Design of temperature/humidity monitoring system based on intelligent sensor

1 Introduction

From the time the ammunition leaves the factory to the time it is used by the troops for combat or training, it is mainly stored in warehouses. During the long-term storage process, although the ammunition is in a macroscopic static state, its quality is constantly changing. This is because the ammunition is mainly composed of metal and charge. During the long-term storage process, due to the influence of temperature and humidity, the metal will rust, thereby reducing the strength of the metal parts. The surface mechanical properties deteriorate, seriously affecting the use of the ammunition, and even causing major accidents. Temperature and humidity will also change the physical and chemical properties of the ammunition charge, thereby causing it to lose its due combat technical requirements. Therefore, temperature and humidity are the main contradictions in the change of ammunition quality. According to the warehouse conditions of our army, the climatic conditions of our country and the current management level of the warehouse, it is usually stipulated that the maximum temperature does not exceed 30℃ and the maximum humidity does not exceed 70%, which is commonly referred to as the "37" line.

The monitoring system of the traditional ammunition warehouse adopts a monitoring system composed of dry-bulb hygrometers, hair hygrometers, hygroresistors or ordinary temperature and humidity sensors, which are usually complex in structure, large in system and poor in accuracy. The author uses the SHT15 ultra-small, self-calibration, multi-functional intelligent sensor launched by Sensirion, Switzerland. It measures parameters such as relative humidity, temperature and dew point, which is suitable for the design of the intelligent monitoring system of temperature and humidity in ammunition warehouses.

2 Performance and characteristics of SHT15

The SHT15 sensor is a single-chip, multi-purpose intelligent sensor, which not only contains a micro relative humidity sensor based on a humidity-sensitive capacitor and a micro temperature sensor based on a bandgap circuit, but also has a 14-bit A/D converter and a 2-wire serial interface. It can output calibrated serial data of relative humidity and temperature, so the system no longer uses the multi-way conversion switch, A/D converter and signal conditioning circuit required by the traditional design. The system structure is relatively compact and simple. SHT15 can measure relative humidity and temperature at the same location. Its internal structure is shown in Figure 1.

SHT15 has relatively few pins and is easy to use. Its pin arrangement is shown in Figure 2. The pin functions are shown in Table 1. The relative humidity measurement range of SHT15 intelligent sensor is 0~100%. The resolution is 0.03%, the highest accuracy is +2%RH, the temperature measurement range is -40℃ to 123.8℃, the resolution is 0.1℃, the power supply voltage range is +2.5V to +5.5V, and the response time is less than 3s.

3 System structure and principle

The design system is mainly composed of single-chip microcomputer, sensor and actuator. It adopts AT89C51 low voltage and high performance CMOS 8-bit single-chip microcomputer. It contains 4Kbytes of repeatedly erasable read-only program memory (PEROM) and 128bytes of random access memory (RAM). It is produced by ATmel's high-density, non-volatile storage technology. It is compatible with the standard MCS-51 machine instruction system, and there are also general 8-bit central processing unit and Flash storage unit on the chip. The powerful AT89C51 microcomputer provides a cost-effective solution. Its main features are as follows:

compatible with MCS-51 instruction system;
4K FlashROM that can be repeatedly erased (>1000 times);
32 bidirectional I/O ports;
programmable UARL channel;
2 16-bit programmable timer/counters;
full static operation 0-24MHz;
1 serial interrupt;
128x8bit internal RAM;
2 external interrupt sources;
a total of 6 interrupt sources;
can directly drive LED;
3-level encryption bit;
low power idle and power-down mode;
software set sleep and wake-up function.

AT89C51 and SHT15 communicate through a serial bus. The actuator is mainly composed of a motor-controlled cooler, a warmer, a dehumidifier, a humidifier, etc. The temperature/humidity monitoring system of the ammunition warehouse is shown in Figure 3. [page]

AT89C51 cyclically selects multiple SHT15 in real time to detect temperature and relative humidity. Then read out the data value and display the data value at this time. When the relative humidity exceeds 70%, the microcontroller controls the dehumidifier to start working. If the relative humidity is lower than 40%, the humidifier starts working; when the temperature exceeds 30℃, the cooling machine starts working, and when the temperature is lower than -12℃, the heating machine starts working. Every 3 cycles, print a temperature and humidity value report.

4 System Programming

The system software is written in MCS-51 assembly language. A modular programming method is adopted. It mainly includes initialization module, data acquisition, data processing, display, alarm, printing and other modules. The system program flowchart is shown in Figure 4.

Since SHT15 contains memory, the initialization program is mainly to set the initial values ​​of some data in the memory of the microcontroller and SHT15, and initialize the initial state of the microcontroller. This system is a multi-channel measurement system. SHT15 enters the dormant state after 10ms after power-on. Only when the microcontroller issues a measurement command, SHT15 is "woken up" and works. Therefore, the microcontroller cyclically issues temperature and humidity measurement commands to each sensor to realize multi-channel temperature and humidity measurement. The command set of SHT15 is listed in Table 2.

Since the data output by SHT15 is nonlinear with the measured value, in order to obtain accurate data, the reading value must be compensated nonlinearly. For nonlinear compensation of humidity, 8-bit data is usually compensated by the following formula:

RH=(1.43N-5.127), 256(0≤IV≤107)(1)

RH=(1.11N-28.93)/256(108≤N≤255)(2)

For temperature sensor, the following formula is usually used for nonlinear compensation:

T=d+d2M(3)

In the formula, N and M are the output values ​​of relative humidity sensor and temperature sensor respectively, d1 and d2 are constants, which are determined according to the power supply voltage and the number of bits of temperature data.

Each measured data should be compared with the specified value. If it is within the allowable range, the program will proceed to the next step. If it does not meet the requirements, an alarm should be issued and the data value at the moment will be displayed. After each cycle, a delay of 1 hour (which can be flexibly selected) is required. This is mainly because there are many wooden structures in the ammunition warehouse, and there is a process of volatilization and absorption of moisture. Measurement is performed after a certain delay. It can avoid the long-term operation of the sensor and the repeated start and stop of the actuator, save resources, and extend the service life of the system. After every 3 cycles of measurement (appropriate selection can be made according to needs), the single-chip microcomputer controls the printer to print out the temperature/humidity data of the 3 measurements, which is convenient for reporting and archiving.

5 Conclusion

The temperature and humidity monitoring system of the ammunition warehouse designed by the author is based on the SHT15 intelligent sensor. SHT15 is a new type of sensor based on the design concept of intelligent sensors. It realizes the digital output of temperature and humidity sensors, and has the characteristics of free debugging, free calibration, and no peripheral circuits, which greatly facilitates the application in the field of embedded measurement and control. It is the future development direction of sensors. The structure of the system is relatively simple. The volume is relatively small and the accuracy is high, which overcomes the shortcomings of the traditional ammunition warehouse temperature/humidity measurement system. (end)