A nonlinear PI controller for climate control of animal buildings

Abstract

Climate control for housed animals brings the added dimension of a biological system into a physical system control situation. The animals in a livestock building impose their own needs, significantly affect their ambient conditions in a non-linear way, and add long time constants to the system response. Moreover, the thermally dynamic nature of a livestock building suggests that disturbance attenuation (load control of external temperature, humidity, heat emanated by the animals and water-vapor production of animals) is far more important than is the case for controlling other types of buildings. This paper presents a set of nonlinear control design methods, resulting in an inherently robust adaptive nonlinear PI control law for simultaneous temperature/humidity control of livestock buildings in both summer and winter conditions. The proposed method consists of four parts: (a) the transformation of user-defined desired settings for temperature and humidity into feasible controller set-points, taking into account the constraints imposed by the capacities of the actuators, the psychrometric laws and the animal welfare; (b) a model based feedback/feed-forward compensation of external disturbances (loads) on the basis of input-output linearization, decoupling and Lyapunov theory; (c) a robust nonlinear compensation of large errors (output deviations from desired set-point values) on the basis of nonlinear damping terms; and (d) an adaptive nonlinear control law incorporating integral action to cope for parametric model uncertainty and unknown disturbances. The proposed method is applicable to any air-conditioning system and is expected to gain wide acceptance in modern climate control systems.