Learning Task Constraints from Demonstration for Hybrid Force/Position Control

We present a novel method for learning hybrid force/position control from demonstration. We learn a dynamic constraint frame aligned to the direction of desired force using Cartesian Dynamic Movement Primitives. In contrast to approaches that utilize a fixed constraint frame, our approach easily accommodates tasks with rapidly changing task constraints over time. We activate only one degree of freedom for force control at any given time, ensuring motion is always possible orthogonal to the direction of desired force. Since we utilize demonstrated forces to learn the constraint frame, we are able to compensate for forces not detected by methods that learn only from the demonstrated kinematic motion, such as frictional forces between the end-effector and the contact surface. We additionally propose novel extensions to the Dynamic Movement Primitive (DMP) framework that encourage robust transition from free-space motion to in-contact motion in spite of environment uncertainty. We incorporate force feedback and a dynamically shifting goal to reduce forces applied to the environment and retain stable contact while enabling force control. Our methods exhibit low impact forces on contact and low steady-state tracking error.Comment: Under revie

Will Expanding Health Care Coverage for People with Disabilities Increase their Employment and Earnings? Evidence from an Analysis of the SSI Work Incentive Program

Reports findings from study that examines the effect that increases in the income threshold under the Supplemental Security Income (SSI) work incentive program have on the employment, earnings, and program participation patterns of SSI recipients. Under Section 1619 of the Social Security Act, SSI recipients may maintain Medicaid eligibility, even if their income is above the level that makes them ineligible for SSI payments. If earnings increase beyond the threshold, however, the person loses their SSI and Medicaid eligibility. Findings indicate that many SSI recipients restrain their earnings to stay below the threshold, supporting the hypothesis that lack of access to health insurance is an important work disincentive for people with disabilities

Virtually Dead: The Extension of Social Agency to Corpses and the Dead on Facebook

Facebook and other social networking technologies are become vastly powerful and expansive social forces that are exerting an influence on corpses, the dead, and death communications. Since its invention in 2004, millions of users of Facebook have died, leaving durable profiles subject to the site’s changing regulatory policies and socially present for the network’s living users as well. Because of their ability to enmesh, mutate, and interact with the social interactions of the living—as well as their durability as a site of continuing bonds—Facebook profiles retain social agency consistent with the theories of agency of Alfred Gell (1998) and Bruno Latour (2005). Close textual examination of the styles of communication, the durability of communication over time, the profile layout and composition, and the continuing nature of social contexts and content of the pages of dead users review that living users memorialize and grant social agency and distributed personhood to the profiles—creating a new sense of social agency for Facebook profiles whether living or dead, and the possibility of a virtual social afterlife

Twisted Electromagnetic Modes and Sagnac Ring-Lasers

A new approximation scheme, designed to solve the covariant Maxwell equations inside a rotating hollow slender conducting cavity (modelling a ring-laser), is constructed. It is shown that for well-defined conditions there exist TE and TM modes with respect to the longitudinal axis of the cavity. A twisted mode spectrum is found to depend on the integrated Frenet torsion of the cavity and this in turn may affect the Sagnac beat frequency induced by a non-zero rotation of the cavity. The analysis is motivated by attempts to use ring-lasers to measure terrestrial gravito-magnetism or the Lense-Thirring effect produced by the rotation of the Earth.Comment: LaTeX 31 pages, 3 Figure

Active Learning of Probabilistic Movement Primitives

A Probabilistic Movement Primitive (ProMP) defines a distribution over trajectories with an associated feedback policy. ProMPs are typically initialized from human demonstrations and achieve task generalization through probabilistic operations. However, there is currently no principled guidance in the literature to determine how many demonstrations a teacher should provide and what constitutes a "good'" demonstration for promoting generalization. In this paper, we present an active learning approach to learning a library of ProMPs capable of task generalization over a given space. We utilize uncertainty sampling techniques to generate a task instance for which a teacher should provide a demonstration. The provided demonstration is incorporated into an existing ProMP if possible, or a new ProMP is created from the demonstration if it is determined that it is too dissimilar from existing demonstrations. We provide a qualitative comparison between common active learning metrics; motivated by this comparison we present a novel uncertainty sampling approach named "Greatest Mahalanobis Distance.'' We perform grasping experiments on a real KUKA robot and show our novel active learning measure achieves better task generalization with fewer demonstrations than a random sampling over the space.Comment: Under revie

Top-Down Constitutional Conventions

Many scholars think that all conventions of the constitution emerge from the bottom up, out of the practices of constitutional actors. Here we develop the first systematic account of conventions that are imposed from the top down, through prescriptions by constitutional actors. We show that ‘top‐down conventions’ (as we term them) can be created through the use of normative rule‐making powers; that powers of the right kind are sometimes conferred by ‘bottom‐up conventions’; that these powers are often exercised; and, as a result, that top‐down conventions are increasingly common. We show, too, that as the formal, systematic, and intentional products of a small number of constitutional actors, top‐down conventions are a potentially illegitimate form of constitutional regulation

COMPARISON OF MOMENTS AND SHEAR FORCES OF THE HIP AND KNEE DURING STEP UPS AND SINGLE LEG SQUATS

The purpose of this study was to evaluate moments and shear forces at the hip and knee during step ups and single leg squats. Subjects were 12 weight trained adults. Each subject performed 3 repetitions of step ups and single leg squats using 70% of 1 repetition maximum. 2 X 2 (lift X phase (eccentric/concentric)) repeated measures ANOVAs were performed for each dependent variable. Significant interaction effects (