Suppression of co-frequency interference in induction wireless data communication

1 Introduction

In industrial automation, for data communication between mobile locomotive and central control room, the cable communication mode is inconvenient because of the towing communication cable. Wireless communication mode has a high error rate due to bad environment in the industrial field. Induction Wireless Data Communication (Data Transmission by Induction Radio)^[1]Because there is only a wireless communication distance of 5-20cm between the encoded cable (also called induction bus) and the induction antenna, it can not only ensure the flexibility of locomotive movement but also ensure the reliability of the communication quality. It can also detect the location of the locomotive synchronously during the communication process.^[2].

Electrical equipment in industrial field, especially variable frequency speed control devices on mobile locomotives, can produce strong harmonics with the same or similar frequency as carrier frequency of inductive wireless data communication. This same frequency interference noise cannot be attenuated by band-pass filter. If effective measures are not taken to suppress it at the input side, the error rate of inductive wireless data communication will be greatly increased, or even will not work properly. Baosteel Phase I coke oven third power transformation is applied to imported equipment from Japan. In practice, "There is frequent interruption of induced bus communication. The reason is random strong interference and antenna detection distortion."^[3]"." Therefore, in some practical applications, inductive wireless technology abandons inductive wireless data communication and only uses inductive wireless location detection technology.

In order to suppress interference in inductive wireless data communication, many beneficial studies have been done by peers. Document [4] puts forward an induction wireless differential receiving antenna device. Reference [5] presents a method for dual-receiving antenna single transmission line. In this paper, the same frequency interference suppression technology for induction wireless data communication with "dual transmission lines crossing with single receiving antenna at the same distance" is presented.^[6]It can effectively suppress the same frequency interference noise, improve the signal-to-noise ratio, and is suitable for ground location detection.

2 Basic principles of inductive wireless data communication

In order to analyze the principle of the same frequency interference suppression technology to improve the signal-to-noise ratio in inductive wireless data communication, the basic principle of inductive wireless data communication is briefly analyzed and introduced.

2.1 Coded Cable and Induction Antenna

The coded cable has a flat shape and several pairs of transmission lines inside, which cross at different locations according to certain coding rules. The coded cable is installed along the track of the moving locomotive, and the starting end is connected to the central control room.

An induction antenna consists of two sets of coils, one as a sending antenna and the other as a receiving antenna, which are encapsulated in a plastic box, commonly known as an antenna box. The antenna box is installed on the mobile locomotive and connected with the control cabinet on the locomotive. The antenna box moves with the locomotive and is always 5-20cm away from the encoded cable. See Figure 1.

When the antenna box is close to the coded cable, each pair of transmission lines in the coded cable and the coils in the antenna box are induced to each other, thus forming a short-distance wireless communication channel between the antenna box and the coded cable.

Amplitude and Phase Analysis of 2.2 Induction Signal

Figure 2 is a diagram of transmission line L and antenna coil tiling. In Figure 2, the antenna width and the spacing between two intersections of the communication transmission line in the coded cable are equal to W, W=2r.

Definition: The central point of the antenna coil is the position of the antenna coil; The area between the two intersections of transmission line L is called the K region of transmission line L (K=I, II, III,...), and the distance between the antenna coil position X and the center line of the K region where x is located is d.

The induction electromotive force E generated by the communication transmission line is analyzed using the antenna coil as the sending coil. According to the theory of electromagnetic induction, the current in the antenna coil is i=I_MSin ω At t, the induced electromotive force E = di/dt generated by the transmission line. Here, the mutual inductance factor M is a function of the position of the antenna coil (x, y, z). Assuming that y and Z do not change when the antenna coil moves in X direction, then:

Induction EMF e generated in area I of the transmission line due to a crossover_IInduction EMF e generated with Zone II_IIThe phase is opposite. If e_IAs a standard, the phase of:Then, when n is even, the induced EMF E and e generated by the transmission line_IThe phase is the same; When n is odd, E and e_IIn contrast, the phase factor is (-1)^N.

When the distance Z between the sending coil and the encoding cable is small, the magnetic line generated by the approximate task sending coil is evenly distributed along the X direction and passes through the transmission line vertically. Therefore, the amplitude A of the inductive electromotive force E generated by the transmission line is proportional to the inductive area of the transmission line. As shown in Figure 2, the position d = 0 of the antenna coil and the effective inductive area S = W × B is, A = A_Max. Antenna coil position 3 d=r, effective induction area S=0, A=0. Antenna coil 2 position, effective induction area S=(W-2d) × B. Get:

Conversely, if the current is passed through the communication transmission line and the antenna coil is used as the receiving coil, the formula (1) - (3) is still valid according to the mutual inductance principle.

3 Interference Noise Suppression Technology

In order to suppress the interference, especially the same frequency interference noise, the most effective way is to keep the interference noise out of the receiver. Therefore, the design idea is: the receiving end of the control room - the transmission line of the coded cable and the receiving end of the vehicle - should be rationally designed to attenuate the interference noise, while attenuating, not attenuating or even enhancing the communication signal as little as possible, so as to improve the signal-to-noise ratio.

3.1 Intersection Design of Double Transmission Line and Single Receive Antenna with Same Spacing

"Double transmission lines intersect with single receiving antenna at the same distance" design, arrange two pairs of intersecting communication transmission lines L in encoding cable₀, L₁; Using a sending antenna and a receiving antenna, the receiving antenna is multi-winded by the wires in a crossover manner, so it can be seen as having a receiving coil 1 and a receiving coil 2. The transmission line intersection spacing, the receiving antenna intersection spacing, and the sending coil width are all W. As shown in Figure 3.

Figure 3 (a) shows the actual structure and working diagram. Figure 3 (b) is a transmission line L for analysis purposes₀, L₁In the practical application, W = 20cm is chosen.

3.2 Transmission Line Receiving Interference Suppression Analysis

When a signal current is added to the transmission antenna on the locomotive, the central control room receives the signal from the communication transmission line. In order to suppress the interference noise, the communication transmission line L0 is crossed at a distance of W. From a long distance, this is a pair of twisted pairs, which can suppress the interference noise up to several dB to 30 dB, with an average of 15 dB.^[7].

For communication signals, according to formula (3), communication transmission line L₀Up-sensing signal amplitude AL₀Is a function of the antenna position x, when the center of the sending coil is aligned to L₀At any of the above intersections, A_L0=0, channel dead zone appears. To avoid this, arrange another pair of communication transmission lines L in the encoding cable₁, its intersection with L₀Staggered, see Figure 3. Make D₀,d₁Represents L where the sending coil position x offset x is located, respectively₀Transmission Line, L₁The distance of the central line in the region of the transmission line is r=d₀+d₁. Make eL0 represent transmission line L₀Induction signal, eL1 for transmission line L₁Induced signal, in the central control room electronic equipment, the signal E'after 90 degree phase shift of eL1_L1And e_L0The sum gives the synthetic signal E. According to formula (2):

Amplitude of synthetic signal e A to D₀Derivation, get the right d₀=d₁There is a minimum value when = r/2In this case, the sending antenna is in the most adverse position. The vector diagram of E is shown in Figure 4.

The above analysis shows that the dual transmission line reception with the crossover shown in Figure 3 can suppress the interference noise effectively. For communication signals, when the sending antenna is in the worst position, there is a 3dB attenuation.

3.3 Analysis of Interference Suppression in Receiving Antenna

For interference noise, the traditional receiving antenna is a single coil without crossing, and has no anti-jamming capability. Because the receiving coil 1 crosses the receiving coil 2, the interference noise EMF eN1 and eN2 induced by the two coils are in opposite phase when working in the field. If the noise electromagnetic wave is evenly distributed within a small range of 2W along the X direction of the receiving antenna, then e_N1=-e_N2Receive the noise EMF extracted by the antenna e_N=e_N1+e_N2=0.

For communication signals, the modulation signal f to be sent by the central control room₀After power amplification, from transmission line L₀Issue; F₀90 degree phase shifted signal f₁Amplified power from transmission line L₁Sent out, these two signals are generated in an electromagnetic field near the coded cable and are sensed by the receiving antenna near the coded cable, because f₀And F₁Orthogonal to avoid channel dead zone. The induction signal generated in the conventional receiving antenna is described in equation (6). The receiving antenna shown in Figure 3 generates the induction electromotive force E from the receiving coil 1 and the receiving coil 2.⁽¹⁾,e⁽²⁾. Because of the intersection of the same spacing, the receiving antenna has:

(1)d₀⁽¹⁾=d₀⁽²⁾,d₁⁽¹⁾=d₁⁽²⁾According to formula (6), e⁽¹⁾,e⁽²⁾Equal range;

(2) If transmission line L_I(i=0,1) The electromagnetic field produced by the K region plays a dominant role in receiving coil 1. The electromagnetic field produced by the K+1 region plays a dominant role in receiving coil 2. Due to the crossing of transmission lines, the electromagnetic field produced by the K+1 region is opposite to the phase of the electromagnetic field produced by the K region. The receiving coil 2 crosses the receiving coil 1n and passes through two reverse phases.⁽¹⁾,e⁽²⁾Same phase.

Therefore, the inductive EMF extracted from the receiving antenna for the communication signal e=e⁽¹⁾+e⁽²⁾=2e⁽¹⁾Is twice as large as a conventional receiving antenna.

In addition, when the sending coil sends a signal, the voltage at both ends of the sending coil is 200V._P-PIn order to prevent the strong signal sent by the receiving coil from damaging the receiving preamplifier circuit, the sending coil is placed between the two coils of the receiving antenna, so that the EMF of the receiving antenna sensing the sending antenna signal is approximately zero.

Analysis of 3.4 Receiving Antenna Interference Suppression Experiment

The experimental conditions are: total length of transmission line is 3m, W=20m M. With a set of actual induction wireless data communication devices, the communication rate is 4800 b/s, the modulation mode is FSK, the carrier frequency is 49 KHz, when working normally, in L₀The peak current of the modulated signal passed through in 0.07A; The peak current of the modulated signal passed through the transmitting antenna coil is 0.38A.

In the experiment, keep the distance between the sending coil and the encoding cable z=200 mm, and keep the center of the sending coil aligned to L.₀One cross does not move. In this case, the transmission line L is measured₁Up-induction signal voltage amplitude V_L1=25mV_P-PInduction signal voltage amplitude V on receiving antenna_A=20mV_P-P.

If the signal generator is used as the interference source, a pair of parallel conductor coupling is used to interfere, as shown in Fig. 5. Signal generator output interference voltage v=V_MSin2pift, f=49KHz, R=130_.

The experiment shown in Fig. 5 (a) corresponds to the interference of a conventional receiving antenna. The experiment shown in Fig. 5 (b) refers to the interference of the cross coil of the receiving antenna. The interference induction EMF extracted from the receiving antenna is set to V._Nm(Peak-Peak). Table 1 shows data from two experiments.

The experimental results show that it can suppress the noise by up to 48dB.

The above theoretical and experimental analysis shows that the same spacing crossed receiving antenna not only has a strong effect of suppressing noise, but also has a 6dB gain compared with the traditional receiving antenna, which greatly improves the signal-to-noise ratio.

4 Conclusion

The interference suppression technology of "double transmission line crossing with single receiving antenna with the same spacing" has been applied in the centralized control and management system of mobile locomotive computer composed of induction wireless technology, and has been used in many industrial fields such as coking plant of Laiwu Iron and Steel Company. In practical application, it can really suppress the interference in industrial field, especially the same frequency interference produced by variable frequency speed control device, which ensures the reliability of data communication. Of course, the interference suppression technology proposed in this paper for inductive wireless data communication only suppresses the interference noise at the receiving end, and for electronic devices working in harsh industrial sites, measures such as grounding and shielding must also be taken, which is not discussed in this paper.