Analysis, Design, and Experimental Verification of a Parallel Wireless Power and Data Transmission Method for Rotary Steering Systems

Abstract

In the rotary steering system of oil drilling, both power and data transmission are needed. This paper presents a parallel power and data transmission method for a rotating steering system with output voltage control. To reduce the size of the system, power and data are transmitted through the same rotational coupling mechanism. A power transfer resonance circuit is used to suppress the influence of the power transfer on data transmission. Therefore, crosstalk interference between the power and the data transmission channel is negligible. The experimental prototype is built, and the feasibility of the data transfer method and the closed-loop control method is verified. The experimental results are in good agreement with the theoretical analysis.

1. Introduction

In the rotary steering system of oil drilling, power and signal are transmitted between the rotating drill pipe and the relatively stationary guide cylinder wall through the slip ring. However, due to the wear of the slip ring, the reliability of the system will be reduced, and the service life will be limited [1].

Wireless power transfer (WPT) technology provides an effective way to solve the problem of contact power transfer of a rotating mechanism. This technology realizes power transfer with the help of a high-frequency electromagnetic field, and the power transmitting end and receiving end can be completely insulated and closed. Although WPT technology has the disadvantage of lower efficiency and higher cost compared with the contact power transmission method, the whole power transfer process is not affected by any humidity, dust, vibration, or other factors, and has high safety and reliability. At the same time, this technology can also provide higher-flexibility, ease-to-use, and maintenance-free electrical equipment, essentially eliminating mechanical wear and greatly improving the service life of the rotating mechanism [2,3,4].

At present, the application of WPT technology in rotary steering systems has attracted the attention of researchers. It can be used in applications such as high-speed rotating transmitters [5], robotic arm joints [6], and non-contact rotary slip rings [7] to improve power supply reliability and equipment life.

In the application of the WPT system for a rotary steering system, the rotating mechanism needs not only to complete the transmission of power, but also to complete the transmission of data at the same time, i.e., realizing the power and data transfer by using a coupling mechanism [8,9,10,11,12,13].

The simultaneous transmission of power and data in WPT systems has been studied by previous researchers. At present, there are two realization methods: The power modulation method and the independent modulation method. The power modulation method mainly realizes signal modulation by changing parameters such as the amplitude, frequency, and phase of the power wave [9,10]. However, the power modulation method has the problem of power fluctuation, and the signal modulation process will lead to unstable power transmission.

The independent modulation method uses an independent carrier wave much higher than the power resonant frequency to modulate the signal. Because of the existence of the power wave and signal modulation wave on the power coupling coil, the power channel and signal channel influence each other at two frequencies, so the key of the independent modulation method is the injection and separation of the signal modulation wave. At present, the most widely used methods are series and parallel, which connect the signal modulated wave to the power coupling coil in series or in parallel by means of a mutual inductance coupler. References [1,11] adopt parallel signal transmission and reception, which can improve the increase in signal transmission, but the crosstalk between the power and signal is serious. The crosstalk between the two can be reduced by adding a wave-blocking network and adopting a composite resonance network. In references [8,12], a serial injection signal bidirectional transmission WPT system is adopted, and the serial transmission gain is small. Therefore, it is necessary to optimize the parameters of the mutual inductance coupler to improve the stability of the signal transmission. In addition, reference [13] studies the parallel transmission of inductance and capacitance combination of power and data, and the data are transmitted through parasitic capacitance, but not all applications have the required aluminum plate. Compared with power modulation, the independent modulation method can achieve stable power transmission, but the signal transmission is easily disturbed by power harmonics and external noise, which can easily lead to signal transmission failure.

Generally, the WPT system for a rotary steering system is designed to provide a constant output voltage [1]. The output voltage can be kept constant through an open loop to ensure the output voltage is insensitive to load and the coupling coefficient [14,15,16], or the output voltage can be adjusted through a closed loop. The closed-loop regulation can be carried out on the primary side or secondary side [17,18]. The primary-side regulation is realized by changing the input power, and the secondary-side regulation is realized by changing the load impedance.

In this paper, a parallel power and data transmission method of a rotary steering system with output voltage control is proposed. In order to reduce the volume of the system, power and data are transmitted through the same rotary coupling mechanism. A power resonance circuit is applied to suppress the influence of power on data transmission, and then stable and reliable data transmission is realized.

2. System Overview

2.1. The Proposed System

Figure 1 shows the proposed power and data parallel transmission circuit, where L_T, C_T, C_P, and C_S constitute the LCC-S power compensation network, while C_DP and C_DS constitute the S-S data compensation network. Q₁–Q₄ constitute a full bridge inverter and D₁–D₄ constitute a rectifier. The output voltage control is employed by using backward data transmission (i.e., from the secondary side to the primary side). It should be noted that in the rotary steering system, forward data transmission (i.e., from the primary side to the secondary side) is also needed to issue control instructions. The data-processing circuits for backward and forward data are symmetrical. To simplify the analysis, only the backward data transmission circuit is shown in Figure 1, and the parasitic resistances of the coils are ignored. The power resonant circuit, composed of L_PP, C_P1, and L_SS, C_S1, is added to the power transfer channel to suppress the influence of power transfer on data transfer. The data resonant circuit composed of L_PP, C_P2, and L_SS, C_S2 is added to ensure that the data are loaded onto L_S. Here, L_pp and L_ss are not magnetically coupled, and they are additional reactive elements incorporated into the primary- and secondary-site circuits. M is the mutual inductance between L_P and L_S. To control the output voltage, the secondary controller uses the A/D module to sample the output voltage after the voltage divider, and then transmits the information to the primary side through the data transmission channel. After receiving the output voltage data, the primary-side controller controls the inverter through the proportional integral (PI) controller.

2.2. Power Transfer Principle

For the LCC-S compensation circuit, C_T, C_P, and C_S satisfy the following equations [19]:

$$\{\begin{array}{c}\begin{array}{ccc}{C}_{\mathrm{T}}={\left({\omega }^2{L}_{\mathrm{T}}\right)}^{-1}& C_{\mathrm{P}}={\left[{\omega }^2\left(L_{\mathrm{P}}-L_{\mathrm{T}}\right)\right]}^{-1}& C_{\mathrm{S}}={\left({\omega }^2{L}_{\mathrm{S}}\right)}^{-1}\end{array}\\ \begin{array}{l}\begin{array}{c}\begin{array}{cc}{C}_{\mathrm{DP}}={\left({\omega }_{\mathrm{D}}{}^2{L}_{\mathrm{P}}\right)}^{-1}& C_{\mathrm{DS}}={\left({\omega }_{\mathrm{D}}{}^2{L}_{\mathrm{S}}\right)}^{-1}\end{array}\\ \begin{array}{cc}{C}_{\mathrm{P}1}={\left({\omega }^2{L}_{\mathrm{PP}}\right)}^{-1}& C_{\mathrm{S}1}={\left({\omega }^2{L}_{\mathrm{SS}}\right)}^{-1}\end{array}\end{array}\\ \begin{array}{cc}{C}_{\mathrm{P}2}={\left({\omega }_{\mathrm{D}}{}^2{L}_{\mathrm{PP}}\right)}^{-1}& C_{\mathrm{S}2}={\left({\omega }_{\mathrm{D}}{}^2{L}_{\mathrm{SS}}\right)}^{-1}\end{array}\end{array}\end{array}$$

(1)

where ω and ω_D are the operating angular frequencies of power transfer and data transfer, respectively, and satisfy $\omega =2\pi f,{\omega }_{\mathrm{D}}=2\pi f_{\mathrm{D}}$; f and f_D are the operating frequencies of power transfer and data transfer, respectively. The phase shift control strategy is adopted for power control, and the fundamental output voltage in phasor form can be expressed as [8]:

$${\dot{U}}_{\mathrm{in}}=\frac{2\sqrt{2}{U}_{\mathrm{dc}}}{\pi }\sin \frac{\delta }{2}\angle 0^{\circ }$$

(2)

where ${\dot{U}}_{\mathrm{in}}$ is the output voltage vector of the inverter, U_dc is the input DC voltage, and $\delta$ is the conduction angle.

By using Kirchhoff Voltage Law (KVL) in the power transfer channel, the following equations can be derived:

$$\{\begin{array}{c}{\dot{U}}_{\mathrm{in}}=\left(j\omega L_{\mathrm{T}}+\frac{1}{j\omega C_{\mathrm{T}}}\right){\dot{I}}_{\mathrm{T}}-\frac{1}{j\omega C_{\mathrm{T}}}{\dot{I}}_{\mathrm{P}}\\ 0=\left(j\omega L_{\mathrm{P}}+\frac{1}{j\omega C_{\mathrm{P}}}+\frac{1}{j\omega C_{\mathrm{T}}}+\frac{1}{j\omega C_{\mathrm{P}1}}+j\omega L_{\mathrm{PP}}\right){\dot{I}}_{\mathrm{P}}-\frac{1}{j\omega C_{\mathrm{T}}}{\dot{I}}_{\mathrm{T}}-j\omega M{\dot{I}}_{\mathrm{S}}\\ 0=-j\omega M{\dot{I}}_{\mathrm{P}}+\left(j\omega L_{\mathrm{S}}+\frac{1}{j\omega C_{\mathrm{S}}}+\frac{1}{j\omega C_{\mathrm{S}1}}+j\omega L_{\mathrm{SS}}+R_{\mathrm{eq}}\right){\dot{I}}_{\mathrm{S}}\\ {\dot{U}}_{\mathrm{R}}=R_{\mathrm{eq}}{\dot{I}}_{\mathrm{S}}\\ U_{\mathrm{L}}=\frac{\pi \sqrt{2}}{4}{U}_{\mathrm{R}}\end{array}$$

(3)

where ${\dot{U}}_{\mathrm{R}}$ and U_R are the input voltage vector and the RMS value of the rectifier, respectively. R_eq is the equivalent input resistance of the rectifier. Because the value of C_P2 and C_S2 is very small, it approximates an open circuit for low frequency, so it can be ignored for power transfer.

By submitting (1) and (2) into (3), the output voltage U_L can be obtained:

$$U_{\mathrm{L}}=\frac{M{U}_{\mathrm{dc}}}{L_{\mathrm{T}}}\sin \frac{\delta }{2}$$

(4)

2.3. Data Transfer Principle

As can be seen from Figure 1, the data transfer circuit can be divided into the transmitting-side circuit and the receiving-side circuit. The transmitting-side circuit includes an amplitude shift keying (ASK) modulation module and a power amplifier. The receiving-side circuit includes a bandpass filter, an envelope detector, and a comparator. The generation of the ASK modulation can be expressed as:

$$y\left(t\right)=\{\begin{array}{c}A\cos \left(2\pi f_{\mathrm{D}}t\right),\hspace{1em}1\\ 0,\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em} 0\end{array}$$

(5)

where A is the amplitude of the data carrier. The principle of data modulation is shown in Figure 2.

By using KVL in the backward data transfer channel as shown in Figure 1, one can obtain:

$$\{\begin{array}{c}{\dot{U}}_{\mathrm{DS}}=\left(j{\omega }_{\mathrm{D}}{L}_{\mathrm{S}}+\frac{1}{j{\omega }_{\mathrm{D}}{C}_{\mathrm{DS}}}+\frac{1}{j{\omega }_{\mathrm{D}}{C}_{\mathrm{S}}}+R_{\mathrm{eq}}\right){\dot{I}}_{\mathrm{DS}}-j{\omega }_{\mathrm{D}}M{\dot{I}}_{\mathrm{DP}}\\ \begin{array}{l}0=-j{\omega }_{\mathrm{D}}M{\dot{I}}_{\mathrm{DS}}+\left(j{\omega }_{\mathrm{D}}{L}_{\mathrm{P}}+\frac{1}{j{\omega }_{\mathrm{D}}{C}_{\mathrm{DP}}}+\frac{1}{j{\omega }_{\mathrm{D}}{C}_{\mathrm{P}}}+\frac{1}{j{\omega }_{\mathrm{D}}{C}_{\mathrm{T}}}+R_{\mathrm{Peq}}\right){\dot{I}}_{\mathrm{DP}}\\ {\dot{U}}_{\mathrm{DP}}=R_{\mathrm{Peq}}{\dot{I}}_{\mathrm{DP}}\end{array}\end{array}$$

(6)

where ${\dot{U}}_{\mathrm{DS}}$ and ${\dot{U}}_{\mathrm{DP}}$ are the output voltage of the transmitting-side power amplifier and the input voltage of the receiving-side bandpass filter, respectively. R_Peq is the equivalent input resistance of the receiving-side bandpass filter.

By submitting (1) into (6), ${\dot{U}}_{\mathrm{DP}}$ can be calculated as:

$${\dot{U}}_{\mathrm{DP}}=\frac{j{\dot{U}}_{\mathrm{DS}}{R}_{\mathrm{Peq}}M{C}_{\mathrm{P}}{C}_{\mathrm{S}}{C}_{\mathrm{T}}{\omega }_{\mathrm{D}}{}^3}{M^2{C}_{\mathrm{P}}{C}_{\mathrm{S}}{C}_{\mathrm{T}}{\omega }_{\mathrm{D}}{}^4+C_{\mathrm{P}}{C}_{\mathrm{S}}{C}_{\mathrm{T}}{\omega }_{\mathrm{D}}{}^2{R}_{\mathrm{Peq}}{R}_{\mathrm{eq}}-j{C}_{\mathrm{P}}{C}_{\mathrm{S}}{\omega }_{\mathrm{D}}{R}_{\mathrm{eq}}-j{C}_{\mathrm{P}}{C}_{\mathrm{T}}{\omega }_{\mathrm{D}}{R}_{\mathrm{Peq}}-j{C}_{\mathrm{S}}{C}_{\mathrm{T}}{\omega }_{\mathrm{D}}{R}_{\mathrm{eq}}-C_{\mathrm{T}}-C_{\mathrm{P}}}$$

(7)

3. Rotary Coupling Mechanism

Figure 3 depicts the proposed coupling mechanism, where (a) indicates the schematic diagram of the primary-side coupling mechanism, and (b) and (c) show the COMSOL simulation model of the primary side and the whole system. Power and data are transmitted through this pair of coils.

In the COMSOL simulation process, the type of three-dimensional magnetic field simulation is selected, and some simplification is made in the simulation process. Both the coil and the magnetic core are equivalent to a cylinder, and the simulation boundary is a sphere, while its radius is the longest size of the coupling mechanism. In the COMSOL simulation model, the primary magnetic core of the WPT system is closely attached to the outer surface of the rotating mechanism’s inner cylinder. The transmitting coil composed of litz wires is wound on the primary magnetic core. At the same time, aluminum rings are placed on both sides of the magnetic core to reduce the coupling mechanism for outside electromagnetic interference. Similarly, the secondary magnetic core of the WPT system is closely attached to the inner surface of the outer cylinder, the receiving coil is wound on the secondary magnetic core, and there are also aluminum rings on both sides of the magnetic core. The specific parameters of the coupling mechanism are shown in

Table 1. Coupling mechanism parameters.

	Primary Part	Secondary Part
Litz wire diameter (mm)	3	3
Number of turns	12	12
Coil diameter (mm)	120	140
Magnetic core diameter (mm)	117	143
Magnetic core length (mm)	120	100
Aluminum ring length (mm)	10	10
Coil electrical conductivity (S/m)	5.998 × 107
Aluminum electrical conductivity (S/m)	3.774 × 107
Magnetic core relative permeability	2300
The relative permittivity of all materials	1

.

In COMSOL, by adding the materials of each part, selecting the operating frequency at 25 kHz, and setting the current of the primary and secondary coils to 1A, the simulated magnetic field distribution is obtained as shown in Figure 4. It can be seen from Figure 4 that the largest part of the magnetic field is located in the middle of the coupling mechanism, and the magnetic flux in the air gap is small. Therefore, the magnetic flux leakage of this structure is small.

4. Experimental Verification

The system model was built in MATLAB/Simulink for simulation analysis, and the parameters of the system are listed in

Table 2. System parameters.

Experiment parameters	Udc (V)	ULref (V)	f (kHz)	fD (MHz)	CP (μF)	CT (μF)	CS (μF)
	48	48	25	1.25	1.69	0.28	0.24
	CDP (pF)	CDS (pF)	CP1 (μF)	CP2 (pF)	CS1 (μF)	CS2 (pF)
	95.95	97.01	1.01	405.31	1.01	405.31
Measurement parameters	LP (μH)	LS (μH)	LPP/LSS (μH)	M (μH)	LT (μH)
	169	167	40	155	145

. The ferrite core and litz wire parameters of the rotary coupling mechanism are consistent with the simulation. The data output U_DP comparison with or without parallel resonance (L_PP, C_P2, L_SS, C_S2) and series resonant capacitance (C_DP, C_DS) was obtained, as shown in Figure 5.

It can be seen that the role of the parallel resonant circuit (L_PP, C_P2, L_SS, C_S2) is to prevent the data from passing, so it is loaded onto the transmitting and receiving coils; if removed, the data will not be transmitted. The function of the series resonant capacitor (C_DP, C_DS) is to suppress the interference of power on data transmission; if removed, the data interference will be very large.

A laboratory prototype was built, as shown in Figure 6. The experimental parameters are consistent with the simulation.

First, the effectiveness of the added power resonance circuit to suppress the influence of power on data transfer is verified, as shown in Figure 7, where data are transferred from the secondary side to the primary side. There is no power resonance circuit in Figure 7a, but there is in Figure 7b. Channel 3 indicates U_DP while channel 2 indicates U_DS. It can be seen from Figure 7 that the transfer gain from U_DS to U_DP is essentially unchanged, regardless of whether there is a power resonant circuit or not. It can also be seen that when the power transmission is turned on, a small amount of clutter is mixed into the data, which is caused by the switching noise of the MOSFET in the power transmission loop. However, it can be filtered through the filter and does not affect the data transmission.

To further verify the data transfer capacity, data transfer waveforms are tested when the power transfer is on and off, and the results are shown in Figure 8. Whereas Figure 8a indicates the forward data transfer results when the power transfer is off, Figure 8b indicates the forward data transfer results when the power transfer is on, Figure 8c indicates the backward data transfer results when the power transfer is off, and Figure 8d indicates the backward data transfer results when the power transfer is on. Channel 1 shows the primary-side modulation waveform, channel 2 is the secondary-side pick-up waveform, channel 3 is the secondary-side envelope demodulated waveform, and channel 4 is the output waveform shaped by the comparator. The data transfer rate was selected to be 19,200 bps.

As can be seen from Figure 8, with the power transfer resonance circuit, the data transfer capacity is not affected by the power transfer. The power transfer channel and the data transfer channel do not affect each other. Figure 9 shows the bit error rate (BER) test results of the data transfer. It can be seen that in the bidirectional data transmission test, the data can be transmitted correctly.

For the rotary steering system of oil drilling, the system shall withstand high temperatures.

Table 3. System experimental parameters.

Temperature	Output Power (W)	Input Power (W)	Output Voltage (V)	Efficiency	BER
25 °C	50	60.4	48.2	82.8%	100%
	100	118.5	48.1	84.4%	100%
	150	176.3	48.0	85.1%	100%
	200	233.1	48.0	85.8%	100%
135 °C	50	60.3	48.1	82.9%	100%
	100	118.8	48.1	84.2%	100%
	150	176.9	48.0	84.8%	100%
	200	236.1	47.9	84.7%	100%

shows the comparison data of normal temperature (i.e., 25 °C) and high temperature (i.e., 135 °C, where the temperature is simulated by the high-temperature test box).

As can be seen from Table 3, under the tested temperature conditions, with the increase in output power, the output voltage of the system remains basically constant at 48 V under the action of the closed-loop control circuit. The working efficiency of the system is maintained at more than 80%, and the data transmission is error-free.

5. Conclusions

This paper proposed a parallel power and data transmission method for a rotary steering system with output voltage control. Power and data are transmitted through the same rotary coupling mechanism. A power resonance circuit is applied to suppress the influence of power on the data transfer. Thus, the crosstalk interference between the power and the data transfer channel can almost be neglected. An experimental prototype was built to verify the feasibility of the data transfer method and the closed-loop control method, and the experimental results show good consistency with the theoretical analysis.