Erick J. Rodríguez-Seda

Bilateral Teleoperation of Multiple Mobile Agents: Coordinated Motion and Collision Avoidance

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.

Experimental Comparison Study of Control Architectures for Bilateral Teleoperators

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.

Trajectory tracking with collision avoidance for nonholonomic vehicles with acceleration constraints and limited sensing

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.

Collision avoidance control with sensing uncertainties

Sensing and localization mechanisms, employed by mobile robots for the detection of obstacles and other nearby agents, may inaccurately estimate the position of obstacles due to noise, delays, and interferences incurred during the detection process. Therefore, it is critical to design collision avoidance strategies that are robust to the presence of measurement errors. In this paper, we present a decentralized, cooperative collision avoidance strategy for a pair of agents considering bounded sensing uncertainties and acceleration constraints. The avoidance control can be appended to any other stable control law (i.e., main control objective) and is active only when the vehicle is close to the other agent. A numerical example is presented that validates the proposed avoidance strategy.

Teleoperation of multi-agent systems with nonuniform control input delays

Consensus and coordination of multiple teleoperated agents has been a control topic of growing interest over the last decade due, in part, to its many potential applications and related complex challenges. One of these challenges is to guarantee stability and motion coordination in the presence of inherent communication and input delays as well as system nonlinearities in the multi-agent system. Addressing this challenge herein, we report on a model reference robust control framework that guarantees stability, motion coordination, and formation control of N output strictly passive, nonlinear Lagrangian systems with arbitrarily large, nonuniform control input delays. The control framework is comprised of a reference model coupled to the nonlinear systems via the use of N modified scattering transformation blocks. It is shown that the overall control architecture is stable for any large constant delay, robust to system uncertainties, and that the control parameters are delay-independent. A numerical example with four nonlinear planar manipulators and another example with six ground vehicles are finally presented to illustrate the performance of the proposed controller.

A time-varying wave impedance approach for transparency compensation in bilateral teleoperation

Among the still existing issues in bilateral teleoperation, there is the inability by force-feedback control schemes to guarantee delay-independent stability and achieve both position coordination and force reflection independently of the remote environmental dynamics. Particularly, most bilateral control frameworks fail to address position coordination when interacting with rigid environments. In this paper we present a novel control strategy that aims to passively compensate for position errors that arise during contact tasks and, in general, achieve stability and transparency when alternating between unobstructed (free) and obstructed (contact) environments. The proposed control framework exploits the wave impedance independent passivity property of the scattering transformation to guarantee stability and transparency by gradually switching between a low wave impedance, ideal for free motion, and a sufficiently large impedance, suitable for contact tasks. The validity of the control framework is verified through simulations and experiments on a pair of nonlinear robots.

An experimental comparison study for bilateral internet-based teleoperation

This paper presents a detailed experimental comparison of several published algorithms for motion and force control of bilateral internet teleoperators. Different control techniques based on wave variables, smith predictors, and recent algorithms on synchronization are compared under variable time delays, packet losses and environmental disturbances. The experiments are performed using a pair of two-link direct drive arms equipped with force/torque sensors and connected in a master-slave configuration. Comparing different control schemes on the same physical hardware allows a detailed comparison of their respective performance.

Lyapunov-based cooperative avoidance control for multiple Lagrangian systems with bounded sensing uncertainties

We present a decentralized, real-time, cooperative avoidance control law for a group of nonlinear Lagrangian systems with bounded control inputs, limited sensing ranges, and bounded sensing errors. The control formulation builds on the concept of avoidance control and uses Lyapunov-based analysis to guarantee collision-free trajectories for a group of N vehicles with sensing uncertainties. Advantages of the cooperative avoidance strategy include its easy synthesis with other stable control laws and its null effect on the agents main task when other vehicles and obstacles are sufficiently away. Two numerical examples are finally presented that illustrate the performance of the proposed control framework.

Decentralized trajectory tracking with collision avoidance control for teams of unmanned vehicles with constant speed

In this paper, we present a decentralized trajectory tracking control with collision avoidance for an arbitrarily large group of heterogeneous, nonholonomic vehicles. We consider vehicles with different constant speeds and bounded turning rates (e.g., airplanes), for which their avoidance maneuverability is limited. The proposed controller is divided in two parts: an auxiliary system and a local heading control. The auxiliary system is in charge of guiding the vehicle toward the desired trajectory while guaranteeing collision avoidance with other agents at all times. The local heading control is designed such that the vehicle remains continuously within a bounded, short distance of the auxiliary system. To enforce the convergence of the vehicles to a proximity of their desired trajectories and avoid deadlocks (a common issue within most decentralized navigation methods), an alternative discontinuous control strategy is presented. Finally, a numerical example is provided to illustrate the performance of the proposed control strategy.

Guaranteed Safe Motion of Multiple Lagrangian Systems with Limited Actuation

In this paper we present decentralized, reactive, cooperative collision avoidance strategies for a group of nonlinear Lagrangian vehicles with limited actuation and bounded sensing range. The avoidance control strategies, which are continuous, build on the concept of avoidance functions and are synthesized with a time-varying set-point control law that drives the agents towards their desired final configurations (possibly time-varying) while avoiding deadlocks and unwanted local minima. We show, by using results from packing optimization and Lyapunov-based analysis, that the overall control strategies are bounded, comply with the agentss dynamic constraints (i.e., limited actuation and acceleration), and guarantee collision-free trajectories for all agents. Moreover, we provide a bound on the number of agents that can interact with a single vehicle at any given time and illustrate their performance via a numerical example with a group of twelve vehicles.