Actuator Length Research Articles

Dual measurement self-sensing technique of NiTi actuators for use in robust control

Using a shape memory alloy actuator as both an actuator and a sensor provides huge benefits in cost reduction and miniaturization of robotic devices. Despite much effort, reliable and robust self-sensing (using the actuator as a position sensor) had not been achieved for general temperature, loading, hysteresis path, and fatigue conditions. Prior research has sought to model the intricacies of the electrical resistivity changes within the NiTi material. However, for the models to be solvable, nearly every previous technique only models the actuator within very specific boundary conditions. Here, we measure both the voltage across the entire NiTi wire and of a fixed-length segment of it; these dual measurements allow direct calculation of the actuator length without a material model. We review previous self-sensing literature, illustrate the mechanism design that makes the new technique possible, and use the dual measurement technique to determine the length of a single straight wire actuator under controlled conditions. This robust measurement can be used for feedback control in unknown ambient and loading conditions.

Improvement of effectiveness of EMHD flow separation control

The present numerical study shows that the effectiveness of flow separation control using Electro-Magneto-Hydro-Dynamic (EMHD) method can be improved by choosing the dimensions of the actuator based on the Reynolds number and the shape of the model studied. The width of the electrode/magnet for most effective flow separation control should be approximately 2.5 times the boundary layer thickness, which in turn is dependent on the Reynolds number. The length of the EMHD actuator for most effective flow separation control can be the shortest based on manufacturing constraints as well as on the possibility of shifting of separation point. The application of Lorentz force inside the boundary layer near the separation point provides the most effective flow separation control. The studies are done for flow over a cylinder in low Reynolds number regime (Reynolds number = 200) and for flow over an elliptical airfoil in high Reynolds number regime (Reynolds number = 1.6 × 105).

On the development of rod-based models for pneumatically actuated soft robot arms: A five-parameter constitutive relation

While soft robots have many attractive features compared to their hard counterparts, developing tractable models for these highly deformable, nonlinear, systems is challenging. In a recent paper, the authors published a non-classic, five-parameter constitutive relation for a rod-based model of a widely used, pneumatically actuated soft robot arm. It is natural to ask if the complexity of the relation can be eliminated by redesigning the actuator? To this end, finite element models and experimental results are used to further explore the five-parameter constitutive relation. For multiple designs of the pneumatically actuated soft robot arm, we are able to demonstrate how finite element models can be employed in place of experiments to specify the constitutive relations and how the relations are scalable by actuator length and applied pressure. Our primary result is the finding that the five-parameter constitutive relation is germane to pneumatically actuated soft robot arms and the parameters for this relation can be determined by three finite element simulations.

A new approach using hybrid power series – cuckoo search optimization algorithm to solve electrostatic pull-in instability and deflection of nano cantilever switches subject to van der waals attractions

A hybrid Power Series (PS) and Cuckoo Search via L´evy Flights (CS) optimization algorithm (PS-CS) method is utilized to obtain a solution for the deflection and pull-in instability of a nano cantilever switch in the presence of the van der Waals attractions, electrostatic forces and fringing filed effects. In order to obtain a relation for deflection of the beam, a trial solution including adjustable coefficients, satisfying the boundary conditions of the governing, is proposed. The cuckoo search optimization algorithm is executed to find the ad-justable parameters of the trial solution satisfying the governing equation of the nanobeam. The results are compared with the available results in the literature as well as numerical solution. The results indicate the remarkable accuracy of the present approach. The minimum initial gap and the critical free standing detachment length of the nano actuator that does not stick to the substrate due to the van der Waals attractions, as an important parameter in pull-in instability of the nano switches, is calculated. Utilizing the results of the PS-CS, the stress distribution inside the nano actuator is determined at the onset of the pull-in instability.

A Novel Slack-Enabling Tendon Drive That Improves Efficiency, Size, and Safety in Soft Wearable Robots

Tendon drives are widely used in robotics. The compliance of the tendon in such drives suits them for soft robots, including soft wearable robots, but several issues impede their use. Generally, the tendon should always maintain tension to prevent derailment from the spool. However, in soft robots, tendon tension induces high friction forces owing to the absence of ball bearings. Because the kinematics of the soft robot is basically nonlinear and changed by the deformation of the structure, the kinematic difference between the soft structure and the spool causes derailment of the tendon. Moreover, continuously maintained tension in soft wearable robots causes safety issues. The linear actuator can be an option. However, the need to increase the length of the linear actuator to accommodate the excursion length of its tendon is a barrier to its use in small-scale applications. To preclude this issue, a slack-enabling actuator that employs a spool is proposed. The space efficiency of the spool enables the mechanism to be small, and a one-way clutch applies unidirectional friction force to the tendon to tighten the tendon around the spool. This paper describes the design concept for the slack-enabling mechanism, its design optimization, and system identification for force control.

Dynamic Control of the Bionic Handling Assistant

In the previous decade, multiple approaches for kinematic controllers of continuum manipulators were successfully developed. However, for fast and exact motions of continuum robots that are hardly seen so far, dynamic controllers are necessary—especially for spatial manipulators with multiple sections. Therefore, a dynamic controller in actuator space is developed that both exploits input constraints and decouples the actuators in order to provide a good and intuitive dynamic behavior of the manipulator. Having a pneumatically actuated manipulator with multiple actuators, this can be achieved using a cascaded control approach with a centralized actuator length controller and an underlying decoupled pneumatic controller. For both the outer multiple-input multiple-output (MIMO) controller and the inner single-input single-output (SISO) controllers, model-based controllers are developed based on suitable models using feedback linearization and a combination of feedforward control and linear PD-controllers. The resulting actuator controller can be used for tracking optimized trajectories in actuator space and also for possible extensions towards direct tool center point feedback.

Fundamental Study of Soft Actuator Using Anisotropic Gel Hybridized with Nanosheet Liquid Crystal: Analysis of Heat Characteristics and Length Control

The fundamental heat characteristics of a gel actuator that uses nanosheet liquid crystals are studied. Owing to the structural anisotropy of nanosheets, the gel actuator exhibits anisotropic thermo-induced deformation, which makes it possible for the gel actuator to be applied to new materials for molecular robotics. As a fundamental study for the development, a relationship between the contraction length of the gel actuator and water temperature is investigated, and the mapping between them is obtained. Furthermore, using the mapping, fundamental controller methods are proposed to control the contraction length of the gel actuator. Controller performances are assessed through experimental results.

Measurement Experiments and Analysis for Modeling of McKibben Pneumatic Actuator

[abstFig src='/00280006/06.jpg' width='300' text='Relation betweenV,Landfmon constant pressure condition' ] In this study, we investigate the dynamic relationship between tension, contraction velocity, and length of the McKibben pneumatic actuators (MPAs) via experiments. Although MPAs are widely used in biomimetic robots or rehabilitation machinery, it has not been verified why they can exhibit stable motion from their simple structure. We analyze the relation of output tension, contraction velocity, and length. From the results of experiments, we conclude that the tension is related to both the velocity and the length of the actuator and the general form of this relation can be approximated as a plane. Moreover, we confirm that the parameters of the plane depend on the MPA pressure and valve condition.

Damage Detection of Beams by a Vibration Characteristic Tuning Technique Through an Optimal Design of Piezoelectric Layers

An advanced technique for damage detection in beam structures, using a vibration characteristic tuning procedure is developed by an optimal design of piezoelectric materials. Piezoelectric sensors and actuators are mounted on the surface of the host beam to generate excitations for the tuning via a feedback process. The excitations induced by the piezoelectric effect are used to magnify the effect of the damage in the vibration characteristics of the damaged structure to realize an effective damage detection process. To describe the detection process, theoretical models of the cantilevered beams with and without tuning induced by piezoelectric effect are built first, while the damage is represented by a crack at the fixed end. From the established models, the natural frequencies of the tuned beams with and without the crack are obtained to study the sensitivity of the proposed technique. As a result of the tuning process, more obvious changes on the natural frequencies due to the existence of a crack are observed. The results also indicate a shorter distance from the piezoelectric actuators to the crack leads to a higher detection sensitivity. An optimal length of the piezoelectric actuators is also obtained for better detection.

Parallel kinematic mechanisms for distributed actuation of future structures

Future machines will require distributed actuation integrated with load-bearing structures, so that they are lighter, move faster, use less energy, and are more adaptable. Good examples are shape-changing aircraft wings which can adapt precisely to the ideal aerodynamic form for current flying conditions, and light but powerful robotic manipulators which can interact safely with human co-workers. A 'tensegrity structure' is a good candidate for this application due to its potentially excellent stiffness and strength-to-weight ratio and a multi-element structure into which actuators could be embedded. This paper presents results of an analysis of an example practical actuated tensegrity structure consisting of 3 ‘unit cells’. A numerical method is used to determine the stability of the structure with varying actuator length, showing how four actuators can be used to control movement in three degrees of freedom as well as simultaneously maintaining the structural pre-load. An experimental prototype has been built, in which 4 pneumatic artificial muscles (PAMs) are embedded in one unit cell. The PAMs are controlled antagonistically, by high speed switching of on-off valves, to achieve control of position and structure pre-load. Experimental and simulation results are presented, and future prospects for the approach are discussed.

Bifurcation and chaotic vibration for an electromechanical integrated harmonic piezodrive system

In this paper, the nonlinear coupled dynamics equations of the electromechanical integrated harmonic piezodrive system are proposed. Using these equations, the bifurcation and chaotic vibrations of the drive system are investigated. The results show that chaotic vibrations occur in the drive system under some parameters. The voltage on the piezoelectric actuator, the stiffness factor and the length of the piezoelectric actuator significantly affect the nonlinear vibrations of the drive system. The ranges for the system parameters that lead to a drive system with bad dynamics are shown. These results can be used to predict the dynamic load in a drive system and are useful for maximizing the power density of a drive system.

Construction of a Fish‐like Robot Based on High Performance Graphene/PVDF Bimorph Actuation Materials

Smart actuators have many potential applications in various areas, so the development of novel actuation materials, with facile fabricating methods and excellent performances, are still urgent needs. In this work, a novel electromechanical bimorph actuator constituted by a graphene layer and a PVDF layer, is fabricated through a simple yet versatile solution approach. The bimorph actuator can deflect toward the graphene side under electrical stimulus, due to the differences in coefficient of thermal expansion between the two layers and the converse piezoelectric effect and electrostrictive property of the PVDF layer. Under low voltage stimulus, the actuator (length: 20 mm, width: 3 mm) can generate large actuation motion with a maximum deflection of about 14.0 mm within 0.262 s and produce high actuation stress (more than 312.7 MPa/g). The bimorph actuator also can display reversible swing behavior with long cycle life under high frequencies. on this basis, a fish‐like robot that can swim at the speed of 5.02 mm/s is designed and demonstrated. The designed graphene‐PVDF bimorph actuator exhibits the overall novel performance compared with many other electromechanical avtuators, and may contribute to the practical actuation applications of graphene‐based materials at a macro scale.

35 Hz shape memory alloy actuator with bending-twisting mode.

Shape Memory Alloy (SMA) materials are widely used as an actuating source for bending actuators due to their high power density. However, due to the slow actuation speed of SMAs, there are limitations in their range of possible applications. This paper proposes a smart soft composite (SSC) actuator capable of fast bending actuation with large deformations. To increase the actuation speed of SMA actuator, multiple thin SMA wires are used to increase the heat dissipation for faster cooling. The actuation characteristics of the actuator at different frequencies are measured with different actuator lengths and results show that resonance can be used to realize large deformations up to 35 Hz. The actuation characteristics of the actuator can be modified by changing the design of the layered reinforcement structure embedded in the actuator, thus the natural frequency and length of an actuator can be optimized for a specific actuation speed. A model is used to compare with the experimental results of actuators with different layered reinforcement structure designs. Also, a bend-twist coupled motion using an anisotropic layered reinforcement structure at a speed of 10 Hz is also realized. By increasing their range of actuation characteristics, the proposed actuator extends the range of application of SMA bending actuators.

Characterization of Thermal Expansion Coefficient of LPCVD Polycrystalline SiC Thin Films Using Two Section V-beam Actuators

In this paper we present the characterization of the coefficient of thermal expansion (CTE) of in-situ doped polycrystalline SiC thin films, obtained by low pressure chemical vapor deposition (LPCVD). The material is characterized using V-beam actuators on which the temperature coefficient of resistance (TCR) and the in-plane displacement versus current are measured. A CTE value of 4.3 ± 0.4 ppm/K is obtained in the temperature region of 20°C to 300°C. This value is used in a finite element modeling (FEM) simulation of vertical SiC-SiO2 bimorph beams. For an actuator length of 700 μm, width of 100 μm and layer thickness of 2 μm, a displacement up to 200 μm can be obtained.

Study of electrical characterization and induced flow by nanosecond pulsed dielectric barrier discharge actuator

Focusing on the basic issues of nanosecond pulse dielectric barrier discharge (NS DBD), different actuators are used to investigate the electrical properties, induced flows and shock wave by NS DBD. The results indicate that the coupled energy per unit length is basically controlled by the peak voltage and is nearly independent of the actuator length and the pulse frequency. The peak powers can up to several hundred kW, while the average power for the NS DBD is slightly low, on the orders of W. The very small induced flow velocity of NS DBD is on the order of 0.1 m/s. The compression waves are observed after discharge, initially spread faster than the speed of sound. Phenomenological simulation validates that deposited energy at different levels can bring shock wave propagates at different speed. Deposited energy corresponding to the experiment induces compression wave that propagates close to the speed of sound.

A large-stroke shape memory alloy spring actuator using double-coil configuration

One way to increase the range of motion of shape memory alloy (SMA) actuators is to create displacements of the SMA associated with not only the deformation from straining but also rigid-body motion from translation and rotation. Rigid-body motion allows the SMA to create larger displacements without exceeding the maximum recovery strain so that the SMA actuators can have a larger shape recovery ratio. To improve the linear actuation stroke of SMA wire actuators, a novel SMA spring actuator is proposed that employs a double-coil geometry that allows the displacement of the SMA to be mainly induced by rigid-body motion. A double-coil SMA spring actuator is fabricated by coiling an SMA wire twice so that the double coiling results in a reduction of the initial length of the double-coil SMA spring actuator. The effects of the geometric parameters on the actuation characteristic of a double-coil SMA spring actuator are verified numerically by finite element analysis and experimentally according to a parametric study of the geometric parameters. The displacement-to-force profile of the double-coil SMA spring actuator is nonlinear, and the spring stiffness changes when the actuator transforms its configuration from a double-coil shape to a single-coil shape. According to the results of the parametric study, increasing the wire diameter increases both primary and secondary coil stiffness, and increasing the primary inner coil diameter decreases both primary and secondary coil stiffness, whereas increasing the secondary inner coil diameter decreases only the secondary coil stiffness. The result shows that one of the double-coil SMA spring actuators with an initial length of 8 mm has a recovery ratio of 1250%, while the recovery ratio of the single-coil SMA spring actuator with the same geometric parameters is 432%.

MODELING OF SURFACE STRESS EFFECTS ON THE DYNAMIC BEHAVIOR OF ACTUATED NON-CLASSICAL NANO-BRIDGES

The influence of surface stress and small scale on the dynamic pull-in behavior of nano-bridges is investigated in this paper. For this purpose, the governing equation of motion is derived based on the modified couple stress theory and Homotopy Perturbation Method with an auxiliary term is employed to produce the approximate solution of nano-beam vibrations. The effects of actuation voltage, initial conditions, surface energy and length scale parameter on the pull-in instability and fundamental frequency of the system are studied. The accuracy of proposed asymptotic approach is validated with numerical simulations. The obtained results from asymptotic analysis reveal that two terms in series expansions are sufficient to produce an acceptable approximation. The nano-actuator dynamics exhibit periodic and homoclinic orbits.

Design and Simulation of a Bending Actuator Based on Super-Aligned Carbon Nanotube/Polymer Composites

Super-aligned carbon nanotube films are carbon nanotube macrostructures which have excellent orientations. The bending actuator based on super-aligned carbon nanotube/polymer composites can make a significant controllable bending deformation under a very low DC voltage (< 700 V/m). In this paper, we explored how to make the thermal induced actuator reach maximal deformation. By theoretical modeling and simulation through Mathematica software, the relationship between free-end displacement of the actuator and actuator length, thickness (or thickness ratio) of two layers, difference of coefficient of thermal expansion between two layers, temperature variation and other parameters were studied. Simulation results showed that the deformation is greatly influenced by the thickness ratio of the two layers of the actuator. The deformation displacement reaches a maximum value with a specified thickness ratio. This study may provide valuable theoretical references for the experimental design of carbon nanotube composite actuators.

1A1-B05 McKibben型空気圧人工筋の動的特性のモデル化に向けた測定実験と解析

In this paper, we investigate the relation of dynamic property between tension, contraction velocity and length of McKibben pneumatic actuators via some experiments. Although robots with McKibben pneumatic actuators are widely applied to rehabilitation, it has not been well verified how do their properties exhibit stable motions from its simple structure. We analyze the relation of output tension, contraction velocity and length. From the results of experiments, we conclude that the dynamic property depending on velocity of the actuator has relation to both the velocity and the length of the actuator and the general form from the relation can be approximated as the plane.

Contact damping in microelectromechanical actuators

We examine the significance of the energy loss mechanisms active in electrostatic MEMS actuators. We find that the dominant loss mechanism changes depending on the actuator mode of operation. We find that the active mechanisms in the order of their significance are: fluid-structure interactions dominant for actuators operating in air, actuator-substrate interactions dominant for actuators in contact with a substrate under vacuum, and intrinsic loss mechanisms dominant for actuators in-flight under vacuum. Further, experimental results show that the quality factor of an electrostatic MEMS actuator drops drastically as the actuator first comes into line contact with a substrate. As the contact area expands along the actuator length, the quality factor increases. Measurements under 1 Torr vacuum show a three-fold increase in the quality factor as the contact area expands from a line to 30% of the actuator area. This increase in the quality factor is attributed to the drop in the contribution of friction forces into energy losses as contact expands and adhesion forces increase.