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Byline: Abhijit. Chandra, Prabhu. Singh, Saket. Kumar, Nikhil. Chopra, Vishal. Gupta, Pradeep. Joshi, Vivek. Gupta Introduction: Laparoscopic ventral mesh rectopexy (LVMR) has gained widespread acceptance for the management of complete [...]

Byline: Nikhil. Chopra, Vivek. Gupta, Rahul, Saket. Kumar, Pradeep. Joshi, Vishal. Gupta, Abhijit. Chandra Introduction: Most centres offer laparoscopic treatment for liver hydatid cyst (LHC). There have been concerns about [...]

In this technical note we study the problem of exponential synchronization for one of the most popular models of coupled phase oscillators, the Kuramoto model. We consider the special case of finite oscillators with distinct, bounded natural frequencies. Our first result derives a lower bound on the coupling gain which is necessary for the onset of synchronization. This bound improves the one derived by Jadbabaie et al. [8]. We then calculate a lower bound on the coupling gain that is sufficient to guarantee oscillator synchronization and derive further sufficient conditions to ensure exponential synchronization of the angular frequencies of all oscillators to the mean natural frequency of the group. We also characterize the coupling gain that is sufficient for the oscillator phase differences to approach any desired compact set in finite time. Index Terms--Cooperative control, exponential stabilization, Kuramoto oscillators, multi-agent systems, synchronization.

This paper addresses the problem of steady-state position and force tracking in bilateral teleoperation. Passivity-based control schemes for bilateral teleoperation provide robust stability against network delays in the feedback loop and velocity tracking, but do not guarantee steady-state position and force tracking in general. Position drift due to data loss and offset of initial conditions is a well-known problem in such systems. In this paper, we introduce a new architecture, which builds upon the traditional passivity-based configuration by using additional position control on both the master and slave robots, to solve the steady-state position and force-tracking problem. Lyapunov stability methods are used to establish the range of the position control gains on the master and slave sides. Experimental results using a single-degree-of-freedom master/slave system are presented, showing the performance of the resulting system. Index Terms--Bilateral teleoperation, force tracking, position tracking, time delay.

Gradient-based optimization and control frameworks have been utilized in several applications. The learning rate parameter is typically chosen following a schedule or using methods such as line search to enhance the convergence rate. Recently, the machine learning community has developed methodologies for automated tuning of the learning rate, known as adaptive gradient methods. This paper develops a control theory-inspired framework for modeling adaptive gradient methods that solve non-convex optimization problems. We first model the adaptive gradient methods in a state–space framework, which allows us to present simpler convergence proofs of prominent adaptive optimizers, such as AdaGrad, Adam, and AdaBelief. The proposed framework is constructive because it allows synthesizing new adaptive optimizers. To illustrate this fact, we then utilize the transfer function paradigm from classical control to propose a new variant of Adam, coined AdamSSM, and prove its convergence. We add an appropriate pole-zero pair in the transfer function from squared gradients to the second moment estimate. Applications on benchmark machine learning tasks of image classification using CNN architectures and language modeling using LSTM architecture demonstrate that the AdamSSM algorithm improves the gap between generalization accuracy and faster convergence than the recent adaptive gradient methods. [ABSTRACT FROM AUTHOR]

My first ever "painting" was a copy of my grandfather's painting of Kashmir. Charcoal and pastel on rice paper 11 × 8 in. Courtesy of the artist and Chatterjee & Lal PHOTO (COLOR): Nikhil Chopra (b. 1974) Lands, Waters & Skies: Sundersar, 2018. Could you describe the paintings of your grandfather and your memories of Kashmir? It's quite a dark song, and I sing it in the Sackler wing, with the projections of archival footage of America, museums, and institutional Met museum people, in Egypt in the early part of the twentieth century, digging graves in Egypt, excavating priceless pieces from burial sites. [Extracted from the article]

Images courtesy Chatterjee & Lal and the artist PHOTO (COLOR): Performance Lands, Waters and Skies by Nikhil Chopra, Metropolitan Museum of Art, New York. Images courtesy Chatterjee & Lal, Galleria Continua and the artist PHOTO (COLOR): Performance Drawing a Line through Landscape by Nikhil Chopra, documenta 14, 2017. Images courtesy Chatterjee & Lal, Galleria Continua and the artist PHOTO (COLOR): Performance Drawing a Line through Landscape by Nikhil Chopra, documenta 14, 2017. Images courtesy Chatterjee & Lal, Galleria Continua and the artist PHOTO (COLOR): Performance Lands, Waters and Skies by Nikhil Chopra, Metropolitan Museum of Art, New York. [Extracted from the article]

In this brief, false data injection attacks in bilateral teleoperation systems (BTOSs) are studied, where the attacker can inject false data into the states being exchanged between the master and slave robots. To demonstrate the vulnerability of BTOS, a destabilizing false data injection attack (DFDIA) is designed and experimentally implemented. To detect any general false data injection attack, including DFDIA as a special case, a physics-based detection scheme with an encoding–decoding structure is proposed. The efficacy of the proposed attack detection scheme is demonstrated in experiments. [ABSTRACT FROM AUTHOR]

In this paper, vulnerability of a distributed consensus seeking multi-agent system (MAS) with double-integrator dynamics against edge-bound content modification cyber attacks is studied. In particular, we define a specific edge-bound content modification cyber attack called malignant content modification attack (MCoMA), which results in unbounded growth of an appropriately defined group disagreement vector. Properties of MCoMA are utilized to design detection and mitigation algorithms so as to impart resilience in the considered MAS against MCoMA. Additionally, the proposed detection mechanism is extended to detect the general edge-bound content modification attacks (not just MCoMA). Finally, the efficacies of the proposed results are illustrated through numerical simulations. [ABSTRACT FROM AUTHOR]

In order to accomplish cooperative tasks, decentralized systems are required to communicate among each other. Thus, maintaining the connectivity of the communication graph is a fundamental issue. Connectivity maintenance has been extensively studied in the last few years, but generally considering undirected communication graphs. In this paper, we introduce a decentralized control and estimation strategy to maintain the strong connectivity property of directed communication graphs. In particular, we introduce a hierarchical estimation procedure that implements power iteration in a decentralized manner, exploiting an algorithm for balancing strongly connected directed graphs. The output of the estimation system is then utilized for guaranteeing preservation of the strong connectivity property. The control strategy is validated by means of analytical proofs and simulation results. [ABSTRACT FROM PUBLISHER]

In order to accomplish cooperative tasks, multi-robot systems are required to communicate among each other. This inter-robot communication can be modeled as a graph. Thus, maintaining the connectivity of the communication graph is a fundamental issue in cooperative robotic systems. Connectivity maintenance has been extensively studied in the last few years, but generally considering only kinematic agents. In this paper we introduce a control strategy which, by exploiting a decentralized procedure for the estimation of the algebraic connectivity of the graph, ensures connectivity maintenance for groups of Lagrangian systems. Collision avoidance control actions are also integrated with the connectivity maintenance algorithm. The control strategy is validated by means of analytical proofs and simulation results. [ABSTRACT FROM AUTHOR]

Abstract: Networked control of robotic systems is widely recognized as a potentially transformative technological enabler for several applications. However, the issues of time delays in communication and recovery from data losses have emerged as the pivotal issues that have stymied practical deployment. The study for control of robotic system with input/output communication delays has attracted many researchers’ attention, but the existing results have been primarily developed for rigid-joint robots. Since joint flexibility is largely unavoidable in practical manipulators, in this paper the set-point control problem for flexible-joint robots with input/output communication delays is studied. It is demonstrated that the scattering variables address the stability problem for unknown constant delays, however, in contrast to the rigid-robot case, they cannot guarantee set-point regulation. In addition, we compute the explicit dependence of the regulation errors on the communication delays, control gains, and the desired set-point configuration. Without exact knowledge of time delays, a scattering variable based controller with position feedback is subsequently studied in this paper to guarantee stability with improved regulation performance. The control architecture is further extended to the case with time-varying delays. Simulation results are presented to validate the efficacy of the proposed control algorithms. [Copyright &y& Elsevier]

Input/output (I/O) delays in a closed-loop robotic system can significantly deteriorate both its stability and performance. In this paper, we study the position control problem for robotic manipulators under I/O delays, where the delays manifest as a result of communication with a non-collocated controller. Previous research on this topic has assumed the gravitational torque to be pre-compensated in a collocated manner. This paper studies the stability and regulation performance of the closed-loop system when the compensation for the gravitational torque is provided by a non-collocated controller. We demonstrate that simply utilizing scattering transformation for robotic systems under I/O communication delays results in position drift due to gravity compensation. Hence, a new control algorithm incorporating delayed position feedback and scattering variables is studied subsequently. Stability and performance margins dependent on the round-trip delays and the gravitational model are proposed to guarantee stable position regulation. Since the control architecture with scattering transformation does not address the stability problem due to sensing-actuation delays, a new controller is developed to cope with robotic control systems under time-varying sensing-actuation delays. The proposed control algorithms are studied in this paper via experiments on a manipulator with three degrees of freedom. [ABSTRACT FROM AUTHOR]

To accomplish cooperative tasks, robotic systems are often required to communicate with each other. Thus, maintaining connectivity of the communication graph is a fundamental issue in the field of multi-robot systems. In this paper we present a completely decentralized control strategy for global connectivity maintenance of the communication graph. Considering the disk communication model, we describe a gradient-based control strategy that exploits decentralized estimation of the algebraic connectivity. Unlike previous approaches available in the literature, the proposed control algorithm solves the global connectivity problem in a decentralized manner providing theoretical guarantees, without requiring maintenance of the local connectivity between robotic systems. Moreover, results obtained with simulations and experiments on real robots are described for demonstrating the efficacy of the proposed algorithm. [ABSTRACT FROM AUTHOR]

This study introduces a control algorithm that, exploiting a completely decentralized estimation strategy for the algebraic connectivity of the graph, ensures the connectivity maintenance property for multi robot systems, in the presence of a generic (bounded) additional control term. This result is obtained by driving the robots along the negative gradient of an appropriately defined function of the algebraic connectivity. The proposed strategy is then enhanced with the introduction of the concept of critical robots, that is robots for which the loss of a single communication link might cause the disconnection of the communication graph. Limiting the control action to critical robots will be shown to reduce the control effort that is introduced by the proposed connectivity maintenance control law and to mitigate its effect on the additional (desired) control term. [ABSTRACT FROM PUBLISHER]