Publikationen

Abstract: A sustainable, uniform, and utility-maximizing operation of energy-harvesting sensor networks requires methods for aligning consumption with harvest. This article presents a lightweight algorithm for online load adaptation of energy-harvesting sensor nodes using supercapacitors as energy buffers. The algorithm capitalizes on the elementary relationship between state of charge and voltage that is characteristic for supercapacitors. It is particularly designed to handle the nonlinear system model, and it is lightweight enough to run on low-power sensor node hardware. We define two energy policies, evaluate their performance using real-world solar-harvesting traces, and analyze the influence of the supercapacitor’s capacity and imprecisions in harvest forecasts. To show the practical merit of our algorithm, we devise a load adaptation scheme for multihop data collection sensor networks and run a 4-week field test. The results show that (i) choosing a duty cycle a priori is infeasible, (ii) our algorithm increases the achievable work load of a node when using forecasts, (iii) uniform and steady operation is achieved, and (iv) depletion can be prevented in most cases.

Abstract: This paper considers the use of wireless mesh networks as a communication infrastructure for monitoring and controlling industrial plants and processes such as solar power plants. In the latter, several thousands, tiltable and rotatable flat mirrors (heliostats) focus sunlight on a receiver tower. Solar power plants require a reliable and highly scalable communication infrastructure. Especially in case of a hazardous situation, the safety system must rapidly alert all heliostats, so that they drive into a stow position. The paper picks up this topic and presents a concept for a reliable wirelessly controlled emergency shutdown, even in cases when the connectivity of the network no longer holds. At this juncture, we face the challenge of communication in large-scale and extremely dense wireless networks. To this end, we introduce a robust broadcasting algorithm based on connected dominating sets. Analytical results of this paper are substantiated by simulations. Finally, prospects of wireless controlled safety systems are discussed.

Abstract: This paper presents and evaluates ORiNoCo, a novel data-collection and event-reporting protocol for sensor networks. ORiNoCo is built upon the asynchronous duty-cycle protocol RI-MAC and breaks with the tradition of exchanging extensive neighborhood information, a cornerstone of many competing collection protocols and one of their major source of communication overhead and energy expenditure. The merit of this venture is an opportunistic, energy-efficient, latency-reducing, and self-stabilizing protocol. ORiNoCo comes at virtually no extra costs in terms of memory demand and communication overhead compared to RI-MAC. We derive theoretical boundaries for the improvements in radio efficiency, latency, and energy-consumption. ORiNoCo is verified with these findings via simulation and compared with CTP. ORiNoCo achieves lower energy-consumption while reducing end-to-end delays.

Abstract: The HELIOMESH project pursues the goal to evaluate and validate the feasibility of a wireless mesh network as control technology in a field of self-powered heliostats, thus eliminating the need for cabling. To enhance precision of control, an auto-calibration method was implemented. The team chose a combination of a practical approach combined with simulations to ensure scalability for large heliostats field to be build in the future: About 100 small communication devices, so called HelioNodes, are deployed in the DLR Solar Tower Demonstration Plant heliostat field, controlled by a base station located in the tower. The deployment validates the feasibility and the industrial capability of the wireless mesh control system. In simulations, the good performance of the wireless communication is shown for huge fields. Additionally, the auto-calibration technology was optimized and successfully tested using self-developed, self-powered 8m² heliostats. These heliostats were tested and optimized, with a focus on power management and drive efficiency. Results prove that stepper motors are a good choice in case state of the art electronics are used for control.

Abstract: This dissertation examines concepts for wireless communication in large-scale, dense mesh networks, which can be used in future for plant control or building automation. It is shown that recent communication approaches for resource-constrained, IEEE 802.15.4 hardware do not scale well. This motivates the development of methodologies and protocols for reliable wireless communication in large-scale networks. This work covers the following topics: a "toolbox" for the development, programming, testing, and simulation of communication protocols for embedded systems; the decomposition of large networks into smaller subnets; concepts for scalable broadcasting and routing; the realization of selected network services such as emergency shutdowns and software updates.

Abstract: In large-scale, heavy loaded sensor networks the hidden node problem significantly restricts the attainable throughput. This paper examines this issue and depicts why most TDMA as well as dedicated hy- brid MAC protocols are still negatively affected by this phenomenon. The concept of probabilistic self-stabilization is adopted to provide a framework for implementable reservation MAC protocols that avoid packet loss caused by signal interferences even under high load. These protocols base upon two main primitives: continuity in channel access, allowing predictability, and acknowledgments, permitting to discover packet loss. The designed TDMA and CSMA protocols are able to cope with the hidden node problem and with topology changes and achieve a high throughput in a steady state. The protocols are simulated and compared with IEEE’s 802.15.4 unslotted CSMA/CA protocol.

Abstract: This paper proposes and assesses analytical tools for large-scale monitoring applications with wireless sensor networks powered by energy-harvesting supplies. We introduce the concept of an energy budget, the amount of energy available to a sensor node for a given period of time. The presented tools can be utilized to realize distributed algorithms that determine a schedule to perform the monitoring task and the inherent communication. Scheduling is based on the energy budgets of the nodes or on latency requirements. In this context, we derive theoretical results for the energy consumption of the individual nodes plus the latency of event-reporting. These results are verified by simulations and a real testbed implementation.

Abstract: Neighborhood relations are changing over time in wireless sensor networks due to different hardware or environmental effects. These effects and memory limitations require a balanced neighborhood management to ensure agility, stability, symmetry, and connectivity. The proposed neighborhood management protocol Mahalle is optimized with regard to these four criteria. Agility and stability are achieved by ALE, a new adaptive link estimator.