Introduction
The central air conditioning subsystem plays a crucial role in building automation systems. Currently, there are various methods for automating central air conditioning systems. Some use microcontrollers with interfaces such as RS485, fieldbus, or Ethernet to achieve remote monitoring. Others use PLCs, such as Siemens' S7-200, for data acquisition and monitoring. There are many types of microcontrollers available, and a wide range of chips can be selected to achieve this data acquisition and monitoring function, such as the MEGA series and Freescale series. High-end chips offer rich interfaces, making implementation easier, but they are more expensive. Furthermore, the cost bottleneck of PLC-based central air conditioning monitoring systems limits their further adoption. Therefore, developing a low-cost, high-reliability remote monitoring system for central air conditioning is essential.
Option Two
In recent years, the functionality of microcontrollers has been greatly improved, including storage capacity, data processing speed, peripheral expansion capabilities, and communication functions. With its functions gradually becoming more complete and its low cost advantage, its market share has continued to rise.
The main function of this system is to remotely acquire and monitor data from the central air conditioning system. The parameters for data acquisition and monitoring include air conditioning temperature, on/off status, fan speed, and cooling/heating status. A microcontroller is selected as the lower-level device, and Microsoft Visual Basic is sufficient for the upper-level monitoring software to meet the control requirements.
Three-system design concept
Currently, central air conditioning systems are mainly classified into three types based on the transport medium: air, water, and refrigerant. Therefore, corresponding central air conditioning systems are mainly divided into duct systems, chilled/hot water systems, and refrigerant systems. This solution is primarily applicable to chilled/hot water systems. Chilled/hot water systems consist of a main unit and fan coil units. The main working principle is that the outdoor unit generates chilled/hot water for air conditioning, which is then delivered to various terminal units indoors via a piping system. At the terminals, the chilled/hot water exchanges heat with the indoor air, producing hot or cold air, thereby eliminating the room's air conditioning load. The terminals of a chilled/hot water air conditioning system are usually equipped with fan coil units. The control principle of the fan coil units uses a thermostat and electric valve structure, as shown in Figure 1. Therefore, the amount of heating or cooling delivered to the room can be adjusted by regulating the speed of the terminal fan. Thus, the characteristic of this system is that it allows for individual control and adjustment of each terminal (room).
The indoor temperature can be adjusted by an electric three-way valve that is interlocked with the temperature sensor in each room and installed on the return water branch pipe of each fan coil unit, or by a three-speed switch of the fan coil unit.
Figure 1. Control principle diagram of fan coil unit
Introduction to this fan coil unit (as shown in Figure 2):
(1) System control-------The temperature controller is placed in the room where the temperature needs to be regulated. It has two on/off states, ON and OFF, which can directly control the opening and closing of the system.
(2) Temperature control -------- The temperature controller is equipped with a temperature setting button. There are two pairs of contacts inside the thermostat. In summer, the temperature controller selector switch is set to the "COOL" position to supply chilled water to the coil. When the temperature is lower than the set value, one pair of contacts opens and the electric valve is de-energized. When the room temperature is higher than the set value, the other pair of contacts closes and the electric valve is energized. Conversely, in winter, the temperature controller selector switch is set to the "HEAT" position to supply hot water to the coil. When the room temperature is higher than the set value, one pair of contacts of the electric valve opens and the electric valve is de-energized. When the room temperature is lower than the set value, the other pair of contacts closes and the electric valve is energized, thus maintaining the room temperature within a certain range in winter and summer.
(3) Electric valve control------ The operation of the electric valve is directly controlled by the thermostat. When the electric valve is energized, it opens and supplies hot and cold water to the fan coil unit; when the power is lost, the electric valve closes. This ensures that the temperature is controlled within a certain range.
(4) Fan control --------- When the thermostat is in the "ON" state, the fan can be adjusted to high, medium and low levels through another set of changeover switches.
Figure 2 Control principle and operation of fan coil air conditioner
The lead wires for the fan coil units in this system are shown in Figure 3.
Figure 3 Fan coil unit leads
The lower-level part of this system consists of a temperature controller, a data acquisition unit, an intermediate station, and a host computer monitoring unit.
The terminal controller (thermostat) collects valid signals from lower-level devices, such as temperature values, air conditioner on/off status, air conditioner cooling/heating status, and fan speed, and transmits them to the data acquisition unit via an RS485 serial bus. The data acquisition unit is responsible for data acquisition on one hand and receiving commands from the host computer on the other.
If there are a large number of data collectors, intermediate stations can be added. These have similar functions to the data collectors, including data acquisition and command transmission. For a single building, the intermediate station can achieve centralized control of the building's central air conditioning without an Ethernet interface. If there are multiple buildings, the intermediate station can be expanded with Ethernet interface modules to achieve remote centralized control of the central air conditioning systems in multiple buildings.
A remote computer is used as a client, and the data acquisition and monitoring are implemented using the visual programming software Visual Basic.
Overall Design of Four Systems
1 Network structure diagram
The network structure diagram of the remote monitoring system is shown in Figure 4.
Figure 4. Network structure diagram of the central air conditioning remote monitoring system
2 Hardware Selection
A high-end 8-bit Atmega series microcontroller from Atmel was selected, with expanded serial interfaces (RS485 interface) and Ethernet interfaces. The Ethernet controller used was the Microship ENC28J60. Based on centralized control of the central air conditioning in each building, a client/server architecture was adopted to achieve network control, enabling remote centralized control of the central air conditioning systems in multiple buildings.
Specifically, the temperature controller uses an Atmega8 chip and an 18B20 temperature sensor chip, with an additional digital tube display and button control; the data acquisition unit uses an Atmega162; and the intermediate station chips use an Atmega64 and an ENC28J60.
3. Software Configuration
The temperature controller, data acquisition unit, and intermediate station are all programmed in C language, while the upper-level monitoring part is programmed in VB.
4. Database Selection
Given the wide range of database options, this system uses the Access database included in the Microsoft Office suite to store the collected data, including the real-time temperature values, temperature setpoints, on/off status, fan speed settings (high, medium, and low), and the running time of each status for each air conditioner.
5. System structure diagram
(1) Thermostat layer
Figure 5.1 Temperature controller layer structure diagram
Temperature control at the terminal units (individual rooms) is achieved by a thermostat (as shown in Figure 5.1), and the design of the thermostat is one of the key aspects of the system. In this system, the temperature of each room in the central air conditioning system is controlled by switching the fan coil units on and off. The microcontroller's I/O controls three relays to achieve high-speed, medium-speed, and low-speed fan control, thereby regulating the temperature.
The buttons are mainly used for setting various parameters. There are 5 buttons: MODE, fan speed selection, power switch, and temperature adjustment button.
Figure 6. Actual picture of the thermostat
Button description (as shown in Figure 6): The MODE button is used to select the cooling or heating mode;
Pressing the fan speed control button 1, 2, 3, 4 times represents high speed, medium speed, low speed, and automatic operation of the fan.
The power button is used to control the operation and shutdown of the air conditioner;
Each press of the temperature up or temperature down button increases or decreases the set temperature by 1 degree Celsius.
The display uses an LCD screen to show the current temperature, set temperature, lock status, cooling/heating status, and fan speed. These parameters can also be set centrally by a higher-level administrator.
Chip selection: Due to the small amount of data collected, Atmega8 was selected.
(2): Collector layer
Figure 5.2 Data Acquisition Layer Structure Diagram
The data acquisition unit (as shown in Figure 5.2) is responsible for collecting data uploaded by the terminal temperature controllers and control commands transmitted from the intermediate station or host computer. The data acquisition unit has dual serial ports for communication with both the terminal temperature controllers and the intermediate station. In this layer design, one data acquisition unit is responsible for receiving data from eight terminal devices, functioning similarly to a hub. The data acquisition unit uses a polling method.
Chip selection: Considering that the data acquisition unit must communicate with the terminal and intermediate stations via RS485 respectively, the Atmega162 with dual serial port interfaces is selected.
(3) Intermediate station level
Figure 5.3 Intermediate layer structure diagram
The intermediate station layer (as shown in Figure 5.3) is responsible for communicating with the data collector and the host computer, receiving data from the data collector and transmitting control commands from the host computer. The intermediate station is added to enable remote monitoring of multiple buildings. By adding an Ethernet control module and embedding the TCP/IP protocol into the main chip, a good solution for long-distance data transmission is provided.
Chip selection: The Ethernet communication module is selected as ENC28J60. This Ethernet controller is compatible with IEEE 802.3, integrates a MAC and 10BASE-TPHY, and has only 28 pins, occupying a small footprint, as shown in Figure 7. The main chip is selected as Atmega64, with 64KB of FLASH, rich peripheral interfaces, and high cost-performance ratio.
(4) Upper computer layer
The host computer acts as a client, collecting data from various intermediate stations and remotely controlling the temperature values of each terminal. This enables temperature control of a single terminal, as well as temperature setting and data collection for one or more floors or an entire building, as shown in Table 1. An Access database is selected for data storage. Considering the 2GB storage capacity of an Access database, this choice is sufficient to meet the data storage requirements.
Figure 7 External Circuit of ENC28J60
Table 1 Functions implemented by the host computer software
1: Selectively set the air conditioner's on/off time. |
2: Fully set temperature, turn on/off, cooling/heating, and lock the unit. |
3: Single-transmission, full-transmission, full-reception data |
4: User Management |
5: Timed sampling (time needs to be determined experimentally), manual sampling. |
6: By building number, floor, and terminal, you can select to collect terminal temperature, fan speed, power on/off, and set temperature value. |
7: Tables and charts showing the time at each wind speed setting for each terminal. |
Five key technologies
1. Porting of TCP/IP Protocol
To enable communication between the intermediate station and the remote computer, and ultimately achieve remote control, an Ethernet communication module was added to the intermediate station, and the TCP/IP protocol was ported to the main chip Atmega64. This was one of the key design considerations.
2. Design of Terminal Temperature Controller
The proper allocation of button functions and their software implementation are the foundation for the completeness of system functionality.
3. Reliability and Scalability of Supervisory Control Software
To prevent prolonged standby or system crashes due to line faults or communication errors, the host software must include delay timeout checks and communication counting methods.
In addition, considering the differences in the number of floors, number of rooms, and number of floors in different situations, the design must be flexible.
VI. Conclusion
This paper proposes a cost-effective design scheme for a remote monitoring system for central air conditioning, which is particularly suitable for applications with a large number of terminal units, floors, or buildings. The design incorporates comprehensive fault handling measures, significantly improving the system's stability and practicality, and providing valuable insights for the design of remote monitoring systems for central air conditioning in building automation systems.
