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Abstract This study employs a high-fidelity numerical framework to determine the plastic material flow patterns and temperature distributions that lead to void formation during friction stir welding (FSW), and to relate the void morphologies to the underlying alloy material properties and process conditions. Three aluminum alloys, viz., 6061-T6, 7075-T6, and 5053-H18, were investigated under varying traverse speeds. The choice of aluminum alloys enables the investigation of a wide range of thermal and mechanical properties. The numerical simulations were validated using experimental observations of void morphologies in these three alloys. Temperatures, plastic strain rates, and material flow patterns are considered. The key results from this study are as follows: (1) the predicted stir zone and void morphology are in good agreement with the experimental observations, (2) the temperature and plastic strain rate maps in the steady-state process conditions show a strong dependency on the alloy type and traverse speeds, (3) the material velocity contours provide a good insight into the material flow in the stir zone for the FSW process conditions that result in voids as well as those that do not result in voids. The numerical model and the ensuing parametric studies presented in this study provide a framework for understanding material flow under different process conditions in aluminum alloys and potentially in other alloys. Furthermore, the utility of the numerical model for making quantitative predictions and investigating different process parameters to reduce void formation is demonstrated. 

This work investigates the stability and rendering limitations of admittance-type haptic devices. We investigated a wider range of impedances than had previously been considered, including stiffness, damping, mass, and combinations thereof. The coupled human driving impedance, actuator position control bandwidth, and loop delay are identified as major factors affecting the range of stable impedances. Finally, the theoretical results are experimentally verified using a custom one degree of freedom admittance type haptic device. 

Despite the large amount of research on kinesthetic haptic devices and haptic effect modeling, there is limited work assessing the perceived realism of kinesthetic model renderings. Identifying the impact of haptic effect parameters in perceived realism can help to inform the required accuracy of kinesthetic renderings. In this work, we model common kinesthetic haptic effects and evaluate the perceived realism of varying model parameters via a user study. Our results suggest that parameter accuracy requirements to achieve realistic ratings vary depending on the specific haptic parameter. 

Impedance based kinesthetic haptic devices have been a focus of study for many years. Factors such as delay and the dynamics of the device itself affect the stable rendering range of traditional active kinesthetic devices. A parallel hybrid actuation approach, which combines active energy supplying actuators and passive energy absorbing actuators into a single actuator, has recently been experimentally shown to increase the range of stable virtual stiffness a haptic device can achieve when compared to the active component of the actuator alone. This work presents both a stability and rendering range analysis that aims to identify the mechanisms and limitations by which parallel hybrid actuation increases the stable rendering range of virtual stiffness. Increases in actuator stability are analytically and experimentally shown to be linked to the stiffness of the passive actuator. 

Abstract The goal of this research was to examine how altering the amount of friction stir tool eccentricity while controlling the amount of slant in the tool shoulder (drivers of oscillatory process forces) effects the generation of process force transients during sub-surface void interaction. The knowledge gained will help improve the accuracy of force-based void monitoring methods that have the potential to reduce the need for post-weld inspection. Process force transients during sub-surface void formation were examined for multiple tools with varying magnitudes of kinematic runout. The eccentric motion of the tool produced oscillations in the process forces at the tools rotational frequency that became distorted when features (flats) on the tool probe interacted with voided volumes, generating an amplitude in the force signals at three times the tool rotational frequency (for three-flat tools). A larger tool eccentricity generates a larger amplitude in the force signals at the tool’s rotational frequency that holds a larger potential to create a distortion during void interaction. It was determined that once void becomes large enough to produce an interaction that generates an amplitude at the third harmonic larger than 30% of the amplitude at the rotational frequency in a weld with no interaction (amplitude solely at rotational frequency), the trailing edge of the tool shoulder cannot fully consolidate the void, i.e., it will remain in the final weld. Additionally, once the void exceeds a certain size, the amplitudes of the third harmonics saturate at 70% of the amplitude at the rotational frequency during full consolidation. The interaction between the eccentric probe and sub-surface void was isolated by ensuring any geometric imperfection in the shoulder (slant) with respect to the rotational axis was removed. The results suggest that geometric imperfections (eccentricity and slant) with respect to the tool’s rotational axis must be known when developing a void monitoring method from force transients of this nature. 

This work investigates the stability of admittance type haptic devices in the context of a wider range of impedances than previously considered. More specifically, we consider the stable range of mass and damping. The coupled human driving impedance, actuator position control bandwidth, and loop delay are identified as major factors affecting the range of stable impedances. Finally, theoretical results are experimentally verified using a custom one degree of freedom admittance type haptic device. 

To understand how the realism of a kinesthetic haptic rendering is affected by the accurate selection of the rendering model parameters, we conducted a preliminary user study where subjects compared three real-world objects to their equivalent haptic rendering. The subjects rated the rendering realism as the model parameters were varied about their nominal values. The results suggest that the required accuracy of various haptic rendering parameters is not equally important when considering the perceived realism. 

Manipulations of a constrained object often use a non-rigid grasp that allows the object to rotate relative to the end effector. This orientation slip strategy is often present in natural human demonstrations, yet it is generally overlooked in methods to identify constraints from such demonstrations. In this paper, we present a method to model and recognize prehensile orientation slip in human demonstrations of constrained interactions. Using only observations of an end effector, we can detect the type of constraint, parameters of the constraint, and orientation slip properties. Our method uses a novel hierarchical model selection method that is informed by multiple origins of physics-based evidence. A study with eight participants shows that orientation slip occurs in natural demonstrations and confirms that it can be detected by our method. 

Cooperative robots or “cobots” promise to allow humans and robots to work together more closely while maintaining safety. However, to date the capabilities of cobots are greatly diminished compared to industrial robots in terms of the force and power they are able to safely produce. This is in part due to the actuation choices of cobots. Low impedance robotic actuators aim to solve this problem by attempting to provide an actuator with a combination of low output impedance and a large bandwidth of force control. In short the ideal actuator has a large dynamic range. Existing actuators success and performance has been limited. We propose a high force and high power balanced hybrid active-passive actuator which aims to increase the actuation capability of low impedance actuators and to safely enable high performance larger force and workspace robots. Our balanced hybrid actuator does so, by combining and controlling a series elastic actuator, a small DC motor, and a particle brake in parallel. The actuator provides low and high frequency power producing active torques, along with power absorbing passive torques. Control challenges and advantages of hybrid actuators are discussed and overcome through the use of trajectory optimization, and the safety of the new actuator is evaluated.