Journal of Aerospace Mechanics
Year:2007 | Volume:3 | Issue:3|Papers Count:8

Occurrence of faults in hydraulic systems may cause serious problems in hydraulic machineries, especially where human life is involved. Therefore, online detection of faults is an important issue. In this article, occurance of fault has been detected wing a high-gain observer for an experimental servo hydraulic setup. State-space model of the dynamic system has been derived. Leunberger non-linear observer is designed for the servo hydraulic system. State parameters of the system, such as internal pressures, are estimated by the proposed observer.Observer's gains and stability conditions are derived based on Lipshitz and Lyapanov methods, respectively. Fault detection has been carried out by multiplicate and residual functions. Using the proposed method, existence of leakage and trashes in the system can be detected in real time. Simulation results show that the application of Leunberger observer can reach a high performance fault detection method for servo hydraulic systems in presence of external noise.

In this paper, an anti-ice system for a turbojet is designed. Speed, altitude, ambient temperature and moisture content of the air are considered in the design. Critical conditions, in which icing is expected, are determined. Capture region, where ice formation is most probable, is investigated and protected by hot air from compressor exit. A control system based on PID controller is designed to control the flow rate of the hot air. In this design, surfaces like lip, bullet nose, and struts are protected against ice formation.

In this article, speed control of a servo hydraulic rotary actuator is investigated, using flexible structure neural network (NN). The applied architecture of NN is a feedback error learning (FEL), whose underlying learning strategy is based on the inverse dynamics of the system under control. The classic control output was taken as the cost function to be minimized by the NN. A three-layer feed forward NN was applied and a flexible sigmoid activation function was used for the hidden layer nodes. The learning paradigm was online, making use of the back propagation of error with momentum. A simulation was performed and the results obtained indicated the high capability of the flexible NN in learning inverse dynamics in real time in controlling servo hydraulic systems.

A numerical investigation of unsteady laminar mixed convection heat transfer in a lid driven cavity of aspect ratio of 5 is performed. The forced convective flow inside the cavity is attained by a mechanically induced sliding lid, which is set to oscillate horizontally. The natural convection effect is sustained by subjecting the bottom wall to a higher temperature than its top counterpart. In addition, the two vertical walls are kept insulated. Discretization of the governing equations is achieved through a finite volume method. Fluid flow and heat transfer characteristics are examined in the domain at Richardson number, Grash of number and dimensionless lid oscillation frequency of: 10-3 £ Ri £103 , Gr = 104, and S=0.001, 01, respectively the working fluid is assigned a Prandtl number of 0.7 throughout this investigation. Temporal variations of streamlines, isotherms, and Nusselt number are presented for various dimensionless groups. The results show that with decreasing the value of oscillation frequency, while Ri<1, the amplitude and the period of the rate of heat transfer oscillations increase.

In this paper, two position controllers have been developed for a double-acting pneumatic cylinder with two 3/2 on/off solenoid valves, using PWM method. The first one is a PID controller, whose gains have been obtained by Zighler-Nicoltz method. The second one - a Sliding Mode Controller (SMC) - has been designed based on a specific mathematical model, where an appropriate PWM method has been applied to overcome non-linearity’s such as time delay, during opening or closing of valves, and dead band due to station. The results of experimental tests on the closed loop system with the PID controller illustrate that the PWM method is suitable for servo control purposes with high accuracy. Furthermore, high accuracy was also achieved in experimental results in response to step inputs. Small values of maximum and Root Mean Square (RMS) positioning errors in response to sinusoidal inputs with various frequencies show better accuracy of SMC controller in comparison with the PID one. Good performance of the developed SMC is even more evident as frequency increases. These results also show considerable improvements in accuracy in comparison with the results of previous works on costly proportional valves.

The suitable design of the keel and skis is of great significance in reduction of impact load on the bottom surface of the flying boats and in hydrodynamic stability during take-off and landing. No theoretical tools are available to handle the effects of viscosity, gravity, and surface tension. Also, the experimental procedures in the laboratory are both time-consuming and expensive. In this paper, numerical simulation of two-phase flow in the water impact problem of a WIG with VOF method and finite volume methods are taken into account and the effect of changing the dead-rise angle on the slamming force is investigated. An algebraic relation based on the dimensionless parameters for the maximum slam force is developed. This is a new step towards choosing the optimum dead-rise angle of the keel and skis of flying boats, faster and easier and decreases the cost of numerical calculations.

In this paper, an optimization technique is used for finding the thermal power of heaters over the heater surface on a radiant oven to obtain the desired emissive power and heat flux distributions over the product surface (design surface). The direct problem is solved by a quasi-steady method and the view factors are calculated by the Hotel’s crossed string method. The conjugate gradient method is used to minimize the objective function, which is defined by the sum of square residuals between the desired and the estimated heat fluxes over the design surface. The aim of the inverse problem is to optimum the design of thermal systems, such as industrial furnaces, in such a way that the desired transient conditions are achieved over the surface of the product in a thermal process (such as coating, baking and drying).