Joan Albesa

Powering wireless sensor nodes: Primary batteries versus energy harvesting

Wireless sensor networks (WSNs) are increasingly used in many fields. Still, power supply of the nodes remains a challenge. Primary batteries are mainly used but energy harvesting offers an alternative, although not free of problems. This paper compares the use of primary batteries against solar cells. Basic principles are first enunciated, then generic design examples are presented and finally actual deployed nodes of a WSN are illustrated.

Wireless Power Transmission for Autonomous Sensors in Removable Vehicle Seats

This work proposes the use of magnetic coupling for powering autonomous sensors in space-constrained applications, such as occupancy and belt detection in removable vehicle seats. The power demand of the autonomous sensor is considered between tens and hundreds of milliwatts. A theoretical analysis first highlights the critical parameters in order to achieve a large powering range and high efficiency. Series-resonant tanks are considered for both the primary and secondary networks. Because the intended application is space-constrained, small coils have to be used. In order to increase their quality factor, commercial ferrite-core coils are used. A class D power amplifier is proposed for the primary network. Experimental results show that a power of tens of milliwatts can be transferred to a 100 §U load placed at the secondary network up to a distance of 2cm, near seven times the radius of the coils (3mm). The addition of a rectifier and a voltage regulator at the secondary network in order to properly power an autonomous sensor (3V@30mA) limits the powering range to 1cm. Overall power efficiencies around 45% and 20% are achieved respectively at distances of 5mm and 1cm.

Smartphone remote control for home automation applications based on acoustic wake-up receivers

In most home automation scenarios electronic devices like shutters or entertainment products (Hifi, TV) are constantly in a standby mode that consumes a considerable amount of energy. The standby mode is necessary to react to commands triggered by the user. To reduce the standby current we present a node that can be attached to the plug of electronic devices and that can turn them on and off. The node contains a wake-up receiver module that reacts to an acoustic 18 kHz tone and that switches the node from active to passive mode. In active mode the node can turn on or off the respectively power source for the device under control. The acoustic wake-up signal can be sent out by any kind of speaker which enables a commercial smartphone to act as an universal acoustic remote control without line-of-sight requirement. Our wake-up receiver consists of an 18 kHz LF receiver and an MEMs-Microphone. A wake-up range of 7.5 m using a smartphone as a sender was achieved. The overall power consumption was measured to 56 μW in standby mode. Using a 230 mAh coin cell as the energy supply a theoretical lifetime of 500 days is possible.

Inductive power transfer for autonomous sensors in presence of metallic structures

This work proposes powering autonomous sensors via inductive links in presence of metallic structures. The intended application is occupancy and belt detection in removable vehicle seats. The final aim is to maximize the powering distance and minimize the influence of the metallic structures, which was achieved by using resonant networks and magnetic core materials for the coils. First, a theoretical analysis was carried out in order to identify critical parameters. Then, simulations were performed at a frequency of 130 kHz with three different coil types, two with ferrite-core coils and one with an air-core coil. Numerical results show that ferrite-core coils, in especial that with an ETD-core coil, are less affected by the presence of metallic structures. Finally, both the maximum achieved distance and the effect of a nearby metallic plate were assessed experimentally. Without the metallic plate, the air-core coils provided the maximum powering distance, thanks to its much larger winding diameter. However, with the metallic plate present the transferred power with the air-core coils to the load was insufficient for the intended application. On the other hand, the ferrite-core coils barely noticed the presence of the metallic plate, achieving the ETD-core coils the highest powering distance, around 3 cm.

Voltage regulation on inductive power links for autonomous sensors

This work is focused on the voltage regulation stage of autonomous sensors powered via inductive links. At the receiving network, a rectifier and a voltage regulator are required to provide a DC voltage supply to the autonomous sensor. Three type of voltage regulators are compared from the point of view of the system power efficiency. Both a theoretical analysis and experimental results are presented. Results show that shunt regulators provide the best power efficiency over the two other alternatives, which are series regulators and switching buck regulators.

Pervasive Computing Approaches to Environmental Sustainability

This issues Works in Progress department lists eight projects with a focus on environmental sustainability. The first three projects explore sensing and pervasive computing techniques for monitoring environmental conditions in outdoor situations. The next four projects use pervasive computing in indoor environments to inform individuals about their energy and resource consumption with the goal of positively influencing their behaviors. The final project aims to develop an energy generation infrastructure that combines multiple types of renewable energy sources.

Efficient inductive powering of brain implanted sensors

This paper describes a size and tissue absorption based comprehensive approach to optimize a pair of coils for the purpose of wireless powering of brain implanted sensors. In the first step, the optimum transmission frequency is determined by considering tolerable coil size, power transmission efficiency and tissue absorption effects. After modeling the important quantities at the frequency of interest, a numerical analysis is performed, revealing a set of coils suitable for efficient inductive powering. This numerical analysis was verified by both FEM simulation and concluding measurements. All simulations account for the layered structure of the human head, modeling the dielectric properties with Cole-Cole dispersion effects. Furthermore, a strategy of boosting power transmission efficiency is covered in simulation and measurement, particularly the application of a ferrite shielding to the transmission coil. In consequence, a link efficiency of 80% at a coil separation distance of 5mm and 20% at 20 mm using a 10mm planar receiving coil can be achieved, contributing to a higher integration density of multi-channel brain implanted sensors.

Wireless power distribution system for brain implants

Implants like brain pacemakers or brain computer interfaces (BCI) fundamentally requires an improved and efficient wireless power distribution system. This work therefore presents a novel concept based on an intermediate resonator, which provides the opportunity to power multiple implants and to minimize furthermore the dimensions of the external power transmitter. Numerical computations specify requirements to the antenna configuration and a model show the electrical behavior. Finally, a prototype system presents an initial implementation, allowing the evaluation of the concept.

Occupancy and Belt Detection in Removable Vehicle Seats Via Inductive Power Transmission

This work proposes the use of inductive links to wirelessly power an autonomous sensor in a vehicle application. The selected application is intended for occupancy and belt detection in removable vehicle seats, where wiring the seat detectors from the vehicle chassis is impractical. The autonomous sensor includes seat detectors and a wireless transceiver to transfer the data on the state of detectors. To compensate the loose coupling between the coupled coils, resonant tanks were used. To drive the transmitting resonant network, a commercial class-D amplifier was used. Working frequency was restricted to 150 kHz. Commercial magnetic-core coils were selected as they provide high coil values and quality factors in a small-size factor, which is a requirement for the intended application. At the receiving network, a rectifier and a voltage regulator were used to provide dc voltage supply to the autonomous sensor. Three kinds of voltage regulators were compared from the point of view of the power efficiency. Both a theoretical analysis and experimental results are presented for different combinations of coils and working frequencies. Theoretical analysis shows that the operating points for the linear shunt regulator always lead to higher power efficiencies compared with other alternatives such as linear series and switching buck regulators. Experimental tests were carried out using a mechanical setup to fix the coil-to-coil distances. Experimental results agree with the theoretical analysis. Achieved power efficiencies ranged from around 50% to 10% for coil-to-coil distances from one to three times the inner diameter of the coils. Experimental tests also showed that the autonomous sensor was properly powered up to coil-to-coil distances of 2.5 cm, i.e., more than four times the inner diameter of the coils.

Seat Occupancy and Belt Detection in Removable Seats via Inductive Coupling

Seat Belt Reminder (SBR) systems are effective in avoiding deaths and injuries in traffic accidents. However, their implementation can be unpractical in removable vehicle seats because of the difficulty in wiring the associated sensors, e.g. a seat occupancy sensor and a seat belt detector, to an electronic control unit (ECU). This paper proposes the use of inductive links to avoid wiring the sensors. Both sensors, which can be roughly modeled as switches, form part of a secondary resonant network. Their state (open- or short-circuit) is attained by estimating the resonance frequency of the equivalent input resistance of a primary network inductively coupled to the secondary network. Attending to the possible states of the sensors, four different resonant frequencies result. Because the application is space-constrained, small coils have been used. Commercial ferrite-core models were selected in order to achieve high coil inductance and quality factor. Furthermore, computer simulations showed the higher coupling factor achieved with respect to that achieved with air-core coils. Experimental tests were carried out using an impedance analyzer and commercial sensors for seat occupancy and belt detection. Detection was feasible at distances up to 2 cm between the primary and secondary coils.