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Loading system mechanism for dielectric elastomer generators with equi-biaxial state of deformation
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Proceedings of SPIE - The International Society for Optical Engineering
Volume 9056, 2014, Article number 90561F
Electroactive Polymer Actuators and Devices, EAPAD 2014; San Diego, CA; United States; 10 March 2014 through 13 March 2014; Code 106848
Loading system mechanism for dielectric elastomer generators with equi-biaxial state of deformation (Conference Paper)
Fontana, M. , Moretti, G., Lenzo, B., Vertechy, R.
PERCRO SEES, TeCIP Institute, Scuola Superiore sant'Anna, Piazza Martiri della Libertà 33, Pisa, 5612, Italy
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Abstract
Dielectric Elastomer Generators (DEGs) are devices that employ a cyclically variable membrane capacitor to produce electricity from oscillating sources of mechanical energy. Capacitance variation is obtained thanks to the use of dielectric and conductive layers that can undergo different states of deformation including: uniform or non-uniform and uni- or multi-axial stretching. Among them, uniform equi-biaxial stretching is reputed as being the most effective state of deformation that maximizes the amount of energy that can be extracted in a cycle by a unit volume of Dielectric Elastomer (DE) material. This paper presents a DEG concept, with linear input motion and tunable impedance, that is based on a mechanical loading system for inducing uniform equi-biaxial states of deformation. The presented system employs two circular DE membrane capacitors that are arranged in an agonist-antagonist configuration. An analytical model of the overall system is developed and used to find the optimal design parameters that make it possible to tune the elastic response of the generator over the range of motion of interest. An apparatus is developed for the equi-biaxial testing of DE membranes and used for the experimental verification of the employed numerical models
Robust, Fast and Accurate Solution of the Direct Position Analysis of Parallel Manipulators by Extra-Sensors
No full textParallel manipulators (PMs) are closed kinematic chains with one or more loops where only some pairs are actuated while the remaining are passive. In particular, they feature a fixed link (base) and an output moving link (platform) interconnected by at least two independent kinematic chains (legs) to form one loop. The most well known and commonly employed PMs (hereafter called UPS-PMs) feature n variable-length legs of type UPS (where U, P and S are for universal, spherical and prismatic pairs respectively). Equivalently, a revolute pair R could be used instead of the prismatic pair P in order to make the leg length variable (in this case the leg would be of type URS). These leg topologies provide the platform with six degrees of freedom with respect to the base.
Although the definition of UPS-PMs requires n  2, in practice, neglecting overconstrained and redundantly-actuated manipulators, performance issues recommend 3  n  6. Indeed, UPS-PMs with only two UPS legs might exhibit a low stiffness against torques acting along the line joining the centers of the two spherical pairs, and their control would require the in-series placement of at least three actuators/sensors (one of them placed to control/measure at least one out of the three degrees of freedom of the spherical pairs) which reduces the overall manipulator dynamic and accuracy capabilities. On the other side, the use of more than six legs reduces the exploitable manipulator workspace for the increase of leg interference.
Different sub-classes of manipulator architectures can be obtained according to the location of the centers of the U and S pairs in the base and in the platform respectively (Innocenti & Parenti-Castelli, 1994; Faugere & Lazard, 1995). General UPS-PM architectures feature distinct joint centers. Special architectures can be devised by setting some of the joint centers to be coincident.
A schematic of a 6-DOF UPS-PM having six legs (n = 6) and general architecture is shown in Fig. 1. In the figure, the U pairs (connecting the legs to the base) and S pairs (connecting the legs to the platform) are depicted as grey and white dots respectively. Points Bi and Pi (i = 1, ..., 6) represent the centers of the U and S pairs of the i-th leg on the base and on the platform respectively. The six legs of type UPS are represented by the telescopic rods BiPi (i = 1, ..., 6). Accordingly, the length of the i-th leg is defined as the distance li = Pi - Bi.
Manipulators with less than six DOF can be obtained from UPS-PMs by suitably eliminating or locking some of the leg kinematic pairs. For instance, considering a 6-DOF UPS-PM having six legs, elimination of four P pairs yields a 2-DOF PM having two legs of type UPS and four legs of type US.
Well-known examples of UPS-PMs are as follows: 1) the 6-DOF UPS-PMs (Gough & Whitehall, 1962; Stewart, 1965; Cappel, 1967); 2) the 3-DOF spherical PMs (Innocenti & Parenti-Castelli, 1993); 3) the 2-DOF spherical PMs (Vertechy & Parenti-Castelli, 2006); and 4) the 1-DOF helicoidal PMs (Jacobsen, 1975). Because of their parallel architecture, UPS-PMs exhibit large payload-to-weight ratio, high accuracy, high structural rigidity and high dynamic capabilities, which make them excel as: a) fast and high precision robots in vehicle simulators (Gough & Whitehall, 1962; Stewart, 1965; Cappel, 1967), machine tools (Charles, 1995) and positioning systems (Schmidt-Kaler, 1992); b) passive Cartesian input devices in joysticks, master-slave teleoperation systems (Daniel et al., 1993) and other tracking devices (Geng & Haynes, 1994); c) force/torque sensors and generators in multi-axis sensors and motors (Gaillet & Reboulet, 1983; Nguyen et al., 1991; Lewis et al., 2002); d) mechanical transmissions in motion converters (Jacobsen, 1975); and e) orthopedic devices in fixations systems (Taylor & Taylor, 2000; Di Gregorio & Parenti-Castelli, 2002).
Practical use of UPS-P..
Electromechanical characterization of a new synthetic rubber membrane for dielectric elastomer transducers
No full textDielectric Elastomers (DE) are incompressible polymeric solids that experience finite elastic deformations and are electrically non-conductive. Stacking multiple DE films separated by compliant electrodes makes a deformable capacitor transducer, namely a DE Transducer (DET), which can expand in area while shrinking in thickness and vice versa. DETs can be used as solid-state actuators, sensors and generators. The development of an effective DET requires the accurate knowledge of the constitutive behavior of the employed DE material. In this context, this paper reports the experimental results of the electromechanical characterization of a new synthetic rubber membrane (TheraBanTM Latex Free Resistance Band Yellow (P/N #11726), or TheraBand LFRB-Y in short) to be used as elastic dielectric in DETs. Comparison of the obtained results with those of the best quoted Natural Rubber membrane (OPPO BAND 8003) is also provided that shows the superior performances of TheraBand LFRB-Y both in terms of reduced mechanical hysteresis and of higher dielectric strength stability to ambient wetness conditions
Modeling of static reliability assessment in dielectric elastomer transducers subject to electric loads
No full textIn recent years, Dielectric Elastomer Transducers (DETs) have been of top-notch interest as an alternative solution to conventional mechatronic transduction systems, thanks to their features as low-cost and affordable materials, silence operation, low-power consumption, and high level of energy density. Generally, in their most uncomplicated layout, these devices form an electrostatic system, composed of a Dielectric Elastomer (DE) membrane, embedded between two opposite compliant electrodes, constituting a highly deformable capacitor capable of transforming electrical energy into mechanical and vice versa. However, DETs applicability is strongly affected by several engineering constraints. One of their principal failure modes is related to the electrical breakdown of the DE membrane, which occurs when an applied input electrical load exceeds the dielectric strength of the DE. In order to address this problem, the materials and the input load conditions must be chosen appropriately to assure a desired lifetime of operation. For this purpose, this work proposes a preliminary static reliability assessment procedure to evaluate the failure probability of a DET for static events as the electrical breakdown, with specific electric input load conditions. The resulting reliability model comprises the stochastic comparison of the dielectric strength of the DE material with the extreme values distribution of electrical loads in a specific period, aiming to forecast the reliability evaluation for a more extended period, multiple of the original one
Modelling and Control of Lozenge-Shaped Dielectric Elastomer Generators
No full textDielectric Elastomers (DEs) are a very promising technology for the development of energy harvesting devices based on the variable-capacitance electrostatic generator principle. As compared to other technologies, DE Generators (DEGs) are solid-state energy conversion systems which potentially feature: 1) large energy densities, 2) good energy conversion efficiency that is rather independent of cycle frequency, 3) easiness of manufacturing and assembling, 4) high shock resistance, 5) silent operation, 6) low cost. Envisioned applications for DEGs are in devices that convert ocean wave energy into usable electricity.This paper introduces the Lozenge-Shaped DEG (LS-DEG) that is a specific type of planar DE transducer with one degree of freedom. A LS-DEG consists of a planar DE membrane that is connected along its perimeter to the links of a parallelogram four-bar mechanism. As the mechanism is put into reciprocal motion, the DE membrane varies its capacitance that is then employed as a charge pump to convert external mechanical work into usable electricity.Specifically, this paper describes the functioning principle of LS-DEGs, and provides a comparison between different hyper-elastic models that can be used to predict the energy harvesting performances of realistic prototypes. Case studies are presented which address the constrained optimization of LS-DEGs subjected to failure criteria and practical design constraints
A New Parameter Selection Method for Power Skiving Tools
No full textThis paper presents a new selection method of the teeth number and the helix angle of a power skiving tool, which are basic parameters for machining internal gears. It is based on screw theory, whose use for power skiving has not been yet documented in the literature. The tool parameters are selected based on an optimization procedure of the machining process kinematics, differently from the common practice that defines them by experience. In addition, specific power skiving requirements are also considered in the selection. A working example that shows the effectiveness of the method is provided, and the optimal tool parameters are found. In particular, the results show that two different types of tool can machine the same internal gear
Power skiving manufacturing process: A review
No full textGears are the most widely used mechanical components for motion and power transmission; thus, gear manufacturing plays a crucial role in many industrial sectors. Amongst the different methods for gear machining, power skiving has become a highly competitive gear manufacturing process in the last few decades. This is mainly due to advances in manufacturing engineering and improvements in numerical control of electric drives. This article presents a comprehensive review on the research and development activities on power skiving that is missing in the literature. In particular, it aims at presenting the current state of the art of this manufacturing process and highlighting new advancements. The study encompasses some of the major topics, namely: new tool designs, influence of working parameters on the cutting operation, chip geometry, determination of cutting forces and tool wear. Finally, study deficiencies, practical limitations and new research directions in the field of power skiving are discussed which can serve as guidelines for new research on the topic
On Locally Optimal Redundancy Resolution using the Basis of the Null Space
No full textThis paper presents two methods for the computation of the null space velocity command in redundant robots. Both these methods resort to the solution of a constrained optimization problem. The first one is a formalization of the traditional Gradient Projection Method (GPM) which guarantees the respect of the joint bounds and a gradual activation/deactivation of the null space command. The second one, called Null Space Basis Optimal Linear Combination Method (NSBM), finds the optimal coefficients of a basis of the null space of the Jacobian, ensuring in turn that the joint bounds are respected and that the null space is activated and deactivated gradually. The two methods are applied to the case study of a welding application in which the null space command must avoid the collision between the robot and an obstacle. The comparison of the results of the case study shows that NSBM performs better than GPM. The proposed algorithms are also tested on a real robotic platform to demonstrate that their computational time is compatible with the real-time requirements of the robot
Open-access dielectric elastomer material database
No full textDielectric Elastomer Transducers (DETs) are deformable capacitors that can be used as sensors, actuators and generators. The design of effective and optimized DETs requires the knowledge of a set of relevant properties of the employed Dielectric Elastomer (DE) material, which make it possible to accurately predict their electromechanical dynamic behavior. In this context, an open-access database for DE materials has been created with the aim of providing the practicing engineer with the essential information for the design and optimization of new kinds of DET. Among the electrical properties, dielectric susceptibility, dielectric strength and conductivity are considered along with their dependence on mechanical strain. As regards mechanical behavior, experimental stress-strain curves are provided to predict hyperelasticity, plasticity, viscosity, Mullins effect and mechanical rupture. Properties of commercial elastomeric membranes have been entered in the database and made available to the research community. This paper describes the instrumentations, experimental setups and procedures that have been employed for the characterization of the considered DE materials. To provide an example, the experimental data acquired for a commercially available natural rubber membrane (OPPO Band Red 8012) are presented
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