Variable Stiffness Device Research Articles

Nonlinear structural control using integrated DDA/ISMP and semi-active tuned mass damper

In the recent decade, the robustness, reliability and operability of semi-active tuned mass dampers (STMD) against frequency changes in linear structures has primarily been investigated in the frequency domain. Under severe environmental excitations such as strong earthquakes or winds, structures are expected to respond in the nonlinear range due to material nonlinearity or occurrence of damage which necessitates time domain analyses. In this study, a new proposed STMD controller is presented based on integrating damage detection and semi-active control strategies. The variable stiffness of the STMD is provided by a semi-active independently variable stiffness (SAIVS) device. The stiffness variation of the SAIVS device is implemented by adapting the frequency of the STMD to the fundamental frequency of the bare structure (without STMD), which is obtained using real-time tracking of the damage in the structure based on the measured dynamic responses of the coupled main structure and the STMD system. In this paper, the structural parameters of the bare structure are estimated using the damage detection algorithm based on identified system Markov parameters (DDA/ISMP) method. The effectiveness of the proposed scheme is evaluated via numerical analysis of a nonlinear ten-story building considering seven performance criteria under real earthquakes. In comparison to the passive case, the results reveal that the proposed STMD controller is robust against any structural frequency variations which effectively mitigates damages and nonlinear dynamic responses of the structure under strong ground motions.

Variable Negative Stiffness Device for Seismic Protection of Building Structures through Apparent Weakening

AbstractIn the present work, the variable negative stiffness device (VNSD) is presented for seismic protection through apparent weakening. Equations relating the force-displacement behavior for two...

A Soft, Controllable, High Force Density Linear Brake Utilizing Layer Jamming

While much work has focused on the design of actuators for inputting energy into robotic systems, less work has been dedicated to devices that remove energy in a controlled manner, especially in the field of soft robotics. Such devices have the potential to significantly modulate the dynamics of a system with very low required input power. In this letter, we leverage the concept of layer jamming, previously used for variable stiffness devices, to create a controllable, high force density, soft layer jamming brake (SLJB). We introduce the design, modeling, and performance analysis of the SLJB and demonstrate variable tensile resisting forces through the regulation of vacuum pressure. Further, we measure and model the tensile force with respect to different layer materials, vacuum pressures, and lengthening velocities, and show its ability to absorb energy during collisions. We hope to apply the SLJB in a number of applications in wearable technology.

JammJoint: A Variable Stiffness Device Based on Granular Jamming for Wearable Joint Support

In robotics, controlling the stiffness of the joints that contribute to the robots’ degree of freedom dictates the adaptability, versatility, and safety of the whole system. We can achieve variable stiffness or impedance in a robotic system purely by the control or by introducing new material or mechanisms to address cases that require innate safety through system compliancy. This paper presents JammJoint, a compliant and flexible wearable robot, which uses jamming of granular media to vary its stiffness. It consists of a silicone sleeve with hollow sections that are filled with cubic rubber granules and subjected to different levels of vacuum pressure. Unlike contemporary vacuum-based actuators or systems, JammJoint is wearable, portable, and autonomous: It uses a powerful miniature vacuum pump, a small battery, and bluetooth-enabled electronics. Experiments revolving around bending and torsional stiffness show that the system is able to achieve up to a fourfold increase in spring stiffness. Further measurements of individual variable stiffness structures indicate that for other modes of deformation, including simply supported bending or compression for alternative linear applications, higher changes in stiffness over a factor of seven are possible. These aspects make mobile jamming-based stiffness variation as wearable joint assistance promising for future applications such as rehabilitation after injuries and joint support in challenging working conditions.

Scalable Electroactive Polymer for Variable Stiffness Suspensions

We present an approach for creating a reliable variable stiffness suspension using prestretched electroactive polymer films. In comparison to other variable-stiffness devices, the design is compact, lightweight, consumes low energy, and can change its stiffness in approximately 10 ms. Attention to the main causes of failure increases the reliability of the device so that multiple layers can be combined for increased loads and stiffness. A device with 20 layers supports loads of over 15 N at minimum stiffness and maximum voltage for displacements of $\pm 7$ mm and up to 138 N at maximum stiffness with a displacement of 20 mm. Multilayer devices have demonstrated 3 million mechanical load cycles and hundreds of thousands of stiffness variation (charging and discharging) cycles, respectively.

Developing a smart structure using integrated DDA/ISMP and semi-active variable stiffness device

Recent studies integrating vibration control and structural health monitoring (SHM) use control devices and control algorithms to enable system identification and damage detection. In this study real-time SHM is used to enhance structural vibration control and reduce damage. A newly proposed control algorithm, including integrated real-time SHM and semi-active control strategy, is presented to mitigate both damage and seismic response of the main structure under strong seismic ground motion. The semi-active independently variable stiffness (SAIVS) device is used as semi-active control device in this investigation. The proper stiffness of SAIVS device is obtained using a new developed semi-active control algorithm based on real-time damage tracking of structure by damage detection algorithm based on identified system Markov parameters (DDA/ISMP) method. A three bay five story steel braced frame structure, which is equipped with one SAIVS device at each story, is employed to illustrate the efficiency of the proposed algorithm. The obtained results show that the proposed control algorithm could significantly decrease damage in most parts of the structure. Also, the dynamic response of the structure is effectively reduced by using the proposed control algorithm during four strong earthquakes. In comparison to passive on and off cases, the results demonstrate that the performance of the proposed control algorithm in decreasing both damage and dynamic responses of structure is significantly enhanced than the passive cases. Furthermore, from the energy consumption point of view the maximum and the cumulative control force in the proposed control algorithm is less than the passive-on case, considerably.

From modeling to control of a variable stiffness device based on a cable-driven tensegrity mechanism

Cable-driven tensegrity mechanisms are now considered for various applications in which their reconfiguration capacities are required together with their inherent lightness. Moreover, they can exhibit interesting variable stiffness capacities through the modification of their level of prestress when composed of deformable cables. Control schemes that deal with both reconfiguration and stiffness variation have however not been developed in the literature yet.This paper presents two control strategies for that purpose to transform a cable-driven tensegrity mechanism into a variable stiffness device. The mechanism is a planar tensegrity mechanism allowing us to control an angular position and the associated stiffness. Relying on the properties of the mechanism models, the proposed control strategies allow a modulation of the stiffness or of its first time derivative. The interest of both propositions is outlined and an experimental investigation of their characteristics is performed. Encouraging results are obtained in terms of reconfiguration capabilities and stiffness variation.

A multi-layered variable stiffness device based on smart form closure actuators

This contribution describes the properties and limitations of multi-layered mechanical devices with variable flexural stiffness. Such structures are supposed to be components of new smart, self-sensing and self-controlling composite materials for lightweight constructions. To enable a proper stiffness control, reliable actuators with high actuation capabilities based on smart materials are used. Those actuators are either driven by electroactive polymers (EAPs) or shape memory alloy (SMA) wires. They control the area moment of inertia of the multi-layered bending structures. To change the area moment of inertia and, hence, the flexural stiffness of an multi-layered beam within a wide range, it is necessary to stack as many layers as possible over each other. The fundamental function of this approach is demonstrated with a three-layer stack consisting of three independent layers and four form closure actuators driven by SMAs. This experimental set-up was able to change its bending stiffness k by a factor of 14.6, with a minimum and maximum stiffness of kmin = 0.11 N mm−1 and kmax = 1.73 N mm−1, respectively. The usage of four independently controllable actuators yields nine independent flexural-stiffness states of the beam. Both analytical and numerical calculations have shown good agreement with the measured stiffness values.

Helicopter Vibratory Loads Alleviation through Combined Action of Trailing-Edge Flap and Variable-Stiffness Devices

The aim of this paper is the assessment of the capability of controllers based on the combined actuation of flaps and variable-stiffness devices to alleviate helicopter main rotor vibratory hub loads. Trailing-edge flaps are positioned at the rotor blade tip region, whereas variable-stiffness devices are located at the pitch link and at the blade root. Control laws are derived by an optimal control procedure based on the best trade-off between control effectiveness and control effort, under the constraint of satisfaction of the equations governing rotor blade aeroelastic response. The numerical investigation concerns the analysis of performance and robustness of the control techniques developed, through application to a four-bladed helicopter rotor in level flight. The identification of the most efficient control configuration is also attempted.

Vibration control performance of tuned mass dampers with resettable variable stiffness

Vibration control of civil engineering structures using tuned mass dampers (TMDs) is a widely acknowledged control strategy based on numerous analytical and experimental verifications. Although the design and application of traditional linear TMD systems are well established, nonlinear TMD systems that may have better control performance remain in the developmental stage. The TMD systems have two main problems, i.e. detuning effect and excessive TMD’s stroke. To improve the overall performance of TMD systems, a novel semi-active TMD named resettable variable stiffness TMD (RVS-TMD), is proposed. The RVS-TMD consists of an undamped TMD and a resettable variable stiffness device (RVSD). This RVSD is composed of a resettable element and a controllable stiffness element. By varying the stiffness of controllable stiffness element in the RVSD, force produced by the RVSD can be controlled smoothly through a semi-active control law. By operating the resettable element, the hysteretic loops of the RVSD can cover all four quadrants in the force–deformation diagram and, thus, increases energy dissipation. In other words, both stiffness and damping of the RVS-TMD are adjustable via only the RVSD device. The harmonic and seismic responses of a building equipped with the RVS-TMD were investigated. Numerical results show that the proposed RVS-TMD system can avoid detuning effect automatically and assure the optimal control performance as desired when the frequency of primary structure is changed.

A novel semi-active TMD with folding variable stiffness spring

An innovative variable stiffness device is proposed and investigated based on numerical simulations. The device, called a folding variable stiffness spring (FVSS), can be widely used, especially in tuned mass dampers (TMDs) with adaptive stiffness. An important characteristic of FVSS is its capability to change the stiffness between lower and upper bounds through a small change of distance between its supports. This special feature results in lower time-lag errors and readjustment in shorter time intervals. The governing equations of the device are derived and simplified for a symmetrical FVSS with similar elements. This device is then used to control a single-degree-of-freedom (SDOF) structure as well as a multi-degree-of-freedom (MDOF) structure via a semi-active TMD. Numerical simulations are conducted to compare several control cases for these structures. To make it more realistic, a real direct current motor with its own limitations is simulated in addition to an ideal control case with no limitations and both the results are compared. It is shown that the proposed device can be effectively used to suppress undesirable vibrations of a structure and considerably improves the performance of the controller compared to a passive device.

Family of smart tuned mass dampers with variable frequency under harmonic excitations and ground motions: closed-form evaluation

A family of smart tuned mass dampers (STMDs) with variable frequency and damping properties is analyzed under harmonic excitations and ground motions. Two types of STMDs are studied: one is realized by a semi-active independently variable stiffness (SAIVS) device and the other is realized by a pendulum with an adjustable length. Based on the feedback signal, the angle of the SAIVS device or the length of the pendulum is adjusted by using a servomotor such that the frequency of the STMD matches the dominant excitation frequency in real-time. Closed-form solutions are derived for the two types of STMDs under harmonic excitations and ground motions. Results indicate that a small damping ratio (zero damping is the best theoretically) and an appropriate mass ratio can produce significant reduction when compared to the case with no tuned mass damper. Experiments are conducted to verify the theoretical result of the smart pendulum TMD (SPTMD). Frequency tuning of the SPTMD is implemented through tracking and analyzing the signal of the excitation using a short time Fourier transformation (STFT) based control algorithm. It is found that the theoretical model can predict the structural responses well. Both the SAIVS STMD and the SPTMD can significantly attenuate the structural responses and outperform the conventional passive TMDs.

1A23 Vibration Control of Variable Stiffness Tuned Mass Damper with Consideration of Time Delay(The 12th International Conference on Motion and Vibration Control)

This study employed a resettable variable stiffness TMD (RVS-TMD) to alleviate detuning effect. The RVS-TMD is composed of a TMD and a resettable variable stiffness device (RVSD), which consists of a resettable element and a controllable stiffness element. By varying the stiffness element of RVSD, the detuning effect can be mitigated. While, by actuating the resettable element, the hysteresis loop of the RVSD can cover all four quadrants in the force-deformation diagram and thus result in more energy dissipation. However, to adjust the stiffness of the TMD system, the problem of time-delay is inevitable. For an active control system, time is consumed in data acquisition, data processing, on-line calculation, and control force execution. There is always a delay between the time at which the control force is assumed to be applied and actually applied, which may cause degradation in control efficiency. Therefore, the control design of a RVS-TMD system should take into account the effect of time delay. This study applied delayed control gains to alleviate the time delay effect based on the LQR optimal output feedback control algorithm. A parametric study was conducted to demonstrate the relationships of delayed gains with intentionally-added delay time. The results of numerical simulation show the application of optimally-delayed gains significantly alleviates the time delay effect and assure the desired control efficacy.

On a high-potential variable-stiffness device

There are great efforts in developing effective composite structures for lightweight constructions for nearly every field of engineering. This concerns for example aeronautics, but also automotive industry and energy harvesting applications. Modern concepts of lightweight components try to use structures with adjustable properties. However, classic composite materials can only slightly adapt to varying environmental conditions because most materials, like carbon- or glass-fiber composites, show properties which are time-constant and not changeable. This contribution describes the development, the potential and the limitations of novel smart, self-controlling structures which can change their mechanical properties—in particular their flexural stiffness—by more than one order of magnitude. These structures use a multi-layer approach consisting of a ten-layer stack of 0.75 mm thick polycarbonate layers. The set-up is analytically described and its mechanical behavior is predicted by finite element analysis performed with ABAQUS. The individual layers are braided together by an array of shape memory alloy wires, which can be activated either all together or independently. Depending on the temperature applied by an electrical current flowing through the wires and the corresponding contraction, the wires can control the area moment of inertia of the whole stack, and with it the bending stiffness. First experimental investigations have shown a maximum stiffness change by a factor of 60, which is close to the theoretically predicted value.

Semi-active Control of Building Structures using Variable Stiffness Device and Fuzzy Logic

Semi-active control devices, also called “Intelligent” control devices, constitute the positive aspects of both the passive and active control devices. A semi-active control strategy is similar to the active control strategy, but this control device has been shown to be more energy-efficient than active devices. A particular type of semi-active control device, the Variable Stiffness Device (VSD), consists of a hydraulic cylinder with a normally closed solenoid control valve inserted in the tube connecting the two cylinder chambers. This paper emphasizes on the application of Semi-active Fuzzy Logic Controller (SFLC) of this system for getting the best results in the reduction of the building responses under earthquake excitations. For the numerical example, a 12-story building, located in the city of Rasht, Iran, is modeled as 3-D frame and the problem is solved in state space. The results obtained from the proposed control scheme (SFLC) are compared with those obtained from the ON-OFF control method. It is found that the SFLC is highly effective in reducing the responses of the example building than the ON-OFF algorithm. In this study, the optimal values of the fuzzy rule bases, membership functions, and the location of the control device are determined.

Design and analysis of cable-driven manipulators with variable stiffness

A manipulator with variable stiffness allows the manipulator to adjust its stiffness to suit for different task requirements. In this paper, a cable-driven manipulator with the ability to significantly regulate its stiffness through tension resolution is introduced. Variable stiffness for the cable-driven manipulators is achieved by attaching a novel variable stiffness device along each driving cable, in which the stiffness of the device is a function of the cable tension. As the cable-driven manipulator has actuation redundancy, the tension distribution can be manipulated even at a stationary pose. Such property allows the cable-driven manipulator to adjust the stiffness of each variable stiffness device, thereby regulating the stiffness of the manipulator. The design and analysis of the variable stiffness device are presented. The variable stiffness device uses commercial torsion springs, and has a compact and light-weight design. For a particular pose, a stiffness-oriented tension resolution algorithm is proposed to optimize the tension distribution to achieve an isotropic stiffness matrix with high stiffness magnitude. Experimental and simulation results verified that cable-driven manipulators with such variable stiffness devices are able to achieve significant stiffness regulation.

Hybrid nonlinear control of a tall tower with a pendulum absorber

Pendulums can be used as passive vibration control devices in several structures and machines. In the present work, the nonlinear behavior of a pendulum-tower system is studied. The tower is modeled as a bar with variable cross-section with concentrated masses. First, the vibration modes and frequencies of the tower are obtained analytically. The primary structure and absorber together constitute a coupled system which is discretized as a two degrees of freedom nonlinear system, using the normalized eigenfunctions and the Rayleigh-Ritz method. The analysis shows the influence of the geometric nonlinearity of the pendulum absorber on the response of the tower. A parametric analysis also shows that, with an appropriate choice of the absorber parameters, a pendulum can decrease the vibration amplitudes of the tower in the main resonance region. The results also show that the pendulum nonlinearity cannot be neglected in this type of problem, leading to multiplicity of solutions, dynamic jumps and instability. In order to improve the effectiveness of the control during the transient response, a hybrid control system is suggested. The added control force is implemented as a non-linear variable stiffness device based on position and velocity feedback. The obtained results show that this strategy of nonlinear control is attractive, has a good potential and can be used to minimize the response of slender structures under various types of excitation.

Variable stiffness smart structure systems to mitigate seismic induced building damages

SUMMARYUpgrading noncode conforming buildings to mitigate seismic induced damages is important in moderate to high seismic hazard regions. The damage, can be mitigated by using conventional (e.g. FRP wrapping) and emerging (e.g. smart structures) retrofit techniques. A model for the structure to be retrofitted should include relevant performance indicators. This paper proposes a variable stiffness smart structure device known as the Smart Spring to be integrated on building structures to mitigate seismic induced damage. The variable stiffness capability is of importance to structures that exhibit vertical (e.g. soft storey) irregularities and to meet different performance levels under seismic excitation. To demonstrate the utility of the proposed retrofitting technique, a four‐storey steel building is modelled in MATLAB and appropriate performance indicators are chosen. Various return period seismic hazards are generated from past earthquake event records to predict the structure's performance. The performance improvement because of the retrofitting of building structures using the variable stiffness device is presented. Copyright © 2012 John Wiley & Sons, Ltd.

Upgrading Pantograph Performance Using Variable Stiffness Devices (2<sup>nd</sup> Report: Development of Variable Stiffness Device and Control of Dynamic Characteristics of Pantograph)

Upgrading Pantograph Performance Using Variable Stiffness Devices (2<sup>nd</sup> Report: Development of Variable Stiffness Device and Control of Dynamic Characteristics of Pantograph)

Development of Variable Stiffness Colonoscope Consisting of Pneumatic Drive Devices

Colonoscopy is important and effective medical procedure to detect colonic disorder including cancer of the colon. However, because the large intestine is soft and complex shape, insertion of conventional colonoscopes into the large intestine is difficult, and it depends on doctors’ skill strongly. In many cases, patients feel strong pain. In this research, we aim at development of a novel colonoscope which can change own stiffness partially and realize safe insertion without special techniques. The colonoscope consists of variable stiffness devices. The device is made from silicone rubber and can change its stiffness by pneumatic pressure. In this report, two kinds of variable stiffness devices made from different silicone rubber materials have been developed by molding, and stiffness change characteristics of them are shown experimentally. By applying not only positive pneumatic pressure but also negative pressure, widely stiffness change range is realized. Additionally colonoscopes have been fabricated using them and FMA (Flexible Microactuator). From insertion experiments into the large intestine phantom, advantages and effectiveness have been recognized.