Composite active disturbance rejection robust control for a prototype of an active damping artificial ankle prosthesis

The problem of robust control applied to adjust the configuration of an ankle prosthesis based on disturbance estimation has been addressed in this study. Active disturbance rejection control was the paradigm used for controlling the robotic prosthesis by means of a direct active estimation. Based on this active estimation, the robust controller implemented the disturbance cancellation providing a fast converge to the origin of the tracking error. The uncertainties affecting the prosthesis dynamics were identified by a high‐order extended state high gain observer. This identification was used to force the tracking between the actual position and force needed in the ankle prosthesis and some reference values obtained by a biomechanical gait cycle analysis. Therefore, the estimated states were used to implement a robust output feedback controller that was effective to reject actively the perturbations. This rejection implemented within the controller forced the trajectory tracking to a small vicinity of the origin. A strategy based on composite Lyapunov function served to prove that tracking problem for the prosthesis was successfully solved despite the switching nature of the gait cycle. The controller was implemented in numerical simulations for showing the convergence of the tracking error. The convergence of this tracking error to the region around the origin was obtained within the first second of simulation.     

Persuasive social features that promote knowledge sharing among tertiary students on social networking sites: An empirical study

Persuasive system features have been widely adopted to encourage attitude and behaviour change. Recently, most social networking sites (SNS) adopt some form of persuasive system features that leverage social influence to deliberately induce prescribed behaviours in their users. However, studies on how these features can be used to promote knowledge sharing are inadequate; particularly, regarding how SNS that have been developed solely for academic purposes can adopt these features to promote knowledge sharing. To address this knowledge gap, this study integrates constructs from the social capital theory and persuasive systems design model to investigate the impact of persuasive social features on knowledge sharing among students of tertiary institutions on academic social networking sites. Data are quantitatively gathered from 218 respondents from tertiary institutions and statistically analyzed. The results suggest that perceived dialogue support and perceived social support have strong influences on knowledge sharing behaviour.     

The influence of clearance on friction, lubrication and squeaking in large diameter metal-on-metal hip replacements

Large diameter metal-on-metal bearings (MOM) are becoming increasingly popular, addressing the needs of young and more active patients. Clinical data has shown excellent short-to-mid-term results, though incidences of transient squeaking have been noted between implantation and up to 2 years post-operative. Geometric design features, such as clearance, have been significant in influencing the performance of the bearings. Sets of MOM bearings with different clearances were investigated in this study using a hip friction simulator to examine the influence of clearance on friction, lubrication and squeaking. The friction factor was found to be highest in the largest clearance bearings under all test conditions. The incidence of squeaking was also highest in the large clearance bearings, with all bearings in this group squeaking throughout the study. A very low incidence of squeaking was observed in the other two clearance groups. The measured lubricating film was found to be lowest in the large clearance bearings. This study suggests that increasing the bearing clearance results in reduced lubricant film thickness, increased friction and an increased incidence of squeaking.     

Non Uniform Multiresolution Method for Optical Flow and Phase Portrait Models: Environmental Applications

In this paper we define a complete framework for processing large image sequences for a global monitoring of short range oceanographic and atmospheric processes. This framework is based on the use of a non quadratic regularization technique for optical flow computation that preserves flow discontinuities. We also show that using an appropriate tessellation of the image according to an estimate of the motion field can improve optical flow accuracy and yields more reliable flows. This method defines a non uniform multiresolution approach for coarse to fine grid generation. It allows to locally increase the resolution of the grid according to the studied problem. Each added node refines the grid in a region of interest and increases the numerical accuracy of the solution in this region. We make use of such a method for solving the optical flow equation with a non quadratic regularization scheme allowing the computation of optical flow field while preserving its discontinuities. The second part of the paper deals with the interpretation of the obtained displacement field. For this purpose a phase portrait model used along with a new formulation of the approximation of an oriented flow field allowing to consider arbitrary polynomial phase portrait models for characterizing salient flow features. This new framework is used for processing oceanographic and atmospheric image sequences and presents an alternative to complex physical modeling techniques.     

Studies on the alcoholysis of some seed oils

The effects of time and temperature on the alcoholysis of rubber seed, melon seed, linseed, and soyabean oils have been studied. The following temperatures were investigated: 200, 220, 245, and 260°C. Litharge (PbO) was used as the alcoholysis catalyst. The optimum alcoholysis temperature was found to be 245 ± 2 °C for each of the oils. At lower alcoholysis temperatures (<245°C), there is the preferential alcoholysis of seed oils derived from unsaturated acid; and the general alcoholysis rates were found to be in the following order: linseed oil ≈ rubber seed oil ≥ soyabean oil ≈ melon seed oil. The alcohol‐solubility of the oils is generally observed to begin at 42–45% conversion of oils to monoglycerides. The α‐monoglyceride contents of the alcoholysis mixtures of rubber seed and linseed oils were generally similar at methanol tolerance, and higher than those of melon seed and soyabean oils. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1826–1832, 2000     

QFT design of multi‐loop nonlinear control systems

In control practice, one of the fundamental limitations of feedback is given by the sensor noise effect. This problem is still more important in uncertain nonlinear control systems. This work extends the previous multi‐loop QFT technique, specifically designed to accommodate bandwidth limitation, to the nonlinear case. Copyright © 2000 John Wiley & Sons, Ltd.     

Neugebauer and Demichel: Dependence and independence in n‐screen superpositions for colour printing

The Neugebauer equations and the Demichel equations on which they are based are one of the basic tools for modeling colour printing systems that use the halftoning technique. However, these equations implicitly assume that the colour ink distributions in the screen superposition are statistically independent. We show that this condition is not satisfied in the conventional screen superposition used for colour printing, and we discuss the consequences of this fact. Furthermore, we give a precise criterion that determines, for any number of superposed regular screens, in which cases the Demichel (and hence the Neugebauer) equations are satisfied, and in which cases they fail: The Demichel equations fail in all cases where the screen superposition is singular, and they are satisfied in all nonsingular screen superpositions. We illustrate our results with several examples of both cases. © 2000 John Wiley & Sons, Inc. Col Res Appl, 25, 267–277, 2000     

Multi-leader coordination algorithm for networks with switching topology and quantized information

This paper analyzes the stability and convergence properties of a proportional–integral protocol for coordination of a network of agents with dynamic information flow and quantized information exchange. In the setup adopted, each agent is only required to exchange its coordination state with its neighboring agents, and the desired reference rate is only available to a group of leaders. We show that the integral term of the protocol allows the agents to learn the reference rate, rather than have it available a priori, and also provides disturbance rejection capabilities. The paper addresses the case where the graph that captures the underlying network topology is not connected during some interval of time or even fails to be connected at all times.