Tracking of Stiffness Variation in Structural Members Using Input Error Function Observers

This study evaluates input error function observers for tracking of stiffness variation in real-time. The input error function is an Analytical Redundancy (AR)-based diagnosis method and necessitates a mathematical model of the system and system identification techniques. In practice, mathematical models used during numerical simulations differ from the actual status of the structure, and thus, accurate mathematical models are rarely available for reference. Noise is an unwanted signal in the input–output measurements but unavoidable in real-world applications (as in long span bridge trusses) and hard to imitate during numerical simulations. Simulation data from the truss system clearly indicates the effectiveness of the proposed structural damage detection method for estimating the severity of the damage. Optimization of the input error function can further automate the stiffness estimation in structural members and address critical aspects such as system uncertainties and the presence of noise in input–output measurements. Stiffness tracking in one of the planar truss members indicates the potential of optimization of the input error function for online structural health monitoring and implementing condition-based maintenance.

A new tunable elastic metamaterial structure for manipulating band gaps/wave propagation

A one-dimensional mechanical lattice system with local resonators is proposed as an elastic metamaterial model, which shows negative mass and negative modulus under specific frequency ranges. The proposed representative units, consisting of accurately arranged rigid components, can generate controllable translational resonance and achieve negative mass and negative modulus by adjusting the local structural parameters. A shape memory polymer is adopted as a spring component, whose Young’s modulus is obviously affected by temperature, and the proposed metamaterials’ tunable ability is achieved by adjusting temperature. The effect of the shape memory polymer’s stiffness variation on the band gaps is investigated detailedly, and the special phenomenon of intersecting dispersion curves is discussed, which can be designed and controlled by adjusting temperature. The dispersion relationship of the continuum metamaterial model affected by temperature is obtained, which shows great tunable ability to manipulate wave propagation.

Fully-Printable Soft Actuator with Variable Stiffness by Phase Transition and Hydraulic Regulations

Actuators with variable stiffness have vast potential in the field of compliant robotics. Morphological shape changes in the actuators are possible, while they retain their structural strength. They can shift between a rigid load-carrying state and a soft flexible state in a short transition period. This work presents a hydraulically actuated soft actuator fabricated by a fully 3D printing of shape memory polymer (SMP). The actuator shows a stiffness of 519 mN/mm at 20 ∘C and 45 mN/mm at 50 ∘C at the same pressure (0.2 MPa). This actuator demonstrates a high stiffness variation of 474 mN/mm (10 times the baseline stiffness) for a temperature change of 30 ∘C and a large variation (≈1150%) in average stiffness. A combined variation of both temperature (20–50 ∘C) and pressure (0–0.2 MPa) displays a stiffness variation of 501 mN/mm. The pressure variation (0–0.2 MPa) in the actuator also shows a large variation in the output force (1.46 N) at 50 ∘C compared to the output force variation (0.16 N) at 20 ∘C. The pressure variation is further utilized for bending the actuator. Varying the pressure (0–0.2 MPa) at 20 ∘C displayed no bending in the actuator. In contrast, the same variation of pressure at 50 ∘C displayed a bending angle of 80∘. A combined variation of both temperature (20–50 ∘C) and pressure (0–0.2 MPa) shows the ability to bend 80∘. At the same time, an additional weight (300 g) suspended to the actuator could increase its bending capability to 160∘. We demonstrated a soft robotic gripper varying its stiffness to carry objects (≈100 g) using two individual actuators.

Hydrostatic Bearing Characteristics Investigation of a Spherical Piston Pair with an Annular Orifice Damper in Spherical Pump

The spherical pump is a totally new hydraulic concept, with spherical piston and hydrostatic bearing, in order to eliminate the direct contact between the piston and cylinder cover. In this paper, the governing Reynolds equation under spherical coordinates has been solved and the hydrostatic bearing characteristics are systematically investigated. The operating sensitivities of the proposed spherical hydrostatic bearing, with respect to the piston radius, film beginning angle, film ending angle, film thickness, and temperature, are studied. The load carrying capacity, pressure drop coefficient, stiffness variation of the lubricating films, leakage properties, and leakage flow rates are comprehensively discussed. The related findings provide a fundamental basis for designing the high-efficient spherical pump under multiple operating conditions. Besides, these related results and mechanisms can also be utilized to design and improve other kinds of annular orifice damper spherical hydraulic bearing systems.

A novel tunable stiffness mechanism using filament jamming

The jamming mechanism is an important method to tune the stiffness of soft-bodied machines to enable them to adapt to their surroundings. However, it is difficult for the present jamming structures to integrate them into complicated structures such as twist, cylinder, and spiral. This paper introduces a novel jamming mechanism termed a filament jamming technique, which varies stiffness using jamming of a cluster of tiny and compliant filaments. The jamming structure demonstrated a variety of characteristics such as softness, shape compatibility, lightweight, and high stiffness, which these feats can meet to a variety of application scenarios that the traditional jamming one cannot afford. The mechanical behavior of the jamming structure was studied with an experimental test, in which the experimental results illustrated that its structural and material factors affect stiffness variation and dynamic performance. To demonstrate the advantage of the jamming technique, we constructed a soft gripper and a torsional actuator to demonstrate how the mechanics of filament jamming can enhance the performance of real-world robotics systems. Therefore, the filament jamming mechanism provides a variety of machines and structures with additional properties to increase forces transmitted to the environment and to tune response and damping. This study aims to foster a new generation of mechanically versatile machines and structures with both softness and stiffness.

Dynamic analysis of railway track on variable foundation under harmonic moving load

This paper aims to investigate the effect of variable foundation stiffness on the dynamic response of an infinite railway track under the action of a harmonic moving load. In this study, harmonic variation in the foundation stiffness along the track length is considered. Here, the dynamic response of the finite element (FE) model is obtained with the help of Newmark Beta method using programming in MATLAB. It is ascertained that in the central region of the long FE model, the response is repetitive, thereby ensuring that boundary conditions do not influence the response in the central region. In this problem, two frequencies, i.e. frequency of moving load and the spatial frequency of variable foundation stiffness, are involved. Their combined influence on the dynamic characteristics such as resonant frequency, critical velocity, displacement time history, displacement below load, and bifurcation curve are investigated. It is shown that the dynamic response is qualitatively and quantitatively affected by the wavelength (λ) and amplitude (ε) of the variation of foundation stiffness. It is also shown that the important dynamic properties, i.e., the critical velocity and the resonant frequency reduce with the increase in wavelength of stiffness variation. This reduction is significant for the large amplitude of harmonic stiffness variation.

Research on Variable-Stiffness Mechanisms of Robot Wrists for Compliant Assembling-Clamping

The stiffness requirements of robot wrists vary with processes during automatic assembling-clamping of robots. The precision of robots moving workpieces to operating positions in the process of rigid localization is achieved if robot wrists equip with a large stiffness. The pose errors of workpieces in the process of compliant assembling-clamping can easily be compensated if robot wrists with a low stiffness is utilized. The present compliant wrist can not meet the stiffness requirements of different processes. A robot wrist with a large stiffness variation is proposed and its mechanisms of rigid localization and compliant assembling-clamping are studied. The pose models of wrists caused by deformations are established. The influences of wrist stiffness on the deformation of itself are researched. The mechanism of modulating wrist stiffness during compliant assembling-clamping is revealed. A structure of 3-DOF (degrees of freedom) robot wrist with a stiffness variation is proposed. The wrist stiffness is changed by modulating the pretension. The influences of pretensions and geometrical parameters on the variable-stiffness characteristics and the stiffness distribution of a wrist are researched. Finally, the experiments are carried out to verify the feasibility of the wrists finishing assembling-clamping operations by modulating the stiffness.