Reactive, provably safe collision avoidance methods for multivehicle systems commonly require continuous monitoring of other agents and tend to yield conservative, low-efficient responses where ...vehicles may try to avoid other agents that pose very little threat. To counteract the conservatism of traditional methods, this article presents a cooperative, asynchronous, decentralized collision avoidance strategy that uses self-triggered control to reduce the frequency at which nearby agents are observed and velocity information to decrease the distance at which obstacles are detected and avoided. The avoidance strategy triggers new observations at irregular time intervals using the last available relative position of nearby agents. Similarly, it adjusts both the detection range at each discrete sampling instance and the distance at which a vehicle starts avoiding an obstacle using the vehicle's velocity and the relative distance. The control strategy is rigorously proven to guarantee the safety of an arbitrarily large group of agents at all times, and the observation triggering rule is shown to avoid Zeno behavior. It is also shown via simulation and experiments that the control strategy can generate smoother vehicle's trajectories and reduce the power consumed by obstacle detection systems.
Nowadays, autonomously operated nonholonomic vehicles are employed in a wide range of applications, ranging from relatively simple household chores (e.g. carpet vacuuming and lawn mowing) to highly ...sophisticated assignments (e.g. outer space exploration and combat missions). Each application may require different levels of accuracy and capabilities from the vehicles, yet, all expect the same critical outcome: to safely complete the task while avoiding collisions with obstacles and the environment. Herein, we report on a bounded control law for nonholonomic systems of unicycle-type that satisfactorily drive a vehicle along a desired trajectory while guaranteeing a minimum safe distance from another vehicle or obstacle at all times. The control law is comprised of two parts. The first is a trajectory tracking and set-point stabilization control law that accounts for the vehicle’s kinematic and dynamic constraints (i.e. restrictions on velocity and acceleration). We show that the bounded tracking control law enforces global asymptotic convergence to the desired trajectory and local exponential stability of the full state vector in the case of set-point stabilization. The second part is a real-time avoidance control law that guarantees collision-free transit for the vehicle in noncooperative and cooperative scenarios independently of bounded uncertainties and errors in the obstacles’ detection process. The avoidance control acts locally, meaning that it is only active when an obstacle is close and null when the obstacle is safely away. Moreover, the avoidance control is designed according to the vehicle’s acceleration limits to compensate for lags in the vehicle’s reaction time. The performance of the synthesized control law is then evaluated and validated via simulation and experimental tests.
A detailed experimental comparison study of several published algorithms for motion and force control of bilateral teleoperators, with emphasis on Internet-based teleoperation, is presented. The ...study investigates the effects of data losses, communication delays, and environmental constraints on a teleoperation system for different control techniques, which are based on wave variables, Smith predictors, and recent algorithms on synchronization. The controllers are compared on stability, transparency, and complexity using two identical nonlinear robots coupled via a stochastic network model that allowed transmission round-trip delays and data-loss rates to range from 8 to 1088 ms and 0% to 50%, respectively. A total of 18 subjects, which were distributed among 26 experiments with the aims of regulating the effects of the operators learning process and dynamic properties, participated in this study. Overall, the comparison study reports a deteriorating effect in the performance (i.e., larger position errors and lower fidelity of contact information) from delays and data losses. Yet, the effect of data losses is less critical when compared with time delays. In addition, the preference for a particular control framework is shown to strongly depend on the operational conditions of the system, such as the characteristics of the coupling channel, the specifics of the remote task, and the computational capabilities of the manipulators.
This paper presents theoretical and experimental results on bilateral teleoperation of multiple mobile slave agents coupled to a single master robot. We first design a passifying ...proportional-derivative (PD) controller to enforce motion tracking and formation control of master and slave vehicles under constant, bounded communication delays. Then, we incorporate avoidance functions to guarantee collision-free transit through obstructed spaces. The unified control framework is validated by experiments with two coaxial helicopters as slave agents and a haptic device as the master robot.
Portal vein thrombosis (PVT) may occur at any time following liver transplantation. We describe our experience with portal vein recanalization in cases of thrombosis after liver transplantation. ...Twenty‐eight children (5%) out of 566 liver transplant recipients underwent portal vein recanalization using a transmesenteric approach. All children received left hepatic segments, developed PVT, and had symptoms or signs of portal hypertension. Portal vein recanalization was performed via the transmesenteric route in all cases. Twenty‐two (78.6%) patients underwent successful recanalization and stent placement. They received oral anticoagulants after the procedure, and clinical symptoms subsided. Symptoms recurred due to portal vein restenosis/thrombosis in seven patients. On an intention‐to‐treat basis, the success rate of the proposed treatment was 60.7%. Only 17 out of 28 children with posttransplant chronic PVT retained stent patency (primary + assisted) at the end of the study period. In cases of portal vein obstruction, the transmesenteric approach via minilaparotomy is technically feasible with good clinical and hemodynamic results. It is an alternative procedure to reestablish the portal flow to the liver graft that can be performed in selected cases and a therapeutic addition to other treatment strategies currently used to treat chronic PVT.
The authors demonstrate the long‐term outcomes of portal vein recanalization with stent placement using a transmesenteric approach in cases of chronic portal vein thrombosis after pediatric liver transplantation.
In biliary atresia (BA), efforts to prevent premature liver transplantation (LT) are aimed at early diagnosis, timing of Kasai-portoenterostomy (KPE), and centralization of care. This report presents ...the clinical picture, treatment strategies, and outcomes of BA patients with no previous treatment. A retrospective cohort study (Jan/2001 to Jan/2021) was conducted to evaluate the outcome of patients with BA referred to a single team. Study groups were: 1) Kasai-only group (K-only) n=9), 2) LT-only group (n=7), and 3) Kasai+LT group (K+LT) (n=23). Survival with native liver and overall survival were 22.9 and 94.8%, respectively, at 120 months of follow-up. There was no difference in age at KPE in the K-only group (46.8±21.8 days) vs K+LT (52.1±22 days), P=0.4. Ten (25.6%) patients were babies conceived through in vitro fertilization (IVF). Four IVF patients (40%) presented associated congenital heart disease vs 5 patients (17%) in the remaining group (P=0.14). Two of the IVF patients were premature (<37 weeks). Median maternal age at birth was 35 years (33 to 41 years). Excellent patient survival is expected for patients with BA with the available treatment strategies. IVF+BA was an unexpected prevalent association in this cohort, and further studies are required to better understand these findings.
Contribution: This article presents a project-based learning (PBL) activity for use in the instruction of network attacks on cyber-physical systems. Student learning is analyzed to determine the ...project's contribution to learning outcome attainment. Background: The literature contains a significant amount of research on the benefits of PBL as a technique that enhances student attainment of learning outcomes. However, minimal published work currently exists on the use of PBL activities in cybersecurity of control systems curricula. Intended Outcomes: The proposed activity is intended to facilitate student attainment of cybersecurity curriculum learning outcomes and for use in assessing student learning. Application Design: A group PBL method is selected to develop student teamwork in addition to attaining technical learning outcomes. Project design seeks to minimize laboratory costs while providing flexibility for future enhancement. Findings: Analysis of the proposed activity shows that students are able to attain specified learning outcomes at or above expectations and are engaged while completing the activity.
A set of cooperative and noncooperative collision avoidance strategies for a pair of interacting agents with acceleration constraints, bounded sensing uncertainties, and limited sensing ranges is ...presented. We explicitly consider the case in which position information from the other agent is unreliable, and develop bounded control inputs using Lyapunov-based analysis, that guarantee collision-free trajectories for both agents. The proposed avoidance control strategies can be appended to any other stable control law (i.e., main control objective) and are active only when the agents are close to each other. As an application, we study in detail the synthesis of the avoidance strategies with a set-point stabilization control law and prove that the agents converge to the desired configurations while avoiding collisions and deadlocks (i.e., unwanted local minima). Simulation results are presented to validate the proposed control formulation.
Maple syrup urine disease (MSUD) is an autosomal recessive disease associated with high levels of branched-chain amino acids. Children with MSUD can present severe neurological damage, but liver ...transplantation (LT) allows the patient to resume a normal diet and avoid further neurological damage. The use of living related donors has been controversial because parents are obligatory heterozygotes. We report a case of a 2-year-old child with MSUD who underwent a living donor LT. The donor was the patient's mother, and his liver was then used as a domino graft. The postoperative course was uneventful in all three subjects. DNA analysis performed after the transplantation (sequencing of the coding regions of BCKDHA, BCKDHB, and DBT genes) showed that the MSUD patient was heterozygous for a pathogenic mutation in the BCKDHB gene. This mutation was not found in his mother, who is an obligatory carrier for MSUD according to the family history and, as expected, presented both normal clinical phenotype and levels of branched-chain amino acids. In conclusion, our data suggest that the use of a related donor in LT for MSUD was effective, and the liver of the MSUD patient was successfully used in domino transplantation. Routine donor genotyping may not be feasible, because the test is not widely available, and, most importantly, the disease is associated with both the presence of allelic and locus heterogeneity. Further studies with this population of patients are required to expand the use of related donors in MSUD.
In this paper, a novel cooperative control strategy with relative velocity information is derived to guarantee collision-free trajectories for multi-agent systems with Lagrangian dynamics. An ...important feature of this method is that the avoidance control input of an agent depends not only on its proximity to other agents/obstacles but also on their relative motions. For instance, agents approaching at high speeds might be more critical than slow moving yet physically closer agents. The main advantage of using this additional velocity information is that the collision avoidance maneuvers of agents are smoother, and less conservative in the sense that the agents do not spread out as much while avoiding collisions with one another. A Lyapunov-based analysis is adopted to guarantee that the agents meet their desired objectives without colliding. Finally, simulation results on three different systems are provided to illustrate the effectiveness of the proposed control strategy.
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