Synthesis of a three-channel PID controller for controlling a three-zone medium-temperature furnace

Currently, in metrological laboratories there is a need to transfer temperature units from standards of the highest category by reproducing the fixed points of the international temperature scale. The reproduction process of fixed point involves heating, melting, overheating and solidification of the metal and can sometimes take more than one working day. The purpose of this work was to create and practically test an algorithm that takes into account heat flows in a three-zone furnace for uniform heating and relatively fast reaching the solidification area. Over the past twenty years, several algorithms have been used making it possible to go through all four stages within one working day. This paper describes a practically tested algorithm for controlling three-zone furnaces for reproducing the fixed points of the temperature scale in the range from 156.5985 °C to 1084.62 °C. It takes into account the requirements for maintaining the minimum temperature gradient along the height of the fixed point cell. This algorithm has been used in the furnaces of the West Siberian branch of FSUE "VNIIFTRI". The controller model is based on the principles of automatic control theory using a multi-channel discrete proportional-integral-derivative or PID controller. The model was modified in accordance with the system structure. The main idea is to represent the model of a three-zone furnace as a three-channel control object with mutual influence between the channels. The corresponding controller model is a three-channel control system. In each channel of it there is a PID controller. Depending on the phase of the playback process, the three-channel controller changes its structure and tunes its coefficients. Such a structure made it possible to achieve sufficient productivity of the furnace.