Two critical parameters effect the quality and consistency of a weld between two or more work pieces, weld gun pressure and the current density in the region to be welded. Weld gun pressure is readily regulated by pneumatic, hydraulic, or other mechanical means. Current density regulation requires an electronic solution. Many methods have been utilized to regulate and maintain the current density constant within the contact area between weld gun contact tips and the material to be welded. As the contact tips deteriorate, the contact area increases, resulting in a decrease in the current density at the weld nugget. This results in a decreased heat input and can result in weld defects. Compensation for this decrease in current density over the life of the tips can be accomplished through several different methods to increase or boost the current. Less heat and thus less current, is required during the first or early stage of the contact tips' life. Once the contact tips have settled in, during a second stage, a gradual increase in heat is required. During the last stage, as the contact tips start to deform, even more heat is required. These three stages form a user profile for the current or heat boost.
Earlier weld controllers modified the firing angle of SCR switches to regulate the conduction angle of the SCRs to a particular percentage of full or 180 degree conduction. This mode of operation, known as a voltage compensation method, does not regulate current directly. A second method measures the available heat as a function of the overall system power factor, and provides a user programmed percentage of that available heat following a profile based on the above mentioned three stages of the contact tip lifetime. This method will or could provide for line voltage variations. The user adjusts either the percent conduction angle or the percent heat in either of these methods to achieve a desired weld current as measured by external means.
Some weld controllers provide a third method which uses a constant current control which will adjust the firing angle of the SCRs to maintain a predetermined current flow to the contact tips based on the user profile. The use of stepper programs implements this method by increasing the current in equal increments according to the user profile. Some prior art weld controllers employ a manual stepper to adjust for the current boost, which typically is increased as a series of scheduled linear steps as specified by a weld engineer to obtain metallurgically sound welds during the life of the weld contact tips. For example, the first stage may be programmed to reach a 5 percent current boost in one percent increments after 200 weld cycles, the second stage may be programmed to reach a 10 percent current boost after 2000 weld cycles, and the last stage may be programmed for a 15 percent current boost after 8000 weld cycles. Commonly assigned U.S. Pat. No. 5,083,003 discloses an adaptive stepper which increases the heat boost and thus the current density as a function of not only the number of weld cycles but also as a function of expulsions. Expulsions, also known as spitting, generally indicate that too much heat is being applied during the weld cycle. Molten material is blown away from the weld zone during expulsion, resulting in a significant drop in resistance at the primary of the weld transformer supplying the contact tips.
In all of these cases, it becomes difficult to detect process variations in the welding cycle that could indicate other fault conditions. These variations occur as a result of either short term or long term impedance changes in the welder system. Short term changes are caused by variations in the workpiece and contact tip interface such as oxidation of the surface of the workpieces or poor part fit Long term effects are associated with tool or weld cable degradation or poor connections. With the first and second methods, this increased impedance will result in lower weld current. In the third method, the weld controller will attempt to directly compensate for the increase and regulate the current to maintain it at the constant, preset level, providing more and more power to the weld system, with possible expulsions occurring. The user would have no indication of a problem until it may be too late, as the system would have to detect an inability to regulate current to reach a current limit before error messages are generated. At that time, it would only indicate a failure of the control to regulate the weld current, which could be from many sources. There would be no indication of a change in system impedances. The current controlled system will continue to make metallurgically sound welds until it fails catastrophically from lack of maintenance. A method to indicate changes in system impedances is therefore desirable.
Quality and strength of a spot weld can be correlated with a change in resistance as measured through the weld as the weld progresses during the fusion process. This will effect a change in the power factor of the circuit which will be reflected from the secondary circuit of the welding transformer coupling power to the contact tips to the primary circuits and will result in a significant drop in resistance at the primary circuit. The timing changes resulting from this change can be sensed by the microprocessor based weld controller system. The amount of increase or decrease in the current conduction angle can be determined from these changes and can become a basis for controlling the welding heat applied to the workpiece being welded. U.S. Pat. No. 4,399,511 describes one such type of control. The controller measures the weld current conduction time from the point of initiation to the point of extinction. The conduction time, when added to the original time delay of the initiation signal, is related to the power factor. Using a numerical representation a characteristic resistance curve that relates the ohmic resistance of the work piece as a function of time and weld cycles, the measured and calculated power factor changes can be compared with this curve to determine if more or less energy is required to be supplied to the welding tips. This requires the use of look-up tables or complicated calculations that are non-trivial to calculate the power factor for each weld cycle.