This paper explores the modeling and analysis of the effect of minimum actuation pressure previously observed in literature. This minimum pressure is similar in kind to the minimum amplitude of the signal for muscle actuation seen in mammalian muscles. This minimum actuation, or “threshold” pressure is used a method of mechanically encoding the control of FAM engagement and the actuation efficiency of a group, or ‘bundle’ of muscles with differing threshold pressures is compared with a single muscle of equivalent force and strain. The results of this analysis indicate the efficacy of using this design and control method; it is advantageous in cases where a range of displacements and forces are necessary.
Researchers have performed theoretical investigations of flow induced limit cycle oscillations (LCOs) of tensioned ribbons. Furthermore, attempts have been made to tap into the energy harvesting capability of such ribbons, owing to its structural simplicity, low weight and ease of fabrication. However, in order to tune the ribbon to perform optimally at a given location, a robust, reliable model of the ribbon is essential to predict the limit cycle behavior. The model needs validation across a broad spectrum of its operating envelope based on experimentally obtained results. This paper seeks to provide experimental data for a sample tensioned ribbon in cross flow to serve as basis for validation of an aeroelastic model. This paper experimentally characterizes a PTFE (polytetrafluoroethylene) ribbon of aspect ratio 18 across a range of applied axial preload tension and wind speeds.
Interest in clean, stable, and renewable energy harvesting devices has increased dramatically with the volatility of
petroleum markets. Specifically, research in aero/hydro kinetic devices has created numerous new horizontal and vertical
axis wind turbines, and oscillating wing turbines. Oscillating wing turbines (OWTs) differ from their wind turbine cousins
by having a rectangular swept area compared to a circular swept area. The OWT systems also possess a lower tip speed
that reduces the overall noise produced by the system. OWTs have undergone significant computational analysis to
uncover the underlying flow physics that can drive the system to high efficiencies for single wing oscillations. When two
of these devices are placed in tandem configuration, i.e. one placed downstream of the other, they either can constructively
or destructively interact. When constructive interactions occurred, they enhance the system efficiency to greater than that
of two devices on their own. A new experimental design investigates the dependency of interaction modes on the pitch
stiffness of the downstream wing. The experimental results demonstrated that interaction modes are functions of
convective time scale and downstream wing pitch stiffness. Heterogeneous combinations of pitch stiffness exhibited
constructive and destructive lock-in phenomena whereas the homogeneous combination exhibited only destructive
A multifunctional compliant structure is proposed that can harvest electrical power from both incident sunlight and ambient mechanical energy including wind flow or vibration. The proposed energy harvesting device consists of a slender, ribbon-like, flexible thin film solar cell that is laminated with piezoelectric patches at either ends and mounted in the cross flow of wind in a clamped-clamped end condition with an adjustable axial preload. Taking this motivation forward a system model of the energy harvester is developed which captures the structural response of the solar ribbon and couples it with Theodorsen unsteady aerodynamics to predict the flutter boundary conditions as a function of applied axial preload tension. The model also accounts for geometric and material discontinuities, by effective use of Transfer Matrix Method (TMM) modeling technique both in bending and torsional degrees of freedom. This paper also derives TMM technique for torsional vibrations with an applied axial load from first principles, verifies the method and presents its applicability for the proposed energy harvester. The paper also points out that the flutter instability arises out of different structural modes at different values applied axial tension, with the help of a sample modal convergence plot. The analysis also presents the possibility to tune the solar ribbon to operate at an optimal reduced frequency by adjusting the applied axial preload.
This study characterizes hybrid control approaches for the variable recruitment of fluidic artificial muscles with double acting (antagonistic) actuation. Fluidic artificial muscle actuators have been explored by researchers due to their natural compliance, high force-to-weight ratio, and low cost of fabrication. Previous studies have attempted to improve system efficiency of the actuators through variable recruitment, i.e. using discrete changes in the number of active actuators. While current variable recruitment research utilizes manual valve switching, this paper details the current development of an online variable recruitment control scheme. By continuously controlling applied pressure and discretely controlling the number of active actuators, operation in the lowest possible recruitment state is ensured and working fluid consumption is minimized. Results provide insight into switching control scheme effects on working fluids, fabrication material choices, actuator modeling, and controller development decisions.
This paper presents the system design, construction, and testing of an active variable buoyancy system (VBS) actuator with applications to unmanned multi-domain vehicles. Unmanned multi-domain vehicles require nontraditional VBS designs because of their unique operation requirements. We present a VBS actuator design that targets multi-domain vehicle design objectives of high endurance, stealth, and underwater loitering. The design features a rigid ballast tank with an inner elastic bladder connected to a hydraulic pump and a proportionally controlled vent valve. The system working fluid is obtained from the ambient surrounding water and the elastic bladder separates the water from pressurized gas, thus preventing any gas from escaping during a venting operation. An analytic model of the VBS characterizing the system dynamics is derived. Ballast tank prototype design and construction is discussed. A VBS test platform vehicle is presented, featuring two ballast tanks, motor, pump, and RF receiver for control.
Increasing demand to harvest energy from renewable resources has caused significant research interest in unsteady aerodynamic and hydrodynamic phenomena. Apart from the traditional horizontal axis wind turbines, there has been significant growth in the study of bio-inspired oscillating wings for energy harvesting. These systems are being built to harvest electricity for wireless devices, as well as for large scale mega-watt power generation. Such systems can be driven by aeroelastic flutter phenomena which, beyond a critical wind speed, will cause the system to enter into limitcycle oscillations. When the airfoil enters large amplitude, high frequency motion, leading and trailing edge vortices form and, when properly synchronized with the airfoil kinematics, enhance the energy extraction efficiency of the device. A reduced order dynamic stall model is employed on a nonlinear aeroelastic structural model to investigate whether the parameters of a fully passive aeroelastic device can be tuned to produce limit cycle oscillations at desired kinematics. This process is done through an optimization technique to find the necessary structural parameters to achieve desired structural forces and moments corresponding to a target limit cycle. Structural nonlinearities are explored to determine the essential nonlinearities such that the system’s limit cycle closely matches the desired kinematic trajectory. The results from this process demonstrate that it is possible to tune system parameters such that a desired limit cycle trajectory can be achieved. The simulations also demonstrate that the high efficiencies predicted by previous computational aerodynamics studies can be achieved in fully passive aeroelastic devices.
This paper proposes a multifunctional compliant structure that can harvest electrical power from both incident sunlight and ambient mechanical energy including wind flow or vibration. The energy harvesting device consists of a slender, ribbon-like, flexible thin film solar cell that is laminated with piezoelectric patches. The harvester is mounted in longitudinal tension and subjected to a transverse wind flow to excite flow-induced aeroelastic vibrations. This paper formulates an analytic model of the bending dynamics of the device. We present a Transfer Matrix formulation that also accounts for the changes in natural frequencies and mode shapes of the system when subjected to axial loads in a beam. It also observed that mode shape obtained using TMM formulation shows numerical stability even for very high tensile loads providing results consistent with the geometric boundary conditions applied at the ends of a beam. This article also discusses about structurally modeling a piezo - solar energy harvester using TMM methodology, where a thin clampedclamped solar film is bonded with piezo patches having a much higher bending stiffness. Additionally, the effect of axial tension on the mode shape of the thin host structure of the piezo-solar ribbon is presented and it is shown how this tension can be used advantageously to affect the strain distribution of the entire structure and introduce higher strains at the piezo patches.
McKibben artificial muscles are often utilized in mobile robotic applications that require compliant and light weight actuation capable of producing large forces. In order to increase the endurance of these mobile robotic platforms, actuation efficiency must be addressed. Since pneumatic systems are rarely more than 30% efficient due to the compressibility of the working fluid, the McKibben muscles are hydraulically powered. Additionally, these McKibben artificial muscles utilize an inelastic bladder to reduce the energy losses associated with elastic energy storage in the usual rubber tube bladders. The largest energy losses in traditional valve-controlled hydraulic systems are found in the valving implementation to match the required loads. This is performed by throttling, which results in large pressure drops over the control valves and significant fluid power being wasted as heat. This paper discusses how these throttling losses are reduced by grouping multiple artificial muscles to form a muscle bundle where, like in skeletal muscle, more elements that make up the muscle bundle are recruited to match the load. This greatly lessens the pressure drops by effectively changing the actuator area, leading to much higher efficiencies over a broader operation envelope. Simulations of several different loading scenarios are discussed that reveal the benefits of such an actuation scheme.
With increasing population and proliferation of wireless electronics, significant research attention has turned to harvesting energy from ambient sources such as wind and water flows at scales ranging from micro-watt to mega-watt levels. One technique that has recently attracted attention is the application of bio-inspired flapping wings for energy harvesting. This type of system uses a heaving and pitching airfoil to extract flow energy and generate electricity. Such a device can be realized using passive devices excited by aeroelastic flutter phenomena, kinematic mechanisms driven by mechanical linkages, or semi-active devices that are actively controlled in one degree of freedom and passively driven in another. For these types of systems, numerical simulations have showed strong dependence on efficiency and vortex interaction. In this paper we propose a new apparatus for reproducing arbitrary pitch-heave waveforms to perform flow visualization experiments in a soap film tunnel. The vertically falling, gravity driven soap film tunnel is used to replicate flows with a chord Reynolds number on the order of 4x10<sup>4</sup>. The soap film tunnel is used to investigate leading edge vortex (LEV) and trailing edge vortex (TEV) interactions for sinusoidal and non-sinusoidal waveforms. From a qualitative analysis of the fluid structure interaction, we have been able to demonstrate that the LEVs for non-sinusoidal motion convect faster over the airfoil compared with sinusoidal motion. Signifying that optimal flapping frequency is dependent on the motion profile.
This paper proposes and experimentally investigates applying piezoelectric energy harvesting devices driven by flow induced vibrations to create self-powered actuation of aerostructure surfaces such as tabs, flaps, spoilers, or morphing devices. Recently, we have investigated flow-induced vibrations and limit cycle oscillations due to aeroelastic flutter phenomena in piezoelectric structures as a mechanism to harvest energy from an ambient fluid flow. We will describe how our experimental investigations in a wind tunnel have demonstrated that this harvested energy can be stored and used on-demand to actuate a control surface such as a trailing edge flap in the airflow. This actuated control surface could take the form of a separate and discrete actuated flap, or could constitute rotating or deflecting the oscillating energy harvester itself to produce a non-zero mean angle of attack. Such a rotation of the energy harvester and the associated change in aerodynamic force is shown to influence the operating wind speed range of the device, its limit cycle oscillation (LCO) amplitude, and its harvested power output; hence creating a coupling between the device’s performance as an energy harvester and as a control surface. Finally, the induced changes in the lift, pitching moment, and drag acting on a wing model are quantified and compared for a control surface equipped with an oscillating energy harvester and a traditional, static control surface of the same geometry. The results show that when operated in small amplitude LCO the energy harvester adds negligible aerodynamic drag.
This paper presents the design, construction, experimental characterization, and system testing of a legged, wall-climbing robot actuated by meso-scale hydraulic artificial muscles. While small wall-climbing robots have seen increased research attention in recent years, most authors have primarily focused on designs for the gripping and adhesion of the robot to the wall, while using only standard DC servo-motors for actuation. This project seeks to explore and demonstrate a different actuation mechanism that utilizes hydraulic artificial muscles. A four-limb climbing robot platform that includes a full closed-loop hydraulic power and control system, custom hydraulic artificial muscles for actuation, an on-board microcontroller and RF receiver for control, and compliant claws with integrated sensing for gripping a variety of wall surfaces has been constructed and is currently being tested to investigate this actuation method. On-board power consumption data-logging during climbing operation, analysis of the robot kinematics and climbing behavior, and artificial muscle force-displacement characterization are presented to investigate and this actuation method.
This paper presents experimental energy harvesting efficiency analysis of a piezoelectric device driven to limit cycle oscillations by an aeroelastic flutter instability. Wind tunnel testing of the flutter energy harvester was used to measure the power extracted through a matched resistive load as well as the variation in the device swept area over a range of wind speeds. The efficiency of this energy harvester was shown to be maximized at a wind speed of about 2.4 m/s, which corresponds to a limit cycle oscillation (LCO) frequency that matches the first natural frequency of the piezoelectric structure. At this wind speed, the overall system efficiency was 2.6%, which exceeds the peak efficiency of other comparably sized oscillator-based wind energy harvesters using either piezoelectric or electromagnetic transduction. Active synchronized switching techniques are proposed as a method to further increase the overall efficiency of this device by both boosting the electrical output and also reducing the swept area by introducing additional electrical energy dissipation. Real-time peak detection and switch control is the major technical challenge to implementing such active power electronics schemes in a practical system where the wind speed and the corresponding LCO frequency are not generally known or constant. A promising microcontroller (MCU) based peak detector is implemented and tested over a range of operating wind speeds.
The investigation of unsteady aerodynamics is becoming a more attractive topic of research in enhancing flight capabilities. Natural flyers such as birds and insects can undergo flight maneuvers that are very difficult or impossible to accomplish with man-made flyers and current classical aerodynamic theory. Modeling the unsteady phenomena produced by flapping wings is important to the understanding of these maneuvers, with possible applications to aircraft flight. We investigate numerically simulating the unsteady aerodynamics generated by flapping wings using the two seperate approaches of rotational lift and dynamic stall. A low order quasi-steady model based on rotational lift and a revised version based on dynamic stall are presented. Both concepts are analyzed using simulated results, with experimental data produced with matching kinematics as a basis of comparison. The numerically generated force curves are used to explore the characteristics and distinguishing features of both approaches, as well as how well they capture the salient features of the experimentally produced forces.
Energy harvesting from flowing fluids using flapping wings and fluttering aeroelastic structures has recently gained
significant research attention as a possible alternative to traditional rotary turbines, especially at and below the
centimeter scale. One promising approach uses an aeroelastic flutter instability to drive limit cycle oscillations of a
flexible piezoelectric energy harvesting structure. Such a system is well suited to miniaturization and could be used to
create self-powered wireless sensors wherever ambient flows are available. In this paper, we examine modeling of the
aerodynamic forces, power extraction, and efficiency of such a flapping wing energy harvester at a low Reynolds
number on the order of 1000. Two modeling approaches are considered, a quasi-steady method generalized from
existing models of insect flight and a modified model that includes terms to account to the effects of dynamic stall. The
modified model is shown to provide better agreement with CFD simulations of a flapping energy harvester.
This paper investigates a novel mechanism for powering wireless sensors or low power electronics by extracting energy
from an ambient fluid flow using a piezoelectric energy harvester driven by aeroelastic flutter vibrations. The energy
harvester makes use of a modal convergence flutter instability to generate limit cycle bending oscillations of a
cantilevered piezoelectric beam with a small flap connected to its free end by a revolute joint. The critical flow speed at
which destabilizing aerodynamic effects cause self-excited vibrations of the structure to emerge is essential to the design
of the energy harvester. This value sets the lower bound on the operating wind speed and frequency range of the system.
A system of coupled equations that describe the structural, aerodynamic, and electromechanical aspects of the system are
used to model the system dynamics. The model uses unsteady aerodynamic modeling to predict the aerodynamic forces
and moments acting on the structure and to account for the effects of vortices shed by the flapping wing, while a modal
summation technique is used to model the flexible piezoelectric structure. This model is applied to examine the effects
on the cut-in wind speed of the system when several design parameters are tuned and the size and mass of the system is
held fixed. The effects on the aeroelastic system dynamics and relative sensitivity of the flutter stability boundary are
presented and discussed. Experimental wind tunnel results are included to validate the model predictions.
A novel energy harvesting device powered by aeroelastic flutter vibrations is proposed to generate power for embedded
wireless sensors on a helicopter rotor blade. Such wireless sensing and on-board power generation system would
eliminate the need for maintenance intensive slip ring systems that are required for hardwired sensors. A model of the
system has been developed to predict the response and output of the device as a function of the incident wind speed. A
system of coupled equations that describe the structural, aerodynamic, and electromechanical aspects of the system are
presented. The model uses semi-empirical, unsteady, nonlinear aerodynamics modeling to predict the aerodynamic
forces and moments acting on the structure and to account for the effects of vortex shedding and dynamic stall. These nonlinear effects are included to predict the limit cycle behavior of the system over a range of wind speeds. The model results are compared to preliminary wind tunnel tests of a low speed aeroelastic energy harvesting experiment.
Energy harvesting has enabled new operational concepts in the growing field of wireless sensing. A novel energy
harvesting device driven by aeroelastic flutter vibrations has been developed and could be used to complement existing
environmental energy harvesters such as solar cells in wireless sensing applications. An analytical model of the
mechanical, electromechanical, and aerodynamic systems suitable for designing aeroelastic energy harvesters for various
flow applications are derived and presented. Wind tunnel testing was performed with a prototype energy harvester to
characterize the power output and flutter frequency response of the device over its entire range of operating wind speeds.
Finally, two wing geometries, a flat plate and a NACA 0012 airfoil were tested and compared.
Aeroelastic vibration of structures represents a novel energy harvesting opportunity that may offer significant advantages
over traditional wind power devices in many applications. Such a system could complement existing alternative energy
sources by allowing for distributed power generation and placement in urban areas. The device configuration of a
simple two degree aeroelastic system suitable for piezoelectric power harvesting is presented. The mechanical,
electromechanical, and aerodynamic equations of motion governing the dynamics and electrical output of the system as a
function of incident wind speed are derived. The response and current output of one design for a bench top scale
harvester are simulated and presented. Finally, a strategy for expanding the operating envelope of the power harvester is
proposed and discussed.