Research on Highly Reliable Self-Powered Vibration Sensors for Geological Drilling

Abstract

Vibration signals at the bottom of the drill string during geological drilling are crucial for lithological identification and drilling parameter optimization. However, existing downhole vibration sensors suffer from limitations in power supply and reliability. This study proposes a self-powered vibration sensor with high redundancy based on the triboelectric nanogenerator principle, which is capable of measuring both axial and transverse vibrations, thereby reducing the dependence on external power sources. The experimental results show that the sensor can measure axial vibration frequencies ranging from 0 to 11 Hz with an error of less than 4% and transverse vibration frequencies ranging from 0 to 5 Hz with an error of less than 5%. It can operate stably in temperatures from 0 to 180 °C and relative humidities from 0 to 95%. The sensor’s axial vibration measurement features six identical measurement structures, providing high redundancy and effectively enhancing its reliability. Furthermore, the sensor exhibits power generation capabilities. When an external load of 1 MΩ is applied to the axial measurement module and 10 MΩ to the transverse measurement module, the sensor achieves its maximum power output for both axial and transverse measurements, reaching 32.4 × 10⁻⁹ W and 2.1 × 10⁻⁹ W, respectively. Compared to traditional bottom-of-the-hole vibration sensors, this sensor possesses self-powering capabilities and high reliability, which can improve the operational efficiency and hold significant practical value for future applications.

1. Introduction

Geological drilling is an indispensable tool in obtaining subsurface geological information, exploring mineral resources, generating energy, and constructing infrastructure [1,2]. During the geological drilling process, vibration signals reflect the interaction between the drill bit and the formation, providing crucial information for the geological structure, lithology identification, and drilling parameter optimization [3]. Therefore, real-time monitoring of vibration signals during drilling is crucial for enhancing the drilling efficiency and reducing risks. However, traditional vibration sensors are chip-based and are susceptible to damage and failure in the complex downhole environment due to factors like impact and temperature variations [4,5], requiring regular replacement. Replacing a damaged sensor requires lifting the entire drill string from the bottom of the borehole to the surface. This process can take several days when the borehole depth reaches several kilometers. This not only increases maintenance costs but also potentially leads to data loss, compromising the safety and reliability of drilling operations. Therefore, developing highly reliable vibration sensors holds significant importance for geological drilling.

Downhole vibration measurements typically involve mounting the sensor on the drill string, sealing it, and lowering it to the bottom of the borehole along with the drill string. After collecting the data, it is retrieved by tripping out the drill. There are two main power supply methods for downhole vibration sensors: battery-powered and cable-powered [6]. Battery-powered systems face limitations regarding limited battery life, difficult replacement, and environmental pollution. Conversely, cable-powered systems encounter challenges such as cable damage, restricted transmission distance, and complex installation procedures. Self-powered vibration sensors can effectively overcome these issues. Self-powered sensors can generate electricity from ambient energy sources, eliminating the need for external power sources and enabling long-term stable monitoring. This contributes to enhanced drilling efficiency, cost reduction, and reduced environmental impact, aligning with green and sustainable principles.

In recent years, the triboelectric nanogenerator (TENG), as a novel energy harvester, has demonstrated significant potential in both generator and sensor applications [7,8,9]. For instance, in the field of generators, TENGs have been successfully employed to harvest energy from human motion [10], wind [11], rainfall [12], wave energy [13,14], vibration [15,16], and even bubble energy [17]. Due to the generation of pulse signals during TENG operation [18,19,20], and the fact that the frequency characteristics of these pulses can reflect changes in the inducing factors, TENGs have also found wide applications in various sensor fields, such as motion monitoring [21,22,23], sweat monitoring [24], pressure monitoring [25,26], ethanol monitoring [27], gas monitoring [28,29,30], landslide monitoring [31,32], velocity monitoring [33], posture monitoring [34], and tactile sensing [35,36,37]. Triboelectric nanogenerators hold promise for achieving both self-power generation and sensing functionality simultaneously, offering a novel research approach for the development of underground vibration sensors.

Based on these advantages, this research proposes a highly reliable self-powered vibration sensor based on the TENG, which effectively addresses the challenges of power supply and reliability for sensors. This research also provides a novel solution for vibration measurements at the bottom of geological boreholes.

2. Structure and Working Principle

2.1. Structure of Sensor

As illustrated in Figure 1a, the sensor is strategically positioned near the drill bit within the nipple, preventing it from being directly exposed to the harsh downhole environment and to accurately acquire realistic vibration data. The sensor, characterized by its cylindrical form (Φ120 mm × 130 mm, wall thickness 5 mm), is fabricated using 3D printing with an acrylonitrile butadiene styrene material, enabling it to withstand high temperatures up to 180 °C. It is suitable for large-diameter drilling with drill pipe specifications greater than 120 mm. The sensor incorporates a spring vibrator system, featuring six axial vibration modules and one transverse vibration module that are distributed evenly. A rigid connection is employed between the transverse vibration module and the spring vibrator, while the axial vibration modules utilize a ball head rod connection and a hard connection rod. This configuration effectively mitigates the impact of transverse vibration on the axial vibration modules, particularly during instances of significant transverse vibration.

Figure 1b presents a comprehensive view of the sensor, highlighting its overall structure and key components. To facilitate signal transmission, the upper end of each axial vibration module is coated with Kapton friction material (the thickness is 0.05 mm), while the bottom end (Φ12 mm × 30 mm) is equipped with a copper electrode for grounding. Similarly, the transverse vibration module’s upper ellipsoid is coated with Kapton material, and its bottom end (Φ16 mm × 30 mm) features a copper electrode. This configuration allows for a single electrode working mode in both modules, simplifying the installation of the upper-end conducting wires. A two-dimensional representation of the sensor’s internal module is depicted in Figure 1c.

2.2. Working Principle of Sensor

Both axial and transverse vibration measurements of the sensor are based on the principles of triboelectric charging and electrostatic induction. When vibration occurs, it induces triboelectric charging within the sensor’s internal structure, generating a triboelectric signal that directly corresponds to the vibration frequency. Therefore, the vibration frequency can be measured by statistically counting the number of triboelectric signal outputs from the sensor.

The working principle of both the axial and transverse vibration modules is illustrated in Figure 2. The axial vibration module measures the frequency of axial vibration using the principles of triboelectric charging and electrostatic induction. Initially, the Kapton and copper electrodes are in contact, creating a potential difference due to their differing electronegativity (Figure 2a(i)). As axial vibration commences, the separation distance between the electrodes increases, inducing a corresponding increase in the potential difference (Figure 2a(ii)). This potential difference reaches its peak when the separation distance is maximized (Figure 2a(iii)). As the electrodes return to their initial contact position (Figure 2a(iv)), the potential difference decreases, completing a vibration cycle. This cycle generates a voltage and current pulse signal that serves as the basis for detecting the vibration frequency.

The working principle of the transverse vibration module is illustrated in Figure 2b. Initially, the ellipsoid is separated from the inner wall of the lower end (Figure 2b(i)). Transverse vibration causes the spring vibrator to move the upper end of the module laterally, bringing the ellipsoid into contact with the inner wall (Figure 2b(ii)). This contact generates triboelectric charges, with the Kapton surface acquiring a negative charge and the copper electrode acquiring a positive charge. The oscillating upper end intermittently contacts and separates from the inner wall (Figure 2b(iii,iv)). This cyclical contact and separation induces a charge transfer, generating a current and a corresponding voltage pulse output and enabling the detection of the transverse vibration frequency.

When subjected to mixed motion, the spring effectively drives both the axial and transverse vibration modules, generating separate output signals. The ball head connection minimizes the influence of transverse vibration on the axial vibration. Moreover, the integrated large-area electrode on the inner wall of the transverse vibration module mitigates the influence of axial vibration on the transverse vibration measurement. These design features enhance the accuracy of vibration detection, validating the theoretical soundness of the sensor design.

3. Sensor Performance Testing

To systematically evaluate the sensor’s performance, we conducted a series of tests within a simulated indoor environment. These tests encompassed sensing performance, redundancy, power generation, and environmental adaptability, as detailed below.

3.1. Experimental Setup

Figure 3 depicts the indoor experimental setup. Figure 3a presents a schematic diagram of the experimental apparatus, while Figure 3b shows a photograph of the assembled setup. As illustrated, the experimental apparatus consists of a vibration console, an electromagnetic shaker, a data acquisition card, a 6517 electrometer, and a computer. The sensor is mounted on the electromagnetic shaker, which generates mechanical vibrations through electromagnetic force to simulate downhole vibrations. Precise control over the axial and lateral amplitudes and vibration frequencies of the electromagnetic shaker is achieved by adjusting the parameters of the vibration console. A data acquisition card is a hardware device that collects signals from external systems and converts them into digital data. The 6517 electrometer, produced by Keithley, is suitable for measuring electrical parameters under high-impedance conditions. The sensor data are sequentially processed by the data acquisition card and 6517 electrometer before being transmitted to the computer. The computer runs software developed in LabVIEW language, which enables real-time display and storage of the collected data.

3.2. Sensing Performance Tests

The sensor is capable of measuring both axial and transverse vibration frequencies. Therefore, the sensing performance of the sensor was tested under both conditions, and the results are shown in Figure 4. As shown in Figure 4a–d, the output voltage and current signals of the sensor under axial vibration are both pulse signals. The amplitudes of both the output voltage and current signals increase gradually with the increase in vibration frequency. When the axial vibration frequency is 11 Hz, the maximum output voltage and current signal amplitudes reach 11.3 V and 0.6 µA, respectively. When the vibration frequency exceeds 11 Hz, the output waveform of the sensor becomes irregular and chaotic. Therefore, the measurement range of the sensor for axial vibration frequency is defined as 0 to 11 Hz. Similarly, when the transverse vibration frequency is 5 Hz, the maximum output voltage and current signal amplitudes of the sensor reach 2.6 V and 0.024 µA, respectively. When the transverse vibration frequency exceeds 5 Hz, the output waveform of the sensor becomes irregular and chaotic. Therefore, the measurement range of the sensor for transverse vibration frequency is defined as 0 to 5 Hz. Further testing was conducted to evaluate the sensor’s measurement error under combined-vibration conditions, as depicted in Figure 4e,f. It is observed that within the measurement range, the sensor’s measurement error for axial vibration frequency is less than 4%, while the measurement error for transverse vibration frequency is less than 5%. These error values meet the requirements of actual drilling operations. This indicates the high adaptability of the sensor in real working environments.

3.3. Redundancy Capability Tests

The axial vibration measurement of the sensor comprises six identical axial vibration measurement modules, providing redundancy in axial vibration frequency measurement and thus ensuring high reliability. The number of axial vibration measurement modules is defined as redundancy. Therefore, this sensor has a redundancy of six, with six axial vibration measurement modules. The redundancy characteristics of the sensor were tested, and the results are presented in Figure 5. As shown in Figure 5a, the output voltage under different redundancy levels was measured at an axial vibration frequency of 11 Hz. The output voltage remains relatively constant with increasing redundancy values. This is because the different axial vibration measurement units of the sensor are connected in parallel. Therefore, the output voltage of multiple vibration measurement units connected in parallel is the same as that of a single unit, resulting in a constant output voltage. Further testing was conducted to evaluate the measurement error at an axial vibration frequency of 11 Hz under different redundancy levels. As shown in Figure 5b, the measurement error exhibits slight fluctuations when the redundancy is less than six. However, the error remains below 4%. When the redundancy exceeds six, the measurement error increases with increasing redundancy values. This is because as the number of measurement units increases, the vibrations of the different units do not remain perfectly synchronized. Consequently, the voltage pulse obtained from the parallel connection is the superposition of all pulses, affecting the voltage pulse frequency and leading to increased measurement error. Therefore, a redundancy of six was chosen to ensure a measurement error below 4%, meeting the engineering requirements. The redundancy characteristics of the sensor were further tested under combined-axial and transverse-vibration conditions with frequencies of 11 Hz and 5 Hz, respectively. The results are shown in Figure 5c,d. It can be observed that the output voltage and measurement accuracy of the sensor under combined-vibration conditions are consistent with those under single-axial-vibration conditions. This demonstrates the sensor’s excellent adaptability to practical working conditions.

3.4. Power Generation Tests

Further tests were conducted to evaluate the power generation capabilities of the sensor. The results are presented in Figure 6. As shown in Figure 6a, the axial output voltage of the sensor increases gradually, while the output current decreases with increasing external load resistance values. The maximum voltage and current values were 11.8 V and 0.6 µA, respectively. Figure 6b illustrates a non-linear relationship between the axial output power of the sensor and the external load. The maximum axial output power is achieved at an external load of 1 MΩ, reaching a value of 32.4 × 10⁻⁹ W. The power generation characteristics of the transverse module are similar to those of the axial module. As shown in Figure 6c, the transverse output voltage of the sensor increases gradually, while the output current decreases with increasing external load resistance values. The maximum voltage and current values were 2.6 V and 24 nA, respectively. Further testing of the transverse module’s power generation revealed that the maximum transverse output power is reached at an external load of 10 MΩ, with a maximum value of 2.1 × 10⁻⁹ W, as shown in Figure 6d.

3.5. Working Condition Adaptability Tests

The underground environment is subject to variations in temperature and humidity. Therefore, the sensor’s output characteristics under different temperatures and relative humidities were evaluated. The test results are presented in Figure 7. As shown in Figure 7a,b, the amplitude of the axial vibration output voltage gradually decreases within the temperature range of 0 to 180 °C and the relative humidity range of 0 to 95%, with respective attenuation rates of 11.8% and 17.1%. Figure 7c,d illustrate a similar trend for the transverse vibration output voltage, with attenuation rates of 12.5% and 14.3% within the same temperature and humidity ranges.

As a pulse–output type sensor, its subsequent circuit is typically connected to the pulse input port of a microprocessor. The pulse input port counts the number of pulses to measure the vibration frequency. Microprocessors follow the transistor–transistor logic (TTL) level standard for input pulse signals, which defines any input voltage exceeding 2 V as a high level. Consequently, as long as the sensor’s output voltage amplitude remains above 2 V, its sensing performance is unaffected. This indicates that the sensor’s output performance remains unaffected within the specified temperature and humidity range, demonstrating its high environmental adaptability. Therefore, the temperature and humidity adaptability ranges of the sensor are defined as 0 to 180 °C and 0 to 95%, respectively.

Further testing was conducted to evaluate the sensor’s stability after multiple operating cycles, with the results shown in Figure 8, as shown in Figure 8a,b. After 50,000 cycles of axial vibration, the output voltage amplitude was 9.8 V, and the current amplitude was 0.28 µA, representing decreases of 3% and 1%, respectively, compared to the initial values before cycling. Similarly, after 50,000 cycles of transverse vibration, the output voltage amplitude was 2 V, and the current amplitude was 0.01 µA, with decreases of 2% and 3%, respectively.

3.6. Comparison of Sensor Types

After completing the test, we conducted a comparative study with different vibration sensors.

As shown in

Table 1. Comparison of different types of sensors.

Sensor Types	Error Rate	Operating Temperature	Generated Power
Chip-based [38]	0.6%	150 °C	None
Friction-based	4%	180 °C	32.4 × 10−9 W

, traditional chip-based vibration sensors offer high measurement accuracy and low error rates, but their electronic components are not well suited for high temperatures, leading to lower reliability and a continuous need for an external power supply. In contrast, the sensor developed in this study has higher redundancy and incorporates a self-powering feature, making it more suitable for real-world downhole environments.

4. Conclusions and Discussions

This study proposes a highly reliable vibration sensor with self-powering capability. The experimental results show that the sensor can measure axial vibration frequencies in the range of 0 to 11 Hz with an error below 4% and lateral vibration frequencies in the range of 0 to 5 Hz with an error below 5%. The sensor can operate stably within a temperature range of 0 to 180 °C and a relative humidity range of 0 to 95%. The sensor possesses energy harvesting capabilities. When the axial and lateral modules are externally connected to loads of 1 MΩ and 10 MΩ, respectively, the maximum power output is achieved for both the axial and lateral directions, reaching 32.4 × 10⁻⁹ Ww and 2.1 × 10⁻⁹ W, respectively.

Compared to traditional bottom-hole vibration sensors, this sensor offers two key innovations. Firstly, it features self-powering capability, effectively reducing its dependence on external power sources and minimizing the time wasted on battery replacement during drilling operations, making it more suitable for real-world applications. Secondly, the sensor boasts high reliability. The axial measurement module adopts a redundant design with six identical structures. As long as the number of damaged measurement structures is less than six, the sensor can still operate normally, effectively preventing drilling interruptions and time wasted on replacement due to sensor failure, thereby enhancing drilling efficiency.

However, the current power output of the sensor is relatively low, limiting its use to self-powered vibration sensing. To further enhance the sensor’s power output, two areas require further investigation. Firstly, altering the structural design of the sensor could enable it to harvest energy from mudflow and drill string rotation in addition to bottom-hole vibration, thereby broadening the energy harvesting range and boosting the power output. Secondly, optimizing the surface microstructure of nanomaterials, such as arranging dot arrays, can increase the contact area of the material, effectively enhancing the power density per unit of nanomaterial and thereby boosting the output power. In addition, the introduction of an electromagnetic generation module can further enhance the power output.