Vittal S. Rao

OEDSR: Optimized Energy-Delay Sub-network Routing in Wireless Sensor Network

A novel optimized energy-delay sub-network routing (OEDSR) protocol for wireless sensor networks (WSN) is presented. Routing is based on a link cost factor, which is defined using the quality of service (QoS) metric selected as a ratio of the remaining energy of the nodes and end-to-end delay times the distance from the base station (BS). Initially in OEDSR, the nodes are either in idle or sleep mode, but once an event is detected, the nodes near the event become active and start forming sub-networks. Formation of the inactive network into a sub-network saves energy because only a portion of the network is active in response to an event. Subsequently, the sub-networks organize themselves into clusters with cluster heads (CHs). The data from the CHs are sent to the BS via relay nodes that are located outside the sub-networks in a multi-hop manner by using the proposed routing protocol. This routing protocol improves the lifetime of the network and the scalability. GloMoSim results indicate that the OEDSR protocol results in lower average end-to-end delay, fewer collisions and less energy consumed when compared with DSR, AODV, and Bellman Ford routing protocols

Detection and characterization of high-velocity impact damage in advanced composite plates using multi-sensing techniques

Abstract Advanced composites are increasingly used in aerospace, naval, and automotive vehicles due to their high specific strength and stiffness. However, the mechanical properties of composite materials may degrade severely in the presence of damage. Damage due to impact in composite plates is often difficult to detect using any single technique. In this paper, the use of multiple sensing techniques to characterize high-velocity impact damage in advanced composites is reported. Broadband wave-based acoustic emission (AE) sensors are used to capture wave signals due to impact while shearography and ultrasonic (UT) immersion techniques are used to assess location and extent of damage after the impact. Five 48-ply [0/+45/90/−45]6s laminated AS4/PEEK composite plates were used as test specimens. Shearography images of all five test specimens were taken before impact testing to detect any pre-existing internal damage from fabrication. Three broadband AE sensors were mounted on the surface of the composite plates to capture the AE signals due to impact. A 3/8-inch diameter stainless steel ball fired from a gas gun facility was used as a projectile to inflict damage to the composite plates. The AE signals were instantaneously acquired during the impact tests and stored in a computer. The AE signals show existence of both the extensional and flexural modes, with extensional modes typically showing first. AE energy also increases to a threshold as the kinetic energy of impact increases. Shearography fringe patterns show existence of damage and this is confirmed and quantified with C-scan images from the UT immersion test. There is good correlation between AE parameters such as AE energy, AE amplitude, and AE count with impact energy and with damage on the composite plates. Due to the low contrast of the shearograms, UT C-scans are used to show extent of damage. This research demonstrates how multiple sensing techniques can be used to characterize high-velocity impact damage in advanced composites.

Self-organizing wireless sensor networks for structural health monitoring

A smart sensor node has been developed which has (a) the ability to sense strain of the structure under observation, (b) process this raw sensor data in cooperation with its neighbors and (c) transmit the information to the end user. This network is designed to be self organizing in the sense of establishing and maintaining the inter node connectivity without the need for human intervention. For the envisioned application of structural health monitoring, wireless communication is the most practical solution for node interconnectivity not only because they eliminate interconnecting cables but also for their ability to establish communication links even in inaccessible regions. But wireless nework brings with it a number of issues such as interference, fault tolerant self organizing, multi-hop communication, energy effieiciency, routing and finally reliable operation in spite of massive complexity of the sysetm. This paper addresses the issue of fault tolerant self organiing in wireless sensor networks. We propose a new architecture called the Redundant Tree Network (RTN). RTN is a hierarchical network which exploits redundant links between nodes to provide reliability.

Structural health monitoring using wavelet transforms

A new method of damage detection using wavelet transforms and curvature mode shapes is proposed in this paper. A damage in the structure results in changing its dynamic characteristics such as natural frequencies, damping, and mode shapes. A number of researchers have investigated structural health monitoring techniques for identifying, locating, and quantifying the damage using the changes in the dynamic response of a damaged structure. Curvature mode shape and wavelet maps are two such methods that have already been used to locate damages. These methods have some limitations in determining the exact location of the damages. We have developed a technique by combining these two methods for enhancing the sensitivity and accuracy in damage location. The mode shapes are double differentiated using the central difference approximation to obtain the curvature mode shape. Then a wavelet map is constructed for the curvature mode shape. It is shown that this method can be used to determine the location of the damages. The proposed method is applied to detect damage in an experimental lattice structure and a cantilever beam with multiple damages. The mode shapes are obtained analytically using finite element analysis and also experimentally using laser vibrometer. The experimental results obtained are satisfactory.

Robust control of flexible structures using multiple shape memory alloy actuators

The design and implementation of control strategies for large, flexible smart struc tures presents challenging problems. To demonstrate the capabilities of shape-memory-alloy (SMA) actuators, we have designed and fabricated a three-mass test article with multiple shape-memory- alloy, NiTiNOL, actuators. Both force and moment actuators were implemented on the structure to examine the effects of control structure interaction and to increase actuation force. These SMA actu ators exhibit nonlinear effects due to dead band and saturation. The first step in the modeling process was the experimental determination of the transfer function matrix derived from frequency response data. A minimal state space representation was determined based on this transfer function matrix. Finally, a reduced order state space model was derived from the minimal state space representation. The simplified analytical models were compared with models developed by structural identification techniques based on vibration test data. From the reduced order model, a controller was designed to dampen vibrations in the test bed. To minimize the effects of uncertainties on the closed-loop system performance of smart structures, a linear quadratic Gaussian loop transfer recovery, LQG / LTR, control methodology was utilized. A standard LQG/LTR controller was designed; however, this controller could not achieve the desired performance robustness due to saturation effects. Therefore, a modified LQG / LTR design methodology was implemented to accommodate for the limited control force provided by the actua tors. The closed-loop system response of the multiple input multiple output (MIMO) test article with robustness verification was experimentally obtained and is presented in this paper. The modified LQG / LTR controller demonstrated performance and stability robustness to both sensor noise and parameter variations.

A state space modeling and control method for multivariable smart structural systems

A system identification technique for the derivation of minimal, continuous time state variable models for multivariable smart structural systems is presented. The structural identification technique is based on the measurement of eigenvalues and eigenvectors of the structure. Two sensors are required for each mode included in the structural system model. Unlike computational system identification techniques, the relatively large number of sensors simplifies the identification process making it ideal for systems with several inputs and outputs. Additionally, the identification technique allows the implementation of multi-input multi-output full state feedback controllers with simple analog hardware. The amount of hardware required for the implementation of an analog linear quadratic regulator is significantly reduced from standard discrete control implementation methods and stability margins are retained. The eigenvectors of distributed parameter structural systems are examined. For a general unknown system, the eigenvalues and eigenvectors cannot be directly measured. For the lightly damped structural systems considered in this paper however, it is shown that these measurements are possible. Eigenvalues are conspicuous in the frequency domain and the eigenvectors exist at near steady state conditions. By utilizing a priori knowledge of the structural system, the eigenvectors can be estimated from steady state sinusoidal amplitude measurements. The identification procedure utilizes n measurement variables of the structural system with n/2 modes to produce a nth order model. This allows for the measurements to be defined as the model states. It is shown that an array consisting of n/2 sensors on the structure and some simple analog hardware suffice for the identification. For symmetrical systems, it is shown that the number of sensors required for the model identification is reduced further. A variety of measurement devices and techniques are discussed in relation to the proposed system identification technique. A sensor array consisting of shaped and segmented polyvinylidene fluoride film is presented as an inexpensive and practical measurement device. A procedure for the generation of distributed sensors for state variable measurement is presented. Identification and control are successfully implemented on a multivariable cantilever plate system and experimental results are presented.

Lessons learned about wireless technologies for data acquistion

In recent years the electronics for developing sensor networks have become compact and cheaper. This has led to an interest in creating communities of distributed sensors that can collect and share data over a large area without being physically connected by wires. The Intelligent Systems Center at the University of Missouri-Rolla (UMR) has for several years been using commercial off-the-shelf (COTS) hardware and custom software to develop a system of stationary sensing nodes capable of pre processing their data locally and sharing processed data to produce global details. This distributed sensing and processing array is targeted for use in monitoring a wide variety of infrastructures. It has been laboratory tested for use in civil, automotive, and airframe monitoring. This paper is an overview of the technologies investigated and the level of functionality obtained from each hardware/sensor/target set. The current system consists of a web server, a central cluster and a collection of satellite clusters. The central cluster is a PC104 X 86 based computer with the satellite clusters being 8051 based single board computers. The satellite clusters are of the order 6 inch X 5 inch X 2 inch in size. There is an effort under way to place a short-range radio with a processor and a PZT sensor into a 2 inch X 1.5 inch X.5 inch package. Exercises have been carried out to demonstrate the ability of the central clusters to remotely control the satellite clusters and the web servers ability to control the central cluster. Further work is under way to integrate the entire system into a web server attached to the Internet and to a long distance communication device, currently employed is a cellular modem into the monitoring array. The web server communicates over standard phone lines to the central cluster, which is equipped with a cellular modem. The central cluster communicates with the satellite clusters using short-range wireless equipment. Proxim rangelan, Erickson Bluetooth, and Linx Technologies RF modules have all been tested as short-range wireless communication solutions. We have demonstrated a system that consists of a structure with an array of smart sensors, preprocess and collect data, and post this data on a web server for global inspection and manipulation. This will enable data sharing and collaborative data analysis to extend the knowledge of structural health monitoring.

Web-controlled Wireless Network Sensors for structural health monitoring

Wireless network sensors are being implemented for applications in transportation, manufacturing, security, and structural health monitoring. This paper describes an approach for data acquisition for damage detection in structures. The proposed Web-Controlled Wireless Network Sensors (WCWNS) is the integration of wireless network sensors and a web interface that allows easy remote access and operation from user-friendly HTML screens. The WCWNS is highly flexible in terms of functions and applications. Algorithms and tools for data analysis can be directly installed on and executed from the web server. This means WCWNS will have unlimited capabilities in performing data analysis. Data can be analyzed for damage detection either on site distributed amongst the intelligent sensors or off site either in the web server or at an end users location after downloading from the web server. This feature allows for a variety of health monitoring algorithms to be investigated by researchers of all backgrounds and abilities. In addition, both short-range and long-range communications devices handle data exchange and communications in WCWNS. The system can be setup to operate efficiently in any topological arrangement. Short-range communications devices facilitate fast and low-power local data transfer, while long-range communications devices support high quality long-distance data exchange. The proposed system is demonstrated on an experimental setup.

Microsensors for health monitoring of smart structures

Health monitoring of structural systems has gained a lot of interest in recent times. In this paper, we consider the wireless data acquisition for health monitoring of smart structures. Some of the work done towards development of micro sensors for wireless health monitoring of smart structures is presented. The concept of smart sensors is demonstrated with the help of commercially available micro controller and wireless Rx/Tx modules. Application of these smart sensors in health monitoring is also demonstrated on a laboratory set up. A subspace system identification method known as N4SID is used for getting the state space matrices of the nominal and the damaged systems. The concepts are demonstrated on simple test article. Finally, the future goals in the development of micro sensors are given.

Active control of smart structures with optimal actuator and sensor locations

Sensors and actuators used in active control of smart structures have to be located appropriately in order to ensure maximum control and measurement effectiveness. Many placement techniques are based on the structure itself and overlook the effects of the applied control law. The optimal locations determined from open-loop system can not guarantee the best performance of the closed-loop system because the performance is closely related with the design requirements and applied controller. In this paper, we presented a method of obtaining the optimal locations of actuators/sensors by combining the open-loop and closed-loop optimal criterions. First, for open-loop system, location indices of the controlled modes are calculated on the basis of modal controllability and observability. The controlled modes are weighted based on the controller design requirements. To reduce the spill-over effect of uncontrolled modes, the location index values of uncontrolled modes are added as penalty terms. Locations with high index values are chosen as candidate locations of actuator/sensor for the next determining step on the closed-loop system. Three control techniques, optimal H2, H(infinity ) norms and optimal pole-placement, are utilized for two different control objectives, disturbance rejection and damping property enhancement. Linear matrix inequality (LMI) techniques are utilized to formulate the control problems and synthesize the controllers. For each candidate location of actuator/sensor, a controller is designed and the obtained performance is taken as location index. By solving the location problem in two steps, we reduced the computational burden and ensured good control performance of the closed-loop system. The proposed method is tested on a clamped plate with piezoelectric actuators and sensors.