Real time adaptive control for resistance spot welding process

A method of adaptively controlling a resistance spot welding process performed on a machine having an adjustable heat command to control the welding current and heat generated in a workpiece comprising:

• providing an off-line model of the spot welding process and calculating, given said heat command as an input, process model normalized displacement;
• measuring workpiece displacement due to thermal growth;
• monitoring the process and calculating, from N samples of workpiece displacement and process model normalized displacement, an estimated linear process gain, defined as the relation between displacement and heat command input, and a bias displacement;
• determining estimated displacement from the product of the estimated process gain and process model normalized displacement to which is added the bias displacement;
• generating a shaped displacement trajectory command which gives a desired displacement versus time to yield acceptable spot welds under ideal process conditions using a constant heat;
• and
• comparing said estimated and desired displacements and generating an error, compensating the latter, and regulating said heat command with the compensated error to achieve acceptable spot welds over a range of process variability.

An adaptive control system for a resistance spot welding machine comprising:

• a model reference feedback control whose output is a dynamic welding machine heat command to control welding current and heat generated in a workpiece over a range of process variability;
• a sensor on said machine to measure workpiece displacement due to thermal growth;
• an estimator to monitor the process and calculate an estimated real time process gain for every displacement measurement;
• a precompensator that generates a desired displacement trajectory command to produce acceptable spot welds under ideal conditions using constant heat;
• said model reference control having a mathematical model of the spot welding process which computes process model normalized displacement resulting from said heat command, means to calculate estimated displacement from said process model normalized displacement and estimated process gain, means to compare said desired and estimated displacements and generate an error, and means to compensate said error and sum the result with an initial heat setting to yield said welding machine heat command.