William M. Healy

Impacts of 2.4-GHz ISM Band Interference on IEEE 802.15.4 Wireless Sensor Network Reliability in Buildings

With the recent emergence of standards, wireless solutions are ready to be deployed in building automation networks. IEEE 802.15.4 is a standard for short-range wireless networks. Its major application fields are home and building automation, as well as industrial sensor and actuator networks. It operates primarily in the license-free 2.4-GHz industrial, scientific, and medical band. This feature makes the technology not only easily applicable but also potentially vulnerable to interference by other technologies in this band, e.g., Bluetooth and microwave ovens. There are many possible coexistence scenarios with different network sizes, configurations, interference sources (ISs), and environmental conditions. To investigate the impacts of ISs on the performance of IEEE 802.15.4 wireless sensor networks, this paper performs several experiments with commercially available equipment. The results give a rough indication of the mutual interference of the different systems when any two of the networks operate simultaneously and in range. This work collects important communication parameters to evaluate the impacts of other ISs on IEEE 802.15.4 networks. These results should help designers better understand the challenges of building wireless applications.

On the validity of the adiabatic spreading assumption in droplet impact cooling

Abstract The effect of heat transfer on the spreading of a water droplet impacting a heated surface has been numerically investigated. The aim is to assess the validity of commonly made assumptions in spray cooling models which ignore heat transfer during the droplet spreading period. The results of this study indicate that while the amount of heat transfer during the droplet spreading period may, itself, be small, its impact on the liquid coverage area and the subsequent heat transfer to the resulting liquid film may be substantial.

Surface wetting effects on the spreading of liquid droplets impacting a solid surface at low Weber numbers

Abstract A modification to the Kurabayashi–Yang equation, for predicting the maximum spreading ratio of a liquid droplet impacting a solid surface, has been made to account for effects of the contact angle between the spreading liquid and the impact surface. A computational fluid dynamics model was used to generate the correction factor, and comparison of the corrected model to experimental data from the literature shows that predictions improved significantly. The average error between the model’s predictions and the experimental values dropped from 12.2 ± 24.8% for the original equation to 3.6 ± 12.2% for the corrected equation.

An experimental study of interference impacts on ZigBee-based wireless communication inside buildings

ZigBee is a wireless technology developed as an open global standard to address the unique needs of low-cost, low-power wireless sensor networks. This standard takes full advantages of the IEEE 802.15.4 physical radio specification and operates in unlicensed bands (e.g., 2.4 GHz) worldwide at different frequencies. As more and more companies make products that use the 2.4 GHz portion of the radio spectrum, network designers have had to deal with increased signals from collocated networks operating over the same frequency range. This paper aims to highlight the issues affecting co-existence of ZigBee systems in the presence of different interferences. We present an experimental study of ZigBee-based wireless communication over a period of time with WiFi, BlueTooth and microwave ovens. Results are presented for several different link configurations. Based on observations of the Packet Error Rate, we propose interference prediction algorithms to explore the impacts of WiFi/microwave oven on ZigBee communications.

ZigBee-wireless mesh networks for building automation and control

ZigBee is a wireless technology developed as an open global standard to address the unique needs of low-cost low-power wireless sensor networks. This standard takes full advantages of the IEEE 802.15.4 physical radio specification and operates in unlicensed bands worldwide at different frequencies. ZigBee-Wireless Mesh Networks (ZigBee-WMNs) are recognized as a cost-effective and flexible solution for building automation and control. They have the potential to unify the methods of data communication for sensors, actuators, appliances, and asset-tracking devices. They offer a means to build a reliable but affordable network backbone that supports battery-operated devices with a low data rate and a low duty cycle to facilitate building automation and control systems (BACs). This paper illustrates the possibilities and advantages of applying ZigBee-WMNs in BACs to benefit building occupants.

Battery discharge characteristics of wireless sensors in building applications

Sensor nodes in wireless networks often use batteries as their source of energy, but replacing or recharging exhausted batteries in a deployed network can be difficult and costly. Therefore, prolonging battery life becomes a principal objective in the design of wireless sensor networks (WSNs). There is little published data that quantitatively analyze a sensor nodes lifetime under different operating conditions. This paper presents several experiments to quantify the impact of key wireless sensor network design and environmental parameters on battery performance. Our testbed consists of MicaZ motes, commercial alkaline batteries, and a suite of techniques for measuring battery performance. We evaluate known parameters, such as communication distance, working channel and operating power that play key roles in battery performance. Through extensive real battery discharge measurements, we expect our results to serve as a quantitative basis for future research in designing and implementing battery-efficient sensing applications and protocols.

Assessment of Performance Metrics for Use of WSNs in Buildings

This paper is part of an effort to develop measurement systems and methods to better predict the performance of wireless sensor and control networks in building applications. Because of the variability in application requirements, the development of measurement methods to assess the performance of wireless sensor networks in buildings is extremely challenging. This paper presents the key challenges in using wireless technology in practical building applications through the results of a literature survey and interactions with building industry professionals. It is anticipated that the findings will provide potential users of wireless technology a clear metric as to how the technology will perform in a particular, and to understand the barriers to adoption of wireless sensors in buildings when appropriate.

Design Concepts for a New Guarded Hot Plate Apparatus for Use Over an Extended Temperature Range

The National Institute of Standards and Technology is building a new guarded hot plate apparatus (GHP) for use at temperatures from 90 to 900 K, with provision to conduct tests in various gases at controlled pressures from 0.013 Pa to 0.105 MPa (= 1.04 atm). Important features of the design of the new NIST GHP include: enclosure of the entire apparatus in a vacuum chamber; solid metal hot plates and cold surface plates to provide highly isothermal surfaces in contact with the test specimens; an integral closefitting edge guard to minimize the effects of edge heat losses or gains; connection guard blocks to minimize the effects of heat conduction along coolant lines, heater leads, thermometry wells, and sensor leads coming from the hot plate and the cold plates; provision of a system to provide a known clamping force between the specimens and the contacting hot and cold plate surfaces; provision of an accurate system for in-situ measurement of specimen thickness during a test; and the use ofthree long-stem standard platinum resistance thermometers to measure the average temperature of the meter plate and the two cold plates.

Variability in energy factor test results for residential electric water heaters

Recent modifications to the minimum energy efficiency requirements for residential water heaters have spurred an investigation into the variability in testing high-efficiency electric water heaters. While initial interlaboratory comparisons showed excellent agreement between test results from different labs, subsequent interlaboratory comparisons show differences between measured energy factors of up to 0.040. To determine the source of these differences, analyses of various parts of the test procedure are performed. For one case studied, the uncertainty in test results can be as high as +0.028 if instrument accuracies reach the minimum level allowed in the test procedure. Other areas of the test procedure where variability is introduced are the optional use of pre-draws, the location of the lower tank temperature-measuring device, the use of insulation on tank fittings, and the use of a warm-up period before the simulated-use test commences. The implications of these issues on test results are discussed.

Performance measurement and analysis of low data rate wireless communication under interference sources in buildings

Interference from collocated networks operating over the same frequency range becomes an increasingly severe problem as the number of networks overlapping geographically increases within commercial and residential buildings. This paper aims to highlight the issues affecting co-existence of IEEE 802.15.4 (ZigBee) systems in the presence of interference. ZigBee uses the IEEE 802.15.4 PHY and MAC layer standards to handle devices. The practical performance of ZigBee systems are established with reference to supporting empirical and simulated data. Our experiments show that, among different interferers, interference from microwave ovens is indeed a major problem. Guidelines are provided for installing sensors inside buildings.