Alex Elvin

A self-powered mechanical strain energy sensor

With the growing use of sensors in various structural and mechanical systems, the powering and communication of these sensors will become a critical factor. Wireless communication electronics are becoming ubiquitous and with the decreasing electrical power requirements for these circuits it is now feasible to generate power from the conversion of mechanical energy into electrical energy. This paper focuses on the theoretical and experimental analysis of a simple mechanical strain energy sensor with wireless communication. A simple beam bending experiment is given to illustrate some of the characteristics of the self-powered strain energy sensor.

A General Equivalent Circuit Model for Piezoelectric Generators

This article presents an equivalent circuit model for a piezoelectric generator which can include any number of vibrational modes. First the electromechanical equations are formulated using an assumed mode (for example, the Rayleigh-Ritz method), the mechanical equations are then decoupled by the standard eigenvector approach. A set of single degree of freedom equations are thus produced. The electromechanical coupling terms are modeled in the equivalent circuit using an ideal transformer, or a set of current-and voltage-dependent sources. To validate the equivalent circuit model, the results show excellent agreement with published analytical solutions for the first three vibration modes of a cantilever unimorph generator. The main advantage of the new method is that it can be used to simulate any circuit topology, for which there is no analytical solution, using a standard electronic simulation program. To demonstrate this, the analysis and design of a more complicated diode bridge circuit is presented.

Feasibility of structural monitoring with vibration powered sensors

Wireless sensors and sensor networks are beginning to be used to monitor structures. In general, the longevity, and hence the efficacy, of these sensors are severely limited by their stored power. The ability to convert abundant ambient energy into electric power would eliminate the problem of drained electrical supply, and would allow indefinite monitoring. This paper focuses on vibration in civil engineering structures as a source of ambient energy; the key question is can sufficient energy be produced from vibrations? Earthquake, wind and traffic loads are used as realistic sources of vibration. The theoretical maximum energy levels that can be extracted from these dynamic loads are computed. The same dynamic loads are applied to a piezoelectric generator; the energy is measured experimentally and computed using a mathematical model. The collected energy levels are compared to the energy requirements of various electronic subsystems in a wireless sensor. For a 5 cm3 sensor node (the volume of a typical concrete stone), it is found that only extreme events such as earthquakes can provide sufficient energy to power wireless sensors consisting of modern electronic chips. The results show that the optimal generated electrical power increases approximately linearly with increasing sensor mass. With current technology, it would be possible to self-power a sensor node with a mass between 100 and 1000 g for a bridge under traffic load. Lowering the energy consumption of electronic components is an ongoing research effort. It is likely that, as electronics becomes more efficient in the future, it will be possible to power a wireless sensor node by harvesting vibrations from a volume generator smaller than 5 cm3.

A self-powered damage detection sensor

All existing methods of embedded damage-detecting sensors require an external power source and a means of transmitting the data to a central processor. This paper presents a novel self-powered strain sensor capable of transmitting data wirelessly to a remote receiver. This paper illustrates the performance of the sensor through the theoretical and experimental analysis of a simple damaged beam. The results show that a sensor powered through the conversion of mechanical to electrical energy is viable for detecting damage. The potential benefits of this sensor include ease of implementation during manufacture of the structure, and the use of an environmentally safe and renewable power source.

A Coupled Finite Element—Circuit Simulation Model for Analyzing Piezoelectric Energy Generators

A coupled finite element method (FEM) and circuit simulation approach for analyzing piezoelectric energy harvesters is presented. The advantage of the proposed method is that the mechanical analysis of the generator can be done using available FEM packages, while the circuit analysis can be performed using standard circuit simulation software (e.g., SPICE). The electromechanical coupling between the two physical domains is achieved by applying equivalent piezoelectric loads in the mechanical model, and equivalent electrical voltages in the electric model. This approach allows for the modeling of complex mechanical geometries and sophisticated, non-linear circuits. The solutions of two example problems are presented: (1) a beam generator with a resistive load, which is compared to an existing analytical solution, and (2) a plate generator with a non-linear diode bridge circuit. Though relatively easy to implement, the explicit solution technique presented in this article can be computationally expensive for complicated models with long simulation time-histories.

Vibrational Energy Harvesting From Human Gait

Driven by the necessity to provide energy to wearable computing devices, the conversion of human movement into useful electrical energy has become a topic of extensive study. This paper presents a framework of calculating the maximal energy conversion from a resonant vibrational harvester during human gait. Acceleration measurements from both recreational and elite athletes are used to estimate power output for various gait speeds. Significant power density was found to occur at the harmonics of the gait cadence with the maximum power density occurring at twice the gait frequency. Though relatively large output power can occur at the first and second harmonics of the gait cadence, the resulting generator displacements are too large for practical use. Constraining the generator displacement to a root-mean-square magnitude of 25 mm provides approximately 28 mW of power for a 30-g device at optimal generator tuning conditions. As expected, the maximum power output increases with increasing electromechanical coupling and decreases with increasing damping.

The Flutter Response of a Piezoelectrically Damped Cantilever Pipe

The investigation of passively damped piezoelectric structures within fluid flows is important for two reasons: (a) to increase the critical flutter speed and (b) conversely to generate electrical energy to power small scale electronic systems. In this article, a cantilever pipe with resistive piezoelectric damping is chosen as the model structure to demonstrate behavior over a range of fluid/structure mass ratios. The effects of piezoelectric coupling on the critical flutter velocity, capacitance, load resistance, and piezoelectric location are investigated. The modeling shows that depending on the piezoelectric parameters chosen and the attached electrical load resistance, the addition of a passive piezoelectric element can either increase or decrease stability of the system (i.e., the critical flutter speed of the cantilever pipe can be altered and hence controlled).

Piezo-powered floating gate injector for self-powered fatigue monitoring in biomechanical implants

In this paper we describe an implementation and modeling of a novel fatigue monitoring sensor based on integration of piezoelectric transduction with floating gate avalanche injection. The miniaturized sensor enables continuous battery-less monitoring and failure predictions of biomechanical implants. Measured results from a fabricated prototype in a 0.5mum CMOS process demonstrate excellent agreement with the theoretical model in computing cumulative statistics of electrical signals generated by the piezoelectric transducer. The power dissipation of the sensor is less than 1muW which makes it attractive for integration with biocompatible poly-vinylidene diflouride (PVDF) based transducers.

Large deflection effects in flexible energy harvesters

The effect of large deflection on the mechanical and electrical behaviors of flexible piezoelectric energy harvesters has not been well studied. A generalized nonlinear coupled finite element circuit simulation approach is presented in this article to study the performance of energy harvesters subjected to large deflections. The method presented is validated experimentally using three test examples consisting of (a) a static case, (b) a free vibration case, and (c) a forced vibration case. Under static conditions (when the transverse tip deflection exceeds a quarter of the cantilever length), large deflections cause geometric stiffening of the structure that reduces the tip deflection of the generator when compared to linear (i.e. small-deflection) behavior. For a cantilever generator under dynamic conditions, geometric stiffening, inertial softening, and nonlinear damping effects become significant. Large deflections both shift the resonant frequency and increase damping and can thus cause a significant reduction in output voltage when compared with small-deflection linear theory. In the finite element generator model studied in this article, these nonlinear dynamic effects become significant when the transverse tip deflection exceeds 35% of the beam length.

Optimizing Structures Subject to Multiple Deflection Constraints and Load Cases Using the Principle of Virtual Work

This paper presents an iterative automated method for optimizing structures with multiple deflection criteria and load cases. The method is based on the principle of virtual work. Discrete sections are selected for structures with fixed geometries. An optimal structure is one which meets all strength and deflection criteria using minimal material. Four case studies are considered in this paper. A simple portal frame is presented to show how the method works. A 60-story frame is optimized to demonstrate the effectiveness of the method for large structures. A warehouse designed by professional engineers is presented to show how the method can be used for structures subjected to complex loading conditions and deflection criteria. The automated method’s solution is 4.5% lighter than the engineers’. Finally, a stepped cantilever is optimized and compared to results in literature. Material savings of up to 14.4% are realized.