Design of wireless temperature sensing system based on Si4432 fire monitoring system

In recent years, with the substantial increase in electricity consumption, the incidence of electrical fires has ranked first among all types of fires. The main causes of electrical fires are as follows: 1) leakage; 2) short circuit; 3) overload. Electrical fires caused by heating of power equipment account for more than 60% of all electrical fires; in addition, leakage current faults caused by damaged insulation, humid environment, etc. (as well as leakage current sparks) account for more than 10% of the causes of electrical fires.
When the leakage current generated by the damaged insulation line reaches 300-500mA, the damaged part will produce spark discharge with the adjacent grounding conductor. At this time, the released electric sparks can easily ignite the surrounding combustibles and flammables, thus causing fire accidents. Even if there are no flammable or combustible materials around, the spark discharge generated by the leakage current can also accelerate the damaged area of ​​insulation, causing the insulation layer around the damaged point to carbonize rapidly and become a flammable object, which is then ignited by the spark and caught fire.
The high-voltage switchgear is equipped with a large number of switch electrical equipment and has complex electrical connection lines. How to conduct real-time and remote detection of the temperature of key parts or electrical connections in the high-voltage switchgear and ensure the safety and reliability of the detection is one of the key technical problems we need to solve.

1 System composition
In order to solve the above technical problems, a wireless temperature sensing system of a fire monitoring system with a simple structure, high safety and reliability is provided. The wireless temperature sensing system of the fire monitoring system includes: a temperature sensor 1, a first logic control circuit 2 connected to the temperature signal output end of the temperature sensor 1 and suitable for timed activation, a first wireless transceiver circuit 3 for converting the temperature signal into a wireless signal connected to the temperature signal output and transceiver control end of the first logic control circuit 2, and the first logic control circuit 2 is connected to a battery power supply circuit 4. The
temperature sensor 1 is arranged at the upper and lower interfaces of the circuit breaker of the incoming cabinet in the power distribution room, the upper and lower interfaces of the three-stage load incoming switch circuit breaker, the upper and lower interfaces of the fan circuit breaker, the upper and lower interfaces of the chiller circuit, or the live contacts in the switch cabinet. Application fire The wireless temperature sensing system of the disaster monitoring system also includes a second wireless transceiver circuit 11 for receiving the wireless signal, a wireless interactive control unit 5 connected to the second wireless transceiver circuit 11, and a central control unit 6 connected to the data output end of the wireless interactive control unit 5 for judging whether the temperature measured by the temperature sensor 1 exceeds the preset range; the temperature signal output end of the central control unit 6 is connected to the host computer 10 for remote monitoring via the communication interface 8.
The temperature display output end of the central control unit 6 is connected to a display circuit 9 for real-time display of the temperature value, that is, an LCD display screen. The control input end of the central control unit 6 is connected to a first keyboard input circuit 7 for inputting the frequency of wireless transceiver and the address table associated with the first logic control circuit 2. The specific transceiver block diagram is shown in Figure 1.

2 Design Principles
2.1 Hardware Design
The wireless temperature sensing system of the fire monitoring system is mainly composed of the microprocessor C8051F930 (MCU for short) and the RF transceiver chip Si4432. The performance indicators show that it is the industry's "highest" embedded wireless solution in terms of cost performance. C8051F930 is the industry's first MCU that can work in the voltage range of 0.9 to 3.6 V recently launched by Silicon Labs. It integrates 64 kB of Flash, 4352B of RAM, 1 UART, 2 SPIs, 4 timers, 10 b ADC, internal oscillator, etc., providing a true SOC (System On Chip) solution.
The Si4432 wireless transceiver is a very representative chip in the EZRadloPro product line of Silicon Labs, supporting a frequency range of 240 to 930 MHz, with a maximum output power of +20 dBm and a sensitivity of -117 dBm. The communication between Si4432 and host MCU is realized through SPI bus, mainly involving SCLK, SDI, SDO and nSEL4 pins.
Si443091/32 provides advanced radio features including frequency coverage in the range of 240-960 MHz and 156 Hz or 312 Hz steps that allow precise tuning control. Other system features such as automatic wake-up timer, low battery detector, 64-byte TX/RX stack, automatic datagram processing, header detection reduce the total current consumption, making it possible to use inexpensive system MCU. An integrated temperature sensor, general-purpose ADC, power-on reset (POR) and GPIO (general-purpose input/output) further reduce the total system cost and scale. [page]

Si4432 is controlled by C8051F930 microcontroller to realize wireless data transmission and reception. C8051F930 in the sending module transmits data to Si4432 for encoding and processing, and sends it to the receiving module via the antenna in a specific format. After the receiving module amplifies and demodulates the received RF signal, it sends the data to the main controller C8051F930 for corresponding processing, such as sending it to the LCD display. The system provides human-machine interaction interfaces such as buttons and LCDs, and also has an RS232 interface for communication with a PC.
The data transceiver circuit of the wireless temperature sensor system of the fire monitoring system is shown in Figure 2, where U9 is S14432. Due to the use of a single antenna, the RF transceiver conversion switch UPG2214T (U6 in Figure 2) is used for transceiver switching.

2.2 Software Design
The system software design uses μC/OS-Ⅱ or RL-RTX real-time operating system (RTOS) to realize multi-tasking embedded program applications. Using RTOS can simplify task scheduling and maintenance, and flexibly configure system resources such as CPU and memory. Program development, compilation and simulation use the new version μVision 4 of ARM's RealView MDK development tool set. The software adopts a modular structure design and can be divided into automatic calibration module, data sampling and processing module, data communication module, and signal generation and shifting module. The structure is clear and reasonable, which is convenient for development, maintenance and future upgrades. At the same time, it helps to enhance the portability of the software and improve development efficiency.

3 Features
In the wireless temperature sensing of the fire monitoring system, the first logic control circuit sends the temperature information measured by the temperature sensor in the form of a wireless signal by controlling the first wireless transceiver circuit. The wireless interactive control unit receives the wireless signal through the second wireless transceiver circuit, obtains the corresponding temperature information after processing and sends it to the central control unit, and finally displays it through the display circuit or sends it to the host computer through the communication interface; therefore, the wireless temperature sensing system of the fire monitoring system has a simple structure, high safety and reliability, and is suitable for monitoring the temperature of contacts in high-voltage switch cabinets. The wireless temperature sensing system can continuously monitor the operating temperature of the equipment contacts, determine the degree of overheating at the contact joints, and issue an alarm indication when overheating occurs. The temperature sensor is installed on the live parts of the equipment (such as the upper and lower interfaces of the circuit breaker and the knife switch), and the digital panel meter of the monitor host displays the temperature value of each live contact, and the temperature value can be transmitted to the host computer through the RS-485 interface for further processing, so as to realize the centralized monitoring of the equipment temperature. The wireless temperature sensing system of the fire monitoring system can be applied to places such as subways, substations, power plants, cables, tunnels, heating pipelines, oil and gas pipelines, etc., especially the equipment safety monitoring of 1500 V DC switches, rectifier systems and 400 V important loads in subways. For example, it is used to monitor the temperature of high and low voltage transmission cables and busbar joints, the temperature of high and low voltage (500 kV~380 V) switch contacts, the temperature of oil pipelines and petrochemical process points, and the temperature monitoring of dry-type transformers/oil-type transformers.

4 Conclusion
The wireless temperature sensing system of the fire monitoring system, in which the main chip of the wireless transceiver system uses Si4432, has the advantages of power up to 100 mW (+20 dBm), and the unobstructed transmission distance can reach more than 500 m when the baud rate is reduced, or penetrate two floors. The wireless temperature sensing system of the fire monitoring system has the advantages of simple structure, high safety and reliability, and wide range of application. It can provide accurate information for maintenance engineers to avoid the occurrence of electrical fires. The design scheme should have a wide range of applications and good application prospects.