Design of Automobile Air Conditioning Control System Based on ZigBee

Abstract: Aiming at the real-time requirements of automobile air-conditioning system, a car air-conditioning control system based on ZigBee technology is proposed. The network structure, sensor nodes and main node circuit of the control system are given, and the network protocol of the control system and the program flow of sensor nodes and main control nodes are designed. It provides a new technology for automobile air-conditioning control.

The automobile air conditioning system is a special device for adjusting the air environment in the vehicle to meet the requirements of the passengers for riding comfort [1]. The comfort of the passengers in the vehicle is related to many factors such as the relative humidity, temperature, air flow rate and surface temperature of the objects in the vehicle [2]. Therefore, the automobile air conditioning system needs to sense various environmental parameters inside and outside the vehicle and make corresponding adjustments according to the requirements. However, the automobile air conditioning system is affected by factors such as vehicle space and vehicle working conditions, and works in a harsh environment and under a large workload. Therefore, it has high requirements for the anti-interference and stability of the control system. Generally, traditional automobile air conditioning systems are controlled manually or automatically, and the control lines of these control methods are connected by wire. Due to the large number of air conditioning control functions, the system control lines are complex, the control accuracy is also low, and there are disadvantages such as high cost, complex monitoring system, and poor anti-interference, which still greatly affects the feasibility and control accuracy of the air conditioning control system.

ZigBee technology[3] is an emerging intelligent sensor and control technology, which is a combination of sensor technology and wireless network technology. It is widely used in environmental monitoring and industrial control. Due to its low cost, small size, strong real-time performance, low power consumption, strong anti-interference, and good embeddability[4], it is particularly suitable for use in harsh working environments, small data transmission volume, strong real-time performance, short transmission distance, and inconvenient wiring. In the automotive air-conditioning control system, ZigBee technology is used to collect and transmit environmental parameters of the air-conditioning system and transmit and control control signals, avoiding the interference and influence of the harsh industrial field environment on the wired transmission method (such as electromagnetic interference, humidity, vibration, etc.), improving the reliability and anti-interference ability of the control system, and having certain significance in reducing the energy consumption of the automotive air-conditioning system and improving the comfort of passengers.

In this paper, a car air conditioning control system based on ZigBee is designed according to the requirements of the car for the air conditioning system, which greatly reduces the cost and complexity of the control system, reduces the energy consumption of the system, and improves the control accuracy and feasibility of the system.

1 System Design

1.1 Control system design

The principle of the automobile air conditioning control system is shown in Figure 1. The system consists of sensor nodes, system main control nodes, action nodes, etc. Sensor nodes include in-car temperature and humidity sensors, outdoor temperature sensors, CO2 concentration sensors, light sensors, various damper sensors, etc. Action control nodes include compressor control nodes, blower control nodes, defrost dampers, mixed dampers, circulation dampers, and other damper control nodes, warm water valves, and humidifier control nodes. The system processes the monitored temperature, humidity, CO2 concentration, and in-car and out-of-car environment data through sensor nodes, converts them into digital signals, and sends them to the main control node, which performs corresponding processing and then sends the corresponding instructions to the action node to control the temperature, humidity, and air quality in the car within the set range.

When designing the system, the system network structure is a star topology, the system master control node is the network controller, and the other nodes are slave nodes. The network topology is shown in Figure 2. The master node is set as a full-function node (FFD), which is responsible for the management and control of the system; the sensor and control nodes are set as reduced function nodes (RFD), which are responsible for environmental parameter data collection and air conditioning system control.

1.2 System circuit design

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The circuit design of the automobile air-conditioning control system includes: (1) environmental data acquisition circuit, including the in-vehicle temperature, humidity and CO2 concentration acquisition nodes, light acquisition nodes, outside temperature acquisition nodes, damper position detection nodes, etc.; (2) air-conditioning system working and control circuit, including compressor working control node, evaporator and condenser fan control node, damper position control node, defrost control node, humidification control node, heating control node and other circuits; (3) main control node circuit, mainly including control and display circuit, operation control circuit, etc.

The basic circuit of the in-vehicle environmental parameter sensor is shown in Figure 3. The sensor circuit consists of a CO2 concentration sensor and signal amplification circuit, a temperature and humidity sensor, a power supply circuit, and a CC2430 processor. The power supply voltages are 5 V and 3 V respectively. The CO2 concentration detection uses the TGS4161 sensor, which has the characteristics of small size, long life, good selectivity and stability, and is also resistant to high humidity and low temperature. It can be widely used in automatic ventilation systems or long-term monitoring of CO2 gas [5]. The weak voltage output by the CO2 sensor is amplified by the amplifier U3 (LM386) and output to P0_2 of U5 for A/D conversion and stored in the storage unit specified by CC2430. PR1 adjusts the gain of the amplifier so that the concentration output signal voltage varies between 0 and 3 V. In order to keep the sensor at the most sensitive temperature, it is necessary to provide a heating voltage to the heater for heating.

The temperature and humidity detection uses the digital temperature/humidity sensor SHT75[6](U6), which has the characteristics of small size, simple and reliable, low price, digital output, no debugging, no calibration and strong interchangeability. It integrates A/D converter and memory, and can automatically calibrate the relative humidity during the measurement process. The DATA and SCK pins of U6 are connected to the P0_0 and P0_1 pins of U5 respectively. The SCK pin of U6 is controlled by P0_1 of U5 to decide whether to read the temperature or humidity data from the memory of U6, and then store the temperature/humidity parameters in the storage unit specified by CC2430. The illumination detection uses an illumination measurement circuit composed of photodiodes. The photosensitive element D1 is input to P0_3 of U5 through the U4 amplifier for A/D conversion.

The basic circuits of the evaporator, condenser, ventilation fan motor and compressor control node are shown in Figure 4. After receiving the control signal from the main controller, U2 calls the motor speed control interrupt program in the processor, outputs a control signal with a certain duty cycle from P0_0 through the photocoupler U3, controls the conduction current of Q1, and thus controls the speed of the air supply motor.

The main control node circuit of the system is shown in Figure 5. The circuit mainly consists of a main node processor, control buttons and parameter display circuit.

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2 System Software Design

2.1 Network Protocol and Data Frame Design

Through the application analysis of this system, the ZigBee protocol is simplified to save the program storage space of the node. The security mechanism is omitted in the system protocol, the device type of the FFD node is set to a full-function node without a security mechanism (FFDNS[5]), the device type of the RFD node is set to a reduced-function node without a security mechanism (RFDNS), and the primitives in the system node that are not related to this application are omitted to improve the protocol efficiency. The sensor and control node protocols and implementation primitives are shown in Figure 6.

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The data frame format of the system data transmission uses the ZigBee MAC layer packet format [6], and its structure is shown in Figure 7. The frame payload in the data packet is defined as: slave node number + node type + detection parameters (or action instructions), where the slave node number is bound to the sensor or control node ID.

Figure 7 Digital frame structure

2.2 System Software Design

The control system program design uses a modular programming method, which consists of a main control module and a wireless node module. The system software flow is shown in Figure 8, and the wireless network node flow chart is shown in Figure 9.

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3 System Test Results and Discussion

After the design of each node of the system was completed, the system designed in this paper was tested in a certain model of minibus, and the test results are shown in Table 1. The test results show that the system parameters meet the requirements. Since there are various electromagnetic interferences at the system working site, corresponding anti-interference measures need to be taken at each node, such as taking corresponding anti-interference measures for the system power supply.

As an emerging monitoring technology in recent years, wireless sensor networks have been widely used in industrial automatic process control. The automobile air conditioning control system based on ZigBee wireless sensors designed in this paper has the characteristics of low cost, reliable operation, strong real-time performance, and less wiring. It is of certain significance to improve the feasibility and stability of automobile air conditioning control systems, improve control accuracy, and reduce air conditioning system energy consumption. The air conditioning control system based on ZigBee has good application prospects in automobile air conditioning systems due to its low cost, good real-time performance, and strong embeddability.

References

[1] Zhang Lei. Automobile Air Conditioning[M]. Beijing: Machinery Industry Press, 2006: 3-10.

2] He Ping. Design of a new type of automobile air conditioning energy-saving controller [J]. Application of Electronic Technology. 2008, 34(1): 47.

[3] Zigbee Alliance. Network Specification(Draft Version 1.0), [EB/OL].2004.http://www.ZigBee.org. 2009-02-13.

[4] Ren Xiuli, Yu Haibin. Research on security of wireless sensor networks based on ZigBee technology[J]. Chinese Journal of Scientific Instruments, 2007, 28(12): 2133.

[5] Zigbee Alliance. Network Specification(Draft Version 1.0), [EB/OL]. http://www.ZigBee.org. 2004.

[6] IEEE. 802.15.4-2003.pdf [EB/OL]. IEEE Standards Association, 2003. http://standards.ieee.org/getieee 802/download/802.15.4-2003.pdf.2009-03 -16.