Method and apparatus for monitoring and controlling a resistance welding operation

In a resistance welding controller having means for monitoring the resistance across a pair of weld electrodes and means for controlling the application of welding current to the electrodes;

• the method of controlling the formation of a plurality of successive welds in accordance with a target weld resistance curve that is characterized by a predetermined minimum resistance value (target Rmin) and a predetermined rate of increase in resistance value (target dR/dt) during the initial heat-up phase of the weld, a predetermined decline in resistance value (desired .DELTA.R) during the nugget growth phase of the weld, and a target weld time, including the steps of:
• (a) initiating a first weld at a first weld current setting,
• (b) monitoring the weld resistance of the weld and detecting the minimum resistance value (Rmin) of the weld and the subsequent rate of increase in resistance value (dR/dt) during the heat-up phase of the weld and comparing the observed values for Rmin and dR/dt with the target values for Rmin and dR/dt,
• (c) dynamically changing the weld current setting during the heat-up phase of the weld in accordance with said comparison,
• (d) terminating the weld when the desired .DELTA.R value is satisfied,
• (e) initiating the succeeding weld at the revised weld current setting,
• (f) repeating steps (b)-(e) for each succeeding weld;
• and
• (g) automatically reverting to said first weld current setting during step (c) whenever the observed values for Rmin and dR/dt bear a predetermined relationship to the target values for Rmin and dR/dt.

In a resistance welding controller having means for monitoring the resistance across a pair of weld electrodes and means for controlling the application of welding current to the electrodes, the method of controlling the formation of a weld including the steps of:

• establishing a target weld resistance curve characterized by a predetermined minimum resistance value (target Rmin) and a predetermined rate of increase in resistance value (target dR/dt) during the initial heat-up phase of the weld, a desired decline in resistance value during the nugget growth phase of the weld, and a target weld time;
• monitoring the resistance of the weld and detecting the minimum resistance value (Rmin) of the weld;
• comparing the observed Rmin value with the target Rmin value and determining whether said observed Rmin value is within a predetermined range of said target Rmin value;
• determining the rate of increase in resistance value (dR/dt) during the heat-up phase of the weld and comparing the observed dR/dt value with said target dR/dt value;
• dynamically correcting during the heat-up phase of the weld the weld current applied to the electrodes in accordance with the comparison of said dR/dt values when said observed Rmin value is within said predetermined range of said target Rmin value;
• and
• continuing to monitor the resistance of the weld and terminating the weld when the desired .DELTA.R value is satisfied.

In a resistance welding controller having means for monitoring the resistance across a pair of weld electrodes and means for controlling the application of welding current to the electrodes, the method of controlling the formation of a weld including the steps of:

• establishing a target weld resistance curve characterized by a predetermined minimum resistance value (target Rmin) and a predetermined rate of increase in resistance value (target dR/dt) during the initial heat-up phase of the weld, a desired decline in resistance value during the nugget growth phase of the weld (desired .DELTA.R value), and a target weld time;
• monitoring the resistance of the weld and detecting the minimum resistance value (Rmin) of the weld and the subsequent rate of increase in resistance value (dR/dt) during the heat-up phase of the weld and comparing the observed values for Rmin and dR/dt with the target values for Rmin and dR/dt;
• dynamically correcting a first time during the heat-up phase of the weld the weld current applied to said electrodes in accordance with said comparisons;
• terminating the weld when the desired .DELTA.R value is satisfied;
• comparing upon termination of the weld the actual weld time observed with said target weld time and making a second correction in the weld current setting in accordance therewith;
• and
• utilizing the twice corrected weld current setting to initiate the next weld.

A resistance welding controller for controlling the application of welding current to a pair of electrodes including voltage sensing means for sensing the voltage across the electrodes, current sensing means for sensing the weld current, and means responsive to said voltage sensing and current sensing means for producing an output signal proportional to the resistance across the electrodes;

• the improvement comprising:
• control circuit means responsive to said output signal for controlling the application of welding current to the electrodes so as to achieve a desired decline in resistance value during the nugget growth phase of the weld (desired .DELTA.R value) within a substantially uniform period of time (target weld time), including means for establishing a target resistance curve characterized by a minimum resistance value (target Rmin) and a rate of increase in resistance during the heat-up phase of the weld (target dR/dt), means for determining the minimum resistance value (Rmin) and rate of increase in resistance value (dR/dt) during the heat-up phase of subsequent welds and comparing the observed values for Rmin and dR/dt with said target values for Rmin and dR/dt, means for dynamically correcting the weld current setting during the heat-up phase of a weld in accordance with said comparisions, and means for terminating the weld when the desired .DELTA.R value is satisfied.

In a resistance welding controller having means for monitoring the resistance across a pair of weld electrodes and means for controlling the application of welding current to the electrodes, the method of controlling the formation of a weld including the steps of:

• establishing a desired decline in weld resistance during the nugget growth phase of the weld (desired .DELTA.R value), and a target weld time;
• initiating the weld at a predetermined weld current setting;
• monitoring the resistance of the weld and terminating the weld when the desired .DELTA.R value is satisfied;
• and
• comparing upon termination of the weld the actual weld time observed with said target weld time and adjusting accordingly said predetermined weld current setting in preparation for the start of the next weld.