Alejo Avello

Natural coordinates for the computer analysis of multibody systems

Abstract In this paper we will describe a new method for the computer kinematic and dynamic analysis of a wide range of three-dimensional mechanisms or multibody systems. This method is based on a new system of non-independent coordinates that use Cartesian coordinates of points and Cartesian components of unitary vectors in order to describe the position and the motion of the system. Angular coordinates are not used. The kinematic constraint equation comes in two ways, from the rigid-body condition for each element and from the joints or kinematic pairs. The consideration of unitary vectors facilitates considerably the formulation of pair constraints when the pair is associated with a particular direction, as is the case with revolute (R), cylindrical (C), or prismatic (P) pairs. The constraint equations are quadratic in the problem coordinates and they never involve transcendental functions. The dynamic differential equations are obtained in a very simple and effective way from the theorem of virtual power. Finally, two examples will be presented.

Two compact robots for remote inspection of hazardous areas in nuclear power plants

Two mobile robots for the inspection of radioactive areas in nuclear power plants are described. Robicen III is a compact pneumatic robot of 3 kg designed for the inspection of radioactive cylindrical tanks. With a novel locomotive mechanism based on pneumatic actuators and suction pads, it is able to climb vertical walls at speeds close to 110 mm/s. MonoCaRob is a rail-guided autonomous robot for inspection in the drywell of BWR power plants. Copper rails and brushes provide a rugged and robust means for power supply and communications. A video camera and a variety of sensors can be carried by the robot during drywell inspections.

An efficient computational method for real time multibody dynamic simulation in fully cartesian coordinates

Abstract An algorithm is presented for the dynamic analysis of mechanisms that is based on the combination of fully cartesian coordinates for the definition of the mechanism, a penalty and augmented Lagrangian formulation for the satisfaction of the constraint equations and the trapezoidal rule for numerical integration with the positions — rather than accelerations — as primary variables. The new method is very systematic and general, and shows very good convergence characteristics even for large time steps. The facts that the Jacobian is linear, that the mass matrix is constant, and that neither Coriolis nor centrifugal terms are present in this formulation make the algorithm be very efficient computationally and therefore suitable for real time simulations. A series of numerical simulations are performed which demonstrate the capabilities of the proposed method.

A simple and highly parallelizable method for real-time dynamic simulation based on velocity transformations

Abstract A semi-recursive and easy-to-parallelize algorithm for real-time dynamic simulation of open- and closed-loop multi-rigid-body systems is presented. The equations of motion are obtained in terms of a minimal set of relative joint coordinates using an efficient implementation of the velocity transformation method. The open-loop velocity transformation matrix, which relates body translational and rotational velocities to joint relative velocities, is computed in parallel using an extremely simple and intuitive idea. Similarly, the open-loop projected mass matrix is computed in parallel. Fine grain parallelization and optimum use of the cache memory are achieved by using a body-by-body procedure for the computation of vectors and matrices. Closed-loop systems are transformed into open-loop systems through the penalty formulation. The performance of the method is tested through an arithmetic operation count of a 45 degree of freedom open-loop model of a human body and an 18 degree of freedom closed-loop model of a heavy truck.

Real Time Simulation of Complex 3-D Multibody Systems With Realistic Graphics

In the last few years a new method for kinematic and dynamic simulation of multibody problems has been developed by the authors at CEIT and University of Navarra. The most distinctive feature of this method is the use of a fully cartesian set of dependent coordinates; instead of describing the spatial position of a rigid body through the cartesian coordinates of a point and Euler angles or Euler parameters, this method uses the cartesian coordinates of two or more points and the cartesian components of one or more unit vectors rigidly attached to the body. Points and vectors can be shared between contiguous elements, keeping the number of variables moderate and contributing to the definition of pair constraints. With these coordinates the formulation has important advantages: constant mass matrix in the global reference frame, absence of Coriolis and centrifugal inertia forces in the dependent coordinates and a jacobian matrix much more easy to evaluate. The result is a very general and very efficient dynamic formulation.

High-Performance Linear Cable Transmission

Cable transmissions offer several advantages such as high stiffness to weight ratio, high strength, low friction, and absence of backlash, which makes them appropriate for demanding mechanical applications. However, while extensively used as rotational transmissions, there are only a few examples of linear cable transmissions in the literature. The reason is that the up-to-date designs are based on a cable layout that leads to cable length changes during movement. This, in turn, produces negative effects such as transmission nonlinearity and cable fatigue. In this paper, an alternative design for linear cable transmissions is presented. The new design overcomes the aforementioned problems through a proper cable layout. Different applications of the new transmission are reported, validating the proposed design.

Adaptive impedance modification of a master-slave manipulator

Master-slave teleoperators have to cope with tasks so different such as unconstrained motion and hard contact tasks. If the controller is tuned on one of them, the other will not perform well and may, eventually, lead to an unstable system. In this paper, an adaptive modification of the desired robot behaviour in the orthogonal direction to the contact task is proposed. This strategy makes the system stable in hard contact tasks while easy to control in unconstrained motion. Experimental results obtained with a 2-degree-of-freedom manipulator prove the effectiveness of the method.

Multibody Dynamics Optimization by Direct Differentiation Methods Using Object Oriented Programming

In this work, general purpose methodologies and formulations are developed for the optimization of the dynamic behaviour of 3D multibody systems. In order to achieve these goals, the latest advances in three fields are used: Dynamic formulations, object oriented programming languages and symbolic computation techniques.

Involving the operator in a singularity avoidance strategy for a redundant slave manipulator in a teleoperated application

This paper proposes a control scheme to apply in a redundant slave robot in a teleoperated application. Singularities are avoided through a damped least-squares formulation of the inverse kinematics problem. A direct approach for selecting the damping factor is proposed. The redundant degree of freedom (DOF) is used to keep the manipulator in the most manipulable position and within the joint limits. In addition, all the redundancy resolution is performed whilst feeding back information to the operator who can help to improve the performance and robustness of the whole system.

On the use of virtual springs to avoid singularities and workspace boundaries in force-feedback teleoperation

A force-force controller of a master-slave force-feedback teleoperation system is proposed. This scheme is especially suitable for non-backdriveable manipulators. Forces and torques exerted by the operator and the environment and measured by a six axis force sensor are used to compute in real time a desired trajectory which is tracked by a PD controller. This system has been applied to a master-slave system, in which the master robot is a Stewart platform. The use of virtual springs prevents the platform from running into singularities and from getting close to workspace boundaries. Experimental results the satisfactory performance of control algorithm with virtual springs.