Design of Artillery Weather Instrument Based on MSP430F149 Single Chip Microcomputer

Aiming at the problems of the simple comprehensive meteorological observation instrument currently used, such as poor real-time performance, insufficient accuracy, long operation time and inability to directly calculate the correction of firing conditions and ground wind, this paper proposes to design an artillery meteorological instrument based on MSP430F149 single-chip microcomputer to meet the needs of simple meteorological conditions and ground wind correction of rocket launchers when the artillery unit is firing.

Functions and basic working principles of weather instruments

The functions of the meteorological instrument include: 1. Real-time measurement of meteorological conditions such as wind direction, wind speed, temperature and air pressure, and providing simple meteorological condition correction for artillery shooting; 2. Real-time measurement of the ground wind direction and wind speed of the rocket launcher position, and providing ground wind correction for the rocket launcher unit.

The artillery meteorological instrument is used to realize the automatic measurement of ground meteorological conditions such as wind direction, wind speed, temperature, and air pressure, and the automatic calculation of shooting condition correction and rocket launcher ground wind correction. Its basic working principle is: the meteorological instrument is connected to the processor through a data line, and after the meteorological instrument is raised to the required height through the lifting rod, the wind makes the wind vane point to the direction of the wind, and the magnetic azimuth sensor senses the magnetic azimuth of the wind direction; the wind speed impeller rotates at different speeds with the wind speed, and each revolution generates a pulse signal, and the wind speed can be calculated by counting per unit time; the temperature and air pressure sensors sense the temperature and air pressure at that time. The above data is processed by the meteorological instrument data and transmitted to the processor, and the processor displays it in a fixed format as required. The composition of the meteorological instrument is shown in Figure 1.

Figure 1 Meteorological instrument block diagram

Weather Instrument Hardware Design

The task of the meteorological instrument is to realize real-time measurement of atmospheric pressure, temperature, wind direction and wind speed. It is composed of pressure sensor, temperature sensor, wind speed sensor, magnetic azimuth wind direction sensor, data acquisition and processing circuit, weather vane, connector, etc. Its basic design concept is to have a small size and light weight while meeting the requirements of accuracy and use environment, so as to be easy to use under field conditions.

Sensor selection

(1) Pressure and temperature sensors

The pressure and temperature sensors use analog output sensors with pre-processing circuits to simplify the design of subsequent circuits.

The pressure sensor is the ASDX015 absolute pressure sensor from Honeywell, USA.

The temperature sensor uses the digital thermometer chip DS1820 based on single bus technology launched by Dallas Corporation in the United States, which is easy to adjust the position and direction of the sensor so that it can effectively sense the external temperature.

(2) Wind speed sensor

There are many types of sensors that can be used to measure wind speed. After comprehensive comparison, the impeller wind speed sensor is selected to measure wind speed using the Hall effect. Compared with photoelectric encoding, this method can make the meteorological instrument more compact and easy to carry and use.

(3) Wind direction sensor

Magnetic direction sensor, also known as electronic compass, is widely used in military and civilian products. This article uses a magnetic direction sensor composed of a flat electronic compass XW3200 and a wind vane.

The sensor measures the planar geomagnetic field through a two-axis magnetoresistive sensor, dual-axis tilt compensation, and a built-in microprocessor to calculate the angle between the sensor and magnetic north. The direction of the two-axis magnetoresistive sensor is driven by the wind vane and always points to the direction of the wind. The output angle is the magnetic azimuth of the wind direction. The entire measurement process does not require orientation, and the operation is simple and flexible.

Circuit Design

The task of the meteorological instrument circuit design is to count the pulse sequence output by the wind speed sensor per unit time, perform A/D conversion on the analog signal output by the pressure sensor, read temperature data based on single bus technology, receive data output by the wind direction and magnetic azimuth sensor through RS-232S, and transmit data to and from the processor.

Introduction to the main control chip

The data processing part uses MSP430F149 as the main chip of the system. This chip is an ultra-low power microprocessor launched by TI of the United States. MSP430F149 has 60KB+256 bytes of FLASH, 2KB RAM, and includes the following modules:

① Basic clock module: includes 1 digitally controlled oscillator (DCO) and 2 crystal oscillators.

② Watchdog Timer Watchdog Timer can be used as a general timer.

③16-bit timer Timer A with 3 capture/compare registers and PWM output.

④16-bit timer Timer B with 7 capture/compare registers and PWM output.

⑤2 8-bit parallel ports with interrupt function: P1 and P2.

⑥4 8-bit parallel ports: P3, P4, P5 and P6.

⑦Analog comparator Comparator A.

⑧12-bit A/D converter ADC 12.

⑨2 serial communication interfaces USART0 and USART1. [page]

Power circuit design

The weather meter is a battery-powered system, so it has high requirements on the overall power consumption of the system. In order to reduce power consumption, the MSP430 series of microcontrollers are selected. The operating voltage of this series of microcontrollers is 1.8V~3.6V, and it has a sleep function. The current consumption is between 0.1µA~400µA, with extremely low power consumption.

The power supply voltage of the main control chip MSP430 microcontroller is 3.3V, the working voltage requirement of the peripheral sensor is 5V, and the voltage of two No. 7 rechargeable batteries is 2.4V. Therefore, the design of the power supply circuit adopts the boost DC-DC chips NCP1400ASN30T1 and NCP1400ASN50T1, which can increase the voltage of 0.8V~3V to 3.3V and 5V, respectively, to power the microcontroller and peripheral devices. At the same time, it is designed with a power shutdown function. When the peripheral devices are not needed to work, the power supply of the peripheral devices is completely cut off, reducing the overall power consumption. The principle of the power supply circuit is shown in Figure 2.

Figure 2 Power supply circuit schematic

Main control circuit design

The main control circuit can be divided into temperature measurement circuit, pressure measurement circuit, wind speed measurement circuit, wind direction measurement circuit and communication circuit, etc.

(1) Temperature measurement circuit

The temperature measurement circuit is shown in Figure 3. The digital temperature sensor DS1820 is used for measurement, and the collected temperature data is transmitted to the main chip through the P6.5 pin of the main chip MSP430F149. According to the needs and actual conditions of the system, an external power supply is used, and the P6.5 pin of the main chip is used as the data input/output pin of the DS1820.

Figure 3 Temperature measurement circuit diagram

(2) Pressure measurement circuit

The pressure measurement circuit is shown in Figure 4. The pressure is measured using the absolute pressure sensor ASDX015, which collects analog signals and transmits analog voltage to the main chip. The analog signal is converted into a digital signal through the A/D conversion module of the main chip, so it must be connected to the pin of the main chip with A/D conversion function. The P6 port of MSP430F149 has 8 A/D conversion channels, and the pressure sensor is connected to the P6.4 pin of the main chip. When implementing the A/D conversion function, the internal reference voltage 2.5V is used as the voltage reference for A/D conversion, but the output range of the pressure sensor is 0~4V, so a voltage divider circuit needs to be designed at the output end of the pressure sensor.

Figure 4 Pressure measurement circuit diagram

(3) Wind speed measurement circuit

The wind speed measurement circuit is shown in Figure 5. The wind speed is measured using a vane wind speed sensor. The connection between the wind speed sensor and the main chip only requires a common I/O interface, which is connected to the P1.0 pin. The P1.0 pin of the main chip receives the electromagnetic pulse generated by the Hall sensor.

Figure 5 Wind speed measurement circuit diagram

(4) Wind direction measurement circuit

The wind direction measurement circuit is shown in Figure 6. The system uses a magnetic azimuth wind direction sensor composed of a flat electronic compass XW3200 and a wind vane to measure wind direction, and uses an integrated circuit conversion chip MAX3232 to achieve level conversion. The MAX3232 chip can achieve bidirectional level conversion. The chip contains two receivers and a driver IC chip, and has a power converter inside, which can convert the input +5V voltage into the -10V to +10V voltage required for the RS232 output level.

Figure 6 Wind direction measurement circuit diagram

(5) Communication circuit

The weather instrument communicates with the data processing system through RS485. RS485 is a multi-point communication standard that uses differential signals for transmission. It has high resistance to common-mode interference. It can detect 200mV voltage and has high sensitivity. The maximum data transmission rate is 10Mb/s. MAX485 is connected to USART0 of the main chip, P3.3 is connected to the control pin of MAX485. When P3.3 is low, it receives data, and when it is high, it sends data. C1 is a power filter capacitor. A and B terminals are connected with 120Ω resistors. The communication circuit is shown in Figure 7. [page]

Figure 7 Communication circuit diagram

(6) Crystal oscillator circuit and reset circuit

MSP430F149 can access two external oscillators, one is a low-speed crystal oscillator, connected through the two pins XIN and XOUT, and the other is a high-speed crystal oscillator, through the two pins XT2IN and XT2OUT. The external capacitor can be connected as needed, and the range can be 450KHz~8MHz. The choice of external crystal frequency is closely related to the system power supply voltage. According to the actual needs of the meteorological instrument, a 4M external active crystal oscillator is selected, and the output is directly connected to the XT2IN pin of the crystal oscillator, and XT2OUT is empty.

The reset circuit is designed to enable the system to be reset reliably and work stably under various complex conditions. There is an RST reset pin in the MSP430F149 microcontroller, which is multiplexed with the non-maskable interrupt function pin. Its function can be selected by software. Under normal circumstances, it is a reset function. The system uses an external chip reset, and the reset chip HT7027 is connected to the reset pin.

Weather Instrument Software Design

The software design of the weather instrument has the following tasks:

(1) Complete the initialization of hardware systems such as special function registers and I/O ports.

(2) It has the function of initializing and checking the temperature, pressure, wind speed and wind direction sensors.

(3) It has the function of receiving and storing the temperature digital signal collected by the temperature sensor.

(4) It has the function of performing A/D conversion and storing the analog signal collected by the pressure sensor.

(5) It has the function of counting, calculating and storing the pulse signals generated by the wind speed sensor.

(6) Complete the initialization function of the communication interface and realize data transmission with the wind direction sensor and the data processing system.

(7) It has the function of power detection and conversion to low power consumption state.

(8) It has a data filtering function to eliminate the collected data with large errors.

The meteorological instrument software includes the main program module, temperature data acquisition module, pressure data acquisition module, wind speed data acquisition module, wind direction data acquisition module, communication module and data filtering module.

The basic process of the main program is that after the meteorological instrument receives the command from the processor, it collects data through various sensors, stores the data in the specified register, performs data filtering, and transmits the collected data to the processor through RS485. The process is shown in Figure 8.

Figure 8 Flowchart of the main program of the meteorological instrument

The data collected from the wind direction, wind speed, temperature and pressure sensors may contain data that does not meet the conditions or has large measurement errors due to the influence of various factors. In order to ensure the accuracy of the measurement, the obtained data must be screened and the data with poor accuracy must be eliminated. To this end, the average value of every ten collected data is calculated, and the data with a large difference from the average value is eliminated. The remaining accurate data is then averaged, and the average value is the wind speed, wind direction, pressure and temperature data required for measurement.

Conclusion

This paper designs an efficient, high-precision, and automated artillery meteorological instrument based on the MSP430 series ultra-low power microcontroller to meet the needs of artillery training. According to the system requirements, this paper selects appropriate pressure sensors, temperature sensors, wind speed sensors, and wind direction sensors, and completes the hardware and software design of the meteorological instrument. The meteorological instrument has reliable performance and stable operation, and can meet the accuracy and technical performance index requirements of troop training.