Lance Doherty

Convex position estimation in wireless sensor networks

A method for estimating unknown node positions in a sensor network based exclusively on connectivity-induced constraints is described. Known peer-to-peer communication in the network is modeled as a set of geometric constraints on the node positions. The global solution of a feasibility problem for these constraints yields estimates for the unknown positions of the nodes in the network. Providing that the constraints are tight enough, simulation illustrates that this estimate becomes close to the actual node positions. Additionally, a method for placing rectangular bounds around the possible positions for all unknown nodes in the network is given. The area of the bounding rectangles decreases as additional or tighter constraints are included in the problem. Specific models are suggested and simulated for isotropic and directional communication, representative of broadcast-based and optical transmission respectively, though the methods presented are not limited to these simple cases.

Flexible power scheduling for sensor networks

We propose a distributed on-demand power-management protocol for collecting data in sensor networks. The protocol aims to reduce power consumption while supporting fluctuating demand in the network and provide local routing information and synchronicity without global control. Energy savings are achieved by powering down nodes during idle times identified through dynamic scheduling. We present a real implementation on wireless sensor nodes based on a novel, two-level architecture. We evaluate our approach through measurements and simulation, and show how the protocol allows adaptive scheduling and enables a smooth trade-off between energy savings and latency. An example current measurement shows an energy savings of 83% on an intermediate node.

Distributed compression for sensor networks

We consider the problem of efficiently transmitting sets of spatially correlated observations in a distributed sensor network without requiring inter-node communication to exploit the correlation. Specifically, we provide a construction for quantizer design given a training set, and a distributed compression scheme to efficiently relay the quantized observations to a central decoder.

Micromachining technology for lateral field emission devices

We demonstrate a range of novel applications of micromachining and microelectromechanical systems (MEMS) for achieving efficient and tunable field emission devices (FEDs). Arrays of lateral field emission tips are fabricated with submicron spacing utilizing deep reactive ion etch (DRIE). Current densities above 150 A/cm/sup 2/ are achieved with over 150/spl middot/10/sup 6/ tips/cm/sup 2/. With sacrificial sidewall spacing, electrodes can be placed at arbitrarily close distances to reduce turn-on voltages. We further utilize MEMS actuators to laterally adjust electrode distances. To improve the integration capability of FEDs, we demonstrate batch bump-transfer of working lateral FEDs onto a quartz target substrate.

Towards 100% reliability in wireless monitoring networks

High end-to-end reliability is a quality demanded by those with critical monitoring and actuation requirements. To date, Wireless Sensor Network (WSN) users have often accepted sub-optimal reliability as being intrinsic to wireless technologies. We describe a centralized monitoring TDMA network with policies chosen to maximize the number of received packets while maintaining low power characteristics. The methods for detecting and diagnosing packet loss are presented along with expected bounds on their relative impacts. This diagnosis allows for a cataloguing of all the known loss mechanisms and for the analysis of loss in a 50-node network running at 99.99% steady-state end-to-end reliability

Deep reactive ion etching for lateral field emission devices

The authors describe the design, fabrication and testing of lateral field emission diodes utilizing the deep reactive ion etch (DRIE). Devices were fabricated on silicon-on-insulator (SOI) wafers of varied thickness, by etching the device silicon in the STS DRIE system in a single mask process. After subsequent oxidation sharpening and oxide removal, diodes were tested on a probing station under vacuum. A typical diode exhibited very high currents on the order of /spl sim/100 /spl mu/A at 60 V, and turn-on voltage between 35 V and 40 V. The high electron current is emitted in such a diode by multiple sharp tips vertically spaced by 450 nm along the etched sidewall due to the pulsed nature of the DRIE process.

A Simple Process for Lateral Single Crystal Silicon Nanowires

In a single-mask standard photolithography based process and a single etch step, lateral silicon nanowires are fabricated according to arbitrary layout and over a range of diameters and lengths. Nanowires with diameters from ∼20 nm and with lengths ranging from 2 μm to 100 μm were fabricated in direct contact with two silicon probing pads for measurement. These nanowires are electrically isolated in the silicon-on-insulator wafer device layer, and suspended over the substrate, thereby increasing thermal and electrical isolation. Because they are formed from single crystal silicon, minimal defects are expected. The addition of a polysilicon deposition and patterning further enhances the process by allowing coaxial silicon nanostructures.Copyright

Scattered data selection for dense sensor networks

An evaluation methodology is presented for the performance of reporting node self-selection in wireless sensor networks. Five cost metrics are proposed along with several methods for self-selection that involve little or no collaboration with other nodes. These costs are used to evaluate how efficiently the various algorithms allow for node self-selection as simulated on different field complexities. Analysis of different methods over 100 test fields sampled by 2000 nodes indicates that there is no single method that is superior in all respects. Trade-offs in latency and overall energy consumption are revealed to be highly dependent on the selection method and the field complexity.

Application of MEMS technologies to nanodevices

A process methodology enabling the fabrication of various nanodevices is demonstrated that is compatible with standard integrated circuit processes and recently developed MEMS technologies. The basic devices are laterally suspended single-crystal silicon nanowires with diameters from /spl sim/20 nm formed by a single DRIE etch step and oxidation thinning cycles. These nanowires can further serve as molds for conformal polysilicon and silicon nitride deposition, resulting in coaxial nanowires and fluidic devices such as nanocapillaries and nanopores with <100 nm inner diameters. The above nanodevices are being investigated for use in thermoelectric and biomedical applications as well as MEMS actuator integration.