Design and implementation of indoor temperature control system based on ARM

Today, when we are vigorously advocating energy conservation and emission reduction and pursuing high-quality life, the shortcomings of winter heating systems are becoming increasingly apparent. Most cities in northern China use centralized heating. During the entire heating period, the system provides continuous heating all day long, regardless of whether there are people indoors or not. The system's heat energy transmission is constant and cannot be adjusted accordingly according to the changes in indoor and outdoor temperatures and the different requirements of individuals for room temperature. This has caused serious waste of heat energy and inhumane heating. This paper designs and implements an indoor intelligent temperature control system based on the embedded system ARM-Linux platform. By real-time detection of indoor and outdoor temperature changes, CONTROL ENGINEERING China Copyright, it reasonably adjusts the indoor temperature, reduces heat energy consumption, and improves people's quality of life.
1 System Function and Overall Structure
The hardware block diagram of this system is shown in Figure 1. It is mainly composed of an ARM9 controller, a temperature detection module, a touch screen display module, a ZigBee wireless transceiver module, a heating control module, a GPRS module, etc.

Figure 1 System hardware block diagram

The temperature detection includes outdoor temperature detection, indoor temperature detection and heating temperature detection. In order to avoid cumbersome wiring, each temperature detection point is wirelessly connected to the ARM controller through Zigbee technology to form a star wireless network. The temperature value of each detection point is transmitted to the ARM controller through Zigbee wireless. The ARM controller performs comprehensive processing and analysis based on the received temperature values ​​of each point, and outputs the corresponding control signal to the heating control module, thereby realizing intelligent adjustment of the room temperature. The information display and input module is implemented by an LCD touch screen, which is used to display the current indoor temperature and the input temperature value, and can set different working modes such as low temperature and room temperature. The ARM controller is wirelessly connected to the outside through GPRS, and the user can remotely control the system's working mode at any time through a mobile phone. For example, on the way home, the user can switch the system working mode through the mobile phone. When returning home, the room temperature has returned to normal temperature, which brings great convenience to the user.
1.1 ARM intelligent control module
The ARM intelligent control module consists of an ARM9 controller, FLASH, SDRAM, power and reset module, LCD touch screen and related peripheral circuits. The system uses the S3C2440 processor based on the ARM920T core of SAMSUNG as the controller. The S3C2440 processor is powerful, cost-effective, and low in power consumption. In addition to the bus and SDRAM controller of general embedded chips, it also has a rich expansion function interface, which is convenient for building peripheral circuits.
The LCD touch screen uses the TFT LCD module TD035STEB1, which uses LTPS TFT as a switching device Copyright by Control Engineering Network , integrates a four-wire resistive touch screen and backlight circuit, thereby simplifying the peripheral circuit. The system establishes a graphical user interface (GUI) based on QT/Embedded on the basis of the touch screen to realize information display and control input, optimize the human-computer interaction environment, and bring great convenience to users.
1.2 Temperature detection and transmission module
The temperature detection and transmission module consists of a temperature detection module and a Zigbee wireless transmission module. The temperature detection module uses a digital temperature sensor DS18B20. Its measurement range is -55℃~125℃, and within the range of -10℃~85℃, the accuracy is ±0.5℃, which fully meets the requirements of this design. ZigBee is an emerging short-distance, low-power, low-cost two-way wireless communication technology www.cechina.cn , which is very suitable for building a small wireless network. The ZigBee module uses the CC2430 chip, which supports the IEEE802.15.4 protocol and has mature technology. Its high-performance processing capability and rich interface resources bring great convenience to the hardware design work.
In the temperature detection module, the indoor temperature detection places multiple temperature sensors in different rooms in the room to detect the temperature of multiple locations in the room; the outdoor temperature detection places the sensor outdoors to detect the outdoor temperature. The sensor for heating temperature detection is placed on the outer wall of the heating water pipe to detect the temperature of the hot water in the water pipe. The indoor, outdoor and heating temperature information is transmitted to the ARM controller through Zigbee wireless. After comprehensive processing and analysis, the ARM controller gives the heating control module the best control amount to achieve intelligent control of the indoor temperature.
1.3 Heating control module
The heating control module uses a digital flow valve as an actuator. A digital flow valve is a valve that controls liquid flow, with high controllable flow resolution and fast response speed; the drive signal is a binary signal that can be directly connected to the ARM controller. The ARM controller outputs the corresponding binary signal to control the digital flow valve based on the received temperature values ​​of each monitoring point and the input control information, thereby adjusting the flow of heating hot water and realizing intelligent adjustment of indoor temperature.
1.4 GPRS module
GPRS, or general packet radio service technology, is a data transmission technology based on GSM. Users are always online and billed by flow and time, and the communication cost is low, making GPRS technology the best choice for wireless data transmission in home intelligent control systems. The main function of the GPRS module is to realize data exchange between the ARM controller and the owner's mobile phone through the GPRS network. After comprehensive consideration of performance and cost, the system uses Siemens' MC55 GPRS module.
2 Design of adaptive fuzzy controller
Because the indoor temperature system is a large pure lag system and it is impossible to establish an accurate mathematical model, this system uses fuzzy control technology to control the indoor temperature to improve the control accuracy of the indoor temperature. For the indoor temperature system, as the outdoor temperature and heating temperature change, the original perfect fuzzy control rules may not be suitable for the new environment after the change, resulting in poor control effect. Therefore, this design adopts an adaptive fuzzy control system to adapt to the changing environment. The structure of the adaptive fuzzy control system is shown in Figure 2.

Figure 2 Adaptive fuzzy control system [page]

The adaptive fuzzy control system can continuously and automatically measure the dynamic characteristics of the controlled object and compare them with the dynamic characteristics of the ideal model, and then use the difference between the two to change the adjustable parameters such as the proportional factor and the fuzzy control rule to optimize the system performance. This system uses the method of dynamically changing the relevant proportional factors to realize the self-adjustment of the fuzzy control system. As shown in Figure 2, in order to reduce the complexity of the fuzzy controller and reduce the amount of calculation, the system adopts a two-input single-output model. The deviation e and the deviation rate ec between the system set temperature value T0 and the current indoor temperature value T are input variables, and the control quantity u is the output variable. The outdoor temperature value T1 and the heating temperature value T2 are the system reference quantities. e= T0-T, ec=de/dt.ke and kec are the proportional factors of temperature deviation and deviation change rate, respectively, and ku is the proportional factor of the control quantity. The system conducts comprehensive analysis and performance identification based on the changes of e and ec and refers to the values ​​of T1 and T2, and then dynamically adjusts the proportional factors ke and kec, thereby realizing the self-adjustment of the fuzzy control system to adapt to the changing environment.

Figure 3. Adaptive fuzzy control system simulation waveform

According to expert knowledge and actual tests, appropriate domains, membership functions and related proportional factors are selected, reasonable fuzzy control rules are established, and a system simulation model is established under Simulink in Matlab7.1.
The system setting temperature is 24℃, and the output waveform is shown in Figure 3. The overshoot does not exceed 0.5℃. In the indoor heating environment with large pure lag, the control quality is quite good. The actual test shows that the system has good control effect and strong robustness.
3 System software design
The design of the system software part is mainly based on ARM-Linux. Compared with other embedded operating systems, the Linux operating system has a complete TCP/IP protocol, good stability and real-time performance, which well meets the requirements of intelligent control systems for system reliability; in addition, Linux is easy to transplant and cut, has a small kernel, high efficiency, open source code and many developers, providing good technical support for system development.
System development first establishes a cross-compilation environment, then boots the bootloader, transplants the operating system, loads the file system, develops a graphical interface, and finally writes an application. This system uses the Linux 2.6 kernel, which has powerful process, interrupt, memory and device management functions and supports various file systems. The system uses a graphical user interface based on QT/E. Q/E continues all the functions of Qt in the desktop system. The rich API interface and component-based programming model make the application development in the embedded Linux system more convenient.
The system program flow is shown in Figure 4. The system application is mainly composed of a series of subroutines used to implement corresponding functions, including temperature detection program, ZigBee wireless communication program, fuzzy control program, GPRS wireless communication program, etc.

Figure 4 System program flow

4 Conclusion
This paper introduces an indoor intelligent temperature control system based on ARM, gives a detailed system architecture scheme, and explains the design ideas and implementation methods from both software and hardware aspects. The system uses ZigBee technology to build a small wireless network, realizes multi-point temperature collection, and avoids cumbersome wiring; uses fuzzy control technology to improve the control accuracy of indoor temperature; establishes a QT user interface to optimize the human-computer interaction environment; and uses GPRS technology to achieve remote control of the system. This system makes up for the shortcomings of winter heating systems in northern China. With the gradual implementation of metered heating in China, it has good practical application value.