Large Actuation Forces Research Articles

Fluid-driven artificial muscles: bio-design, manufacturing, sensing, control, and applications

Developing artificial muscles that can replace biological muscles to accomplish various tasks is what we have long been aiming for. Recent advances in flexible materials and 3D printing technology greatly promote the development of artificial muscle technology. A variety of flexible material-based artificial muscles that are driven by different external stimuli, including pressure, voltage, light, magnetism, temperature, etc., have been developed. Among these, fluid-driven artificial muscles (FAMs), which can convert the power of fluid (gas or liquid) into the force output and displacement of flexible materials, are the most widely used actuation methods for industrial robots, medical instruments, and human-assisted devices due to their simplicity, excellent safety, large actuation force, high energy efficiency, and low cost. Herein, the bio-design, manufacturing, sensing, control, and applications of FAMs are introduced, including conventional pneumatic/hydraulic artificial muscles and several innovative artificial muscles driven by functional fluids. What’s more, the challenges and future directions of FAMs are discussed.

A Hybrid RF MEMS Switch Actuated by the Combination of Bidirectional Thermal Actuations and Electrostatic Holding

Radio frequency microelectro-mechanical system (RF MEMS) switches have received attention due to their higher performances at high frequencies compared with semiconductor-based switches. However, most of the previous MEMS switches used electrostatic actuation, which requires a high actuation voltage and has a low actuation force. Herein, we propose a new hybrid RF MEMS switch for low-voltage and low-power operation that is actuated by the combination of bidirectional thermal actuation and electrostatic holding. The proposed hybrid RF MEMS switch is actuated by the combination of two actuation mechanisms, i.e.: 1) bidirectional thermal actuation of the large force with low actuation voltage and 2) electrostatic holding for low-power consumption. The large actuation force enables high signal isolation at the “OFF” state of the switch because of the high initial gap between the signal line and the contact metal. Also, the bidirectional thermal actuator that enables upward actuation prevents any stiction of the membrane, which enhances both the power handling capability and the reliability of the actuator. In this work, we successfully fabricated the proposed RF MEMS switch and measured both its mechanical characteristics and RF characteristics. The required actuation voltage for the switch is less than 0.3 V for thermal actuation, and the required voltage for electrostatic holding is 15.4 V. The power consumption was measured as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.24~\mu \text{J}$ </tex-math></inline-formula> per switching. Also, we measured the RF characteristics, and the insertion loss of signals was 0.23 dB at 2.4 GHz, and the isolation of signals was 38.80 dB at 2.4 GHz.

Integrated Capacitance Sensing for Miniature Artificial Muscle Actuators

The use of McKibben pneumatic artificial muscles in surgical instruments is increasing due to their light weight, miniaturization potential, compliance and large actuation force. However, precise position and force control is still challenging due to the intrinsic muscle hysteresis. The present work proposes an easy-to-make integrated capacitance sensor for miniature artificial muscles by replacing two strands of the muscle braid with two conductive wires. By measuring the capacitance at high excitation frequency between both wires, the length of the muscle can be deduced. This new integrated sensor still allows the miniaturization of McKibben muscles. The miniature muscle with integrated capacitance sensing only loses 3% of its contraction length with regard to an original McKibben muscle. The sensor can predict the muscle length with a 0.31mm precision over 80% of the muscle total achievable contraction range in average.

A bundled staggering patterned pneumatic muscle actuator for improved working efficiency

Recently many new actuators have been developed in order to overcome the limitations of conventional actuators. The pneumatic artificial muscle actuator (PMA) is one of the typical examples. Although many studies have been conducted on the single PMA, there are still many limitations of the single PMA operation. The typical problems are low efficiency and limited force output. In order to overcome these limitations, the parallel band PMA (PPMA) consisting of a bundle of PMAs has been developed instead of the single PMA. However, due to the simple actuator driving principle, the parallel PMA causes another problem such as an interference between each PMAs placed side by side. In this paper, we proposed a new bundled staggering patterned pneumatic artificial muscle actuator (SPMA) to resolve the interference problem. The SPMA consists of a bundle of staggered pattern actuators that ensures the large actuation force with a small form factor. To verify the performance of the proposed SPMA, output force and operating range have been compared to those of the existing PPMA by conducting simulation and experiments.

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.

Improving Legged Robot Hopping by Using Coupling-Based Series Elastic Actuation

This paper investigates the use of discrete couplings in series elastic actuation in order to increase variability of dynamic legged robot locomotion. The couplings are capable of engaging and disengaging the connection between an actuation element and a mechanical spring in real time such that the series elasticity could be used to load, store, and retain energy, and release it in the necessary timings. Such a mechanism is particularly beneficial for hopping or running locomotion, because it allows preloading of spring in flight phase. Through a series of simulated and physical robot experiments, this paper explains how such variability of actuation dynamics can improve performance of hopping locomotion under the limitation of actuator forces, as well as the controllability of behaviors. Compared to the conventional hopping robots, the proposed control strategy makes locomotion robots capable of higher hopping without larger actuation forces, and changing behaviors for more aggressive manoeuvres.

Hindwings of insects as concept generator for hingeless foldable shading systems

Hingeless shading systems inspired by nature are increasingly the focus of architectural research. In contrast to traditional systems, these compliant mechanisms can reduce the amount of maintenance-intensive parts and can easily be adapted to irregular, doubly curved, facade geometries. Previous mechanisms rely merely on the reversible material deformation of composite structures with almost homogeneous material properties. This leads to large actuation forces and an inherent conflict between the requirements of movement and the capacity to carry external loads. To enhance the performance of such systems, current research is directed at natural mechanisms with concentrated compliance and distinct hinge zones with high load-bearing capacity. Here, we provide insights into our biological findings and the development of a deployable structure inspired by the Flexagon model of hindwings of insects in general and the hierarchical structure of the wing cuticle of the shield bug (Graphosoma lineatum). By using technical fibre-reinforced plastics in combination with an elastomer foil, natural principles have been partially transferred into a multi-layered structure with locally adapted stiffness. Initial small prototypes have been produced in a vacuum-assisted hot press and sustain this functionality. Initial theoretical studies on test surfaces outline the advantages of these bio-inspired structures as deployable external shading systems for doubly curved facades.

Modeling and test of a kinaesthetic actuator based on MR fluid for haptic applications.

Haptic display units have been widely used for conveying button sensations to users, primarily employing vibrotactile actuators. However, the human feeling for pressing buttons mainly relies on kinaesthetic sensations (rather than vibrotactile sensations), and little studies exist on small-scale kinaesthetic haptic units. Thus, the primary goals of this paper are to design a miniature kinaesthetic actuator based on Magneto-Rheological (MR) fluid that can convey various button-clicking sensations and to experimentally evaluate its haptic performance. The design focuses of the proposed actuator were to produce sufficiently large actuation forces (resistive forces) for human users in a given size constraint and to offer a wide range of actuation forces for conveying vivid haptic sensations to users. To this end, this study first performed a series of parametric studies using mathematical force models for multiple operating modes of MR fluid in conjunction with finite element electromagnetism analysis. After selecting design parameters based on parametric studies, a prototype actuator was constructed, and its performance was evaluated using a dynamic mechanical analyzer. It measured the actuator's resistive force with a varying stroke (pressed depth) up to 1 mm and a varying input current from 0 A to 200 mA. The results show that the proposed actuator creates a wide range of resistive forces from around 2 N (off-state) to over 9.5 N at 200 mA. In order to assess the prototype's performance in the terms of the haptic application prospective, a maximum force rate was calculated to determine just noticeable difference in force changes for the 1 mm stoke of the actuator. The results show that the force rate is sufficient to mimic various levels of button sensations, indicating that the proposed kinaesthetic actuator can offer a wide range of resistive force changes that can be conveyed to human operators.

Approaches for Reduced-Order Modeling of Electrically Actuated von-Karman Microplates

This article presents and compares different approaches to develop reduced-order models for the nonlinear von-Karman rectangular microplates actuated by nonlinear electrostatic forces. The reduced-order models aim to investigate the static and dynamic behavior of the plate under small and large actuation forces. A fully clamped microplate is considered. Different types of basis functions are used in conjunction with the Galerkin method to discretize the governing equations. First, we investigate the convergence with the number of modes retained in the model. Then for validation purpose, a comparison of the static results is made with the results calculated by a nonlinear finite element model. The linear eigenvalue problem for the plate under the electrostatic force is solved for a wide range of voltages up to pull-in. Results among the various reduced-order modes are compared and are also validated by comparing to results of the finite-element model. Further, the reduced-order models are employed to capture the forced dynamic response of the microplate under small and large vibration amplitudes. Comparison of the different approaches is made for this case.

Novel Arrangements for High Performance and Durable Dielectric Elastomer Actuation

This paper advances the design of Rod Pre-strained Dielectric Elastomer Actuators (RP-DEAs) in their capability to generate comparatively large static actuation forces with increased lifetime via optimized electrode arrangements. RP-DEAs utilize thin stiff rods to constrain the expansion of the elastomer and maintain the in-plane pre-strain in the rod longitudinal direction. The aim is to study both the force output and the durability of the RP-DEA. Initial design of the RP-DEA had poor durability, however, it generated significantly larger force compared with the conventional DEA due to the effects of pre-strain and rod constraints. The durability study identifies the in-electro-active-region (in-AR) lead contact and the non-uniform deformation of the structure as causes of pre-mature failure of the RP-DEA. An optimized AR configuration is proposed to avoid actuating undesired areas in the structure. The results show that with the optimized AR, the RP-DEA can be effectively stabilized and survive operation at least four times longer than with a conventional electrode arrangement. Finally, a Finite Element simulation was also performed to demonstrate that such AR design and optimization can be guided by analyzing the DEA structure in the state of pre-activation.

Using Pot-Magnets to Enable Stable and Scalable Electromagnetic Tactile Displays.

We present the design, fabrication, characterization, and psychophysical testing of a scalable haptic display based on electromagnetic (EM) actuators. The display consists of a 4 × 4 array of taxels, each of which can be in a raised or a lowered position, thus generating different static configurations. One of the most challenging aspects when designing densely-packed arrays of EM actuators is obtaining large actuation forces while simultaneously generating only weak interactions between neighboring taxels. In this work, we introduce a lightweight and effective magnetic shielding architecture. The moving part of each taxel is a cylindrical permanent magnet embedded in a ferromagnetic pot, forming a pot-magnet. An array of planar microcoils attracts or repels each pot-magnet. This configuration reduces the interaction between neighboring magnets by more than one order of magnitude, while the coil/magnet interaction is only reduced by 10 percent. For 4 mm diameter pins on an 8 mm pitch, we obtained displacements of 0.55 mm and forces of 40 mN using 1.7 W. We measured the accuracy of human perception under two actuation configurations which differed in the force versus displacement curve. We obtained 91 percent of correct answers in pulling configuration and 100 percent in pushing configuration.

High-performance transmission mechanism for robotic applications

High-performance transmission mechanisms are essential for robotic applications, since they affect overall weight, motion speed and actuation force. An innovative high-performance transmission mechanism with a single motor has been designed, analyzed and developed. The proposed mechanism can achieve a high-speed motion in Mode I and generate a large actuation force in Mode II. It is used to actuate an anthropomorphic finger, allowing it to produce fast bending motions and large contact forces, but it could be adopted in all those applications where high speed and large force are required. Detailed performance analysis is presented as well as experimental measurements showing successful fulfilment of requirements. The maximum velocity of the transmission mechanism is about 96.1mm/s, and the measured actuation force is about 377N.

Control Power Reduction and Frequency Bandwidth Enlargement of Robotic Legs by Nonlinear Resonance

Legged robots are regarded as a new means for transportation at uneven terrains and dangerous situations. In particular, quadruped robots that are capable of high-speed running are receiving great attention due to their superior mobility. The legs of such high-speed running robots rapidly repeat the swing and stance motions. Therefore, the legs of high-speed running robots should exhibit low impedance and friction for fast swing motions, while it is required to produce a significantly large actuation force for propulsion of the robot body in a stance phase. For this purpose, a direct-driven actuation mechanism was proposed for the Cheetaroid robot in our previous work. In this paper, in order to further enlarge the frequency bandwidth of the leg module and to reduce the required control (i.e., actuation) power, a dual-stage spring is designed to introduce a motion-adaptive resonance into the system. The structure and parameters of the dual-stage spring are determined in an optimal manner to minimize the control power. The resonance due to the dual-stage spring occurs only during high-speed locomotion, since the overall mechanism is designed such that the spring force is applied to the leg for high-speed locomotion only. The proposed system is verified by simulation studies and experimental results in this paper.

Electrostatic comb-drive actuator for MEMS relays/switches with double-tilt comb fingers and tilted parallelogram beams

MEMS relays/switches have been some of the most interesting hotspots in all kinds of MEMS devices for decades. Electrostatic actuation is the most popular actuation principle. A novel in-plane electrostatic comb-drive actuator for MEMS relays/switches was designed and fabricated. Double-tilt comb fingers and tilted parallelogram beams were proposed to achieve the requirements of MEMS relays/switches. The restoring force curve, actuation force curve, and pull-in voltage were measured. Double-tilt fingers were found to have a larger actuation force than traditional rectangular fingers, and the pull-in voltage was decreased by 25% by replacing rectangular fingers with double-tilt fingers. In addition, tilted parallelogram beams provided a convex upward restoring force curve, which was more suitable for MEMS relays/switches than the conventional force curve.

Development of Miniature Stewart Platform Using TiNiCu Shape-Memory-Alloy Actuators

A Stewart platform is a parallel manipulator robot that is able to perform three linear movements, lateral, longitudinal, and vertical, and three rotations, pitch, yaw, and roll. This paper reports a 30 mm × 30 mm × 34 mm miniscale Stewart platform using TiNiCu shape-memory-alloy (SMA) actuators. The proposed Stewart platform possesses various advantages, such as large actuation force and high robustness with a simple mechanical structure. This Stewart platform uses four SMA actuators and four bias springs and performs a linearz-axis movement and tilting motions. The SMA actuators are activated by passing a current through the SMA wires using a heating circuit that generates a pulse width modulation (PWM) signal. This signal is varied to control the level of the displacement and tilting angle of the platform. The tilting direction depends on the SMA actuator that is activated, while all four SMA actuators are activated to achieve the linearz-axis movement. Each SMA actuator exerts a maximum force of 0.6 N at PWM duty cycle of 100%. The fabricated miniature Stewart platform yields a full actuation of 12 mm in thez-axis at 55°C, with a maximum tilting angle of 30° in 4 s.

Low voltage actuator using ionic polymer metal nanocomposites based on a miscible polymer blend

An actuator based on a miscible [P(VDF-TrFE)]/PVP/PSSA polymer blend membrane shows a large actuation displacement and force at a low voltage of 1 V compared to those of commercial Nafion and PVDF based actuators.

Multifunctional atomic force microscope cantilevers with Lorentz force actuation and self-heating capability

This paper reports the development of microcantilevers capable of self-heating and Lorentz-force actuation, and demonstrates applications to thermal topography imaging. Electrical current passing through a U-shaped cantilever in the presence of a magnetic field induces a Lorentz force on the cantilever free end, resulting in cantilever actuation. This same current flowing through a resistive heater induces a controllable temperature increase. We present cantilevers designed for large actuation forces for a given cantilever temperature increase. We analyze the designs of two new cantilevers, along with a legacy cantilever design. The cantilevers are designed to have a spring constant of about 1.5 N m−1, a resonant frequency near 100 kHz, and self-heating capability with temperature controllable over the range 25–600 °C. Compared to previous reports on self-heating cantilevers, the Lorentz–thermal cantilevers generate up to seven times as much Lorentz force and two times as much oscillation amplitude. When used for thermal topography imaging, the Lorentz–thermal cantilevers can measure topography with a vertical resolution of 0.2 nm.

Review on miniaturized paraffin phase change actuators, valves, and pumps

During the last 15 years, miniaturized paraffin actuation has evolved through the need of a simple actuation principle, still able to deliver large strokes and high actuation forces at small scales. This is achieved by the large and rather incompressible volume expansion associated with the solid-to-liquid phase transition of paraffin. The common approach has been to encapsulate the paraffin by a stiff surrounding that directs the volume expansion toward a flexible membrane, which deflects in a directed stroke. However, a number of alternative methods have also been used in the literature. The most common applications to this date have been switches, positioning actuators, and microfluidic valves and pumps. This review will treat the historical background, as well as the fundamentals in paraffin actuation, including material properties of paraffin. Besides reviewing the three major groups of paraffin actuator applications—actuators, valves, and pumps—the modelling done on paraffin actuation will be explored. Furthermore, a section focusing on fabrication of paraffin microactuators is also included. The review ends with conclusions and outlook of the field, identifying unexplored potential of paraffin actuation.

Fabrication of 3D soft morphing structure using shape memory alloy (SMA) wire/polymer skeleton composite

The most common method of fabricating a smart structure using a shape memory alloy (SMA) is to create an SMA-embedded structure. However, if the structure is too thick, actuation is decreased significantly. Hence, SMA-embedded structures and robots are usually thin, leaving no space for additional parts. In this research, an SMA-embedded soft morphing structure with large thickness and deformation was developed. A skeletal structure and hinges, which could amplify actuating displacement, were used to increase the overall actuation by maximizing the actuation in a specific area. Also payload of prototype is enough to lift additional weight. A prototype of the design was fabricated via rapid prototyping (RP) and casting. The performance of the prototype was evaluated, and large deformation and actuation force were demonstrated. A cell phone robot was suggested as an application, and the resulting fabricated prototype exhibited crawling actuation.

Large actuation and high dielectric strength in metallized dielectric elastomer actuators

Metal films are seldom used as compliant electrodes for dielectric elastomer actuator (DEA) because they tend to restrain deformation of soft dielectrics. This work showed that silver film electrodes formed by electroless deposition (ELD) are indeed stretchable, and the DEA using ELD silver electrodes is able to generate an actuation up to 50% areal strain. Such ELD silver electrodes can self-heal, remain conductive at up to 33% uni-axial strain, and do not stiffen the dielectric layer as much as the sputtered silver electrodes. This metallized DEA can sustain a high breakdown field up to 350 MV/m, which is good for generating a large actuation force.