Kevin M. Farinholt

Energy harvesting from a backpack instrumented with piezoelectric shoulder straps

Over the past few decades the use of portable and wearable electronics has grown steadily. These devices are becoming increasingly more powerful. However, the gains that have been made in the device performance have resulted in the need for significantly higher power to operate the electronics. This issue has been further complicated due to the stagnant growth of battery technology over the past decade. In order to increase the life of these electronics, researchers have begun investigating methods of generating energy from ambient sources such that the life of the electronics can be prolonged. Recent developments in the field have led to the design of a number of mechanisms that can be used to generate electrical energy, from a variety of sources including thermal, solar, strain, inertia, etc. Many of these energy sources are available for use with humans, but their use must be carefully considered such that parasitic effects that could disrupt the users gait or endurance are avoided. These issues have arisen from previous attempts to integrate power harvesting mechanisms into a shoe such that the energy released during a heal strike could be harvested. This study develops a novel energy harvesting backpack that can generate electrical energy from the differential forces between the wearer and the pack. The goal of this system is to make the energy harvesting device transparent to the wearer such that his or her endurance and dexterity is not compromised. This will be accomplished by replacing the traditional strap of the backpack with one made of the piezoelectric polymer polyvinylidene fluoride (PVDF). Piezoelectric materials have a structure such that an applied electrical potential results in a mechanical strain. Conversely, an applied stress results in the generation of an electrical charge, which makes the material useful for power harvesting applications. PVDF is highly flexible and has a high strength, allowing it to effectively act as the load bearing member. In order to preserve the performance of the backpack and user, the design of the pack will be held as close to existing systems as possible. This paper develops a theoretical model of the piezoelectric strap and uses experimental testing to identify its performance in this application.

A mobile-agent-based wireless sensing network for structural monitoring applications

A new wireless sensing network paradigm is presented for structural monitoring applications. In this approach, both power and data interrogation commands are conveyed via a mobile agent that is sent to sensor nodes to perform intended interrogations, which can alleviate several limitations of the traditional sensing networks. Furthermore, the mobile agent provides computational power to make near real-time assessments on the structural conditions. This paper will discuss such prototype systems, which are used to interrogate impedance-based sensors for structural health monitoring applications. Our wireless sensor node is specifically designed to accept various energy sources, including wireless energy transmission, and to be wirelessly triggered on an as-needed basis by the mobile agent or other sensor nodes. The capabilities of this proposed sensing network paradigm are demonstrated in the laboratory and the field.

RF Energy Transmission for a Low-Power Wireless Impedance Sensor Node

The proper management of energy resources is essential for any wireless sensing system. With applications that span industrial, civil, and aerospace infrastructure, it is necessary for sensors and sensor nodes to be physically robust and power efficient. In many applications, a sensor network must operate in locations that are difficult to access, and often these systems have a desired operational lifespan which exceeds that of conventional battery technologies. In the present study, the use of microwave energy is examined as an alternate method for powering compact, deployable wireless sensor nodes. A prototype microstrip patch antenna has been designed to operate in the 2.4 GHz ISM band and is used to collect directed radio frequency (RF) energy to power a wireless impedance device that provides active sensing capabilities for structural health monitoring applications. The system has been demonstrated in the laboratory, and was deployed in field experiments on the Alamosa Canyon Bridge in New Mexico in August 2007. The transmitted power was limited to 1 W in field tests, and was able to charge the sensor node to 3.6 V in 27 s. This power level was sufficient to measure two piezoelectric sensors and transmit data back to a base station on the bridge.

Development of an Extremely Compact Impedance-Based Wireless Sensing Device

This paper describes the development of the next generation of an extremely compact, wireless impedance sensor node for use in structural health monitoring (SHM) and piezoelectric active-sensor self-diagnostics. The sensor node uses a recently developed, low-cost integrated circuit that can measure and record the electrical impedance of a piezoelectric transducer. The sensor node also integrates several components, including a microcontroller for local computing, telemetry for wirelessly transmitting data, multiplexers for managing up to seven piezoelectric transducers per node, energy harvesting and storage mediums, and a wireless triggering circuit into one package to truly realize a comprehensive, self-contained wireless active-sensor node for various SHM applications. It is estimated that the developed sensor node requires less than 60 mW of total power for measurement, computation, and transmission. In addition, the sensor node is equipped with active-sensor self-diagnostic capabilities that can monitor the condition of piezoelectric transducers used in SHM applications. The performance of this miniaturized device is compared to our previous results and its broader capabilities are demonstrated.

An Energy Harvesting Comparison of Piezoelectric and Ionically Conductive Polymers

With advances in wireless communications and low power electronics there is an ever increasing need for efficient self-contained power systems. Traditional batteries are often selected for this purpose; however, there are limitations due to finite life-spans and the need to periodically recharge or replace the spent power source. One method to address this issue is the inclusion of an energy harvesting strategy that can scavenge energy from the surrounding environment and convert it into usable electrical energy. Since civil, industrial, and aerospace applications are often plagued with an overabundance of ambient vibrations, electromechanical transducers are often considered a viable choice for energy scavengers. In this study, two classes of transducer are considered: the piezoelectric polymer polyvinylidene fluoride and the ionically conductive ionic polymer transducer. Analytical models are formed for each material assuming axial loading and simulation results are compared with experimental results for each test. Each material is then compared to examine the effectiveness of their mechanoelectric conversion properties.

Piezoelectric Active-Sensor Diagnostics and Validation Using Instantaneous Baseline Data

This paper presents a signal processing tool that efficiently performs piezoelectric (PZT) sensor diagnostic and validation. Validation of the sensor/actuator functionality during structural health monitoring (SHM) operation is a critical component to successfully implement a complete and robust SHM system, especially with an array of PZT active-sensors involved. The basis of this method is to track the capacitive value of PZT transducers, which manifests in the imaginary part of the measured electrical admittance. Both degradation of the mechanical/electrical properties of a PZT transducer and the bonding defects between a PZT patch and a host structure can be identified by the proposed process. However, it is found that the temperature variations in sensor boundary conditions manifest themselves in similar ways in the measured electrical admittances. Therefore, we examine the effects of temperature variation on the sensor diagnostic process and develop an efficient signal processing tool that enables the identification of a sensor validation feature that can be obtained instantaneously without relying on prestored baselines. This paper concludes with experimental results to demonstrate the effectiveness of the proposed technique.

Structural Health Monitoring With Autoregressive Support Vector Machines

The use of statistical methods for anomaly detection has become of interest to researchers in many subject areas. Structural health monitoring in particular has benefited from the versatility of statistical damage-detection techniques. We propose modeling structural vibration sensor output data using nonlinear time-series models. We demonstrate the improved performance of these models over currently used linear models. Whereas existing methods typically use a single sensors output for damage detection, we create a combined sensor analysis to maximize the efficiency of damage detection. From this combined analysis we may also identify the individual sensors that are most influenced by structural damage.

Energy Harvesting and Wireless Energy Transmission for Embedded SHM Sensor Nodes

In this article, we present experimental investigations using energy harvesting and wireless energy transmission to power wireless structural health monitoring sensor nodes. The goal of this study is to develop sensing systems that can be permanently embedded within a host structure without the need for an on-board power source. With this approach the required energy will be harvested from the ambient environment, or periodically delivered by a radio-frequency energy source to supplement conventional harvesting approaches. This approach combines several transducer types to harvest energy from multiple sources, providing a more robust solution that does not rely on a single energy source. Both piezoelectric and thermoelectric transducers are considered as energy harvesters to extract the ambient energy commonly available on civil structures such as bridges. Methods of increasing the efficiency, energy storage medium, target applications and the integrated use of energy harvesting sources with wireless energy transmission will be presented.

A low-power wireless sensing device for remote inspection of bolted joints

Abstract A new bolted-joint monitoring system is presented. This system consists of structural joint members equipped with piezoelectric (PZT) sensing elements and a wireless impedance device for data acquisition and communication. PZT enhanced washers are used to continuously monitor the condition of the joint by monitoring its dynamic characteristics. The mechanical impedance matching between the PZT enhanced devices and the joint connections is used as a key feature to monitor the preload changes and to prevent further failure. The dynamic response is readily measured using the electromechanical coupling property of the PZT patch, in which its electrical impedance is directly coupled with the mechanical impedance of the structure. A new miniaturized and portable impedance measuring device is implemented for the practical implementation of the proposed method. The proposed system can be used for the remote and rapid inspection of bolt tension and connection damage. Both theoretical modelling and experimental verification are presented to demonstrate the effectiveness of the proposed concept.

Modeling the electrical impedance response of ionic polymer transducers

An analytical study is presented that investigates the electrical impedance response of the ionic polymer transducer. Experimental studies have shown that the electromechanical response of these active materials is highly dependent upon internal parameters such as neutralizing counterion, diluent, electrode treatment, as well as environmental factors such as ambient temperature. Further examination has shown that these variations are introduced predominantly through the polymer’s ability to convert voltage into charge migration. This relationship can easily be represented by the polymer’s electrical impedance as measured across the outer electrodes of the transducer. In the first half of this study an analytical model is developed which predicts the time and frequency domain characteristics of the electrical response of the ionic polymer transducer. Transport equations serve as the basis for this model, from which a series of relationships are developed to describe internal potential, internal charge dens...