With the fast development of the fifth-generation cellular network technology (5G), the future sensors and microelectromechanical systems (MEMS)/nanoelectromechanical systems (NEMS) are presenting a more and more critical role to provide information in our daily life. This review paper introduces the development trends and perspectives of the future sensors and MEMS/NEMS. Starting from the issues of the MEMS fabrication, we introduced typical MEMS sensors for their applications in the Internet of Things (IoTs), such as MEMS physical sensor, MEMS acoustic sensor, and MEMS gas sensor. Toward the trends in intelligence and less power consumption, MEMS components including MEMS/NEMS switch, piezoelectric micromachined ultrasonic transducer (PMUT), and MEMS energy harvesting were investigated to assist the future sensors, such as event-based or almost zero-power. Furthermore, MEMS rigid substrate toward NEMS flexible-based for flexibility and interface was discussed as another important development trend for next-generation wearable or multi-functional sensors. Around the issues about the big data and human-machine realization for human beings’ manipulation, artificial intelligence (AI) and virtual reality (VR) technologies were finally realized using sensor nodes and its wave identification as future trends for various scenarios.

Development Trends and Perspectives of Future Sensors and MEMS/NEMS.

The importance of thermoelastic damping as a fundamental dissipation mechanism for small-scale mechanical resonators is evaluated in light of recent efforts to design high-Q micrometer- and nanometer-scale electromechanical systems. The equations of linear thermoelasticity are used to give a simple derivation for thermoelastic damping of small flexural vibrations in thin beams. It is shown that Zener’s well-known approximation by a Lorentzian with a single thermal relaxation time slightly deviates from the exact expression.

Thermoelastic Damping in Micro- and Nano-Mechanical Systems.

A wearable energy harvester technology is developed for generating electricity from the movement of human joints. A micro-electroplated ferromagnetic nickel cantilever is integrated with a piezoelectric element and bonded on a flexible substrate. Based on the magnetic interaction between the magnetized cantilever and a magnet on the substrate, a novel vertical-vibration frequency-up-conversion (FUC) structure is formed to generate stable amounts of electric energy per cycle from the horizontal substrate stretching/rebounding. The two ends of the flexible substrate are attached on both sides of a limb joint to transform joint rotation into substrate stretching. During limb movement, the flexible substrate is horizontally stretched and rebounded, causing the cantilever to vertically release from and return to the magnet, thereby exciting the piezoelectric cantilever into resonant generation. Since the horizontal low-frequency limb movement is perpendicular to the vertical high-frequency resonance, the stretch has little influence on the resonance of the cantilever. Thus the generated energy is always stable within a wide frequency range of limb movements. The performance of the novel harvester is experimentally verified using a stretching/rebounding movement cycle, where the cycle corresponds to the frequency range of 0.5–5.0 Hz. Within one stretching/rebounding movement cycle, the generated electric energy is stable in the approximate range of 0.56–0.69 μJ for the whole frequency range. Two flexible harvesters are worn on the human elbow and knee for a body kinetic energy harvesting test. Considerable power can always be generated under typical low-frequency limb movements, such as squatting, walking, jogging, and fast running, where the peak-to-peak generated voltages are always approximately 4.0 V. Additionally, energy harvesting under two-directional area stretching is also realized by adjusting the FUC structure layout. The flexible-substrate harvester is promising for various wearable applications.

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Wearable energy harvesters generating electricity from low-frequency human limb movement

Energy harvesting in wireless sensor networks: A taxonomic survey

This paper presents energy-efficient solutions for wireless sensor systems intended for the monitoring of water quality at water stations. Energy problems, such as energy scarcity and energy consumption in wireless sensor networks, are critical issues that raise concerns when it comes to defining the lifetime of a network. This can be attributed to the finite energy budget of water quality sensor nodes and also the lack of energy-efficient schemes. To address these energy problems, a detailed study has been carried out in this paper on some of the major approaches developed to address energy issues in wireless sensor systems, and dedicated to the monitoring of water quality applications in a way that contextualizes the considered solution methods. Furthermore, the solution methods are classified, while their strengths and weaknesses are also identified. Also, several suggestions are made on solution methods toward further improvement. Therefore, this paper suggests a reasonable direction for future studies on developing energy-efficient solutions in wireless sensor systems for monitoring water quality applications.

Energy Efficient Solutions in Wireless Sensor Systems for Water Quality Monitoring: A Review

Misalignment angle is an important error in the integration of the strapdown inertial navigation system (SINS) and the global positioning system (GPS). Currently, the popular methods for SINS/GPS integrated system are based on the ground velocity. There are two major defects: 1) not all GPS receivers provide the velocity outputs and 2) the contained outliers in the velocity will degrade the performance of the alignment methods. In this paper, an in-motion coarse alignment method for SINS/GPS integrated system using position loci is proposed. First, the observation vector for the in-motion coarse alignment with the position loci is derived. Different from the current popular methods, the constructed observation vector is just relative with the GPS positioning information, which can be obtained from any GPS receiver directly. Thus, the applicable range of the in-motion coarse alignment method is extended. Second, based on the advantages of the position loci, the proposed method can suppress the interferences of the outliers, which are contained in the GPS positioning outputs. Finally, the simulation and field tests are designed to verify the performance of the proposed method. The test results show that the proposed method is more robust than the current popular methods.

In-Motion Coarse Alignment Method for SINS/GPS Using Position Loci

A novel self-powered wireless sensing micro-system is proposed and prototyped. The micro-system can autonomously monitor the amplitude of concerning vibration when it reaches a preset threshold, and wirelessly transmit alarming/notification signals when the vibration lasts for a considerable period. The event-driven sensing/alarming function is enabled by a piezoelectric energy harvester that starts to generate electric power only when the monitored vibration level reaches a critical threshold. The harvester consists of two stages of vibratory structures that interact with each other via magnetic repulsion. The bistability leads to the threshold-triggered power-generating function. For adapting to various applications, the threshold can be preset by adjusting the gap-distance between the two magnets on the two vibration stages; thereby, the power-generating action can be switched on by the concerning vibration level. The working mechanism is verified by both theoretical analysis and test for the prototyped miniature harvesters. Based on the smart energy-harvesting function, an application specific electric-energy control circuit and an RF-transmitter are designed to form a nonsupply wireless alarming system. Experimental results validate the event-driven alarming function. The abnormal vibration event-induced wireless alarming signal is autonomously transmitted to over 2 km away when the vibratory excitation lasts for 1 min to generate electrical energy of about 2 mJ.

Wireless Alarm Microsystem Self-Powered by Vibration-Threshold-Triggered Energy Harvester

Advanced doped‐silicon‐layer‐based passivating contacts have boosted the power conversion efficiency (PCE) of single‐junction crystalline silicon (c‐Si) solar cells to over 26%. However, the inevitable parasitic light absorption of the doped silicon layers impedes further PCE improvement. To this end, alternative passivating contacts based on wide‐bandgap metal compounds (so‐called dopant‐free passivating contacts (DFPCs)) have attracted great attention, thanks to their potential merits in terms of parasitic absorption loss, ease‐of‐deposition, and cost. Intensive research activity has surrounded this topic with significant progress made in recent years. Various electron‐selective and hole‐selective contacts based on metal compounds have been successfully developed, and a champion PCE of 23.5% has been achieved for a c‐Si solar cell with a MoOx‐based hole‐selective contact. In this work, the fundamentals and development status of DFPCs are reviewed and the challenges and potential solutions for enhancing the carrier selectivity of DFPCs are discussed. Based on comprehensive and in‐depth analysis and simulations, the improvement strategies and future prospects for DFPCs design and device implementation are pointed out. By tuning the carrier concentration of the metal compound and the work function of the capping transparent electrode, high PCEs over 26% can be achieved for c‐Si solar cells with DFPCs.

Progress and Future Prospects of Wide‐Bandgap Metal‐Compound‐Based Passivating Contacts for Silicon Solar Cells

Frequency symmetry of a cobweb-like disk resonator gyroscope (CDRG) has been compared with a conventional ring-like disk resonant gyroscope (RDRG) in this letter to corroborate that linear beam structures have a better process tolerance than curved beam structures for micro-fabrication processes. The CDRG comprises multiple concentric cobweb-like rings and each is formed by 16 identical straight beams joined end-to-end. It exhibits as-fabricated symmetric vibration behavior superior to RDRGs. To compare as-fabricated structural symmetry, CDRG and RDRG are placed side-by-side on the same wafer and simulated using COMSOL. A tested frequency split as low as 0.56 Hz is achieved for CDRG, and the average relative frequency split is 79.7 ppm, which is approximately seven times smaller than that of RDRG. Additionally, a temperature characteristics test demonstrates that the as-fabricated CDRG has excellent temperature stability, and the relative frequency split varied by 2.83 ppm in the temperature ranging from −20 °C to +80 °C. The proposed concept is verified by the simulation and experimental results.

Frequency Symmetry Comparison of Cobweb-Like Disk Resonator Gyroscope With Ring-Like Disk Resonator Gyroscope

This paper presents a dual quantization lateral-axis capacitive electromechanical sigma–delta modulator (EM- $\Sigma \Delta {\mathrm{ M}}$ ) vibratory wheel gyroscope under mode-matched condition. A symmetric wheel gyroscope structure was designed, fabricated, and wafer-level packaged under vacuum. The EM- $\Sigma \Delta {\mathrm{ M}}$ interface loop adopts a fourth-order multifeedback and local resonator $\Sigma \Delta {\mathrm{ M}}$ noise shaping structure, taking advantage of single-bit and multibit quantization in a single-loop modulator. To evaluate the influence of quantization noise, various numbers of bits of the multibit quantizator were compared and the optimal number of bits was calculated. The interface circuit system was implemented in hardware based on a low-noise front-end application-specified integrated circuit and a back-end discrete-time EM- $\Sigma \Delta {\mathrm{ M}}$ realized in a field-programmable gate array chip. The system clock was generated using a self-clocking scheme, which allows interfacing to gyroscope with a wide range of drive frequencies. Experimental evaluation indicated that the digital gyroscope system achieved a scale factor of 2073LSB(4.1 mV)/deg/s, with a nonlinearity of 0.046%, a noise floor of 3.1 m°/s/ $\surd $ Hz within a bandwidth of 100 Hz, and a bias instability of 1.3 deg/hr. [2017-0081]

Dacheng Xu

Papers

In-Motion Coarse Alignment Method for SINS/GPS Using Position Loci

Wireless Alarm Microsystem Self-Powered by Vibration-Threshold-Triggered Energy Harvester

Progress and Future Prospects of Wide‐Bandgap Metal‐Compound‐Based Passivating Contacts for Silicon Solar Cells

Frequency Symmetry Comparison of Cobweb-Like Disk Resonator Gyroscope With Ring-Like Disk Resonator Gyroscope

Design of a Dual Quantization Electromechanical Sigma–Delta Modulator MEMS Vibratory Wheel Gyroscope