1. Brief introduction to the working principle of proton precession magnetometer
(1) measurement principle
Magnetometer utilizes the magnetic moment of proton spin in hydrogen-containing atoms such as kerosene, water or alcohol, magnetizes by artificial magnetic field H0, and arranges along H0 direction to generate magnetic moment mP. After cutting off the artificial magnetic field, the direction of the magnetic moment gradually tends to the direction of the geomagnetic field, and in this process, it tends to the direction of T by precession around T. This precession is called larmor precession, and its precession frequency is directly proportional to the intensity of the geomagnetic field. Its expression: T=23.4874? Therefore, we only need to measure its precession frequency (? T can be measured, so the magnetometer is a frequency detector.
(2) Frequency measurement
Fig. 2- 13 attenuation of induced signal with t
The probe for receiving signals is a non-magnetic glass cylinder filled with kerosene and wound with a set of coils. The frequency signal is excited and received by artificial magnetic field. When working, an artificial magnetic field H0(H0? T), when working, H0 direction is perpendicular to the geomagnetic field. After switching off, the proton magnetic moment cuts the signal coil in larmor precession, thus generating induced electromotive force. When mP rotates once, the direction of induced electromotive force changes vertically (backward inclination) → parallel → reverse → parallel → return to the original direction, and the electromotive force changes repeatedly from maximum to zero → negative maximum to zero → maximum (return to the origin) accordingly, forming a sine wave pattern with decreasing amplitude as shown in Figure 2- 13. Since mP gradually moved to T0, the electromotive force gradually decreased to zero. Our task is to determine the number of pulses in t seconds. Because the signal only takes 2 ~ 3 s from generation to failure, the measurement speed is fast.
According to the previous formula, the intensity of geomagnetic field changes by 1nT, and the frequency changes by only 0.0 16 cycles, so it is difficult to achieve high-precision measurement. Therefore, the instrument adopts the method of multi-stage frequency division, and the frequency multiplication is greater than 128 times. Secondly, the control time of the electronic gate must be absolutely accurate, so the standard frequency is provided by the synchronous oscillator to control the gate time accurately. Finally, how to convert larmor precession frequency into geomagnetic intensity reading, according to: T=23.4874×N/? (nT) relationship, select the product of frequency doubling number n and gating time t to adjust, so that T=N (instrument reading).
2. Brief introduction of instrument operation method
The instrument includes a host, a probe and a charger. There is a keyboard and a display (window) on the host computer. See figure 2- 14 for the block diagram of instrument system.
Figure 2- 14 czm-3 block diagram of instrument system
Figure 2- 15 CZM-3 Microcomputer Keyboard Schematic Diagram
The keyboard layout is shown in Figure 2- 15, with 16 operation keys, a power switch and *** 17 keys. Every time you press the key, there will be a sound prompt.
The operation is done on the keyboard. According to the screen prompts, you can operate one by one. The operation is divided into four parts: parameter setting, display measurement, data query and data transmission. There is a screen display after booting. Click any key to display the system menu (Figure 2- 14).
Click the system menu [1] to open the parameter setting home page. On the home page, the cursor first points to the date column. If you enter date data, the date number will be displayed in the date field. After setting this item, point the cursor to the next item, then set this item, and so on. Among them, the magnetic field measurement item can be set to automatic and manual modes. When measuring, just press the sampling key. After setting, return to the system menu.
Click [2] to enter the display measurement page, where setting items and measurement results can be displayed. The display modes can be divided into "automatic" and "manual", and the modes have been determined in the settings. According to the display result, the display result can be modified or cleared.
Click [3] to enter the data query page to view all the data of the day.
After returning to the system menu, click [4] to enter the "Waiting to Send" page. At this point, the system has completed the preparation of sending data and is waiting for computer instructions. This computer has a CZM-3 data processing program. When the magnetometer is connected with the special cable of the computer, the computer sends out the receiving instruction, and the field data enters the computer for processing.
Please refer to the instruction manual for the function and detailed operation of each key of the keyboard.