Variable Stiffness Device Research Articles

Nonlinear Vibration Isolation of Spacecraft System by a Bionic Variable-Stiffness Device Enhanced by Electromagnetic Component

Nonlinear Vibration Isolation of Spacecraft System by a Bionic Variable-Stiffness Device Enhanced by Electromagnetic Component

A nonlinear energy sink enhanced by active varying stiffness for spacecraft structure: theory, simulation, and experiment

A conventional nonlinear energy sink (NES) proposed for spacecraft structure can achieve vibration reduction without changing the resonant frequencies. However, large additional mass limits its engineering applications, and the robustness needs to be improved. A novel nonlinear energy sink with active varying stiffness (NES-AVS) device is proposed for reducing the mass of the conventional NES and enhancing the vibration reduction effects. A two degrees-of-freedom equivalent model of the scaled spacecraft structure is proposed. A harmonic balance method is applied to analyze the effect of the vibration reduction of the equivalent model, and the results are verified numerical simulations. The results show that the NES-AVS can achieve better vibration reduction effects, especially in small damping and mass. A finite element model of the scaled spacecraft structure with the NES-AVS is developed. The NES-AVS device is experimentally embedded in the scaled spacecraft structure. The variable stiffness device is realized by a steel plate with the compression force adjusted by a piezoelectric actuator. An active controller is used to control the actuator by the dSPACE module. The experimental results demonstrate that the NES-AVS has a better effect of the vibration reduction of the spacecraft structure than that of the conventional NES in multiple modes and different excitations. Compared with the conventional NES, the mass of the NES-AVS is reduced by 90%. The experiment and the theory have the same trends that the vibration reduction performance of the NES-AVS can be improved by adjusting the active control parameters. This work devotes to providing a new design scheme of the NES.

Seismic control of adaptive variable stiffness intelligent structures using fuzzy control strategy combined with LSTM

A novel semi-active control algorithm for adaptive variable stiffness intelligent structures that combines the fuzzy control strategy with Long Short-Term Memory (LSTM) is proposed in this study, with the aim of mitigating structural responses under earthquake excitations. The combined Fuzzy-LSTM strategy utilizes an LSTM neural network to predict the displacement and velocity responses of the structure at the next moment, and uses the predicted results as input for fuzzy control to obtain active control forces. Due to the difference between the displacement-velocity response and the fuzzy control domain, the genetic algorithm is used to optimize scaling factors of displacement and velocity responses and the control force, which is implemented by exploiting stiffness variability and structural inter-shift. The Fuzzy-LSTM strategy can compensate for the time delay effect on active control caused by stiffness variations, algorithmic computations and sensor measurements, and therefore achieves a better control effect through prediction. The semi-active independently variable stiffness (SAIVS) device is used to implement the presented control algorithm, which is a diamond device consisting of four linear springs and an electromagnetic actuator. The SAIVS device can achieve a continuous and smooth stiffness variation and meet the application requirements of the developed algorithm. Therefore, active control forces can be obtained through structural stiffness variations and interlayer displacements under seismic excitation, to achieve an approximate active control effect through a semi-active method. A 10-story building is presented as a case study for numerical simulations. Results show that the Fuzzy-LSTM strategy can significantly improve the control effect compared to the conventional fuzzy control strategy, and therefore achieve a better seismic response mitigation performance.

Kinematics and stiffness analysis of a wheelchair-based cable-actuated ankle rehabilitation robot with flexure-based variable-stiffness devices

Kinematics and stiffness analysis of a wheelchair-based cable-actuated ankle rehabilitation robot with flexure-based variable-stiffness devices

Liquid metal enabled conformal electronics.

The application of three-dimensional common electronics that can be directly pasted on arbitrary surfaces in the fields of human health monitoring, intelligent robots and wearable electronic devices has aroused people's interest, especially in achieving stable adhesion of electronic devices on biological dynamic three-dimensional interfaces and high-quality signal acquisition. In recent years, liquid metal (LM) materials have been widely used in the manufacture of flexible sensors and wearable electronic devices because of their excellent tensile properties and electrical conductivity at room temperature. In addition, LM has good biocompatibility and can be used in a variety of biomedical applications. Here, the recent development of LM flexible electronic printing methods for the fabrication of three-dimensional conformal electronic devices on the surface of human tissue is discussed. These printing methods attach LM to the deformable substrate in the form of bulk or micro-nano particles, so that electronic devices can adapt to the deformation of human tissue and other three-dimensional surfaces, and maintain stable electrical properties. Representative examples of applications such as self-healing devices, degradable devices, flexible hybrid electronic devices, variable stiffness devices and multi-layer large area circuits are reviewed. The current challenges and prospects for further development are also discussed.

Design and analysis of passive variable stiffness device based on shear stiffening gel

Shear thickening materials are utilized as the core of variable stiffness devices in various engineering applications in recent years. However, due to the used materials being liquid, most of these devices are suffered from the problem of liquid leakage and coagulation. To address this issue, this paper proposes a design method of a passive variable stiffness device based on shear stiffening gel (SSG), which integrates the core SSG and spring into a compact structure to obtain well stability. The used SSG can affect the force transfer within the device, thereby influencing the system’s stiffness dynamically over the external impact load. Due to the core SSG being good shear stiffening properties, the device is highly sensitive to frequency and shows variable stiffness characteristics. When the frequency of external load increased from 0.1 Hz to 5 Hz, the equivalent stiffness of the device could be increased by 105.74%. An equivalent nonlinear model is used to represent the overall stiffness of the device as a function of the frequency and displacement amplitude of the load. The output of the equivalent nonlinear model agrees well with the experimental data. The cross-validation also proves the accuracy of the model fitting, which provides a quantitative description of the dynamic performance of the variable stiffness device. By combining different types of SSGs and springs with different stiffness, the design principle of the variable stiffness device proposed in this paper can be utilized in various impact-loading applications, such as designing the exoskeletal load carriage supporting mechanism and vehicle suspension systems. The work of this paper has guiding significance for the design of adaptive variable stiffness devices.

Biohybrid Variable-Stiffness Soft Actuators that Self-Create Bone.

Inspired by the dynamic process of initial bone development, in which a soft tissue turns into a solid load-bearing structure, the fabrication, optimization, and characterization of bioinduced variable-stiffness actuators that can morph in various shapes and change their properties from soft to rigid are hereby presented. Bilayer devices are prepared by combining the electromechanically active properties of polypyrrole with the compliant behavior of alginate gels that are uniquely functionalized with cell-derived plasma membrane nanofragments (PMNFs), previously shown to mineralize within 2 days, which promotes the mineralization in the gel layer to achieve the soft to stiff change by growing their own bone. The mineralized actuator shows an evident frozen state compared to the movement before mineralization. Next, patterned devices show programmed directional and fixated morphing. These variable-stiffness devices can wrap around and, after the PMNF-induced mineralization in and on the gel layer, adhere and integrate onto bone tissue. The developed biohybrid variable-stiffness actuators can be used in soft (micro-)robotics and as potential tools for bone repair or bone tissue engineering.

Stretchable and conformable variable stiffness device through an electrorheological fluid.

Stiffness variations extend creatures' functions and capabilities to deal with complex environments. In this study, we proposed an electrorheological fluid-based variable stiffness device, named VSERF, made up of soft materials. Our device is soft, thin, and stretchable so that it can conform to surfaces with complex morphologies. The stiffness of the VSERF device can be continuously, independently, and reversibly adjusted by applying an electric field. It achieves 14.8-times compressive stiffness variation and 3.5-times tangential stiffness variation when the electric field intensity increases from 0 V mm-1 to 750 V mm-1. The VSERF device is able to return to its initial shape after removing the external force and electric field, allowing it to be reused. The effects of stretching and bending on the device's capability of stiffness variations are investigated experimentally and the results show that the stiffness variation is unaffected by a stretching strain of up to 20% and a bending curvature of up to 50 m-1. Finally, we show that the VSERF device is capable of conforming to complex surfaces (coral stones, pencils, and 3D printed cubes) in its inactive state, hanging on them with a weight of up to 80 g (19 times of its own weight) in its active state, and detaching when the electric field is removed. The device's short-term and long-term stabilities are experimentally investigated as well. The demonstration of the VSERF's attaching and detaching ability shows that the stiffness-variation device's adaptability to complex environments can be improved.

Design methodology for the development of variable stiffness devices based on layer jamming transition

Variable stiffness mechanisms as Jamming Transition draw huge attention recently in Soft Robotics. This paper proposes a comprehensive design methodology for developing variable stiffness devices based on layer jamming. Starting from pre-existing modelling, we highlight the design parameters that should be considered, extracting them from literature and our direct experience with the phenomenon. Then we validated the methodology applying the design process to previous layer jamming cases presented in literature. The comparison between the results obtained from our methodology and those presented in the analyzed previous works highlights a good predictive capability, demonstrating that this methodology can be used as a valid tool to design variable stiffness devices based on layer jamming transition. Finally, in order to provide the scientific community with an easily usable tool to design variable stiffness structures based on layer jamming transition, we have elaborated a Matlab script that guides the user through the main design parameters implementing the proposed methodology in an interactive process.

A lattice sandwich structure with the active variable stiffness device under aerodynamical condition

Herein, a novel pyramid lattice sandwich structure with the active variable stiffness (AVS) is proposed, which consists of a proportion integration (PI) active controller, a piezoelectric actuator, and a variable stiffness device. The principle of AVS is to process the real-time vibration feedback signal of the lattice sandwich beam by using the PI controller and the piezoelectric actuator to change structural stiffness continuously. The structure has excellent self-adjusting and active vibration reduction effect. Based on the supersonic piston theory, Hamilton's principle, and the assumed modes method, the dynamics equations of the whole structure are established. The vibration reduction and energy dissipation of lattice sandwich beams with AVS and without AVS are analyzed by using the Runge-Kutta algorithm. Moreover, the relevant parameters of PI controller, positive stiffness, negative stiffness, incoming flow velocity, damping coefficient, and installation position are compared and analyzed, respectively. The results indicate that the novel lattice sandwich structure with AVS can achieve ideal vibration reduction effect, and the efficiency can be improved by adjusting the parameters appropriately.

Control Force Compensation and Wear Monitoring of Variable Stiffness Joints in Drive Machining Process

Aiming at the joint flexibility and wear state existing in the process of driving mechanical parts, this paper first proposes a stiffness and position decoupling control method for variable stiffness joints, which realizes the joint position control and the unity of joint compliance. The joint stiffness model was obtained by using the static relationship between the Jacobian matrix and the model, and the nonlinear equations composed of the mechanical model and the stiffness model of the variable stiffness device were solved by the optimization method to realize the nonlinear decoupling of the stiffness and position of the variable stiffness joint. Secondly, this paper proposes an online monitoring method of wear state in the machining process based on machine tool information. In this method, OPC-UA communication technology was used to collect and store the information of CNC machine tools online, and the internal process information related to the wear of the machine tools was obtained. Based on such information and the corresponding wear information, a wear state recognition model is established by using a convolutional neural network. The feasibility and effectiveness of the proposed compliance control scheme and the performance of online monitoring of wear condition are analysed and verified by simulation experiments.

Variable Stiffness Devices Using Fiber Jamming for Application in Soft Robotics and Wearable Haptics.

Variable stiffness actuation has applications in a wide range of fields, including wearable haptics, soft robots, and minimally invasive surgical devices. There have been numerous design approaches to control and tune stiffness and rigidity; however, most have relatively low specific load-carrying capacities (especially for flexural loads) in the most rigid state that restricts their use in small or slender devices. In this article, we present an approach to the design of slender, high flexural stiffness modules based on the principle of fiber jamming. The proposed fiber jamming modules (FJMs) consist of axially packed fibers in an airtight envelope that transition from a flexible to a rigid beam when a vacuum is created inside the envelope. This FJM can provide the flexural stiffness of up to eight times that of a particle jamming module in the rigid state. Unlike layer jamming modules, the design of FJMs further allows them to control stiffness while bending in space. We present an analytical model to guide the parameter choices for the design of fiber jamming devices. Finally, we demonstrate applications of FJMs, including as a versatile tool, as part of a kinesthetic force feedback haptic glove and as a programmable structure.

A semi-active variable equivalent stiffness and inertance device implemented by an electrical network

Semi-active devices have been widely investigated and applied due to their advantages in control systems. However, for satisfying complicated applications, some semi-active devices need to be capable of controlling multiple mechanical properties, which is hard to implement with a mechanical network considering the space limitation. In this paper, a semi-active variable equivalent stiffness and inertance (VESI) device is developed by an electrical network and an electromagnetic device instead of a mechanical system. Due to that the inerter and spring can store energy, the dynamic control of their mechanical properties without the external power to compensate the stored energy will cause the system discontinuity, which will affect the control performance. Therefore, the variable damping (VD) device is applied to connect with spring or inerter in series to obtain variable equivalent stiffness or inertance properties. Then, according to the force-current analogy, an electrical “VESI” network is designed to simulate the mechanical VESI network. An electromagnetic device equips with the variable electrical system can achieve the VESI mechanical properties, which is validated with experiments. The imaginary part of the admittance of the proposed device can vary to the positive phase and also the negative phase by controlling two resistors. The application of VESI device in the vehicle is investigated and analysed. A semi-active control strategy is designed for vibration control and rollover prevention of vehicles with VESI suspensions. According to the simulation result, the proposed semi-active system can significantly improve the vibration reduction performance at both vertical and roll directions; also, it can decrease the roll angel when vehicles in the cornering maneuver. Besides, the semi-active VESI device with an electrical network has the advantages in installation and maintenance. It has great potential in practical applications.

Alignment of magnetic particles in hydrogel matrix: A novel anisotropic magnetic hydrogels for soft robotics

Magnetic hydrogels are composed of magnetic particles and hydrogel matrix. In recent years, the magnetic hydrogels have been developed rapidly because they have shown promising applications in drug release and artificial muscle. In this paper, we proposed a study to develop novel anisotropic magnetic hydrogels and investigate their mechanical and sensing properties for possible applications in soft robotics. In preparing the anisotropic magnetic hydrogels, the polyacrylamide (PAAm) hydrogel is chosen as a model hydrogel because of its popular application in soft electronics and ionic conductors. A method of free radicals copolymerization is employed to fill (polyacrylic acid/acrylamide) polymers in preparing anisotropic hydrogels under the magnetic field. Unlike most of the previous studies which incorporated magnetic nanoparticles into hydrogels, we mixed the micro-size carbonyl iron particles (CIPs) with the hydrogel and cured them under a magnetic field to form anisotropic structures within its crosslinking polymer chains. The particles and formed particle chains will not only improve the mechanical properties of the hydrogels but also provide sensing function as the electrical resistance changed from mechanical deformation referred to piezoresistivity. We experimentally evaluated the magnetorheological and the piezoresistive behaviors of the magnetic hydrogels, and demonstrated their potential use in soft robots as flexible touch sensors and variable-stiffness devices.

Improving the performance of conventional base isolation systems by an external variable negative stiffness device under near-fault and long-period ground motions

Recent studies have shown that base-isolated objects with long fundamental natural periods are highly influenced by long-period earthquakes. These long-period waves result in large displacements for isolators, possibly leading to exceedance of the allowable displacement limits. Conventional isolation systems, in general, fail to resist such large displacements. This has prompted the need to modify conventional base isolation systems. The current work focuses on the development of an external device, comprising a unit of negative and positive springs, for improving the performance of conventional base isolation systems. This unit accelerates the change in the stiffness of the isolation system where the stiffness of the positive spring varies linearly in terms of the displacement response of the isolated objects. The target objects of the present study are small structures such as computer servers, sensitive instruments and machinery. Numerical studies show that the increase in the damping of the system and the slope of the linear function is effective in reducing the displacement response. An optimal range of damping values and slope, satisfying the stability condition and the allowable limits of both displacement and acceleration responses when the system is subjected to near-fault and long-period ground motions simultaneously, is proposed.

An Electromagnetic Variable Stiffness Device for Semiactive Seat Suspension Vibration Control

This article introduces a rotary semiactive variable stiffness (VS) device, including its system design and model validation. A variable damping (VD) device and springs can form mechanical networks with specific topologies, and the VD device can control the equivalent stiffness of the network by varying its damping. The proposed rotary electromagnetic VS device consists of an electromagnetic VD device and a torsion spring, which are in series, while a planetary gear reducer is used to amplify the torque output of the VS device. The models of the electromagnetic VD and VS devices are both built and validated with tests. Then, a seat suspension applies the VS device by installing the device in the center of its scissors structure. Bump and random vibration experiments are implemented to validate the effectiveness of the proposed VS system for seat suspension vibration control. The new system has merits in controllability and energy efficiency. It is easier to design the controller than the conventional one with magnetorheological damper due to the linear damping of the electromagnetic VD device. Besides, the system energy consumption is low and has energy harvesting capability. The proposed semiactive electromagnetic VS device also has potential in other practical applicationss.

Cyber-physical design and optimization of tall building dynamics using aeroelastic wind tunnel modeling

This study explores the use of an automated cyber-physical system (CPS) framework for the design and optimization of tall building dynamics through aeroelastic boundary layer wind tunnel (BLWT) testing. The framework is fully automated, integrating experimental BLWT modeling with numerical optimization algorithms to rapidly evaluate a wide range of candidate designs. In this study, candidate tall building designs are generated by physically adjusting the dynamic properties of a multi-degree-of-freedom (MDOF) aeroelastic model. The specimen is equipped with an actuation system consisting of a series of variable stiffness devices (VSD), which enable precise control of the lower natural frequencies. The specimen is also instrumented with accelerometers and displacement sensors to capture wind-induced building response. A stochastic optimization algorithm evaluates the fitness of each candidate design based on code-based and/or user-specified serviceability limit states related to occupant comfort and building drift (e.g., sway). Wind tunnel experiments were conducted at the University of Florida Natural Hazard Engineering Research Infrastructure (NHERI) Experimental Facility. Results show that the CPS framework can reliably drive the aeroelastic specimen to the optimal solution which minimizes stiffness (i.e., natural frequency) while satisfying multiple acceleration and drift constraints.

A Novel Electrical Variable Stiffness Device for Vehicle Seat Suspension Control With Mismatched Disturbance Compensation

This paper presents a novel electrical variable stiffness device (EVSD) and its application in seat suspensions. The EVSD is inspired by the conventional mechanical variable stiffness device (VSD) and the force-current analogy, which applies an inductor and a variable resistor to simulate the functions of the spring and variable damper in the mechanical VSD. The electrical system has less physical limitation than the mechanical one in the practical application. Besides, the EVSD requires no power supply except for control signals at milliwatts level. The test result shows the stiffness controllability of the proposed EVSD and matches with the system model in the simulation. Seat suspensions have been extensively utilized in vehicles, especially heavy-duty ones, to protect drivers’ health and improve the ride comfort. An EVSD-based seat suspension is investigated in this paper. The seat suspension applies the suspension deflection, circuit current, and seat acceleration, which are all easy to measure with sensors, as feedback to estimate the mismatched disturbance in the system; and a ${\boldsymbol {H}_\infty }$ controller with compensation of the mismatched disturbance is designed for the seat suspension. The experiments validate the effectiveness of the proposed seat suspension and controller. This EVSD-based seat suspension consumes ignorable energy and has great potential in practical applications.

Study on Stiffness-Oriented Cable Tension Distribution for a Symmetrical Cable-Driven Mechanism

In this paper, we focus on the issues pertaining to stiffness-oriented cable tension distribution for a symmetrical 6-cable-driven spherical joint module (6-CSJM), which can be employed to construct modular cable-driven manipulators. Due to the redundant actuation of the 6-CSJM, three cables are employed for position regulation by adjusting the cable lengths, and the remaining three cables are utilized for stiffness regulation by adjusting the cable tensions, i.e., the position and stiffness can be regulated simultaneously. To increase the range of stiffness regulation, a variable stiffness device (VSD) is designed, which is serially connected to the driving cable. Since the stiffness model of the 6-CSJM with VSDs is very complicated, it is difficult to directly solve the cable tensions from the desired stiffness. The stiffness-oriented cable tension distribution issue is formulated as a nonlinear constrained optimization problem, and the Complex method is employed to obtain optimal tension distributions. Furthermore, to significantly improve the computation efficiency, a decision variable elimination technique is proposed to deal with the equality constraints, which reduces decision variables from 6 to 3. A comprehensive simulation study is conducted to verify the effectiveness of the proposed method, showing that the 6-CSJM can accurately achieve the desired stiffness through cable tension optimization.

An electromagnetic variable inertance device for seat suspension vibration control

Abstract This paper designs and builds an electromagnetic variable inertance (VI) device for a heavy duty vehicle seat suspension to reduce the vibration transferred to driver bodies. The conventional semi-active vibration control mainly applies the variable damping (VD) and variable stiffness (VS) devices. The VI device is equivalent to an inerter with the capability to vary its inertance, which is a new semi-active system inspired by the VS device design. The VI device consists of a transmission device, a VD device and two flywheels. By varying the damping of the VD device, the equivalent inertance of the VI device is controllable in real time. The VI device prototype is built with an electromagnetic VD device which is controlled by the duty cycle of the pulse width modulation signal. The test results verify the proposed system model and identify the unknown model parameters. Then, a seat suspension applies the VI device prototype for improving the ride comfort. Based on the system model, an implementable controller is proposed and experimentally validated. The proposed seat suspension system shows excellent performance in a random vibration experiment. Compared with a conventional passive seat suspension, the frequency-weighted root mean square acceleration of the proposed one reduces 35.7%. The VI device has expanded the application of the inerter and the area of the semi-active research; it shows great potential in vibration control.