Ruud Vullers

RF Energy Harvesting and Transport for Wireless Sensor Network Applications: Principles and Requirements

This paper presents an overview of principles and requirements for powering wireless sensors by radio-frequency (RF) energy harvesting or transport. The feasibility of harvesting is discussed, leading to the conclusion that RF energy transport is preferred for powering small sized sensors. These sensors are foreseen in future Smart Buildings. Transmitting in the ISM frequency bands, respecting the transmit power limits ensures that the International Commission on Non-Ionizing Radiation Protection (ICNIRP) exposure limits are not exceeded. With the transmit side limitations being explored, the propagation channel is next discussed, leading to the observation that a better than free-space attenuation may be achieved in indoors line-of-sight environments. Then, the components of the rectifying antenna (rectenna) are being discussed: rectifier, dc-dc boost converter, and antenna. The power efficiencies of all these rectenna subcomponents are being analyzed and finally some examples are shown. To make RF energy transport a feasible powering technology for low-power sensors, a number of precautions need to be taken. The propagation channel characteristics need to be taken into account by creating an appropriate transmit antenna radiation pattern. All subcomponents of the rectenna need to be impedance matched, and the power transfer efficiencies of the rectifier and the boost converter need to be optimized.

Wearable electronics self-powered by using human body heat: The state of the art and the perspective

In this paper, we present our vision of what kind of wearable devices and how they can be powered by the heat of human beings and by using ambient light. The basic principles of designing body-powered devices and ways of their hybridizing with photovoltaic cells are discussed. The mechanisms of thermoregulation in humans and the laws of thermodynamics enable placing a distinct boarder between realistic targets and the science fiction. These allow prediction of application areas for wearable energy harvesters accounting for competitive batteries with long service life. The existing family of body-powered wearable devices and new technologies for thermopiles are discussed. The theory and practice point at the necessity of using microelectronic and microelectromechanical system technologies for the target application area. These technologies for thermopiles offer the possibility of reduced production cost. Therefore, autonomous systems powered thermoelectrically could be successfully marketed. The related as...

Human++: From technology to emerging health monitoring concepts

This paper gives an overview of the recent results from the Human++ research program, which targets the realization of miniaturized, intelligent and autonomous wireless sensor nodes for body area networks. It combines expertise in micro-power harvesting techniques, ultra-low-power radio, ultra-low-power DSP and sensors and actuators. This paper illustrates how technological breakthroughs in these areas lead to the emergence of new health monitoring concepts.

Human++: Emerging Technology for Body Area Networks

This paper gives an overview of results of the Human++ research program [1]. This research aims to achieve highly miniaturized and autonomous transducer systems that assist our health and comfort. It combines expertise in wireless ultra-low power communications, 3D integration technologies, MEMS energy scavenging techniques and low-power design techniques.

A batch process micromachined thermoelectric energy harvester: fabrication and characterization

Micromachined thermopiles are considered as a cost-effective solution for energy harvesters working at a small temperature difference and weak heat flows typical for, e.g., the human body. They can be used for powering autonomous wireless sensor nodes in a body area network. In this paper, a micromachined thermoelectric energy harvester with 6 μm high polycrystalline silicon germanium (poly-SiGe) thermocouples fabricated on a 6 inch wafer is presented. An open circuit voltage of 1.49 V and an output power of 0.4 μW can be generated with 3.5 K temperature difference in a model of a wearable micromachined energy harvester of the discussed design, which has a die size of 1.0 mm × 2.5 mm inside a watch-size generator.

Shock induced energy harvesting with a MEMS harvester for automotive applications

In this work we report shock induced measurement and simulations on AlN based piezoelectric vibration energy harvesters. We compare the result with sinusoidal input vibrations, where we obtain a record power of 489 µW. The vacuum packaged harvesters have high quality factors and high sensitivity. We validate the potential of piezoelectric vibration harvesters for car tire applications by measurements and simulations.

Ultra low power wireless and energy harvesting technologies — An ideal combination

Rapid developments of energy harvesting in the past decade have significantly increased the efficiency of devices in converting ambient free energy into usable electrical energy, thus offering opportunities to design energy autonomous systems nowadays. To achieve such energy autonomous systems, a good understanding of the harvesting capability from the source side strongly motivates the design of ultra low power (ULP) systems. In this paper, we focus on wireless body area networks (WBAN) applications and show that ULP wireless is the key technology to enable wireless autonomous transducer solutions (WATS). We first show that the current energy harvesters cannot provide sufficient power for a typical wireless sensor node based on off-the-shelf components. We then point out that the wireless module is the main component whose power consumption needs to be significantly reduced. To address this problem, we present a ULP wireless module that could satisfy the typical performance requirement of WBAN. Using this ULP wireless module, we demonstrate the feasibility of energy autonomous sensor nodes (i.e. WATS) with the current energy harvesting technology. Moreover, with this ULP module, we point out some new research trends on the miniaturization and cost reduction of energy harvesters. Therefore, we conclude that ultra low power wireless system is an ideal application for energy harvesting.

Micropower energy scavenging

More than a decade of research in the field of thermal, motion, and vibrational energy scavenging has yielded increasing power output and smaller embodiments. Power management circuits for rectification and DC-DC conversion are becoming able to efficiently convert the power from these energy scavengers. This paper summarizes recent energy scavenging results and their power management circuits.

Thermoelectric and Hybrid Generators in Wearable Devices and Clothes

This paper discusses the necessity and ways of replacing batteries in BSNs and other wearable devices with energy scavengers. The stresses are made on thermoelectric energy converters of human body heat into electrical power and on rules of their designing. The reasons for and possible ways of hybridizing wearable thermoelectric converters with photovoltaic cells are discussed, too. The examples of energy scavengers, both wearable and in clothing, for self-powered wireless sensors are described.

Optimum power and efficiency of piezoelectric vibration energy harvesters with sinusoidal and random vibrations

Assuming a sinusoidal vibration as input, an inertial piezoelectric harvester designed for maximum efficiency of the electromechanical energy conversion does not always lead to maximum power generation. In this case, what can be gained by optimizing the efficiency of the device? Detailing an answer to this question is the backbone of this paper. It is shown that, while the maximum efficiency operating condition does not always lead to maximum power generation, it corresponds always to maximum power per square unit deflection of the piezoelectric harvester. This understanding allows better optimization of the generated power when the deflection of the device is limited by hard stops. This is illustrated by experimental measurements on vacuum-packaged MEMS harvesters based on AlN as piezoelectric material. The results obtained for a sinusoidal vibration are extended to random vibrations. In this case, we demonstrate that the optimum generated power is directly proportional to the efficiency of the harvester, thus answering the initial question. For both types of studied vibrations, simple closed-form formulas describing the generated power and efficiency in optimum operating conditions are elaborated. These formulas are based on parameters that are easily measured or modeled. Therefore, they are useful performance metrics for existing piezoelectric harvesters.