Nonminimum Phase Behavior of Laser Material Processing

Optical sensors are increasingly applied in laser material processing to monitor and control the lasermaterial interaction zone. Dynamic models, relating the sensor signals (e.g. as temperature or molten area) to the process inputs (e.g. laser power or beam velocity), provide the basis for the design and tuning of a feedback controller. These models can show nonminimum phase (NMP) behavior. This means that the sensor signal of a minimum phase process directly changes in the direction of its steady-state value, whereas the sensor signal of the NMP process is initially in the opposite direction. This paper illustrates and discusses the NMP behavior found in three different laser processes. Firstly, the behavior is shown theoretically for laser heating, using a Finite Element Model (FEM). Here the beam velocity is used as an input and the temperature (as well as molten area) is the model output. Secondly, the NMP behavior is shown experimentally for laser alloying of titanium. In this case again the beam velocity is applied as input, whereas a pyrometer signal is considered as output. Finally, laser welding of mild steel is discussed. Here the laser power is considered as input, and the intensity of the plasma radiation as output. Whether or not a process shows NMP behavior is essential information in the design and tuning of model based feedback controllers.