فصلنامه مکانیک هوافضا
سال هجدهم شماره 4 (پیاپی 70، زمستان 1401)

In order to evaluate and improve the accuracy of the distance traveled by the laser beam with a fixed distance, first the relation of the pulsed laser reflection signal reflected from the surface of the objects should be calculated and analyzed. Then, the broadening angle of the reflection signal and the relative broadening factor should be determined. In this research, an analytical relationship for distance measurement and detection for pulsed laser and statistical distance measurement has been designed, in which the minimum measurable distance and the necessary calculations for its quantitative estimation are presented. In this regard, the relationship between the broadening coefficient and relevant parameters such as detection distance, radiation angle to the target surface, and radiation reflection signal have been determined and evaluated. Simulation results and experimental results show that the maximum laser range for reliable distance measurement is 36.5 m, which is associated with a statistical error of 0.24 to 0.48 m. As the measurement threshold increases, the distance measurement error and the reliable range decrease. As the laser radiation angle increases, the divergence angle and the broadening coefficient increase, and as the laser pulse width decreases, the broadening coefficient decreases. Since increasing the angle of the target surface increases the divergence of the reflection signal, the mean values ​​and variance of the reflection signal will also increase.

The understanding some characteristics of the influence of an interior gaseous detonation within a confined space on plate deformation is valuable for production purposes. In this study, the center of mass displacement of thin steel triangular plates subjected to acetylene and oxygen mixture detonation loading is investigated theoretically. Empirical modeling is presented to find the relationships between effective parameters in the gas detonation forming process such as mechanical properties of plate, impulse of applied load, plate geometry and strain-rate sensitivity effect and center of mass displacement of the triangular plate. Some important parameters, including plate thickness and yield strength, different triangular clamp sizes were studied to show the exposed area effect on the center of mass displacement. The results from non-dimensional analysis demonstrated a good agreement compared with experimental data and showed that the center of mass displacement decreases greatly with involving thicker plate. Non-dimensional analysis as a profitable tool in the evaluation of the center of mass displacement demonstrates that midpoint deflection was decreased by the smaller size of the exposed area of the plate.

In this article, using the constrained predictive controller, the tracking of a moving target by a group of flying robots while controlling the group arrangement is presented. The maintenance and control of the group arrangement is based on position control with the assumption of decentralized architecture and leader-follower communication structure between the agents. Target tracking is done by the group leader and the followers are responsible for tracking the group leader. The leader of the group constantly sends the current position and the estimate of his position in the next steps to the followers to use it to calculate the optimal current position and the next moments. In the following, rewriting the constraints of the problem, such as ensuring that the distance is maintained and that the followers do not collide with the group leader, and the limitations of inputs and their changes on all members of the group, as well as providing an innovative geometric method for crossing obstacles and systematically applying all of them in the control process. has been Simulations are presented for a group of five flying robots that seek to intercept a moving ground target while maintaining the formation of the group. The results show that the controller has a good resistance to the disturbances in the group's movement path and the group interception is carried out in a favorable way. Predicting the movement path of the moving target and using it in the design of the predictive controller also improves and smooths the movement path of the group.

This paper proposes robust hybrid sliding mode control with a time-varying sliding surface and active vibration control for a flexible spacecraft during attitude maneuver. The fully coupled nonlinear dynamic model of the rigid-flexible system includes the three-axis rotation of the rigid body in interaction with the transverse deformation of the smart PZT-mounted flexible appendages. The smooth control signal includes a hyperbolic tangent and a sharpness function to reduce the effects of chattering and high-frequency interactions of the flexible parts and external disturbances in interaction with the rigid body and controller. The structure of the variable sliding surface with time has made it possible to adjust the effect of attitude parameters (quaternions and angular velocities) on the control performance. Also, the residual vibrations during and after the reaching phase are suppressed using a robust active vibration control algorithm. The simulations in the form of a comparative study show the performance and superiority of the proposed approach compared to conventional sliding mode control approaches for systems with structural flexibility in the presence of external perturbations and uncertainties.

In this research, the main goal is to investigate the energy absorption parameters of cylindrical steel tubes filled with polyethylene. For this purpose, experimental tests and numerical simulation were used. In order to compare the performance of energy absorption, empty steel cylindrical tube specimens and four type of steel cylindrical tube filled with polyethylene specimens with different axial perforations in the filling part were subjected to quasi-static compression test and compared with each other. Numerical simulation was also done with the help of ABAQUS software and experimental results were used for the mechanical properties of materials. The results of the numerical simulation showed a good agreement with the experimental results. Also, the results showed that the use of polyethylene filler in the filled samples causes a decrease in the initial peak crushing force values, an increase in the mean crushing force values and also a decrease in energy absorption, and an increase in the crushing force efficiency compared to the empty samples. For example, in the sample filled with polyethylene and having five axial holes, there was a 60.5% decrease in the maximum crushing force and an increase in the crushing force efficiency by 166.77% compared to the empty sample. Also, the samples with five holes have the highest crushing force efficiency among the samples filled with polyethylene.

In recent years, considerable efforts have been deducted to evaluate the quality of various products. In this regard, quality control of products fabricated by the additive manufacturing methods has become a new interest. In the 3D printing industry, Defect detection could have a vital role in the final evaluation of products. In this study, artificial defects are located in PLA samples printed with different infills via the Fused Deposition Modeling (FDM) method. In order to detection of the mentioned defects in these parts, the Active IR Thermography was employed. Thermal stimulation was selected as excitation method. Two different excitation formation including transmission and reflection mode are used. In order to validate the results, the temperature-pixel diagram for each sample were illustrated. The effect of Gaussian filter on thermal images was also investigated to facilitate the identification of the obtained images. The experimental results indicated the capability of thermally stimulated thermographic inspection in detection of defects in FDM printed parts.

In this paper, we propose a method for sub-optimal control of time-varying polynomial systems and use it for pursuits guidance law design. Since, engagement equations between pursuit and target are depend on the range between them and this range is varying during the flight, guidance law designer is faced with a time-varying system. The developed methods for control of time-invariant systems are not directly applicable for time-varying systems. One of the conventional approaches for pursuits guidance law design is the optimal control. Approximate dynamic programming is a well-known method for solving the optimal control problem. One of the challenges of using this method for control of nonlinear time-varying systems is the difficulty of solving the Bellman equation. In the proposed method of this paper, solving the Bellman equation has been relaxed with solving a sum-of-squares optimization problem. It will be proved that the designed control policy with this method is globally exponentially stabilizing. Finally, performance of the proposed method for pursuits guidance will be illustrated with numerical simulations.

The navigation system of vehicles calculates the speed, position and attitude of the moving device relative to a reference frame and provides it to the guidance system. One of the most widely used navigation systems is the inertial navigation system. Due to the increasing error of the navigation system over time, the integrated navigation system is usually used for long-term navigation. One of the most common integrated navigation systems is the INS integrated navigation system with GPS, each of them which has advantages and disadvantages that cover the other. In this paper, two GPS and INS data integration algorithms with loosely and tightly coupled integration are implemented and compared. In the loosely coupled method, GPS measurements include positions and speeds. In the tightly coupled method, a model for GPS error is considered, which includes bias dynamics and GPS clock drift. The result of combining GPS and INS data in this way is closer to the truth, but in the method of loosely coupled, the result of the combination follows the average of GPS data. In the implementation of the combined algorithm with tightly coupled, raw GPS data is used, which is pseudo-range and pseudo-range rate along with astronomical information. In this paper, an extended Kalman filter is used to integrate the data of two measurement data. The simulation results show the superiority of tightly over loosely connection performance. Also, the integration algorithm with a loosely approach has been implemented in hardware and car testing has been done in two scenarios of connecting and disconnecting GPS.

Selective laser sintering is one of the methods of additive manufacturing based on powder bed which is widely used in various industries today. In this process, the amount of laminated mass has a high impact on reducing the porosity and quality of the final parts produced. For this purpose, in this study, a mechanism was designed and developed to investigate and achieve the maximum amount of laminated mass in a laminating process. The optimal state was obtained using the central composite design method. The results show that the parameters of blade velocity, angle under blade, and excess powder percentage have a greater effect on the amount of laminated mass, respectively. The optimized laminated mass was predicted by the experimental model to be 0.811 g, in which case the velocity is 2.78 cm / s, the blade angle is 3.1 ° and the initial powder content is 159%. To validate the model, the experimental experiment was performed based on the parameters of the optimal state, in which the laminated mass was measured 0.83 g, which shows that the error of the obtained model is 2.2%.

4D printing is an emerging technology in which the printed parts can change their shapes when exposed to an external stimulus. In this research, a relationship between the pre-stress stored in the printed discs and the amount of shape recovery was obtained using FE simulations. Then, the effect of layer height, bed temperature, nozzle temperature, and printing speed on the amount of pre-stress applied in the printed discs based on polylactic acid material in the Fused deposition modeling 3D printing was investigated. In this paper, 27 experimental tests were performed using a central composite design, and modeling was done with the least cost and time and the most reliability and accuracy. Then a relationship between the mentioned parameters and the amount of pre-stress applied to the printed discs was obtained. R2 and Adj R2 of the model were obtained more than 99% which shows the high accuracy of the model. In order to validate the experimental model, simulation and experimental tests were carried out in the conditions of the highest amount of shape recovery. The results showed that the experimental model error is 5.9% and the error between the applied pre-stress and the amount of shape recovery is 2.95%.

By investigation of the research conducted in the field of stepping planing Craft, almost no research has been done that studies the wet surfaces that are created by water jumping from the step and can have a direct impact on the design of the step. Calculating the area of these wetted Surfaces can help the designer in solving the dynamic longitudinal instability (porpoising), which is one of the most important issues in the design of stepped high-speed Crafts. In this regard, experimental tests have been carried out on the Fridesma model at three different elevation angles as well as at different speeds and geometrical parameters of the step (heights and different places of the step). The purpose of this research is to obtain an analytical and computational formula to calculate these wet surfaces based on the geometric parameters of the body, the geometric parameters of the step and other hydrodynamic parameters that are measured during the tests, including the wetted length of the keel and Chine, dynamic trim, is the amount of water jumping from the Steps. These relations can play a very important role in the design of step in stepped vessels.

In this paper, yaw stability increment of a three-wheeled vehicle has been considered using a new control system. Therefore, a nonlinear dynamic model with twelve-degrees-of-freedom to simulate lateral dynamics of the three-wheeled vehicle has been developed and dynamic model validation is done by means of CarSim software during a standard maneuver. The degrees of freedom are the longitudinal, lateral and vertical velocity and the roll, pitch and yaw angle of the sprung mass, three degrees of freedom for the vertical displacement and three degrees of freedom for rotational movement of the unspring masses. Moreover, to improve handling and lateral stability increment, an active steering control system based on the lateral tire forces has been designed. In the control system, the yaw rate and the lateral velocity of the dynamic model are studied as control states in which must track their desired values. Then, to avoid yaw instability during severe maneuvers, linear quadratic control system (LQR) has been used. Furthermore, to evaluate the performance of the developed control system, an active steering control system has been proposed employing linearization feedback control method. Then, the performance of the dynamic model has been evaluated during without control and controlled conditions. The simulation results show that the LQR control system improves maneuverability and lateral stability during critical maneuver by elimination the error between the yaw rate and its desired value and the lateral velocity restriction.