Further investigation on LuGre friction force model under normal load variations

A large number of friction force models has been proposed in the literature to include the different properties and represent the frictional behavior with more detail [1, 2]. Generally, the friction models are divided into two groups, i.e., the static and dynamic models. The former group describes the steady-state behavior of friction force by enforcing a constant relationship between the relative tangential velocity and the corresponding friction force. The dynamic models are usually more complex since they consider an extra state variable, which describes the friction state, governed by a differential equation. Among the dynamic friction model, the LuGre model [3] has been gaining popularity and acceptance by the scientific community, since it presents a reasonable trade-off between easiness of implementation, range of modeled frictional phenomena, computational efficiency, and ability for parameters identification. These characteristics make LuGre model suitable for many applications in the modeling of multibody mechanical systems. Despite its wide utilization, the LuGre model presents limitations under normal load variation, which made its authors to present an amended version [4] to overcome some of those shortcomings. However, even the amended version has revealed some physical inconsistencies due to the occurrence of a drift during the sticking phase [5, 6]. In this work, a modification to the LuGre friction model is proposed to deal with normal load oscillations without presenting the shortcomings of both the original and amended versions of the model

Subunit asymmetry and roles of conformational switching in the hexameric AAA+ ring of ClpX

The hexameric AAA+ ring of Escherichia coli ClpX, an ATP-dependent machine for protein unfolding and translocation, functions with the ClpP peptidase to degrade target substrates. For efficient function, ClpX subunits must switch between nucleotide-loadable (L) and nucleotide-unloadable (U) conformations, but the roles of switching are uncertain. Moreover, it is controversial whether working AAA+-ring enzymes assume symmetric or asymmetric conformations. Here, we show that a covalent ClpX ring with one subunit locked in the U conformation catalyzes robust ATP hydrolysis, with each unlocked subunit able to bind and hydrolyze ATP, albeit with highly asymmetric position-specific affinities. Preventing U↔L interconversion in one subunit alters the cooperativity of ATP hydrolysis and reduces the efficiency of substrate binding, unfolding and degradation, showing that conformational switching enhances multiple aspects of wild-type ClpX function. These results support an asymmetric and probabilistic model of AAA+-ring activity.National Institutes of Health (U.S.) (Grant GM-101988)Massachusetts Institute of Technology (Poitras Predoctoral Fellowship

On the constraints violation in forward dynamics of multibody systems

It is known that the dynamic equations of motion for constrained mechanical multibody systems are frequently formulated using the Newton-Euler’s approach, which is augmented with the acceleration constraint equations. This formulation results in the establishment of a mixed set of partial differential and algebraic equations, which are solved in order to predict the dynamic behavior of general multibody systems. The classical resolution of the equations of motion is highly prone to constraints violation because the position and velocity constraint equations are not fulfilled. In this work, a general and comprehensive methodology to eliminate the constraints violation at the position and velocity levels is offered. The basic idea of the described approach is to add corrective terms to the position and velocity vectors with the intent to satisfy the corresponding kinematic constraint equations. These corrective terms are evaluated as function of the Moore-Penrose generalized inverse of the Jacobian matrix and of the kinematic constraint equations. The described methodology is embedded in the standard method to solve the equations of motion based on the technique of Lagrange multipliers. Finally, the effectiveness of the described methodology is demonstrated through the dynamic modeling and simulation of different planar and spatial multibody systems. The outcomes in terms of constraints violation at the position and velocity levels, conservation of the total energy and computational efficiency are analyzed and compared with those obtained with the standard Lagrange multipliers method, the Baumgarte stabilization method, the augmented Lagrangian formulation, the index-1 augmented Lagrangian and the coordinate partitioning method.The first author expresses his gratitude to the Portuguese Foundation for Science and Technology through the PhD grant (PD/BD/114154/2016). This work has been supported by the Portuguese Foundation for Science and Technology with the reference project UID/EEA/04436/2013, by FEDER funds through the COMPETE 2020 – Programa Operacional Competitividade e Internacionalização (POCI) with the reference project POCI-01-0145-FEDER-006941.info:eu-repo/semantics/publishedVersio

Joint reactions in rigid or flexible body mechanisms with redundant constraints

The problem of joint reactions indeterminacy, in engineering simulations of rigid body mechanisms is most often caused by redundant constraints which are defined as constraints that can be removed without changing the kinematics of the system. In order to find a unique set of all joint reactions in an overconstrained system, it is necessary to reject the assumption that all bodies are rigid. Flexible bodies introduce additional degrees of freedom to the mechanism, which usually makes the constraint equations independent. Quite often only selected bodies are modelled as the flexible ones, whereas the other remain rigid. In this contribution it is shown that taking into account flexibility of selected mechanism bodies does not guarantee that unique joint reactions can be found. Problems typical for redundant constraints existence are encountered in partially flexible models, which are not overconstrained. A case study of a redundantly constrained spatial mechanism is presented. Different approaches to the mechanism modelling, ranging from a purely rigid body model to a fully flexible one, are investigated and the obtained results are compared and discussed

Detonacja nadziei. O fundamentalnej frakturze opowiadania przemocy w twórczości Heinera Müllera

In the paper, Detonation of Hope, translated by Wiktoria Wojtyra, Frank M. Raddatz attempts to explain the origins of Heiner Müller’s late texts, renowned for their non-dialogue characteristics that have become the basis for the post-dramatic theater. However, this basis has been corrupted by the capitulation to violence now deeply integrated onto it. Furthermore, in Hamletmachine, Müller utilized post-utopian aesthetics as a manifestation of the thwarted hope of the end to violence in history. This fundamental structure of his narrative of violence is preceded by the long struggle against violence and the possibility of overcoming or eradicating it entirely. A profound shock ensues from a sensitization stemming from the regulation of violence set against a pandemic. However, this only succeeds until the moment in which the fight against bestiality will give way for a new desire for violence. History, it turns out, becomes a cul-de-sac as represented by the renaissance of the ice-age which marks a farewell to a utopian world free of violence. At the same time fails an alliance of art and politics. The same alliance, that on behalf of The Absolute Other once hammered out the historical expectations horizon

Manipulator of POLYCRANK robot

W pracy przedstawiono koncepcję manipulatora o nieograniczonych zakresach ruchów obrotowych w przegubach. Istotną nowością tej koncepcji są człony w kształcie ukośnych korb o lekkiej konstrukcji powłokowej, połączone ze sobą za pomocą pojedynczych łożysk krzyżowo- rolkowych oraz ulokowane wewnątrz tych korb równoległowody ukośne służące do transmisji napędu. Trzy pierwsze człony tworzą płaski łańcuch kinematyczny o pionowych osiach obrotu, napędzany przez silniki bezpośredniego napędu DD (Direct Drive) zamontowane współosiowo w podstawie. Dwa pierwsze człony są ukośnymi korbami, a trzeci ma kształt kolumny. Trzy następne człony tworzą płaski łańcuch kinematyczny o poziomych osiach obrotu, którego ostatni człon jest pierwszym elementem kiści. Jest on wraz z kiścią napędzany przez silniki AC z przekładniami związanymi z członem trzecim. Opisano główne problemy konstrukcyjne i technologiczne oraz wyniki wstępnych badań prototypu o sześciu stopniach swobody.Design of direct drive manipulator POLYCRANK without joint limits is presented in the paper. The main links of the arm are in the form of light hollow cranks and are connected to each other by cross-roller bearings. Three first DOF with vertical axes are driven by electric direct drive motors mounted coaxial in the base.The first link is dreiven directly, the second and third are driven remotely by using special parallelogram mechanisms with unlimited range of transmitted rotation. The other DOF's are driven in a similar way by light motors with gears mounted on the third link. High speed of cyclic gross motion in the convex workspace and good isotropic properties are the main advantages of this manipulator. Due to application parallelograms in the special kinematic scheme of the arm and proper mass distribution, kinematic and dynamic decoupling of some DOF was achived. Dynamic coupling of two first DOF is profitable to realisation of tipical high speed motions. One of the main problem of this project was concerne with high mechanical performance of the parallelograms adequately to the DD motors and to obtain the required performance of the arm, i.e. stiffness, lost motion, frequency and damping of natural vibrations. Experimental design, initial tests of the prototype and some remarks concerning dynamic performance are described in the paper

SYNTHESIS OF 3-PYRIDYL SUBSTITUTED 4-THIAZOLIDINONES AS POTENTIAL BIOLOGICALLY ACTIVE COMPOUNDS

The aim of the work. The synthesis of 3-pyridylsubstituted 5-arylidene-4-thiazolidinones as potential biologically active compounds. Materials and Methods. All starting materials were purchased from commercial sources and used without puri cation. The 1H NMR spectra were recorded on a Varian Gemini 400-MHz instrument. Melting points are uncorrected and were measured in open capillary tubes on a BŰCHI B-545 melting point apparatus. Results and Discussion. Novel 3-pyridylsubstituted 5-arylidene-2-thioxo-4-thiazolidinones were synthesized using Holmberg method and Knoevenagel reaction. For the synthesis of structurally similar 3-(2-pyridyl)-2.4-thiazolidinediones two-stage approach were proposed. This approach is based on «one-pot» three-component reaction of 1-benzoyl-3-(pyridine-2-yl)-2-thiourea, chloroacetic acid and aromatic aldehyde yielding 5-arylidene-2-benzoylimino-3-(2-pyridyl)-4-thiazolidinones which via acid hydrolysis in high yields form the target products. The structure of synthesized compounds was con rmed by NMR spectroscopy. Conclusions. A series of novel 3-pyridylsubstituted rhodanine, thiazolidinedione and psudothiohydantoine derivatives were synthesized. The 1H NMR spectra futures con rmed stereoselectivity of Knoevenagel reaction and obtaining of 5-(Z)-arylidene-4-thiazolidinones