With the rapid advance of the Internet of Things (IoT), more and more wearable devices are being developed for real-time monitoring. Most of these existing monitors are powered by chemical batteries. Replacing and disposing batteries for an exponentially increasing number of IoT nodes prohibitively results in labor-intensive maintenance. It is also environmentally unfriendly. Gls EH, reclaiming the wasted ambient energy, is a promising technology for battery-free IoT. This article presents a novel motion-powered real-time human activity recognition (HAR) system called motion-powered HAR system (MP-HAR), where the harvester works as both an energy source and sensor. MP-HAR emphasizes low-power as well as low-cost characteristics, encompassing four necessary units: 1) energy transduction unit (ETU); 2) energy management unit (EMU); 3) energy user unit (EUU); and 4) edge computing unit (ECU). In particular, the unique intermittent operation based on the reconfigurable on/off threshold voltages given by the well-rounded energy-aware circuit has been discussed in detail. The balance between energy supply and information demand in MP-HAR has been achieved by using a handy design. Utilizing the unique correspondence between human arm swing frequency and harvested energy, the information flows with energy inside the system. By knowing the interval between transmitted packets, MP-HAR has realized HAR in real time. Moreover, an all-in-one prototype has been fabricated to validate the performance of the proposed system. Lab and field tests have demonstrated that MP-HAR can reliably recognize different human activities, such as standing, walking, jogging, and running. As a cyber-electro-mechanical co-design, MP-HAR has brought a promising solution for pervasive HAR and ubiquitous IoT.

Energy harvesting (EH) provides a promising solution for powering distributed Internet of Things (IoT) devices. Due to the low-level and sporadic ambient energy supply, an EH-powered device should operate in an intermittent and energy-driven mode. Commercial voltage supervisors were not optimized for the EH scenario, making it difficult to satisfy all new demands. The conventional energy management (EM) circuit has a risk of locking up during the turn-ON transient; therefore, it might fail to power the IoT load device. Previous technologies have used a relatively large circuit to solve this problem. In this article, a concise discrete three-transistor EM (3T-EM) circuit is proposed. It can track stored energy, switch ON/OFF to the load device, and provide a regulated voltage output. These key functions are realized by utilizing a minimum number of components; therefore, power consumption and manufacturing cost are largely cut. The voltage thresholds and minimum input current are theoretically derived. In experiments, the ON/OFF thresholds can be adjusted accurately, as predicted by the theory. The 3T-EM circuit can ensure the correct operation when the input current is as low as$0.4~ \mu \text{A}$. Control experiments also prove the effectiveness and performance of the 3T-EM circuit. The proposed 3T-EM circuit shows the characteristics of low cost, low power, inherent regulation, high voltage rating, and good predictability. It is a good candidate to perform the EM task in widely distributed EH-powered IoT devices.

The increasing number of distributed Internet of Things (IoT) devices makes the power supply a prominent issue, which limits the extent and lifetime of ubiquitous IoT networks. Mechanical energy harvesting (MEH) technology transforms the local mechanical energy into useful electricity. It provides a solution for the realization of self-sustainable motion-powered IoT applications. Given the volatile feature of most ambient vibrations, energy management methods are necessary for matching the unstable energy supply from the MEH sources and the energy demand of timely IoT tasks. This paper summarizes and analyzes two energy buffer-release mechanisms (EBRM) from mechanical and electrical aspects, respectively, for ensuring the robust operation of motion-powered IoT systems. Rather than emphasizing harvesting more energy from vibrations, as most of the previous studies did, we focus on energy neutrality among the mechanical, electrical, and cyber ingredients. A preset energy release threshold realized in either a mechanical or electrical way ensures the completion of every fundamental atomic task. The necessity of EBRM is demonstrated in three controlled experiments. Only those systems with a mechanical or electrical buffer-release mechanism can operate correctly.

Owing to the advancement of vibration energy harvesting technology, many motion-powered battery-free Internet of things (IoT) applications have been reported in recent years. Since the ambient energy is usually weak, these IoT devices (a) mostly operate in intermittent or burst mode. The operation of these devices heavily depends on the level of stored energy. Conventional energy-aware solutions are carried out based on discrete general-purpose comparators or analog-to-digital converters (ADC) inside the system on chip (SoC) to monitor the storage level. Their power consumption is considerable in a low-power system. This paper presents a new energy-aware circuit, which can wake up or turn off the IoT devices according to the stored energy. Its standby quiescent current is at the sub-$\mu$A level. The proposed circuit also provides a regulated output voltage for IoT applications. This design is built with discrete analog components. It is low cost and has high feasibility. It is compatible with different kinds of micro-power generators.

Wireless sensor network (WSN) enables the continuous monitoring of environmental conditions. These systems are usually powered by batteries. Given that there might be tremendous distributed sensor nodes in a network, battery maintenance must become one of the major challenges for their massive deployment. Energy harvesting technology, by which energy is extracted from the ambient environment, is developed for powering the ubiquitous Internet of Things (IoT) devices using different types of local energy, such as solar, vibration, and wind. In this paper, based on the vibration energy harvesting technology, we introduce a battery-free pavement roughness estimation system (BF-PRES), which provides road roughness information by linking the driving vibration and wireless packet count. Instead of harvesting energy to power an off-the-shelf commercial accelerometer and implementing some algorithms for vibration estimation, BF-PRES simply sends out a BLE Beacon packet, when the accumulated energy arrives at a sufficient level. Given that larger vibration intensity gives higher harvested power, the packet count within a constant time interval should be positively related to the road roughness. Lab testing shows the feasibility of the proposed design. In addition, this study also provides a new design scheme and easy implementation of battery-free or energy-constrained IoT systems.

In piezoelectric energy harvesting (PEH), utilizing the synchronized switch interface circuits can significantly enhance the energy harvesting capability. Vibration sensing and synchronization are necessary functions for carrying out the synchronized switching actions. Those functions were usually implemented with an external displacement sensor in the early designs, which is not friendly for self-contained applications. This paper proposes a compact time-sharing solution for energy harvesting and vibration sensing by making full use of a modified buck-boost design. The circuit works in a strong discontinuous conduction mode (DCM). It is based on the principle that, under DCM operation, the voltage levels of the piezoelectric source side and the storage side are proportional to the actively switch- on and passively freewheeling intervals, respectively. Compared with the other self-powered synchronized switch PEH solutions, which rely on analog peak detectors, this self-sensing solution not only provides synchronized triggering signals for carrying out the switching actions but also tells more information about the driven vibration, such as its frequency and displacement magnitude. With the detailed vibration information, such a time-sharing solution offers more design convenience towards future multi-functional battery-free IoT applications.

In this article, we introduce a vibration-powered sensing node (ViPSN), a programmable Internet-of-Things (IoT) platform for the development of vibration-powered or motion-powered sensing and transmitting systems. It leverages the exploitation and utilization of ambient vibration energy by using a piezoelectric transducer. The roles and relations of six necessary modules, including energy generation unit (EGU), energy transduction unit (ETU), energy enhancement unit (EEU), energy management unit (EMU), energy user unit (EUU), and edge demonstration unit (EDU) are discussed in detail. In particular, an enhanced EMU is proposed by making necessary complements to an extensively used off-the-shelf integrated circuit (IC) solution for piezoelectric transducers. It provides more comprehensive energy storage indicating signals, such that the sensing, computing, and transmitting tasks can be carried out more robustly by keeping a good awareness of the remaining energy. Owing to the enhanced EMU design, vibration energy in various forms, such as intermittent and transient ones, can be more effectively harvested and utilized. The performance of ViPSN is evaluated, in terms of its lifetime and Quality of Service (QoS), under different vibration scenarios. The inclusive design and affiliated opensource project of ViPSN help build a new ecosystem for the research and development of vibration- or motion-powered IoT systems.

In piezoelectric energy harvesting (PEH), synchronous electric charge extraction (SECE) has the benefit of load independence. To reduce the energy dissipation on parasitic resistance, multi-shot SECE (MCE) was implemented to split the charge extraction process. In this paper, a multiple charge extraction with bias-flip (MCEBF) interface circuit is proposed. It not only reduces the dissipation in charge extraction but also enlarges the extractable energy of piezoelectric structure by adding a bias-flip action. MCEBF can also generate positive and negative voltage rails via buck-boost topology to supply power for double-rail devices. The analytical and experimental comparison among SECE, MCE, and MCEBF under different controls are also provided. In experiments, MCEBF offers a 57.3% improvement over SECE and 24% over MCE, in terms of harvested power.