode material from which the arc emanates. The distance from the end of the contact tip to the base metal equals the stick-out length plus the actual arc length. Therefore, any variation in the distance between the end of the contact tip and the base metal causes a variation in the stick-out and the actual arc length, which variation corresponding causes variations in the actual arc voltage.
In actual practice, other undesirable effects may occur due to the variation of the distance between the contact tip and base metal. If the torch to base metal distance increases, a long electrode stick-out will result in greater electrode preheating because more power is imparted to the electrode due to greater IRe voltage drop. This, in turn, causes the arc current to decrease. Also, the extent and variation of electrode cast affects the exact positioning of the arc. Therefore, there is a need to maintain a constant electrode stick-out length or a constant contact tip to base metal proximity.
There are a number of present methods for controlling the height or proximity of the welding torch with respect to the base metal. Existing systems are based on contacting mechanical followers, contacting electro-mechanical sensors, non-contacting eddy current transducers, non-contacting optical sensors, as well as through-the-arc current sensing techniques. All of the present techniques suffer various disadvantages.
The key to accurately controlling torch to workpiece proximity lies in the ability to obtain distance related information in the immediate vicinity of the arc. Applicant has now discovered a new through-the-arc sensing or detecting technique that provides the needed information while resolving the deficiencies of the prior art.