Design of inductive coupling channel analysis system based on LabVIEW

QIU Zu-rong， ZHANG Qi， LI Hong-zhi， ZHANG Xiao-wei

(1. School of Precision Instrument and Optoelectronic Engineering, Tianjin University, Tianjin 300072, China；2. National Ocean Technology Center， Tianjin 300112, China)

Abstract： Inductive coupling transmission system is an important measurement device for acquiring and transmitting marine environmental information. However, low transmission rate cannot meet the current demand for large data transmission in marine environment detection at home. In order to improve the transmission performance of the system in practical communication system, optimizing the design by directly changing the circuit parameters is time-consuming and expensive. Therefore, a set of inductive coupling transmission channel analysis system is designed based on virtual instrument to improve the transmission rate and reliability of inductive coupling transmission system. The bit error rate of channel system at different frequency and noise levels are tested by using three kinds of digital modulation mode including amplitude shift keying (ASK), frequency shift keying (FSK) and differential phase shift keying (DPSK), taking square wave and sine wave as a carrier. Finally, the sine wave is selected to be carrier signal and DPSK is chosen to be modulation mode. The reliable transmission of signal with the error rate less than 0.005 and the transmission rate of 9 600 bps, at the noise level of -10 dB, is realized and verified by the debugging circuit experiments with multi-nodes in the laboratory. The study provides an important experimental evidence for improving signal transmission reliability of inductive coupling transmission system.

Key words： inductive coupling transmission; transmission rate; virtual instrument; modulation and demodulation

0 Introduction

Covering approximate 71% of the Earth’s surface and representing 90% of the Earth’s biosphere, oceans play a major role in the world[1]. As an important instrument for deep sea data measurement, inductive coupling transmission system is used in anchor system such as mooring buoys and submersible buoys to realize communication between multi-node underwater equipment and surface buoys[2]. At present, the SEA-BIRD Corporation in the United States has mastered inductive coupling transmission technology and applied it to actual marine monitoring and its data rate is up to 9 600 bps[3]. The transmission rate of inductive coupling data transmission system of RBR Ltd. in Canada can reach 4 800 bps[4]. In China, the National Ocean Technology Center has designed the inductive coupling transmission chain control system with low power consumption. At present, this technology has been successfully applied in the 1 000 m deep ocean environment, and the transmission rate is 1 200 bps[5]. From this it appears that there is a big disparity between the current inductive coupling transmission technology at home and abroad. With the increasing demand for transmission of the marine environment information, existing inductive coupling transmission systems cannot meet the needs of deep sea measurements， therefore they need to be tested and optimized to meet the requirements of high speed and stable transmission of large amounts of monitoring data[6].

The traditional channel test adopts the method of changing the traditional circuit parameters, debugging and conducting sea trial. This method has the disadvantages of long test period, high cost and restraints of sea trial conditions. In order to be more convenient to test the channel, an inductive coupling transmission channel test and analysis system based on LabVIEW is presented in this paper. A multi-function virtual instrument system, which takes the computer and data acquisition card as the main hardware and LabVIEW as the software platform, for digital modulation and demodulation and frequency analysis is constructed. Users can customize the design of transmission schemes and evaluate it through waveform characteristics and bit error rate. Three digital modulation modes, including ASK, FSK and DPSK, which use square wave and sine wave as a carrier, are built to test the bit error rate of transmission channel. In this way, the transmission channel is tested, and its transmission rate can be up to 9 600 bps. Compared with the traditional circuit debugging method, the channel analysis system has the advantages of simple structure, convenient result reading and friendly man-machine interface, therefore it has great application value. The results of this study can provide an experimental evidence for improving the transmission rate of the channel and have important guiding significance for the subsequent implementation of the circuit design.

1 Inductive coupling principle

The inductive coupling transmission system, using seawater and the cable of fixed point observation platform as the transmission medium, is based on the principle of electromagnetic coupling. Underwater equipment and overwater terminal circuit are connected to an induction magnetic ring respectively. The steel cable with electrodes at both ends passes through the inner hole of the magnetic rings. The cable and sea water constitute a closed loop, as shown in Fig.1.

Fig.1 Inductive coupling transmission system

Its transmission principle is equivalent to two transformers, and data information of each underwater instrument is added to the primary winding of the underwater circular magnetic ring through the carrier. The data is sent to the overwater data terminal using magnetic coupling technology[7].

Based on this principle, the inductive coupling physical channel platform is built in laboratory. Lab-built inductive coupling physical channel consists of water tank, coupling ring, cable and resistance box. The channel uses double magnetic ring-a single winding structure. The seawater environment is simulated by the water tank filled with sea water and the plastic cables with different lengths are simulated by the resistance box, as shown in Fig.2.

Fig.2 Lab-built physical channel

2 System structure

The structure of the inductive coupling transmission channel test system is shown in Fig.3.

Fig.3 Channel analysis system

The whole system consists of three parts: transmission channel, data acquisition card and analysis model based on LabVIEW.

2.1 Hardware system design

The hardware platform includes PC platform, high-speed data acquisition card and lab-built physical channel. The PC is used to install the driver software, configure management software and application software LabVIEW to complete interaction with the data acquisition card for the sending and receiving control of the collection task and the modulation and demodulation, analysis of the signal.

The reception and transmission of signal is achieved by the data acquisition card, NI-USB6353. It has the advantages of small size, portability and low power consumption. Moreover, it has differential input voltage range of ±15 V, 32 analog input channels and 4 analog output channels for simultaneously spontaneous signal self-receiving. The maximum sampling rate of single-channel is 1.25 m of samplings per second and the acquisition card resolution is 16 bit, which fully meet the data acquisition needs of the analysis system.

2.2 Software system design

The software mainly includes driver, configuration management software and LabVIEW application[8-10]. The driver software uses NI DAQmx for data acquisition hardware device. The configuration management software uses NI measurement and automation explorer for interaction with the hardware. LabVIEW is located at the top of the software system for writing codec program, and it can design a professional interface with the functions of parameter settings and graphical display.

3 System implementation

Due to the interference of ocean noise and the attenuation characteristics of the channel, the transmission of underwater data cannot be carried out in baseband. Therefore, it is necessary to complete the modulation and demodulation of the transmitted signal by means of the encoding and decoding algorithms. The system uses virtual instrument to simulate the modulation and demodulation processes and achieve the three kinds of modulation and demodulation algorithms. The system also simulates the impact of ocean noise on channel transmission. The system has the advantages of convenient debugging, low cost and high reliability of experimental results.

3.1 Design of LabVIEW back panel

The inductive coupling transmission channel analysis system based on LabVIEW realizes three kinds of commonly used digital modulation and demodulation techniques: ASK, FSK and DPSK. This design uses the choice structure of LabVIEW to realize the choice of the modulation and demodulation ways. The transmission scheme is determined by the number of selected codes, carrier type, carrier frequency, symbol width and sampling frequency.

3.1.1 ASK modulation and demodulation

ASK modulation is a digital modulation mode in which the amplitude of the carrier varies with the digital baseband signal[11]. Sub-VI that generates random codes is used to generate random binary data. After the spread spectrum, sub-VI gets the sequence of waveforms. ASK modulation signal can be acquired by the product of the sequence of the carrier. ASK demodulation uses coherent demodulation. The multiplier achieves to move the digital modulation signal spectrum back near to zero. The low-pass filter removes the high frequency components generated by the multiplier. The filtered waveform is judged by the sampling adjudicator sub-VI, and the original binary data is obtained. The block diagram is shown in Fig.4.

Fig.4 Block diagram of ASK

3.1.2 FSK modulation and demodulation

FSK modulation is a digital modulation using digital baseband signal to control the frequency of high-frequency carrier[12]. Its signal is equivalent to the sum of two different frequency modulation ASK signals. Its modem block diagram is shown in Fig.5.

Fig.5 Block diagram of FSK

3.1.3 DPSK modulation and demodulation

DPSK modulation uses the changes of carrier phase to represent digital information[13]. It converts original binary code to relative code first, and then do phase modulation.

DPSK demodulation uses polarity comparison method. DPSK modem block diagram is shown in Fig.6.

Fig.6 Block diagram of DPSK

3.2 Design of LabVIEW front panel

The front panel is used to design the transmission scheme and show the transmission results. It is divided into five parts: modulation parameter setting, demodulation parameter setting, input and output data comparison, waveform display of modulation and demodulation processes, and channel spectrum analysis. As shown in Fig.7, when the code rate is 9 600 bps, channel noise is -10 dB and the carrier is a sinusoidal wave of ±5 V, DPSK modulation and demodulation processes and the results show that the whole system is operating normally.

4 System test and result

Different transmission schemes, including the choice of modulation and demodulation methods, code rate, carrier type, carrier frequency, symbol width and other parameters, are designed to test the inductive coupling transmission channel. The transmission scheme of the original inductive coupling transmission scheme with a code transmission rate of 1 200 bps uses a square wave carrier of 19.2 kHz[7]. Due to hardware limitations, the sampling frequency of the digital signal is set to be consistent with the sampling frequency of the acquisition card. If you want to restore the signal in the time domain, the sampling frequency should be 5 to 10 times of the carrier frequency. The carrier frequency of the transmission scheme cannot be higher than 125 kHz. Assuming that baseband signal code rate isRs, sampling rate isFsand symbol width isB, there is the following relationship, namely

(1)

According to the transmission characteristics of the channel, four transmission schemes are designed, as shown in Table 1.

Table 1 Transmission schemes

The results tested with these four schemes are shown in Fig.8.

Fig.8 Bit error rates of four schemes

It can be observed from Fig.8(a) that when ASK modulation uses the square wave as the carrier, the error rate increases first and then decreases with the increase of the frequency. When the symbol rate is 1 200 bps, the error rate is 0.026. When the symbol rate is 2 400 bps, the bit error rate increases to 0.031. When code rates are 4 800 bps and 9 600 bps, the error rates are 0.014 and 0.015 respectively. The variation trends of the error rates of FSK and DPSK are the same as that of ASK. In general, it seems that the bit error rate of FSK is greaster than that of ASK, and the bit error rate of DPSK is the minimum among them. It proved that DPSK modulation is obviously more suitable for inductive coupling transmission than the other two.

It can be observed from Fig.8(b) that the bit error rate of modulation using sine wave is less than that of modulation using the square wave, but they have the same change trend of bit error rate. When the modulation mode is DPSK, the bit error rate is 0. The attenuations of the channel are different at different frequencies. A square wave has an abundant frequency component, while the frequency of sine wave is single, so it is more difficult to be distorted than the square wave.

For the complicated environment in the ocean, it is necessary to consider the anti-noise performance of different modulation and demodulation methods. In general, the noise of ocean is Gaussian white noise[14]. On the basis of the fourth scheme, the bit error rates of different modulation and demodulations under different levels of Gaussian white noise are investigated. As shown in Fig.9, it can be observed that as the noise power increases from -60 to -10 dB, the bit error rate of ASK modulation increases from 0.015 to 0.045, the bit error rate of FSK modulation increases from 0.03 to 0.077, and the bit error rate of DPSK is 0. It proved that anti-noise performance is DPSK> ASK> FSK. It is further illustrated that DPSK signal is more suitable for inductive coupling transmission channel.

Fig.9 Bit error rates at different noise levels

5 Verification

The design scheme is implemented and tested by the debugging circuit experiments with multiple nodes in the laboratory. The experimental device is shown in Fig.10.

Fig.10 Diagram of experimental device

The transmission test has a transmission rate of 9 600 bps with quasi-realtime communication and has 24 sensor nodes. The total duration of the experiment was 15 days, averaging 30 times per hour, and each monitoring data was 24 groups. The total number of data groups isN=15×24×30×24=259 200. Table 2 is a collection of seawater temperature and depth data directly copied from the measuring machine. The measuring time is 2018-02-24 16:50:28.

Among them,Ris the resistance of thermistor,Tis the temperature that is converted byR,Uis the output voltage of pressure sensor, andDis the depth of measuring point converted byU. In all data, there are 26 sets of wrong data, and the data efficiency is as high as 0.999 8, which indicates that the transmission scheme has been successfully implemented and the data stability has been improved significantly.

Table 2 Result of measured data

6 Conclusion

This paper introduces a method based on virtual instrument for channel analysis, instead of the traditional circuit debugging. It has the advantages of convenient debugging, low cost, high confidence in the experimental results. The system has successfully implemented three kinds of modulation and demodulation techniques and has obtained the optimal transmission scheme of the inductive coupling transmission system and meets requirement of channel analysis. The system can be used for inductive coupling transmission system testing as well as other physical or analog channels test.