Adaptive Stiffness Research Articles

Environmental adaptive control of a snake-like robot with variable stiffness actuators

This work investigates adaptive stiffness control and motion optimization of a snake-like robot with variable stiffness actuators. The robot can vary its stiffness by controlling magnetorheological fluid (MRF) around actuators. In order to improve the robot's physical stability in complex environments, this work proposes an adaptive stiffness control strategy. This strategy is also useful for the robot to avoid disturbing caused by emergency situations such as collisions. In addition, to obtain optimal stiffness and reduce energy consumption, both torques of actuators and stiffness of the MRF braker are considered and optimized by using an evolutionary optimization algorithm. Simulations and experiments are conducted to verify the proposed adaptive stiffness control and optimization methods for a variable stiffness snake-like robots.

Neuro-controller for antagonistic bi-articular muscle actuation in robotic arms based on terminal attractors

The conventional limitations of the robotic actuation mechanisms have led to many researchers needing to explore novel biomimetic motor mechanisms as the antagonistic human motor system. In this way, it is of interest to understand the inherent adaptive stiffness, or compliance, and modulation, in different alternative actuation architectures such as the antagonistic bi-articular (AbA) system. These novel AbA actuation mechanisms are characterized by resembling the efficient tendon and muscle build-up over our skeletal structure. In this paper, we propose a Cartesian neuro-controller for a robot manipulator actuated by a simplified adaptive viscoelastic linear AbA system. It is shown that the adaptive closed-loop system enforces terminal attractors, induced by a continuous model-free sliding mode control, simultaneously with a learning algorithm to compensate parametric uncertainties of AbA system through a low dimensional neural network. Numerical simulation results exhibit the feasibility of this approach.

Mathematical modeling and finite element analysis of the mechanical behavior of hybrid structures in complex materials

In this research work, the mechanical behavior of a hybrid sandwich beam in magnetorheological elastomer loaded with 40% ferromagnetic particles has been studied. A finite element model has been developed for the three layers. The sandwich beam is modeled using transverse displacement at core layer. The finite element model of the damped three-layer beam is assumed that every layer has the same transverse displacement, and it is derived using the Golla–hughes–McTavish’s principle. Different specimens have been modeled by varying the magnetic field intensity and static force and studied under the clamped-free and cantilever boundary conditions for modal analysis. Results show the influence of MRE adaptive stiffness and the loss factor in the static behavior of the sandwich studied. The structure proposed can be directly applied to civil engineering, for example, aeronautics, aerospace, and building foundations.

Assessment of pseudo-plastic and dilatant materials flow in channel driven cavity: application of metallurgical processes

Non-Newtonian fluid rheology representing the properties of pseudoplastic and dilatant materials has received overwhelming attention due to extensive applications in industrial and technological sectors like in metallurgical processes, shock absorbing materials, smart structures, and devices with adaptive stiffness, damping, emulsions, suspensions and so forth. Thus, in current communication characteristics of power law fluid elucidating attributes of pseudoplastic and dilatant materials in channel driven cavity is addressed. Finite element method (FEM) is implemented to interpret rheological features of power law fluid by varying flow controlling parameters. Discretization of domain at coarse level is performed by using stable first and second order polynomial (P2−P1) shape functions. Square shaped cylinder is placed at (1, 1.5) above the cavity. Hydrodynamics forces like pressure difference drag and lift variations are measured at outer surface of cylinder. The impact of primitive parameters like power law index (n) and Reynold number on velocity, pressure and viscosity for shear thinning and thickening cases is adorned. It is deduced that pressure difference increased against the variation in power law index. In similar way the impact of drag and lift forces mounts by increasing power law index. Reynold number has delineating impact on drag and lift forces near the obstacle. It is also seen that pressure shows optimized non-linear behavior near the obstacle and becomes linear along the downstream as expected in channel flow. It is divulged that pressure drops more rapidly for increasing magnitude of Reynold number. Velocity of fluid increases when power law fluid flow is behaving as shear thinning fluid in comparison to shear thickening case.

Concurrent multi-scale optimization of hybrid composite plates and shells for vibration

In this paper, a concurrent multi-scale optimization framework is established for hybrid composite plates and shells, where fiber volume, fiber orientation and stacking sequence can be optimized simultaneously. Firstly, a finite element model of shell structure that contains patches is established. Then, the candidate material set of hybrid composites is calculated and assembled by different combinations of fiber volume, fiber orientation and stacking sequence at the material and laminate scales. Furthermore, the Discrete Material Optimization (DMO) method is employed to perform the concurrent multi-scale optimization. Two illustrative examples are used to verify the effectiveness of the proposed optimization framework, including a simple example of a hybrid composite plate and a complex engineering example of a double serpentine nozzle. In comparison to optimal results by the traditional constant-stiffness design method, the optimal results of the proposed framework achieve significant 19.8% and 14.0% improvements in the fundamental frequency under the constraint of material cost, respectively. It can be concluded that the proposed concurrent multi-scale optimization framework has huge potential in adaptive stiffness tailoring for hybrid composite plates and shells with complex multi-scale design variables, which can make full use of hybrid composite materials to improve the structural performance against vibration while maintaining the low material cost.

Variable Stiffness Iterative Learning Control for Flexible-Joint Robot

Flexible-Joint (FJ) robots have attracted researchers’ attention in these years for their good compliance and the safety. But as a matter of fact, it is hard to design a suitable controller with stiffness adaptation for FJ robots due to the model uncertainties and the nonlinear systems when using elastic components. In this paper, an iterative learning control method for flexible-joint robot with model uncertainties in both robot dynamics and actuator dynamics is proposed, which involves the use of mechanical elastic elements with adjustable stiffness. Adjusting the stiffness of the joints by using the positions of the roller installed inside of the actuator, the joints generate fixed periodic motions and reduced the tracking error. We show the theory of the variable stiffness and the adaptive law, which involve the iterative learning controller. Simulation results also demonstrate that we achieve a perfect result while generating the desired motions.

АДАПТИВНЫЕ ВОЗМОЖНОСТИ ГИДРОМЕХАНИЧЕСКИХ МУФТ С ДИФФЕРЕНЦИАЛЬНЫМ ПЕРЕДАТОЧНЫМ МЕХАНИЗМОМ

For accident free operation of a technological machine it is necessary to protect it against dynamic loads of high intensity. High dynamic loads are often stipulated by an engineering process and during the process of work a frequency of their impact may change in a wide range. In this case the elastic clutches used in the machine driving gear are not always efficient because of resonance phenomena. The design of a hydro-mechanical coupling with a clogged differential driving gear allows controlling clutch stiffness by means of changing parameters of its hydro-system in the course of technological machine operation. 
 There are carried out investigations of stiffness adaptation potentialities of a hydro-mechanical coupling with a differential driving gear to a changing fre-quency of dynamic load impacts of the technological machine on the basis of a developed simulator of coupling operation dynamics taking into account stress-strain characteristics of the driving gear and processes taking place in the coupling hydro-system. The dependence of a stiffness coefficient of the coupling upon such hydro-system parameters as initial pressure in a hydro-accumulator and liquid consumption. 
 As a result of numerical computation of coupling dynamic characteristics in the Matlab application program package there are obtained dependencies allowing the definition of rational values of hydro-system parameters at which the amplitude of the dynamic load influencing a driving gear is minimum. There are shown the results of the investigations of the coupling under the analysis on the experimental bench developed specially which confirmed coupling functional working capacity and simulator adequacy.

Snap-through and stiffness adaptation of a multi-stable Kirigami composite module

Multi-stable composite laminates have been used to create adaptive and shape-morphing structures for many applications. However, it remains challenging to use these composites to obtain complicated shape changes and mechanical property adaptations. This study proposes a “Kirigami composite” concept to help address this challenge. By strategically placing slit cuts into the composite laminate with carefully designed fiber layout, one can release the internal constraints and significantly enrich the achievable shapes and adaptive functions. This study focuses on the elementary Kirigami composite module consisting of a single slit cut and two bistable patches. Experiments and finite element simulations show that this Kirigami module exhibits four different stable equilibria and its snap-through instability originates from a rapid “run-away” growth of surface curvature inversions due to connecting tab. This study also investigates a stiffness adaptation function. These results can be used for creating more sophisticated Kirigami composite structures with multiple patches and cuts.

Triceps Surae Muscle-Tendon Unit Properties in Preadolescent Children: A Comparison of Artistic Gymnastic Athletes and Non-athletes

Knowledge regarding the effects of athletic training on the properties of muscle and tendon in preadolescent children is scarce. The current study compared Achilles tendon stiffness, plantar flexor muscle strength and vertical jumping performance of preadolescent athletes and non-athletes to provide insight into the potential effects of systematic athletic training. Twenty-one preadolescent artistic gymnastic athletes (9.2 ± 1.6 years, 15 girls) and 11 similar-aged non-athlete controls (9.0 ± 1.7 years, 6 girls) participated in the study. The training intensity and volume of the athletes was documented for the last 6 months before the measurements. Subsequently, vertical ground reaction forces were measured with a force plate to assess jumping performance during squat (SJ) and countermovement jumps (CMJ) in both groups. Muscle strength of the plantar flexor muscles and Achilles tendon stiffness were examined using ultrasound, electromyography, and dynamometry. The athletes trained 6 days per week with a total of 20 h of training per week. Athletes generated significantly greater plantar flexion moments normalized to body mass compared to non-athletes (1.75 ± 0.32 Nm/kg vs. 1.31 ± 0.33 Nm/kg; p = 0.001) and achieved a significantly greater jump height in both types of jumps (21.2 ± 3.62 cm vs. 14.9 ± 2.32 cm; p < 0.001 in SJ and 23.4 ± 4.1 cm vs. 16.4 ± 4.1 cm; p < 0.001 in CMJ). Achilles tendon stiffness did not show any statistically significant differences (p = 0.413) between athletes (116.3 ± 32.5 N/mm) and non-athletes (106.4 ± 32.8 N/mm). Athletes were more likely to reach strain magnitudes close to or higher than 8.5% strain compared to non-athletes (frequency: 24% vs. 9%) indicating an increased mechanical demand for the tendon. Although normalized muscle strength and jumping performance were greater in athletes, gymnastic-specific training in preadolescence did not cause a significant adaptation of Achilles tendon stiffness. The potential contribution of the high mechanical demand for the tendon to the increasing risk of tendon overuse call for the implementation of specific exercises in the athletic training of preadolescent athletes that increase tendon stiffness and support a balanced adaptation within the muscle-tendon unit.

Analytical Modeling of Multi-sectioned Bi-stable Composites: Stiffness Variability and Embeddability

Multi-stable composite laminates have been widely investigated for morphing applications due to distinct geometrical characteristics about each stable configuration. Recently, a mechanism for realizing on-demand stiffness adaptation in compliant structures has been proposed exploiting the stiffness variation from switching between the stable states of embedded bi-stable laminates. This allows for reducing the coupling between loading and morphing deformation modes, broadening the overall design space for morphing structures. However, the design of such embeddable laminates requires the study of multiple lamination domains to allow embedding within larger compliant structures. This process is currently done by computationally expensive simulations. We present an analytical model to predict the stable shapes and structural characteristics of multi-section, multi-stable composite laminates subject to two clamped boundary conditions. The analysis is conducted employing the Rayleigh-Ritz method using polynomial approximations of the displacement fields. The structural characteristics are predicted while constraining the laminate edges, thus extending previous models designed to account mainly for free edge boundary conditions. Our model is subsequently applied to explore the design space for characterizing the stability and variable stiffness properties of two different classes of bi-stable laminates. The presented model allows for the efficient design of multi-stable laminates embeddable within compliant structures.

Adaptive Stiffness and Damping Impedance Control for Environmental Interactive Systems With Unknown Uncertainty and Disturbance

This paper presents an impedance controller with adaptive stiffness and damping terms for environmental interactive system with unknown modeling error and external disturbance. The objective is to compensate the uncertainty and disturbance and form a desired close-loop impedance architecture. A disturbance observer for nonlinear system is employed to estimate the lumped uncertainty and disturbance. The stiffness and damping are adjusted by the state-variable-related negative power type parameters. Consequently, the control input is designed with the consideration of the estimation of lumped uncertainty and disturbance as well as the regulated stiffness and damping. The stability is proved and the criterion of parameter selection is pointed out. Although the state-variable-related stiffness and damping parameters will intuitively be singular, the system works without any singularity due to the independence of these parameters in stability analysis. To verify the proposed approach, a one degree of freedom mass system is adopted and numerical simulation is performed accordingly. The simulation results demonstrate the effectiveness and the superiority of the proposed control scheme for environmental interactive system with unknown modeling error and external disturbance.

Study of a novel adaptive passive stiffness device and its application for seismic protection

The present paper proposes a novel adaptive passive stiffness (APS) device which can produce variable stiffness in an adaptive fashion. This APS device consists of four springs arranged in a rhombus configuration and a guidance template. The stiffness of the device is controlled through varying the angle of the rhombus configuration. The function of the guidance template is to relate the angle of the rhombus to the targeted system response such that the stiffness can be varied in an adaptive manner, without the need for feedback or external energy. Two analytical models are developed to capture the behavior of the APS device which shows good agreement with the experimental data. Experimental result indicates that the proposed APS device can produce adaptive stiffness as expected. A cubic nonlinearity is produced and deployed in a single-degree-of-freedom (SDOF) system to form a hardening Düffing oscillator. Dynamic tests show that the experimental characteristics of the Düffing system agrees well with the numerical results, thereby demonstrating that the APS device is effective in producing smoothly and continuously adaptive stiffness in an adaptive fashion. In addition, a fourth order even function is used as the guidance template to produce a softening-hardening adaptive stiffness. The performance of this adaptive stiffness is numerically evaluated for seismic protection. It is found that the structural responses can be reduced effectively when using this softening-hardening adaptive stiffness. The key value of this APS device is that the guidance template can be easily changed to any desired continuous function such that the targeted adaptive stiffness can be obtained. This provides possibility for controlling the vibrations of structures in a passively adaptive manner.

Adaptive stiffness control of passivity-based biped robot on compliant ground using double deep Q network

Passive dynamic walking exhibits human-like and energy-efficient gait. Biologically inspired compliance introduced to flexible passivity-based robot would be helpful to generate stable locomotion. However, designing adaptive controller for flexible biped on compliant ground still remains a challenge. This paper aims to design an adaptive and model-free stiffness controller for passivity-based flexible biped on compliant ground, where the hip stiffness is modulated by double deep Q network. One benefit of the double deep Q network is that adaptive stiffness control policy could be directly learned from inputs. At first, passive dynamic walking gait is utilized as a reference trajectory during double deep Q network training. Then the trained double deep Q network is used as adaptive stiffness controller for biped on compliant ground. Simulation results show that the passivity-based biped robot could walk in such walking cases as disturbed initial condition, level compliant ground, downslope slippery compliant surface, and varying compliance environments. The adaptive stiffness controller would be used to make the passivity-based biped robot adapt to the environmental changes.

Harnessing the anisotropic multistability of stacked-origami mechanical metamaterials for effective modulus programming

This study examines a three-dimensional, anisotropic multistability of a mechanical meta material based on a stacked Miura-ori architecture, and investigates how such a unique stability property can impart stiffness and effective modulus programming functions. The unit cell of this metamaterial can be bistable due to the nonlinear relationship between rigid-folding and crease material bending. Such bistability possesses an unorthodox property: the arrangement of elastically stable and unstable equilibria are different along different principal axes of the unit cell, so that along certain axes the unit cell exhibits two force–deformation relationships concurrently within the same range of deformation. Therefore, one can achieve a notable stiffness adaptation via switching between the two stable states. As multiple unit cells are assembled into a metamaterial, the stiffness adaptation can be aggregated into an on-demand modulus programming capability. That is, via strategically switching different unit cells between stable states, one can control the overall effective modulus. This research examines the underlying principles of anisotropic multistability, experimentally validates the feasibility of stiffness adaptation, and conducts parametric analyses to reveal the correlations between the effective modulus programming and Miura-ori designs. The results can advance many adaptive systems such as morphing structures and soft robotics.

Algorithmic Design of Low-Power Variable-Stiffness Mechanisms

Compliant actuators enabling low-power stiffness adaptation are missing ingredients and key enablers of next generation robotic systems. One of the key components of these actuators is the mechanism implementing stiffness adaptation that requires sophisticated control and nontrivial mechanical design. However, despite recent advances in controlling these systems, their design remains experience based and not well understood. In this paper, we present an optimization-based computational framework for the design of intrinsically low-power compliant variable stiffness mechanisms. The core ingredient of this framework is the mathematical formulation of the design problem—provided by a constrained nonlinear parameter optimization—which is computationally solved here to identify optimal variable stiffness designs. We show the basic capability of this formulation in finding parameters for variable stiffness mechanisms that require the least power by design. Further, we demonstrate the generality of this method in cross-comparing mechanisms with different kinematic topology to identify the one that requires the least power by design.

Control of a magnetorheological tuned vibration absorber for wind turbine application utilising the refined force tracking algorithm

This work covers selected control issues, including refined force tracking algorithm formulation, concerning the wind turbine tower-nacelle laboratory model equipped with a magnetorheological damper based tuned vibration absorber. The objective of the current research is a development and experimental implementation of the control algorithm that couples basic adaptive stiffness solution with stock magnetorheological damper force tracking concept to obtain a quality tower vibration reduction system. The experiments were conducted assuming monoharmonic, horizontal excitation applied to the assembly modelling the nacelle. The frequency range comprised the neighbourhood of the first bending mode of the tower-nacelle system. The results proved the effectiveness of the adopted algorithm referring to other high-performance solutions.

A User Study on Personalized Stiffness Control and Task Specificity in Physical Human–Robot Interaction

An ideal physical Human-Robot Interaction (pHRI) should offer the users robotic systems that are easy to handle, intuitive to use, ergonomic and adaptive to human habits and preferences. But the variance in the user behavior is often high and rather unpredictable which hinders the development of such systems. This article introduces a Personalized Adaptive Stiffness controller for pHRI which is calibrated for the user's force profile and validates its performance in an extensive user study with 49 participants on two different tasks. The user study compares the new scheme to conventional fixed stiffness or gravitation compensation controllers on the 7-DOF KUKA LWR IVb by employing two typical joint-manipulation tasks. The results clearly point out the importance of considering task specific parameters and human specific parameters while designing control modes for pHRI. The analysis shows that for simpler tasks a standard fixed controller may perform sufficiently well and that respective task dependency strongly prevails over individual differences. In the more complex task, quantitative and qualitative results reveal differences between the respective control modes, where the Personalized Adaptive Stiffness controller excels in terms of both performance gain and user preference. Further analysis shows that human and task parameters can be combined and quantified by considering the manipulability of a simplified human arm model. The analysis of user's interaction force profiles confirms this finding.

Dynamical Analysis and Realization of an Adaptive Isolator

An adaptive vibration isolation system is proposed in this paper to combine the advantages of both linear and nonlinear isolators. Because of the proposed structural piecewise characteristics for different levels of response, the stiffness and damping properties could be designed according to the vibration performances. The adaptive stiffness and damping properties are achieved by the joined utilization of symmetrical precompression triangle-like structure (TLS) and column frame with cam. In order to design the control mechanism with optimum structural parameters, nonlinear vibration performances are analyzed by using averaging method and singularity theory. The parameter plane is divided into transition sets, and then the optimization criterions for structural design are provided according to multiple nonlinear vibration performances including frequency band for effective isolation, multisteady state band and resonance peak, etc. The experiment is carried out to verify the theoretical selection of desirable parameters and indicates the advantages and improvement of vibration isolation/suppression brought by the structural property adaptation. This study provides a novel method of achieving structural property adaptation for the improvement of isolation effectiveness, which shows the intelligent realization by passive components.

A user-specific human-machine interaction strategy for a prosthetic shank adapter

Abstract For people with lower limb amputation, a user-specific human-machine interaction with their prostheses is required to ensure safe and comfortable assistance. Especially during dynamic turning manoeuvres, users experience high loads at the stump, which decreases comfort and may lead to long-term tissue damage. Preliminary experiments with users wearing a configurable, passive torsional adaptor indicate increased comfort and safety achieved by adaptation of torsional stiffness and foot alignment. Moreover, the results show that the individual preference regarding both parameters depend on gait situation and individual preference. Hence, measured loads in the structure of the prosthesis and subjective feedback regarding comfort and safety during different turning motions are considered in a user-specific human-machine interaction strategy for a prosthetic shank adaptor. Therefore, the interrelations of gait parameters with optimal configuration are stored in an individual preference-setting matrix. Stiffness and foot alignment are actively adjusted to the optimal parameters by a parallel elastic actuator. Two subjects reported that they experienced appropriate variation of stiffness and foot alignment, a noticeable reduction of load at the stump and that they could turn with less effort.

Passive material properties of stroke-impaired plantarflexor and dorsiflexor muscles

BackgroundFollowing a stroke, intrinsic muscle properties such as stiffness may be altered, which is accompanied by increased spasticity and contractures. Previously, quantification of muscle stiffness has been based off of indirect measurements. Using shear wave ultrasound elastography, direct measurements of muscle material properties can be made. MethodsOur aim was to evaluate material properties, specifically passive stiffness, using shear wave ultrasound elastography across a range of muscle lengths, in the medial gastrocnemius and the tibialis anterior in chronic stroke survivors. FindingsOur main results show significant increases of 27.7% and 26.9% in shear wave velocity of stroke-impaired medial gastrocnemius compared to the unimpaired contralateral side at 90° ankle angle (P=0.033) and 15° plantarflexion (P=0.001), respectively. However, no significant difference was found in the tibialis anterior between the two sides. Relatively weak correlations were found between SW velocity in the medial gastrocnemius and joint stiffness for both the non-paretic (ρ=0.384, P=0.001), and paretic side (ρ=0.363, P=0.002). Additionally, muscle stiffness estimates of stroke-impaired tibialis anterior from joint torque and angle measurements were significantly greater by 23.1% (P=0.033) than the unimpaired contralateral side. However, no significant difference was found in the medial gastrocnemius. InterpretationThese results indicate that there are non-uniform changes in passive stiffness of stroke-impaired muscle. Therefore, muscles need to be evaluated individually to assess alterations. Additionally, interpretation of joint-based calculations of muscle stiffness should be made cautiously. Having the ability to non-invasively assess muscle stiffness adaptations in vivo would aid in prognosis, evaluation, and treatment following a stroke.