Research On Control Method Of Air Handling Unit In HVAC System

The air handling units,one of the key components of air conditioning system,play a role in regulating indoor temperature,humidity,and carbon dioxide concentration.The operational status of the air handling units not only affects the indoor thermal comfort but also has significant implications for energy conservation and emission reduction.Since the model of the air handling units is a complex nonlinear multi-input multi-output system,its control is difficult and challenging.At present,the study on the control methods of air handling units is insufficient.Classic control methods,such as on/off control and PID control,are simple in structure and widely applied,but have poor anti-interference ability and low control accuracy.The nonlinear models are usually linearized before designing controllers in many model-based controls,which results in the loss of the original system’s nonlinear characteristics.Data-based optimization control often fails to achieve satisfactory results due to data insufficiency or difficulties in data processing.By analyzing the working process and control principle of air handling units,the article studies the problem of control design for air handling units.Due to the inherent complex nonlinearity of the system,nonlinear control methods are used to design controllers.This preserves the system’s nonlinear characteristics and achieves effective control of the air handling units.Additionally,different methods are proposed for cases when system parameters are uncertain or unknown,and the corresponding controllers are designed for each case.The main research contents are listed as follows:(1)The temperature and humidity regulation control problem is investigated for a third-order air handling units.Firstly,the system is transformed into state-space form for design conveniently.Then,the controller is designed using backstepping method which is given step-by-step manner.By selecting appropriate design parameters,precise control of indoor temperature and humidity is achieved.Stability analysis of the closed-loop system is carried out,and a rigorous proof is provided.Three different expected temperature and humidity change curves(step function,ramp function,and mixed function)are set according to the actual changes in temperature and humidity in daily life,and simulation experiments are conducted using MATLAB.Furthermore,the controller designed in this study is compared with those obtained using feedback linearization and PID methods,and the control effects of the three methods are evaluated using four error indicators: MAE,MAPE,MSE,and RMSE.Overall analysis of the temperature and humidity tracking curves and error data shows that the proposed backstepping controller can achieve control objectives more quickly and accurately compared to the other two methods.(2)The temperature,humidity,and carbon dioxide concentration regulation control problem is investigated for air handling units with unknown indoor humidity load.On the one hand,the concentration of carbon dioxide is a significant factor that affects indoor thermal comfort,yet it is often neglected in control design of the current air handling units.On the other hand,in practical systems,the variability of the indoor environment usually leads to changes in the indoor humidity load,so the indoor humidity load is generally unknown.Firstly,a state transformation with a gain L is introduced to deal with the unknown humidity load.Then,the improved backstepping method is applied,and a controller is designed in a step-by-step manner.By selecting appropriate design parameters,effective control of temperature,humidity,and carbon dioxide concentration is achieved.Besides,the stability analysis of the closed-loop system is given,and the rigorous proof is provided.Simulation results show that the designed controller can achieve the control objective of indoor temperature,humidity,and carbon dioxide concentration for air handling units with unknown indoor humidity load.By comparison,the control effect of the designed controller is better than those of the decoupled sliding mode controller and the PID controller.(3)The temperature,humidity,and carbon dioxide concentration regulation control problem is investigated for air handling units with uncertain temperature gradient between supply and return water.In actual operation of air handling units,there exists uncertainty in the temperature gradient between supply and return water.Due to this uncertainty,the control input coefficient is unknown according to analysis of the mathematical model of the air handling units.Therefore,it is impossible to directly design a controller.To handle the above problem,an adaptive method is proposed to design the controller.Firstly,state transformation is introduced for the nonlinear mathematical model of the air handling units.Then,an adaptive control law is introduced to estimate the function of the control coefficient,and the controller is gradually designed.By selecting appropriate adaptive control law and design parameters,effective control of indoor temperature,humidity,and carbon dioxide concentration is achieved.Finally,simulation experiments are conducted to verify the effectiveness of the proposed controller.The simulation results show that the indoor temperature,humidity,and carbon dioxide concentration converge to the desired values in the presence of uncertain supply and return water temperature gradient.