The Principle and Simulation of Moving-coil Velocity Detector. Yong-hui ZHAO, Li-ming WANG and Xiao-ling YAN

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1 17 nd International Conference on Electrical and Electronics: Techniques and Applications (EETA 17) ISBN: The Principle and Simulation of Moving-coil Velocity Detector Yong-hui ZHAO, Li-ming WANG and Xiao-ling YAN Naval University of Engineering, 4333, Wuhan, Province Hubei, China Keywords: Moving-coil velocity detector, Circuit simulation. Abstract. Geophone is a kind of typical seismic sensors, after years of development, the structure and technology has been quite mature, but in recent years, with the development of measuring needs for micro-seismic events, the traditional geophone appeared insufficient, this article through the analysis and Research on its structural principle, based on the similarity principle of electric system and mechanical system and circuit model of this type of detector is established by Multisim software of NI (National Instruments), and it is validated by the corresponding simulation results. Introduction Moving-coil velocity detector is a typical seismic sensor, which is a transducer in the form of transforming mechanical vibrations of seismic wave signal into a voltage signal output. But with the change in application development environment, part of the performance of the sensor can no longer meet the appropriate requirements, need to be improved and development [1]. In this paper, its structure, working principle and its transfer function studies were analyzed for seismic detectors pickup model system, using the similarity principle of electrical and mechanical systems, the circuit simulation model was established circuit simulation in Multisim, which provides the basis for analysis and improvement of circuit. The Structure and Working Principle: Most of them are moving-coil geophones, the main constituent of the system can be divided into two parts magnet circuit system and vibration sensing system, the specific structure shown in fig.1. Wherein the magnetic circuit system consists of a permanent magnet fixed to the housing and magnetic boots composition; vibration sensor system consists of a coil, the coil spring stent and slices. Figure 1. Internal structure of detector. The basic principle of moving-coil geophones is Faraday's law of electromagnetic induction. When the operation of the detector, the vibration is transmitted to the surface of the detector housing. At this time, since the spring of elastic body inertia and the inertia of its own body, so that the inertial body has a tendency to maintain the original state, and therefore, the inertia body movement relative lag in the housing movement, which produce the relative movement between the coil and the casing. The relative motion leads at both ends of the coil induced electromotive force, 69

2 the shell vibration determines the electromotive force of the size, the larger the amplitude, the induced electromotive force is greater, and vice versa is also true []. The Simplified Model and Analysis of the Transfer Function of the Detector Figure. "mass spring damper" system. Dynamic model of moving-coil geophone can be simplified into a single degree of freedom system consisting of a mass, spring and damping, the model shown in fig.. Among them, F is the force, M is the body mass inertia, k is the stiffness coefficient for spring, is the damping coefficient. The vertical is the reference direction, y (t) is the ground displacement, x(t) is the mass s displacement relative to the ground, y (t)+ x(t) is the displacement of mass relative rest reference system, the analysis of the force is as follows: The spring force Fk, the inertial force Fa, the damping force F F kx() t (1) k ( yt ( ) xt ( )) Fa m x () () F dx t dt According to Darren Bell principle, there are: (3) F F F (4) a k That is: () () () m yt kx() t dxt m xt x dt x The ratio h(t) y(t) and x(t) is obtained by using the Laplasse transform: (5) Xs () H () s Ys () s h s s The internal structure of the detector shown in fig.1, and the fixed magnet coil and with a mass motion detector constitutes "mechanical energy - electric energy" of the transducer structure [4]. (6) 7

3 Figure 3. Working principle model of detector. Detector works model shown in Fig.3, it is based on the seismometer system "mass-springdamper", and joined the transducer structure of permanent magnets and coils. When the pendulum mass and the ground relative motion occurs, the coil cutting magnetic induction lines, according to the Faraday's law of electromagnetic induction, the produced induced electromotive force is eg d() x t dt G is the mechanical and electrical constant of electromagnetic energy transfer system, units as V/(m/s),represents the ability to convert the mass block and the relative movement speed of the ground into the induced electromotive force. The input impedance of the detector is Rpreamp, the output voltage is Uout, the coil resistance is Rcoil, and the gain of the preamplifier amplifier is A: U R out A e R R coil preamp preamp Induced electromotive force e generated the current flow through the detector coil, according to Lenz's law, the electromagnetic force of the coil is (7) (8) F L coil G d() x t R R dt preamp We can find that, similar to the mechanical damping force, the electromagnetic force FL, is proportional to the relative motion velocity of mass and the shell, so FL is an electronic damping force. Therefore, the motion state of the mass can be described as d x( t) y( t) Fk Fμ+ FL m (1) dt Therefore, in the introduction of electromagnetic energy transfer structure, the transfer function of the system is Xs () H () s Ys () s hs s Among them, the system damping coefficient h is the sum of inherent damping coefficient h and the electronic damping coefficient he: (9) (11) G h h he m ( R R ) m coil preamp By formula (1), (5) and (1), can get the geophone speed transfer function, which is the ratio of the output voltage and input speed for earthquake [5] (1) 71

4 H Uout () s e X() s G As T-v () s e X() s Y() s s hs HT-v(s) unit for V/(m/s), means sensor system ability in transforming different frequency ground motion into voltage. Table 1 shows the different types of detector working parameters. This paper mainly used detector is the resonant frequency is 4.5Hz the SM-6-type made in the United States ION, its stability and consistency is great, widely used in seismic instruments. Table 1. The United States ION company's detector SM-6 and Chongqing Geological Instrument Factory s CDJ series of detectors. h Parameter SM-6 CDJ- Z1 Resonance frequency f/hz 4.5±1 Sensitivity G/V/(m/s) 8.8±5 CDJ-ZG1 1±1 1±1 8±5 1±5 Coil resistance Rcoil/Ω 375±5 38±5 35±5 Inherent damping coefficient.56±5.58±1 Body mass m/g Maximum displacement P- P/mm 4 Spurious frequency f/hz 14 > >.55±1 (13) Modeling and Simulation by Multisim In general, the mechanical system with the nonlinear factors, the simulation is very difficult, in most of the cases we have adopted similar electrical system to simplify simulation, through the previous experiments, the distortion of the approximate simulation is very low, and therefore it can be trusted. At the same time, because this topic mainly uses the circuit improved method to the subsequent circuit system, so using electric system simulation provides a unified platform for the joint simulation. Multisim Software Multisim is a simulation tool of the United States National Instruments (NI) based on Windows, for analog/digital circuit board design on board level. It contains the input way of graphics circuit principle diagram and circuit hardware description language, with rich simulation ability. Engineers can use Multisim to build interactive circuit principle diagram. Multisim is a refinement of the complex ability of the SPICE simulation, without deep spice expertise, the user will be able to quickly capture, simulation and analysis of the new design. We use Multisim simulation software, use the computer's storage capacity, speed, can complete the things human difficult to complete such as simulate assessment, design verification, optimization and improvement, and data processing work content. So we can test the rationality of the simulation, and continuously optimize the parameters of the circuit, so that it can achieve a most excellent value [6]. Simulation of the Detectors Pick-up System Generally, in order for the entire electro-mechanical system simulation, we used an equivalent circuit to replace the mechanical part to study this type of electric - magnetic - mechanical systems, which use network analysis to simplify the calculation. Namely the use an electric system as an analogy of mechanical systems. In this problem, we use the kind of analogy by Byrne. In the 7

5 analogy, the current equivalent to force, the voltage source is equivalent to the speed of the earth, the mass equivalent to the output voltage. Table. Similarity between the equations of the two order mechanical and electrical systems. Mechanical system F Mu (Inertial force) i C Electric system F kz (Restoring force) 1 i dt L F dz (Damping force) i 1 R k / M h d / M 1/ LC h 1/ RC Table 3. Parameters of the system based on the two different types of detector. Detector type 4.5Hz C=M.111f L=1/k 11.8mH R=1/d.85 Transformation ratio 1:G 1:9 The final equation that describe the ground mass movement in mechanical system is exactly same to that the final description of the relationship between the voltage source and the output voltage in electric system. Fig.4 is the equivalent circuit diagram of the geophone seismic system of the 4.5Hz detector. Figure 4. The equivalent circuit diagram of the 4.5 Hz detector. To verify the parameters of the seismometer system design is reasonable, we use the simulation of the software Multisim to observe compliance with our requirements, and assess the design parameters. Firstly, the simulation of the 4.5Hz geophone seismometer geophone system are follows: 1) By inputting AC voltage in the seismometer system, we observe if the output waveform of the seismometer system is sinusoidal; Operation: seismometer system access input-voltage of mv, 4.5Hz frequency AC voltage source, and then use an oscilloscope to observe the voltage source waveform at both ends and output waveform of the system. Figure 5. Waveform of the output and input of the geophone system. 73

6 Figure 6. The amplitude frequency and phase frequency characteristic of the geophone system. As shown, it can be seen both curves are sinusoidal, but not the same magnitude. Source voltage waveform s peak-peak value is 4mV, and the peak-peak value of the output terminal voltage waveform if 1.mV, the ratio between the output terminal and the input terminal is 5.55, that is 8.14dB, and the phase difference is 9 degrees, according to the Formula 6, which are same to the amplitude frequency and phase frequency theoretical calculation, so the simulation was valid. ) We make an AC analysis to the earthquake system output to the input, then we can get the amplitude frequency and phase frequency characteristic curve of the transfer function of the vibration geophone system, and compare with the theoretical curve of geophone system, verify the rationality of the simulation; Operation: Using the options AC-analysis of Multisim to drawn amplitude-frequency characteristics and phase-frequency characteristic curve, and use Matlab to draw the theoretical curve of geophone system. Figure 7. Amplitude frequency and phase frequency characteristic of geophone system. By the transfer function of the geophone system, we can draw the theoretical amplitude frequency and phase frequency characteristic curve of 4.5Hz detector, which are shown in the following figure 7. Compared with the actual measured curve, the theoretical curve and the measured curve are consistent, which means it meet the requirements of the transfer function, meet the design expectations. Conclusion Through the analysis of structure and principle of moving-coil geophones, this paper established a geophone transfer function model. Then it use an electric system make the circuit model in Multisim to the mechanical systems, which based on the similar principle, and then make some 74

7 preliminary verification analysis. Next, using the Multisim and Matlab, we get the Bode diagram analysis respectively, and the corresponding results prove the simulation model for moving-coil velocity detector is accurate and reliable. This model will establish a unified platform and foundation for the geophone s improved analytical research and simulation. References [1] Chen Ying, Song Jun-lei*(Faculty of Mechanical & Electronic Information, China University of Geoscience, Wuhan 4374, China). [] Cao Shuang-lan, Design and Calibration of Low-frequency Geophones Based on Pole-zero Compensation [D] Changchun China: Jinlin University. [3] Dong Shixue, Zhang Chun-yu, Chuangchun University of Science and Technology, Chuangchun 136. [4] Fang-bing, Research of the Performance improvement of coil-moving geophone [D] Changchun China: Jinlin University. [5] Zhang Xiao-peng, Li Shi-wei, Wang Tong-dong, et al, 14 An improved short period geophone-based seismometer. Progress in Geophysics (in Chinese), 9(5). [6] Xv Yubao; 15 Design of Virtual Electronic System Based on Multisim, Natural Science Journal of Harbin Normal University, 31(1). 75