Identification and improved control of GTAW voltages utilizing digital series compensation

Present methods for automatically controlling the voltage of a Gas-Tungsten-Arc-Welding (GTAW) process utilize an analog servo system to regulate the welding electrode to workpiece distance. The controllers currently being used in this facility are characterized by large over-shoot to a step input, and steady-state error due to deadband. An attempt to use microprocessor technology to improve the performance is underway. Such a method of improving the performance of the voltage controllers has been developed using an on-line identification procedure to determine the mathematical model of the present controller. Once an adequate model has been determined, a digital compensation scheme is designed to acquire the desired performance. The identification procedure used was the Model Reference Adaptive Technique, by which a model is postulated and its performance is compared to the actual plant performance for a set of given inputs. The squared error between the plant performance and the predicted performance is then computed over the identification interval. The model parameters are adjusted by an optimization routine to improve the model of the plant. Once the model is optimized to the point of diminishing returns, a digital PI or PID controller is designed. The result is that now the machine behaves in a manner that can be easily adjusted to give significantly improved performance. A relatively inexpensive M6800 microcomputer is used for both the on-line model identification, including the error minimizing routines, and for the realization of the series compensating controller. The software used is Motorola 6800 Assembly Language and Motorola Fortran. The method of improved control is being expanded to include the other welding parameters (current and speed of torch travel) and to other methods of control and state reconstruction, such as the linear regulator and the Kalman filter. Performance comparisons between the linear regulator and PID controller are made.