Multivariable geometry control of welding. Part II. Control implementation

Abstract

The problem of controlling several geometric attributes of a Gas Metal Arc Welding Press is addressed. In particular, the weld bead width and height are chosen as outputs, owing to the existence of robust measurement methods. The process has been shown to exhibit significant but smooth non-linearity, and a control system is designed using scheduled gains to achieve cancellation of non-stationary system poles. The 'zero scheduling' is based on knowledge of the velocity of welding, while gain scheduling is not attempted, and is dealt with instead by robust design. Two controller forms are implemented, one based on two independently designed non-linear PI loops, and one using the process model to create a decoupling matrix. Both systems can produce stable transient response, but the bi-valued nature of the input output mapping of the process can produce non-steady control inputs. This 'hunting' of the closed-loop system is demonstrated experimentally and can be avoided only by limiting the range of modulation of the inputs. Also apparent are irreducible process delays that are non-constant, causing the decoupling control to have input magnitude-dependent performance. This is partially alleviated by employing Smith predictor design. Within the limited range of operation possible, the performance is acceptable, but little or no disturbance rejection is demonstrated. The latter is shown to be a function of process capability rather than a fundamental limitation of the controller itself.