Orthogonal experiment design of EMI of security monitoring system in coal mines

Security monitoring system of coal mines is indispensable to ensure the safe and efficient production of colliery. Due to the special and narrow underground field of the coal mine, the electromagnetic interference can cause a series of misstatements and false positives on the monitoring system, which will severely hamper the safe production of coal industry. In this paper, first, the frequency characteristics of the interference source on the power line are extracted when equipment runs normally. Then the finite difference time domain method is introduced to analyze the effects of the electromagnetic interference parameters on the security monitoring signal line. And the interference voltage of the two terminal sides on the single line is taken as evaluating indexes. Finally, the electromagnetic interference parameters are optimized by orthogonal experimental design based on the MATLAB simulation on the normal operation of equipment.


Introduction
With the development of coal mine automation, more and more high power electrical instruments have been used in coal mines. The underground electromagnetic interference has become particularly severe because the on-the-spot space is relatively narrow. At times EMI results in errors of the security monitoring system. Currently, there are hardly any research has been carried out in the field of coal mine EMC. Delogne (1991) generated a regulation of electromagnetic wave transmissions and indicated the difficulty of studying practical transmissions in tunnels. Hill (1989) studied the traveling wave in tunnels and radiation impedance of dipoles. Rappaport (1989) conducted a prediction using a ray tracing method. The references (Sun 1999;Sun et al. 2006) about the investigation into the wireless communication and channel studied the measurement for improving EMC system. These studies largely focused on wireless communication, which did not involve in the EMC studies in coal mines. Sun carried out a study of harsh classification on electromagnetic compatible measures of mining monitoring and communication equipment (Sun et al. 2008), as well as an investigation in coal mining EMC (Liang 2007;Zhang et al. 2007). References (Gao 2004;Liu et al. 2009Liu et al. , 2010 researched the region electromagnetic environment while the immunity of the monitoring was not involved. However, there has been no report about the effect of the electromagnetic interference parameters on the security monitoring system so far. Security monitoring signal from sensors is often relatively weak, and its anti-interference ability is very poor in the process of transmission, which resulted in vulnerability to external electromagnetic interference. Therefore, it is very significant and important to reduce the interference voltage of security monitoring single line. In this paper, for the normal operation of equipment, the electromagnetic interference parameters of security monitoring signal line are optimized by using orthogonal experiment design method based on the finite difference time domain method (FDTD) analyses of multiple conductor transmission line (MTL) model, then the simulation experiments are carried out, and the electromagnetic interference voltage are decreased.
2 The basic electromagnetic interference theory of security monitoring system 2.1 Measurement and extraction of interference sources The data of interferential voltage on device's power line while working normally can be obtained for many times by using the Amway 2711 spectrum analyzer. For all spectrum data measured, frequency-domain characteristic parameters are mainly analyzed, including the frequency band range of interference and the frequency distribution, etc. In order to investigate the average interference level on device's power line, the amplitude of each frequency point in the frequency domain is measured by using statistical analysis methods. Statistical analysis methods used are as follows: the interference data of the same type measured is analyzed; the average value at each frequency point is calculated; then connect all the points and draw the mean envelope. With analyzing all interference voltage spectrums on device's power line, the results of a statistical test are shown in Table 1. Figure 1 shows that the average value of envelope spectrum of interferential voltage on device's power line while working normally. With the increase of the frequency, the interferential voltage decreases rapidly when the frequency grows from 0.05 to 0.1 MHz; the interferential voltage increases briefly, then it decreases sharply when the frequency is between 0.1 and 0.2 MHz; the interferential voltage begins to flatten when the frequency is in the range of 0.1-0.7 MHz, and then to decline when the frequency is among 0.7-0.8 MHz; finally, when the frequency is higher than 0.8 MHz, the interferential voltage begins to flatten out again gradually, and it is the minimum value at this time.
As shown in Fig. 1, the maximum interferential voltage value is 8.2327 V when the frequency is 0.05 MHz on device's power line while working normally. Therefore, interference sources can be taken as follows:

Numerical prediction model
The multi-conductor transmission line (MTL) structure can be described by the voltage and current iteration equations as follows (Erdin et al. 1998;Paul 2008;Jiao et al. 2012): where V(z, t) = [V 1 (Z, T), V 2 (z, t),…, V n (z, t)] T and I(z, t) = [I 1 (Z,T), I 2 (z,t),…, I n (z,t)] T represent the voltage and current vectors on the z point of line at moment t respectively; R, G, L, and C are resistive, inductive, conductive, and capacitive per-unit-length (p.u.l.) parameters of the line respectively; V F (z, t) and I F (z, t), respectively, denotes the excitation voltage source and motivation current source vectors on the z point at time t.
In this paper, the three-conductor transmission line is used as an example based on the coal mine security monitoring system, one of these is power line, the other two are signal lines as shown in Fig. 2. S-terminal and L-terminal represent two terminal sides of the transmission line. Infinite ground plane can be seen as reference, and the three lines are parallel.
There are two kinds of crosstalk of MTL: capacitive crosstalk and inductive crosstalk. The segment of the isometric lines is shown in Fig. 3, where V S is the excitation source, V 1 and I 1 are the starting point of voltage and current, dz is the space step.
Because R and G in the lossless uniformed MTL are zero, the spatial discretization for MTL is along the transmission line based on the method of FDTD. The following results are obtained as Formula (3) and (4): where subscript k denotes the spatial discretization sequence, superscript n is the time step point. Use 0 as the voltage superscript initial value, 1/2 as the current superscript initial value. We assume that the initial value of the voltage and current of transmission line is zero. It can be seen in the formula, the current on the conductor could be solved by iteration of the current moment voltage and before a moment voltage. Otherwise the voltage can be solved by iteration of current. Combine with the terminal conditions of the transmission line, we can get the difference equation of both Sterminal and L-terminal side as Formula (5) and (6): The Formula (3), (4), (5) and (6) constitute the basic iterative formula of the FDTD method to analysis of MTL.

Electromagnetic interference parameters by using orthogonal experiment design
The electromagnetic interference parameters for security monitoring system in coal mines during devices' normal working are optimized by orthogonal experimental design methods based on the MATLAB simulation on the interference processes.

Analysis of orthogonal experiment result
Considering the equipment to work normally, the interference voltage of S-terminal for each parameter at levels 1-4 and the difference between the lowest value and the highest value are listed in Table 4. By comparing the value of each parameter, it can be found that the three most significant parameters which influence the interference voltage of S-terminal are as follows: distance between the power line and signal line, load of terminal side of the transmission line and length of transmission line. The interference voltage of S-terminal for the three parameters at four levels is plotted in Fig. 4. Figure 4a shows the variation of the interference voltage of S-terminal with the distance between the power line and signal line. With the increasing of distance between the power line and signal line, the interference voltage of S-terminal decreases at levels 1-4. Figure 4b shows the effect of the load of terminal side of the transmission line on the interference voltage of S-terminal. With the increasing of the load of terminal side of the transmission line, the interference voltage of S-terminal increases at levels 1-4. Figure 4c shows that the variation of the interference voltage of S-terminal with length of transmission line. It is observed that the length of transmission line at level 2 and level 3 are inflection points of the variation of the interference voltage of S-terminal, with the increasing of the length of transmission line; the interference voltage of S-terminal increases when the length of transmission line is lower than 100 X, then decreases when the length of transmission line is higher than 100 X and lower than 1000 X; finally, it increases when the length of transmission line is higher than 100 X.
Considering the equipment working in normal, the interference voltage of L-terminal for each parameter at levels 1-4 and the difference between the lowest value and the highest value are listed in Table 5. By comparing the value of each parameter, it can be found that the three most significant parameters which influences the interference voltage of L-terminal are as follows: the distance between the power line and signal line, load of terminal side of the transmission line and length of transmission line. The 1.89 9 10 -7 8.62 9 10 -7 3.61 9 10 -7 2.22 9 10 -7 2.83 9 10 -8 Level 2 1.00 9 10 -6 4.26 9 10 -7 8.37 9 10 -7 9.15 9 10 -7 2.12 9 10 -7 Level 3 2.97 9 10 -7 1.74 9 10 -7 4.01 9 10 -7 8.93 9 10 -7 1.02 9 10 -6 Level 4 1.05 9 10 -6 1.93 9 10 -7 9.41 9 10 -7 5.09 9 10 -7 1.28 9 10 -6 Maximal difference 8.62 9 10 -7 1.57 9 10 -6 5.80 9 10 -7 6.94 9 10 -7 1.25 9 10 -6 interference voltage of L-terminal for the three parameters at four levels is plotted in Fig. 5. Figure 5 shows the variation of the interference voltage of L-terminal with the distance between the power line and signal line, the load of terminal side of the transmission line and the length of transmission line. It is observed that the variation of the interference voltage of L-terminal is similar to the variation of the interference voltage of S-terminal.
From Table 4 and Fig. 4, it is clear that the interference voltage of S-terminal is a minimal value at the third level of distance between the power line and signal line, at the    Table 5 and Fig. 5, it is clear that the interference voltage of S-terminal is a minimal value at the forth level of distance between the power line and signal line, at the first level of the load of terminal side of the transmission line, and at the first level of the length of transmission line.
To sum up, the optimal parameters for the interference voltage of S-terminal and L-terminal are as follows: u = 90 m, d 1 = 50 cm, d 2 = 2 mm, d 0 = 10 cm, R = 10 X.

Simulation experiment
From Table 3, it is observed that the minimal value of interference voltage of S-terminal and L-terminal among all the trials occur during the No.8 trial. To verify the optimization results of orthogonal experimental design, the comparison-interference voltage of S-terminal and L-terminal between the optimization result and No.8 trial as shown in Fig. 6.
Based on the orthogonal experimental design, the comparison between the optimized and No. 8 simulation results were performed and the results are listed in Table 6. As observed from the table, after the optimization, the interference voltage of the security supervision signal line was reduced at least 34.20 % before optimization.

Conclusions
In this paper, the frequency characteristics of interference source of the power line are extracted when equipment runs normally, the interference parameters are optimized by using orthogonal experiment design method based on the FDTD analyses of MTL on device's power line while working normally, and the results of experimental design are confirmed by MATLAB simulation. Conclusions are as follows: (1) The interference source on device's the power line while working normally can be obtained by using the Amway 2711 spectrum analyzer; the optimal interference parameters obtained using orthogonal experiment design are as follows: length of transmission line is 90 m, distance between the power line and signal line is 50 cm, distance between two signal lines is 2 mm, distance between the transmission line and ground is 10 cm, load of signal line port is 10 X.
(2) Among the interference parameters, the distance between the power line and signal line is the most important one to affect the interference voltage of S-terminal and L-terminal; with the increase of the distance, the interference voltage of S-terminal and L-terminal decreases.
(3) The experiments on the electromagnetic interference of coal mine power system to security monitoring system show that electromagnetic interference can be decreased at least 34.20 %, which providing certain theoretical guidance for engineering practice. Interference voltage of S-terminal 5.35 9 10 -10 9.42 9 10 -10 -43.21 Interference voltage of L-terminal 2.79 9 10 -11 4.24 9 10 -11 -34.20 Orthogonal experiment design of EMI of security monitoring system in coal mines 331