Bistable Mechanism Research Articles

Erratum to “Performance analysis of a raft-type wave energy converter with a torsion bi-stable mechanism” [Energy 227 (2021) 120388

Erratum to “Performance analysis of a raft-type wave energy converter with a torsion bi-stable mechanism” [Energy 227 (2021) 120388

A theory for bistable vibration isolators

Bistable structures have been widely used for vibrational energy harvesting, the corresponding potential vibration isolation application has also received attention recently. This paper presents a general theory for bistable vibration isolators (BVIs) and reveals the bistable vibration isolation mechanism. The nonlinear restoring force and potential energy are obtained through analyzing the general model of BVIs with “three-spring” quasi-zero stiffness (QZS) model. The generalized force and displacement transmissibilities of BVIs are deduced. The theoretical and numerical transmissibilities are calculated. A feasible model of BVIs is applied to quantitatively verify the advantages of BVIs in vibration isolation. The results demonstrate that BVIs can provide negative and softening stiffness characteristics to broaden the isolation band, and a shallower potential barrier and intrawell oscillation are beneficial to vibration isolation. BVIs should be carefully designed to avoid the interwall oscillation to ensure a considerable vibration isolation performance due to the effects of initial equilibrium position. This paper provides a guideline for the design, analysis, and optimization of BVIs.

Bistable Valves for MR Fluid-Based Soft Robotic Actuation Systems

Magnetorheological (MR) fluids have unique characteristics that make them well-suited for soft robotic actuation systems. Two particularly interesting applications are bistable valves and force-amplified grippers, which only require power to switch states. These design topologies can not only make MR fluid actuation systems more efficient, but can also allow them to be constructed from entirely compliant materials. In this work, we describe new bistable valve mechanism concepts; the first design is constructed using traditional rigid components while the second design is composed of entirely compliant materials. This includes liquid metal coils, compliant magnetic composites, and silicone flexures. The MR valve concept is modeled using magnetic field theory and scaling laws are determined from MR fluid holding pressure experiments. Then, experimental results are used to show that rough tubing surfaces can withhold an average of 5× more pressure than smooth tubing surfaces. Both rigid and compliant valve topologies are then constructed and tested, showing the ability to hold up to 88 kPa and 37 kPa of pressure, respectively. These valves are then implemented for controlling a soft multi-fingered PneuNet gripper and a new force-amplified MR fluid gripper, which has the ability to provide up to 324$\%$ higher load capacity than purely pressure-driven mechanisms (with the addition of a 25 mm diameter and 12.7 mm thick Nd-Fe-B magnet). Finally, we show how these bistable valves and grippers can be used for grasping and manipulation applications.

Design of an Anti-Slip Mechanism for Wheels of Step Climbing Robots

This manuscript presents a shape memory alloy (SMA) actuated anti-slip mechanism for the wheels of step climbing robots. The proposed mechanism comprises three kinematic chains considering the Lazy Tong and the bi-stable four-bar mechanism. Chain 1 of the mechanism is used to clamp on the edges of the stairs to avoid slipping. The second chain of the mechanism is used to switch the mechanism between two stable positions, i.e., open position and closed position, of chain 1. For activating the mechanism, the third chain is employed which is based on SMA wire. Furthermore, the mechanism is designed to achieve passive switching from the open position to the closed position. Equations are developed to determine the dimensions of various members. Using those parameters, a 3D model of the proposed mechanism is developed. Stress analysis is performed and the model is found to be safe under a load of 250 N with a factor of safety of 3.025. The mechanism is attached to either side of a wheel of the outer radius of 290 mm. To analyze the kinematics of the mechanism, a three-dimensional model in MSC Adams is developed and studied. The force required by SMA actuator is found to be less than 5 N. The proposed mechanism may be used for various unmanned robotic systems while mitigating step-like obstacles in the path.

Programmable Multistable Perforated Shellular.

Developing bistable metamaterials has recently offered a new design paradigm for deployable structures and reusable dampers. While most bistable mechanisms possess inclined/curved struts, a new 3D multistable shellular metamaterial is developed by introducing delicate perforations on the surface of Schwarz's Primitive shellular, integrating the unique properties of shellular materials such as high surface area, stiffness, and energy absorption with the multistability concept. Denoting the fundamental snapping part by motif, certain shellular motifs with elliptical perforations exhibit mechanical bistability. To bring the concept of multistability to a single motif, multistable shellular motifs are developed by introducing multilayer staggered perforations that form hinges and facilitate local instability. Adopting an n-layer staggered perforation (n hinges) design leads to a maximum 2n-1 stable states within one shellular motif during loading and unloading. Three-directional multistable shellulars are attained by extending the perforation design in three orthogonal directions. Harnessing snap-through and snap-back behaviors and self-contact, the introduced multistable perforated shellulars exhibit strong rigidity both in loading and unloading, and enhanced energy dissipation. The introduced design strategy opens up new horizons for creating multidirectional multistable metamaterials with load bearing capabilities for applications in soft robotics, shape-morphing architectures, and reusable and deployable energy absorbers/dampers.

Homeostatic model of human thermoregulation with bi-stability

All homoiothermic organisms are capable of maintaining a stable body temperature using various negative feedback mechanisms. However, current models cannot satisfactorily describe the thermal adaptation of homoiothermic living systems in a physiologically meaningful way. Previously, we introduced stress entropic load, a novel variable designed to quantify adaptation costs, i.e. the stress of the organism, using a thermodynamic approach. In this study, we use stress entropic load as a starting point for the construction of a novel dynamical model of human thermoregulation. This model exhibits bi-stable mechanisms, a physiologically plausible features which has thus far not been demonstrated using a mathematical model. This finding allows us to predict critical points at which a living system, in this case a human body, may proceed towards two stabilities, only one of which is compatible with being alive. In the future, this may allow us to quantify not only the direction but rather the extent of therapeutic intervention in critical care patients.

Theoretical investigation and experiment of a disc-shaped triboelectric energy harvester with a magnetic bistable mechanism

Triboelectric energy harvesting has emerged as a promising route to scavenge ambient mechanical energy for cost-effective, clean and sustainable electricity. Disc-shaped triboelectric energy harvesters are suitable for two kinds of mechanical energy sources: continuous rotation and vibration. A majority of current studies about disc-shaped triboelectric energy harvesters focus on scavenging energy in continuous rotation, but there is a lack of investigations on angular vibration, especially in structural dynamics. In this work, a new disc-shaped triboelectric energy harvester with a bistable mechanism enabled by two repulsive magnets is developed for harvesting vibration energy. There are two discs in the harvester, one stationary and the other undergoing angular oscillation. Both have segmented triboelectric films on their contact surfaces. The magnetic bistable mechanism is utilized for the first time in a disc-shaped triboelectric energy harvester for efficiency enhancement. A comprehensive theoretical model coupling both structural dynamic and electric dynamic domains is established. A comparison between the coupled and uncoupled models reveals that the ET between electrodes can be ignored. Numerical simulations are carried out to investigate the effect of the potential wells due to the two magnets, basins of attractors and the influence of damping from the perspective of structural dynamics. A prototype is fabricated for experimental investigations, which demonstrate that the harvester with the bistable mechanism can achieve a better performance than the corresponding harvester without the bistable mechanism, and the output voltage of the harvester increases with the increase of excitation amplitude. Theoretical and experimental comparisons about the electric outputs between the triboelectric films with different segmentation structures reveal that increasing the number of sectors on the films effectively improves the harvesting efficiency. This work establishes a link between the structural dynamics and electric dynamics for the vibration-based disc-shaped triboelectric energy harvester, providing guidelines for its design and fabrication.

Evaluation of long-term power capture performance of a bistable point absorber wave energy converter in South China Sea

Nowadays, extensive research has been conducted to generate electricity from ocean waves, but there are not too many studies on the capture of wave energy in a specific ocean area. As the largest sea area in China's offshore waters, the South China Sea contains a large amount of wave energy and is regarded as a natural wave energy power plant. A comparative study was conducted to investigate the effects of wave energy converter (WEC) parameters on the long-term power capture performance of a bistable point absorber WEC installed in the South China Sea in this paper. Combined with the joint probability distribution of significant wave height Hs and peak wave period Tp in South China Sea, a mathematical model was developed for the evaluation of long-term power performance of WECs. The governing equations of the point absorber WEC were established using the hybrid frequency-time domain method. The dynamic response and captured power can be obtained after numerically solving the equations of motion with the 4th order Runge-Kutta Method. The results show that the bistable mechanism has great advantages in the long-term captured power of WEC. In general, a circular truncated cone-shaped buoy WEC with a larger buoy size and smaller nonlinear parameter tend to result in better long-term power capture performance. The insights given in this study provide a motivation and foundation for developing a complete set of performance optimization methods of point absorber WECs installed in South China Sea.

Design of a spatial constant-force end-effector for polishing/deburring operations

Controlling the contact force on workpieces is a challenging task for industrial deburring operations. To solve this issue, a novel constant force mechanism (CFM) based on the combination of positive and negative stiffness mechanism is proposed by using folding beam and bi-stable beam mechanisms. Without using any additional sensors and control algorithms, the proposed CFM can produce a travel range in constant force manner. In this paper, the design concepts, analytical model, finite element analysis (FEA) simulation and experimental studies are presented and discussed. Firstly, a novel spatial CFM is proposed and the pseudo rigid body (PRB) method is used to establish the mathematical model of the whole mechanism. Then, the FEA simulation is performed to validate the correctness of theoretical analysis. In addition, to eliminate the force variation, particle swarm optimization (PSO) method is utilized to find optimal architectural parameters solutions of the CFM. Finally, the experimental tests are performed to verify the performance of the designed CFM. The configuration design and parameter optimization proposed in this paper can be further applied to the design of other types of CFM mechanisms for polishing operations as well.

Bistable auxetic surface structures

We present Bistable Auxetic Surface Structures , a novel deployable material system based on optimized bistable auxetic cells. Such a structure can be flat-fabricated from elastic sheet material, then deployed towards a desired double-curved target shape by activating the bistable mechanism of its component cells. A unique feature is that the deployed model is by design in a stable state. This facilitates deployment without the need of complex external supports or boundary constraints. We introduce a computational solution for the inverse design of our Bistable Auxetic Surface Structures. Our algorithm first precomputes a library of bistable auxetic cells to cover a range of in-plane expansion / contraction ratios, while maximizing the bistability and stiffness of the cell to ensure robust deployment. We then use metric distortion analysis of the target surface to compute the planar fabrication state as a composition of cells that best matches the desired deployment deformation. As each cell expands or contracts during deployment, metric frustration forces the surface towards its target equilibrium state. We validate our method with several physical prototypes.

Nonlinear dynamic response of an isolation system with superelastic hysteresis and negative stiffness

The negative stiffness exhibited by bi-stable mechanisms together with the tunable superelasticity offered by shape memory alloy (SMA) wires can enhance the dynamic resilience of a structure in the context of vibration isolation. The effects of negative stiffness and superelastic damping in base-isolated structures are here explored by carrying out an extensive study of the nonlinear dynamic response via pathfollowing, bifurcation analysis, and time integration. The frequency-response curves of the isolated structure, with and without the negative stiffness contribution, are numerically obtained for different excitation amplitudes to construct the acceleration and displacement transmissibility curves. The advantages of negative stiffness, such as damping augmentation and reduced acceleration/displacement transmissibility, as well as the existence of rich bifurcation scenarios toward quasi-periodicity and chaos, are discussed.

A bistable X-structured electromagnetic wave energy converter with a novel mechanical-motion-rectifier: Design, analysis, and experimental tests

Beneficial nonlinearities have been intentionally introduced to improve energy harvesting performance. However, few results are reported on the issues of exploiting bistable and quasi-zero stiffness structures or mechanisms simultaneously to enhance ocean energy conversion in the literature. In this study, a novel bistable electromagnetic wave energy converter is proposed, which includes a bio-inspired X-shaped supporting structure/mechanism and a mechanical-motion-rectifier. This is for the first time that the bistable stiffness and quasi-zero stiffness are employed together (in all working range) to strengthen wave energy conversion, by developing a novel bistable X-structured system as a wave energy converter, which can bear with large vibration stroke simultaneously. Additionally, a novel mechanical-motion-rectifier with a novel energy-storing-releasing mechanism is specially designed as the power take-off system to convert linear motion to rotary motion, which greatly improves the instant voltage and power generated. The novel X-structured system demonstrates very designable nonlinear bistable and quasi-zero stiffness and thus can significantly enhance the wave energy conversion performance. Compared with traditional linear converters, much more power can be generated, especially in the low-frequency range due to the quasi-zero and bistable properties. The experiments show that, with the wave height of 0.15 m as well as the wave period of 4 s, it can generate a voltage peak at around 15 V and a power peak at about 1 W. The present results indicate that, the proposed nonlinear bistable X-structured system holds promising potentials in acting as a floating-point energy converter, which could power seashore environmental monitoring systems and wireless communication devices for smart oceans.

Functional Morphology of the Flight Apparatus of Ephemera vulgata (Ephemeroptera: Ephemeridae) and Implications for the Evolution of Flight in Insects

Abstract The mesothoracic flight apparatus of preserved and freshly killed specimens of Ephemera vulgata is examined in detail by means of functional morphological examinations. The basic drive mechanism of the forewing of Ephemera vulgata constitutes a four-bar kinematic chain, which encompasses the whole mesothorax. Based on this reassessment, the operating principles of the different subsystems of the flight apparatus of the mayfly were newly analysed. As in the Neoptera, the indirect dorsolongitudinal downstroke and dorsoventral upstroke muscles rotate the wing around an axis which runs through the main wing joint of the fulcrum. In contrast to the Neoptera, the wing is driven via the posterior notal wing process and fourth axillary. A strong direct subalar muscle is able to move the subalare (together with the pleural ridge) inwards and to affect the wingstroke via a bistable click mechanism in this unusual way. The axillary sclerites, two frontal first axillaries and one caudal fourth axillary, permit the radioanal plate to rotate forward–backward on the fulcrum. This motion system, which is superimposed on the kinematic chain mechanism, permits alterations of the wingstroke plane. A short muscle of the radioanal plate allows adjusting the passive pronation of the wing during the downstroke and achieving an increase of the aerodynamical angle of attack. The step-by-step derivability of basal flight mechanisms of the main groups of Pterygota indicates a paranotal origin of the wings. The possibility that the Ephemeroptera and Neoptera constitute sister groups is discussed.

Design and Prototyping of Rotational Bi-Stable Mechanism Using Permanent Magnets

Abstract Diverse applications including switches, deployable structures, and reconfigurable robots can benefit from bi-stability characteristics. However, the complexity of the implementation and the limitation of the structural configuration makes it difficult to apply conventional bi-stable mechanisms to the structures that require rotational bi-stability. In this paper, an implementation method using cylindrical magnets for the rotational bi-stable mechanism is proposed. The proposed bi-stable mechanism consists of a revolute joint with two links. It has rotational bi-stability through the magnetic force relationship between the array of magnets on each link. To identify the characteristics of the proposed bi-stable mechanism, a cylindrical permanent magnet is considered as an electromagnet model that consists of one ring with a virtual electric current. The magnetic field of the cylindrical permanent magnet can be calculated using the Biot–Savart law. Similarly, the magnetic force between two cylindrical permanent magnets is calculated using the Lorentz force law. The criteria of the magnet array for symmetric bi-stability are described and the potential energy diagram of the rotation link is considered as the performance criterion to identify the stable state. The proposed bi-stable mechanism was applied to the prototype of a deployable structure consisting of two links. The load testing of the structure against external torque was performed and it was obtained that the rotation link can stay within 5 deg angle to the maximum load applied and was experimentally verified with good agreement.

Improved bistable mechanism for wave energy harvesting

As one of the renewable and clean energy sources, wave energy has high density and all-day availability characteristics. However, the development of wave energy converters has been hampered, mainly because of the low efficiency induced high levelized energy costs. The present study introduces an improved bistable mechanism composed of three linear springs, aiming to enhance the performance of a point absorber employed as a wave energy converter. The improved bistable mechanism features an asymmetric configuration of three linear stiffness springs: one horizontal and two obliques on a vertical plane. Numerical simulations based on Cummins equations in the time domain describe the point absorber dynamic behavior in regular and irregular waves. The improved bistable wave energy converter featured a wider low equivalent stiffness range than linear and conventional bistable counterparts, improving its ability to frequency shifting. Its lower potential barrier greatly improves the capture width ratio and frequency bandwidth at low excitations. Such characteristics facilitate the improved bistable wave energy converter to achieve higher efficiency than its linear and conventional bistable counterparts at low frequencies, and enhance its robustness against power-take-off damping detuning and sea state changes. These features altogether make the improved bistable mechanism an efficient alternative to explore the benefits of the bistable dynamics applied to wave energy converters.

Autonomous shock sensing using bi-stable triboelectric generators and MEMS electrostatic levitation actuators

This work presents an automatic threshold shock-sensing trigger system that consists of a bi-stable triboelectric transducer and a levitation-based electrostatic mechanism. The bi-stable mechanism is sensitive to mechanical shocks and releases impact energy when the shock is strong enough. A triboelectric generator produces voltage when it receives a mechanical shock. The voltage is proportional to the mechanical shock. When the voltage exceed a certain level, the initially pulled-in microelectromechanical system (MEMS) switch is opened and can disconnect the current in a safety electronic system. The MEMS switch combines two mechanisms of gap-closing (parallel-plate electrodes) with electrostatic levitation (side electrodes) to provide bi-directional motions. The switch is initially closed from a small bias voltage on the gap-closing electrodes. The voltage from the bi-stable generator is connected to the side electrodes. When the shock goes beyond a threshold, the upward force caused by the side electrodes on the switch becomes strong enough to peel off the switch from the closed position. The threshold shock the system can detect is tunable using two control parameters. These two tuning parameters are the axial force on the bi-stable system (clamped-clamped beam) and the bias voltage on the MEMS switch (gap-closing electrodes). The actuation in macro-scale is thus directly connected to a sensor-switch mechanism in micro-scale. This chain makes an autonomous actuation and sensing stand-alone system that has potential application on air bag deployment devices and powerline protection systems. We provide a theoretical frame work of the entire system validated by experimental results.

Buckling Control of Piezolaminated Beams for Compliant Bistable Mechanisms

Buckling Control of Piezolaminated Beams for Compliant Bistable Mechanisms

Fully compliant bistable mechanisms with enhanced pitch stiffness

For compliant bistable mechanisms with planar configurations and exhibiting two in-plane stable states, their flexible beams are supposed to undergo in-plane bending deflections, but the out-of-plane deflections can significantly influence the kinetostatics of the mechanism, resulting in inconsistent bistable behaviors when off-axis or eccentric loads present. In this paper, we propose a novel bistable mechanism utilizing torsion-stiffness enhanced flexible beams to restrain the shuttle from out-of-plane rotation. An analytical kinetostatic model is developed for the proposed bistable mechanism. Five bistable examples are presented to demonstrate the effectiveness of the proposed bistable mechanism in suppressing unwanted motions and the correctness of the proposed kinetostatic model. The results show that the novel bistable designs are much stiffer along the out-of-plane rotation thus are more robust against off-axis or eccentric loading condition, especially when the shuttle is displaced to a position around the second equilibrium stable position (where the flexible beams are in the post-buckling states).

Bio-inspired bistable piezoelectric vibration energy harvester: Design and experimental investigation

Inspired by the flight mechanism of dipteran, a novel bionic-dipteran energy harvester (BDEH) was proposed to collect ultralow-frequency vibration energy. Mimicking the skeletal structure of dipteran wing, two flexible beams and two rigid links are hinged together to form a bistable mechanism. Piezoelectric patches attached onto the flexible beams generate electricity as the proof mass is driven to oscillate by base excitations. A lumped-mass-spring model is built for theoretical analysis, which is verified by both finite element simulations and experiments. The dynamic responses of displacement and output voltage are achieved under different conditions, which indicate that the BDEH has the highest efficiency when the system undergoes cross-well vibration pattern. Both light damping and large excitation are beneficial to trigger cross-well vibration and thus enhance energy harvesting efficiency. A rectifier circuit is built to measure output power of the prototype under different excitations. Maximum power of 0.143 mW is generated under cross-well vibration pattern at an excitation frequency of 4 Hz and acceleration of 0.7g (g = 9.8 m/s2), which implies that the BDEH should be a feasible solution for ultralow-frequency energy harvesting even under low excitation.

Performance analysis of a raft-type wave energy converter with a torsion bi-stable mechanism

In this study, a torsion bi-stable mechanism is proposed to enhance the absorption efficiency of the raft-type wave energy converter (WEC). The torsion bi-stable mechanism and a power take-off (PTO) system are settled at the joint between hinged rafts. The dynamic model in the time domain of the raft-type WEC has been established in conjunction with a simplified model of the bi-stable mechanism. In order to analyze the performance of the whole system more rapidly, the convolution integral term of the dynamic model has been replaced by the state space model. The performances of the WECs are analyzed in both regular and irregular sea conditions. Compared with the corresponding linear model, the numerical results indicate that the bi-stable mechanism can enhance the capture efficiency of the raft-type WEC significantly and extend the range of the high efficiency to the lower frequencies.