Kim D. Listmann

Consensus for formation control of nonholonomic mobile robots

In this article we present novel formation control laws based on artificial potential fields and consensus algorithms for a group of unicycles enabling arbitrary formation patterns for these nonholonomic vehicles. Given connected and balanced graphs we are able to prove stability of the rendezvous controller by applying the LaSalle-Krasovskii invariance principle. Further, we introduce obstacle avoidance, enabling a reactive behavior of the robotic group in unknown environments. The effectiveness of the proposed controllers is shown using computer simulations and finally, a classification w.r.t. existing solutions is done.

DisCoverage: A new paradigm for multi-robot exploration

The main aspect in multi-robot exploration is the efficient coordination of a group of robots. Inspired by previous results on the coverage problem, we propose a novel, frontier-based approach for multi-robot exploration. This approach merges the step of choosing appropriate target points with the step of planning a collision-free path. This is achieved by optimizing an objective function consisting of distance and orientation costs as well as an estimated information gain. The optimization yields motion control laws directly solving the exploration task. Using a Voronoi partition of the environment ensures, that each robot autonomously creates and optimizes the objective function to obtain a collision-free path in a distributed fashion. Simulations demonstrate the effectiveness of our approach.

Partial-state synchronization of linear heterogeneous multi-agent systems

This article is devoted to the synchronization of heterogeneous, and hence non-identical, linear systems. The proposed solution results in a fully distributed control law solely relying on local measurements. In order to achieve this, it is shown that the problem can be decomposed into three sub-problems: First, the synchronization of identical exosystems provides a feasible trajectory which turns out to be the synchronization manifold. Second, a decentralized observer is proposed for estimating the absolute state based on the local measurements performed. Third, this estimate is used to derive a tracking control law, based on the full information output regulation problem, to follow the generated exosystem reference. The concept is illustrated by an example.

Output synchronization of systems in chained form

In this article we extend previous results for back-stepping and passivity-based design of cooperative control laws to a class of chained form systems that includes certain drift-free nonholonomic systems. We exploit the cascaded structure and feedback equivalence to passive systems to derive suitable control laws using a modified backstepping methodology. A virtual output is obtained and shared within the group in such a way that full state synchronization for the group is achieved. Given a static, strongly connected and balanced communication network we prove stability of the proposed controller. Further, an extension to higher order chained forms is shown, providing a methodology to coordinate general nonholonomic systems. The effectiveness of the method is demonstrated by synchronizing a collective of unicycles.

Synchronisierung identischer linearer Systeme — ein Zugang über LMIs

Zusammenfassung Dieser Beitrag stellt ein neues Verfahren zur Synchronisierung identischer linearer Systeme vor, die über statische Kommunikationsnetzwerke verbunden sind. Dazu wird das Entwurfsproblem auf ein simultanes Stabilisierungsproblem zurückgeführt. Aus diesem werden dann neue notwendige und hinreichende Bedingungen für den Entwurf abgeleitet. Diese sind konstruktiv, so dass auf Basis linearer Matrixungleichungen, geeignete Rückführungen für Multi-Agenten-Systeme mit stabilisierbarer Dynamik und zusammenhängendem Kommunikationsnetzwerk entworfen werden können. Es wird gezeigt, dass dies sogar dann möglich ist, wenn die Agenten nur einen Teil ihrer Zustandsinformation austauschen dürfen. Abstract This article presents a new method for the synchronization of identical linear systems linked over static communication channels. Based on a transformation of the synchronization problem into a simultaneous stabilization problem novel and constructive necessary and sufficient conditions are obtained. By using linear matrix inequalities, appropriate feedback design is facilitated for all multi-agent systems with stabilizable dynamics and connected communication topologies. In addition to full-state coupling, synchronization is also achieved if only a part of the state of each agent is communicated over the network.

DisCoverage for non-convex environments with arbitrary obstacles

DisCoverage is a distributed strategy for frontier-based multi-robot exploration. The robots coordinate by a partition of the environment, and choose their target points by optimizing a locally decomposable objective function. In [9] DisCoverage for convex regions was proposed. In this work, we extend DisCoverage to support arbitrary non-convex real-world environments with obstacles. Therefore, we introduce a transformation of non-convex environments to robot centric star-shaped domains. This results in a general solution with broader applications for exploration and path planning. Simulations as well as experiments with real robots demonstrate the exploration progress.

Motor-Current-Based Estimation of Cartesian Contact Forces and Torques for Robotic Manipulators and Its Application to Force Control

We present a Kalman filter-based approach for estimating external forces and torques relying on a dynamic model of a serial-chain robotic manipulator where only motor signals (currents, joint angles, and joint speeds) are measurable. The method does not require any additional sensing compared to standard robot control systems. The approach exploits redundancy in 7DOF arms, but also applies to traditional 6DOF manipulators. Automatic filter calibration routines are presented minimizing the number of parameters that must be tuned in order to successfully apply the proposed scheme and to optimize estimation quality. The approach is verified by measurement data gathered from an ABB YuMi, a dual-arm collaborative robot with 7DOF each arm. Furthermore, measurement results are presented employing force and torque estimates in a compliance control scheme, verifying that the estimation quality achieved is improved compared to existing approaches and is sufficient to employ the estimates in force-controlled applications.Note to Practitioners—More and more robotic applications involve contact with at least partially unknown environments. As a consequence, they require control approaches that go beyond the traditional position control. In particular, information about contact forces and torques has to be taken into account. However, integrating additional sensing equipment to obtain the required force/torque information is often technically challenging and expensive. Cartesian contact force and torque estimation allows obtaining force/torque information solely from available sensors. The estimation technique can be regarded as a virtual sensor, and hence this brief deals with a key technology enabling force controlled robotic applications such as assembly, grinding, and deburring without the need for expensive additional sensing.

DisCoverage: From Coverage to Distributed Multi-Robot Exploration

DisCoverage transfers the well-known solution to the coverage problem to the exploration problem. Essentially, DisCoverage solves the multi-robot exploration problem through a spatially distributed optimization problem. Our contribution is a new objective function for DisCoverage based on the centroidal search. Each robot continuously creates and optimizes the proposed objective function, obtaining a gradient-based control law that leads into unexplored regions. A proof of convergence is given as well as a simulation and a statistical evaluation demonstrating DisCoverage.

An experimental validation of magnetometer integration into a GPS-aided helicopter UAV navigation system

This paper considers the integration of a triaxial magnetometer sensor into an existing navigation system for a helicopter Unmanned Aerial Vehicle (UAV). An existing inertial navigation system uses triaxial accelerometer and rate gyro measurements and is aided by GPS. A novel calibration method for the magnetometer is implemented. The integration of the calibrated magnetometer into the existing design is experimentally shown to improve navigation filter accuracy.

A comparison of methods for higher-order numerical differentiation

This article compares three different methods for the online computation of higher-order derivatives from a measurement signal. In general such measurements are noise corrupted and the application of finite difference schemes is inappropriate. Thus, all methods are compared w.r.t. their noise suppression capabilities for different noise types and levels, their computational complexity to compute a derivative and their tuning effort for proper commissioning. Finally, a recommendation is provided which of the differentiators is best used when.