Denis Dondi

Modeling and Optimization of a Solar Energy Harvester System for Self-Powered Wireless Sensor Networks

In this paper, we propose a methodology for optimizing a solar harvester with maximum power point tracking for self-powered wireless sensor network (WSN) nodes. We focus on maximizing the harvesters efficiency in transferring energy from the solar panel to the energy storing device. A photovoltaic panel analytical model, based on a simplified parameter extraction procedure, is adopted. This model predicts the instantaneous power collected by the panel helping the harvester design and optimization procedure. Moreover, a detailed modeling of the harvester is proposed to understand basic harvester behavior and optimize the circuit. Experimental results based on the presented design guidelines demonstrate the effectiveness of the adopted methodology. This design procedure helps in boosting efficiency, allowing to reach a maximum efficiency of 85% with discrete components. The application field of this circuit is not limited to self-powered WSN nodes; it can easily be extended in embedded portable applications to extend the battery life.

A solar energy harvesting circuit for low power applications

In this paper we present a solar energy harvesting circuit for low-power applications describing circuit architecture and guidelines for an optimal design. We evaluate the performance of two implemented prototypes intended to power a wireless embedded system under different light intensities and different switching frequencies. Measurements show that higher switching frequencies allow reaching the maximum efficiency (~90%) at higher light intensities, whereas lower operating frequencies perform better under lower irradiance. Experimental results show that circuit optimization depends on light conditions and the proposed solar energy harvester can autonomously supply the nodes of a wireless sensor network WSN.

Photovoltaic cell modeling for solar energy powered sensor networks

Photovoltaic scavenging circuits have been presented to reduce installation and maintenance costs of wireless sensor networks. When small-size photovoltaic modules are adopted, optimizing the efficiency of the harvesting process and tracking the maximum power point (MPP) becomes very difficult, and the development of a photovoltaic harvester has to be preceded by extensive simulations. The paper focuses on the definition of the model for a small PV cell allowing the simulation of harvester systems. The model is validated on a case study of MPPT circuit for sensor networks.

Photovoltaic scavenging systems: Modeling and optimization

The interest in embedded portable systems and wireless sensor networks (WSNs) that scavenge energy from the environment has been increasing over the last years. Thanks to the progress in the design of low-power circuits, such devices consume less and less power and are promising candidates to perform continued operation by the use of renewable energy sources. The adoption of maximum power point tracking (MPPT) techniques in photovoltaic scavengers increases the energy harvesting efficiency and leads to several benefits such as the possibility to shrink the size of photovoltaic modules and energy reservoirs. Unfortunately, the optimization of this process under non-stationary light conditions is still a key design challenge and the development of a photovoltaic harvester has to be preceded by extensive simulations. We propose a detailed model of the solar cell that predicts the instantaneous power collected by the panel and improves the simulation of harvester systems. Furthermore, the paper focuses on a methodology for optimizing the design of MPPT solar harvesters for self-powered embedded systems and presents improvements in the circuit architecture with respect to our previous implementation. Experimental results show that the proposed design guidelines allow to increment global efficiency and to reduce the power consumption of the scavenger.

Load optimization of an inductive power link for remote powering of biomedical implants

This article presents the analysis of the power efficiency of the inductive links used for remote powering of the biomedical implants by considering the effect of the load resistance on the efficiency. The optimum load condition for the inductive links is calculated from the analysis and the coils are optimized accordingly. A remote powering link topology with a matching network between the inductive link and the rectifier has been proposed to operate the inductive link near its optimum load condition to improve overall efficiency. Simulation and measurement results are presented and compared for different configurations. It is shown that, the overall efficiency of the remote powering link can be increased from 9.84% to 20.85% for 6 mW and from 13.16% to 18.85% for 10 mW power delivered to the regulator, respectively.

An autonomous wireless sensor network device powered by a RF energy harvesting system

In this paper, we present an energetically autonomous wireless sensor network (WSN) device designed to enhance safety in vehicles capable to connect extra gear/equipment to the main chassis. The proposed system allows the vehicle stability control system to automatically recognize the connected trailer or implement through a purposely designed WSN device, which is integrated into trailer/implement and wirelessly sends its identification number. The WSN device we developed integrates also a novel RF energy harvesting circuit which gathers the energy from an 868MHz RF signal source, which is purposely transmitted from the vehicle towards the trailer or implement for remote powering. Measurements performed on fabricated WSN system prototypes show that the RF harvester can gather up to ≈50uW@3m from the RF power source with efficiency higher than 30% over a range of 10dBm. The combination of the RF energy harvesting circuit with the ultra-low power architecture and a custom task manager designed for the WSN system allows to further increase primary battery lifetime, making the wireless system capable to operate autonomously for several years.

AlN-based MEMS devices for vibrational energy harvesting applications

This paper presents a new AlN-based MEMS devices suitable for vibrational energy harvesting applications. Due to their particular shape and unlike traditional cantilever which efficiently harvest energy only if subjected to stimulus in the proper direction, the proposed devices have 3D generation capabilities solving the problem of device orientation and placement in real applications. Thanks to their particular shape, the realized devices present more than one fundamental resonance frequencies in a range comprised between 500 Hz and 1.5 kHz, with a voltage generation higher than 300μV and an output power up to 0.4 pW for single MEMS device.

RF to DC CMOS rectifier with high efficiency over a wide input power range for RFID applications

In this paper we present a RF-DC rectifier which operates over a wide range of input power by providing a regulated output DC voltage. The circuit solution we propose is based on a novel active load circuit which adjusts the output current as a function of the incoming RF power. This allows maximizing both the efficiency and sensitivity of the circuit. Circuit prototypes fabricated in 130nm CMOS technology start to operate at −14dBm, providing a regulated output voltage of 1.6÷1.8V in the −14÷1dBm RF input power at 868MHz. Noticeably, the circuit efficiency of the rectifier peaks at 45%, remaining above 30% in the −12÷+1dBm input power range.

Optimized Energy-Aware Wireless System for Identification of the Relative Positioning of Articulated Systems in the Free Space

In this paper, a low-cost solution to identify the relative positioning of articulated systems in the free space is presented. To prove the effectiveness of the proposed solution, the system has been applied to a real case study of a tractor connected with a baler. Differently from other solutions, the implemented system can monitor the working conditions of the whole machinery while warning the driver when the machinery gets into a dangerous situation. The system is comprised of two wireless devices called Wireless Master Device (WMD) and Wireless End Device (WED) installed on the tractor and on the baler, respectively. To identify instantaneously the dangerous working conditions, each of the two wireless devices exploits a MEMS inertial sensor measuring 3-D linear accelerations and 3-D magnetic fields components integrated in the devices. Very low power consumption has been obtained by exploiting a hardware-software codesign approach implementing an optimized algorithm combined with a smart task manager. Furthermore, a vibrational energy harvester has been designed and integrated on the WED in order to make the device autonomous from an energetic point of view.

A vibration-powered wireless system to enhance safety in agricultural machinery

In this paper, we present a wireless sensing system capable to enhance safety in agricultural machinery. Modern farm tractors adopt vehicle stability control algorithms to enhance safety and prevent accidents. The main limitation in current approach is that the tractor has no information about the implement connected on the front/rear. The system we propose provides information on the connected implement to the tractor control unit allowing the vehicle characteristics update, which allows enhancing vehicle safety. The system we developed is comprised of two wireless devices. The first one, called Master Device (MD), is mounted on the tractor and receives power supply from on-board electrical system. The second one, called End-Device (ED), is mounted on the implement and gathers the supply energy from implement natural vibrations by using a vibrational energy harvester and a piezoelectric transducer. With this approach, the device can recharge an energy reservoir (e.g. a battery) during the implement usage, thus avoiding the need of frequent battery replacement and leading the wireless system to autonomously work for several years.