Design and simulation of a discrete time controller for regulating temperature in a scale greenhouse

This paper describes the process of designing a temperature controller working in discrete time, using as manipulated variables: thermal power applied from a heat flow supply, in combination with the variation of outdoor air volume flow. As a preliminary step, a dynamic model was analytically derived to describe the behavior of the temperature as a function of the variables: thermal power and air volume flow. An experimental procedure was performed to determine the particular parameters of the real system. Among others specific thermal power, surface thermal conductivity between the greenhouse cover and the environment, heat dissipation coefficient of the incoming airflow. With the values obtained proceeded to make a simulation of the greenhouse on scale as a representative of a physical system. The proposed controller was designed based on the limitations and operating ranges of the real system variables for manipulating : up to 20 watts for the heater in a volume of 5750 cm3, from 0°C of external temperature up to 12°C. By including the air flow as an input variable the system became nonlinear but designed controller was able to maintain the inner temperature between acceptable range for tested potato plants. At a simulation work, it was added a warm air supply at a higher temperature than the outside one. In addition to heating, warm air was also used to increase temperature. This work is the basis to build a mechanism that will be implemented for temperature control which shall carry out, in a full-scale greenhouse in which the efficiency of the control system will be evaluated to protect a crop against the frost weather phenomenon.