Bioinspired Soft Tube-Foot Array With Variable Stiffness: Design, Characterization, and Application

In order to improve the environmental adaptability and interaction stability of industrial robotic arms, a flexible joint with variable damping and stiffness based on magnetorheological (MR) technology was proposed. The mechanical models for variable stiffness and damping of MR flexible joint were established, and the relationship between cam surface and variable stiffness element was derived based on the energy method. Subsequently, the variation patterns of equivalent stiffness and damping of the prototype under different excitations were experimentally tested, and the test results show that its mechanical properties meet the design requirements. To further verify the performance of the MR flexible joint on the robotic arm, we setup a collision buffering test system and investigated the buffering performance of the system under different working conditions. The buffering test results indicate that when the current of variable stiffness element is 2.0 A, the peak collision acceleration of the system after installing the MR flexible joint decreases by about 40.19% compare with the rigid collision.

This paper presents an innovative seat suspension with variable stiffness and variable damping (VSVD) via magnetorheological fluid dampers to enhance ride comfort. The VSVD seat suspension is developed based on an innovative structure with springs in series and damping units, and its principles of VSVD and vibration attenuation are evaluated theoretically. The proposed VSVD seat suspension is then manufactured and tested. The test results illustrate that the equivalent stiffness of the seat suspension with power on (Is = 2.0 A current) increased to 186% and the equivalent damping (Id = 1.0 A) raised to 520% compared to the seat suspension with no current applied. The evaluation of vibration-attenuation capability of the seat suspension is conducted by employing Fuzzy logic controls and both harmonic and random signals are considered as vibration excitations. The experimental data show that the proposed seat suspension with controlled stiffness and damping results in lower seat acceleration compared to a passive seat or one with traditional damping control.

Vibration isolation methods that vary damping and stiffness have demonstrated excellent authority over system vibration, thus, potentially making them attractive for many applications. However, conventional devices for controlling variable stiffness are typically complicated and difficult to implement in most applications. To address this issue, a new method is proposed that requires two magnetorheological (MR) fluid dampers placed in series. With this configuration, variable damping and stiffness vibration control is simultaneously achieved by varying a small current to the MR dampers. This paper presents a theoretical and experimental analysis of a two degree-of-freedom system that is controlled by the MR dampers. Five different control schemes involving the variable damping and stiffness are explored. The time and frequency responses of the two degree-of-freedom system to a random input show that combined variable damping and stiffness control provides the best vibration isolation over a frequency range spanning the system’s two structural vibration modes. The experimental results agree well with the theoretical analysis.

A quasi-zero-stiffness (QZS) vibration isolation system presents the characteristics of high-static-low-dynamic, which can effectively isolate ultralow-frequency vibrations. However, most QZS vibration isolation systems are highly susceptible to external loads, causing deviations from the equilibrium position, which degrades their vibration isolation performance and severely restricts the range of QZS. To overcome these limitations, a novel vibration isolator was proposed by integrating a QZS structure with a variable stiffness and variable damping (VSVD) structure. The stiffness and damping of the vibration isolation system can be regulated to resist variable mass loads, and the QZS vibration isolation band can be broadened using the nonlinear positive stiffness of the conical rubber spring to effectively suppress ultralow-frequency vibrations. The dynamic characteristics of the QZS-VSVD vibration isolator were analyzed using the harmonic balance method. The effects of damping coefficients, excitation amplitude, and mass load on the amplitude–frequency characteristics and displacement transmissibility of the vibration isolation system were investigated. A virtual prototype of the QZS-VSVD vibration isolator was established using the ADAMS software, and vibration simulations were conducted. The results indicate that the QZS-VSVD vibration isolator can achieve ultralow-frequency vibration isolation at approximately 1 Hz under variable mass loads ranging from 3300 N to 3700 N. When the mass load was 3500 N, the amplitudes of the isolated object were attenuated by 70.6% and 86.2% under displacement excitation at 3 Hz and 5 Hz, respectively. Compared to linear isolators and a typical QZS structure, the proposed QZS-VSVD vibration isolator exhibited a greater load-bearing capacity, stronger anti-interference capability, and broader vibration isolation bandwidth.

Lighter structures are increasingly required for flexible spacecraft. However, low frequency and large amplitude vibration problems are unavoidable due to external disturbances or attitude maneuvering especially when working under a microgravity environment. Therefore, a new methodology involving a semi-active technique using an actively tunable joint with variable stiffness and damping control, capable of handling such issues is proposed in this study. The incentive active joint was conceived with a compact structure, based on the electromagnetic direct drive principle. First, a dynamic model of rotating flexible plate was established. Then, a prototype was fabricated and tested. Finally, on the one hand, numerical simulations and experimental results indicated that, when the joint torsional stiffness changed, a frequency shift phenomenon occurred. On the other hand, two types of noncontact periodic vibration excitation methods along the rotation direction were proposed and the experimental results validated that the major frequency bandwidth of interference signal was 0.08–0.52 Hz with a significant vibration attenuation of 5.5–31.5 dB in effective bandwidth. Moreover, in the low frequency range (0.08–0.43 Hz), variable damping was found to be the main factor and variable stiffness was the secondary factor. However, in the high frequency range (around 0.52 Hz), variable stiffness was dominant and variable damping was inferior. These findings are expected to effectively suppress the low frequency and large amplitude vibration of solar panels with flexible joints.

The capability of a variable stiffness and damping (VSVD) suspension system via a magnetorheological (MR) damper on improving vehicle lateral stability is investigated in this paper. First, a type of the VSVD suspension system is briefly introduced with the application of variable damping to realize the capability of VSVD. Then, variable damping via the MR damper is extended, and the characteristics of damping force are presented with the Bouc-Wen model. Then, the proposed VSVD system is inserted into a full vehicle model as a front suspension system to control tire normal forces. A control strategy composed of a fuzzy controller that the output is wheel slip ratio and a simple on/off controller to model the application of VSVD is developed to illustrate its ability in improving vehicle stability. It is shown from simulation that VSVD can increase tire normal forces and make vehicles present better performance on lateral stability.

For Variable Stiffness (VS) composites with steered curvilinear tow paths, the fiber orientation angle varies continuously throughout the laminate, and is not required to be straight, parallel and uniform within each ply as in conventional composite laminates. Hence, the thermal properties (conduction), as well as the structural stiffness and strength, vary as functions of location in the laminate, and the associated composite structure is often called a “variable stiffness” composite structure. The steered fibers lead not only to the alteration of mechanical load paths, but also to the alteration of thermal paths that may result in favorable temperature distributions within the laminate and improve the laminate performance. Evaluation of VS laminate performance under thermal loading is the focus of this chapter. Thermal performance evaluations require experimental and numerical analysis of VS laminates under different processing and loading conditions. One of the advantages of using composite materials in many applications is the tailoring capability of the laminate, not only during the design phase but also for manufacturing. Heat transfer through variable conduction and chemical reaction (degree of cure) occurring during manufacturing (curing) plays an important role in the final thermal and mechanical performance, and shape of composite structures. Three case studies are presented in this chapter to evaluate the thermal performance of VS laminates prior to and after manufacturing. The first case study is a numerical analysis that investigates the effect of variable conductivity within the VS laminate on the temperature and degree of cure distribution during its cure cycle. The second case study is a numerical analysis that investigates the transient thermal performance of a rectangular VS composite laminate. Variable thermal conductivity will affect the temperature profile and results are compared to a unidirectional composite. The effect of fiber steering on transient time and steady state solutions is compared to the effect of unidirectional fibers. The third case study is an experimental one that was conducted to evaluate the thermal performance of two variable stiffness panels fabricated using an Advanced Fiber Placement (AFP) Machine.

Semi-active vibration isolation system with variable stiffness and damping control

Wrist-worn photoplethysmography (PPG) sensors allow for continuous heart rate (HR) measurement without the inconveniences of wearing a chest belt. Although green light PPG technology reduces HR measurement motion artifacts, only a limited number of studies have investigated the reliability and accuracy of wearables in non-laboratory-controlled conditions with actual specific and various physical activity movements. The purpose of this study was to (1) assess the reliability and accuracy of the PPG-based HR sensor of the Fitbit Charge 4 (FC4) in ecological conditions and (2) quantify the potential variability caused by the nature of activities. We collected HR data from participants who performed badminton, tennis, orienteering running, running, cycling, and soccer while simultaneously wearing the FC4 and the Polar H10 chest belt (criterion sensor). Skin tone was assessed with the Fitzpatrick Skin Scale. Once data from the FC4 and criterion data were synchronized, accuracy and reliability analyses were performed, using intraclass correlation coefficients (ICCs), Lin concordance correlation coefficients (CCCs), mean absolute percentage errors (MAPEs), and Bland-Altman tests. A linear univariate model was also used to evaluate the effect of skin tone on bias. All analyses were stratified by activity and pooled activity types (racket sports and running sports). A total of 77.5 hours of HR recordings from 26 participants (age: mean 21.1, SD 5.8 years) were analyzed. The highest reliability was found for running sports, with ICCs and CCCs of 0.90 and 0.99 for running and 0.80 and 0.93 for orienteering running, respectively, whereas the ICCs and CCCs were 0.37 and 0.78, 0.42 and 0.88, 0.65 and 0.97, and 0.49 and 0.81 for badminton, tennis, cycling, and soccer, respectively. We found the highest accuracy for running (bias: 0.1 beats per minute [bpm]; MAPE 1.2%, SD 4.6%) and the lowest for badminton (bias: -16.5 bpm; MAPE 16.2%, SD 14.4%) and soccer (bias: -16.5 bpm; MAPE 17.5%, SD 20.8%). Limit of agreement (LOA) width and artifact rate followed the same trend. No effect of skin tone was observed on bias. LOA width, bias, and MAPE results found for racket sports and soccer suggest a high sensitivity to motion artifacts for activities that involve "sharp" and random arm movements. In this study, we did not measure arm motion, which limits our results. However, whereas individuals might benefit from using the FC4 for casual training in aerobic sports, we cannot recommend the use of the FC4 for specific purposes requiring high reliability and accuracy, such as research purposes.

The system of shear wall as an additive in retrofitting of soft storey structure has been investigated in previous and present research to improve the strength of structures due to seismic load. The aim of this research is to conduct comparative study on Equivalent Static Analysis (ESA), Response Spectrum Analysis (RSA) and Time History Analysis (THA) of structure between reference sample and shear wall systems was performed in order to determine the strength and stiffness of soft storey structure. The locations and shapes of shear wall system were used as retrofitting method. The retrofitting of soft storey structure was done for fives (5) models consist of one (1) model without shear wall as reference sample and the other four (4) models of different location and shape of shear wall systems. The models were tested by using SAP200 computer software to performed strength of structure. The models had been checking by value of lateral displacement and base shear resistance. Based on the collected data, the additional of shear wall was found to increase the strength and stiffness of soft storey structure. The swastika shape of shear wall and provide at centre experienced the maximum base shear resistance in all X, Y and Z direction and the least displacement compared to other model. Through this study had shown the improvement of strength of soft storey structure by applying shear wall system as an additive strength and stiffness of structure due earthquake event.

In this paper, we present a novel compliant robotic gripper with three variable stiffness fingers. While the shape morphing of the grippers is cable-driven, the stiffness variation is enabled by layer jamming. The inherent flexibility makes compliant grippers suitable for tasks such as grasping soft and irregular objects. However, their relatively low load capacity due to low structural stiffness limits their applications. Variable stiffness robotic grippers have the potential to address this challenge as their stiffness can be tuned on demand based on the needs of tasks. Layer jamming is an emerging method for variable stiffness due to its advantages of light weight, simple and quick actuation. In our design, the compliant backbone of the fingers is made of 3d printed PLA material. Four thin film materials are attached to each side of the skeleton. The working process of the robotic gripper follows two basic steps. First, the compliant skeleton is bent to a desired shape by actuating a tension cable via a servo motor. Second, upon application of a negative pressure by a vacuum pump, the finger is stiffened up owing to the increasing of the friction between contact surfaces of layers preventing their relative movement. Since the structural stiffness of the fingers is increased, their load capacity will be increased proportionally. When the air pressure is sufficiently large, the morphed shape can even be locked (no slipping). Test for stiffness of individual finger and load capacity of the robotic gripper are conducted to validate capability of the design. The results showed a 69-fold increase in stiffness of individual finger and a 30-fold increase in gripper’s load capacity.

The nitrogen adsorption capacity is of great significance for the characterization of porous materials, carbon sequestration, and coalbed methane development. Essentially, this is a typical research scenario of solid-gas interface interaction in the broad field of interface science. Accurate prediction of adsorption volume had proven challenging due to the highly nonlinear relationship between coal rocks complex characteristics and adsorption behavior. This study investigated coal rocks of varying particle sizes. Based on low-temperature liquid nitrogen adsorption experimental data, the pore structure characteristics were systematically analyzed. Random Forest (RF) and Support Vector Machine (SVM) models were subsequently employed, in combination with four optimization algorithms (Sparrow Search Algorithm (SSA), Snake Optimization (SO), Chameleon Swarm Algorithm (CSA), and Northern Goshawk Optimization (NGO)), to predict nitrogen adsorption volume. The results demonstrated that optimized models, particularly those enhanced by NGO and CSA, outperformed alternative approaches. Compared with SVM, the RF model achieved superior prediction accuracy. Among optimization algorithms, the RF model enhanced by NGO/CSA exhibited the best performance, attaining a coefficient of determination (R2) value of 0.9804. Regarding validation set performance, the optimized RF model showed substantial improvements, with a mean absolute error (MAE) of 0.1119, a mean absolute percentage error (MAPE) of 0.0545, a mean squared error (MSE) of 0.0389, and a root mean squared error (RMSE) of 0.1972. The R2 value of the optimal model on the validation set was improved by nearly 10% compared with the original RF model. Coal rock size constitutes a key predictive factor for nitrogen adsorption volume. 120 to 200 mesh exhibits the most significant influence. This research highlights the importance of particle size as a predictive factor and validates the effectiveness of integrating optimization algorithms with machine learning models for predicting nitrogen adsorption capacity in coal rocks. It also provides a novel quantitative method for the study of the interaction between solid and gas interfaces in interface science.

Compared with a rigid robotic hand, the soft robotic hand has the advantages of strong safety and high adaptability, but it has problems such as insufficient grasping force, poor attitude retention ability, and poor dynamic stability. Therefore, it is of great significance to introduce a variable stiffness mechanism so that the soft robotic hand can flexibly change its stiffness under different task requirements, thus improving its grasping stability. An innovative design of a novel particle clogging variable stiffness soft robotic hand, featuring superior stiffness performance, is presented in this article, which incorporates a novel particle-clogging variable stiffness structure coupled with a ball-jointed skeleton and particles, resulting in a 29.4% stiffness enhancement over traditional designs lacking a ball-jointed skeleton, while boasting a 217.7% increase in single-finger fingertip force, and an elevation of embracing grasping performance by 110.56% along with a 280.2% improvement in fingertip grasp performance, thereby significantly improving grasping force, stability and effectiveness for daily food, and tool grasping.

SUMMARYThis paper presents the conceptual designs, kinematics and dynamics modeling of a cooperative re-configurable Dual-Arm Cam-Lock Manipulator. A cam-lock manipulator is a robotics structure with a pair of multi-degree of freedom planar arms jointed together at a shared base. This manipulator is designed to be capable of performing a wide variety of tasks by automatically re-configuring itself to form a variable geometry, stiffness, damping, and workspace robotics structure by the virtue of a novel link/joint design along i''s arms, labeled as the cam-lock design. The kinematics and dynamics of this manipulator is described using admissible variables (i.e., variables that define the constrained admissible motion). Along with the dynamic relations of this manipulator, a generic equation was also developed for the joint servo system.

SUMMARY In the present study, a semi-active tuned mass damper (STMD) with variable damping coefficient and stiffness is evaluated under seismic excitations. Variation of the damping ratio of the STMD is implemented through tracking the displacement of the STMD.If the tracked amplitude of the STMD is increasing, damping ratio of the STMD is set to zero, or else it is set to an appropriate nonzero value. Stiffness of the STMD is tuned through tracking the displacement of the primary structure, which is analyzed using a short-time Fourier transform-based control algorithm. Both far-field and near-fault ground motions are used to examine the effectiveness of the STMD and the control algorithm. Displacement time history and response (displacement and acceleration) spectra are obtained for the cases of an optimal passive TMD and an STMD. It is found that the STMD with variable damping ratio and frequency can effectively attenuate the seismic responses and outperform the optimal passive TMD. In addition, results are obtained for the case that damage occurs to the primary structure during an earthquake. The study indicates that the STMD controlled by the proposed algorithm can rapidly capture the variation of the structure and remains tuned with the primary structure, whereas the optimal TMD becomes off-tuned when damage occurs. Copyright © 2013 John Wiley & Sons, Ltd.