Disturbance rejection of offshore drilling platforms: An equivalent-input-disturbance-based dynamic positioning control method.

Offshore drilling platforms are exposed to wind, waves, currents, and other unknown disturbances. Accurately estimating and rejecting these disturbances is the key to ensuring reliable station-keeping of the platforms. In this study, a novel dynamic positioning method using an improved equivalent-input-disturbance (EID) approach is proposed for offshore drilling platforms. An improved EID estimator is employed to estimate and suppress unknown disturbances, significantly enhancing the disturbance-rejection performance of the dynamic positioning system. The input channels are decoupled through linear transformation, and the parameter tuning process of the observer and controller is optimized, thus improving system performance. The bounded-input bounded-output stability of the closed-loop system is proved. This study provides insights into the design of dynamic positioning systems for offshore drilling platforms.

Interfacial Co-O-Cu bonds prompt electrochemical nitrate reduction to ammonia in neutral electrolyte.

In this work, the formed interfacial Co-O-Cu bonds in Co-doped Cu(OH)2 (Co2-Cu(OH)2) sufficiently expose active sites and improve the reaction kinetics. As a result, the optimal Co2-Cu(OH)2 provides an amazing faradaic efficiency (91.6%), high selectivity (93.2%) and robust stability toward the NO3RR.

Modeling and tracking control of dielectric elastomer actuators based on fractional calculus.

Dielectric elastomer actuators (DEAs) show broad application prospects in the area of soft robots since they offer merits of fast response, large deformation, light weight and high energy conversion efficiency. Practical soft robot applications would usually require the study on the modeling and control of the DEA. However, the DEA has a memory property, which results in a highly nonlinear characteristics, bringing difficulties to the subject. To address this issue, an effective strategy is the utilization of the fractional order model, which is a type of modeling approach that can accurately characterize the memory property of a material with a small amount of parameters. Meanwhile, the fractional order controller can better handler the memory property, and it owns a better flexibility than traditional integer order controllers. With the above considerations, this paper proposes a modeling strategy and a tracking control strategy for the DEA on the basis of the fractional calculus. In the proposed strategy, a fractional order model is established to characterize the complicated nonlinear characteristics of the DEA. Then, to facilitate the computer simulation, the Oustaloup filter is used to construct an integer order approximation model (IOAM) of the fractional order model. Since the IOAM is difficult to be employed in the system controller design due to its high order, the IOAM is further simplified into a reduced order model based on the square root balance truncation algorithm. To realize the high accuracy control of the DEA, a feedforward-feedback combined controller is devised, which is composed of a feedforward controller and a fractional order proportional integral feedback controller (FOPIFC). Among which, the feedforward controller is devised based on the analytical inverse of the reduced order model to compensate the complicated nonlinear characteristics of the DEA, and the FOPIFC is devised to handle the bad influence from the modeling error and uncertainties on the control performance. Based on the proposed strategy, control experiment was conducted, and the root-mean-square errors in experiment are all below 0.7%, indicating the superiority of the presented modeling and tracking control strategies.

Modeling Based on a Two-Step Parameter Identification Strategy for Liquid Crystal Elastomer Actuator Considering Dynamic Phase Transition Process.

Liquid crystal elastomer (LCE) is a promising candidate for actuation in light-driven soft robot applications. Due to the fact that LCE has complex hysteretic nonlinearities, which are highly dependent on the environment, modeling of actuators made of LCE is a very challenging issue. In this article, a model is proposed to describe the deformation of the LCE actuator accurately and analytically by considering the dynamic phase transition process of LCE molecules. First, an overview of the physical process of LCE's deformation is presented, and the schematic of the LCE actuator, as well as the modeling scheme are then introduced. Next, a thermodynamic analysis of the system's free energy is performed to establish the model for the LCE actuator, which gives the relationship between the system's deformation and the temperature. Here, to describe the complex hysteretic nonlinearity in the model, the dynamic process of the phase transition of LCE molecules is exploited. To effectively identify the model parameters, a two-step parameter identification strategy based on the differential evolution algorithm and nonlinear least-squares method is utilized. Finally, experimental results verify the validity of the proposed model. This modeling provides an approach to describe LCE's deformation with high accuracy and can fully reflect the physical nature of LCE's deformation, especially hysteresis. It can be utilized as a basis for accurate control over LCE actuators in photoresponsive soft robot applications.

A General Position Control Method for Planar Underactuated Manipulators With Second-Order Nonholonomic Constraints.

The study on the stabilization of planar underactuated manipulators without gravity is well recognized as a major challenge since the system includes a second-order nonholonomic constraint when the passive link is not located at the first link. It is important to solve this difficulty for applications such as systems working in aerospace or underwater. This article presents a position control method based on bidirectional motion planning and intelligent optimization for this kind of system. The control objective is to move the end effector of the manipulator from its initial position to a given target position. The differential evolution algorithm is applied to solve the target angles of all links corresponding to the target position of the end effector. Then, bidirectional motion planning is performed, which consists of forward and backward motions. Each motion is planned by designing a trajectory for every active link based on their initial and target angles. During the forward motion, all active links except the first one are moved to their target angles, and the first active link and the passive link to the intermediate angles. For the backward motion, the first active link is moved to its target angle, the other active links remain at their target angles, and the passive link will be moved to its target angle at the same time. The planned trajectories are chosen based on the time-scaling method and differential evolution algorithm to make sure that the forward and backward planned motions can be connected smoothly. Finally, the trajectory tracking controllers for all active links are designed based on the sliding-mode control method. The proposed control method is verified on planar four-link underactuated manipulators with different passive joints. This strategy has the advantage that it works for planar underactuated manipulators with a second-order nonholonomic constraint whose passive link can be at different positions. Meanwhile, by combining intelligent optimization with bidirectional motion planning, the control process becomes simpler and more effective.

Dynamic modeling of dielectric elastomer actuator with conical shape.

With desirable physical performances of impressive actuation strain, high energy density, high degree of electromechanical coupling and high mechanical compliance, dielectric elastomer actuators (DEAs) are widely employed to actuate the soft robots. However, there are many challenges to establish the dynamic models for DEAs, such as their inherent nonlinearity, complex electromechanical coupling, and time-dependent viscoelastic behavior. Moreover, most previous studies concentrated on the planar DEAs, but the studies on DEAs with some other functional shapes are insufficient. In this paper, by investigating a conical DEA with the material of polydimethylsiloxane and considering the influence of inertia, we propose a dynamic model based on the principles of nonequilibrium thermodynamics. This dynamic model can describe the complex motion characteristics of the conical DEA. Based on the experimental data, the differential evolution algorithm is employed to identify the undetermined parameters of the developed dynamic model. The result of the model validation demonstrates the effectiveness of the model.

Position control for planar four-link underactuated manipulator with a passive third joint.

This paper presents a position control strategy based on the differential evolution (DE) algorithm for a planar four-link underactuated manipulator (PFUM) with a passive third joint, which is to move its end-point from any initial position to any target position. Based on the structural characteristic of the PFUM, a model reduction method is conceived to reduce the PFUM to a planar virtual three-link manipulator and a planar Acrobot in turn. Considering the existence of the angle constraint in the planar Acrobot, the DE algorithm is used to optimize and coordinate the control objective of each reduced system, and also to ensure the target angles of the planar Acrobot corresponding to the target position of the PFUM can be found. Simulations demonstrate the validity of the proposed control strategy.

Fabrication of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) microstructures using soft lithography for scaffold applications.

This paper reports two soft lithographic methods, micromolding and hot embossing, to produce biodegradable poly (3-hydroxybutyrate-co-3-ftydroxyhexanoate) (PHBHHx) arrays of microstructures for hosting and culturing cells in a local microenvironment by controlled shape. Silicon masters with high-aspect-ratio microfeatures were fabricated using KOH and DRIE anisotropic etching. These silicon masters were used as molds to construct PHBHHx microstructures using micromolding and hot embossing. Using silicon rather than conventional PDMS as molds allowed microstructures with feature size of 20 microm and height of 100 microm to be realized. PHBHHx microstructures with different configurations including circles, rectangles, and octagons were fabricated to investigate the effects of topography on cell culture. Mouse fibroblast cell lines L929 were cultured on PHBHHx microstructures in vitro to investigate the biocompatibility. This study demonstrates the feasibility of microfabrication of PHBHHx structures with micro-scale feature size using soft lithography, and the results show that PHBHHx microstructures can be created to mimic cellular microenvironment for cell culture, providing a convenient means to investigate relationships of microstructures and cell functions.

The mechanical properties and in vitro biodegradation and biocompatibility of UV-treated poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).

Strong mechanical properties and controllable biodegradability, together with biocompatibility, are the important requirement for the development of medical implant materials. In this study, an ultraviolet (UV) radiation method was developed to achieve controlled degradation for bacterial biopolyester poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) which has a low biodegradation rate that limits its application for many implant applications required quick degradation. When UV radiation was applied directly to PHBHHx powder, significant molecular weight (Mw) losses were observed with the powder, Mw reduction depended on the UV radiation time. At the same time, a broad PHBHHx Mw distribution was the result of inhomogeneous radiation. Interestingly, this inhomogeneous radiation helped maintain the mechanical properties of films made of the UV-radiated powder. In comparison, the PHBHHx films subjected to direct UV radiation became very brittle although their degradation was faster than that of the PHBHHx powders subjected to direct UV radiation. After 15 weeks of degradation in simulated body fluid (SBF), films prepared from 8 and 16h UV-treated PHBHHx powders maintained 92% and 87% of their original weights, respectively, while the untreated PHBHHx films lost only 1% of its weight. Significant increases in growth of fibroblast L929 were observed on films prepared from UV-radiated powders. This improved biocompatibility could be attributed to increasing hydrophilic functional groups generated by increasing polar groups C-O and CO. In general, UV-treated PHBHHx powder had a broad Mw distribution, which contributed to fast degradation due to dissolution of low Mw polymer fragments, and strong mechanical property due to high Mw polymer chains. Combined with its improved biocompatibility, PHBHHx is one more step close to become a biomedical implant material.

Gelatin blending improves the performance of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) films for biomedical application.

To improve the performance of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx), gelatin was blended with PHBHHx at different ratios. With increasing gelatin content, the weight loss of gelatin/PHBHHx blend in simulated body fluid at 37 degrees C was accelerated. After 2 months, there was about 15% weight loss in PHBHHx blending with 30% gelatin. Scanning electron microscopy and X-ray diffraction results showed that gelatin blending increased the surface porosity and decreased the crystallinity, which may be responsible for the acceleration of the weight loss. Second harmonic generation results indicated that 10% gelatin blending had less disruption to PHBHHx spatial structure, resulting in better tensile mechanical properties. At the same time, increased surface porosity and decreased crystallinity caused by gelatin incorporation may be beneficial for cell growth compared with pure PHBHHx. All these indicated that gelatin incorporation may improve the performances of PHBHHx to meet the need of different situations during medical implantation.

Effect of composition of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) on growth of fibroblast and osteoblast.

Films made of poly (3-hydroxybutyrate) (PHB), poly(3-hydroxybutyrate- co-3-hydroxyhexanoate) (PHBHHx) consisting of 5%, 12% and 20% hydroxyhexanoate (HHx), respectively, were evaluated for biomedical application in comparison with poly (L-Lactide) (PLA). With the increase of HHx content in PHBHHx, the polymer surface properties changed accordingly. P(HB-co-20%-HHx) had the smoothest surface while PHB surface was most hydrophilic among the evaluated PHB and all the PHBHHx. All PHBHHx also showed strong protein affinity and biocompatibility. It was found that fibroblast and osteoblast had different responses to these polymers: fibroblast cells favored P(HB-co-20%-HHx), yet osteoblast cells preferred P(HB-co-12%-HHx). PHB and all PHBHHx appeared to have better biocompatibility for fibroblast and osteoblast compared with PLA. Polymers possessing different surface properties may help meet different cellular requirements. Combined with their good mechanical properties for elongation and adjustable biocompatibility, PHBHHx may meet the needs of growth requirements of different tissues and cells.

Evaluation of three-dimensional scaffolds made of blends of hydroxyapatite and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) for bone reconstruction.

Hydroxyapatite (HAP) was blended into poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) to make films and scaffolds. After HAP blending, mechanical properties of PHB including compressive elastic modulus and maximum stress showed improvement and osteoblast responses including cell growth and alkaline phosphatase activity were also strengthened. On the other hand, scaffolds made of PHBHHx blended with HAP had an adverse effect. No remarkable change on degradation of PHB or PHBHHx blended with HAP, respectively, was observed in simulated body fluid. Scanning electron microscopy examination revealed that osteoblast responses to HAP incorporation may be related to surface morphology and to the exposed HAP particles on polymer surface. All these results indicated that the blending of HAP particles into PHBHHx scaffolds fabricated by salt leaching was unable to either strengthen its mechanical properties or enhance osteoblast responses. Although HAP is bioactive and osteoconductive, its blending with PHBHHx did not generate a better performance on bone reconstruction.

Attachment, proliferation and differentiation of osteoblasts on random biopolyester poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) scaffolds.

Rabbit bone marrow cells were inoculated on 3D scaffolds of poly(lactic acid) (PLA), poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) to evaluate their in vitro biocompatibilities. It was found that PHBHHx had the best performance on attachment, proliferation of bone marrow cells. The cells on PHBHHx scaffolds presented typical osteoblast phenotypes: round cell shape, high alkaline phosphotase (ALP) activity, strong calcium deposition, and fibrillar collagen synthesis. After incubation for 10 days, cells grown on PHBHHx scaffolds were approximately 2x10(5)ml(-1), 40% more than that on PHB scaffolds and 60% more than that on PLA scaffolds. ALP activity of the cells grown on PHBHHx scaffolds was up to about 65U/g scaffolds, 50% higher than that of PHB and PLA, respectively. The scanning electronic microscopy (SEM) results showed that PHBHHx scaffolds had the appropriate roughness for osteoblast attachment and proliferation comparing with PHB and PLA. All these indicated that PHBHHx was a suitable biomaterial for osteoblast attachment, proliferation and differentiation from bone marrow cells.

Reduced mouse fibroblast cell growth by increased hydrophilicity of microbial polyhydroxyalkanoates via hyaluronan coating.

The mouse fibroblast cell line L929 was inoculated on 3D scaffolds of microbial polyesters, namely polyhydroxybutyrate (PHB) and poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBHHx) to evaluate their in vitro biocompatibility. It was found that both polyhydroxyalkanoates (PHA) subjected to lipase treatment and hyaluronan (HA) coating decreased the contact angle of water to the material surface approximately 30%, meaning an increased hydrophilicity on the PHA surface. At the same time, both the lipase treatment and the HA coating smoothened the PHA surface. After the lipase treatment or HA coating, the ratio of PHA hydrophilic groups including hydroxyl and carboxyl to carbonyl of PHA was approximately 1:1 or 2:1. Cells grown on scaffolds treated with lipase were approximately 4 x 10(5)/ml, twice in number of the control. However, PHA scaffolds coated with HA were observed with a 40% decrease in cell growth compared with that of the control. HA coating reduced the cell attachment and proliferation on PHA although the materials had increased hydrophilicity. In comparison, lipase treatment promoted the cell growth on PHA although the treatment did not lead to better hydrophilicity compared with HA coating. It appeared that an appropriate combination of hydrophilicity and hydrophobicity was important for the biocompatibility of PHBHHx, especially for the growth of L929 cells on the surface of this material. This may have instructive significance for biomaterial selection and design.