Long range wireless connectivity opens the door for new IoT applications. Low energy consumption is essential to enable long autonomy of devices powered by batteries or even relying on harvested energy. This paper introduces technologies for long-distance interaction with energy-constraint embedded devices. It proposes sleepy strategies for power management for the platform and the transmission. Adequate signal processing on the remote modules is demonstrated to play a crucial role to sustain the autonomy of these systems. The presented open-source development platform invites the signal processing community to a smooth validation of new algorithms and applications.

In recent years, the number of connected embedded systems with sensors is increasing exponentially. Many challenges remain to provide all these devices with energy. A common solution is to use a battery or super capacitor. However, these energy sources get drained over time. Therefore, a human intervention is necessary to recharge or replace the battery. As the number of sensors increases and they may get embedded in difficult to reach places, the human intervention generates a high overhead. To alleviate this problem, an Unmanned Vehicle (UV)-based approach exploiting Wireless Power Transfer (WPT) was conceived. Its main target is to recharge IoT nodes. Thereto energy is transferred from the charge station along the UV to the IoT nodes through two WPT stages. We present an efficient wireless power system developed for charging the batteries inside the UV. Furthermore a WPT system to charge the batteries or super capacitors at the nodes was implemented. Load modulation enables communication to the charge station and results in a reduction of the energy loss.

Many commercial platforms for fast prototyping have gained support for lpwan technologies. However, these solutions do not meet the low-cost and low-power requirements for a large-scale distribution of battery-powered sensor nodes. This paper presents the design, realization and validation of an open-source lpwan versatile platform. Energy and cost are considered key constraints for this hardware design. A power-efficient LoRa radio interface is implemented by hosting MAC functionality on the application microcontroller, eliminating the need for a modem. In the system architecture, power and cost savings are obtained by omitting and controlling lossy power circuitry. The resulting platform allows entry-level prototyping, while featuring an ultra-low sleep power of 25.2 μ W . This makes lpwan sensor applications accessible in domains that would otherwise require custom hardware development. The proposed design is validated by an illustrative but functional example of sensor nodes deployed in the field.

Advances in energy efficient electronic components create new opportunities for wireless acoustic sensor networks. Such sensors can be deployed to localize unwanted and unexpected sound events in surveillance applications, home assisted living, etc. This research focused on a wireless acoustic sensor network with low-profile low-power linear MEMS microphone arrays, enabling the retrieval of angular information of sound events. The angular information was wirelessly transmitted to a central server, which estimated the location of the sound event. Common angle-of-arrival localization approaches use triangulation, however this article presents a way of using angular probability density functions combined with a matching algorithm to localize sound events. First, two computationally efficient delay-based angle-of-arrival calculation methods were investigated. The matching algorithm is described and compared to a common triangulation approach. The two localization algorithms were experimentally evaluated in a 4.25 m by 9.20 m room, localizing white noise and vocal sounds. The results demonstrate the superior accuracy of the proposed matching algorithm over a common triangulation approach. When localizing a white noise source, an accuracy improvement of up to 114% was achieved.

Indoor positioning systems are mostly supported by Radio Frequency (RF) technologies, due to the omnipresence of wireless communication infrastructure and handsets. Most of these localization systems rely on a Line Of Sight (LOS) connection to calculate a position based on signal strength, delay or direction. However, indoor environments are characterized by Non Line Of Sight (NLOS) connections and multipath effects such as reflections, scattering, diffraction and refraction. Multipath components are generally considered as undesirable and therefore most techniques focus on mitigation. This research applies an Angle of Arrival (AoA) multipath assistance approach, using ray tracing in a known environment to exploit signal reflections. Because the complete propagation channel is taken into account, a mobile node can already be localized by a single fixed antenna array. In order to assess the performance of the system, LOS and NLOS experiments are performed in three indoor environments in the 2.4 GHz and 5 GHz frequency bands. All measurements are processed by the multipath assisted AoA method, as well as a standard triangulation approach. These tests indicate the superior accuracy of the multipath assisted method, especially in NLOS conditions. Also, the tests demonstrate that the systems performs significantly better at 2.4 GHz than at 5 GHz. Finally, the performance of the presented system is compared to results that can be found in related work.

The increase of herd sizes hinders the capability of the dairy farmer to timely detect illnesses. Therefore, automatic health monitoring systems are deployed, but due to their high energy consumption, the application possibilities remain limited. In this work, a wireless, inductive charging solution for dairy cow monitoring is designed. This system is mounted at the eating trough, and the amount of energy transferred each eating turn is determined experimentally. For the first time, inductive wireless power transfer is used to charge on-body sensor networks for cattle. Measurements at a research farm on 40 dairy cows show an average energy transfer of 96 J per meal, for an average eating time of 160 s. It is demonstrated that inductive power transfer is a viable technology to resolve the energy provision challenge for the automatic and real-time health monitoring of dairy cows.

Indoor localization of unknown acoustic events with MEMS microphone arrays have a huge potential in applications like home assisted living and surveillance. This article presents an Angle of Arrival (AoA) fingerprinting method for use in Wireless Acoustic Sensor Networks (WASNs) with low-profile microphone arrays. In a first research phase, acoustic measurements are performed in an anechoic room to evaluate two computationally efficient time domain delay-based AoA algorithms: one based on dot product calculations and another based on dot products with a PHAse Transform (PHAT). The evaluation of the algorithms is conducted with two sound events: white noise and a female voice. The algorithms are able to calculate the AoA with Root Mean Square Errors (RMSEs) of 3.5° for white noise and 9.8° to 16° for female vocal sounds. In the second research phase, an AoA fingerprinting algorithm is developed for acoustic event localization. The proposed solution is experimentally verified in a room of 4.25 m by 9.20 m with 4 acoustic sensor nodes. Acoustic fingerprints of white noise, recorded along a predefined grid in the room, are used to localize white noise and vocal sounds. The localization errors are evaluated using one node at a time, resulting in mean localization errors between 0.65 m and 0.98 m for white noise and between 1.18 m and 1.52 m for vocal sounds.

Wireless acuusuc sensor networks (wasns) are a promising technology for performing acoustic surveillance because of their flexibility and low cost. However, their commercialization is nowadays limited due to their high energy consumption, which is mainly a result of the high data rates required to stream audio data between sensor nodes. In order to improve energy efficiency in WASNs, we explore the benefits of introducing capabilities to reconfigure and collaborate. More specifically we consider an application for localizing and recording sound events in a warehouse environment. For studying this, we introduce an energy consumption model for a WASN node, which includes the cost of sensing, processing and wireless communications. Presented results show that the centralized features allow important energy savings, while maintaining a sufficient Quality of Service. For the considered setup, our simulations show that a centralized approach saves 22% of energy. Furthermore, by using a smart activation of nodes, the energy consumed by all the nodes can be balanced, resulting in a longer network lifetime.

In order to remain cost competitive, dairy farmers are equipping their animals with automatic health monitoring systems. An important obstacle for integrating these systems is the high energy consumption of the on-cow components. A solution is wireless charging of the automated system at a feeding trough by inductive coupling. We developed an inductively powered system that is charged each time the cow eats at a feeding trough. We study which energy buffer is preferable for this application: rechargeable Li-ion batteries or supercapacitors. From measurements at a dairy farm, we obtain that the rate at which energy is captured is too high for an efficient use of Li-ion batteries. Supercapacitors are able to store energy at a very high rate, making them better suited for this application.

This paper presents an omnidirectional ultra low-power MEMS microphone array for use in low-power distributed wireless sensor networks. The microphone array is used for determining the Angle-of-Arrival (AoA) of sound events in an indoor environment. When the angles, measured by several wireless sensor nodes, are combined, a sound event can be located. This array consisting of 4 microphone elements, each with their own switchable two-stage amplifier, is completely configurable using an on board low-power microcontroller. This means that only the necessary elements are active when they are actually needed. Due to the selection of ultra low-power components and smart switching, the power consumption of the array is much lower than other similar designs, consuming only 0.8 mW when the array is activated and 66 nW during sleep.

Commercial wireless power systems based on inductive coupling are gaining more popularity. These systems rely on a static alignment between a receiver and a transmitter coil for a certain amount of time. To make the technology more responsive when the device has to be charged while moving randomly in a larger area than the size of the transmitter coil, a dedicated communication scheme is required. In the presented solution, the receiver itself is equipped with a supercapacitor on board, in order to provide a constant energy supply to power the receiver functionality. The communication as described in this work is developed in order to meet the requirements of fast detection and quick charging. It has been implemented in the real-life configuration of a commercially available wireless mouse where the battery compartment was replaced with the receiver circuitry. Excellent functionality of the device was observed. It was demonstrated experimentally that the time it takes to force the transmitter in high-power mode equals 450 ms in case the receiver energy storage is completely drained, and on average, 50 ms when the receiver's microcontroller is still powered. These results contrast significantly with the well-known static receiver Qi-protocol, where these values rise to an average of respectively 2500 and 250 ms. The obtained timing values meet the requirements of rapidly collecting energy and a high degree of autonomy, even when the receiver structure is only a short amount of time above the transmitting structure.

Electronic devices for measuring body parameters in healthcare environments generally exhibit hygienic problems. Furthermore, they require regular battery replacement or recharging. This paper presents an electronic wristband for medical measurements that can be charged wirelessly. Its power supply is equipped with a Qi compatible inductive power receiver, an Electrochemical Double-Layer Capacitor (EDLC) energy buffer and a series of voltage regulators that can be disconnected from the energy buffer by a microcontroller. Because the wireless power receiver was placed behind a display, a study was performed of the effects of the display on receiver coil characteristics and system performance. For the selection of the EDLC, a selection process is presented, making a trade-off between device autonomy and charging time. The result is a completely sealable and thus sterilizable device that can be charged in less than 5 seconds while being worn on the wrist.

Present-day commercial wireless inductive power transfer systems rely on a low frequency alternating magnetic field, generated by a transmitter coil and picked up by a receiver coil. In general, these inductively coupled coils are perfectly aligned, in order to obtain a constant high coupling factor. In some cases though, the receiver can move on a regular basis above the transmitter. This paper discusses a method to wirelessly power a device moving on a surface. The transmitter consists of a single coil that is significantly smaller than the operation radius of the moving receiver. Therefore, the receiver is not always in the range of the transmitter and the power transfer occurs intermittent. In order to provide the receiver with a constant energy supply, a supercapacitor buffer is implemented. When the receiver approaches the transmitter, the buffer is charged in a short period of time. Furthermore, a communication channel is set up from the power receiver to the power transmitter, enabling receiver identification and power management.

In this paper, we describe how the quality factor of a coil depends on the frequency in the case of litz wires for wireless, inductive power applications. The importance of this work relates to the observation that the link efficiency of a induced coupled system depends critically on the quality factors of the transmitter and receiver coils. An important design choice in these systems is the frequency at which the application operates. The winding of the litz wire and the presence of ferrite shifts the frequency at which the maximal quality factor, and thus link efficiency, is obtained.