Alvin M. Strauss

Weld modeling and control using artificial neural networks

Artificial neural networks were evaluated for monitoring and control of the variable polarity plasma arc welding (VPPAW) process. Three areas of welding application were investigated: weld process modeling, weld process control, and weld bead profile analysis for quality control. Experiments and analysis confirm that artificial neural networks are powerful tools for analysis, modeling, and control applications. They are particularly attractive in view of their capabilities to process nonlinear and noisy data, learn from actual welding data, and execute at relatively high speed. It is shown that neural networks are capable of modeling parameters of the VPPAW process to on the order of 10% accuracy or better. The same was observed when neural networks were used to select welding equipment parameters and the resulting bead geometries were estimated. These performance figures suggest that a VPPA welding control system can be implemented based on neural network models and control mechanisms. >

An anthropomorphic hand exoskeleton to prevent astronaut hand fatigue during extravehicular activities

This correspondence presents a prototype of a powered hand exoskeleton that is designed to fit over the gloved hand of an astronaut and offset the stiffness of the pressurized space suit. This will keep the productive time spent in extravehicular activity from being constrained by hand fatigue. The exoskeleton has a three-finger design, the third and fourth fingers being combined to lighten and simplify the assembly. The motions of the hand are monitored by an array of pressure sensors mounted between the exoskeleton and the hand. Controller commands are determined by a state-of-the-art programmable microcontroller using pressure sensor input. These commands are applied to a PWM driven dc motor array which provides the motive power to move the exoskeleton fingers. The resultant motion of the exoskeleton allows the astronaut to perform both precision grasping tasks with the thumb and forefinger, as well as a power grasp with the entire hand.

Helical motion analysis of the knee--II. Kinematics of uninjured and injured knees during walking and pivoting

The knee kinematics of eight individuals with uninjured knees and of seven individuals with ruptured anterior cruciate ligaments have been investigated during walking and pivoting. The kinematics were measured using a six degree of freedom goniometer and quantitated using helical motion analysis. The helical motion variables reveal clearly that the knee is definitely neither a hinge nor a planar joint and its dynamic behavior changes over the stride. Ligamentous loss results in more adduction and external rotation during certain periods of the stride. Also, the range of translation of the tibia in the medial/lateral direction is reduced, and its average translation is more medial.

Helical motion analysis of the knee—I. Methodology for studying kinematics during locomotion

A technique for investigating the three-dimensional kinematics of knee motion during dynamic functional tasks has been developed. It involves the combined usage of a six degree of freedom goniometer and helical motion analysis. A detailed procedure for coordinate system alignment and calibration must be followed. Once established this entire procedure is routinely implementable. Ensemble averages from multiple walking strides reveal that this technique is sensitive enough to differentiate between the kinematics of an uninjured and injured knee.

Robotic friction stir welding

The forces and torques associated with friction stir welding (FSW) are discussed as they relate to implementation of the welding process with industrial robots. Experimental results are presented that support the conclusions drawn from models developed by others. It is shown that even with heavy‐duty industrial robots with high stiffness, force feedback is important for successful robotic FSW. Methods of implementing force feedback are reviewed. Attention is paid to stability issues that arise with variations in tool rotation and travel speed. Successful implementations of robotic FSW are cited.

Experimental defect analysis and force prediction simulation of high weld pitch friction stir welding

Abstract Experimental data for AA 6061-T6 friction stir welded at rotational and travel speeds ranging from 1000 to 5000 rev min−1 and from 290 to 1600 mm min−1 (11–63 ipm) are presented. The present paper examines the forces and torques during friction stir welding (FSW) with respect to mechanistic defect development owing to process parameter variation. Two types of defects are observed: wormholes and weld deformation in the form of significant excess flash material. A 3D numerical model, implemented using the computational fluids dynamics package Fluent, is used to simulate and investigate the parametric relationship of the forces and torques during FSW. In order to establish a mechanistic quantification of the FSW process, two mechanical models, the Couette and the viscoplastic fluid flow models, were simulated and compared with experimental data for AA 6061-T6.

DYNAMIC ANALYSIS OF SPACE-BASED INFLATED BEAM STRUCTURES

This paper examines the dynamic behaviors of inflated beam aerospace structures. The principal foci of this investigation are the determination of the damping mechanisms active in structures constructed from inflated cylindrical beams, development of a practical modeling method for complex structures, and an examination of the difficulties in predicting on‐orbit dynamic behavior from ground tests. A Euler–Bernoulli model of the inflated beam is used to determine distributed damping coefficients from modal tests. The results show that the viscous damping in the inflated beam is independent of beam pressure, but that the pressurization stress levels in the beam fabric affect the strain‐rate damping. The Euler–Bernoulli inflated beam model is used in conjunction with a conventional finite element package to model the dynamic behavior of a complex inflated beam structure, a 1.7‐m‐diam inflated dish antenna mockup. The model accurately predicts the lower natural frequencies of the dish structure. A comparison ...

Synchronous weld pool oscillation for monitoring and control

A novel approach for inducing and monitoring oscillations in a molten weld pool is presented. Research efforts have illustrated that the weld pool resonates at natural frequencies that are related to its dimensions and state of penetration. This phenomenon may be used to monitor the weld pool, and particularly its depth of penetration, in a closed-loop feedback control system. The approach used to induce pool oscillations was to excite the weld pool with current pulses synchronized to the natural oscillations of the pool. Implementation of this synchronous weld pool pulsing technique was based on the use of a phase locked loop (PLL) system. The natural weld pool oscillations are used as the reference frequency source and a pulsing circuit is controlled by the PLL oscillator so that the arc current pulses repeatedly impact the pool after a fixed number of reference oscillation periods. An optical sensor detects the pool oscillations which are amplified, filtered, and limited to eliminate amplitude variations from the optical signal. A model of the weld pool is developed which uses a fluid droplet formulation for the relation of weld pool geometry and other physical parameters to the natural frequencies of the weld pool. Comparison of the weld pools actual resonant frequency with the expected resonant frequency as predicted by weld pool geometry models and measurements of the pool width (or area) allows and assessment of the state of penetration of the weld pool into the workpiece.<<ETX>>

Statistical process control application to weld process

A statistical weld process monitoring system is described. Using data collected while welding, the welding statistical process control (SPC) tool provides weld process quality control by implementing techniques of data trending analysis, tolerance analysis, and sequential analysis. The SPC system computes the mean, standard deviation, and range of each of the parameters sampled by the data collection system. Changes in the mean, standard deviation, and range are displayed using control (or trend) charts. The control chart displays a function of a parameter with respect to the ordering of the weld records (for a single weld) or weld number (for multiple welds). The SPC tool also permits plotting tolerance charts of the mean, standard deviation, and range for each of the sampled parameters. The tolerance chart is plotted versus the record number (or weld number) and consists of a vertical line for each record (or weld number) showing the minimum and maximum value of that parameter for that record (or weld number). The upper control limit (UCL), lower control limit (LCL), and nominal value may also be displayed on the tolerance chart printout. The SPC also performs sequential analysis, which allows the user to examine the process as it goes along, which in turn may permit the user to locate a possible change in the process before it goes out of control. Work directed toward developing an expert interpreter of the voluminous statistical output generated by the SPC is also described.

Heated Friction Stir Welding: An Experimental and Theoretical Investigation into How Preheating Influences Process Forces

As friction stir welding (FSW) has expanded to welding higher strength materials, large process forces and extreme tool wear have become issues. One possible solution is introducing an additional heating source in front of the FSW tool which softens the material and reduces the tool loads. We investigate the advantages of elevating temperature. Bead on plate welds were performed with a Trivex tool in aluminum alloy (AA 6061) heated to initial material temperatures up to 300°C. Macrograph cross-sections of the welds revealed a slight increase in material flow with increasing temperatures. More significant, the welding forces were analyzed to reveal up to a 43% reduction in the axial force with even moderate heating. An intriguing trend is observed that the process forces do not decrease steadily with increasing initial temperature, as might be expected, but exhibit a more complex polynomial shape, which actually increases for some heating intervals.