Design of cold storage temperature monitoring system based on single bus

With the progress of the times, people's requirements for the quality of refrigerated and frozen foods are constantly increasing. The changes in food appearance and nutritional content are closely related to the refrigeration and freezing temperatures. Different foods have their corresponding optimal refrigeration or freezing temperatures; different storage times require different storage temperatures. Therefore, it is very necessary to develop a cold storage temperature monitoring system that meets practical needs.

Based on the single bus technology, this paper uses the computer serial port, single bus and serial port protocol conversion chip DS2480B, and single bus digital temperature sensor DS18B20 to design a single bus temperature detection network. The system can meet the actual needs of various types of refrigerated and frozen foods, save electricity and manpower, and has functions such as temperature alarm, data storage, drawing dynamic and historical temperature curves, and modifying sensor data. It can effectively improve the operating efficiency of cold storage and has high practical value.

1 Composition of cold storage temperature monitoring system

The system mainly consists of five parts: DS18B20 digital temperature sensor module, single-chip module, RS-485 transmission bus module, RS-485/RS-232 conversion interface module, and upper PC display and control module, as shown in Figure 1. The temperature data is collected by DS18B20 and sent to the lower computer AT89C52. After being converted into RS-485 format, it is transmitted over long distances using the 485 bus. The upper computer receives the temperature data through the RS-232 interface. The control information is also transmitted in the reverse direction through the same channel.

2 Cold Storage Temperature Collection System Hardware Design

2.1 Design of cold storage temperature detection and acquisition circuit

AT89C52 is a low-power, high-performance CMOS 8-bit microcontroller with 8K in-system programmable Flash memory.

Flash is quite flexible and effective.

DS18B20 uses direct VCC power supply. The power supply voltage is the same as the microcontroller power supply. The data collected by DS18B20 is transmitted to the microcontroller through a data line, and the microcontroller's read/write operations on DS18-B20 are also implemented through this pin. Due to the design requirements, 5 temperature measurement points are placed in each cold storage, so 5 temperature sensors are connected to a single bus, and 5 temperature data are measured. The average value is calculated by the PC host software as the temperature of the cold storage. As shown in Figure 2.

Design of RS-485 bus and its interface circuit with single-chip microcomputer#e#2.2 Design of RS-485 bus and its interface circuit with single-chip microcomputer

A 485 bus is added between the lower MCU and the upper PC. The MAX485 chip is used in the MCU system to complete the conversion between the MCU TTL level and the RS-485 level.

Connect a matching resistor at the end to absorb the reflected signal on the bus, so that the normal transmission signal is free of burrs. As long as multiple cold storage temperature measurement system circuits are connected in parallel on the RS-485 bus, multiple cold storages can be monitored in real time at the same time.

2.3 Design of RS-232/RS-485 converter circuit

This system converts the RS-232 interface into the RS-485 interface, and then adds the RS-485 bus between the microcontroller and the PC. RS-485 is used for long-distance, high-speed serial asynchronous communication. It is only necessary to convert the TTL level serial interface into the RS-485 serial interface through the chip. This system uses the MAX485 chip to complete this conversion. For the existing RS-232 interface of the PC, the system uses a conversion circuit composed of a MAX485 chip and a MAX232 chip to realize the conversion between RS-485 level and RS-232. This conversion circuit is connected to the RS-232 standard 9-pin interface on one side and to the RS-485 bus on the other side. As shown in Figure 3. This allows the data to be transmitted to the host computer very well.

3 Design of cold storage temperature collection sub-software

This system adopts single bus device design. Although the hardware circuit design is simple, it comes at the cost of complex software. The software writing must strictly comply with the single bus protocol, otherwise it is easy to make mistakes. The flow chart of the temperature acquisition subroutine is shown in Figure 4.

4 Conclusion

The cold storage temperature monitoring system uses DS18B20 for temperature measurement and AT89C52 microcontroller for temperature control. It can achieve temperature control between -55 and +125℃, thereby realizing real-time temperature acquisition, display and control functions. It is an ideal intelligent control system. Suitable for communication distances of tens of meters to thousands of meters. The RS-485 serial bus uses balanced transmission and differential reception, so it has the ability to suppress common-mode interference. RS-485 can be used to form a distributed system, which allows up to 32 drivers and 32 receivers to be connected in parallel. At the same time, the system has strong scalability. When the temperature monitoring points increase, whether adding temperature sensors or adding data collectors to the system can be directly connected. Moreover, the system temperature measurement is more accurate, and multi-point temperature measurement avoids the error of treating local temperature as the overall temperature. In short, the system displays the real-time temperature of each warehouse and each monitoring point in a graphical way, which is convenient for corresponding to the site and has a high use value.