Design of Logging While Drilling System Based on FSK Wireless Communication

introduction

Oil and natural gas are natural resources that humans rely on for survival. During the drilling and mining process, it is necessary to have a real-time understanding of the high-temperature and high-pressure environment underground, so the real-time requirements for signal transmission are very high. However, the underground environment is harsh, and there are various interferences in the communication system, so it is crucial to design a system that can resist interference and transmit signals at a faster baud rate for this type of operation. This article uses the AT89C51 microcontroller as the controller, XR2206 and XR22111 as FSK modulation and demodulation chips, and builds and simulates the entire communication process in the laboratory.

1 Theoretical Analysis

The system mainly involves two theories: 2FSK modulation theory and electromagnetic induction theory.

1.1 2FSK Modulation Theory

To conduct wireless communication, the signal must be modulated. There are many digital modulation methods, such as ASK, FSK, PSK, etc. After comprehensive consideration, 2FSK modulation is selected here, which has a certain anti-interference ability and is simple and easy to use. 2FSK uses sine waves of different frequencies to represent digital signals "0" and "1". The frequency of the carrier changes between the two frequencies of f1 and f2 with the binary baseband signal. Its expression is shown in formula (1), and the 2FSK signal waveform is shown in Figure 1.

1.2 Electromagnetic Induction Theory

The wireless communication here is actually an inductive communication. Two coils are placed adjacent to the two drill pipes. The change of current in one coil (primary coil) will generate an alternating magnetic field around it. This alternating magnetic field causes the other coil (secondary coil) to generate an induced electromotive force. This is the principle of inductive communication. Its schematic diagram is shown in Figure 2.

2 System Hardware Design

The hardware structure diagram of the system is shown in Figure 3. The whole system is based on two single-chip microcomputers, with XR2206 and XR2211 as modulation and demodulation chips, MAX275 as a bandpass filter chip, and the amplifier circuit uses a basic common emitter amplifier circuit. The system can realize two-way communication (not full-duplex, but can only work in half-duplex mode, that is, the channel can be time-division multiplexed). The ground mainly transmits control signals downward, and the underground transmits temperature, pressure and other information upward. It has a symmetrical structure from top to bottom, so only the module of unidirectional signal transmission is analyzed in the following analysis. Of course, in actual drilling operations, some wells are as deep as a kilometer, and multi-level drill pipes need to be cascaded, so the signal needs to be demodulated, amplified and modulated after every 3 to 5 levels of drill pipes. In fact, a relay module is needed, which completes the functions of demodulation, amplification and re-modulation.

2.1 Modulation module and demodulation module design

Through the test of channel characteristics, it is found that when the four-stage drill pipe cascade (40m in total) and the number of turns of the coupling coil is 400 turns (the radius of the copper wire used in the coupling coil is 0.2mm), the amplitude of the output signal is the largest when a 120kHz sine wave is input, indicating that the resonant frequency of the channel formed by the induction coil is 120kHz, and the carrier frequency can be selected near 120kHz. Therefore, the low-cost XR2206 that meets the requirements is used as the modulation chip, and the paired XR2211 is used as the demodulation chip.

XR2206 main performance parameters:

◆Single-chip integrated function generator, capable of generating high-stability, high-precision sine, square, triangle and other waveforms.

◆The operating frequency range is 0.01 Hz to 1 MHz.

◆The operating voltage is 10~26 V, and the frequency temperature stability is 20×10-6/℃.

The modulation module circuit composed of XR2206 is shown in Figure 4.

According to the previous analysis, the two carriers of 2FSK are selected as f1=110 kHz and f2=130 kHz, "1" is modulated on f1, and "0" is modulated on f2. The capacitor between pins 5 and 6 of XR2206 is a timing capacitor, which is 1000pF. In order to obtain f1 and f2, two timing resistors R1=1/f1c=9.09 kHz and R2=1/f2c=7.7 kHz can be obtained. Here, R1 and R2 are respectively selected as a 5 kΩ resistor and a 10 kΩ potentiometer in series for accurate adjustment. The digital signal is input from pin 9, and the modulated signal is output from pin 2; the 500 Ω potentiometer connected in series between pins 13 and 14 can improve the output waveform; the potentiometer R3 connected to pin 3 is used to adjust the output amplitude; the 10 kΩ potentiometer connected in series between pins 15 and 16 can improve the distortion of the output sine waveform.

XR2211 performance parameters:

◆The operating frequency range is 0.01 Hz to 300 kHz.

◆Operating voltage is 4.5～20 V, frequency temperature stability is 20×10-6/℃, HCMOS/TTL/logic compatibility.

◆Wide dynamic range: 10 mV～3 Vrms.

The demodulation module circuit composed of XR2211 is shown in Figure 5. [page]

According to the XR2211 chip manual, calculate the relevant parameters according to its steps. The recommended range of center frequency and R0 is 10-100 kΩ. Here, R0=16.67 kΩ is selected, then C0=1/f0R0=500 pF, R1=2R0·f0/(f2-f1)=200 kΩ, C1=1 250×C0/R10.52=12 pF; Rf≥5R1, take Rf=1 MΩ, RB≥5Rf, take RB=5 MΩ, Rsum=(Rf+R1)RB/(Rf+R1+RB)=967 kΩ; Cf=0.25/(Rsum×baud rate), when the baud rate is 9 600 b/s, Cf=27 pF. When the baud rate is other values, just change the value of Cf accordingly. If the value calculated above is not a nominal value of a resistor or capacitor, the required resistor or capacitor can be formed in series or in parallel.

2.2 Filter circuit design

Due to the bad channel environment and the presence of various noises, the signal is almost drowned in the noise, so the signal needs to be filtered. A bandpass filter is needed here. If an ordinary active filter is selected, it is simple to implement but difficult to adjust the parameters, and it is used in high-frequency occasions. Since the distributed capacitance around the components will seriously affect the characteristics of the filter, its stability is also poor. Here we choose the analog integrated active filter MAX275. Using MAX275 can avoid the shortcomings of active filters. Its main characteristic parameters are as follows:

◆By connecting different external resistors, Butterworth, Chebyshev, and Bessel type low-pass and band-pass filters can be realized.

◆The center frequency range of the filter is 0.01 Hz to 300 kHz.

◆Gain bandwidth product is 16 MHz.

◆Single 5 V power supply or ±5 V power supply.

The bandpass filter circuit composed of MAX275 is shown in Figure 6.

According to the MAX275 manual, the parameters of the peripheral components can be obtained. Since the selected carrier frequencies are 110kHz and 130kHz respectively, the center frequency of the bandpass filter is set to 120kHz, and the two carrier frequencies must be within the passband of the filter. The passband range can be set to 105-135kHz. According to these requirements, the relevant parameters can be obtained: R1=5.1kΩ, R2=16.7kΩ (can be achieved by a 10kΩ resistor in series with a 10kΩ potentiometer), R3=16.7kΩ (can be achieved by a 10kΩ resistor in series with a 10kΩ potentiometer), R4=11.7kΩ (can be achieved by a 11kΩ resistor in series with a 1kΩ potentiometer). Since MAX275 is a two-stage cascade filter, the parameters of the two stages can be selected to be the same, that is, R5=R1, R6=R2, R7=R3, R8=R4. The performance of the filter is tested when debugging the circuit, and the data is plotted using MATLAB. Its amplitude response is shown in Figure 7.

As can be seen from Figure 7, the passband range of the filter is approximately 110 to 133 kHz, and the center frequency is around 121 kHz, which can meet actual requirements.

2.3 Amplification circuit design

Any communication system cannot do without an amplifier circuit, and this system is no exception. After the signal passes through the 4-stage drill pipe, it is only about 30 mV and is completely submerged by noise, so the signal must be amplified at least 100 times to meet the processing requirements of the subsequent circuit. There are many ways to implement the amplifier circuit. You can choose to build it with discrete components or use an integrated operational amplifier. However, it must be noted that the integrated operational amplifier has a gain-bandwidth product. For example, for an operational amplifier with a gain-bandwidth product of 1 MHz, it is not suitable for amplifying signals with a frequency exceeding 100 kHz. Therefore, this article uses an amplifier circuit composed of triodes. When designing an amplifier circuit, pay attention to selecting a triode with a relatively high cutoff frequency. Here, a high-frequency, low-power tube 3DG100 is selected. It is necessary to ensure that the triode will not cause cutoff distortion and saturation distortion when amplifying useful signals. There are many configurations of amplifier circuits. Here, a common emitter amplifier circuit is selected. The first-stage amplifier circuit obviously cannot meet the amplification requirements. The second-stage amplifier circuit can meet the requirements. At the same time, the output of the common emitter amplifier circuit is inverted with the input, and the output and input after the second-stage amplification meet the requirements of being in phase. There are many books and materials available for reference on amplifier circuits, so I will not go into details here.

3 System Software Design

The software in this system is mainly divided into two parts: one is the test software required in the system debugging process, such as generating a square wave signal of a specific frequency to simulate the binary "1" and "0". A keyboard circuit can be added to the periphery of the microcontroller to flexibly select data of different baud rates for testing; the other is the software that plays a control role in the system operation process, which is used to control the transmission direction of information, etc. The writing of the software must comply with the serial port communication protocol between the microcontroller and the computer. If the transmitted data is encoded, although it can improve the reliability of the communication system and reduce the bit error rate, it will affect the rate of effective data transmission, so the channel is not encoded here.

4 Test methods and results

The test of this system follows the principle of from part to whole. Each module is tested first, and then the whole system is tested after each module is tested. One PC sends data and another PC receives data. The sent data and the received data are compared to get the bit error rate. When the baud rate is 9600 b/s, the system bit error rate test results are listed in Table 1.

Conclusion

This paper uses XR2206 and XR2211 as modulation and demodulation chips, and AT89C51 single-chip microcomputer as control chip to design a FSK induction communication system. It has been verified in the laboratory and obtained a baud rate of 9 600 b/s. This system can be used in underground operations such as oil and natural gas, as well as the exploration of marine resources. However, compared with the level of foreign countries, it still needs to be improved. In practical application, there are still many factors to consider in this system: in terms of device selection, resistors and capacitors with poor accuracy will affect the quality of the signal, and there will be problems such as carrier frequency offset; impedance matching of the circuit is the biggest problem of the system. The signal is often transmitted under mismatch conditions, and the signal loss at the coupling coil is quite serious; in addition, due to the limitation of Shannon's theorem, the communication rate of the system cannot be very high. It is possible to consider modulating the signal to a higher frequency band, but at this time, capacitors, inductors and other components should be added to the coupling coil to change the resonance point of the channel so that it resonates at a higher frequency band. The above problems need to be carefully studied in future work.