Harmonie excitation based autotuning: Autotuning usable for controlling biomass combustion processes

Controlling combustion processes is an important task, mainly with respect to energy efficiency and ecological impacts. This problem has become significant with the increased use of biomass-fired boilers, including low-power boilers. The combustion process is a process that we have limited information about. The equations for the chemical reactions are known, but in reality combustion of biomass is much more complicated and more variable than a mathematical model can express. It is therefore not simple to design a controller that is able to control the combustion process effectively in the whole range. Small-scale biomass-fired boilers are usually equipped with simple two-state (on/off) controllers. Many laboratories have attempted to develop a model of the combustion process suitable for controller setting and parameter adaptation, but the results are not yet satisfactory or generally valid. Thus, advanced model-based control strategies are not usable, while the use of a model-free controller is not excluded if well-designed tuning rules can be provided. An approach which tries to satisfy most of the requirements of industrial practice is presented in this paper, which is based on an experimentally performed evaluation of small amplitude excited frequency responses with the aim to achieve recommended values of one or more control quality indicator known e.g. from the course of the Nyquist plot. The indicators can be evaluated experimentally in control loops involving nonlinearities. In this sense, the method has a philosophy similar to that of the popular Ziegler and Nichols method, but no interruption of the control process is necessary and the amplitude of the excited oscillation can be set by operator. The main advantages of the method presented here are: there is no use of any mathematical model, it can be used as an addition to the existing controller purely by software, and there is no need to interrupt the control process while retuning the controller.