An Air Velocity Monitor for Coal Mine Ventilation Based on Vortex-Induced Triboelectric Nanogenerator.

Air velocity of coal mine ventilation is an important consideration that may cause serious damage. This paper proposes a simple, low cost and effective air velocity monitor (AVM) for coal mine ventilation. The AVM uses the lock-in characteristic of vortex-induced vibration (VIV) to sense the air velocity. Amplitude of the VIV is converted into frequency signal of a vortex-induced triboelectric nanogenerator (VI-TENG) to improve the durability. Structure of the AVM are designed, and parameters of the AVM are optimized with experiments. For the upper and lower air velocity thresholds of 3.1 and 3.6 m/s, the optimized flexible beam length, slider weight, electrode space and electrode width are 42.5 mm, 0.4 g, 0.2 mm and 0.5 mm, respectively. Experiments also show that the output frequency of the VI-TENG could represent the amplitude of VIV well with the correlation coefficient of 0.93. Durability test demonstrates that the AVM generates stable output frequency in 120,000 cycles. A prototype and its controller are fabricated. Wind tunnel tests of this prototype show that it can give alarm when the gas velocity goes above the upper threshold or below the lower threshold. The proposed AVM could be a good solution for simple and effective coal mine ventilation alarm.

Triboelectric Rotary Motion Sensor for Industrial-Grade Speed and Angle Monitoring.

Mechanical motion sensing and monitoring is an important component in the field of industrial automation. Rotary motion is one of the most basic forms of mechanical motion, so it is of great significance for the development of the entire industry to realize rotary motion state monitoring. In this paper, a triboelectric rotary motion sensor (TRMS) with variable amplitude differential hybrid electrodes is proposed, and an integrated monitoring system (IMS) is designed to realize real-time monitoring of industrial-grade rotary motion state. First, the operating principle and monitoring characteristics are studied. The experiment results indicate that the TRMS can achieve rotation speed measurement in the range of 10-1000 rpm with good linearity, and the error rate of rotation speed is less than 0.8%. Besides, the TRMS has an angle monitoring range of 360° and its resolution is 1.5° in bidirectional rotation. Finally, the applications of the designed TRMS and IMS prove the feasibility of self-powered rotary motion monitoring. This work further promotes the development of triboelectric sensors (TESs) in industrial application.

Self-Powered Sensors and Systems Based on Nanogenerators.

Sensor networks are essential for the development of the Internet of Things and the smart city. A general sensor, especially a mobile sensor, has to be driven by a power unit. When considering the high mobility, wide distribution and wireless operation of the sensors, their sustainable operation remains a critical challenge owing to the limited lifetime of an energy storage unit. In 2006, Wang proposed the concept of self-powered sensors/system, which harvests ambient energy to continuously drive a sensor without the use of an external power source. Based on the piezoelectric nanogenerator (PENG) and triboelectric nanogenerator (TENG), extensive studies have focused on self-powered sensors. TENG and PENG, as effective mechanical-to-electricity energy conversion technologies, have been used not only as power sources but also as active sensing devices in many application fields, including physical sensors, wearable devices, biomedical and health care, human-machine interface, chemical and environmental monitoring, smart traffic, smart cities, robotics, and fiber and fabric sensors. In this review, we systematically summarize the progress made by TENG and PENG in those application fields. A perspective will be given about the future of self-powered sensors.

Wind Aggregation Enhanced Triboelectric-Electromagnetic Hybrid Generator with Slit Effect.

It is of great significance to establish a low-cost, high-efficiency, self-powered micrometeorological monitoring system for agriculture, animal husbandry, and transportation. However, each additional detection element in the meteorological monitoring system increases the power consumption of the whole system by about 0.7 W. As a renewable energy technology, a triboelectric nanogenerator has the advantages of low price and self-powered sensing. To reduce the power consumption of the micrometeorological monitoring system, this work introduces an innovative solution: the wind-gathering enhanced triboelectric-electromagnetic hybrid generator (WGE-TEHG). Coupling the thin-film vibrating triboelectric nanogenerator (TENG) and electromagnetic generator (EMG), the TENG is used to monitor wind direction and the EMG is used to monitor wind speed and provide energy needed by the system. In particular, the TENG can be used as a self-powered sensor to reduce the power consumption of the sensing system. Besides, the TENG is used to produce slit effect to enhance the output performance of EMG. The experimental results show that the WGE-TEHG can build a self-powered natural environment micrometeorological sensing system. It can monitor the wind direction, wind speed, temperature, and relative humidity. This research has great application value for the self-powered sensing implementation of a hybrid TENG and EMG.

Bionic Fish-Shaped Triboelectric-Electromagnetic Hybrid Generator via a Two-Stage Swing Mechanism for Water Flow Energy Harvesting and Condition Monitoring.

The water flow energy of rivers is an important renewable and clean energy that plays a vital role in human life but is challenging to harvest at low flow velocity. This work proposes a bionic fish-shaped triboelectric-electromagnetic hybrid generator (BF-TEHG) via a two-stage swing mechanism for harvesting water flow energy. It is designed to simulate the shape of fish, effectively improving its ability to utilize low-velocity water flow energy and enabling it to operate at a minimum flow rate of 0.24 m/s. Furthermore, the impact of motion parameters on electrical performance is studied. The triboelectric and electromagnetic power-generation units can generate peak powers of 0.55 and 0.34 mW in the simulated river environments with a flow velocity of 0.98 m/s. In applications, after being immersed in water for 40 days, the BF-TEHG maintains its electrical performance without reduction, indicating excellent water immersion durability. Therefore, this work proposes an efficient strategy to harvest low-velocity water flow energy and provides an acceptable candidate for monitoring water flow conditions.

A Blade-Type Triboelectric-Electromagnetic Hybrid Generator with Double Frequency Up-Conversion Mechanism for Harvesting Breeze Wind Energy.

Triboelectric nanogenerators (TENGs) have garnered substantial attention in breeze wind energy harvesting. However, how to improve the output performance and reduce friction and wear remain challenging. To this end, a blade-type triboelectric-electromagnetic hybrid generator (BT-TEHG) with a double frequency up-conversion (DFUC) mechanism is proposed. The DFUC mechanism enables the TENG to output a high-frequency response that is 15.9 to 300 times higher than the excitation frequency of 10 to 200 rpm. Coupled with the collisions between tribomaterials, a higher surface charge density and better generating performance are achieved. The magnetization direction and dimensional parameters of the BT-TEHG were optimized, and its generating characteristics under varying rotational speeds and electrical boundary conditions were studied. At wind speeds of 2.2 and 10 m/s, the BT-TEHG can generate, respectively, power of 1.30 and 19.01 mW. Further experimentation demonstrates its capacity to charge capacitors, light up light emitting diodes (LEDs), and power wireless temperature and humidity sensors. The demonstrations show that the BT-TEHG has great potential applications in self-powered wireless sensor networks (WSNs) for environmental monitoring of intelligent agriculture.

Development of a piezoelectric actuator based on stick-slip principle inspired by the predation of snake.

A new piezoelectric actuator based on the stick-slip working principle inspired by the predation of the snake is proposed and developed in this work. A lead zirconate titanate (PZT) stack is used and inserted into the stator with an asymmetric configuration. Then, the elongation of the PZT stack can be transmitted into the vertical and horizontal displacements on the driving foot. They are used to press and drive the slider, respectively. In this design, the motion of the actuator imitates the predation process of the snake. The principle of the proposed actuator is clarified in detail. The statics characteristics are conducted by using the FEM method. The dynamics model of the actuator was established to show the motion behavior of the slider in theory. Finally, the output characteristics of the developed piezoelectric actuator are tested. The results stated that this actuator obtained the maximum output speed of 11.44 mm/s under a voltage of 100 V and a frequency of 600 Hz. The output force of the developed actuator was 2.8N under the preload force of 3N. In conclusion, the feasibility of the proposed piezoelectric stick-slip type actuator inspired by the predation of the snake is verified.

Integral back-stepping active disturbance rejection control for piezoelectric stick-slip drive nanopositioning stage.

Piezoelectric stick-slip driven nanopositioning stage (PSSNS) with nanometer resolution has been widely used in the field of micro-operation. However, it is difficult to achieve nanopositioning over large travel, and its positioning accuracy is affected by the hysteresis characteristics of the piezoelectric elements, external uncertain disturbances, and other nonlinear factors. To overcome the above-mentioned problems, a composite control strategy combining stepping mode and scanning mode is proposed in this paper, and an integral back-stepping linear active disturbance rejection control (IB-LADRC) strategy is proposed in the scanning mode control phase. First, the transfer function model of the system in the micromotion part was established, and then the unmodeled part of the system and the external disturbance were treated as the total disturbance and extended to a new system state variable. Second, a linear extended state observer was used as the core of the active disturbance rejection technique to estimate displacement, velocity, and total disturbance in real time. In addition, by introducing virtual control variables, a new control law was designed to replace the original linear control law and improve the positioning accuracy and robustness of the system. Furthermore, the effectiveness of the IB-LADRC algorithm was verified by simulation comparison experiments and experimentally validated on a PSSNS. Finally, experimental results show that the IB-LADRC is a practical solution for a controller capable of handling disturbances during the positioning of a PSSNS with a positioning accuracy of less than 20 nm, which essentially remains constant under load.

Micro-Droplets Parameters Monitoring in a Microfluidic Chip via Liquid-Solid Triboelectric Nanogenerator.

The monitoring of micro-droplets parameters is significant to the development of droplet microfluidics. However, existing monitoring methods have drawbacks such as high cost, interference with droplet movement, and even the potential for cross-contamination. Herein, a micro-droplets monitoring method (MDMM) based on liquid-solid triboelectric nanogenerator (LS-TENG) is proposed, which can realize non-invasive and self-powered monitoring of micro-droplets in a microfluidic chip. The droplet frequency is monitored by voltage pulse frequency and a mathematical model is established to monitor the droplet length and velocity. Furthermore, this work constructs micro-droplets sensor (MDS) based on the MDMM to carry out the experiment. The coefficients of determination (R2 ) of the fitting curves of the micro-droplets frequency, length, and velocity monitoring are 0.998, 0.997, and 0.995, respectively. To prove the universal applicability of the MDMM, the micro-droplets generated by different liquid media and channel structures are monitored. Eventually, a micro-droplet monitoring system is built, which can realize the counting of micro-droplets and the monitoring of droplet frequency and length. This work provides a novel approach for monitoring micro-droplets parameters, which holds the potential to advance developments in the field of microfluidics.

Vehicle-Mounted Pavement Health Monitoring System Based on a Spring-Guide-Assisted Triboelectric Sensor.

The present pavement health monitoring is limited by a professional staff patrol. Herein, a vehicle-mounted pavement health monitoring system (VPHMS) based on a spring-guide-assisted triboelectric sensor (S-TES) is proposed, which uses the vibration generated by vehicles passing through uneven pavements to monitor the pavement damage. The VPHMS consists of S-TES, the integration module, and the terminal display module. The designed S-TES has high stability and can achieve a high linear vibration amplitude measurement within 90 mm. Moreover, the integration module is used to process signals and transmit wireless data transmission. The terminal display module is used to receive signals and display the measurement results on the screen. When a vehicle equipped with VPHMS is driven over uneven pavements, the system can accurately monitor the potholes and upheavals on the pavement in real time. This work has significant application value in fields such as pavement health monitoring and intelligent transportation.

Recent Advances in Triboelectric Nanogenerators: From Technological Progress to Commercial Applications.

Serious climate changes and energy-related environmental problems are currently critical issues in the world. In order to reduce carbon emissions and save our environment, renewable energy harvesting technologies will serve as a key solution in the near future. Among them, triboelectric nanogenerators (TENGs), which is one of the most promising mechanical energy harvesters by means of contact electrification phenomenon, are explosively developing due to abundant wasting mechanical energy sources and a number of superior advantages in a wide availability and selection of materials, relatively simple device configurations, and low-cost processing. Significant experimental and theoretical efforts have been achieved toward understanding fundamental behaviors and a wide range of demonstrations since its report in 2012. As a result, considerable technological advancement has been exhibited and it advances the timeline of achievement in the proposed roadmap. Now, the technology has reached the stage of prototype development with verification of performance beyond the lab scale environment toward its commercialization. In this review, distinguished authors in the world worked together to summarize the state of the art in theory, materials, devices, systems, circuits, and applications in TENG fields. The great research achievements of researchers in this field around the world over the past decade are expected to play a major role in coming to fruition of unexpectedly accelerated technological advances over the next decade.

A stick-slip linear actuator with high speed and nano-resolution by resonance/non-resonance hybrid driving.

To achieve high speed, nano-resolution, and large stroke, a resonance/non-resonance hybrid piezoelectric stick-slip actuator with a lever-type flexure hinge (LTFH-PSSA) is proposed in this work. The actuator can achieve high speed and large stroke in the resonance mode by the stick-slip working principle and achieve nano-resolution in the non-resonant mode by the direct drive working principle. The excitation electrical signals used in the two working modes are the sine waveform and half-sine waveform, respectively. Compared with the traditional sawtooth waveform, the excitation signal of the sine and half-sine waveforms have no sudden change of voltage, which are more conducive to reduce the impact and vibration of the system. Moreover, a series of static analysis and modal analysis of the stator are carried out by the finite element method. The experimental system is built to test the output characteristics of the LTFH-PSSA. In the resonance state by the stick-slip working principle, the impedance analysis and frequency characteristic test of the LTFH-PSSA are carried out, which states that the tested resonance frequency agrees well with the simulated ones. When the locking force, the voltage, and the frequency are 2 N, 100 Vp-p, and 1850 Hz, the speed of the LTFH-PSSA is up to 52.71 mm/s, and the backward motion is suppressed completely as well. In the non-resonance state, the resolution can reach 2.19 nm and 2.69 nm in the forward and backward motion, respectively. So far, the proposed actuator ranks first in speed and resolution among all reported LTFH-PSSAs.

Self-Powered Sensing for Non-Full Pipe Fluidic Flow Based on Triboelectric Nanogenerators.

Fluidic flow monitoring of a non-full pipe is of great significance in the field of energy measurement and pipeline transportation. In this work, a monitoring method based on triboelectric nanogenerators for non-full pipe fluidic flow of large pipelines is proposed. Specifically, a triboelectric non-full pipe flow sensor (TNPFS) is fabricated, which can monitor the flow velocity and the liquid level simultaneously, and then the flow can be obtained by conversion. For flow velocity monitoring, the flexible blades slide between electrodes, generating periodic electrical signals. Interestingly, the frequencies of the voltage and flow velocities show a good linear relationship. For liquid level monitoring, according to the principle of liquid-solid contact electrification, a variable area interdigital electrode with a stable signal distributed on a polytetrafluoroethylene tube is designed. The experiments demonstrate that the peak number and trend of the voltage derivative curve are related to the liquid level. Finally, a real-time flow-monitoring system is established to effectively monitor the flow from 94 to 264 L/min. Compared with the actual measured flow, the error rate is under 1.95%. In addition to this, the TNPFS also has good responsiveness in sewage. This work provides a novel method for fluidic flow monitoring, especially the non-full pipe flow of large pipelines.

Nondestructive Dimension Sorting by Soft Robotic Grippers Integrated with Triboelectric Sensor.

In smart logistics, traditional manual sorting and sorting systems based on rigid manipulators limit the warehousing development and damage the goods. Here, a nondestructive sorting method based on bionic soft fingers is proposed. This method is implemented by the soft robotic gripper (SRG) for grasping of the breakable objects, the triboelectric sensor (TES) for size sorting of the objects, and the signal processing module. In the fabrication of SRG, the silicon rubber is prepared by controlling the material synthesis process, and its Young's modulus is 600.91 kPa, which is comparable to the Young's modulus of skin tissue. Also, the maximum input pressure of SRG is 71.4 kPa. The TES has a linear relationship between pulse number and sliding displacement, and its resolution is 3 mm. It induces pulse signal sequences to quantify the SRG bending state and thus realize the size sorting of objects. Additionally, a nondestructive sorting system based on TES and SRG has been developed for fruit sorting (e.g., apples, oranges), enabling nondestructive grasping and accurate sorting. Its sorting range is 70-120 mm, and the sorting accuracy rate is up to 95%. This work also provides a way for the application of SRG and triboelectric sensors in the sorting field.

Gyroscope-Structured Triboelectric Nanogenerator for Harvesting Multidirectional Ocean Wave Energy.

Wave motion in the ocean can generate plentiful energy, but it is difficult to harvest wave energy for practical use because of the low frequency and random directional characteristics of wave motion. In this paper, a gyroscope-structured triboelectric nanogenerator (GS-TENG) is proposed for harvesting multidirectional ocean wave energy. Its inner and outer generation units can operate independently in different directions, and they all adopt the friction mode of surface contact. While realizing noninterference multidirectional energy harvesting, the power generation area is increased. In the experiments, under acceleration of 6 m/s2 with variations in excitation angle, the GS-TENG can output direct currents of 0.8-3.2 µA, and the open-circuit voltages of the inner and outer generation units can reach 730 and 160 V, respectively. When the devices are networked and placed in the water, the electrical energy generated by the GS-TENGs can enable commercial thermometers to operate normally. The attenuation of direct-current output by the GS-TENG in the experiment of 30 days in water is about 8%, which verifies the good durability of the device in the water environment. Therefore, the GS-TENG has excellent application prospects in the wave energy harvesting field.

High-Performance Dielectric Elastomer Nanogenerator for Efficient Energy Harvesting and Sensing via Alternative Current Method.

Soft, low-cost, high-performance generators are highly desirable for harvesting ambient low frequency mechanical energy. Here, a dielectric elastomer nanogenerator (DENG) is reported, which consists of a dielectric elastomer capacitor, an electret electrostatic voltage source, and a charge pump circuit. Under biaxial stretching, DENG can convert tensile mechanical energy into electrical power without any external power supply. Different from traditional DEG with the charge outward transfer in direct current (DC), the DENG works based on shuttle movement of internal charges in an alternating current (AC). Through alternating current (AC) method, the charge density of the DENG can reach 26 mC m-2 per mechanical cycle, as well as energy density of up to 140 mJ g-1 . Due to the all-solid-state structure without air gap, the DENG is capable of working stably under different ambient humidity (20 RH%-100 RH%). To demonstrate the applications, a water wave harvester based on the DENG is constructed. The integrated device powers a sensing communication module for self-powered remote weather monitoring, showing the potential application in ocean wave energy harvesting.

Nonintrusion Monitoring of Droplet Motion State via Liquid-Solid Contact Electrification.

Droplet motion state monitoring is important in microfluidic applications, such as biomedicine, drug delivery, and metal ion extraction. Here, a nonintrusion monitoring method of droplet motion state via liquid-solid contact electrification is proposed, and a triboelectric droplet motion state sensor (TDMSS) is fabricated. Droplet counting and droplet size monitoring can be realized by signal processing and information extracting of the voltage pulse. The experimental results show that the number of droplets increases linearly with the increase of liquid flow, and the linearity is 0.9854. Moreover, TDMSS can stably monitor the number of droplets in different motion states. In addition, the output pulse width is sensitive to droplet size, and the droplet length ranges from 3 to 13.5 mm. More importantly, TDMSS can realize the function of droplet counting and size monitoring of a conductive liquid and accurate droplet counting under different inclination angles and motion states. This work not only provides a nonintrusion method for droplet motion state monitoring but also makes a solid step for microfluidic sensing technology based on a triboelectric nanogenerator.

Self-Powered Sensing for Smart Agriculture by Electromagnetic-Triboelectric Hybrid Generator.

The lack of efficient, low-cost, distributed energy collection methods is a vital factor restricting the application of the Internet of Things (IoT) in smart agriculture. This paper proposes a method based on triboelectric nanogenerator and electromagnetic generator to realize self-powered IoT nodes and self-powered sensors at the same time. An energy harvesting and sensing device based on electromagnetic-triboelectric hybrid generator (ES-ETHG) is designed. The peak power of ES-ETHG is 32.4 mW, which can supply power to IoT nodes for a long time with power management circuits. In addition, ES-ETHG can critically measure wind speed and wind level within the range of 3-15 m/s, and accurately detect wind direction within 2 s. Furthermore, the self-powered distributed weather sensing system based on ES-ETHG is developed to realize the remote collection of wind speed, wind direction, temperature, and humidity. This work proposes a solution for developing self-powered IoT and sensor in the field of smart agriculture.

Real-Time Monitoring System of Automobile Driver Status and Intelligent Fatigue Warning Based on Triboelectric Nanogenerator.

To improve automobile safety, identifying driver fatigue is considerably crucial because it is one of the main causes of traffic accidents. In this research, smart systems based on a triboelectric nanogenerator are designed, which can provide driver status monitoring and fatigue warning in real time. The smart system consists of a self-powered steering-wheel angle sensor (SSAS) and a signal processing unit. The SSAS, which comprises a stator, a rotor, and a sleeve, is mounted on the steering rod, and the electrodes are designed with a phase difference to improve the resolution of the sensor. The turning angle of the steering wheel operated by the driver is recorded by the SSAS; meanwhile, the number of rotations, the average angle, and other parameters in the driver's recorded data are analyzed by the signal processing unit from which a warning threshold for each parameter is determined. The system assesses the status of the driver in real-time by comparing these parameters and threshold values, and experimental results demonstrate that driver status is accurately judged. This work has important potential applications in the fields of traffic safety and intelligent driving.

Triboelectric Nanogenerator for Ocean Wave Graded Energy Harvesting and Condition Monitoring.

Using triboelectric nanogenerators (TENGs) to harvest blue energy in the ocean is advanced technology at present. In wave environments, the wave magnitude is constantly changing, so designing a TENG that can adjust the energy harvesting ability is necessary. Herein, a graded energy harvesting triboelectric nanogenerator (GEH-TENG) is fabricated, in which double generation units can operate in different transmission states to adapt to wave changes. Under small waves, the GEH-TENG is in the primary transmission state. Once waves are large enough, it enters the secondary transmission state, realizing graded energy harvesting to enhance power generation performance. Experiments show that when the input frequency is 1.0 Hz and the amplitude is 120 mm, the GEH-TENG can generate 0.7 mJ of energy in a single operation cycle, which is 2.3 times of it without grading. Moreover, it can be placed on the shore to monitor ocean wave conditions. An idea of graded energy harvesting is proposed in this study, and the proposal provides useful guidance for practical applications of TENGs in ocean wave condition monitoring.