مجله مکانیک سازه ها و شاره ها
سال سوم شماره 4 (زمستان 1392)

In this paper a new approach is presented for controlling the structural vibrations. The proposed active control method is based on structural dynamics theories in which multi actuators and sensors are utilized. Each actuator force is modeled as an equivalent viscous damper so that several lower vibration modes are damped critically. This subject is achieved by simple mathematical formulation. The proposed method does not depend on the type of dynamic load and it could be applied to control structures with multi degrees of freedom. For numerical verification of proposed method, several criterions such as maximum displacement, maximum kinetic energy and maximum drift are evaluated in five story shear building, excited by harmonic load. This study shows that the proposed method has suitable efficiency for reducing structural vibrations. Also, according to the results, the maximum shift in the structure of the upper floors 5 degrees of freedom are reduced by 75%

In this study, mathematical modeling and dynamic response of nonholonomic wheeled mobile robotic manipulator that consists of a serial manipulator with both Revolute-Prismatic (R-P) joints and an autonomous wheeled mobile platform is considered. To avoid the Lagrange multipliers associated with the nonholonomic constraints the approach of Gibbs-Appell formulation in recursive form is adopted. For modeling the system completely and precisely the coupling effects due to the simultaneous rotating and sliding motion of the rigid arms as well as both nonholonomic constraints associated with the no-slipping and the no-skidding conditions are included. Finally, the analysis of a mobile manipulator with two (R-P) joints is considered.In this study, mathematical modeling and dynamic response of nonholonomic wheeled mobile robotic manipulator that consists of a serial manipulator with both Revolute-Prismatic (R-P) joints and an autonomous wheeled mobile platform is considered. To avoid the Lagrange multipliers associated with the nonholonomic constraints the approach of Gibbs-Appell formulation in recursive form is adopted. For modeling the system completely and precisely the coupling effects due to the simultaneous rotating and sliding motion of the rigid arms as well as both nonholonomic constraints associated with the no-slipping and the no-skidding conditions are included. Finally, the analysis of a mobile manipulator with two (R-P) joints is considered.

Flexible-joint robots have complex dynamics due to the nonlinearity, uncertainty, and flexibility in joints. The conventional sliding mode control for flexible-joint robots is based on the torque control strategy which has a few drawbacks. Because of complexity of the robot dynamics, the control law becomes complex and determining the bounding functions is difficult. In addition, dynamics of actuators are not considered in the control law; thereby the control performance is degraded. Moreover, the chattering problem occurs which excites the unmodeled dynamics. In order to overcome these problems, a novel sliding mode fuzzy control design for robots is presented in this paper. This design employs an adaptive fuzzy estimator for function approximation and has no need to calculate the bounding functions. The chattering phenomenon is reduced, as well. The controller is based on the voltage control strategy which is simpler and more effective than the torque control strategy. Another novelty is that the controller has only one control loop whereas the conventional control approach uses two control loops. Stability of the control system is proven and efficiency of the proposed control approach over the modified fuzzy proportional-integral control is shown through simulations on the flexible-joint articulated robot manipulator.

Underwater robots are a common subject between robotic science and ocean engineering. Their applications are environmental monitoring, mapping and exploring the depths of the water. In this study, navigation and path tracking of an underwater robot by ANFIS and Memetic Algorithm are desired. Memetic Algorithm is a kind of evolutionary algorithms that uses combination of local searches and other evolutionary algorithms to optimize a problem to achieve to better solutions. In this study due to evolution of behavior in solving of an optimization problem, an adaptive strategy for Memetic Algorithm is presented. By determining an adaptive parameter, the number of population, mutation probability and permission to access local search algorithms are adaptive in each iteration. The algorithm can optimize static and dynamic systems. An ANFIS controller and Memetic are used to Offline control of an underwater robot. ANFIS parameters are trained by Memetic to navigate and control of the underwater robot with minimal error on a predefined path with disturbance condition. Also the ANFIS controller was trained by Genetic Algorithm. Comparison of results shows that Memetic Algorithm training is more stable and more precise.

One of the main obstacles of productivity in metal cutting processes is cutting tool vibration. The vibration in metal cutting leads to unsatisfactory surface finishes and poor dimensional quality. The cutting tool vibration also diminishes the useful life of the cutting tool and machine tool’s bearings, spindle, motors, servos and the related components. This paper proposes a method based on wavelet transform and haralic texture features to determine the vibration level in metal cutting. Wavelet Transform is used to filter the effect of cutting tool vibration from other sources (e.g. feed marks, tool wear and surface defects) affecting the workpiece surface. The texture of filtered image in turn is analyzed using haralic features. The vibration level of the cutting tool is controlled by increasing the overhang of the tool and the subsequent irregularity of surface texture is introduced as a criterion to determine the cutting tool vibration. In order to validate the proposed vision based method, an accelerometer was attached to the shank of the cutting tool to measure the vibration in tangential direction.

HVAC systems must provide comfort thermal condition despite the complexity. In these systems the controllers must be able to adapt the changing environmental parameters. Nowadays in the most air conditioning systems use ON/OFF controllers (thermostats) or Proportional - Integral - Derivative (PID) controllers in the advanced process, those are unable to provide the desired environmental conditions. Furthermore, in the long run, these controllers are expensive, because they operate at a very low-energy efficiency and tuning of HVAC systems is difficult and time consuming. In this study we consider the single zone HVAC system and assume the system is operating on the cooling mode and to achieve the desired optimal operating point, we benefit the high order fuzzy method that includes mamdany fuzzy controller for assessing variation of control variables and sugeno fuzzy controller for linearization gain coefficient. We use discrete action reinforcement learning automata to final set the gain. The simulation results shows that this control strategy is very robust and flexible than other methods and able to reduce energy loses.

In this research, an aerodynamic design of axisymmetric diffuser is performed via linking a solver of boundary layer equations, Genetic Algorithm and Ball Spine inverse design algorithm (BSA). A numerical boundary layer code is incorporated to the genetic algorithm to reach an optimum pressure distribution on the diffuser wall in such a way that maximum pressure recovery is obtained without separation. To validate the developed boundary layer code, the calculated quantities are compared with Blasius and Howart’s analytical results. Then, the optimized pressure distribution is considered as the "target pressure distribution" for the inverse design algorithm to find out the relevant optimum geometry. For inverse design, Ball-Spine algorithm as the geometry modification algorithm is compined by the Fluent software as the flow solver. Implementation of this combination is completed through User Defined Function (UDF) feature of Fluent. Having examined the performance of the proposed inverse design method, quantitative effect of this method on the performance improvement of an axisymmetric diffuser is studied. The numerical results of the optimized diffuser shows that its pressure recovery coefficient has been increased considerably.

Essentially, Gas turbines are complex, nonlinear and time-varying systems that their parameters making change by alter working conditions permanently. Prediction of dynamic behavior of these systems has important role in control and health condition monitoring system design and also component design. Thus providing an appropriate computer model to predict the gas turbines behavior is necessary. In this article, the dynamic behavior of gas turbine SGT-600 as a widely used industrial twin-shaft gas turbine in our country is modeled and simulated with considering the cooling effects of turbine blades. For this purpose, first, the design point parameters of gas turbine are determined and in order to validate the proposed model, its results are compared with the results provided by the manufacturer. Then, gas turbine behavior in steady state mode is modeled using the mass and energy conservation equations. Comparing the obtained results with the results provided by GSP software is approved the proposed model accuracy. Finally, in this paper an algorithm for prediction of transient behavior of the gas turbine in both acceleration and deceleration mode is presented and the engine transient behavior is simulated. It should be noted that in this article, the cooling effects of turbine blades is considered for design point, steady and transient modeling.

Boundary layer flow control, is one of the most important issues of aerodynamic.Generally, the purpose of flow control, is control of the boundary layer separation. Nowadays, researchers have considered the using of active electrohydrodynamic actuators due to the access and easy installation, no need for special repairs, very short response time and very low energy consumption. In this paper, using a similar geometry, the effects of dielectric barrier, on induced velocity in still air boundary layer, in the corona wind of direct and alternating fields was studied. The results showed that the using of dielectric barrier in the field of direct current, the flow velocity in the boundary layer is reduced, while with the dielectric barrier in the field of alternating current, greater speeds in boundary layer could be reached. with detection of smoke injection technique, it was found that the barrier dielectric has no effect on the flow pattern,but this pattern is in the direction of electric field in the direct current and is tangential to the boundary layer in the alternating current.finally, the using of the alternating current field, in addition to installing in the exact location due to small size of them, can cause to achieve higher speeds is still air in the boundary layer.

There are different numerical and experimental studies on the flow patterns around cylinders with various cross sections. However, there are only very few papers in which the effects of the cross sectional shape on the flow patterns around different cylinders are considered. This paper, therefore, provides a comparative study of the flow past cylinders with different cross sectional shapes using an immersed boundary method. The immersed boundary method used in this paper is a modified version of a direct forcing method in which the same functions employed in the continuous forcing methods are also used in interpolation and extrapolation processes. A specific approximation has also been used to determine the force at Lagrangian nodal points. All the simulations performed in this study are at Reynolds numbers 50, 100 and 150 and the results include vortex shedding and streamline diagrams as well as diagrams which show the variations of the lift and drag coefficients and Strouhal number. Computational results are satisfactory as compared to similar results reported in the literature.