Bashar El-Khasawneh

Computation of stiffness and stiffness bounds for parallel link manipulators

Parallel link spatial mechanisms in general, and Stewart Platforms in particular, are increasingly being studied for possible use in multi-axis machine-tools. An important consideration in the design of such machines is their stiffness. For a given design, stiffness varies with the direction in which it is computed, the posture (or configuration) of the mechanism and the direction of the actuation or disturbing force. This paper addresses the problem of finding the minimum and maximum stiffnesses and the directions in which they occur for a manipulator in a given posture. In addition, the computation of stiffness in an arbitrary direction is also discussed. Since engineers are often interested in the response of the mechanism in the direction of perturbation (called single-dimensional or engineering stiffness in this paper), this paper proves the computed bounds (maximum and minimum) are tight for such a definition. Stiffness computed using the algebraic formulae derived are compared to those obtained from a Finite Element Analysis model to demonstrate correctness of formulation. Finally, minimum, maximum, arbitrary direction and single-dimensional stiffness maps, are produced for a Stewart Platform and their use in machine tool design is discussed.

Genetic-Fuzzy Sliding Mode Controller for a DC Servomotor system

A Genetic-Fuzzy Sliding Mode Controller is presented for DC Servomotor system control. The fuzzy logic controller was optimized by Genetic Algorithm method to reduce and eliminate the chattering phenomenon. To demonstrate the effectiveness of the presented approach, a comparison between the proposed system, and standard Sliding Mode controller were conducted. Simulation results have shown the advantages of choosing the proposed controller, to achieve the desired results, regardless of the external disturbance, the variation in system parameters, or the feedback noise.

The Tetrahedral Tripod

The Tetrahedral-Tripod is a novel spatial, parallel-link manipulator that offers pure translational motion. In this paper we describe the ideas leading to the development of low degree-of-freedom parallel link mechanisms for use in machine tool applications. The tetrahedral tripod offers superior theoretical characteristics by admitting closed-form solutions for its kinematics and dynamics. Further, its motion and mechanical construction are simple while offering high stiffness and accuracy. It offers a relatively large, singularity-free workspace. It may be used as the basis of a 3-axis machine tool. When used in conjunction with a universal table, it offers the potential for a stiff and accurate 5-axis, series-parallel machine tool. Keywords. Parallel link manipulators, Stewart platform, Machine tools.

Incorporating time standards into generative CAPP: a construction steel case study

Purpose – Time standards are essential to plan and analyze manufacturing processes. A key element of process planning that is not generated from a typical computer‐aided process plan (CAPP) is the process time standards. Generative process planning that includes time standards is particularly needed in the construction steel building (CSB) industry due to variability in projects (orders) size and content. Hence, the purpose of this paper is to focus on incorporating time standards into CAPP of CSB.Design/methodology/approach – Empirical formulas are developed to generate time standards for variant steel beams based on their CAD files (design parameters and geometry) and process parameters (operational conditions). A Motion and Time Study (MTS) is used to estimate times for manual work elements such as load/unload activities and to validate the generated time standards. A generic parametric model is developed with Excel and integrated into the CAPP system to automatically estimate the standard time of each...

The kinematics and calibration of a 5 degrees-of-freedom hybrid serial-parallel kinematics manipulator

Hybrid serial-parallel link manipulators combine the advantages and eliminate disadvantages of their ancestors the pure parallel or serial kinematics manipulators. The particular type of hybrid machine shown in this paper consists of two planar three degrees-of-freedom manipulators assembled together in a special configuration. This paper developed the kinematic analysis of this mechanism, namely the inverse and forward kinematics of these manipulators. Furthermore, this paper developed a calibration model for this type of manipulators, and calibration model results were very promising. A graphical tool was developed to assist user in visualizing postures and for solving the different kinematics problems.

Optimized Planar 3PRR Mechanism for 5 Degrees-of-Freedom Hybrid Kinematics Manipulator

Hybrid kinematics mechanisms combines the advantages of purely serial and purely parallel kinematics mechanisms. Several hybrid kinematics mechanisms have been proposed. This paper proposes a novel hybrid kinematics mechanism using planar 3PRR planar kinematics mechanism, which can be utilized for machine tool. As the common main drawback of parallel mechanism is the workspace, the proposed mechanism has been optimized by using constrained nonlinear optimization. It is shown that the optimization gives significant improvement of the workspace area and shape.Copyright

Continuous-Flow Cell Dipping and Medium Exchange in a Microdevice using Dielectrophoresis

Medium exchange is the process of changing the suspension medium of cells/particles, and has applications in washing, surface modifications, nutrient replenishment, or simply changing the environment of the target entities. Dipping involves diverting the path of target cells in the carrying fluid to immerse them in another fluid for a short duration, and pushing them again into the original medium. In this paper, a simple microfluidic platform is introduced that employs dielectrophoresis to achieve medium exchange and dipping of micro-objects in a continuous manner. The essential feature of the platform is a microchannel that includes two arrays of microelectrodes that partly enter the bottom surface from both sides. In the first step, numerous finite element-based parametric studies are carried out to obtain the optimized geometrical and operational parameters ensuring successful dipping and medium exchange processes. The results of those studies are utilized to fabricate the platform using standard photolithography techniques. The electrodes are patterned on a glass substrate, while the channel, made out of polydimethylsiloxane, is bonded on top of the glass. Trajectories of blood cells from numerical studies and experimentations are reported, and both results exhibited close agreement.

Nonlinear estimation for kinematic calibration of 3PRR planar parallel kinematics manipulator

Calibration is a common procedure to increase the accuracy of machine tools. Estimation as an important part of the calibration has been conducted by using various algorithms. This paper presents the implementation of nonlinear least squares (Gaussian least squares differential correction) algorithm to estimate the geometrical parameters of 3PRR planar parallel kinematics manipulator having nonlinear kinematics which can be used in a hybrid serial-parallel kinematics machine tool. The independent parameters are first estimated followed by the dependent parameters. The convergence to the true values with zero estimation error is guaranteed with any initial estimates provided that no measurement noise is introduced. Subsequently, the estimation by incorporating noise from all measurement devices is conducted which gives the estimates with certain estimation errors. While the estimation errors are affected by the noise level of the measurement devices, it is shown that larger size of measurement samples increases the estimation accuracy. Finally, the uncertainty of the estimates is evaluated by using Monte Carlo simulation.

Multi-objective optimization of planar 3PRR (prismatic-revolute-revolute) parallel mechanism using genetic algorithm

This paper proposes multi-objective optimization of planar 3PRR parallel kinematics mechanism which offers the advantages of lower degree of freedom parallel kinematics mechanisms. Workspace area, minimum eigenvalue across the workspace, and stiffness condition number across the workspace are chosen to be the objectives in the optimization in order to gain as large workspace area as possible while maintaining high stiffness in all directions under the defined kinematics constraints of the mechanism. The multi-objective optimization has been conducted by using multi-objective genetic algorithm. It is shown that the multi-objective optimization compromises the improvement of all objectives by providing non-dominated solutions. A decision maker can pick a preferred solution among those solutions.

Trajectory of Microscale Entities in a Microdevice for Field Flow Fractionation Based on Dielectrophoresis

This article deals with the development of a two-dimensional dynamic model for tracking the path of cells subjected to dielectrophoresis, in a continuous flow microfluidic device, for purposes of field-flow fractionation. The nonuniform electric field exists between the top and bottom surface of the microchannel; the top electrode runs over the entire length of the microchannel while the bottom surface of the same holds multiple finite sized electrodes of opposite polarity. The model consists of two governing equations with each describing the movement of the cell in one of the two dimensions of interest. The equations governing of the cell trajectories as well as that of the electric potential inside the microchannel are solved using finite difference method. The model is subsequently used for parametric study; the parameters considered include cell radii, actuation voltage, microchannel height and volumetric flow rate. The model is particularly useful in the design of microfluidic device employing dielectrophoresis for field flow fractionation.