Search

This pafa considers a three-dimensional micropump based on a piezoelectric actuator. COMSOL software version 6.5 is used to simulate the faformance of the micropump. At first, a reference micropump is defined, which works at a frequency of 60 Hz and a voltage of 1500 V that is capable of pumping a maximum flow rate of 0.038 ml/s. The influence of frequency, voltage, the axial position of input and output of micropump, dimensions of piezoelectric actuator and membrane, and shape of the actuator on the fluid output flow rate and pumped fluid volume is investigated. The results demonstrate that the frequency, voltage, dimensions of the piezoelectric actuator and the membrane has a direct effect on the output flow rate and the pumped fluid volume so that the output flow rate of the fluid and the volume of the pumped fluid increase with the increase of these parameters. For example, the output flow rate of the micropump at 100 and 20 Hz frequency is 0.065 and 0.011 ml/s, respectively. By increasing the voltage from 1500 to 1875 V, the micropump output flow rate increases to 0.047 ml/s. The results show that the rectangular piezoelectric actuator has a lower efficiency than the circular one. [ABSTRACT FROM AUTHOR]

In this paper, a finite-time fault tolerant controller based on sliding mode algorithm, as a robust control method, is presented to control the attitude stabilization of the quadrotor system in the presence of actuator fault and uncertainty. The controller is designed based on the nonlinear model of a quadrotor and its stability analysis is performed according to the Lyapunov stability theorem. Also, regarding some weaknesses of MEMS sensors such as partly high noise and bias error, an extended Kalman filter is designed and implemented in order to merge sensors data and reduce the noise effect on the outputs. To validate the controller performance, the experimental tests is implemented on a full-scale quadrotor in real-time. The evaluation of the designed strategy is carried out in different scenarios, no fault in the actuators and a partial loss of effectiveness of an actuator as well as uncertainty in the quadrotor parameters. The experimental results reveal the superiority of the sliding mode tolerant strategy over feedback linearization in the presence of various faults and uncertainty effects. [ABSTRACT FROM AUTHOR]

Pneumatic Soft bending actuators as safe and highly adaptable robots are being considered by researchers, such that they become an ideal choice for making devices that interact more with humans. These advantages come along with several disadvantages. Their great adaptability is provided by their high degrees of freedom, and consequently, it makes the modeling a significant challenge for researchers. In this paper, the modeling of a soft fiber- reinforced bending actuator that is in contact with the environment has been conducted by utilizing the finite rigid element method. This method provides a context within which the modeling theories of traditional rigid robots, such as the Denavit-Hartenberg method, become usable for a soft continuum actuator. Therefore, in the following, resorting to this theory, the static and dynamic behavior of a soft actuator under the effect of the external load has been investigated and compared with the empirical results derived from a fabricated actuator. The results show a minimum accuracy of 9 percent, although being as simple as can be used for other purposes like implementing control systems based on that. [ABSTRACT FROM AUTHOR]

The comfort of car passengers is one of the most important issues while driving, therefore the suspension system plays a significant role in this matter. To investigate this issue, in this research, the design of a fault-tolerant control system for the active suspension system of the one half- car model has been considered. In this article, the goal is to design a robust-adaptive sliding mode controller to stabilize the vehicle suspension system and converge the output response to the desired response. For this purpose, firstly, the model of the suspension system of the one half of the car is considered in the presence of actuator, sensor, modeling uncertainties and limited external disturbances. Then, with the assumption of not knowing the upper band of the term, including system faults and disturbances, the error tolerant control law has been designed so that the output response of the system in front of any sudden fault converges to the desired(stable) value and also the output of a desired path. [ABSTRACT FROM AUTHOR]

The series elastic actuators make more comfort in the use of assistive exoskeletons. In this paper, an assistive controller is designed for a series-elastic-actuator-driven knee exoskeleton to restore normative mobility of individuals with weak muscles. The main target of the proposed controller is to modify the dynamics performance of the coupled humanexoskeleton system. In other words, the proposed controller modifies the relationship between the net muscle torque exerted by the human and the resulting angular motion. There are fewer sensors in the proposed intent-independent method relative to other methods. Moreover, there are less controller coefficients to regulate where these coefficients are extracted from a type zero Takagi-Sugeno-Kang fuzzy system. The performance of the controller is evaluated by simulations and experiments. The amplitude of the EMG signals decreased in a healthy person worn the SUT-KneeExo. Moreover, the proposed algorithm has a better performance in comparison with integral admittance shaping mothed and output feedback assistive controller. In other words, the amplitude of the integral admittance is more and the phase lag is less than other methods. [ABSTRACT FROM AUTHOR]

Human ankle-foot gait is the result of a complex interaction between nerves and muscles. A significant number of prosthetic ankles-foots (passive, semi-active, active) have been designed to restore an identical function of a real limb. Excluding passive and semi-active prosthesis, that cannot generate any positive work, one of the biggest challenges in creating these prostheses is providing the needed power and energy during movement. Supplying this power and energy, requires a high-torque and high-power actuator having high weight, thereby causing a dramatic increase in the weight and size of the resulted prosthesis. In this paper, a combination of an active actuator (an electrical motor) and a passive stimulus (a spring) is utilized, which can decrease the needed power and energy. This design does not only support walking mode, but also provides running mode up to 2.5m/s. Accordingly, the first stage of this article includes mechanical modeling of the ankle and evaluation of efficiency and power consumption in all presented models. Then, a combination of spring and series actuator is selected as the best and optimal combination, which is different from the previous structures. In this structure, power and energy consumption are dramatically reduced up to 58% and 26% in walking mode and 64% and 57% in running mode. Consequently, a lighter motor and battery can supply the required power, so the prosthesis’s weight is decreased. Thus, in order to optimize power and energy consumption, it is necessary to determine the optimal spring stiffness based on design priorities (maximum power reduction). In the last step, the most suitable segments are developed for receiving the best performance, and then the designed prosthetic ankle is built. The results of simulations and experimental tests show that the produced prosthesis has all the desired performance, and it uses less energy and power while its user is moving. [ABSTRACT FROM AUTHOR]

Today, piezoelectric actuators are widely used in micro-positioning applications due to unique features such as high precision, fast response and high natural frequency. Despite the aforementioned characteristics, nonlinear characteristics such as hysteresis deteriorate the precision of piezoelectric actuators. In order to reduce the effect of hysteresis in control applications, external sensors are used for feedback control schemes. But, high costs and space limitations are prohibitive factors which limit the application of external sensors. Hence, an alternative is using self-sensing methods that is based on electromechanical characteristics of piezoelectric materials which eventually eliminate external sensors. In this research, self-sensing method is applied for position estimation in piezoelectric actuators. The most conventional method is based on the linear relation of electrical charge and actuator position which the position can be estimated by measuring the actuator charge. But this method is faced with serious challenges due to charge drift, especially at low frequencies. For this purpose, a method for modeling and compensating of charge drift is proposed. Then, by linearization of the electric charge-position relation, the self-sensing method is implemented based on the compensated electric charge measurement. Experiments have confirmed that this method can effectively estimate the actuator position with 1.5% estimation error in the presence of charge leakage. [ABSTRACT FROM AUTHOR]

Piezoelectric actuators are the most common choice for position control with ultra-high precision. Despite the significant advantages, the linear and nonlinear dynamics of these actuators, such as hysteresis, could decrease the precision of the control system. In this research, a controller based on the sliding mode method is proposed for position control of piezoelectric actuator. Sliding mode control is a model-based and useful method in nanopositioning systems. In this research, Bouc-Wen model is used for description of the actuator’s behavior. In this model, the linear dynamic is modeled with mass, stiffness and damping terms, and the hysteresis is modeled by its nonlinear dynamics. Usually, there are mismatch and uncertainty between the physical system and mathematical model. For stability analysis of the prevalent sliding mode control, the upper bound of uncertainty must be known. But, in practical systems, this is not possible, simply. On the other hand, selecting the large values for this bound, increases the controller gain and distances it from the optimum value. The proposed adaptive robust control eliminates the dependency to the upper bound of uncertainty. This is done by introducing an online adaptive law for estimating this bound. Proposing this law, asymptotic stability of the closed-loop control system is proven. Implementing the presented method on the laboratory setup and simulator software, its effectiveness is shown by simulation and experimental results. [ABSTRACT FROM AUTHOR]

Hydrogels are the smart polymeric materials, which undergo large deformation when they are subjected to different physical and chemical stimuli in contact with fluids. These materials can be applied as sensors and actuators for instance in microfluidics in which the fluidsolid interactions have an important effect on its performance. On the other hand, the use of graded materials is also important considering their advantages. In this study, the behavior of a functionally graded temperature sensitive hydrogel micro-valve is investigated through considering the fluid-solid interactions. In this regard, the appropriate numerical tool for finite element modeling of a functionally graded hydrogel micro-valve has been developed that it has been implemented in both non fluid-solid interactions and fluid-solid interactions simulation. The homogeneous cases of the micro-valve have also been considered to distinguish the functionally graded temperature sensitive hydrogel micro-valve effect. The results indicate that the effect of fluid-solid interactions was important and have considerable impact on micro-valve operating parameters particularly its closing temperature and fluid flow rate. Thus, a comprehensive study on hydrogel-based micro-valve has been presented considering operating parameters such as inlet pressure and cross linking density of hydrogel. [ABSTRACT FROM AUTHOR]

Maneuvering with the highest speed and low power has always been a challenge to design a satellite and spacecraft control system. In this paper, apart from the complexity of modeling actuators, different control methods were used to control the satellite attitude in the presence of uncertainties and disturbances in satellites, in order to obtain an explicit response to minimize the EULERINT criterion. The EULERINT criterion is the integral of the Euler angles between the body axes and the target around Euler’s axis over time and somehow interprets the speed of the satellite maneuver in the three control axes. First, using the proportional-derivative control, the comparison of the EULERINT criterion in the application of different kinematic representations (Euler, quaternion vectors and direction cosine matrix equations) in linear and nonlinear models of the satellite was carried out. Then the comparison of the EULERINT criterion between the different methods was presented using the quaternion kinematic, which has the least amount of EULERINT, through changing the proportional-derivative controller to linear-quadratic regulator controllers, pole placement, adaptive, fuzzy, and adaptive-fuzzy. The comparison was conducted to achieve the best control method in terms of frequency response, the lowest EULERINT and the least control effort to control the attitude of the satellite in the presence of disturbance and uncertainty. [ABSTRACT FROM AUTHOR]

The main purpose of this paper is to present an adaptive-neural controller for strictfeedback nonlinear systems with unknown time delays and in the presence of external disturbances and actuator failure. The proposed adaptive-neural controller is constructed based on DSC design technique. Radial Basis Functions (RBF) networks are utilized to approximate unknown nonlinear functions. Adaptive rules are obtained based on Lyapunov design for updating the parameters of neural networks. Disturbances are unknown functions which their bounds are partially known. Therefore, continuous robust terms are applied in order to minimize their effects. Furthermore, due to the existence of unknown time delays in the system, Lyapunov-Krasovskii functionals are utilized in the process of designing the controller and proofing the stability of the system. In addition, the controller is designed so that it can compensate its effect if the considered actuator failure happens. For the designed controller, the boundedness of all the closed-loop signals is guaranteed and the tracking error is proved to converge to a small neighborhood of the origin. [ABSTRACT FROM AUTHOR]

Because of the fact that cable-driven parallel robot possess limited moment resisting/ exerting capabilities and relatively small orientation workspaces, in this paper, a method for determination of optimal configuration of reconfigurable cable-driven parallel robots is presented to improve their performance. In such robots, actuators can move the cable attachment points on the base with respect to the motion of the end-effector in its trajectory. In the determined configuration, any external wrench on the end-effector can be balanced, using cable forces for all poses near to a pose of the robot. The largest wrench-closure circular zone centered at an arbitrary point of a trajectory for a given range of orientation around a reference orientation of the end-effector is computed. Taking the area of such zone into account and with the aim of enlarging them, the optimal configuration of the robot is determined. The optimal configuration is found by appropriately changing the position of the moving attachment points on the base of the robots. By applying this procedure on a number of points on a given trajectory iteratively, proper actuation schemes are obtained. In this paper, this method is utilized for reconfigurable planar cable-driven parallel robots and the quality of their actuation schemes is compared with the robots with fixed cable attachment points on base. [ABSTRACT FROM AUTHOR]

Piezoelectric bending actuators have been extensively utilized in recent years. Two major modeling methods, lumped and continuous, have been generally proposed in previous researches for these actuators. The lumped method can only express the transverse vibration of one specified point on the actuator. In addition, the effect of higher vibrational modes has been ignored. Hence, continuous dynamic models have been proposed to rectify the mentioned drawbacks. In this method, linear constitutive equations for low voltage applications are usually applied. But, the main challenge in continuous modeling of piezoelectric actuators is the hysteresis nonlinear phenomenon caused by high excitation voltages. In this paper, piezoelectric nonlinear hysteretic constitutive equations have been employed to carry out the continuous dynamic model for two general types of bimorph bending actuators i.e. Series and Parallel. In addition, zero dynamic analysis for nonlinear systems has been applied to clarify the effect of higher vibrational modes on the actuator dynamic behavior based on the location of target point. Experimental results show the maximum error 1.44 and 1.2% in the identification of first and second vibrational modes, respectively, and the maximum error 2.89% in the modeling of actuator nonlinear behavior by two vibrational modes. These results validate the efficiency of the proposed dynamic model to express the actuator nonlinear behavior, dynamic analysis, and its superiority over conventional models with one vibrational mode. [ABSTRACT FROM AUTHOR]

The scope of the current investigation incorporates the entire process involved in design and development of a Shape Memory Alloy (SMA) actuated wing intended to fulfill morphing missions. At the design step, a two Degree-of-Freedom (DOF) mechanism is designed that is appropriate for morphing wing applications. The mechanism is developed in such a way that it can undergo two different DOF, i.e. gull and sweep, so that the wing can have maneuvers that are more efficient. Smart materials are commonly selected as the actuators due to their suitable thermo-mechanical characteristics. Shape Memory Alloy (SMA) actuators are capable of providing more efficient mechanisms in comparison to the conventional actuators due to their large force/stroke generation, smaller size with high capabilities in limited spaces, and lower weight. As SMA wires have nonlinear hysteresis behavior, their modeling should be implemented in a meticulous way. In this work, after proposing a two DOF morphing wing, an aerodynamic analysis of the whole wing for unmorphed and morphed wings is presented. The results show that the performance of the morphed wing in special flight regimes is improved. [ABSTRACT FROM AUTHOR]

This article describes the details in design and development of a satellite attitude dynamics simulator that is used to validate control algorithms, improve novel control methods, verify the operation of available actuators or sensors and exhibit the fundamentals of attitude dynamics to students or experts. This dumbbell style simulator is based on a spherical air-bearing and is able to produce a frictionless and torque-free environment for simulating spacecraft operational environment. The facility includes a variety of subsystems such as: cold-gas propulsion subsystem, inertial measurement unit, power-supply unit, on-board processor and semi-automatic mass balancing mechanism. The overall design of this system was pretty complicated especially when considering the mission requirements, operating constraints and functional limitations imposed by the ground-based simulation, thus, a detailed design procedure has been deployed and also, this procedure has been performed accurately. An explanation about dynamic equations of motion of simulator with on-off thrusters, software-based simulation, applying reliable control algorithm and balancing methodology is the next part of this article. Finally, the results of the realistic maneuvers of this satellite simulator are presented and investigated in detail. [ABSTRACT FROM AUTHOR]