Daniel White Sexton

Radio Channel Quality in Industrial Wireless Sensor Networks

Wireless mesh sensor networks are being deployed today in various monitoring and control applications. Some radio network designs, such as ZigBee, presume that radio connectivity is reasonably consistent over time. Others take the opposite approach of presuming that links are entirely unreliable, and build large degrees of physical redundancy into the network in the hope that a collection of redundant but unreliable individual links will result in a reliable overall system. Surprisingly little work has been done in the middle ground, endeavoring to understand the root cause of link failure in real-world factory environments and applying this knowledge in the design of protocols that adoptively detect and use workable radio channels. In collaboration under a Department of Energy grant for the Industries of the Future, General Electric and Sensicast Systems have studied theoretical and actual performance of 2.4 GHz IEEE 802.15.4 radio transceivers on the lab bench and on the factory floor, with particular attention to jamming from 802.11 and multipath fading. Temporal and frequency variations in link quality are explored. The implications for network reliability and protocol design are discussed

A vibration energy harvesting sensor platform for increased industrial efficiency

A model for piezoelectric vibration energy harvesting with a piezoelectric cantilever beam is presented. The model incorporates expressions for variable geometry, tip mass, and material constants, and allows the parameterized determination of the voltage and power produced over a purely resistive load. The model is of a lumped-element form, with the base excitation acceleration and voltage representing the effort variables, and the tip velocity and electrical current representing the flow variables. Subsequent to the models derivation, experimental results are presented and demonstrate the accuracy of the model. As peak power output for existing vibration configurations is typically of interest, several simple optimization studies are then performed on a simple generator configuration to demonstrate the effects of several of the driving geometric and material parameters.

Underwater electromagnetic communications using conduction - Channel characterization

Wireless underwater transmission is considered using electric field generated by a pair of electrodes with opposite current and detected by two receiving electrodes. Experiments were conducted at frequencies between 100kHz and 6.35MHz, using orthogonal frequency division multiplexing (OFDM). Our lab tests were performed in a plastic tank filled with salt water, and our sea test at the ocean surface and at 5m depth (boundary free). Magnitude and phase-delay of the channel transfer function were modeled based on inference from dipole radiation theory in conducting medium. An exponential attenuation model fitted to the lab measurements indicated inverse cubic range dependence (near-field compliant). A rational-polynomial model provided the best match for the recorded magnitude, especially at low frequencies. Based on the exponential attenuation model, we estimated that the capacity of this channel is on the order of 10Mbps in the 100kHz-6.35MHz band when inside half a meter radius with 1W of transmit power, suitable for contactless data collection by remotely operated vehicles from single or multiple nodes via spectrum sharing. Finally, estimation of the effect range uncertainty of ?0.5m can have on the achievable data rates showed up to 30% performance downtrend for 1m range.

Underwater electromagnetic communications using conduction: channel characterization

This paper explores the properties of short-range broadband wireless communications for underwater operations using electric conduction. Electric field in the water is generated by a pair of electrodes with opposite current and detected by two receiving electrodes. Ranges of operation can be shorter than 1 m, suitable for contactless data collection by remotely operated vehicles (ROVs), and even as short as 1-10 cm, suitable for contactless riser health monitoring for deep sea drilling sensors. Experiments were conducted at frequencies between 100 kHz and 6.5 MHz, using orthogonal frequency division multiplexing (OFDM). Our lab tests were performed in a plastic tank filled with salt water, and our sea test at the ocean surface and 5 m depth (boundary free). Magnitude and phase-delay of the channel transfer function were modeled based on inference from dipole radiation theory in a conducting medium. An exponential attenuation model fitted to the lab measurements indicated inverse cubic range dependence (near-field compliant). A rational-polynomial model provided the best match for the recorded magnitude, especially at low frequencies. The phase characteristic obtained from the ocean measurements exhibited a minimum around 2 MHz, which agrees with theory.

Remote excitation and readout of a high Q silicon resonator

This paper presents the first published report of a totally passively driven actuation and readout of an electromechanical resonator using inductive coupling. The goal of this work is to remotely excite and read a MEMS comb drive resonator with a focus on low power operation while simultaneously maximizing standoff distance without the use of active electronics at the sensor location. Initial measurements focused on the determining the relationship of the received signal level with the drive parameters. Reading the resonator through integrated piezoresistors, the drive response showed the expected dependence of both the RF power and AM modulation depth and the coil separation matched a simple model of a 6 cm coil radius and B4 dependence. Measurements of an entirely wireless (read and driven) resonator were made to explore the standoff capability with a practical limit of ∼15 dBm RF power to both the drive and receive systems. A standoff of 9 cm was demonstrated limited by power input to the device. The effect of read and drive coil position was also studied.