Research on signal processing method of acoustic short-wave propagation in rotary steerable tools

Preface

Rotary steerable closed-loop drilling technology is a breakthrough and strategically significant technology among various advanced drilling technologies and processes developed at home and abroad. It combines computer control technology with drilling tools and represents the highest level of drilling technology development in the world today. The controllable eccentric studied by the Downhole Measurement and Control Research Institute of Xi'an Shiyou University is an intelligent steerable drilling tool that can realize a downhole closed-loop rotary steerable intelligent drilling system.

The proposal of acoustic short transmission

To realize rotary steerable closed-loop drilling, it is inseparable from the communication between the ground and downhole combination. For the transmission of signals from downhole to the ground, the oil industry has a relatively mature and commercial technology - measurement while drilling (MWD), which is mainly used to monitor the working conditions of the downhole and transmit it to the ground. However, there is no mature technology for short-distance transmission of near-drill-bit data.

Since 1993, the Downhole Measurement and Control Research Institute of Xi'an Shiyou University has been committed to the research and testing of downhole closed-loop rotary steerable intelligent drilling systems, including transmitting the data collected by the position sensor near the drill bit to the spindle. Initially, a slip ring was used between the spindle of the controllable eccentric and the non-rotating sleeve to transmit the signal. However, there are some problems when using this method to transmit signals, so the best solution is to develop a near-drill-bit wireless short-transmission device.

In 2007, the Downhole Measurement and Control Institute of Xi'an Shiyou University was committed to studying electromagnetic telemetry methods, and established a wireless electromagnetic short-transmission system based on the special structure of the controllable eccentric. The result of the study is: when the transmitting coil and the receiving coil are installed inside the drill collar, the mud is completely conductive, and the signal transmission power is 0.3W, the signal-to-noise ratio at the receiving end is -60dB. However, this method is greatly affected by the conductivity of the formation around the wellbore. Therefore, this design decided to use acoustic waves to achieve signal transmission from the non-rotating sleeve to the main shaft.

Acoustic short-transmission in controllable

eccentric Mechanical structure of controllable eccentric

The main shaft passes through the non-rotating sleeve through the coupling of the bearing, and there are electronic cavities, positioning assemblies for controlling the eccentric displacement vector, and ribs on the non-rotating sleeve. One end of the main shaft is connected to the drill bit, and the other end is connected to the stabilizer. There is also a power supply short section connected to the MWD in the stabilizer. The sensors near the drill bit, such as the internal and external pressure of the drill bit and the drill bit posture, are installed in the electronic cavity on the non-rotating sleeve. The transmitting circuit board, power supply battery and transmitting transducer are also installed in the electronic cavity inside the non-rotating sleeve. They are placed in parallel in the electronic cavity. The receiving end power supply battery, receiving circuit board and receiving transducer are in the power short section of the stabilizer.

Composition of the acoustic short transmission system

(1) Transmitter: The transmitting circuit performs FSK (frequency shift keying) modulation on the data sent by the near-drill bit sensor, that is, two frequency points are selected near the optimal frequency point of the transmission signal as the modulated binary data, and then the modulated FSK signal is sent to the power amplifier through the coupling circuit. The signal is sent to the transmitting transducer after power amplification. The transmitting transducer converts the current signal into an acoustic wave signal. This acoustic wave signal is transmitted in the loop formed by the controllable eccentric, drilling fluid and formation.

(2) Receiving device: The receiving transducer at the other end of the transmission channel converts the transmitted acoustic wave signal into a current signal, feeds the captured signal back to the small signal amplifier, and finally obtains the data transmitted to the MWD after a series of processing by functional modules such as amplification, noise filtering, FSK demodulation and signal detection. The data is transmitted to the PC through the 232 port for drawing and display. The principle block diagram of the acoustic short transmission system is shown in Figure 1.



Figure 1 Principle block diagram of the acoustic wave transmission system

Transmitter signal processing

Modulation method and synchronization signal

Considering the acoustic characteristics of the sound wave transmission along the oil pipe and the frequency response curve when the controllable eccentric is transmitted, 2FSK modulation is used to select 6.8kHz and 7.3kHz with the smallest attenuation in the channel as the transmission frequency. In addition, in order to further analyze the frequency characteristics of the oil pipe sound channel in the medium and low frequency bands during synchronization, the linear frequency modulation signal LFM is selected as the synchronization signal.

Software design

Initialize the C8051F060 microcontroller, the I/O interface of AD9833 and the cross switch. The AD9833 initialization flow chart is shown in Figure 2. When performing FSK modulation, the two frequency registers of AD9833 are loaded with different frequency values. In this design, frequency register 0 is loaded with a low frequency of 6830Hz, and frequency register 1 is loaded with a high frequency of 7230Hz. The main program flow is shown in Figure 3.

Figure 2 AD9833 initialization flow chart

Figure 3 Main program flow

Write data to the control register

The timing of the microcontroller transmitting data to AD9833 is shown in Figure 4. The FSYNC pin is an enable pin, level-triggered, and low level is valid. When performing serial data transmission, the FSYNC pin must be set low. It should be noted that before FSYNC starts to become low (when writing data is about to begin), SCLK must be high.

Figure 4 Timing diagram of the unit transmitting data to AD9833 When

the microcontroller writes 16-bit data to AD9833, the high bit is in front and the low bit is in the back. Use software to simulate the clock signal and chip select signal. The procedure for transmitting data is as follows:

SCLK=1;

FSYNC=1;

Delay(100);

FSYNC=0;

for(i=0;i<16;i++)

{ SDATA=datas&0x8000;

SCLK=0;

Delay(50);

SCLK=1;

datas=datas<<1;

}

Delay(50);

FSYNC=1;

SCLK=0;

Output waveform

Observing the FSK modulated signal on the oscilloscope, it can be seen that the sinusoidal signal output with a frequency of 7230Hz is observed, and the actual output frequency is 7.22985kHz.

Receive signal processing

Receive processing process

Since the acoustic wave sensor outputs an analog signal, the signal must be pre-filtered before ADC sampling. Considering the requirement of minimizing signal ripple and the passband filtering characteristics of the filter, a Butterworth filter is used in this design.

The conditioned sensor signal cannot be sampled immediately by the microcontroller. This is because the sensor output signal range is -5V to +5V, while the input voltage range of the microcontroller ADC module is 0V to 2.5V, so the conditioned sensor signal must pass through the voltage conversion circuit, and the precision voltage chip AD780 provides the microcontroller with a 2.5V standard ADC reference signal.

Program Design

The host computer application is developed using MATLAB's class (Serial) and m language, supplemented by MATLAB's GUIDE toolbox, and program development is relatively simple. The SoC program is compiled in C language. After the lower computer MCU receives the command from the upper PC, it switches to the corresponding subroutine through the serial port interrupt program, performs the corresponding operation, and finally returns a handshake signal as a status flag to confirm whether the command is executed correctly.

Frame synchronization recognition

The synchronization process is actually the process of finding the maximum correlation peak.

First, a threshold GATE is determined. Only when the multiplication-accumulation value is greater than GATE, the correlation peak is compared. GATE must be set for two reasons:

(1) to reduce the number of unnecessary comparisons and improve program execution efficiency;

(2) to filter out the pseudo maximum correlation peak obtained by multiplying and accumulating random noise and LFM signals.

When a multiplication-accumulation value greater than the threshold GATE appears, it indicates that the real LFM signal may arrive. By finding the location of the correlation peak, the starting bit of the information code can be known.

Software design of demodulation filter

Design a bandpass filter with a center frequency of 6830Hz, an order of 8, a sampling frequency of 100kHz, an amplitude attenuation of 1dB in the passband range, and a minimum attenuation of 30dB in the stopband. According to the above parameters, the filter is designed as follows:

Convert the high-order IIR filter into a cascade of a series of second-order IIR filters. The numerator coefficient output from FDAtool is Num, the denominator coefficient is Den, and the gain is Gain. The transfer function of the filter is shown as follows:



According to the filtering technology requirements, the filter order is obtained in FDAtool. The coefficients use 4 second-order cascaded IIR filters. The fundamental form of these four second-order cascades is the same, except that their coefficients are different. The filter program can be written in the form of a for loop, and 4 loops can form a point filter. The overall filtering software flow is shown in Figure 5.

Figure 5 Software programming flow chart of the fourth-order bandpass Chebyshev filter

This paper constructs a simulation block diagram of the digital filter by calling various functional modules of Simulink in MATLAB. The filter file designed by FDAtool is imported through the digital filter design module in the Simulink environment, and the simulation is repeated to obtain the best filtering effect. The waveform of the original signal and the waveform of the filtered signal are shown in Figure 6 and Figure 7 respectively. From Figure 7, it can be seen that the 7kHz frequency component is separated after discrete sampling and digital filtering.


Figure 6 Original signal waveform Figure 7 Filtered signal waveform Demodulation

results and analysis

Using the above demodulation method, the demodulation program is written in the single-chip microcomputer C8051F060, the online debugging is completed through the adapter, and the decoding software that is finally debugged successfully is used for decoding test. After the sound wave signal is transmitted in the controllable eccentric, the receiving end conditions and collects the received signal. The time domain diagram of the collected signal is shown in Figure 8, and then enters the demodulation program for demodulation, and the graph drawn in MATLAB is shown in Figure 9. It can be seen that the code element of the receiving end signal after decoding is 101010101010101010, which is consistent with the information code of the transmitting end, indicating that the demodulation is successful.

Figure 8 Signal time domain diagram Figure 9 Decoded waveform diagram

Conclusion

In a complex underground environment, accurate and real-time transmission of information is very important. This paper uses 2FSK technology to process the short-transmission acoustic signal in the rotary steering tool. Compared with the mud pulse transmission method, the signal transmission rate is faster. At the same time, an IIR filter is designed at the receiving end to effectively overcome the problem of severe attenuation when electromagnetic wave transmission is used. In addition, the 2FSK technology equipment is simple and easy to demodulate, which is conducive to promotion and use.