Actuation Modes Research Articles

Active Periodic Magnetic Nanostructures with High Aspect Ratio and Ultrahigh Pillar Density.

Magnetically actuated micro/nanoscale pillars have attracted significant research interest recently because of their dynamic properties. These structures can be used for various applications, such as dry adhesion, cell manipulation, and sensors or actuators in microfluidics. Magnetically actuated structures can be fabricated by mixing magnetic particles and polymers to yield a favorable combination of magnetic permeability and mechanical compliance. However, the pillar density of demonstrated structures is relatively low, which limits the potential applications in active surface manipulation of microscale objects. Here, we demonstrate active periodic nanostructures with a pillar density of 0.25 pillar/μm2, which is the highest density for magnetically actuated pillars so far. Having a structure period of 2 μm, diameter of 600 nm, and high aspect ratio of up to 11, this structure can be magnetically actuated with a displacement of up to 200 nm. The behaviors of the pillars under various cyclic actuation modes have been characterized, demonstrating that the actuation can be well controlled. This work can find potential applications in particle manipulation and tunable photonic elements.

Liquid Crystal Networks on Thermoplastics: Reprogrammable Photo-Responsive Actuators.

Arbitrary shape (re)programming is appealing for fabricating untethered shape‐morphing photo‐actuators with intricate configurations and features. We present re‐programmable light‐responsive thermoplastic actuators with arbitrary initial shapes through spray‐coating of polyethylene terephthalate (PET) with an azobenzene‐doped light‐responsive liquid crystal network (LCN). The initial geometry of the actuator is controlled by thermally shaping and fixing the thermoplastic PET, allowing arbitrary shapes, including origami‐like folds and left‐ and right‐handed helicity within a single sample. The thermally fixed geometries can be reversibly actuated through light exposure, with fast, reversible area‐specific actuation such as winding, unwinding and unfolding. By shape re‐programming, the same sample can be re‐designed and light‐actuated again. The strategy presented here demonstrates easy fabrication of mechanically robust, recyclable, photo‐responsive actuators with highly tuneable geometries and actuation modes.

Liquid Crystal Networks on Thermoplastics: Reprogrammable Photo‐Responsive Actuators

AbstractArbitrary shape (re)programming is appealing for fabricating untethered shape‐morphing photo‐actuators with intricate configurations and features. We present re‐programmable light‐responsive thermoplastic actuators with arbitrary initial shapes through spray‐coating of polyethylene terephthalate (PET) with an azobenzene‐doped light‐responsive liquid crystal network (LCN). The initial geometry of the actuator is controlled by thermally shaping and fixing the thermoplastic PET, allowing arbitrary shapes, including origami‐like folds and left‐ and right‐handed helicity within a single sample. The thermally fixed geometries can be reversibly actuated through light exposure, with fast, reversible area‐specific actuation such as winding, unwinding and unfolding. By shape re‐programming, the same sample can be re‐designed and light‐actuated again. The strategy presented here demonstrates easy fabrication of mechanically robust, recyclable, photo‐responsive actuators with highly tuneable geometries and actuation modes.

Analysis of laminated piezoelectric composite plates using an inverse hyperbolic coupled plate theory

This paper presents a non-polynomial coupled plate theory for smart composite structures employing inverse hyperbolic displacement and electric potential functions. The theory is utilized towards analysis of composite piezoelectric plates operating in sensor and actuator modes. Particularly, the following three cases are studied: (i) passive laminated composite structure, (ii) composite piezoelectric plate actuator and (iii) unimorph and bimorph piezoelectric plate sensors. Analytical solutions are obtained for simply supported plates under static electrical and mechanical loads. These results are validated with existing 3D elasticity solutions and compared with other plate theory solutions. Furthermore, parametric studies are performed to determine the effect of loading, span-to-thickness ratio and lamination sequence on the response of the piezoelectric plate. Finally, the theory is applied to a transverse shear sensing device which utilizes transverse shear-electric field coupling in piezoelectric materials. This effect is often ignored in literature.It is observed that the maximum percentage error of the present theory, when compared with 3D results, is less than 3%, which is lower than other higher order plate theories.

Autonomous Microrobotic Manipulation Using Visual Servo Control.

This describes the application of a visual servo control method to the microrobotic manipulation of polymer beads on a two-dimensional fluid interface. A microrobot, actuated through magnetic fields, is utilized to manipulate a non-magnetic polymer bead into a desired position. The controller utilizes multiple modes of robot actuation to address the different stages of the task. A filtering strategy employed in separation mode allows the robot to spiral from the manipuland in a fashion that promotes the manipulation positioning objective. Experiments demonstrate that our multiphase controller can be used to direct a microrobot to position a manipuland to within an average positional error of approximately 8 pixels (64 µm) over numerous trials.

Design and Tension Modeling of a Novel Cable-Driven Rigid Snake-Like Manipulator

In this study, a cable-driven snake-like manipulator with high load capacity and end-positioning accuracy is designed for applications in complex and narrow environments. The Hooke joint-like two degree-of-freedom joint design and the full actuation mode enhanced the rigidity of the robot. The modular link design increased the local flexibility of the robot. Because the cable tension cannot be ignored under high load on the basis of the kinematics model, a cable tension model is established based on rigid body static equilibrium to describe the relationship between posture and cable tension. This provided a foundation for follow-up studies on obstacle avoidance path planning with optimized tension. At the same time, in order to improve the response speed of the motor position controller to the tension change, this study introduces both the tension as the reference model input and the system state variable into the adaptive control method based on model identification and reinforcement learning. The kinematics model and the cable tension model were validated by experiments on the prototype. The practical results of the two adaptive control methods were compared and the result shows that the method based on model identification has a better effect.

Sensitivity of twin boundary movement to sample orientation and magnetic field direction in Ni-Mn-Ga

When applying a magnetic field parallel or perpendicular to the long edge of a parallelepiped Ni-Mn-Ga stick, twin boundaries move instantaneously or gradually through the sample. We evaluate the sample shape dependence on twin boundary motion with a micromagnetics computational study of magnetic domain structures and their energies. Due to the sample shape, the demagnetization factor varies with the direction of the external magnetic field. When the external magnetic field is applied perpendicular to the long edge of the sample, i.e. in the direction in which the demagnetizing field is highest, the magnetic energy intermittently increases when the strength of the applied magnetic field is low. This energy gain hinders the twin boundary motion and results in a gradual switching, i.e. a gradual magnetization reversal as the applied magnetic field is increased. The formation of 180° magnetic domains offsets this effect partially. In contrast, when the applied magnetic field is parallel to the long edge of the sample, i.e. in the direction in which the demagnetizing field is lowest, the energy decreases with each subsequent magnetization domain reversal and the twin boundary moves instantaneously with ongoing switching. The actuation mode with the field parallel to the long sample edge lends itself for on-off actuators whereas the actuation mode with the field perpendicular to the long sample edge lends itself to gradual positioning devices.

A modified capacitance model of shunt micro switch based on MEMS technology in actuation mode: simulation and analytical study

In this paper, a modified mathematical model for capacitance analysis of micro-electro-mechanical switch based on Agarwal’s Model has been presented. This model is applicable in the condition when the micro-switch is supplied with electrostatic voltages. Effects of perforated structure and fringe field are accounted for accurate capacitance analysis of the micro-switch. The switch parameters such as thickness of dielectric material and actuation voltage for micro switch have also been considered in the present study. The switch is designed and simulated using COMSOL Multiphysics tool and results obtained through simulations have been validated using MATLAB software. It is found that the results obtained by simulation and mathematical model are in agreement closely for the presented study. This model could be used for evaluation of capacitance in downstate of the micro-switch for symmetric structures.

Active Control for Wall Drag Reduction: Methods, Mechanisms and Performance

Active reducing the skin friction drag of vehicles (such as missiles, rockets, aircraft, high-speed train, submarines, etc.) during the navigation process can improve the respond speed effectively, save the energy consumption and increase the endurance time. As lots of active drag reduction methods were proposed for different applications, a review article is urgently needed to guide researchers to choose suitable methods for their pre-research. In this review, the generation mechanisms of skin friction drag under the action of disturbing the turbulent boundary layer are discussed, and the main active drag reduction methods are summarized. According to the actuation modes, we divided the active drag reduction methods into: drag reduction based on wall motion; drag reduction based on volume force control; drag reduction based on wall deformation and drag reduction based on micro vibration generated by piezoelectric actuator. The development status, drag reduction mechanisms, typical structures and drag reduction performance of each active drag reduction method are discussed and summarized in this work, which can provide preliminary research references for those who are engaged in the research of active wall drag reduction.

Material properties identification of a piezoelectric beam using inverse method

This paper presents an inverse method for material properties identification of a piezoelectric beam (piezoelectric charge and relative dielectric coefficients) using a wavelet-based neural network as an inverse tool. The identification analysis is carried out by using two approaches. In the first approach, i.e. sensor mode analysis, the input data for wavelet-based neural network training are measured voltages at several specific points on the beam's top surface resulting from the applied beam tip deflection. In the second approach, i.e. actuation mode analysis, the input data are values of the beam tip deflection caused by applying voltage on the beam's top surface. In this study, the input parameters employed to train the wavelet-based neural network are obtained using the finite element method. The identification results are compared with those of some conventional neural networks including radial basis function and multilayer perceptron. The results show that the proposed neural network is an efficient tool in the material properties identification problem.

Interval sliding mode observer-based fault accommodation for non-minimum phase LPV systems with online control allocation

This paper proposes an interval sliding mode observer (ISMO)-based sliding mode actuator fault accommodation (FA) framework for non-minimum phase linear-parameter-varying (LPV) systems involving online control allocation (CA) problem. Firstly, a specifically designed coordinate transformation is introduced to deal with the non-minimum phase issue. Then, an ISMO is proposed to estimate the set of admissible values for the states of the faulty LPV systems, which combines the interval observer technique and the sliding mode observer technique. A fault-tolerant control (FTC) law with an online CA scheme is subsequently designed by stabilising the ISMO instead of the original faulty LPV system. Finally, a simulation for an inverter used in China Railway High-speed (CRH) is presented to illustrate the effectiveness of the proposed framework.

3D Printing of Interdigitated Dielectric Elastomer Actuators

AbstractDielectric elastomer actuators (DEAs) are soft electromechanical devices that exhibit large energy densities and fast actuation rates. They are typically produced by planar methods and, thus, expand in‐plane when actuated. Here, reported is a method for fabricating 3D interdigitated DEAs that exhibit in‐plane contractile actuation modes. First, a conductive elastomer ink is created with the desired rheology needed for printing high‐fidelity, interdigitated electrodes. Upon curing, the electrodes are then encapsulated in a self‐healing dielectric matrix composed of a plasticized, chemically crosslinked polyurethane acrylate. 3D DEA devices are fabricated with tunable mechanical properties that exhibit breakdown fields of 25 V µm−1 and actuation strains of up to 9%. As exemplars, printed are prestrain‐free rotational actuators and multi‐voxel DEAs with orthogonal actuation directions in large‐area, out‐of‐plane motifs.

4D Printing of a Liquid Crystal Elastomer with a Controllable Orientation Gradient.

Liquid crystal elastomers (LCEs), a class of soft materials capable of a large and reversible change in the shape under the trigger of external stimuli, can be fabricated into diverse architectures with complicated deformation modes through four-dimensional (4D) printing. However, the printable LCE ink is only in the form of monomeric precursors and the deformation mode is limited to contraction/extension deformation. Herein, we report a novel approach to break through these limitations. We achieved 4D printing of a single-component liquid crystal polymer ink in its isotropy state through direct ink writing (DIW) technology. The drawing force imposed by the movement of nozzle in the extruded printing process was able to align the mesogen units along the specific printing path. An orientation gradient perpendicular to the printing direction was obtained due to the existence of a temperature gradient between the two sides of printed samples and could be further fixed by post-photo-cross-linking treatment through the dimerizable groups in the LCE, realizing a new actuation mode in the field of extrusion-based printing of LCE actuators. The printed film was able to change reversibly from a strip to a tightly hollow cylinder and could reversibly lift up an object with roughly 600 times its own weight. The orientation gradient can be patterned through liquid-assistant printing or programmed structure design to integrate both bending and contraction actuation modes on the same printed sample, leading to complex deformation and two-dimensional (2D) planar porous structure to three-dimensional (3D) porous cylinder transition. This study opens up a new prospect to directly print a wide variety of LCE actuators with versatile actuation modes.

Electrically controlled liquid crystal elastomer-based soft tubular actuator with multimodal actuation.

Soft tubular actuators can be widely found both in nature and in engineering applications. The benefits of tubular actuators include (i) multiple actuation modes such as contraction, bending, and expansion; (ii) facile fabrication from a planar sheet; and (iii) a large interior space for accommodating additional components or for transporting fluids. Most recently developed soft tubular actuators are driven by pneumatics, hydraulics, or tendons. Each of these actuation modes has limitations including complex fabrication, integration, and nonuniform strain. Here, we design and construct soft tubular actuators using an emerging artificial muscle material that can be easily patterned with programmable strain: liquid crystal elastomer. Controlled by an externally applied electrical potential, the tubular actuator can exhibit multidirectional bending as well as large (~40%) homogenous contraction. Using multiple tubular actuators, we build a multifunctional soft gripper and an untethered soft robot.

Electrically tunable fast and reversible structural coloration of two-dimensional photonic crystals

Photonic crystals (PCs) can regulate the propagation of light through them and show visible structural coloration when illuminated by white light. We achieve fast (response time less than 1 s) and reversible structural coloration of two dimensional (2D) PCs by integrating them with a dielectric elastomer actuator (DEA). For the in-plane actuation mode of DEA, we study the diffraction angle dependent structural coloration of 2D PCs vertically illuminated by white light under different actuation voltages experimentally and theoretically. We also optimize the DEA by considering its snap-through instability and actuation performance in our theoretical model. Furthermore, we first demonstrate that the out-of-plane actuation mode of DEA can also effectively tune the structural coloration of integrated 2D PCs. The tunable 2D PCs realized by DEA have applications in color changeable soft skin for camouflage and smart optical devices, such as adjustable optical grating.

Symmetry-Breaking Actuation Mechanism for Soft Robotics and Active Metamaterials.

Magnetic-responsive composites that consist of a soft matrix embedded with hard-magnetic particles have recently been demonstrated as robust soft active materials for fast-transforming actuation. However, the deformation of the functional components commonly attains only a single actuation mode under external stimuli, which limits their capability of achieving tunable properties. To greatly enhance the versatility of soft active materials, we exploit a new class of programmable magnetic-responsive composites incorporated with a multifunctional joint design that allows asymmetric multimodal actuation under an external stimulation. We demonstrate that the proposed asymmetric multimodal actuation enables a plethora of novel applications ranging from the basic one-dimensional/two-dimensional (2D) active structures with asymmetric shape-shifting to biomimetic crawling robots, swimming robots with efficient dynamic performance, and 2D metamaterials with tunable properties. This new asymmetric multimodal actuation mechanism will open up new avenues for the design of next-generation multifunctional soft robots, biomedical devices, and acoustic metamaterials.

Design, Analysis, and Control of an XY Parallel Nanomanipulator With Multiple Actuation Modes

In this article, an XY parallel nanomanipulator with multiple actuation modes (PNM) is proposed, analyzed, and tested. Arising from the parallel mechanism of a machine tool, the PNM is featured with multiple actuation modes and configurable parallel chains. Analytical models with different approaches are established to compressively understand the presented PNM. Workspace distribution and reachability are analyzed under different actuation modes in a detail manner. The models also reveal the combined effect of multiple modes and the influence of chain arrangement on the achieved workspace and the stiffness property. Dynamic analysis indicates that PNM is with high bandwidth as well. Finite element analysis and experiments were carried out to verify the above analysis results. Finally, a compound decoupled controller is especially designed for PNM to achieve the decoupling motion control. Planar trajectory-tracking tests demonstrate the feasibility and effectiveness of proposed control method.

LISA Pathfinder Performance Confirmed in an Open-Loop Configuration: Results from the Free-Fall Actuation Mode.

We report on the results of the LISA Pathfinder (LPF) free-fall mode experiment, in which the control force needed to compensate the quasistatic differential force acting on two test masses is applied intermittently as a series of “impulse” forces lasting a few seconds and separated by roughly 350 s periods of true free fall. This represents an alternative to the normal LPF mode of operation in which this balancing force is applied continuously, with the advantage that the acceleration noise during free fall is measured in the absence of the actuation force, thus eliminating associated noise and force calibration errors. The differential acceleration noise measurement presented here with the free-fall mode agrees with noise measured with the continuous actuation scheme, representing an important and independent confirmation of the LPF result. An additional measurement with larger actuation forces also shows that the technique can be used to eliminate actuation noise when this is a dominant factor.

Shear actuation mechanism and shear-based actuation capability of an obliquely reinforced piezoelectric fibre composite in active control of annular plates

In this work, the mechanism of shear actuation and the shear mode actuation capability of an obliquely reinforced piezoelectric fibre composite are investigated in active control of annular plates. This piezoelectric fibre composite is capable of providing extensional and shear actuation forces in a transverse plane of its Cartesian material coordinate system, and these actuation forces are utilized for shear-based actuation of an annular plate by embedding the piezoelectric fibre composite patches at the core of the plate. First, the mechanism of shear actuation of the annular sandwich plate is analysed, and it mainly reveals an adverse effect of the coupling between the transverse normal strain and shear stresses on the shear actuation forces, even though a good shear-actuated bending deformation of the annular plate is observed. So, next, the effectiveness of the piezoelectric fibre composite patches in shear mode active control of flexural vibration of the annular sandwich plate is investigated, where a shear-based feedback control arrangement is proposed for effective active control of the plate according to the velocity feedback control law. The analysis reveals an indicative shear actuation capability of this obliquely reinforced piezoelectric fibre composite, and thus it may be a potential material for shear-based active control of vibration of annular plates.

Active vibration control of a polyvinylidene fluoride laminated membrane plate mirror

Lightweight optical mirrors usually play key roles in aerospace and optical structural systems applied to space telescopes, radars, solar collectors, communication antennas, etc. Due to their high flexibility and low damping properties, external excitations such as orbital maneuver may induce unexpected oscillations and thus reduce their working performance. Active vibration control is therefore essential for the lightweight optical mirror systems. In this spirit, a lightweight mirror structronic system with a linear quadratic optimal controller is presented. The mirror is modeled as a membrane plate with pretension and distributed polyvinylidene fluoride sensors and actuators. The sensing sensitivity of the piezoelectric (PVDF) sensors and the modal actuation factor of the PVDF actuators are derived. The state-space equations are established and the feedback control gains between sensing and control signals are obtained. Sensor and actuator of different shape, size, and position are employed to actively control the first four natural modes of the mirror. The influences of mode order, pretension, and the two weighting factors Q and R on the control performance are also investigated. Analytical results in this paper could guide the design and layout of the PZT sensor and actuator on lightweight membrane plate mirrors.