Achim Berger

Low-Complex Synchronization Algorithms for Embedded Wireless Sensor Networks

In industrial applications of wireless sensor networks (WSNs), synchronized sampling of data on each sensor node is often required. Thus, the wireless communication protocol needs to support accurate timing synchronization. If due to a high sampling rate also high data throughput is required, WSNs based on the IEEE 802.15.4 physical layer often do not provide sufficient data rate. Wireless communications based on the well-established IEEE 802.11 wireless local area network (WLAN) standard provides high data throughput but not an accurate timing synchronization unless the protocol stack is severely changed. We propose two low-complexity consensus-based synchronization algorithms for the hybrid WSN introduced, which are executable at limited embedded computing capacity, e.g., on an 8 bit microcontroller. A time division multiple access-based synchronization packet broadcasting with three-step-controlled or proportional-integral (PI)-controlled clock adjustment enables 1 kHz sensor sampling rate with a sampling jitter <;15 μs for the three-step-controlled synchronization algorithm and <;1 μs for the PI-controlled algorithm.

Energy-efficient and reliable wireless sensor networks - an extension to IEEE 802.15.4e

Collecting sensor data in industrial environments from up to some tenth of battery-powered sensor nodes with sampling rates up to 100 Hz requires energy-aware protocols, which avoid collisions and long listening phases. The IEEE 802.15.4 standard focuses on energy-aware wireless sensor networks (WSNs) and the Task Group 4e has published an amendment to fulfill up to 100 sensor value transmissions per second per sensor node (low latency deterministic network (LLDN) mode) to satisfy demands of factory automation. To improve the reliability of the data collection in the star topology of the LLDN mode, we propose a relay strategy, which can be performed within the LLDN schedule. Furthermore, we propose an extension of the star topology to collect data from two-hop sensor nodes. The proposed retransmission mode enables power savings in the sensor node of more than 33%, while reducing the packet loss by up to 40%. To reach this performance, an optimum spatial distribution is necessary, which is discussed in detail.

Synchronized industrial wireless sensor network with IEEE 802.11 ad hoc data transmission

In industrial applications of wireless sensor networks (WSNs) synchronized sampling of data on each sensor node is often required. Thus the wireless communication protocol needs to support accurate timing synchronization. If due to a high sampling rate also high data throughput is required, WSNs based on the IEEE 802.15.4 physical layer often do not provide sufficient data rate. Wireless communication based on the well established IEEE 802.11 (WLAN) standard provides high data throughput but not an accurate timing synchronization unless the protocol stack is severely changed. We propose a hybrid WSN which uses WLAN for high rate data transmission combined with a proprietary wireless communication system operating on Sub-GHz short range devices for synchronization. A consensus-based synchronization algorithm is used to be independent from the number of WSN nodes and node failure. The proposed hybrid WSN performs a two-step synchronization algorithm which can easily be executed on an 8-bit microcontroller and achieves a sampling jitter smaller than 50 μs.

TDMA proposals for wireless sensor networks for highly reliable and energy efficient data collection in an industrial application

The range of applications for wireless sensor networks (WSNs) is continuously growing and is expanding into new branches and markets rapidly. Largely different requirements regarding network structure, powering possibility, latency, data rates, and reliability, lead to the need of suitable systems and protocols. An approved way to implement an energy aware wireless network is the Time Division Multiple Access (TDMA) strategy. In this work we propose two different TDMA schedules for highly reliable sensor data collection with sample rates of 10 Hz and analyze and compare them regarding reliability and energy consumption. A standard coin cell can power a sensor node for more than 2 month with a sensor data loss smaller than 10-3 percent at 3% PER in the radio channel.

TDMA approach for efficient data collection in wireless sensor networks

Wireless sensor networks (WSNs) are becoming increasingly attractive for industrial applications, where the requirements with respect to data rates, latency, and reliability are especially challenging. Due to cost reasons in such applications Standard hardware has to be used. As well known protocols like ZigBee or Wireless HART are not able to offer high data rates with short and deterministic delays, we propose in this contribution a time division multiple access-based approach for efficient data transmission in WSNs with a broad range of sample- and transmission rates. Our proposal is based on beacon-triggered time-intervals, supports wireless sensor nodes with a wide range of data rates and enables self organized joining and leaving of nodes. We present measurement results of a demonstration system in star topology using evaluation boards of the CC2510, a low-power System-on-Chip solution with integrated micro-controller and RF-transceiver, with two different sleep mode strategies to minimize power consumption.

Sustainable energy harvesting for robust wireless sensor networks in industrial applications

Wireless Sensor Networks (WSNs) are at the verge of a broad acceptance in demanding industrial applications. Nodes must fulfill key requirements like reliability and deterministic communication, but also energy autarky in order to allow maintenance-free systems. In this paper a system combining low power, robust communication with appropriate methods for energy harvesting and energy management is suggested. By comparing two alternative variants for power-management, constraints of a solar-cell powered node design are derived. The resulting system demonstrates energy sufficiency at standard industrial indoor lighting conditions of 1300 lx for sensor nodes sampling temperature values at 10 Hz and transmitting once per second.

TWECIS: A testbed for wireless energy constrained industrial sensor actuator networks

Wireless sensor and actuator networks (WSAN) for industrial applications usually have tighter requirements on dependability, synchronization, and real-time capability as compared applications in environmental monitoring or health care. For WSAN research and development, testbeds are a valuable tool as they enable controlled operation under conditions close to reality. In this contribution we present TWECIS, a testbed targeting the investigation of Wireless Energy Constrained Industrial Sensor and Actuator networks. We describe the concept, its architecture and the used hardware. Unique feature of TWECIS is the use of an industrial Real Time Ethernet system which offers the evaluation of the timing behavior of the nodes with a jitter below 100 ns. In addition TWECIS offers synchronized network-wide current measurements on all testbed nodes in a range from 1 μA to 100 mA for accurate power consumption profiling, remote and parallelized programming of nodes, remote in-circuit debugging, monitoring of UART output, and centralized storage of all captured data.

A power measurement system for accurate energy profiling of embedded wireless systems

In this paper, we present a highly accurate but still cost-effective measurement solution for tracking the highly dynamic power consumption of wireless embedded systems. We extended the conventional measurement of a single shunt resistors voltage drop by using a dual shunt resistor stage with an automatic switch-over between both stages, which leads to a large dynamic measurement range of 50 dB (1μA to 100 mA) with an average measurement error smaller than 1.2 %. Using two zero-drift current sense amplifiers for measuring the voltage drop over two different sized shunt resistors together with a 16 bit SAR ADC for each of both amplifiers leads to a far better thermal noise behaviour and a higher total measurement resolution compared to a single shunt solution, providing a minimum invasive measurement system with a maximum voltage drop of 100 mV. Using a state-of-the-art Ethernet connection allows our measurement system to forward the power samples with up to 250 kHz sampling rate to any remote networked computer.

Timing synchronization of low power wireless sensor nodes with largely differing clock frequencies and variable synchronization intervals

In this paper a novel synchronization method for wireless sensor networks with star topology is presented. We address timing synchronization using low frequency real-time clocks in all nodes. A beacon-driven TDMA-protocol for bidirectional node/base communication is used. Between the beacons, which are sent by the base station, lie the superframe time intervals to handle data transmission from node to base. We discuss the protocol and its energy saving advantages including the challenges of synchronization. We reduce the required communication for synchronization based on long term synchronicity of the node to save energy. Due to the individual node clock, the accurate superframe time interval usually will consist of a rational number of clock ticks. We propose to use a ΔΣ-converter to generate a sequence of superframes with different time durations, but each consisting of integer multiples of clock ticks, which - on average - achieve the accurate superframe duration for any rational number of clock ticks. We show by theory and measurements that our novel approach leads to a variance of the synchronization error which is constant at a value of 0.25 clock cycles. The variance is independent of the rate at which the nodes listen to the beacon of the base station.

Improving IEEE 802.15.4e LLDN performance by relaying and extension of combinatorial testing

Many applications in factory and process automation require robust wireless sensor networks (WSNs) for collecting sensor data with sampling and transmission rates up to 100 Hz from battery powered sensor nodes. To establish real-time communication in a star network topology, the LLDN mode of IEEE 802.15.4e amendment was released. We present a demonstration system which applies a recently introduced relaying method compatible to the LLDN mode, which uses dedicated relay nodes, thus increasing the reliability of the network while reducing the energy consumption of the sensor nodes. Furthermore, packet combining schemes on packets received erroneously from both, sensor node and relay node, are proposed, applied, and analyzed. Measurements with the demonstration system have shown that 96.4% of these erroneous transmissions could be recovered correctly.