Nikos A. Aspragathos

Identification of crack location and magnitude in a cantilever beam from the vibration modes

Measurement of flexural vibrations of a cantilever beam with rectangular cross-section having a transverse surface crack extending uniformly along the width of the beam and analytical results are used to relate the measured vibration modes to the crack location and depth. From the measured amplitudes at two points of the structure vibrating at one of its natural modes, the respective vibration frequency and an analytical solution of the dynamic response, the crack location can be found and depth can be estimated with satisfactory accuracy. The method can be used to identify cracks in structures by measuring its modal characteristics. It is a non-destructive testing method for crack identification, and it is applicable to structures for which a structural analysis is available. The main features of the method are as follows: (a) it requires amplitude measurements at two positions of the structure only; (b) it is applicable to all one-dimensional structures; (c) it demands modest computational effort; (d) it is an accurate, simple and easy to handle method having the advantage that the measurements may be carried out in situ with rather simple equipment.

Design of plane developments of doubly curved surfaces

This paper deals with the approximate design of planar developments of doubly curved surfaces and their refinements in order to derive a final plane pattern with limited gaps and overlaps. The hole problem is divided in three stages; the stage of defining the staring guide-strip, the stage of designing the initial pattern and the stage of its refinement. The introduced method dealing with the first stage of the problem is based on elements of the geodesic and Gaussian curvature. In addition, an alternative technique for generating an initial planar development of a doubly curved surface is presented. Two methods for the refinement of the initial pattern are proposed, supported by a set of indices for measuring the accuracy of the flattened component. The first one is based on the surface geometry and the second one on the concept of generalized inverses. By way of illustration, all the proposed methods are presented in parallel with the generation and the refinement of plane patterns of a torus quadrant. In addition, an application of these methods in the 3D surface of a shoe last is demonstrated.

Geodesic curvature preservation in surface flattening through constrained global optimization

In this paper, two methods for generating a planar development of a three-dimensional (3D) surface are proposed. The first method is based on the solution of a global optimization problem without constraints taking into consideration the geodesic curvature of the surface isoparametric curves. The second method is based on the solution of a global optimization problem subject to constraints which are used to control the local accuracy in the derived planar development. Contrarily to previous approaches the proposed second method does not depend on the surface parameterization, since the formed energy function and the proposed constraints are based only on the triangulation of the object surface. Several experiments illustrate that when the 3D surface is approximated with satisfying accuracy, modifications in the triangulation do not have any significant influence to the result of the proposed method. The proposed second method can be used without any modifications with trimmed surfaces, while the generated planar developments can also be used for minimally distorted texture mapping. Finally, the effectiveness of the proposed second method is illustrated through an indicative application in shoe designing.

Fuzzy logic grasp control using tactile sensors

Abstract This paper presents a method to control a two-fingers gripper for safe grasping of fragile and delicate objects like glass, fruits or vegetables. The proposed control algorithm adjusts the motion of the fingers of a gripper using tactile feedback, so that an object is grasped with the minimal required force without the measurement of this force. The rules set of the control law is derived empirically by investigation of the human finger skills. The formulation of these rules and the inference mechanism are based on fuzzy logic. The controller receives the object velocity and acceleration and the detection of incipient slippage and adjusts the fingers motion. The main advantage of this method is that the designed controller is quite robust because knowledge concerning the size, the weight or the surface texture of the grasped object is not required. The object is stably grasped by applying a relatively low grasping force. Simulated experiments are presented to demonstrate the efficiency of the introduced fuzzy controller of robot grippers, while its performance is compared with a PID controller.

Obstacle representation by Bump-surfaces for optimal motion-planning

Abstract This paper introduces a new method for global, near optimal, motion-planning of a robot (either mobile or redundant manipulator) moving in an environment cluttered with a priori known prohibited areas which have arbitrary shape, size and location. The proposed method is based on the novel notion of Bump-surfaces (or B-surfaces) which represent the entire robot environment through a single mathematical entity. The motion-planning solution is searched on a higher-dimension B-surface in such a way that its inverse image into the robot environment satisfies the given objectives and constraints. The computed solution for a mobile robot consists of a smooth curve without self-loops which connects the starting and destination points with the shortest possible path. The same approach is also used for nth degree-of-freedom manipulators where the end-effector reaches the destination position following a smooth short path avoiding the prohibited areas. For clarity reasons the proposed method is introduced in this paper for the case of a two-dimensional (2D) planar terrain with static obstacles, while a generalization to motion-planning problems on curved terrains is also discussed. Extensive experiments are presented and discussed to illustrate the efficiency and effectiveness of the proposed motion-planning method in a variety of complex environments.

A genetic path planning algorithm for redundant articulated robots

In some daily tasks, such as pick and place, the robot is requested to reach with its hand tip a desired target location while it is operating in its environment. Such tasks become more complex in environments cluttered with obstacles, since the constraint for collision-free movement must be also taken into account. This paper presents a new technique based on genetic algorithms (GAs) to solve the path planning problem of articulated redundant robot manipulators. The efficiency of the proposed GA is demonstrated through multiple experiments carried out on several robots with redundant degrees-of-freedom. Finally, the computational complexity of the proposed solution is estimated, in the worst case.

On using planar developments to perform texture mapping on arbitrarily curved surfaces

Abstract This paper introduces a method for applying texture from a two-dimensional domain to a three-dimensional surface which is divided into two stages: the pre-processing stage, where a proper planar development of the three-dimensional surface is generated and the stage of mapping texture from the plane to the given surface. Several techniques for generating planar developments are studied and a new technique for surface flattening is proposed. Using this technique it is possible to control the local mapping accuracy. A new mapping method and a set of indices for evaluating the mapping accuracy are proposed. The accuracy indices are derived through the numerical method of singular-value decomposition. To reduce aliasing a space-variant filter, which can be incorporated within the two stages of the introduced texture mapping technique is developed. Finally, various applications are worked out to illustrate the effectiveness of the techniques proposed in this paper.

Optimal location of a robot path when considering velocity performance

A method for searching the best location of a path in the robot workspace considering the velocity performance of the robot is presented. After a thorough investigation of the robot performance indices, a new measure for the velocity efficiency of a robot moving its end-effector along a path is introduced. This measure is an approximation of the minimum of the Manipulator Velocity Ratio (MVR) along the path. The minimum is calculated from a finite set of MVRs determined at specified points of the path using an algorithm for an approximate motion of the end effector along the path.The introduced measure is used as the objective function in an optimisation problem, where an optimal location of the path is searched. The objective function is procedural and non continuous, so a Genetic Algorithm is used to search the space for the optimal location of the path. The proposed method is tested using a simulated PUMA-like robot, which has to move its end-effector along a straight-line segment. At the end of the paper, the results obtained in these tests are presented and discussed.

An evolutionary algorithm for generating planar developments of arbitrarily curved surfaces

In this paper, a new method based on constrained global optimization is proposed for generating planar developments of arbitrarily three-dimensional surfaces. With this method an initial planar development is derived which is refined in order to satisfy certain criteria. This refinement is formulated as a global minimization problem under constraints. Generally, the constraints are used to control the local accuracy in the derived planar development. The overall minimization problem is resolved using a real-coded genetic algorithms (GA), which is developed for resolving multi-dimensional minimization problems. Indicative applications are presented to illustrate the effectiveness of the proposed method.

A knowledge-based system for the conceptual design of grippers for handling fabrics

The paper presents a knowledge-based system (KBS) for the conceptual design of grippers for handling fabrics. Its main purpose is the integration of the domain knowledge in a single system for the systematic design of this type of grippers. The knowledge presented, in terms of gripper, material and handling process, are classified. The reasoning strategy is based upon a combination of a depth-first search method and a heuristic method. The heuristic search method finds a final solution from a given set of feasible solutions and can synthesize new solutions to accomplish the required specifications. Details of the main features of the system are given, including its ability to take critical design decisions according to four criteria, weighted by the designer. The knowledge-based system was implemented in the Kappa P. C. 2.3.2 environment. Two examples are given to illustrate some critical aspects concerning the KBS development, to explain the operation of the proposed searching heuristic method, and to show its effectiveness in producing design concepts for grippers.