Task Hierarchy Research Articles

Hierarchical memory management for parallel programs

An important feature of functional programs is that they are parallel by default. Implementing an efficient parallel functional language, however, is a major challenge, in part because the high rate of allocation and freeing associated with functional programs requires an efficient and scalable memory manager. In this paper, we present a technique for parallel memory management for strict functional languages with nested parallelism. At the highest level of abstraction, the approach consists of a technique to organize memory as a hierarchy of heaps, and an algorithm for performing automatic memory reclamation by taking advantage of a disentanglement property of parallel functional programs. More specifically, the idea is to assign to each parallel task its own heap in memory and organize the heaps in a hierarchy/tree that mirrors the hierarchy of tasks. We present a nested-parallel calculus that specifies hierarchical heaps and prove in this calculus a disentanglement property, which prohibits a task from accessing objects allocated by another task that might execute in parallel. Leveraging the disentanglement property, we present a garbage collection technique that can operate on any subtree in the memory hierarchy concurrently as other tasks (and/or other collections) proceed in parallel. We prove the safety of this collector by formalizing it in the context of our parallel calculus. In addition, we describe how the proposed techniques can be implemented on modern shared-memory machines and present a prototype implementation as an extension to MLton, a high-performance compiler for the Standard ML language. Finally, we evaluate the performance of this implementation on a number of parallel benchmarks.

Measurement of the severity of disability in community-dwelling adults and older adults: interval-level measures for accurate comparisons in large survey data sets

ObjectivesTo (1) create a single metric of disability using Rasch modelling to be used for comparing disability severity levels across groups and countries, (2) test whether the interval-level measures were...

Passivity-based whole-body balancing for torque-controlled humanoid robots in multi-contact scenarios

This work presents a new control approach to multi-contact balancing for torque-controlled humanoid robots. The controller includes a non-strict task hierarchy, which allows the robot to use a subset of its end effectors for balancing while the remaining ones can be used for interacting with the environment. The controller creates a passive and compliant behavior for regulating the center of mass (CoM) location, hip orientation and the poses of each end effector assigned to the interaction task. This is achieved by applying a suitable wrench (force and torque) at each one of the end effectors used for interaction. The contact wrenches at the balancing end effectors are chosen such that the sum of the balancing and interaction wrenches produce the desired wrench at the CoM. The algorithm requires the solution of an optimization problem, which distributes the CoM wrench to the end effectors taking into account constraints for unilaterality, friction and position of the center of pressure. Furthermore, the feedback controller is combined with a feedforward control in order to improve performance while tracking a predefined trajectory, leading to a control structure similar to a PD+ control. The controller is evaluated in several experiments with the humanoid robot TORO.

An Approach to Combine Balancing with Hierarchical Whole-Body Control for Legged Humanoid Robots

Legged humanoid robots need to be able to perform a variety of tasks including interaction with the environment while maintaining the balance. The external wrenches, which arise from the physical interaction, must be taken into account in order to achieve robust and compliant balancing. This work presents a new control approach for combining multiobjective hierarchical control based on null space projections with passivity-based multicontact balancing for legged humanoid robots. In order to achieve a proper balancing, all task forces/torques are first distributed to the end effectors and then mapped into joint space considering the task hierarchy. The control approach is evaluated both in simulation and experiment with the humanoid robot TORO.

A Review of Verb Network Strengthening Treatment

This article examines Verb Network Strengthening Treatment (VNeST), a relatively new treatment approach for anomia in people with aphasia. The VNeST protocol aims to promote generalization to increased lexical retrieval of untrained words across a hierarchy of linguistic tasks, including single-word naming of nouns and verbs, sentence production, and discourse. The concept of the verb network relates to the centrality of the verb to the semantics and syntax of a sentence. The VNeST protocol elicits diverse agents (e.g., musician) and patients (e.g., tambourine) around trained verbs (e.g., shake) to activate a range of semantic concepts and personal responses to potentially facilitate generalization to a multitude of lexical items. The pre- and posttreatment generalization results for the 19 participants reported in English are analyzed. Participants represent a range of aphasia severities and types, including Broca's, transcortical motor, anomic, Wernicke's, and conduction aphasia. A previous study that evaluated 3 monolingual Korean speakers on a modified version of VNeST that accommodated Korean's verb final word order is also summarized. The findings across the 5 English studies and the Korean study revealed increased noun and verb naming and lexical retrieval in sentences and discourse on untrained items and tasks for more than half of the participants, suggesting preliminary efficacy for VNeST. Potential predictors and mechanisms of improvement are explored, and clinical implications, including consideration of goals, outcome measures, dosage, inclusion and evaluation of writing, and verb selection, are discussed.

Passivation of Projection-Based Null Space Compliance Control Via Energy Tanks

A manipulator with kinematic redundancy w.r.t. a main task enables to perform an additional, subordinate task in its null space. The classical hierarchy-based approach is to apply dynamically consistent null space projections such that this lower-priority task does not disturb the main task. The major problem is that the control task hierarchy is established at the expense of the passivity of the system, which causes problems in terms of stability, robustness, and safety. Here, we extend this classical approach and passivate the closed-loop system by means of an energy tank, which is filled by the dissipated power of the active damping on both hierarchy levels. We validate the method in simulations and experiments on a torque-controlled manipulator. The concept is predestined for physical human-robot interaction and safety-relevant applications, where passivity must be guaranteed.

Whole-body hierarchical motion and force control for humanoid robots

Robots acting in human environments usually need to perform multiple motion and force tasks while respecting a set of constraints. When a physical contact with the environment is established, the newly activated force task or contact constraint may interfere with other tasks. The objective of this paper is to provide a control framework that can achieve real-time control of humanoid robots performing both strict and non strict prioritized motion and force tasks. It is a torque-based quasi-static control framework, which handles a dynamically changing task hierarchy with simultaneous priority transitions as well as activation or deactivation of tasks. A quadratic programming problem is solved to maintain desired task hierarchies, subject to constraints. A generalized projector is used to quantitatively regulate how much a task can influence or be influenced by other tasks through the modulation of a priority matrix. By the smooth variations of the priority matrix, sudden hierarchy rearrangements can be avoided to reduce the risk of instability. The effectiveness of this approach is demonstrated on both a simulated and a real humanoid robot.

Momentum control with hierarchical inverse dynamics on a torque-controlled humanoid

Hierarchical inverse dynamics based on cascades of quadratic programs have been proposed for the control of legged robots. They have important benefits but to the best of our knowledge have never been implemented on a torque controlled humanoid where model inaccuracies, sensor noise and real-time computation requirements can be problematic. Using a reformulation of existing algorithms, we propose a simplification of the problem that allows to achieve real-time control. Momentum-based control is integrated in the task hierarchy and a LQR design approach is used to compute the desired associated closed-loop behavior and improve performance. Extensive experiments on various balancing and tracking tasks show very robust performance in the face of unknown disturbances, even when the humanoid is standing on one foot. Our results demonstrate that hierarchical inverse dynamics together with momentum control can be efficiently used for feedback control under real robot conditions.

Online Planning for Large Markov Decision Processes with Hierarchical Decomposition

Markov decision processes (MDPs) provide a rich framework for planning under uncertainty. However, exactly solving a large MDP is usually intractable due to the “curse of dimensionality”— the state space grows exponentially with the number of state variables. Online algorithms tackle this problem by avoiding computing a policy for the entire state space. On the other hand, since online algorithm has to find a near-optimal action online in almost real time, the computation time is often very limited. In the context of reinforcement learning, MAXQ is a value function decomposition method that exploits the underlying structure of the original MDP and decomposes it into a combination of smaller subproblems arranged over a task hierarchy. In this article, we present MAXQ-OP—a novel online planning algorithm for large MDPs that utilizes MAXQ hierarchical decomposition in online settings. Compared to traditional online planning algorithms, MAXQ-OP is able to reach much more deeper states in the search tree with relatively less computation time by exploiting MAXQ hierarchical decomposition online. We empirically evaluate our algorithm in the standard Taxi domain—a common benchmark for MDPs—to show the effectiveness of our approach. We have also conducted a long-term case study in a highly complex simulated soccer domain and developed a team named WrightEagle that has won five world champions and five runners-up in the recent 10 years of RoboCup Soccer Simulation 2D annual competitions. The results in the RoboCup domain confirm the scalability of MAXQ-OP to very large domains.

Generalized hierarchical control

Multi-objective control systems for complex robots usually have to handle multiple prioritized tasks. Most existing hierarchical control techniques handle either strict task priorities by using null-space projectors or a sequence of quadratic programs; or non strict task priorities by using a weighting strategy. This paper proposes a novel approach to handle both strict and non-strict priorities of an arbitrary number of tasks. It can achieve multiple priority rearrangements simultaneously. A generalized projector, which makes it possible to completely project a task into the null-space of a set of tasks, while partially projecting it into the null-space of some other tasks, is developed. This projector can be used to perform priority transitions and task insertion or deletion. The control input is computed by solving one quadratic programming problem, where generalized projectors are adopted to maintain a task hierarchy, and equality or inequality constraints can be implemented. The effectiveness of this approach is demonstrated on a simulated robotic manipulator in a dynamic environment.

An overview of null space projections for redundant, torque-controlled robots

One step on the way to approach human performance in robotics is to provide joint torque sensing and control for better interaction capabilities with the environment, and a large number of actuated degrees of freedom (DOFs) for improved versatility. However, the increasing complexity also raises the question of how to resolve the kinematic redundancy which is a direct consequence of the large number of DOFs. Here we give an overview of the most practical and frequently used torque control solutions based on null space projections. Two fundamental structures of task hierarchies are reviewed and compared, namely the successive and the augmented method. Then the projector itself is investigated in terms of its consistency. We analyze static, dynamic, and the new concept of stiffness consistency. In the latter case, stiffness information is used in the pseudoinversion instead of the inertia matrix. In terms of dynamic consistency, we generalize the weighting matrix from the classical operational space approach and show that an infinite number of weighting matrices exist to obtain dynamic consistency. In this context we also analyze another dynamically consistent null space projector with slightly different structure and properties. The redundancy resolutions are finally compared in several simulations and experiments. A thorough discussion of the theoretical and empirical results completes this survey.

What benefits could extension papers and admissions tests have for university mathematics applicants?

It is well documented in both the British press and mathematics education literature that many consider A-levels (qualifications for secondary school leavers in England, Wales and Northern Ireland) to be insufficient preparation for undergraduate mathematics study in the UK. A-level Mathematics and Further Mathematics are not solely intended for those wishing to go on to study undergraduate mathematics, though this is often the only mathematics study and preparation available to, and done by, most students. A number of British universities now require applicants to perform well on entrance examinations or ‘extension’ papers in addition to achieving certain grades in A-level Mathematics and, sometimes, Further Mathematics. This article outlines research which used the Mathematical Assessment Task Hierarchy (Smith et al., 1996) and builds upon the work of Darlington (2014) to describe the mathematical skills required to answer the questions in extension papers. These are then contrasted with skills required at A- and undergraduate level. Each of the three extension papers analysed (Advanced Extension Awards, Sixth Term Examination Papers and university admissions tests) were found to differ from each other, as well as from A-level and undergraduate examinations. This suggests that there are benefits for students doing such papers before university mathematics study. The advantages of doing these papers are not only for admissions tutors, who can use them as additional indicators of candidates’ aptitudes, but also for the students themselves as they gain insights into the nature of undergraduate mathematics and related further mathematical challenges.

Prioritized multi-task compliance control of redundant manipulators

We propose a new approach for dynamic control of redundant manipulators to deal with multiple prioritized tasks at the same time by utilizing null space projection techniques. The compliance control law is based on a new representation of the dynamics wherein specific null space velocity coordinates are introduced. These allow to efficiently exploit the kinematic redundancy according to the task hierarchy and lead to a dynamics formulation with block-diagonal inertia matrix. The compensation of velocity-dependent coupling terms between the tasks by an additional passive feedback action facilitates a stability analysis for the complete hierarchy based on semi-definite Lyapunov functions. No external forces have to be measured. Finally, the performance of the control approach is evaluated in experiments on a torque-controlled robot.

Contrasts in mathematical challenges in A-level Mathematics and Further Mathematics, and undergraduate mathematics examinations

This article describes part of a study which investigated the role of questions in students approaches to learning mathematics at the secondary-tertiary interface, focussing on the enculturation of students at the University of Oxford. Use of the Mathematicasl Assessment Task Hierarchy taxonomy revealed A-level Mathematics and Further Mathematics questions in England and Wales to focus on requiring students to demonstrate a routine use of procedures, whereas those in first-year undergraduate mathematics primarily required students to be able to draw implicatins, conclusions and to justify their answers and make conjectures. While these findings confirm the need for reforms of examinations at this level, questions must also be raised over the nature of undergraduate mathematics assessment, since it is sometimes possible for students to ve awarded a firstclass examination mark solely through stating knoen facts or reproducing something verbatim from lecture notes.

A Passivity-Based Model-Free Force–Motion Control of Underwater Vehicle-Manipulator Systems

A passivity-based model-free control scheme for an underwater fully actuated vehicle-manipulator system (UVMS) in contact tasks is proposed. Orthogonalized motion and force second-order sliding modes are enforced for all time for the redundant noninertial robotic UVMS, including when it is subject to a class of fluid disturbances. To this end, we first determine the constrained dynamics using the quasi-Lagrangian formulation to explicitly characterize hydrodynamic fluid perturbations. Our scheme aims at exploiting the structural properties of Lagrangian systems, then we derive a mapping between the quasi-Lagrangian UVMS to its equivalent Lagrangian form, and study the conditions for open-loop passivity preservation during the interaction of postures and contact constraints between the end effector of the UVMS and the rigid contact surface of bulky objects in cluttered submarine environments. Internal motions are simultaneously computed by solving, in the tangent subspace of the contact manifold, a hierarchy of secondary tasks to satisfy the posture constraints. More importantly, the solution shapes the extended errors that are used to preserve passivity and to enforce dissipativity so as to guarantee local exponential stability without any knowledge of the complex UVMS dynamics, and the energetic performance of the UVMS in closed-loop. Illustrative simulations are discussed to show the feasibility of the proposed scheme.

Prevalence, patterns and predictors of nursing care left undone in European hospitals: results from the multicountry cross-sectional RN4CAST study

BackgroundLittle is known of the extent to which nursing-care tasks are left undone as an international phenomenon.AimThe aim of this study is to describe the prevalence and patterns of nursing...

Ergonomic Evaluation of Aircraft Wing Recovering Tasks in General Aviation Maintenance

General aviation (GA) maintenance ergonomics has been a largely understudied area. It is known that risks for musculoskeletal disorders (MSD) occur in other types of maintenance. This paper presents an approach to ergonomic evaluations in a GA maintenance workshop for tasks of recovering aircraft wings. First, a high-level analysis and resulting job/task hierarchy were completed. The task hierarchy suggested that the recovering tasks for three different types of aircraft wings present different musculoskeletal risks to maintainers. Next, ergonomic screening tools were used to evaluate the work performed on each wing type. Finally, based on the screening, more detailed ergonomic analyses were performed. Results indicated that risks to the low back as well as hands and wrists are present in these tasks with risks largely dependent on task duration. Evaluation tools used and suggestions for further research in this work domain are discussed.

Locality and checkability in wait-free computing

This paper studies notions of locality that are inherent to the specification of distributed tasks by identifying fundamental relationships between the various scales of computation, from the individual process to the whole system. A locality property called projection-closed is identified. This property completely characterizes tasks that are wait-free checkable, where a task $$T =(\mathcal{I },\mathcal{O },\varDelta )$$ is said to be checkable if there exists a distributed algorithm that, given $$s\in \mathcal{I }$$ and $$t\in \mathcal{O }$$ , determines whether $$t\in \varDelta {(s)}$$ , i.e., whether $$t$$ is a valid output for $$s$$ according to the specification of $$T$$ . Projection-closed tasks are proved to form a rich class of tasks. In particular, determining whether a projection-closed task is wait-free solvable is shown to be undecidable. A stronger notion of locality is identified by considering tasks whose outputs “look identical” to the inputs at every process: a task $$T= (\mathcal{I },\mathcal{O },\varDelta )$$ is said to be locality-preserving if $$\mathcal{O }$$ is a covering complex of $$\mathcal{I }$$ . We show that this topological property yields obstacles for wait-free solvability different in nature from the classical impossibility results. On the other hand, locality-preserving tasks are projection-closed, and thus they are wait-free checkable. A classification of locality-preserving tasks in term of their relative computational power is provided. This is achieved by defining a correspondence between subgroups of the edgepath group of an input complex and locality-preserving tasks. This correspondence enables to demonstrate the existence of hierarchies of locality-preserving tasks, each one containing, at the top, the universal task (induced by the universal covering complex), and, at the bottom, the trivial identity task.

Migrancy, Gender and Social Class in Domestic Labour and Social Care in Italy: An Intersectional Analysis of Demand

Since the 1980s, welfare provision in Italy has been dramatically transformed, due to demographic changes, changes in gender orders and, most importantly, the increased employment of migrants for domestic and care work. Drawing first on statistical data, I explore how the intersections of ‘migrancy’ and gender configure in the domestic-work and social-care sector in Italy. I conclude that, even though gender remains the most important stratifying factor in this field, migrancy is almost as important. I then explore the demand for domestic work and the different forms of care work through in-depth interviews with Neapolitan employers in Naples. I posit that the demand for housekeepers is a class-specific phenomenon related to a particular life-style, including the traditional gendered division of labour and a symbolic hierarchy of household tasks according to which certain jobs are deemed too ‘dirty’ for the ‘madams’, or female employers. The demand for childcarers, on the other hand, is more connected to Italian women's increasing labour participation and men's absence from caring responsibilities. However, here, too, social class is not irrelevant. It affects the demand for elderly care in a slightly different way: the availability of an inexpensive migrant labour force, combined with state subventions, has made it possible for families from lower social strata to employ home carers.

Improving a Measure of Mobility‐Related Fatigue (The Mobility‐Tiredness Scale) by Establishing Item Intensity

To improve the construct validity of self-reported fatigue by establishing a formal hierarchy of scale items and to determine whether such a hierarchy could be maintained across time (aged 75-80), sex, and nationality. Cohort study. Two Nordic urban locations: Jyväskylä, Finland, and Glostrup, Denmark. Baseline (1989/90) consisted of a random sample of citizens of Finland or Denmark born in 1914 (n=837). At 5-year follow-up, excluding those lost to follow-up and with baseline disability resulted in a sample of n=690. The Mobility-Tiredness (Mob-T) Scale is a six-item scale that requires subjects to self-report on whether they become tired performing mobility-related tasks. Employing item response theory, an attempt was made to enhance construct validity by confirming a hierarchy of mobility-related fatigue. A formal hierarchy of fatigue tasks, maintained across time, was established using the revised Mob-T Scale. At age 75, the scalability statistics were a homogeneity coefficient (H) of 0.80, Ha T of 3.9% and an H(T) value of 0.66. The corresponding figures at age 80 were 0.75, 6.9% and 0.59. The property of invariant item ordering was maintained across subgroups based on sex and nationality. Establishing a formal hierarchy at age 75 allowed which tasks were most debilitating to be identified more clearly and the individual's "distance" from these tasks to be gauged. Because it was possible to confirm that the item hierarchy was maintained across time (aged 75-80), researchers or clinicians can be more confident that performance over time is the result of real change and has less to do with measurement error.