Sharif Digital Repository

In this paper, a novel, A-shaped microrobot with nanometric resolution for precision positioning applications is addressed. The locomotion concept of the mechanism is founded on the "friction drive principle". To achieve the translational motion, a stack piezoelectric actuates the microrobot near its natural frequency. The dynamic modeling of the mechanism is based on the assumptions of linear behavior of piezo stack actuator and Coulomb friction model at contact points. The suitability of three simple, friction-based locomotion modes for implementation on the proposed device is presented. Influences of different friction coefficients on the behavior of the microrobot, with respect to... 

The friction drive principle, which is based on the superposition of two synchronized perpendicular vibrations at the interface of the robot and the work floor, plays a fundamental role in the locomotion of miniaturized robots. In this paper, the iteration perturbation method proposed by He is used to generate a periodic solution for this type of friction drive microrobot. The equation of motion for the system reveals a parametrically excited oscillator with discontinuity, the elastic force term for which is proportional to a signum function. The obtained solutions are in excellent agreement with those achieved from numerical integration and experiments reported in the literature. Results... 

In this paper, we investigate the influence of nonideal boundary conditions on the nonlinear vibration of damped Euler-Bernoulli beams subjected to harmonic loads. These nonidealities allow for small deflections and/or moments at the supports of the beam. Using the iteration perturbation method, analytical expressions for the case of simply supported beams with immovable end conditions are provided. The results reveal that the first order of approximation obtained by the proposed method is more accurate than the perturbation solutions. Moreover, compared with the previous studies, some interesting phenomenon is predicted. We have shown that in some special combinations of the nonidealities... 

The motion of a stick-slip microrobot propelled by its piezoelectric unimorph legs is mathematically modeled. Using a continuously distributed mass model for the robot's body, the working equation of the mechanism is derived based on the assumption of linear Euler-Bernoulli beam theory and linear piezoelectric behavior. Moreover, the required condition for generating net motion is calculated in terms of physical characteristics of the microrobot. It is demonstrated that the higher the friction constant, then a lower average speed is obtained. Also, it is shown that a microrobot with heavier legs can move in a rougher environment. Regardless of the mass proportion between robot's main body... 

In this work, a powerful analytical method, called Liao's homotopy analysis method is used to study the limit cycle of a two-dimensional nonlinear dynamical system, namely the van der Pol oscillator with delayed amplitude limiting. It is shown that the solutions are valid for a wide range of variation of the system parameters. Comparison of the obtained solutions with those achieved by numerical solutions and by other perturbation techniques shows that the utilized method is effective and convenient to solve this type of problems with the desired order of approximation  

In this paper, we investigate the dynamic analysis of a strongly nonlinear microrobot using a three-term harmonic balance method. The employed locomotion concept, namely "friction drive principle," is based on the superposition of a horizontal vibration at the interface between the robot and work floor and an active variation of friction force, obtained by the vertical vibration of the base at the same interface. The equation of motion for the system reveals a parametrically excited oscillator with discontinuity for which the elastic force term is proportional to a signum function. The obtained periodic solution not only is of high accuracy, but also can predict the contribution of the... 

Among the locomotion concepts employed in the microrobotics, friction-based locomotion principles are of considerable importance. In this study, the dynamic modelling of friction drive microrobots subjected to the tangential and normal excitations is investigated, which have the same frequency, but are shifted in phase. The motion equation of the microrobot reveals a strongly non-linear differential equation with discontinuity for which the elastic force term is proportional to a signum function. Using the homotopy perturbation method, simple expressions are derived for predicting average velocity of the slider. The obtained results are in good agreement with those achieved from numerical... 

This paper presents a novel, sliding, A-shaped microrobot with nanometric resolution for precision positioning applications. The microrobot is actuated near its natural frequency using a piezoelectric stack actuator to produce translational motion. The dynamic modeling of the mechanism is based on the assumptions of the linear piezoelectric behavior and the Coulomb friction model. Using this model the required condition for generating net motion is found. The suitability of three simple, friction-based locomotion modes for implementation on the proposed device is addressed. Influences of some important configuration parameters on the behavior of the microrobot, based on defined criteria, are... 

In this paper, we analytically investigate the dynamic modeling of a nonlinear microrobot based on the "friction drive principle". The superposition of a horizontal vibration at the interface between the robot and work floor and an active variation of friction force, obtained by the vertical vibration of the base, makes the robot to move on the substrate. Periodic solutions obtained from the harmonic balance method can predict the contribution of the friction coefficient on the average velocity of the slider. Unlike traditional analytical techniques and in agreement with both numerical simulations and experiments reported in the literature, results show that the maximum average velocity... 

This study presents a bilayer structure as a skin scaffold comprised of an electrospun sheet layer made of polycaprolactone and polyvinil alcohol and a porous hydrogel layer made of chitosan and gelatin. The hydrogel layer was fabricated by employing the freeze-gelation technique. The bilayer structure was achieved by pouring the hydrogel solution on the electrospun sheet at the bottom of a mold followed by the freeze-gelation technique to obtain a porous structure in the hydrogel. The hydrogel and hydrogel-electrospun samples were characterized by scanning electron microscopy, swelling, tensile strength, in vitro and in vivo analyses. From a mechanical strength standpoint, the combination... 

In this study, fabrication of a three-dimensional porous scaffold was performed using freeze gelation method. Recently, fabrication of scaffolds using polymer blends has become common for many tissue engineering applications due to their unique tunable properties. In this work, we fabricated alginate-gelatin porous hydrogels for wound healing application using a new method based on some modifications to the freeze-gelation method. Alginate and gelatin were mixed in three different ratios and the resulting solutions underwent freeze gelation to obtain 3D porous matrices. We analyzed the samples using different characterization tests. The scanning electron microscopy (SEM) results indicated... 

The nonlinear equation of motion for pre-stretched EulerBernoulli beams is derived. The effect of pre-tension and pre-compression in EulerBernoulli beams is studied. It is shown that compressive strain affects the bending stiffness much more than tensile strain. Based on the derived equation, the dynamic model of bimorph piezo-actuated beams, which is accurate, yet simple, is then developed. Afterwards, the critical voltage, which makes the piezo-actuated microbeam unstable, is numerically investigated. It is shown that the strain of the centerline is proportional to the beam's aspect ratio squared. Results show that the more the aspect ratio, the less the critical voltage. In addition, it... 

Microrobots design and manufacturing has been one of interesting fields in robotics in recent years. Various legged designs have been proposed in the literature. All designs rely on friction for locomotion. In this paper the dynamic model of a planar two-legged microrobot is presented using LuGre friction model. LuGre friction model is more realistic model, reducing uncertainties of the microrobot dynamic model, providing a better prediction for both design and control applications. The proposed microrobot is driven by a piezoelectric actuator mounted between centers of two legs. One of important issues in modeling of microrobots is to determine the friction force between robot and... 

A series of samples with different compositions and parameters were welded together. The alloys were based on Ni-8Al-8Cr-l.5Mo-0.01B %wt composition with Zr additions of 1 and 3 %wt. Crack-free full-penetration welds of castable nickel aluminide alloy were performed using autogenous GTAW technique. It was found that Ni-Ni5Zr eutectic was associated with solidification cracks. Alloys were fully dendritic and also, weld metal consisted of columnar and axial dendrites beside the fusion line and in the middle, respectively. Optical and scanning electron microscopy (SEM) techniques were employed to characterize the composition as well as the weld structure. X-Ray diffraction was utilized to... 

We model the problem of assigning counsel to poor defendants as a matching problem. A novel aspect of this matching problem is the moral hazard component on the part of counsel. Within the model, we show that holding the total expenditure for counsel fixed and changing the matching procedure to accommodate defendants' and attorneys' preferences, i.e., switch from random matching to stable matching, defendants become worse offbecause a stable matching exacerbates the moral hazard problem on the part of counsel. In addition, we show that under suitable conditions random matching is the efficient way to allocate defendants to counsel. © 2018 American Economic Association  

In synchroes and resolvers, brushes and slip-rings produce a lot of noise in the output signal. Compared to encoders, the application of such position sensors in precision control systems are restricted because of their lower accuracy. In this research, a novel scheme of a brushless synchro is introduced. In this scheme, the secondary windings are mounted on the stator and the stator magnetic flux passes a certain path in the rotor and induces voltage in the secondary windings. The operation principle is clearly described in the paper and by using 2D finite element method the novel synchro is initially designed and analyzed. The stator winding's turns is calculated by a method based on... 

A new and general procedure for history matching that uses streamline simulation and a gradual deformation technique has been proposed. Streamline trajectories define the major flow paths and help the algorithm to modify the reservoir model only in the regions that cause the mismatch between simulation results and field observations. The use of a gradual deformation technique enables the algorithm to remain constrained to reservoir geostatistics. It also reduces the number of independent variables in the optimization problem. In our algorithm, effective conductivities are defined along each streamline. The effective streamline conductivities are then adjusted using the values of measured and...