Application of CAN bus in central air conditioning control system

    Abstract: This article introduces the central air-conditioning control system based on CAN bus, and analyzes and discusses the overall design scheme and software and hardware design of the system. The system can achieve high-precision automatic control of temperature and humidity.

    Keywords: field bus CAN bus microcontroller control system

The central air conditioning control system is an indispensable component of smart buildings. The traditional control method is to use DDC (Direct Digital Controller) method to connect each temperature and humidity detection point and control point to one or more DDCs to implement multi-point real-time monitoring. Since modern smart buildings have many floors, multiple air conditioning fans are located on different floors, and temperature and humidity detection points are distributed in various rooms. The use of DDC control has shortcomings such as too long leads, inconvenient construction, and low real-time and reliability of system communication. .

Among various communication methods, fieldbus technology for industrial control is currently the best solution to solve the problem of on-site data communication in industrial control. Fieldbus technology is an advanced field industrial control technology developed in the late 1980s. It integrates digital communications, intelligent instruments, computer technology, and network technology, fundamentally breaking through the limitations of traditional "point-to-point" analog signal or digital-analog signal control, and becoming a true "distributed control, centralized Management" provides technical assurance.

The communication protocol structure of the fieldbus is formulated based on the open system interconnection model (ISO/OSI) provided by the International Organization for Standardization. The CAN bus used in this system is one of the earliest field buses applied in my country. It adopts the physical layer and data link layer in the ISO/OSI seven-layer framework. The CAN bus standard adopts a multi-master mode. Any node on the network can actively send information to other nodes. The network work points can be divided into different priorities according to the real-time requirements of the system. The data link layer adopts a short frame structure, each frame is 8 bytes, which is easy to correct errors. Frames that lose arbitration or have errors during transmission can be automatically retransmitted, and faulty nodes can automatically leave the bus. The CAN bus standard supports full-duplex communication. The transmission medium uses twisted pairs and optical fibers. The transmission rate can reach 1Mbps and the number of nodes can reach 110. Its biggest feature is that it abolishes the traditional station address encoding and replaces it with encoding communication data blocks. It has strong fault tolerance and anti-interference capabilities, and high transmission security.

1 The overall composition of the central air conditioning control system

The overall block diagram of the central air conditioning control system is shown in Figure 1. In the figure, the host computer uses an IBM-PC compatible computer, which is responsible for the reception and management of system control, the sending of control commands, and the real-time display of the system's working process. The controller of each unit serves as the lower computer and uses the AT89C51 single-chip computer produced by ATMEL Company as the microprocessor. It is responsible for the on-site data detection and working status control of the air-conditioning fan unit in the unit. The CAN bus controller SJA1000 of the unit controller is responsible for receiving data from the CAN bus and sending data to the host computer through the CAN bus. The host computer connects to the CAN bus through an intelligent CAN bus communication adapter card inserted in the PC bus expansion slot, and is connected to each unit controller through the CAN bus. The unit controller can also be separated from the host computer and directly perform on-site manual control.

The working principle of the system is: each unit controller performs patrol inspection on each detection point of the unit, and sends the detection data to the host computer in accordance with the CAN bus protocol standard; the host computer receives the data uploaded by each unit controller through the intelligent CAN bus communication adapter card. Data is sent to each unit controller according to the operator's instructions or the system software pre-selected control program, and the unit controller controls each air conditioning fan unit in real time. If separated from the host computer, the unit controller will directly automatically control the air conditioning fan unit according to the control parameters set by the software. The operator can modify the control parameters on site through the small keyboard on the unit controller.

2 Hardware design

The system hardware mainly includes intelligent CAN bus communication adapter card and unit controller. Figure 2 is the schematic block diagram of the intelligent CAN bus communication adapter card. It provides an interface between the host microcomputer and the CAN bus, using high-performance embedded microprocessor 80C188, CAN bus controller 82C200 and CA line transceiver 82C250 to be responsible for data exchange and communication processing. 82C200 is a product of PHILIPS Company, which can complete all functions of the physical layer and data link layer. The application layer of the electronic control unit (ECU) is provided by the microprocessor, and the 82C200 provides a multi-purpose interface for it. As a data sharing area between PC and CAN bus controller, dual-port RAM IDT7230 can provide two mutually independent ports. Each port has its own address line, data line and control line, and has two sets of mutually independent interrupts. Logic to implement handshake control signals between two CPUs. Through software and hardware settings, the dual-port RAM is mapped into the physical memory of the PC, so that sending and receiving data is equivalent to reading and writing data directly to the memory, thereby improving the data exchange rate and ensuring that two CPUs read and write the same memory unit at the same time. Data correctness.

In addition, the intelligent CAN bus communication adapter card also has functions such as interrupt selection, main memory base address selection, LED indication of system status and CAN transceiver status.

When the system is working, the host computer sends control commands to the intelligent CAN bus communication adapter card via the ISA bus, writes them into the dual-port RAM through the driver circuit, and then sends out an interrupt signal. After receiving the interrupt signal, the CAN communication controller 82C200 takes out the data from the dual-port RAM and sends it to the CAN bus through the output drive circuit, photoelectric isolation circuit and CAN bus transceiver 82C250 according to the CAN bus protocol standard, completing the process from the host computer to Data exchange for unit controllers. The process of receiving data by the host computer is similar to that of sending commands, but in the opposite direction.

Figure 3 is a schematic block diagram of the unit controller circuit. In the picture, the microprocessor is AT89C51 microcontroller produced by ATMEL Company, the CAN bus controller is SJA1000, and the CAN bus transceiver is still 82C250 to control the data exchange of the CAN bus. When working, the CAN bus controller SJA1000 receives commands and data from the host computer from the CAN bus and notifies the CPU89C51 in an interrupt manner. After the CPU receives the interrupt signal, it stores the data received by the SJA1000 into the RAM, and issues corresponding commands to the I/O interface circuit based on the data to control the fan unit, humidifier and other execution components to perform corresponding operations. If the host computer needs the status information of each unit, the CPU starts the data acquisition program and controls the I/O interface circuit to perform patrol inspection on the data of each detection point, and sends it to the CAN bus by the CAN bus controller, and is received by the host computer. The I/O interface circuit can provide 8 digital inputs, 8 digital outputs, 16 analog inputs and 4 analog output interfaces, which are used to connect various detection points and control points of the unit.

The hardware watchdog circuit X25045 is used for system monitoring to prevent programs from running away. It also provides a 512-byte EEPROM to protect important system control parameters and improves the anti-interference ability of the unit controller.

The digital display can display the fan's mutual return air humidity, supply air temperature, return air humidity, frequency converter frequency, damper opening, water valve opening, type and number of alarm signals and other system status information. The small keyboard is used to modify the setting parameters and change the display type, so that the unit controller can still complete the control function without being separated from the host computer.

3 Software design

The system software consists of upper computer management software and unit controller control software. The host computer management software was developed using Visual Basic 6.0 on the Windows 98 operating platform. It includes modules such as system monitoring, communication management, data processing, control commands, and dynamic display. It has the advantages of friendly interface, intuitive display, and easy operation. When the system is running, the positions of each detection point and control point are graphically displayed on the host computer monitor. The detection and control data are dynamically displayed next to their respective positions. The operator can control the entire system after simple training. The unit controller control software is programmed using 8051 assembly language. It is solidified in the EEPROM of Wei 89C51 and mainly completes functions such as data acquisition, data communication, I/O interface control, and digital display control. According to the different requirements for indoor temperature and humidity in each season, the control parameters for different seasons are pre-selected and set in the software, and can be modified at any time through the small keyboard. During actual operation, the humidity control accuracy reaches ±0.5°C and the humidity control accuracy reaches ±2%RH.

Using CAN bus for communication between the upper and lower computers of the central air conditioning control system can greatly improve the reliability, real-time and scalability of the system, achieve high-precision humidity and humidity control, and has broad prospects for promotion and application.