Adaptive Stiffness Research Articles

Force Tracking Control With Adaptive Stiffness and Iterative Position of Hip-Assistive Soft Exosuits

Force Tracking Control With Adaptive Stiffness and Iterative Position of Hip-Assistive Soft Exosuits

Improved Assistive Profile Tracking of Exosuit by Considering Adaptive Stiffness Model and Body Movement.

Wearable robots have been developed to assist the physical performance of humans. Specifically, exosuits have attracted attention due to their lightweight and soft nature, which facilitate user movement. Although several types of force controllers have been used in exosuits, it is challenging to control the assistive force due to the material's softness. In this study, we propose three methods to improve the performance of the basic controller using an admittance-based force controller. In method A, the cable was controlled according to the user's thigh motion to eliminate delays in generating the assistive force and improve the control accuracy. In method B, the stiffness feedforward model of the human exosuit was divided into two independent models based on the assistance phase for compensating the nonlinear stiffness more accurately. In method C, the real-time optimization method for the stiffness feedforward model with an adaptive moment estimation method optimizer was proposed. To validate these methods' effectiveness, we designed three new controllers, gradually combined the proposed methods with the basic controller, and compared their performances. We found that controller III, combining all three methods with the basic controller, showed the best performance. By applying controller III in the same exosuit, the root-mean-square error of the assistive force decreased from 39.84 N to 13.72 N, reducing the error by 65.56% compared with the basic controller. Moreover, the time delay for force generation in the gait cycle percentage decreased from 9.99% to 3.41%, reducing the delay by 65.87% compared with the basic controller.

Adaptive Stiffness Visual Servoing for Unmanned Aerial Manipulators With Prescribed Performance

Adaptive Stiffness Visual Servoing for Unmanned Aerial Manipulators With Prescribed Performance

Numerical and experimental study on adaptive stiffness yaw damper for suppressing abnormal vibration of high-speed trains

Numerical and experimental study on adaptive stiffness yaw damper for suppressing abnormal vibration of high-speed trains

Utilising redundancy in musculoskeletal systems for adaptive stiffness and muscle failure compensation: a model-free inverse statics approach

Vertebrates possess a biomechanical structure with redundant muscles, enabling adaptability in uncertain and complex environments. Harnessing this inspiration, musculoskeletal systems offer advantages like variable stiffness and resilience to actuator failure and fatigue. Despite their potential, the complex structure presents modelling challenges that are difficult to explicitly formulate and control. This difficulty arises from the need for comprehensive knowledge of the musculoskeletal system, including details such as muscle arrangement, and fully accessible muscle and joint states. Whilst existing model-free methods do not need explicit formulations, they also underutilise the benefits of muscle redundancy. Consequently, they necessitate retraining in the event of muscle failure and require manual tuning of parameters to control joint stiffness limiting their applications under unknown payloads. Presented here is a model-free local inverse statics controller for musculoskeletal systems, employing a feedforward neural network trained on motor babbling data. Experiments with a musculoskeletal leg model showcase the controller’s adaptability to complex structures, including mono and bi-articulate muscles. The controller can compensate for changes such as weight variations, muscle failures, and environmental interactions, retaining reasonable accuracy without the need for any additional retraining.

Stiffness Modulation and Pulsatile Release in Dual Responsive Hydrogels.

Inspired by nature, self-regulation can be introduced in synthetic hydrogels by incorporating chemo-mechanical signals or coupled chemical reactions to maintain or adapt the material's physico-chemical properties when exposed to external triggers. In this work, we present redox and light dual stimuli responsive hydrogels capable of rapidly adapting the polymer crosslinking network while maintaining hydrogel stability. Upon irradiation with UV light, polymer hydrogels containing redox responsive disulfide crosslinks and light responsive ortho-nitrobenzyl moieties show a release of payload accompanied by adaptation of the hydrogel network towards higher stiffness due to in situ crosslinking by S-nitrosylation. Whereas the hydrogel design allows the network to either become softer in presence of reducing agent glutathione or stiffer upon UV irradiation, simultaneous application of both stimuli induces network self-regulation resulting in a pulsatile form of payload release from the hydrogel. Finally, adaptive stiffness was used to make tunable hydrogels as substrates for different cell lines.

Modulation der Steifigkeit und Pulsierende Freisetzung in Dual‐Responsiven Hydrogelen

AbstractNach dem Vorbild der Natur kann Selbstregulierung synthetischer Hydrogele durch chemomechanische Signale oder chemische Reaktionsnetzwerke eingebaut werden, um die physikochemischen Eigenschaften des Materials aufrecht zu erhalten oder anzupassen, wenn sie externen Stimuli ausgesetzt werden. In dieser Arbeit stellen wir Hydrogele vor, welche auf Redox‐ und Lichtstimuli reagieren und in der Lage sind, das polymere Netzwerk schnell zu verändern und gleichzeitig die Stabilität des Hydrogels zu erhalten. Bei Bestrahlung mit UV‐Licht zeigen die Polymerhydrogele, welche redoxresponsiv über Disulfide quervernetzt sind und lichtresponsive ortho‐Nitrobenzyleinheiten tragen, eine Freisetzung ihrer Ladung mit gleichzeitiger Anpassung des Hydrogelnetzwerks zu höherer Steifigkeit aufgrund der in situ Vernetzung durch S‐Nitrosylierung. Während das Hydrogeldesign es ermöglicht, dass das Netzwerk entweder in Gegenwart des Reduktionsmittels Glutathion weicher oder bei UV‐Bestrahlung steifer wird, führt die gleichzeitige Anwendung beider Stimuli zu einer Selbstregulierung des Netzwerks, was zu einer pulsierenden Freisetzung der Ladung der Hydrogele führt. Schließlich wurde die adaptive Steifigkeit genutzt, um einstellbare Hydrogele als Substrate für verschiedene Zelllinien herzustellen.

Semi-active vibro-acoustic control of vehicle transmission systems using a metal rubber-based isolator

This work is concerned about employing a semi-active metal rubber (MR)-based isolator to suppress noise and vibration for vehicle transmission systems. The adaptive stiffness and damping properties of MR material play a key role in the semi-active control effect. In this regard, a particular automotive transmission component—precisely, a plate dedicated to accommodating bearings—has been selected as the focus of research to control its noise and vibration. A microcomputer-driven control system is applied, featuring characteristics such as high performance, compact size, lightweight design, and energy efficiency, which make it ideal for testing in the automotive sector. The efficacy of the devised control scheme is assessed through the deployment of accelerometers and sound intensity probes to quantify the vibrations and the radiated acoustic power, respectively. Interesting noise and vibration reduction rates are achieved using both open-loop and closed-loop strategies. This research opens the horizons to employ MR material in a semi-active configuration for industrial requirements surpassing its classical passive implementation, notably in terms of broadband attenuation performance.

Personalized tendon loading reduces muscle-tendon imbalances in male adolescent elite athletes.

An imbalanced adaptation of muscle strength and tendon stiffness in response to training may increase tendon strain (i.e., the mechanical demand on the tendon) and consequently tendon injury risk. This study investigated if personalized tendon loading inducing tendon strain within the effective range for adaptation (4.5%-6.5%) can reduce musculotendinous imbalances in male adolescent handball athletes (15-16 years). At four measurement time points during a competitive season, we assessed knee extensor muscle strength and patellar tendon mechanical properties using dynamometry and ultrasonography and estimated the tendon's structural integrity with a peak spatial frequency (PSF) analysis of proximal tendon ultrasound scans. A control group (n = 13) followed their usual training routine, an intervention group (n = 13) integrated tendon exercises into their training (3x/week for ~31 weeks) with a personalized intensity corresponding to an average of ~6.2% tendon strain. We found a significant time by group interaction (p < 0.005) for knee extensor muscle strength and normalized patellar tendon stiffness with significant increases over time only in the intervention group (p < 0.001). There were no group differences or time-dependent changes in patellar tendon strain during maximum voluntary contractions or PSF. At the individual level, the intervention group demonstrated lower fluctuations of maximum patellar tendon strain during the season (p = 0.005) and a descriptively lower frequency of athletes with high-level tendon strain (≥9%). The findings suggest that the personalized tendon loading program reduced muscle-tendon imbalances in male adolescent athletes, which may provide new opportunities for tendon injury prevention.

Adaptive stiffness in lattice metastructures through tensile-buckling inspired topology morphing

This paper explores the use of simultaneous tensile buckling of unit cells to induce a transformation in lattice topology. Under tension, unit cells undergo passive transformation from a rectangle-like to a triangle-/pentagon-like topology, with an associated change in the effective stiffness properties. This behaviour is investigated through finite element analysis and experiments, with analytical results providing insights into the observed behaviour. The analysis identifies (i) that the initial unit cell topology (rectangular) is dominated by membrane effects, (ii) the transformation phase is associated with negative stiffness, and (iii) once formed, the new topology (triangular/pentagonal) exhibits increased stiffness in both compression and tension. Finite element analysis confirms that the unit cell behaviour is also preserved in lattices. Under tension, the lattice undergoes a seven-fold increase in stiffness as it transitions from its initial to the new topology, with a regime of negative stiffness during this transformation accounting for approximately 82% of its total elastic deformation. This new approach to elastically tailor the nonlinear response of (meta-)materials/structures has the potential to contribute to the development of novel tensile energy absorbers.

Design and experiment of a variable stiffness prosthetic knee joint using parallel elastic actuation

Active prosthetic knee joints are proven to provide outstanding actuating performance in various walking situations. Currently, most active prosthetic knee joints are directly actuated via motors, which have the disadvantages of high-power consumption and insufficient joint compliance. In order to solve this problem, this paper proposed an active prosthetic knee joint with a unique variable stiffness parallel elastic actuation (VSPEA) mechanism. The VSPEA prosthetic knee joint is implemented by a geared five-bar mechanism with a motor-screw system. The VSPEA mechanism is implemented via another optimized compact motor-screw-spring system controlled by an adaptive stiffness adjustment algorithm. Numerical verifications show that the VSPEA prosthetic knee joint can reduce its actuation torque and power compared to the prosthetic knee joint without the VSPEA mechanism. Further practical experiments of the VSPEA prosthetic knee joint indicate that the Root Mean Square of its actuation motor power can decline 7.36 %, 8.75 %, and 11.51 % at 1.49 m/s, 1.21 m/s, and 0.85 m/s level-ground walking, respectively, which demonstrated that VSPEA could effectively reduce the energy consumption and improve the endurance of the prosthetic knee joint.

Assembly of low-stiffness parts through admittance control with adaptive stiffness

Existing studies on robot assembly mostly focus on high-stiffness metal parts, which are unsuitable for assembling low-stiffness parts. Additionally, when assembly is achieved by the conventional admittance control method, the number of parameters to be adjusted is significant, and unstable contact force may occur. This study proposes two algorithms, namely admittance control with variable stiffness (ACVS) and admittance control with adaptive stiffness (ACAS), which are suitable for robotic assembly of low-stiffness parts. The proposed ACVS is achieved without a position trajectory, which reduces the number of parameters that need to be adjusted by the user. Furthermore, overshoot is suppressed by calculating the stiffness parameter in real time. The ACAS scheme adjusts the stiffness value in ACVS through a multilayer feedforward neural network; therefore, its overshoot is lower than that of ACVS, and the contact force stability is enhanced. The ACAS scheme thus characterized by a slight overshoot and stable contact force enables stable assembly of low-stiffness parts via the maximum force generated by the robot.

Experimental and numerical investigations on adaptive stiffness double friction pendulum systems for seismic protection of bridges

This study examines the design, development, and characterization of double friction pendulum systems with variable sliding surface geometry. The proposed isolator has a spherical surface with a small radius within an inner sliding range while different geometric functions are used to describe the sliding surface in the outer range. As a result of variable curvature used as sliding surface, the isolators exhibit an adaptive stiffness behavior with increasing displacement amplitude. First, the analytical equations that describe the characteristics of the proposed isolators were derived. Then, a large-scale prototype adaptive isolator that employs a logarithmic function for the sliding surface geometry in the outer range was fabricated. The response of the prototype isolator was characterized under increasing amplitude reversible cyclic loads considering two different levels of vertical isolator loads. A high-fidelity finite element model was developed to capture the response of the prototype isolator. The results obtained from the experimental tests and numerical simulations were used to verify the analytical equations derived for the proposed isolators. Next, a typical multi-span bridge structure was modeled with adaptive stiffness isolators and its response under bi-directional ground motions records were analyzed. In contrast to a conventional double friction pendulum bearing, the suggested isolators yield significantly reduced base shear forces while exhibiting greater isolator displacements. However, the proposed isolators retain the capability to effectively curtail excessive displacements, ensuring the prevention of collapse.

Dynamics of nonlinear energy sink with a polynomial damped frequency converter

In recent years, significant strides have been made in enhancing stiffness design to improve energy dissipation efficiency. Nevertheless, there appears to be limited discourse on the concept of decomposing vibrational energy into multiple frequencies to enable concurrent dissipation. This paper introduces an innovative isolator denoted as NES-C, characterized by polynomial variable stiffness, consisting of a nonlinear energy sink and a polynomial damped frequency converter (PDFC). Tristate NES-C and quad-state NES-C configurations are realized by exploiting the nonreciprocal nature of energy propagation within the PDFC framework. Numerical simulations reveal that NES-C exhibits adaptive stiffness modulation contingent upon vibration intensity, facilitates frequency decomposition, and exhibits superior damping characteristics compared to the conventional Type I NES. Investigation into the influence of various parameters on the vibration mitigation performance of NES-C reveals that quad-state NES-C exhibits heightened robustness relative to tristate NES-C, particularly in cases involving smaller initial impulses. Additionally, the amalgamated attributes of PDFC impart enhanced programmability to NES-C design.

Longitudinal Evidence for High-Level Patellar Tendon Strain as a Risk Factor for Tendinopathy in Adolescent Athletes

BackgroundHigh tendon strain leads to sub-rupture fatigue damage and net-catabolic signaling upon repetitive loading. While high levels of tendon strain occur in adolescent athletes at risk for tendinopathy, a direct association has not yet been established. Therefore, in this prospective longitudinal study, we examined the hypothesis that adolescent athletes who develop patellar tendon pain have shown increased levels of strain in advance.MethodsIn 44 adolescent athletes (12–17 years old), patellar tendon mechanical properties were measured using ultrasonography and inverse dynamics at four time points during a season. Fourteen athletes developed clinically relevant tendon pain (SYM; i.e., reduction of the VISA-P score of at least 13 points), while 23 remained asymptomatic (ASYM; VISA-P score of > 87 points). Seven cases did not fall into one of these categories and were excluded. Tendon mechanical properties of SYM in the session before the development of symptoms were compared to a randomly selected session in ASYM.ResultsTendon strain was significantly higher in SYM compared to ASYM (p = 0.03). The risk ratio for developing symptoms was 2.3-fold higher in athletes with tendon strain ≥9% (p = 0.026). While there was no clear evidence for systematic differences of the force applied to the tendon or tendon stiffness between SYM and ASYM (p > 0.05), subgroup analysis indicated that tendon force increased prior to the development of symptoms only in SYM (p = 0.034).DiscussioThe study provides novel longitudinal evidence that high tendon strain could be an important risk factor for patellar tendinopathy in adolescent athletes. We suggest that inadequate adaptation of tendon stiffness to increases in muscle strength may occur if adolescent athletes are subject to mechanical loading which does not provide effective tendon stimulation.

The performance investigation of flow-induced motion energy-harvesting by applying time-varying stiffness in the dynamic ocean environment

Ocean water exhibits periodic motion both temporally and spatially. However, no time-varying mechanism has been studied for harvesting energy from dynamic ocean. This paper presents an enhanced mechanism for harvesting the energy of ocean flows by applying time-varying stiffness adaptation in dynamic ocean environments, resulting in superior performance. By conducting numerical simulations and experiments, this study investigates the motion response and energy harvesting characteristics of a single oscillating cylinder equipped with time-varying stiffness. The research focuses on examining its behavior in both uniform flow and shear flow conditions, and presents several noteworthy findings. First, in comparison to impulse and trapezoid patterns, the sin-patterned time-varying stiffness demonstrates superior energy harvesting performance in both uniform flow and time-varying flow conditions. Second, the frequency of the sin-patterned time-varying stiffness is a critical parameter that influences the energy harvesting performance. Setting the sin-patterned time-varying stiffness frequency to ωv = 0.75ωn (natural frequency of constant stiffness) yields optimal performance under both uniform and time-varying flow conditions. At ωv = 0.75ωn, the energy-harvesting power can be increased by more than 30 times compared to the case of constant stiffness. Simultaneously, the oscillator's motion frequency experiences a reduction of over 40%. It is within this frequency range that the energy-harvesting power is maximized across the entire frequency spectrum. Third, as the frequency of sin-patterned time-varying stiffness increases, there is a decrease in both the energy harvesting power and efficiency. At a frequency of ωv/ωn* = 1.5, the advantage over the constant stiffness oscillator becomes negligible. Fourth, the energy harvesting performance of time-varying stiffness is superior in a time-varying flow condition compared to a uniform flow condition. The maximum energy harvesting power is achieved at ωv/ωn* = 0.75 in the time-varying flow condition, surpassing the corresponding value in the uniform flow condition. Fifth, the phase-shift resonance between the shedding frequency and the frequency of time-varying stiffness significantly enhances the energy harvesting performance of the time-varying stiffness oscillator. The optimal time-varying frequency is determined to have a phase difference of 0.15ωn. Sixth, the power harvesting efficiency can be improved by up to 1.5 times on average using the time-varying stiffness mechanism (sin-patterned, 10% additional power required) compared to state-of-the-art technologies employing constant stiffness. Finally, matching the form and dominant frequency of the excitation force with those of the time-varying stiffness can greatly improve the energy harvesting power and motion amplitude of the oscillator. This paper explores a performance-based approach to energy harvesting in dynamic ocean environments by applying sin-patterned time-varying stiffness with ωv = 0.75ωn.

Experimental and simulation studies of adaptive stiffness double friction pendulum bearing

To decrease residual displacements and forces on bridges, an adaptive stiffness double friction pendulum bearing (AS-DFPB) was proposed. Its stiffness was adjusted by the geometrical surfaces of the bearing, which has a spherical surface with a small radius in the inner range and an exterior variable surface with an outline of variable slope. Two specimens of the bearing were fabricated and tested. In addition, numerical simulation was employed to examine the mechanical performance of the bearing. In comparison to conventional bearings, the proposed bearing can reduce the residual displacement and forces on bridge piers.

Bi-directional semi-active tuned mass damper for torsional asymmetric structural seismic response control

Tuned mass damper (TMD) is a popular vibration absorber. To improve the seismic protection performance of a bi-directional passive TMD, a bi-directional semi-active TMD (BSTMD) with adaptive stiffness, variable mass and semi-active damping is proposed in this study. The BSTMD can adapt to structural X- and Y-directional instantaneous frequencies at the same time under bi-directional ground motions, which are identified through wavelet transform (WT), and the instantaneous angle of a diamond spring device is adjusted according to the instantaneous frequency ratio of two directions, which means that the stiffness ratio of two directions is retuned first. Then, the mass is retuned to match the exact instantaneous frequency. At last, the semi-active damping is realized according to an extended output signals-based control algorithm. A two-directional asymmetric torsional benchmark model is proposed as a case study. For comparison, two optimal passive TMDs implemented in X and Y directions separately and two unidirectional STMDs with variable stiffness and damping implemented in X and Y directions separately are investigated as well. Numerical results show that under bi-directional earthquake excitations in the benchmark problem, the two optimal passive TMDs have a good control effect. Two unidirectional STMDs have the best aseismic mitigation effect generally, in both lateral and torsional dynamic responses; while the BSTMD has a comparable control effect with less electromechanical devices, and can reduce the stroke of unidirectional STMD to a great degree. Though the BSTMD is aimed at X- and Y- directional dynamic responses, it can control torsional responses effectively as well, and when the eccentricity is enlarged to double, it remains effective for seismic mitigation.

Application of metal rubber for semi-active vibration control of mechanical transmission systems

This study proposes a novel semi-active vibration control technique for mechanical transmission systems. The backbone of current research technology is the adaptive stiffness and damping properties of metal rubber (MR) which plays the role of tuning to the vibrational system eigenmodes. The adaptive properties of MR material were proved by a previous study, but the present research investigates the effect of this feature on the structural response and designs an MR-based semi-active vibration controller for industrial application. For this purpose, the semi-active control strategy is created based on an optimal tuning using experimental harmonic test results. The control strategy is applied in practice using a linear actuator controlled by a double pole double throw (DPDT) relay, and National Instruments hardware and software for reading data and writing the control task. The isolator attachment position is defined through the analysis of eigenmode shapes that are predetermined by numerical simulation. A maximum isolation rate of 55.17% is measured near the vibration isolator position. It is observed that the isolation position has a prominent effect on structure vibration behavior where the elastic wave has been reduced in amplitude by passing the isolator position. The effectiveness of the MR-based semi-active control approach is eminent and its application for vibration isolation of commercial mechanical transmission systems could be studied in the future.

Development and investigation of a self-centering cable brace with variable slip stiffness for braced frame structure

Extremely rarely occurring earthquake (EROE) level is currently not often considered in most national codes or standards, and the earthquake damage has shown that its consequences are serious. To control the seismic responses of the structures under the EROE level, a novel self-centering cable brace with variable slip stiffness (VSS-SCB) is proposed as the lateral force resisting component of braced frame structure. Firstly, the theoretical analysis of the damper in the VSS-SCB was carried out, it is found that the slope angle, friction coefficient, and compressive stiffness of the disc springs are the main parameters affecting the mechanical behavior of the damper. The combination of strong and weak stacks of disc springs can effectively provide a self-centering damper with adaptive stiffness. Then, pseudo-static tests were carried out on the design specimens of the damper. The test results show that the hysteretic curve of the self-centering cable brace with constant slip stiffness (CSS-SCB) presents a classical flag type, and the residual deformation can be ignored. When the disc spring stacks with different stiffness are combined in series, there are significant strengthened points for the loading and unloading stiffness of the brace. Finally, the displacement-based design method was applied to the design of the braced frame structures with the VSS-SCBs. The dynamic analysis shows that the braced frame structure with the CSS-SCBs has larger displacement response than that with the buckling-restrained braces (BRBs), and the hysteretic curves of the CSS-SCB and VSS-SCB show good self-centering capability. When the deformation of the CSS-SCB and VSS-SCB exceeds the strengthened point, the large strengthened stiffness of VSS-SCB can control the seismic response of the braced frame structure. The seismic fragility curves of the braced frame structures with the BRBs and with the VSS-SCBs are comparable, and are significantly smaller than that with the CSS-SCBs. However, since the BRB is prone to significant residual deformation, the braced frame structure with the VSS-SCBs has more advantages in improving the seismic resilience of the structure.