Multifunctional Structures for Attitude Control (MSAC) is a new spacecraft attitude control system that utilizes deployable panels as multifunctional intelligent structures to provide both fine pointing and large slew attitude control. Previous studies introduced MSAC design and operation concepts, simulation-based design studies, and Hardware-in-the-Loop (HIL) validation of a simplified prototype. In this article, we expand the scope of design studies to include individual compliant piezo-electric actuators and associated power electronics. This advance is a step toward high-fidelity MSAC system operation, and reveals new design insights for further performance enhancement. Actuators are designed using pseudo rigid body dynamic models (PRBDMs), and are validated for steady-state and step responses against Finite Element Analysis. The drive electronics model consists of a few distinct topologies that will be used to evaluate system performance for given mechanical and control system designs. Subsequently, a high-fidelity multiphysics multibody MSAC system model, based on the validated compliant actuators and drive electronics, is developed to support implementation of MSAC Control Co-design optimization studies. This model will be used to demonstrate the impact of including the power electronics design in the Optimal Control Co-Design domain. The different control trajectories are compared for slew rates and the vibrational jitter introduced to the satellite. The results from this work will be used to realize closed-loop control trajectories that have minimal jitter introduction while providing high slew rates.

Wearable motion sensors find a great number of applications in the biomedical field by recording real-time movements and transferring data to mobile electronics. Patients with hyperkinetic movements is a group of interest for such sensors to survey their conditions for long periods. Longer and more frequent recording intervals are necessary to diagnose and treat patients’ disease. Mobile battery-operated motion sensors have a limited recording span, and they need to be charged frequently, which is inconvenient for most of the patients. In this study, vibration energy harvesters are employed to extend the battery life of motion sensors: one step closer to make autonomous sensors without chargers. A vibration energy harvester is designed for a motion sensor to harvest energy from involuntary movements of patients with hyperkinetic movements. An analytical model for charging and discharging cycles is developed to predict the battery life based on the amount of harvested power. Preliminary data from commercial devices are used as a foundation for the design and the current feasibility study.

This paper presents the design and the realization of an innovative SMA actuated bistable vacuum suction cup. The sealed, compact and fully integrated design enables the positioning and transport of inherent stable components in mobile and stationary applications. The bistable actuator mechanism based on SMA wires combined with a bistable spring represent an energy-efficient, noiseless gripping system without the need for compressed air. Additionally, the self-sensing effect of the SMA enables a sensorless condition-monitoring and energy-efficient control. The mechanics consists of antagonistic SMA wires, which are laterally arranged and connected to the bistable spring via levers. The membrane is directly connected to the bistable spring. The actuation of the wires leads to a rotational movement of the levers thus changes the state of the bistable spring, which directly deforms the membrane. When the membrane is sealed connected to the workpiece, the deformation of the membrane generates a vacuum. The integrated microcontroller electronics manages the joule heating of the wires by measuring the transmitted electrical energy. By applying an electrical energy to the pre-strained SMA wire, the wire heats up and contracts due to the phase transformation from martensite to austenite. The contraction of the wire is accompanied by a significant change in electrical resistance, which enables a resistance based strain feedback. The integrated electronics is able to correlate this resistance change to the actual state of the bistable spring, which leads to a position feedback of the membrane. This allows an adequate electrical energy deposition in the SMA wire by turning-off the heating directly after the position toggle of the membrane. Thereby, a successful position toggle is ensured independent from the ambient temperature and the real supply voltage. The new position of the membrane is then held by the bistable spring without the use of additional energy. This concept leads to a reliable gripping system with fast actuation times.

An electroactive polymer is a material capable of changing its size and shape when an electric field is present. It is composed of a thin film of dielectric elastomer and two electrodes placed on the top and bottom of the dielectric material. Since the rediscovery of their capabilities, electroactive polymers have been proposed as novel materials for use in numerous fields such as in bioengineering, electronics, hydraulics, and aerospace. It has been demonstrated that the actuation potential of electroactive polymer dielastomers can be significantly enhanced by mechanically pre-straining the material prior to application of an electric field. Application of uniform pre-strain is critical for uniform actuation and is challenging to achieve. This research details methods for constructing an automated uniform stretcher that resulted in the production of a LabView controlled iris stretcher for flexible materials. The high torque stretcher was capable of pre-straining materials with a minimum diameter of 1 inch mm) to a maximum diameter of 16 inches. The stretcher calculates the percent strain and has adjustable speed control through a high torque micro-stepper motor and controller. The stretcher’s capabilities were demonstrated on materials within varying tensile strengths up to 725 psi.

Liquid metal (LM) alloys such as eutectic gallium indium (EGaIn) and gallium-indium-tin (Galinstan) have been used in the fabrication of soft and stretchable electronics during the past several years. The liquid-phase and high electrical conductivity of these materials make them one of the best candidates for fabrication of deformable electronics and multifunctional material systems. While liquid metals are highly reliable for fabrication of simple circuits and stretchable microfluidic devices, their application for producing complex circuits faces fabrication challenges due to their high surface tension and surface oxidization. In this study, we propose a scalable, cost-effective, and versatile technique to print complex circuits using silver nanoparticles and transform them into stretchable electronics by incorporating eutectic gallium indium alloys to the circuit. As a result, the deposited liquid metal considerably increases the electrical conductivity and stretchability of the fabricated electronics. The reliability and performance of these stretchable conductors are demonstrated by studying their electromechanical behavior and integrating them into skin-like electronics, termed electronic tattoos.

Printing technologies are attractive methods for high-throughput additive manufacturing of nanomaterials-based thin film electronics. Roll-to-roll (R2R) compatible techniques such as gravure printing can operate at high-speed (1–10 m/s) and high-resolution (< 10 μm) to drive down manufacturing costs and produce higher quality flexible electronic devices. However, large-scale deployment of printed wireless sensors, flexible displays, and wearable electronics, will require greater understanding of the printing physics of nanomaterial-based inks in order to improve the resolution, reliability, and uniformity of printed systems. In this study, we designed and constructed a custom sheet-fed gravure printer which features registered multilayer printing for nanomaterial exploration and thin film device development. The design allows precise, independent control of the speeds and forces of each of the subprocesses of gravure (ink filling, wiping, and transfer), enabling novel experimental controls for dissecting the printing process fluid mechanics. We use these new capabilities to investigate the primary artifacts which distort printed nanomaterial patterns, such as dragout tails, edge roughness, and pinholes. These artifacts are studied as a function of print parameters such as contact pressure, wiping speed, and transfer speed, by printing silver nanoparticle ink to form continuous features with dimensions in the range of 100 μm to 10 mm. We found that the contact mechanics of the ink transfer process have a strong influence on the formation of dragout artifacts, indicating the presence of a transfer-driven squeezing flow which distorts the trailing edges of features. By engineering the transfer contact mechanics with varying rubber substrate backing stiffness, we found it is also possible to suppress this artifact formation for a particular nanomaterial ink. The improved areal uniformity and print quality achieved using these methods highlight the potential for gravure printing to be a versatile nano-manufacturing tool for patterning a variety of thin film smart materials. We also hope that the open-source printer designs presented here can serve to accelerate the development of high-speed nanomaterial printing.

Total knee replacement has been utilized to restore the functionality of diseased knee joints for more than four decades. Today, despite the relatively high level of patient satisfaction, still about 20% of patients are not fulfilled with their surgical outcomes in terms of function and reduction in pain. There is still an ongoing discussion on correlating the postoperative functionality of the joint to intraoperative alignment, which suffers from lack of in vivo data from the knee after surgery. However, it is necessary to mention that using computer assisted surgical techniques, the outcomes of knee replacement procedures have been remarkably improved. In order to obtain information about the knee function after the operation, the design of a self-powered instrumented knee implant is proposed in this study. The design is a total knee replacement ultra high molecular weight polyethylene insert equipped with four piezoelectric transducers distributed in the medial and lateral compartments of the bearing. The piezoelectric elements are employed to measure the axial force applied on the tibial insert through the femoral component of the joint as well as to track the movement in the center of pressure. In addition, generated voltage from the piezoelectrics is harvested and stored to power embedded electronics for further signal conditioning and data transmitting purposes. The performance of the instrumented implant is investigated via experimental testing on a fabricated prototype in terms of sensing and power harvesting capacity. Piezoelectric force and center of pressure measurements are compared to the actual quantities recorded from the load frame and pressure sensitive films in order to evaluate the performance of the sensing system. The output voltage of the piezoelectric transducers is rectified and stored in a capacitor to evaluate the energy harvesting ability of the system. The results show only a small level of error in sensing the force and the location of center of pressure. Additionally, a 4.9 V constant voltage is stored in a 3.3 mF capacitor after 3333 loading cycles. The sensing and energy harvesting results present the promising potential of this system to be used as an integrated self-powered instrumented knee implant.

This research introduces an integrated vibration energy harvester and electrochemical energy storage device that can effectively convert ambient vibrations directly into stored electrochemical energy. The electrochemical energy storage device is an electrical double layer capacitor (EDLC) with an ionic redox transistor as its membrane separator. This ‘ smart ’ membrane separator directly rectifies the electrical energy generated by the transduction from the nonlinear energy harvester, creating an ionic polarization across the membrane separator for storage. This electrochemical gradient can be subsequently used for powering sensor electronics as required in various applications, including structural condition monitoring. The alternating voltage developed by the energy harvester (+/−5V around 100 Hz) is connected to an aqueous supercapacitor fabricated from nanofibrous carbon paper electrodes and a polypyrrole-based (PPy(DBS)) smart membrane separator. A potential below −400mV from the energy harvester applied to the supercapacitor turns the smart membrane separator ‘ON’ and results in a unidirectional ionic current of Li+ ions. As the potential developed by the harvester cycles above ∼50 mV, the membrane separator switches ‘OFF’ and prevents the discharge of the rectified current. This leads to a continuous polarization of ions towards electrical fields relevant for powering electronics. This article is the first description and demonstration of an energy harvesting and storage system that can directly convert the electrical energy from a vibration energy harvester into electrochemical energy without the use of passive circuit components for power rectification.

In this study, a smart heat pump, which could be used for the cooling of electronics, made of the laminated structure of thermoelectric (TE) and electrocaloric (EC) materials is studied. A simple arrangement of two TE layers sandwiched with one EC layer is modeled. This smart heat pump utilized the newly developed EC materials of giant adiabatic temperature change and the TE materials of high figure of merit, which has the advantages of no moving parts, made of solid state, operable over large working temperature difference, and can be formed into very small size. The operation of the device is numerically modelled considering the three major parametric effects: EC operation as function of time, electric current applied on TE, and temperature difference between the hot and cold sinks. The results on Coefficient of Performance (COP) and heat flow per unit area are discussed. This study validates the feasibility of TE-EC-TE laminated structure heat pump, and extends the understanding by further discussing the performance of structures with more laminated layers.

This paper presents a systematic design of a piezoelectric stack energy harvester from human walking locomotion. The proposed footstep energy harvester is a mobile energy harvesting device that comprises of four sets of piezo-electric stack with force amplification frame assembly with associated power electronics. The objective of this work is to optimize the output power from each piezo-electric stack for which a high-efficiency force amplification frame was developed. Considering the nature of the application, High-Strength A514 Alloy Steel was chosen as the frame material and SONOX SP 505 as the piezo-electric stack in d 33 configuration. The mathematical formulation of real-time human walking force excitation was also vital in the study. In this paper, a real-time equation of human Vertical Ground Reaction Forces (VGRF) was used for the systematic modeling and simulation process. Following the success of piezoelectric electro-mechanical modeling and simulation, a prototype of four sets of force-amplification frames each with a piezoelectric stack installed inside were fabricated and assembled into a unique constrainer box — such an assembled device was fit into the heel of a 12″ Field and Stream ® boot to effectively convert kinetic energy from walking locomotion to electricity and therefore, to power a wireless sensor. The uniqueness of the work is to develop an easy-fit footstep energy harvester with much higher power density than similar design in the literature. In particular, the developed energy harvesting device is not visible externally and does not affect the walking gait pattern of the user. Moreover, our design only adds 0.25 kg to the self-weight of 0.85 kg of the boot. A peak power of 130 mW and peak Voltage of 118 V was recorded for an 80 kg person walking. This type of energy harvester will find its application in clean-energy generation in remote areas without electricity access.

Transducers based on dielectric electroactive polymers (DEAP) offer an attractive balance of work density and electromechanical efficiency. For example in automation and haptic applications, especially multilayer transducers are used to scale up their absolute deformation and force. Depending on the application different transducer controls have to be realized to match the specifications of the particular application. However, analogous to conventional electromechanical drive systems an inner sensor-less force control can be realized for DEAP transducers, too. For this force control the nonlinear relations between voltage and electrostatic pressure as well as the electromechanical coupling have to be considered. The resulting open-loop force control can be used for superimposed motion controls, such as position, vibration and impedance controls. Therefore, within this contribution the authors propose a model-based feedforward force control based on an overall model of the transducer that does not require any force measurement. Finally, the derived open-loop force control interface is experimentally validated using in-house developed DEAP stack-transducers and driving power electronics.

Mechanoluminescence (ML) is a property of inorganic and organic materials that describes the emission of light due from the application of force. Inorganic crystals (mostly phosphors) and certain organic macromolecules exhibit elastico-ML and are a natural fit for structural health monitoring (SHM) of composite structures. Composites with particulate ML crystals enable the visualization of stress distribution over a plane and over contoured surfaces in a spatially continuous manner. Imaging ML composites with affordable high-resolution imaging methods further enables the creation of high-resolution validation method for computational methods. Also, with the embedding of suitable photo-detectors for signal detection, the need for additional wiring, sensor electronics and high-level electronics is eliminated. In this conference proceedings technical publication, the application of commercially available ZnS:Cu, Mn phosphors for SHM of polymer composites will be presented via experimental and structural simulation. Results demonstrate the dependence of intensity of elastico-ML (in cd/cm 2 ) on strain rate, strain and composition (w/w of ML particulates). The experiments show methods to fabricate elastic coupons of phosphors in polydimethylsiloxane (PDMS) and subsequent methods for application in SHM. The structures are excited at 5Hz to 17.5Hz to develop empirical relationships between strain rate and EML intensity and it is shown that the intensity increases nonlinearly with the magnitude of stress/strain rate. A range of stresses transferred to the EML particles by the PDMS matrix is also numerically predicted. The numerical simulations show the importance of interfacial binding in the transfer of stress and subsequent EML emission. These results also provided a basis for validation and improvement of structural simulation models.

Structural health monitoring can enhance reliability, increase safety, and decrease maintenance costs by detecting damage at an early stage. By taking advantage of the electromechanical coupling, piezoelectric materials have the potential to harvest energy from ambient vibration sources to provide low-power electricity for self-powered electronic devices. In comparison with other piezoelectric transducers, zinc oxide (ZnO) nanowires carry the added advantages of structural flexibility, lower cost, compactness, and lighter weight. In this study, the energy harvesting capabilities of nanoscale ZnO piezoelectric nanowires (NW) grown on the surface of glass fiber fabrics are investigated experimentally. A series of cantilevered carbon fiber beams containing a controlled amount of ZnO nanowires is evaluated. The absolute electrical energy dissipation is quantified by measuring the output power over a broad spectrum of known vibratory loads and frequencies. The maximum amount of power extracted is obtained by employing resistive impedance matching. Here, a maximum peak of ∼6.7 mV was generated when the beam containing ZnO nanowires was excited at 2.90g and connected to a 10 MΩ load. At that excitation level, a maximum of 20.0 pW was generated when an optimal resistor of 1 MΩ is connected. A tip mass of ∼0.6 gram added to the sample with ZnO NWs increased the peak-voltage by 2.21 mV and increased the peak-power by 13.3 pW. A series of DC voltage applied to the ZnO sample suggests the equivalence of poling treatment, where the dipole alignment of the ZnO NWs are disrupted. Here, a maximum peak-power of 45 pW is reported, showing promising potential of scaling-up to harvest ambient energy for low-powered electronics.

In applications of vibration energy harvesting to embedded wireless sensing, the available power and energy can be very low. This poses interesting challenges for technological feasibility if the parasitic losses in the electronics used to harvest this energy are prohibitive. In this study, we present a theory for the active control of power generation in energy harvesters in a manner which addresses and compensates for parasitic loss. We conduct the analysis in the context of a single-transducer piezoelectric bimorph cantilever beam subjected to a low-frequency impulse train. The power generation of the vibration energy harvester is maximized while considering mechanical losses, electrical losses, and the static power required to activate control intelligence and facilitate power-electronic conversion. It is shown that the optimal harvesting current can be determined through the use of linear quadratic optimal control techniques. The optimal harvesting time over which energy should be generated, following an impulse, is determined concurrently with the optimal feedback law. We show that this optimal harvesting time exhibits bifurcations as a function of the parameters characterizing the losses in the system.

Harvesting energy from high-shock pulse events such as those experienced during high velocity impact and firing setback acceleration in munitions using piezoelectric-based harvesters and converting it to usable electrical energy is not new. However, efficient collection of the generated charges and its transfer to an electrical storage device such as a capacitor for pulse events with very short duration, such as those that last a small fraction of a millisecond such as 50–100 microseconds is challenging. Another challenge for such piezoelectric-based harvesters is their hardening to survive high levels of shock loading, sometimes in excess of 100–200,000 Gs. In this paper, the basic design and operation of two such piezoelectric-based energy harvesters and their charge collection and storage electronics are described. The novel methods described are shown to achieve this goal with very high efficiency. The paper also describes the development of test-beds to simulate electrical charge generation of the energy harvesting power sources during high velocity impact and munitions firing for use in the design and evaluation of the developed charge collection and storage electronics.

Piezoelectric nanowires (NWs) have recently attracted immense interest due to their excellent electro-mechanical coupling behavior that can efficiently enable conversion of low-intensity mechanical vibrations for powering or augmenting batteries of biomedical devices and portable consumer electronics. Specifically, nano-electromechanical systems (NEMS) composed of piezoelectric NWs offer an exciting potential for energy harvesting applications due to their enhanced flexibility, light weight, and compact size. Compared to the bulk form, high aspect ratio NWs can exhibit higher deformation to produce an enhanced piezoelectric response at a lower stress level. NEMS made of conventional semiconducting vertically aligned, ZnO NW arrays have been investigated thoroughly for energy harvesting; however, ZnO has a lower piezoelectric coupling coefficient as compared to many ferroelectric ceramics which limits its piezoelectric performance. Amidst lead-free ferroelectric materials, environmentally-friendly barium titanate (BaTiO 3 ) possesses one of the highest piezoelectric strain coefficients and thus can enable greater energy transfer when used in vibrational energy harvesters. In this paper, a novel NEMS energy harvester is fabricated using ultra-long (∼40 μm long), vertically aligned BaTiO 3 NW arrays which has a low resonant frequency (below 200 Hz) and its AC power harvesting capacity from low amplitude base vibrations (0.25 g) is demonstrated. The design and fabrication of low resonant frequency vibrational energy harvesters has been challenging in the field of MEMS/NEMS since the high stiffness of the structures results in resonant frequency often greater than 1 kHz. However, ambient mechanical vibrations usually exist in the 1 Hz to 1 kHz range and thus highly complaint ultra-long, NW arrays are beneficial to enable efficient energy conversion. Through the use of this newly developed synthesis process for the growth of highly compliant, ultra-long BaTiO 3 NW arrays, it is shown that piezoelectric NWs based NEMS energy harvesters capable of harnessing this low frequency ambient vibrational energy can be conceived.

Shape Memory Alloys (SMAs) are active metallic materials classified as “smart” or “intelligent” materials along with piezoelectric ceramic and polymers, electro-active plastics, electro-rheological and magneto-rheological fluids and others. SMAs show a multitude of different and dependent properties interesting for technological applications. These properties depend on the peculiar deformation mechanisms, accounting for the so-called shape memory effect. SMAs are nowadays used in quite different fields, like thermo-mechanical devices, anti-loosening systems, biomedical applications, mechanical damping systems, in some cases employed for large scale civil engineering structures. These multifunctional materials can be naturally considered as sensor-actuator elements demonstrating large possibilities for applications in high-tech smart systems. The use of SMAs in actuators offers an excellent technological opportunity to develop reliable, robust, simple and lightweight elements within structures or as stand-alone components that can represent an alternative to electro-magnetic actuators commonly used in several fields of industrial applications, such as automotive, appliances, consumer electronics and aerospace. NiTi-based SMAs demonstrated to have the best combination of properties, especially in terms of the amount of work output per material volume and the large amount of recoverable stress and strain. However, there are several limiting factors to a widespread diffusion of SMAs to technological fields. For instance, SMAs display a critical dependence of the shape-memory related properties, like transition temperatures, on their actual composition. For this reason, a great care in the production steps, mainly based on casting processes, is required. Another critical aspect, that is to be considered when dealing with SMAs, is the strong influence of their thermo-mechanical history on their properties. This may disclose interesting perspectives of application to smart devices in which different aspects of the shape memory phenomenology, like one and two way shape memory effect, pseudoelasticity, damping capacity, etc., are used. Last, but not least, one of the most debated aspects around NiTi alloys is microcleanliness. This concept is becoming increasingly important as the industrial market moves to smaller, lower profile devices with thinner structures. In this work a general overview about the peculiar behavior of NiTi alloys along with their main issues, the shape memory components under development, and the main efforts and directions for materials improvement will be presented and discussed. A bird’s-eye view on the future opportunities of NiTi-based shape memory actuators for industrial applications will also be given.

The development of self-powered wireless microelectromechanical (MEMS) sensors hinges on the ability to harvest adequate energy from the environment. When solar energy is not available, mechanical energy from ambient vibrations, which are typically low frequency, is of particular interest. Here, higher power levels were approached by better coupling mechanical energy into the harvester, using improved piezoelectric layers, and efficiently extracting energy through the use of low voltage rectifiers. Most of the available research on piezoelectric energy harvesters reports Pb(Zr,Ti)O 3 (PZT) or AlN thin films on Si substrates, which are well-utilized for microfabrication. However, to be highly reliable under large vibrations and impacts, flexible passive layers such as metal foil with high fracture strength would be more desirable than brittle Si substrates for MEMS energy harvesting. In addition, metallic substrates readily enable tuning the resonant frequency down by adding proof masses. In order to extract the maximum power from such a device, a high level of (001) film orientation enables an increase in the energy harvesting figures of merit due to the coupling of strong piezoelectricity and low dielectric permittivity. Strongly {001} oriented PZT could be deposited by chemical solution deposition or RF magnetron sputtering and ex situ annealing on (100) oriented LaNiO 3 / HfO 2 / Ni foils. The comparatively high thermal expansion coefficient of the Ni facilitates development of a strong out-of-plane polarization. 31 mode cantilever beam energy harvesters were fabricated using strongly {001} textured 1∼3 μm thick PZT films on Ni foils with dielectric permittivity of ∼ 350 and low loss tangent (<2%) at 100 Hz. The resonance frequency of the cantilevers (50∼75 Hz) was tuned by changing the beam size and proof mass. A cantilever beam with 3 μm thickness of PZT film and 0.4 g proof mass exhibited a maximum output power of 64.5 μW under 1 g acceleration vibration with a 100 kΩ load resistance after poling at 50 V (E C ∼ 16 V) for 10 min at room temperature. Under 0.3g acceleration, the average power of the device is 9 μW at a resonance frequency of ∼70 Hz. Excellent agreement between the measured and modeled data was obtained using a linear analytical model for an energy harvesting system, using an Euler-Bernoulli beam model. It was also demonstrated that up to an order of magnitude more power could be harvested by more efficiently utilizing the available strain using a parabolic mode shape for the vibrating structure. Additionally, voltage rectifying electronics in the form of ZnO thin film transistors are deposited directly on the cantilever. This relieves the role of voltage rectification from the interfacing circuitry and provides a technique improved harvesting relative to solid state diode rectification because the turn-on bias can be reduced to zero.

A new class of self-powered acceleration event detection sensors are presented that are powered by electrical energy harvested during munitions launch by integrated piezoelectric elements. The sensors are provided with a novel safety electronic and logic circuitry that is used to differentiate the firing event from all accidental events such as accidental drops, transportation vibration, and the like. When the launch conditions are detected from the magnitude of the experienced acceleration as well as its duration, the remaining electronics and logics circuitry of the device is enabled. The developed self-powered sensors may also be used in place of G-switches in munitions and other industrial and commercial devices with the advantage of activating not only from the magnitude of the experienced acceleration but also from its duration. The latter capability is essential in many munitions and commercial applications to avoid false switching event. For example in some cases dropping of around over a hard surface may impart higher peak acceleration than actual firing. And in many industrial and commercial devices and equipment, high-G and very short duration shock loadings do not cause damage and G-switches used to deactivate the device may not be desired to trip. Prototypes of the developed piezoelectric-based self-powered event detection sensors as standalone sensors and as switches for detecting and opening or closing circuitry upon detection of shock or vibration loading with prescribed magnitude and duration thresholds with integrated electronics and logics circuitry have been designed, fabricated and successfully tested for a number of munitions and industrial applications. In this paper the design and operation of such devices and their testing are described.

Vibrational energy harvesting has become relevant as a power source for the reduced power requirement of electronics used in wireless sensor networks (WSNs). Vibrational energy harvesters (VEHs) are devices that can convert ambient kinetic energy into electrical energy using three principal transduction mechanisms: piezoelectric, electromagnetic and electrostatic. In this paper, a macroscopic two degree-of-freedom (2Dof) nonlinear energy harvester, which employs velocity amplification to enhance the power scavenged from ambient vibrations, is presented. Velocity amplification is achieved through sequential collisions between free-moving masses, and the final velocity is proportional to the mass ratio and the number of masses. Electromagnetic induction is chosen as the transduction mechanism because it can be readily implemented in a device which uses velocity amplification. The experimental results are presented in Part A of this paper, while in Part B three theoretical models are presented: (1) a coupled model where the two masses of the non-linear oscillator are considered as a coupled harmonic oscillators system; (2) an uncoupled model where the two masses are not linked and collisions between masses can occur; (3) a model that considers both the previous cases. The first two models act as necessary building blocks for the accurate development of the third model. This final model is essential for a better understanding of the dynamics of the 2-Dof device because it can represent the real behaviour of the system and captures the velocity amplification effect which is a key requirement of modelling device of interest in this work. Moreover, this model is essential for a future optimization of geometric and magnetic parameters in order to develop a MEMS scale multi-degree-of-freedom device.