Browsing by Author "Narayanan, Arvind"
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- Development of Low-power Wireless Sensor Nodes based on Assembled Nanowire DevicesNarayanan, Arvind (Virginia Tech, 2000-07-24)Networked wireless sensor systems have the potential to play a major role in critical applications including: environmental monitoring of chemical/biological attacks; condition-based maintenance of vehicles, ships and aircraft; real-time monitoring of civil infrastructure including roads, bridges etc.; security and surveillance for homeland defense systems; and battlefield surveillance and monitoring. Such wireless sensor networks can provide remote monitoring and control of operations of large-scale systems using low-power, low-cost, "throw-away" sensor nodes. This thesis focuses on two aspects of wireless sensor node development: (1) post-IC assembly of nanosensor devices onto prefabricated complementary-metal-oxide-semiconductor (CMOS) integrated circuits using a technique called dielectrophoretic (DEP) assembly; and (2) design of a low-power SiGe BiCMOS multi-band ultra-wideband (UWB) transmitter for wireless communications with other nodes and/or a central control unit in a wireless sensor network. For the first part of this work, a DEP assembly test chip was designed and fabricated using the five-metal core CMOS platform technology of Motorola's HiP6W low-voltage 0.18_m Si/SiGe BiCMOS process. The CMOS chip size was 2.5mm x 2.5 mm. The required AC signal for assembling nanowires is provided to the bottom electrodes defined in the Metal 4 (M4) layer of the IC process. This signal is then capacitively coupled to the top/assembly electrodes defined in the top metal (M5) layer that is also interconnected to appropriate readout circuitry. The placement and alignment of the nanowires on the top electrodes are defined by dielectrophoretic forces that act on the nanowires. For proof of concept purposes, metallic rhodium nanowires ((length = 5μm and diameter = 250 nm) were used in this thesis to demonstrate assembly onto the prefabricated CMOS chip. The rhodium nanowires were manufactured using a nanotemplated electroplating technique. In general, the DEP assembly technique can be used to manipulate a wider range of nanoscale devices (nanowire sensors, nanotubes, etc.), allowing their individual assembly onto prefabricated CMOS chips and can be extended to integrate diverse functionalized nanosensors with sensor readout, data conversion and data communication functionalities in a single-chip environment. In addition, this technique provides a highly-manufacturable platform for the development of multifunctional wireless sensor nodes based on assembled nano-sensor devices. The resistances of the assembled nanowires were measured to be on the order of 110 Ω consistent with prior prototype results. Several issues involved in achieving successful assembly of nanowires and good electrical continuity between the nanowires and metal layers of IC processes are addressed in this thesis. The importance of chemical/mechanical planarization (CMP) technique in modern IC processes and considerations for electrical isolation of readout circuit from the assembly sites are discussed. For the second part of this work, a multi-band hopping ultrawideband transmitter was designed to operate in three different frequency bands namely, 4.8 GHz, 6.4 GHz and 8.0 GHz. As a part of this effort, this thesis includes the design of a CMOS phase/frequency detector (PFD), a CMOS pseudo-random code generator and an on-chip passive loop filter, which were designed for the multi-band PLL frequency synthesizer. The CMOS PFD provided phase tracking over a range of -2π to +2π radians. The on-chip passive loop filter was designed for a 62_ phase margin, 250 μA-charge pump output current and 4 MHz-PLL loop-bandwidth. The CMOS pseudorandom code generator provided a two-bit output that helped switch the frequency bands of the UWB transmitter. With all these components, along with a BiCMOS VCO, a CMOS charge pump and a CMOS frequency divider, the simulated PLL frequency synthesizer locked within a relatively short time of 700ns in all three design frequency bands. The die area for the multi-band UWB transmitter as laid out was 1.5 mm x 1.0 mm. Future work proposed by this thesis includes sequential assembly of diverse functionalized gas/chemical nanosensor elements into arrays in order to realize highly sensitive "electronic noses". With integration of such diverse functionalized nano-scale sensors with low-power read-out and data communication system, a versatile and commercially viable low-power wireless sensor system can be realized.
- A linear programming approach for synthesizing origin-destination (O-D) trip tables based on a partial set of link traffic volumesNarayanan, Arvind (Virginia Tech, 1995)This research effort is motivated by the need to quickly obtain origin-destination (O-D) trip information for an urban area, without expending the excessive time and effort usually accompanying survey-based methods. The proposed approach aims to exploit the information contained even in a "partial set" of available link volumes to estimate an O-D trip table. Recently, a new approach to synthesize a trip table from observed link flows on the network was developed at Virginia Tech. This approach employs a linear programming formulation and is based on a non-proportional assignment, user-equilibrium principle. The model is designed to determine a traffic equilibrium network flow solution that reproduces the link volume data, if such a solution exists. If such alternate solutions exist, then it is designed to find that which most closely resembles a target trip table. A modified column generation technique is employed to solve the problem. The methodology also accommodates a specified prior or target trip table, and drives the solution toward a tendency to match this table using user controlled parameters. The limitation of this approach is that it needs the specification of a complete set of link flows for its accurate operation. Such a requirement limits the applicability of this model to real networks, since link volumes are not always available on all the links of a network. This research work enhances the above linear programming methodology, adding the capability to estimate OD trip tables even when only a "partial set" of link traffic counts are available. The proposed approach formulates a sequence of linear programs to approximate a fundamentally nonlinear optimization problem that is employed to estimate origin-destination flows, given incomplete network flow information. The research suggests techniques for terminating a given linear program in the sequence, as well as criteria for terminating the sequences of such LPs, and also develops a procedure for continually updating the cost vector from one linear program to the next. Modifications in the column generation technique, necessary to solve the revised model formulation, are also developed. The enhanced model is evaluated and compared with the maximum entropy approach, which is a popular approach for OD table estimation. These models are evaluated through tests on an artificial network and a real network. The tests aim to evaluate these models using various sets of link volumes and prior table information. The results indicate that the linear programming approach performs better than the maximum entropy approach for most cases. Conclusions and recommendations for future research are also presented.
- Review and evaluation of models that produce trip tables from ground countsSivanandan, R.; Narayanan, Arvind; Lei, Peng (Virginia Center for Transportation Innovation and Research, 1996-02)This research effort was motivated by the desires of planning agencies to seek alternative methods of deriving current or base year Origin-Destination (O-D) trip tables without adopting conventional O-D surveys that are expensive, time consuming and labor intensive. This study had two objectives: (t) to conduct a review of existing approaches and models that produce trip tables from ground counts, and to select a few models for testing and evaluation, and (2) to perform a detailed testing of selected models based on application to both hypothetical and real networks, and to conduct performance evaluation and sensitivity analyses of these models. Two models, namely, The Highway Emulator (THE), and the Linear Programming (LP) model developed at Virginia Tech, were chosen for comprehensive testing and evaluation. For test purposes, these two models were applied to the following three case studies: (1) Sample Network, (2) Purdue University Network, and (3) Pulaski Town Network. While the first network was a hypothetical one, the other two were real networks. Different cases of targetable information and combinations of percentage available target cells and link volume information were used in the tests. These tests enabled a comprehensive evaluation of the performance and sensitivity analyses of the models. The test results were judged by two criteria: (1) the closeness of the model output tables to the "correct" or "surveyed" tables, and (2) the replication of observed link volumes by the models. The test results led to the following key conclusions: In general, the LP model results have proven to be superior, both in terms of closeness of modeled trip tables to the "correct"/"surveyed" tables, and in terms of replicating observed link volumes, for all the case studies, The exception to this is the structural target case, when THE produced better results, in terms of closeness of output tables to the "correct "l" surveyed" tables. This is based on the assumption that the "correct"/"surveyed" trip tables used for the case studies were in fact "correct"/"true". 2. THE model performed superior to the LP model for the structural target case (almost all the cases), where the target contains 1/0 cell values, 1 for those cells which represent O-D Interchanges that are feasible, and 0 for those that are not. This has practical implications in that if a region does not have a prior table available as target, then a structural target could be used.A word of caution must be noted with regard to conclusion # 2 above. While one would be tempted to use THE with a structural target for applications where a prior table is not available, it must be noted that the modeled results of both THE and LP turned out to be poor when compared with the "correct"/"surveyed" tables for all the cases, even though THE results were better than those of LP. However, these conclusions are based on tests on specific and limited number of networks, and under the assumption that the data used in testing and evaluation were accurate enough. The adoptability of these models and the use of one model versus the other must be decided based on the above facts, and in the context of error rates reported in this study, However, this study has highlighted the value of using such theoretical models for trip table estimation without performing conventional surveys.