Design of proton precession magnetometer based on MSP430

　　Proton precession magnetometers are mainly used for ground magnetic exploration, and are used for the survey, detailed investigation, and geological mapping of iron ore and other metal deposits. In aviation, ocean, and earthquake prediction work, they are used for magnetic variation observation and flow magnetic measurement at geomagnetic stations. At present, most of the magnetometers used in geomagnetic measurements are imported products and are expensive. The domestic magnetometers are mostly CZM-2 proton precession magnetometers produced by Beijing Geological Instrument Factory. This instrument was produced in the 1980s and is simple and convenient to use. However, due to the limitations of technical conditions at the time, the instrument had high power consumption, poor stability, and low accuracy (±1nT). According to the needs of geomagnetic flow measurement and fixed-point observation in my country, it is of great significance to develop a new type of high-precision proton precession magnetometer. The author designed a new type of magnetometer, which has a 1-fold increase in actual measurement accuracy (±0.5nT) compared with the CZM-2 proton precession magnetometer.

　　1 Working principle of proton precession magnetometer

　　The proton magnetometer sensor consists of two identical coils connected in series in reverse order to suppress external interference. The coil skeleton is filled with kerosene or water rich in hydrogen protons. When the instrument is polarized, a DC current of about 1 A is passed through the coil. A polarized magnetic field that is perpendicular to the direction of the geomagnetic field and several orders of magnitude larger than the geomagnetic field will be generated inside the coil. After a few seconds of polarization, the magnetic moment of the hydrogen protons will be neatly arranged along the polarized magnetic field perpendicular to the direction of the geomagnetic field. Then the polarization current is cut off, the polarized magnetic field generated by the coil disappears, and the proton magnetic moment is in a precession state with the geomagnetic field as the axis under the action of the geomagnetic field. During the precession process, the magnetic moment of the proton cuts through the coil and generates an induced electromotive force of several microvolts at both ends of the coil. The relationship between its angular frequency and the geomagnetic field

　　ω0=γH0

　　Where: H0 is the geomagnetic field strength, in nT, f=ω0/2π is the Larmor precession frequency, in Hz. γ is the proportionality coefficient, called the gyromagnetic ratio. For aviation kerosene, γ=（2.67513±0.00002）rad/（Ts）. Its amplitude decays exponentially with time,

　　It is called free induction decay, where µ0 is the magnetic permeability, n is the number of coil turns, A is the cross-sectional area of ​​the coil, M is the proton magnetization, and T2 is the transverse relaxation time. The free induction decay signal is shown in Figure 1.

　　After the induced signal is frequency-selected, amplified, shaped and divided, the period is measured by the single-chip microcomputer, converted into frequency and multiplied by the normalization coefficient to directly read the geomagnetic field value.

　　2 System Overall Design

　　2.1 The role of MSP430 ultra-low power microcontroller

　　The single-chip microcomputer is the control core of the system. According to the functional requirements of the instrument design, in addition to the basic measurement and control functions, the single-chip microcomputer must also realize the following functions: it can query the measurement data; save the date and time when the instrument is measuring; it can automatically measure the magnetic field at a fixed time; automatically complete the processing and storage of data; press keys to complete data display, measurement curve display, serial communication with the host computer, and print drawing output, etc.; the watchdog prevents the program from running away; it has a temperature monitoring function to perform temperature correction on the instrument; and detects the battery power. In addition, the single-chip microcomputer should have low power consumption to be suitable for field operations.

　　The commonly used 51 series single-chip microcomputers, due to the limitation of their internal resources, must be externally expanded to complete the above functions, which not only increases the complexity and failure rate of the circuit, but also increases the power consumption due to the increase of chips.

　　This system uses the MSP430F series of ultra-low power microcontrollers produced by TI of the United States. The MSP430 series of microcontrollers include the following series: MSP430×1×××, MSP430×3××, MSP430×4××, etc., and all members are software compatible, which can be easily transplanted between models in the series. It adopts the von Neumann structure, and the RAM, ROM and all peripheral modules are located in the same address space. Compared with other microcontrollers, the MSP430 series can greatly extend the battery life.

　　The MSP430F149 microcontroller has the following main features:

　　1) Ultra-low power consumption structure, requiring only 250µA operating current at 1MHz clock frequency and 0.1µA standby current;

　　2) Strong antistatic ability;

　　3) On-chip 12-bit A/D conversion;

　　4) On-chip precision comparator;

　　5) Hardware multiplier;

　　6) On-chip temperature measurement diode;

　　7) All MSP430 are industrial grade; 16-bit reduced instruction set MCU operating temperature - 40 ~ 85 ℃.

　　MSP430F149 has 60kbit program memory, 2 kbit data memory, 48 independent I/O pins and a very rich peripheral modules, and the design requirements of this system can be met almost without adding other components.

　　MSP430F149 has 256 bytes of online programmable FLASH ROM inside, which can save and read instrument setting data. The on-chip A/D is used to detect the battery power. The on-chip watchdog timer prevents the program from running away. The internal temperature diode is used to measure the internal temperature of the instrument. The on-chip serial port can communicate with the host computer. The external DS1302 clock chip is used to collect data and record time regularly. An external FLASHROM is used to store a large amount of measurement data. The internal hardware multiplier is used to speed up the signal processing. Its ultra-low power consumption core can extend the battery life.

　　2.2 Probe tuning principle

　　The probe is an inductive element used to measure frequency signals. The proton magnetometer uses an LC parallel resonant circuit for frequency selection measurement. The resonance formula is:

　　In formula (3), f is the center frequency of the LC parallel resonant circuit; L is the inductance of the probe; C is the value of the tuning capacitor in the instrument. As long as the inductance L of the probe and the tuning capacitor C in the instrument are tuned more accurately, the resonant frequency f of the circuit will resonate near the proton precession frequency in the probe.

　　The intensity of the Earth's magnetic field changes little in a short period of time. The proton magnetometer uses this characteristic to realize the automatic tracking function of the instrument's frequency selection measurement. The frequency value f (or magnetic field value T) measured last time is used to calculate the tuning capacitance value for the next frequency selection measurement according to formula (3):

　　Since the probe tuning circuit works under weak signals, the tuning capacitor can only be switched by controlling the relay through the single chip microcomputer, so a group of I/O port P2 is used to control 8 relays to achieve 256 different tuning capacitor values. [page]

　　2.3 Signal Amplifier

　　The signal output by the probe is only a few microvolts, and must be amplified to the order of 10000/volt before the measurement circuit can perform digital frequency measurement. Therefore, the amplifier is required to have low noise and high gain. In order to improve the signal-to-noise ratio of the output signal, the amplifier is designed to have frequency selection characteristics. The center frequency of the amplifier is changed by changing the matching capacitor to measure different magnetic field strengths.

　　The indicators of the amplifier are: operating frequency range 1300 ~ 3100 Hz, gain greater than 118 dB, input impedance greater than 10 k8, and frequency selection passband f = 40 ~ 100 Hz.

　　2.4 Signal frequency measurement

　　There are usually two methods for measuring frequency of proton magnetometers: phase-locked frequency multiplication counting to measure frequency and frequency division to measure signal period.

　　The phase-locked frequency multiplication counting method is widely used in domestic instruments. The signal is shaped and then multiplied, and then counted by a gated counter. The gate time is controlled by a digital circuit so that the count value is exactly the geomagnetic field value. The advantage of this method is that the conversion from frequency to geomagnetic field value can be achieved without complex calculations. However, due to the wide range of signal frequency variation (1000~3000Hz) and the exponential decay of the signal amplitude, it is difficult for the phase-locked loop to achieve accurate frequency multiplication in the entire frequency band, so this method has low accuracy. The method of measuring the period by frequency division is easier to implement (see Figure 2). The shaped signal is divided by 256 by a digital circuit to measure its period, and the result is calculated by a single-chip microcomputer. Since the error is only caused by the rising edge of the last period of the 256-division frequency, the error is very small and the accuracy is high.

　　The frequency measurement circuit uses a frequency division and period measurement logic circuit composed of a programmable logic device (CPLD), and a high-stability crystal oscillator provides the measurement clock source. This circuit performs digital period measurement on the precession signal output by the frequency-selective amplifier. The oscillation frequency of the quartz crystal oscillator is 40 MHz, and the accuracy can reach ±25 ns. It is used as a counting pulse, and the number of pulses is recorded by the counter of the measurement circuit. The precise frequency value can be obtained through calculation and processing by the single-chip microcomputer, as shown in Figure 3.

　　Considering the low power design and 3.3 V power supply, the circuit uses EPM 7064AET I44-10 CPLD chip, and the instrument will turn off this part of the power supply when not measuring to save power. The programming is implemented in Verilog HDL (Verilog High speed integrated circuit hardware Description Language).

　　2.5 Selection of other circuit components

Considering the wide working temperature range of the instrument, the LCD uses a graphic LCD with good low temperature resistance. In order to reduce power consumption and improve power utilization, each circuit uses LM 2674 for power supply. This is a DC-DC chip with small ripple and interference, but the power supply and digital circuit parts must still be shielded and isolated from the analog signal amplifier. FLASH ROM (Flash Read Only Memory) uses Samsung's 64MB memory K9F1208.

　　3. Key points of MCU software development

　　The MSP430 development software has a powerful C compiler, so part of the software is written in C. Here, IAR's EW-430 V2120A version of the MSP430 development software is used.

　　Due to space limitations, the detailed procedures are not listed here, but two points need to be explained.

　　1) The bus of the MSP430 microcontroller is not open to the outside world. To transmit signals to the LCD and FLASH memory, you can only use the I/O port to simulate the bus. This is slightly inconvenient for programming.

　　2) The accuracy of the internal temperature measuring diode of MSP430 is low. There may be an error of several degrees Celsius. However, it has little impact on the accuracy calibration of the instrument.

　　4. Whole machine test

　　Test location: South of Dachanggou, Bayi Township, a certain city.

　　The whole machine current: polarization current 1200 mA; signal amplification measurement 38 mA; display data 15 mA; standby current 0.8 mA. Geomagnetic field measurement method: measure 4 times at the same location and take the average value. The measured geomagnetic field value is shown in Table 1.

　　By comparing the measurement data of EVN I (env ironment) earth measurement system, it can be concluded that the resolution of this instrument is 0.1nT and the accuracy is 0.5nT.

　　5 Conclusion

　　MSP430F149 is used as the processing chip, with fewer peripheral components, low system failure rate, low power consumption, long battery life, small electromagnetic interference of the internal components of the instrument to the amplifier, and small size of the instrument. The indirect frequency measurement method of frequency division and period measurement is adopted, and the instrument has high measurement accuracy. However, there is still a certain gap compared with some high-end imported instruments, and it can be further improved in the future.