Said M. Megahed

Robot workspace estimation and base placement optimisation techniques for the conversion of conventional work cells into autonomous flexible manufacturing systems

Robot arms are used in modern work cells and flexible manufacturing systems (FMS) in handling work pieces and loading/unloading processes. The robot arm links may interfere with the bodies of the system components, which are considered as obstacles in the robot workspace. In order that the robot works in safe conditions, study of robot workspace in a free space and in the presence of obstacles should be investigated. The inverse problem of having an existing working space occupied with a number of CNC machines to which it is supposed to introduce a robot to serve is an interesting problem. This is a very common problem when trying to convert conventional work cells into autonomous systems. There are two main questions when studying this kind of problem: what type of robots is suitable to satisfy the existing working space? And where to place the base of this robot to efficiently serve the existing machines? The main objective of this article is trying to answer these two questions. A computational algorithm is developed to estimate the robot workspace. The optimisation of robot base placement is achieved using genetic algorithms. A comparative study of the suitability of different robots for a specified working area is also included. Finally, robot movement visualisation within a pre-defined FMS using solid edge modelling is presented to verify the proposed algorithm and simulate the robot path within the work cell.

Modeling and simulation of planar flexible link manipulators with rigid tip connections to revolute joints

This paper presents the basis of a mathematical model for simulation of planar flexible-link manipulators, taking into consideration the effect of higher stiffness zones at the link tips. The proposed formulation is a variation of the finite segment multi-body dynamics approach. The formulation employs a consistent mass matrix in order to provide better approximation than the traditional lumped masses often encountered in the finite segment approach. The formulation is implemented into a computational code and tested through three examples; cantilever beam, rotating beam and three-link manipulator. In these examples, the length of the rigid tips at both sides of each link ranges from 0p to 6.25p of the whole link length. The zones of higher stiffness at the link tips are treated as short rigid zones. The effect of the rigid zones is averaged along with some portions of the flexible links, thereby allowing further simplification of the dynamic equations of motion. The simulation results demonstrate the effectiveness of the proposed modeling technique and show the importance of not ignoring the effect of the rigid tips.

A Continuum Based Three-Dimensional Modeling of Wind Turbine Blades

Accurate modeling of large wind turbine blades is an extremely challenging problem. This is due to their tremendous geometric complexity and the turbulent and unpredictable conditions in which they operate. In this paper, a continuum based three dimensional finite element model of an elastic wind turbine blade is derived using the absolute nodal coordinates formulation (ANCF). This formulation is very suitable for modeling of largedeformation, large-rotation structures like wind turbine blades. An efficient model of six thin plate elements is proposed for such blades with non-uniform, and twisted nature. Furthermore, a mapping procedure to construct the ANCF model of NACA (National Advisory Committee for Aeronautics) wind turbine blades airfoils is established to mesh the geometry of a real turbine blade. The complex shape of such blades is approximated using an absolute nodal coordinate thin plate element, to take the blades tapering and twist into account. Three numerical examples are presented to show the transient response of the wind turbine blades due to gravitational/aerodynamics forces. The simulation results are compared with those obtained using ANSYS code with a good agreement. [DOI: 10.1115/1.4007798]

Use of the floating frame of reference formulation in large deformation analysis: Experimental and numerical validation

Abstract The finite-element floating frame of reference (FFR) formulation is used, for the most part, in the small deformation analysis of flexible bodies that undergo large reference displacements. This formulation allows for filtering out systematically complex shapes associated with high frequencies that have no significant effect on the solution in the case of small deformations. The resulting low-order FFR models have been widely used to obtain efficient and accurate solutions for many engineering and physics applications. In this investigation, the use of the FFR formulation in the large deformation analysis is examined, and it is demonstrated that large deformation FFR models can be accurate in applications, where the deformation can be described using simple shapes as it is the case in robot system manipulators. In these cases, the standard finite-element FFR formulation must be used with non-linear strain—displacement relationships that account for the geometric non-linearities. The results obtained using the large deformation FFR models are compared with the results obtained using the large deformation absolute nodal coordinate formulation (ANCF), which does not allow for the use of linear modes. The ANCF models are developed using two different methods for formulating the elastic forces: the basic continuum mechanics approach (ANCF-BC) and the elastic line method (ANCF-EL). While the explicit Adams method can be used to obtain the numerical solution of the FFR model, two implicit integration methods are implemented in order to be able to obtain an efficient solution of the FFR and ANCF models. These implicit integration methods are the RADAU5 method and the Hilber—Hughes—Taylor (HHT) method. In the case of simple large deformation shapes, the simulation results obtained in this study show a good agreement between the FFR and the ANCF solutions. The results also show that, in the case of thin and stiff beams, the coupled deformation modes that result from the use of the ANCF-BC can be a source of numerical and locking problems, as reported in the literature. These ANCF-BC numerical problems can be circumvented using the implicit HHT integration method. Nonetheless, the HHT integrator does not capture high-frequency FFR axial modes which are necessary in order to obtain accurate solutions for high-speed rotating beams. In addition to the comparison with the ANCF solutions, experimental results of a forward dynamics model are used in this study to validate the large deformation FFR numerical solutions. The experimental set-up used in the validation of the numerical solutions is also described in this investigation.

Modeling Slope Discontinuity of Large Size Wind-Turbine Blade Using Absolute Nodal Coordinate Formulation

This paper describes the use of the Absolute Nodal Coordinate Formulation (ANCF) in modeling large-size wind turbine blades. An efficient procedure is developed for mapping NACA airfoil wind-turbine blades into ANCF thin plate models. The procedure concerns the wind turbine blade with non-uniform, twisted nature. As a result, the slope discontinuity problem arises and presents numerical errors in the dynamic simulation. This investigation illustrates a method for modeling slope discontinuity resulting from the variations of the cross sectional layouts across the blade. A method is developed and applied for the gradient-deficient thin plate element in order to account for structural discontinuity. The numerical results show a numerical convergence and satisfy the principle of work and energy in dynamics. The simulation results are compared with those obtained using ANSYS code with a good agreement.Copyright

Inverse kinematics of spherical wrist robot arms: Analysis and simulation

The spherical wrist robot arm is the most common type of industrial robot. This paper presents an efficient analytical computation procedure of its inverse kinematics. It is based on the decomposition of the inverse kinematic problem to two less complex problems; one concerns the robot arm basic structure and the other concerns its hand. The proposed computation procedure is used to obtain the inverse kinematic position models of two robot arms: one contains only revolute joints and the other contains both revolute and prismatic joints. The 1st and 2nd time derivatives of the obtained models give more accurate inverse kinematic velocity and acceleration models than numerical differentiation. These models are verified by simulation for two different trajectories. The obtained results demonstrate the effect of the proposed procedure on reducing the necessary computation time compared to other computation techniques.

FLOATING FRAME OF REFERENCE AND ABSOLUTE NODAL COORDINATE FORMULATIONS IN THE LARGE DEFORMATION ANALYSIS OF ROBOTIC MANIPULATORS: A COMPARATIVE EXPERIMENTAL AND NUMERICAL STUDY

This paper describes the use of flexible multibody system approaches in the dynamic modeling of interconnected rigid-elastic robotic manipulators. Two approaches are used to establish the flexible robot dynamic model; the floating frame of reference formulation (FFR) and the absolute nodal coordinate formulation (ANCF). The ANCF is used with two different methods for formulating the elastic forces; basic continuum mechanics approach (ANCF-BC) and elastic line method (ANCF-EL). The simulation results show that the use of the nonlinear FFR and the ANCF-EL improves the performance of the beam element in the modeling of flexible robotic manipulators. In the case of simple large deformation shape, the simulation results obtained show a good agreement between the FFR and the ANCF solutions. In the case of thin and stiff beams, the coupled deformation modes that result from the use of the ANCF-BC can be a source of numerical problems. These problems can be avoided using the implicit Hilber-Hughes-Taylor (HHT) integration method. On the other hand, HHT integrator does not capture high frequency axial modes when the FFR is used; RADAU5 method is used instead. The experimental results of the direct dynamics model are effectively used in this study to validate the numerical solutions.Copyright

Tonal Tuning of a Variable Inertia Vibration Absorber: A Feasibility Study

The primary goal of this study is to investigate the dynamics of a new class of adaptive Tuned Vibration Absorber (TVA): A variable effective inertia absorber. In particular, the accuracy of linearization of the dynamic equations, the effect of the moving mass on the dynamics of the absorber, and steady state vibration of the primary system are investigated. It is shown that the linearized model is accurate. Two simulations using the nonlinear model are reported. These simulations show effectiveness of the absorber on reducing the displacement and acceleration of the primary system. In a third simulation, the block is moved from a detuned position to a tuned position and nonlinear differential equation of the motion is solved. The results show a significant decrease of the vibration of the primary system.Copyright

Topological and kinematical study of tree structure robot manipulators: Symbolic computation technique

Abstract Tree structure robot manipulators have been seldomly studied because of the complexity of their topological structures compared to simplechain type. This paper presents the topological and kinematical analysis of such systems. The kinematic analysis is based on the idea of decomposing the whole model to a number of submodels, one for each arm of the tree. A complete symbolic kinematical analysis of a five-link structure with two end-effectors is presented. The necessary number of arithmetic operations for computing each model is also computed. This example shows the effectiveness of the approach used for kinematic analysis of this type of robot manipulators.

Novel Contact Sensor Concept and Prototype Based on 2-DOF Vibration Absorber System

Landmines are major problems, waste life and money. Much recent research acknowledges that the contact sensors have promising potential. In this work, a new idea of contact sensor for landmine detection is introduced. The sensor main principle is based on the concept of 2-DOF vibration absorber system (two springs and two masses), to detect the existence of an object (ex: landmine) in sand which is modeled as a 3rd spring. The sand stiffness (the 3rd spring stiffness ko) can be acquired as function of the frequency vibration absorber mode ?Abs (the frequency at which the 2nd mass has the lowest amplitude (mathematically proven: zero)). When the sand stiffness changed due to the presence of the landmine, the vibration absorber frequency ?Abs changes, and consequently the landmine can be detected. The mathematical derivation of the (?Abs-ko) relation is verified by simulations with Matlab and with finite element COMSOL Multi-physics. The system is succeeded to measure the sand stiffness up to 100kN/m. A physical prototype for the sensor is developed with sensitivity 16.85 (N/m)/Hz.