Desired Stiffness Research Articles

Correlation of morphological, dynamic mechanical, and thermal properties in compatibilized polypropylene/ethylene-vinyl acetate copolymer/organoclay nanocomposites

Compatibilized and noncompatibilized nanocomposites based on polypropylene (PP)/ethylene–vinyl acetate copolymer (EVA) blends, having 75/25 (wt %/wt %) PP/EVA ratio, with different contents of organoclay (OMMT) and maleated polypropylene as compatibilizer were made through a melt-mixing process with a twin-screw microcompounder, and their properties were investigated. X-ray analysis revealed mainly intercalated/partially exfoliated structures for the blend-based nanocomposites. Through a thermodynamic theoretical approach and transmission electron microscopy investigation, it was shown that the OMMT nanoparticles were mainly placed at the EVA phase of the nanocomposites. Also, scanning electron microscopy analysis showed that OMMT acted like a compatibilizer and reduced the average size of the EVA dispersed phase by preventing coalescence. Dynamic mechanical analysis revealed that the addition of OMMT in the presence of the compatibilizer increased both the storage modulus and damping factor of the corresponding blends. The critical interparticle distance of the EVA domains for achieving supertoughening behavior was obtained at about 100 nm. The addition of OMMT led to a significant improvement in the thermal stability of the blends, especially in the presence of the compatibilizer. A correlation between the morphological, dynamic mechanical, and thermal properties was established to determine the best performing composition, in which desirable damping and stiffness and good thermal stability could be achieved. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Alkaline peroxide mechanical pulping of fast-growth paulownia wood

Alkaline peroxide mechanical pulping of paulownia wood harvested from exotic tree plantations in northern Iran was investigated. The fiber length, width, and cell wall thickness of this wood were measured as 0.82 mm, 40.3 μm, and 7.1 μm, respectively. The chemical composition including cellulose, lignin, and extractives soluble in ethanol-acetone, 1% NaOH, hot and cold water was determined as 49.5%, 25%, 12.1%, 26.9%, 11.4%, and 8.1% respectively. The ash content of this wood was 0.45%. Pre-washed chips were chemically treated at 70°C for 120 minutes with different combinations of three dosages (1.5, 3, and 4.5%) of hydrogen peroxide and three dosages (1.5, 3, and 4.5%) of sodium hydroxide prior to defibration. Other chemicals including DTPA, sodium silicate, and MgSO4 were constant at 0.5%, 3%, and 0.5%, respectively. The results showed that using a 1.5% hydrogen peroxide and 4.5% sodium hydroxide charge, the brightness of APMP pulp reached 68.7% ISO and higher chemical dosages did not improve the brightness; however, to produce APMP pulp with higher strength, a sodium hydroxide charge of 4.5% was needed. The tensile strength, tear strength, burst strength indices, and bulk density of the APMP pulp produced from 1.5% hydrogen peroxide and 4.5% sodium hydroxide were measured as 15.5Nm/g, 6.54mN.m2/g, 0.56kPa.m2/g, and 3.47cm3/g, respectively. The resulting pulp was bulky and is suitable for use in the middle layer of boxboard to provide the desired stiffness with a lower basis weight.

Closed-form shear flow solution for box–girder bridges under torsion

To provide desired stiffness and strength in torsion, bridge superstructures are often constructed with a cross-section consisting of multiple cells which have thin walls relative to their overall dimensions and resist Saint–Venant torsion through shear flow (force per unit length) that develops around the cell walls. For a single thin-walled cell subject to torsion, shear flow is constant along each of its walls while shear stresses vary around the section based upon changes in wall thickness. When the cross-section contains multiple cells they all contribute resistance to applied torsion and for elastic continuity each cell must twist the same amount. With these considerations, equilibrium and compatibility conditions allow simultaneous equations to be formed and solved to determine the shear flow for each cell. A second approach is a relaxation method that distributes incremental shear flows back and forth between cells, reducing errors with each distribution cycle, until the final shear flows for all cells approximate the correct values. A major advantage to this method is that it does not require setting up and solving simultaneous equations, favoring situations where hand calculation is desired. In this paper, a closed-form approach is introduced to determine, exactly, both the torsional constant and all shear flows for multi-cell cross-sections under torsion; no simultaneous equations are required and there is no need to distribute shear flows back and forth between cells. Simple closed-form equations are derived which give shear flows for cross-sections with any number of cells of arbitrary shape.

A Collagen Peptide‐Based Physical Hydrogel for Cell Encapsulation

Collagen peptide-based hydrogels are prepared and characterized for application in 3D cell growth. Physical hydrogels are formed by covalently linking a collagen-based peptide to an 8-arm poly(ethylene glycol) star polymer. The resulting viscoelastic hydrogels have the ability to melt into a liquid-like state near the melting temperature of the collagen triple helix and reform back into an elastic-state at room temperature, adding a thermoresponsive feature to the material. In addition, the hydrogels possess desirable stiffness, as well as a highly cross-linked network of pores where cells are found to reside, making the hydrogels promising scaffolds for the culture of hMSCs.

Design of a Variable Stiffness Actuator Based on Flexures

Variable stiffness actuators can be used in order to achieve a suitable trade-off between performance and safety in robotic devices for physical human–robot interaction. With the aim of improving the compactness and the flexibility of existing mechanical solutions, a variable stiffness actuator based on the use of flexures is investigated. The proposed concept allows the implementation of a desired stiffness profile and range. In particular, this paper reports a procedure for the synthesis of a fully compliant mechanism used as a nonlinear transmission element, together with its experimental characterization. Finally, a preliminary prototype of the overall joint is depicted.

Stiffness Control of Surgical Continuum Manipulators

This paper introduces the first stiffness controller for continuum robots. The control law is based on an accurate approximation of a continuum robot's coupled kinematic and static force model. To implement a desired tip stiffness, the controller drives the actuators to positions corresponding to a deflected robot configuration that produces the required tip force for the measured tip position. This approach provides several important advantages. First, it enables the use of robot deflection sensing as a means to both sense and control tip forces. Second, it enables stiffness control to be implemented by modification of existing continuum robot position controllers. The proposed controller is demonstrated experimentally in the context of a concentric tube robot. Results show that the stiffness controller achieves the desired stiffness in steady state, provides good dynamic performance, and exhibits stability during contact transitions.

Design of Parallel Bearing Structure for 800MN Forging Press with Consideration of Manufacturing Errors

800MN forging press uses plate laminated parallel structure as the main bearing mode, where the force transfer and the stiffness match play key roles in achieving satisfactory performance of the equipment. Although the traditional design methods can obtain desirable stiffness or strength for bearing structure, they less consider the effect of manufacturing errors on the system performance and thus are ineffective. This is because manufacturing errors can cause local structure variation and heavy stress concentration, which may do great harms to the working performance of the equipment. In this paper, a new design approach is proposed to consider the effect of manufacturing errors to the system performance. First, a three-dimension finite element model of the 800MN forging press is built using Marc, upon which stress distributions and concentrations under different combination modes of manufacturing errors are studied. Then, combination law of manufacturing errors is established and validation is carried out through physic test.

Comparison of EG/AD/S and EG/AD model ice properties

EG/AD/S type model ice was originally selected as the primary model ice material for the MOERI ice tank in Korea. The existence of a sugar component in the EG/AD/S mixture may cause a serious maintenance problem. In order to understand the influence of sugar in the original model ice, a series of tests with EG/AD/S and EG/AD model ices were performed, and their material properties compared. Because the target strength of model ice in the full-scale MOERI ice tank is expensive and difficult to control, tests were performed under cold room conditions using a miniature ice tank. This paper describes the material properties of EG/AD/S and EG/AD model ices, such as flexural strength, compressive strength and elastic modulus. In order to obtain the desired strength and stiffness levels for the model ice, a warm-up process was introduced.

Simultaneous control of the motion and stiffness of redundant closed-loop link mechanisms with elastic elements

This paper describes the position and stiffness control of planar redundant link mechanisms with elastic elements in order to utilize the flexibility of robots. Assigning both of the two-dimensional position and stiffness of an output link as an output vector, the procedure of the forward kineto-static analysis for the planar redundant link mechanisms with elastic elements is formulated. The mechanisms have elastic linearactuators composed of a coil spring and linearactuator and rotary actuators and multi jointed links. An inverse kineto-static analysis to obtain the optimum input motions which can generate the desired position and stiffness of the output link while taking into account the motion range of the linearactuator is also conducted and applied to the optimum motion control of the mechanism. Several simulations and experiments with a prototype of a planar closed-loop manipulator with 5 DOF and 4 output show the effectiveness of the proposed method.

Digital Implementation Issues of Electronic Line Shafting

Electronic line shafting (ELS) is a control methodology for reproducing the desirable intershaft stiffness dynamics of a physical line shaft while adding some features that cannot be achieved with a physical line shaft. There are important challenges that must be addressed before ELS can be implemented. Asynchronous sampling between drives will cause perceived distortion, which negatively affects the degree of motion and tension control available. Relative-state motion control requires high position information resolution to be able to accurately measure and correct for small differences in motion between shafts. In addressing these issues, ELS can be implemented correctly to achieve much more precise tension and speed control in paper machines. The asynchronism between drive sampling frequencies is addressed using a novel observer-based compensation mechanism. This method is shown to work in both simulation and experimentation. The resolution of motion information is increased using a sine/cosine encoder in conjunction with an optical-signal-tracking observer. The improvements are shown both in simulation and experimentation.

Task-Based Analysis on Number of Robotic Fingers for Compliant Manipulations

This paper presents a task-based analysis on the number of independent robotic fingers required for compliant manipulations. Based on the stiffness relation between operational space and fingertip space of a multi-fingered object manipulating system, we describe a technique for modulation of the fingertip stiffness without inter-finger coupling so as to achieve the desired stiffness specified in the operational space. Thus, we provides a guideline how many fingers are basically required for successful multi-fingered compliant tasks. Consequently, this paper enables us to assign effectively the number of fingers for various compliant manipulations by robot hands.

Stability and analysis of configuration-tunable bi-directional MWNT bearings

We report on the energetic and structural stability of configuration-tunable, bi-directionallinear bearings based on cap-less, partial segments engineered within individualmulti-walled carbon nanotubes (MWNTs). Using computational models, we show that anexternally applied excitation force can be used to select an operating bearing configurationwith a desired stiffness and operating frequency. Our models also demonstrate thepossibility of simultaneous, independent operation of multiple bearings within a single NTsegment, paving the way towards ultra-high device densities with molecular-scalefootprints.

Comparison of effective stiffness and compliance for characterizing cracked rocks

The noninteraction approximation (NIA) is commonly used for prediction of the anisotropic elastic stiffnesses of cracked rocks. At large crack density, the NIA has a desirable nonlinear stiffness behavior; however, this is inconsistent with the dilute crack assumption. The nonlinear behavior of stiffness predicted by the NIA at high crack density is produced by defining compliance to be a linear function of crack density and then inverting the compliance tensor to stiffness. The linear behavior of compliance is strictly valid only when there is no crack interaction (at low crack density), so the resulting nonlinear stiffnesses at high crack density are unconstrained extrapolations. Comparison of results from the NIA method, an effective stiffness (T-matrix) method, and numerical modeling shows that: (1) first-order compliance methods are not better than first-order stiffness methods; they are equivalent and valid only under the noninteraction (or dilute crack) assumption; and (2) it is misleading to use the NIA at high crack density; crack interactions (shielding and amplification) should not be assumed to cancel for all crack distributions, so they require explicit consideration, for example, with a high-order T-matrix formulation. Examples of unreliable predictions of the NIA at high crack density include nonzero stiffnesses at 100% porosity for a model with dry cracks, and errors in relative stiffnesses in the isotropic limit.

Low-velocity impact damage on dispersed stacking sequence laminates. Part I: Experiments

The stacking sequence design of composite laminates is often limited to combinations of 0°, 90°, and ±45° fibre angle plies. Furthermore, in order to comply to certain stiffness requirements, clustering of plies becomes unavoidable. Although such laminates might have the desired stiffness properties, they may show poor impact and/or compression-after-impact behaviour. A method to redesign the traditional stacking sequences such that the alternative laminates have improved damage resistance whilst keeping similar in-plane and bending stiffness properties as their original traditional stacking sequences is proposed. This method makes use of optimisation tools based on genetic algorithms. In the alternative laminates, the difference between fibre angles of two consecutive plies is maximised and allowed to vary in the 0–90° fibre angle range at intervals of 5°. Manufacturing of such laminates is practical nowadays as the industry is changing its production techniques into accurate automated fibre-placement and tape-laying technologies. A two-step approach is proposed for the design of laminates. In the first step, the optimal laminate is designed in the traditional fashion to cope with the expected quasi-static loads on the structure. The second step consists of redesigning this laminate to better withstand impact loads by dispersing its stacking sequence while keeping similar stiffness properties as in the first step. A traditional laminate and two dispersed stacking sequence alternative layups were tested under low-velocity impact and compression-after-impact loads in order to compare their impact resistance and damage tolerance characteristics. The evaluation of these laminates will also be carried out by the innovative numerical tools proposed in the follow-up of the present paper.

On the Passivity-Based Impedance Control of Flexible Joint Robots

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, a novel type of impedance controllers for flexible joint robots is proposed. As a target impedance, a desired stiffness and damping are considered without inertia shaping. For this problem, two controllers of different complexity are proposed. Both have a cascaded structure with an inner torque feedback loop and an outer impedance controller. For the torque feedback, a physical interpretation as a scaling of the motor inertia is given, which allows to incorporate the torque feedback into a passivity-based analysis. The outer impedance control law is then designed differently for the two controllers. In the first approach, the stiffness and damping terms and the gravity compensation term are designed separately. This outer control loop uses only the motor position and velocity, but no noncollocated feedback of the joint torques or link side positions. In combination with the physical interpretation of torque feedback, this allows us to give a proof of the asymptotic stability of the closed-loop system based on the passivity properties of the system. The second control law is a refinement of this approach, in which the gravity compensation and the stiffness implementation are designed in a combined way. Thereby, a desired static stiffness relationship is obtained exactly. Additionally, some extensions of the controller to viscoelastic joints and to Cartesian impedance control are given. Finally, some experiments with the German Aerospace Center (DLR) lightweight robots verify the developed controllers and show the efficiency of the proposed control approach. </para>

Development of magnetically preloaded air bearings for a linear slide: Active compensation of three degrees of freedom motion errors

This article describes a linear air-bearing stage that uses active control to compensate for its motion errors. The active control is based on preloads generated by magnetic actuators, which were designed to generate nominal preloads for the air bearings using permanent magnets to maintain the desired stiffness while changing the air-bearing clearance by varying the magnetic flux generated by the current in electromagnetic coils. A single-axis linear stage with a linear motor and 240 mm of travel range was built to verify this design concept and used to test its performance. The motion of the table in three directions was controlled with four magnetic actuators driven by current amplifiers and a DSP (Digital Signal Processor)-based digital controller. The motion errors were measured using a laser interferometer combined with a two-probe method, and had 0.085 microm of repeatability for the straightness error. As a result of feed-forward active compensation, the errors were reduced from 1.09 to 0.11 microm for the vertical motion, from 9.42 to 0.18 arcsec for the pitch motion, and from 2.42 to 0.18 arcsec for the roll motion.

A New Compliance Control Approach for Traveling-Wave Ultrasonic Motors

This paper presents a new approach for compliance control using a traveling-wave ultrasonic motor. Its objective is to demonstrate the effectiveness of using duty ratios and frequencies as control variables for both preload and stiffness control. At the home position, the motor is engaged in a standing-wave mode operation and provides self-locking torque. When the motor is deflected, it is engaged in a traveling-wave mode operation to provide a reacting torque. The torque is varied according to the deflection to provide the desired stiffness. Experiments have confirmed the effectiveness of this method. This approach can also be readily deployed into many force-feedback and haptic applications such as robotic locomotion.

Landing Motion Control of Articulated Hopping Robot

This paper deals with the landing motion of an articulated legged robot. Humans use a peculiar crouching motion to land safely which can be characterized by body stiffness and damping. A stiffness controller formulation is used to realize this human behavior for the robot. Using this method, the landing motion is achieved with only the desired body stiffness and damping values, without desired COG(Center of Gravity) or joint paths. To achieve soft landing, variable body stiffness and damping values were optimized. PBOT, which has four links with flexible joints was used for validation of the landing controller. A body stiffness and damping controller was used as an outer landing control loop and a fast subsystem controller for flexible joints was used as an inner force control loop. Simulations and experimental results about the landing motion are presented to show the performance of the body stiffness and damping controller.

Effect of process parameter on mechanical properties of friction stir welded tailored blanks from aluminium alloy 6181-T4

Tailor welded blanks (TWBs) are produced by joining at least two sheets of dissimilar gauge and/or properties to achieve a lighter blank of desired stiffness and strength. In this way, the properties of the blank may be tailored to meet the design requirements of a panel. As in steel structures, cost and weight can be saved when using the TWB concept for aluminium. However, use of aluminium is a concern in the automotive industry because of well documented weldability and reproducibility problems, since relatively minor changes in welding parameters may affect the quality of the weld. Friction stir welding offers an attractive alternative to conventional fusion welding methods because of the excellent properties (particularly ductility), reproducibility, robustness and surface finish obtained. However, the working envelope for friction stir welded tailored blanks has yet to be fully explored. In the present study TWBs of the alloy 6181-T4 in a thickness combination 1 to 2 mm have been produced. The influence of welding parameters and welding tools on the weld quality and mechanical properties has been determined. Satisfactory surface finish with no detrimental effects to the mechanical properties has been observed. Weld efficiencies >90% have been consistently achieved.

Controlling Cartilaginous Matrix Evolution in Hydrogels with Degradation Triggered by Exogenous Addition of an Enzyme

Crosslinked hydrogels provide an accommodating environment for cartilage regeneration. However, degradation of the crosslinked network is necessary to create gels with an initially desirable mechanical stiffness and long-term distribution of properly assembled matrix molecules. In this study, chondrocytes were encapsulated in crosslinked poly(ethylene glycol) (PEG) hydrogels with caprolactone blocks that enabled an exogenously controlled, enzymatic degradation mechanism. At different stages of in vitro culture, a lipase enzyme was added to culture media to trigger degradation of the gel network. In gel constructs that never received lipase, the large cartilage matrix molecule, type II collagen, was localized to the pericellular region. Constructs that received lipase in the media for at least 1 week degraded enough to allow some distribution of collagen, but the timing and duration of lipase administration affected the outcome of regenerated tissue after 8 weeks of in vitro culture. Degradation that was triggered too early resulted in more significant defects in the cartilaginous matrix. The hydrogels applied in this study allow explicit control over degradation, and therefore provide a useful tool for investigating the effects of specific mass loss profiles on the evolution of neocartilage in vitro.