Impact stabilizing controller for hydraulic actuators with friction: Theory and experiments

Stabilizing manipulators during the transition from free to constraint motion is an important issue in contact task control design. This paper documents the development, theoretical analysis and experimental evaluation of a Lyapunov-based control scheme to regulate the impacts of a hydraulic actuator that comes in contact with a nonmoving environment. Upon sensing a nonzero force, the controller positions the actuator at the location where the force was first sensed, exerting minimal force on the environment. The scheme does not require continuous measurement of force or velocity during the short period of impacts, making it very useful for practical cases. Furthermore, no knowledge of the impact dynamics, friction effects, servovalve dynamics, or hydraulic parameters is required for control action. Stability of the control scheme is verified via analytical analyses. Due to the discontinuous friction model and the discontinuous nature of the proposed control law, the control system is nonsmooth. The existence, continuation and uniqueness of Filippov's solution to the system are, therefore, investigated. The extension of LaSalle's invariance principle to nonsmooth systems is next employed to prove that all the solution trajectories converge to the equilibria. The controller is finally tested experimentally to verify its practicality and effectiveness in collisions with hard and soft environments and with various approach velocities.     

Switching contact task control in hydraulic actuators: Stability analysis and experimental evaluation

A switching contact task control for hydraulic actuators is proposed. The controller is built upon three individually designed control laws for three phases of motion: (1) position regulation in free space, (2) impact suppression and stable transition from free to constrained motion and (3) force regulation in sustained‐contact motion. The position and force control schemes are capable of asymptotic set‐point regulation in the presence of actuator friction and without the complexity of sliding mode or adaptive control techniques. The intermediate impact control scheme is included for the first time to dampen the undesirable impacts and dissipate the impact energy that could potentially drive the whole system unstable. The solution concept and the stability of the complete switching control system are analyzed rigorously using the Filippov's solution concept and the concept of Lyapunov exponents. Both computer simulations and experiments are carried out to demonstrate the efficacy of the designed switching control law. Copyright © 2009 John Wiley & Sons, Ltd.     

Design and experimental evaluation of a robust force controller for an electro-hydraulic actuator via quantitative feedback theory

This paper presents the design and experimental evaluation of an explicit force controller for a hydraulic actuator in the presence of significant system uncertainties and nonlinearities. The nonlinear version of quantitative feedback theory (QFT) is employed to design a robust time-invariant controller. Two approaches are developed to identify linear time-invariant equivalent model that can precisely represent the nonlinear plant, operating over a wide range. The first approach is based on experimental input–output measurements, obtained directly from the actual system. The second approach is model-based, and utilizes the general nonlinear mathematical model of a hydraulic actuator interacting with an uncertain environment. Given the equivalent models, a controller is then designed to satisfy a priori specified tracking and stability specifications. The controller enjoys the simplicity of fixed-gain controllers while exhibiting robustness. Experimental tests are performed on a hydraulic actuator equipped with a low-cost proportional valve. The results show that the compensated system is not sensitive to the variation of parameters such as environmental stiffness or supply pressure and can equally work well for various set-point forces.     

Stores clustering using a data mining approach for distributing automotive spare-parts to reduce transportation costs

Clustering of retail stores in a distribution network with specific geographical limits plays an important and effective role in distribution and transportation costs reduction. In this paper, the relevant data and information for an established automotive spare-parts distribution and after-sales services company (ISACO) for a 3-year period have been analyzed. With respect to the diversity and lot size of the available information such as stores location, order, goods, transportation vehicles and road and traffic information, three effecting factors with specific weights have been defined for the similarity function: 1. Euclidean distance, 2. Lot size 3. Order concurrency. Based on these three factors, the similarity function has been examined through 5 steps using the Association Rules principles, where the clustering of the stores is performed using k-means algorithm and similar stores are allocated to the clusters. These steps include: 1. Similarity function based on the Euclidean distances, 2. Similarity function based on the order concurrency, 3. Similarity function based on the combination of the order concurrency and lot size, 4. Similarity function based on the combination of these three factors and 5. Improved similarity function. The above mentioned clustering operation for each 5 cases addressed in data mining have been carried out using R software and the improved combinational function has been chosen as the optimal clustering function. Then, trend of each retail store have been analyzed using the improved combinational function and along with determining the priority of the depot center establishment for every cluster, the appropriate distribution policies have been formulated for every cluster. The obtained results of this study indicate a significant cost reduction (32%) in automotive spare-parts distribution and transportation costs.     

Electrohydraulic force control design of a hardware-in-the-loop load emulator using a nonlinear QFT technique

This paper presents the design of a robust force control system for an electrohydraulic load emulator utilized as part of a hardware-in-the-loop flight simulation experiment. In this application, the force controlled hydraulic actuator is used to artificially recreate in-service loads upon a second hydraulic flight actuator operated in closed-loop position control. Electrohydraulic force control is more difficult than electrohydraulic position tracking because the load dynamics influence the force transfer function in a way that makes it challenging to develop an accurate force tracking system using simple feedback control. Nonlinear quantitative feedback theory (QFT) is applied in this paper to address this issue. First, an effective and robust feedback controller is designed by nonlinear QFT to desensitize the force control loop to nonlinear servovalve flow/pressure effects and typical system uncertainties. A secondary compensator is also designed within the QFT framework to extend the force tracking bandwidth with respect to the load motion. Experiments demonstrate acceptable force tracking performance within the scope of a representative flight-simulation experiment.     

Self-esteem stability and depressive symptoms in acute stroke rehabilitation: Methodological and conceptual expansion.

Objectives: Explore the relationship of self-esteem level, self-esteem stability, and other moderating variables with depressive symptoms in acute stroke rehabilitation. Measures: One hundred twenty participants completed measures of state self-esteem, perceived recovery, hospitalization-based hassles, impairment-related distress, and tendency to overgeneralize negative self-connotations of bad events. Self-report of depressive symptoms was collected at admission and on discharge. Four regression analyses explored the relationship of self-esteem level and stability and each of 4 moderating variables (perceived recovery, hassles, impairment-related distress, and overgeneralization) with depressive symptoms at discharge. Results: Analyses indicated significant 3-way interactions in the 4 regression models. In general, individuals with unstable high self-esteem endorsed greater depressive symptoms under conditions of vulnerability (e.g., lower perceived recovery) than did individuals with stable high self-esteem. Under conditions of vulnerability, participants with stable low self-esteem indicated the highest levels of depressive symptoms. Implications: Self-esteem level and stability interact with psychological, environmental, and stroke-specific variables to predict depressive symptoms at discharge from stroke rehabilitation. This suggests the viability of self-esteem stability in exploring depressive symptoms in this setting and the complexity of emotional adjustment early after stroke. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Game theory approach for detecting vulnerable data centers in cloud computing network

Nowadays, cloud computing has many benefits to accessibility, scalability, and cost‐effectiveness, leading to network security risks and vulnerabilities. Cloud computing is gaining in popularity with the advances and growth of its systems. Therefore, the security of this system and the identification of vulnerable data centers are more complicated than the past. Definitely discovering vulnerable data centers that are vulnerable to attacks can help to strengthen these data centers and provide a safer and more secure network structure. This paper examines the vulnerability of malware data centers in the infrastructure and cloud computing network structure. Based on the analysis of the cloud computing system in the field of game theory, we introduce a developed model for identifying vulnerable data centers in cloud computing. The developed model in this paper is based on the game theory as a mathematical tool. According to the game theory, we introduce a measure of the degree of vulnerability of data centers in the cloud computing network.     

The evaluation of autothermal methane reforming for hydrogen production over Ni/CeO2 catalysts

Nanocrystalline Ni/CeO₂ catalysts with various loadings of Ni (10, 15, 20, and 25%) were synthesised by a facile solvent deficient precipitation method for methane autothermal reforming process. The characterisation techniques such as XRD, BET, TPH, H₂-TPR were carried out on fresh and spent samples to investigate the catalytic properties of the Ni/CeO₂. On the basis of characterisation results, the 20% Ni/CeO₂ performs the best activity among the catalysts with different Ni contents. The optimal reaction conditions for autothermal methane reforming has been investigated by evaluating the effect of reaction parameters including the reactivity temperature, the gas hourly space velocity (GHSV) and H₂O/CH₄ (S/C) and O₂/CH₄ (O/C) molar ratios. The stability of 20 wt% Ni/CeO₂ catalyst at 700 °C is examined for 20 h on-stream reaction. It reveals that the methane conversion starts a graduate decrease trend from the second 10 h, which is found to be because of the sintering of Ni nanoparticles by TPH and BET analysis.     

Optimization in measurement of MVC and active stiffness in wrist using the quick release method

The quick release method was used for measurement of Biomechanical parameters in the wrist joints. Wrist joint was previously studied in many researches for its biomechanical characteristics. However, more automation in measurement process has been discussed. In this research, a new test rig with a specified control unit was fabricated. Measurement of parameters in few seconds after the release was considered in the design of the new test rig. Based on reliability tests performed, the fabricated test rig showed firmly reliable measurements for maximum voluntary contraction, and active stiffness. These were true for both cases of left and right wrists during flexion and extension.     

Multilevel modeling of self-esteem change during acute inpatient stroke rehabilitation.

Objective: To explore self-esteem change during inpatient stroke rehabilitation and moderators of change. Research Method: One hundred twenty survivors of stroke serially completed the State Self-Esteem Scale (SSES) during inpatient rehabilitation, as well as measures of mood and perceived recovery as potential moderators of change. Age, gender, prior stroke, prestroke depression, stroke laterality, and admission Functional Independence Measure (FIM) self-care, mobility, and cognitive scores were also included as moderators. Results: Multilevel modeling of the repeated administrations of the SSES indicated that self-esteem significantly improved during rehabilitation. Female gender, left hemisphere stroke, prior stroke, and lower admission FIM cognitive scores were associated with lower self-esteem ratings at admission, but only age and admission FIM self-care and mobility scores were associated with self-esteem change. Older individuals showed less self-esteem improvement than younger individuals, and higher self-care and mobility scores at admission were associated with greater self-esteem improvement. While mood change significantly covaried with self-esteem, the rate of mood change did not appear to influence rate of self-esteem change. Greater improvement in self-esteem over time was related to lower levels of perceived recovery, but this was likely because of the relationship between perceived recovery and self-esteem at rehabilitation admission. Implications: These results suggest that self-esteem improves during inpatient rehabilitation, and this change may be partially dependent on functional status. Implications for facilitating self-esteem change by the clinician are discussed, as well as future research directions. (PsycINFO Database Record (c) 2010 APA, all rights reserved)