Design of DDC controller of air handling unit based on STM32

[ Abstract] Discusses the hardware and software design of the central air conditioning direct digital controller (DDC), and proposes an economic and reliable implementation plan.
Keywords central air conditioning DDC controller PID fresh air temperature and humidity
0 Introduction
In recent years, with the development of computer technology, control technology, information technology and the reduction of hardware costs, computers have been widely used in the field of HVAC. Based on the control requirements of the central air conditioning system, giving full play to the characteristics of computer technology in the management of the air conditioning system can effectively improve the operating quality of the central air conditioning system, improve the indoor air quality, save operating energy consumption, and improve the management level. This article will introduce the design of the direct digital controller (DDC) of the central air conditioning processing unit.
1 Central air conditioning system control requirements and DDC introduction
         An important part and core of the central air conditioner is the air handling unit, which is a centralized air handling equipment concentrated in the air conditioning equipment room, including the supply / return fan, filter, cooler, heater, humidifier, etc. The control objects of the central air-conditioning system include the temperature, humidity, fresh air volume, cold (hot) water temperature and pressure of the control area. Therefore, the control of the air handling unit is mainly to control the size and proportion of the temperature, humidity and new / return air volume of the regulated area. The control objective is to maintain the indoor temperature and humidity at an appropriate level and consume as little energy as possible. The monitoring principle of the air handling unit is shown in Figure 1.
Figure 1 The monitoring principle of the air handling unit
         Adopting DDC and auxiliary host computer to control the air handling unit can realize various control functions for multiple controlled devices at the same time, which is unmatched by the traditional control system composed of conventional instruments, and when the control content and scale are the same, Can significantly reduce the overall cost and increase flexibility.
2 DDC function of air handling unit
         The main functions of the air handling unit DDC include fresh air temperature / humidity monitoring, supply air temperature / humidity monitoring, return air temperature / humidity control, filter clogging alarm, new return air proportional control, unit timing start / stop control, chain protection control, Environmental control of important places, etc. The air handling unit DDC is equipped with a communication interface. With the host computer, it can complete comprehensive alarm management, historical record storage, information printing, and manual / automatic control. In addition, the air handling unit DDC can also be connected to a display.
3 controller design
3.1 Hardware design
         The air treatment unit DDC is mainly composed of a main control circuit, a temperature sensor signal acquisition circuit, a DC voltage and current signal acquisition circuit, a switch state detection circuit, a switch output control circuit, an analog output circuit, a clock circuit, a display / button circuit, and data storage The circuit and RS-485 communication circuit are composed as shown in Figure 2.
Figure 2 Air treatment unit DDC structure diagram
         (1) The main control circuit is mainly composed of STM32 microcontroller. A data acquisition and data storage module are externally connected to the single chip microcomputer, and data acquisition, data analysis, state judgment and control execution functions can be completed through the software.
         (2) The temperature sensor signal acquisition circuit is shown in Figure 3. The temperature sensor signal acquisition circuit inputs the resistance signal of the Pt1000 sensor it has collected into the constant current source circuit and converts it into a voltage signal, and then accesses the 12-bit A / D channel of the CPU through the RC filter circuit. After sampling, the current correspondence is obtained. Temperature value. In order to prevent the temperature measurement accuracy from being affected by the external leads of the sensor, the Pt1000 sensor uses a three-wire connection method to eliminate lead resistance errors. The temperature sensor signal can be used to monitor the fresh air, return air, supply air and the temperature of a typical room.
Figure 3 Temperature sensor signal acquisition circuit diagram
         (3) The DC voltage and current signal acquisition circuit connects the collected DC signal to the internal A / D channel of the CPU after resistance division, op amp amplification, and RC filtering.
       
(4) The switch state detection circuit monitors the external input node using optocoupler isolation.
       
         (5) The analog output circuit is shown in Figure 4. The pulse signal output by the PWM function of the single-chip computer outputs a controllable DC 0-10V or 4-20mA signal after optocoupler isolation, transistor expansion, RC rectification and filtering. The DC signal can be used to adjust the opening ratio of the air valve and water valve, and the fan frequency can also be adjusted according to demand.
Figure 4 Analog output circuit diagram
         (6) The switch output control circuit is controlled by the I / O port of the single-chip microcomputer through the triode to expand the output of the multi-channel relay, which increases the drive capacity of the relay and ensures the stability of the control.
         (7) The controller has a 128 × 64 LCD display screen and a button unit, which can realize parameter setting and query, and can also adjust the controller system parameters in real time. The controller provides an RS-485 port to facilitate communication with the host computer.
3.2 Software design
         The control flow of the air handling unit is shown in Figure 5. The air handling unit is a typical large inertial system with pure lag. It has a lot to do with the flow of indoor people, indoor objects, and outdoor climatic conditions. Therefore, it is difficult to describe it with an accurate mathematical model, and it is difficult to achieve the best control and energy-saving purposes with traditional conditions.
Figure 5 Control flow chart of air handling unit
         (1) Under normal circumstances, the fresh air valve is kept in a position where basic fresh air volume can be drawn in. When the CO2 concentration measured by the CO2 sensing element is higher than the system setting, the fresh air valve will automatically adjust to the position corresponding to the predicted value. When the CO2 concentration exceeds a certain critical value, the fresh air valve is fully opened to dilute the indoor air. The control system not only considers the CO2 concentration of the indoor air, but also considers maintaining the indoor positive pressure and the number of people in the room.
         (2) Adjustment and control of fresh air relative humidity. The controller tests the humidity value in the room and compares it with the set value. If there is a difference, a proportional integral link is used to control the humidity electric regulating valve to keep the supply air humidity within the required range.
         (3) Adjustment and control of supply air temperature. The controller compares the indoor temperature collected by the temperature sensor according to the set temperature determined by its internal clock, and uses a PID control algorithm or other algorithms to adjust the three-way regulating valve of the coil to make the indoor temperature consistent with the set temperature.
         (4) Filter alarm control. After the fan is started, a pressure difference will be formed before and after the filter. If the filter is clean, the pressure difference will be less than the set value, and the contactor will be opened. Otherwise, the pressure difference will become larger and the contactor will be closed. The controller will issue a filter alarm signal according to the dry contact condition of the contactor.
         (5) Safety and fire control. The temperature control program will only work after the fan is started and the air flow switch detects the wind pressure. The new fan group and the fire protection system adopt chain control. When there is an alarm signal, the fan should be shut down and the fresh air valve should be closed.
4 Conclusion
         This paper designs an air conditioner energy-saving controller. The controller uses STM32 single-chip microcomputer as the core, and the hardware and software distribution is reasonable. Combined with PID control, it achieves good control and energy-saving effects.
The article comes from: "Electrical Technology" 2012 issue 7.
references
         [1] East China Architectural Design Institute. Intelligent Building Design Technology [M]. Shanghai: Tongji University Press, 2002
         [2] Jing Youyin, Gao Yuefen, Zheng Guozhong. HVAC design and system analysis [M]. Beijing: China Electric Power Press, 2010
         [3] Liu Jinkun. Advanced PID control MATLAB simulation [M]. Beijing: Electronic Industry Press, 2011
         [4] Wang Yonghong, Xu Wei, Hao Liping. STM32 series ARM-Cortex-M3 microprocessor principle and practice [M]. Beijing: Beijing ## Space University Press, 2008

About the Author:

Yu Jing, female, undergraduate, engineer of Wuhan Ankerui Electric Co., Ltd., the main research direction is intelligent power monitoring and power management system

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