Alin Albu-Schäffer

Calibration of Error Distributions in Robot Kinematics for Increased Precision in Manipulation Tasks

The accuracy of robotic forward kinematics is commonly improved by calibration. However, most calibration methods only take deterministic errors, such as inaccurate geometry and unknown stiffnesses, into account and neglect errors with stochastic characteristics, including joint friction, gear backl

Centroidal Angular Momentum-Aware Planning for Legged Locomotion Integrating 3D-DCM and Swing-Leg Trajectory Optimization

In humanoid motion planning, centroidal angular momentum (CAM) is often neglected or simplified due to the complexity arising from its nonlinear and non-holonomic nature. As a result, unmodeled CAM effects can compromise center of pressure (CoP) tracking and, consequently, reduce robustness during m

Discovering Optimal Natural Gaits of Dissipative Systems via Virtual Energy Injection

Legged robots offer several advantages when navigating unstructured environments, but they often fall short of the efficiency achieved by wheeled robots. One promising strategy to improve their energy economy is to leverage their natural (unactuated) dynamics using elastic elements. This work explor

IROSA: Interactive Robot Skill Adaptation Using Natural Language

Foundation models have demonstrated impressive capabilities across diverse domains, while imitation learning provides principled methods for robot skill adaptation from limited data. Combining these approaches holds significant promise for direct application to robotics, yet this combination has rec

Locomotion of an Elastic Snake Robot via Natural Dynamics

Nature suggests that exploiting the elasticities and natural dynamics of robotic systems could increase their locomotion efficiency. Prior work on elastic snake robots supports this hypothesis, but has not fully exploited the nonlinear dynamic behavior of the systems. Recent advances in eigenmanifol

Robot Tracking Control with Natural Task-Space Decoupling

Extraction of Robotic Surface Processing Strategies from Human Demonstrations

Learning from Demonstration (LfD) is a widely used approach for teaching robot motion, but more sophisticated strategies are required to address complex tasks such as surface processing. Sanding is an example where comprehensive strategies are necessary to ensure complete and efficient coverage of t

Interactive Incremental Learning of Generalizable Skills With Local Trajectory Modulation

The problem of generalization in learning from demonstration (LfD) has received considerable attention over the years, particularly within the context of movement primitives, where a number of approaches have emerged. Recently, two important approaches have gained recognition. While one leverages vi

RACCOON: Grounding Embodied Question-Answering with State Summaries from Existing Robot Modules

Explainability is vital for establishing user trust, also in robotics. Recently, foundation models (e.g. vision-language models, VLMs) fostered a wave of embodied agents that answer arbitrary queries about their environment and their interactions with it. However, naively prompting VLMs to answer qu

VSB - Variable Stiffness Based on Bowden Cables: A Simple Mechanism for Soft Robotic Hands

Soft robotic hands compensate for uncertainty in perception and actuation by leveraging passive deformation in their intrinsically compliant hardware, facilitating robust and dexterous interactions with their environment. The ability to adjust the level of compliance during operation has the potenti

A Probabilistic Approach to Multi-Modal Adaptive Virtual Fixtures

Virtual Fixtures (VFs) provide haptic feedback for teleoperation, typically requiring distinct input modalities for different phases of a task. This often results in vision- and position-based fixtures. Vision-based fixtures, particularly, require the handling of visual uncertainty, as well as targe

A Tree-Based World Model for Reducing System Complexity in Autonomous Mobile Manipulation

Mobile manipulation tasks in unstructured environments remain challenging for autonomous robots. The need to employ a diverse set of software and hardware components to solve the various subtasks inevitably increases system complexity. Knowledge exchange among such diverse components renders them hi

CoFlex TOP: A Teleoperation System for Flexible Ureteroscopy

In flexible ureteroscopy (fURS) for kidney stones, the renal collecting system is inspected and stones are removed with a flexible ureteroscope (FU). One alternative to the fragmentation of stones and their removal with the help of graspers or extraction baskets is the dusting of stones with laser l

Feasibility Checking and Constraint Refinement for Shared Control in Assistive Robotics

Shared control enables users with motor impairments to control high-dimensional assistive robots with low-dimensional user interfaces. The challenge is to simultaneously 1) provide support for completing daily living tasks 2) enable sufficient freedom of movement to foster user empowerment 3) ensure

Singularity-Robust Prioritized Whole-Body Tracking and Interaction Control With Smooth Task Transitions

In this work, we propose a singularity-robust whole-body control framework that ensures smooth task transitions while maintaining strict priorities. The weighted generalized inverse is adopted to derive a hierarchical control law compatible with singular and redundant tasks. Moreover, a smooth activ…

CollisionGP: Gaussian Process-Based Collision Checking for Robot Motion Planning

Collision checking is the primitive operation of motion planning that consumes most time. Machine learning algorithms have proven to accelerate collision checking. We propose CollisionGP, a Gaussian process-based algorithm for modeling a robot's configuration space and query collision checks. Collis

Embodying Quasi-Passive Modal Trotting and Pronking in a Sagittal Elastic Quadruped

Animals rely on the elasticity of their tendons and muscles to execute robust and efficient locomotion patterns for a vast and continuous range of velocities. Replicating such capabilities in artificial systems is a long-lasting challenge in robotics. By taking advantage of a pitch dynamics decoupli

Hybrid Force-Impedance Control for Fast End-Effector Motions

Controlling the contact force on various surfaces is essential in many robotic applications such as in service tasks or industrial use cases. Mostly, classical impedance and hybrid motion-force control approaches are employed for these kinds of physical interaction scenarios. In this work, an extend

Model Predictive Control Applied to Different Time-Scale Dynamics of Flexible Joint Robots

Modern Lightweight robots are constructed to be collaborative, which often results in a low structural stiffness compared to conventional rigid robots. Therefore, the controller must be able to handle the dynamic oscillatory effect mainly due to the intrinsic joint elasticity. Singular perturbation

A Robotic System for Solo Surgery in Flexible Ureterorenoscopy

Urolithiasis is a common disease with increasing prevalence across all ages. A common treatment option for smaller kidney stones is flexible ureterorenoscopy (fURS), where a flexible ureteroscope (FU) is used for stone removal and to inspect the renal collecting system. The handling of the flexible

Experimental Closed-Loop Excitation of Nonlinear Normal Modes on an Elastic Industrial Robot

Adding elastic elements to the mechanical structure should enable robots to perform efficient oscillatory tasks. Still, even characterizing natural oscillations in nonlinear systems is a challenge in itself, which nonlinear modal theory promises to solve. Therein eigenmanifolds generalize eigenspace

Impedance Control on Arbitrary Surfaces for Ultrasound Scanning Using Discrete Differential Geometry

We propose an approach to robotic ultrasound scanning and interaction control with arbitrary surfaces using a passivity-based impedance control scheme. First, we introduce task coordinates depending on the geometry of the surface, which enable hands-on guidance of the robot along the surface, as wel

Kinematic Transfer Learning of Sampling Distributions for Manipulator Motion Planning

Recent research has shown that guiding sampling-based planners with sampling distributions, learned from previous experiences via density estimation, can significantly decrease computation times for motion planning. We propose an algorithm that can estimate the density from the experiences of a robo…

Multi-Phase Multi-Modal Haptic Teleoperation

Virtual Fixtures facilitate teleoperation, for in-stance by guiding the human operator. Developing these Virtual Fixtures in tasks with tight tolerances remains challenging. Fixtures with a high stiffness allow for more precise guidance, whereas a lower stiffness is required to allow for corrections…

Strict Modes Everywhere - Bringing Order Into Dynamics of Mechanical Systems by a Potential Compatible With the Geodesic Flow

Strict nonlinear normal modes provide very regular families of oscillations within conservative mechanical systems. However, a strict normal mode will generally be an isolated curve within the configuration space of the system. In this letter, we design a potential that will densely fill the configu

Actuating Eigenmanifolds of Conservative Mechanical Systems via Bounded or Impulsive Control Actions

Eigenmanifolds are two-dimensional submanifolds of the state space, which generalize linear eigenspaces to nonlinear mechanical systems. Initializing a robot on an Eigenmanifold (or driving it there by control) yields hyper-efficient and regular oscillatory behaviors, called modal oscillations. This

Collision Detection, Identification, and Localization on the DLR SARA Robot with Sensing Redundancy

Physical human-robot interaction is known to be a crucial aspect in modern lightweight robotics. Herein, the estimation of external interactions is essential for the effective and safe collaboration. In this work, an extended momentum-based disturbance observer is presented which includes the sensin…

Compliant Floating-Base Control of Space Robots

This letter presents a compliant feedback controller of an arm-equipped spacecraft, which does not enforce requirements on the spacecraft position and attitude. The controller is applicable to the pre-contact, contact, and post-contact phases of a robotic operation. In contrast to conventional float

Efficient and Goal-Directed Oscillations in Articulated Soft Robots: The Point-To-Point Case

Introducing elasticity in the mechanical design can endow robots with the ability of performing efficient and effective periodic motions. Yet, devising controllers that can take advantage of such elasticity is still an open challenge. This letter tackles an instance of this general problem, by propo

Embedding a Nonlinear Strict Oscillatory Mode into a Segmented Leg

Robotic legs often lag behind the performance of their biological counterparts. The inherent passive dynamics of natural legs largely influences the locomotion and can be abstracted through the spring-loaded inverted pendulum (SLIP) model. This model is often approximated in physical robotic legs us…

Stabilization of User-Defined Feedback Controllers in Teleoperation With Passive Coupling Reference

Ensuring stability in time-delayed teleoperation can be difficult in some cases where the feedback forces need to be modified as defined by the user, to increase transparency. In the present letter, we propose an approach to ensure stability of any feedback controller selected by the user by conside

Strict Nonlinear Normal Modes of Systems Characterized by Scalar Functions on Riemannian Manifolds

For the study of highly nonlinear, conservative dynamic systems, finding special periodic solutions which can be seen as generalization of the well-known normal modes of linear systems is very attractive. However, the study of low-dimensional invariant manifolds in the form of nonlinear normal modes

Adaptive Air Density Estimation for Precise Tracking Control and Accurate External Wrench Observation for Flying Robots

Air density changes depending on the local atmosphere and affects the rotor thrust of flying robots. This effect has to be compensated by the flight controller in order to realize precise tracking of a desired trajectory. So far, the influence of the air density has been disregarded or only consider

EDAN: An EMG-controlled Daily Assistant to Help People With Physical Disabilities

Injuries, accidents, strokes, and other diseases can significantly degrade the capabilities to perform even the most simple activities in daily life. A large share of these cases involves neuromuscular diseases, which lead to severely reduced muscle function. However, even though affected people are…

Error Bounds for PD-Controlled Mechanical Systems Under Bounded Disturbances Using Interval Arithmetic

We present a numerical algorithm based on invariant set theory to evaluate the worst-case performance of PD-controlled mechanical systems affected by bounded disturbances. By performing a specific coordinate transformation, the search and computation of positive invariant sets is simplified. It is s

Joint-Level Control of the DLR Lightweight Robot SARA

Lightweight robots are known to be intrinsically elastic in their joints. The established classical approaches to control such systems are mostly based on motor-side coordinates since the joints are comparatively stiff. However, that inevitably introduces errors in the coordinates that actually matt…

Object-Level Impedance Control for Dexterous In-Hand Manipulation

This work presents a novel object-level control framework for the dexterous in-hand manipulation of objects with torque-controlled robotic hands. The proposed impedance-based controller realizes the compliant 6-DOF positioning of a grasped object. Enabled by the in-hand localization, the slippage of

Pattern Recognition for Knowledge Transfer in Robotic Assembly Sequence Planning

The autonomous assembly of customized products is highly demanded in future manufacturing scenarios. This requires robotic systems being able to adapt to individual products without increasing overall production time. However, increasingly complex assemblies lead to a growing number of potential ass

Robust, Locally Guided Peg-in-Hole using Impedance-Controlled Robots

We present an approach for the autonomous, robust execution of peg-in-hole assembly tasks. We build on a sampling-based state estimation framework, in which samples are weighted according to their consistency with the position and joint torque measurements. The key idea is to reuse these samples in…

A Coordinate-based Approach for Static Balancing and Walking Control of Compliantly Actuated Legged Robots

The paper addresses the static balancing and walking of elastically actuated legged robots. The control is realized by commanding the motor positions only and exploiting the bijective relation between motor and link positions at equilibrium under static external forces. The approach is formulated in…

Coordinated Control of Spacecraft's Attitude and End-Effector for Space Robots

This letter addresses the coordinated control of the spacecraft's attitude and the end-effector pose of a manipulator-equipped space robot. A controller is proposed to simultaneously regulate the spacecraft's attitude, the global center-of-mass, and the end-effector pose. The control is based on a t

Experiments with Human-inspired Behaviors in a Humanoid Robot: Quasi-static Balancing using Toe-off Motion and Stretched Knees

Humanoid robots typically display locomotion patterns that include walking with flat foot-ground contact, and knees slightly bent. However, analysis of human gait indicate that several physiological mechanisms like stretched knees, heel-strike and toe push-off increase the step length and energetic…

Iteratively Refined Feasibility Checks in Robotic Assembly Sequence Planning

Due to shorter product life cycles and increasing customization, production lines must be able to quickly adapt to novel product variants. This requires the automatic generation of assembly sequence plans from product specifications, as manual engineering of plans is slow and labor intensive. The ma

Visual Repetition Sampling for Robot Manipulation Planning

One of the main challenges in sampling-based motion planners is to find an efficient sampling strategy. While methods such as Rapidly-exploring Random Tree (RRT) have shown to be more reliable in complex environments than optimization-based methods, they often require longer planning times, which re…

Convex Properties of Center-of-Mass Trajectories for Locomotion Based on Divergent Component of Motion

This letter presents an in-depth analysis of the convex properties of center-of-mass (CoM) trajectories for legged robot locomotion based on the concept of Divergent Component of Motion (DCM). In particular, we show that the union of all possible trajectories forms a bounded convex set under appropr

Data-Driven Discrete Planning for Targeted Hopping of Compliantly Actuated Robotic Legs

Motion planning for fast locomotion of compliantly actuated robotic legs is generally considered to be a challenging issue, posing considerable real-time problems. This is at least the case if time-continuous trajectories need to be generated online. In this paper we take advantage of a simple contr…

Dynamic Locomotion Gaits of a Compliantly Actuated Quadruped With SLIP-Like Articulated Legs Embodied in the Mechanical Design

The spring loaded inverted pendulum (SLIP) model has been extensively shown to be fundamental for legged locomotion. However, the way this low-order template model dynamics is anchored in high-dimensional articulated multibody systems describing compliantly actuated robots (and animals) is not obvio

Fusing Joint Measurements and Visual Features for In-Hand Object Pose Estimation

For a robot to perform complex manipulation tasks, such as an in-hand manipulation, knowledge about the state of the grasp is required at all times. Moreover, even simple pick-and-place tasks may fail because unexpected motions of the object during the grasp are not accounted for. This letter propos

Inferring Semantic State Transitions During Telerobotic Manipulation

Human teleoperation of robots and autonomous operations go hand in hand in today's service robots. While robot teleoperation is typically performed on low to medium levels of abstraction, automated planning has to take place on a higher abstraction level, i.e. by means of semantic reasoning. Accordi…

Passivity Analysis and Control of Humanoid Robots on Movable Ground

Reliable and robust balancing controllers are key elements of whole-body control frameworks for humanoid robots. Passivity-based balancing controllers have been proposed and tested successfully in various scenarios involving different types of ground surfaces. This letter extends the passivity consi

The Repetition Roadmap for Repetitive Constrained Motion Planning

We present the Repetition Roadmap, a motion planner that effectively exploits the repetitiveness of a set of tasks with small variations to efficiently compute new motions. The method learns an abstract roadmap of probability distributions for the configuration space of a particular task set from pr

Tracking Control for the Grasping of a Tumbling Satellite With a Free-Floating Robot

In this letter, a novel method is presented for grasping a partially cooperative tumbling satellite with a free-floating robot. A reference trajectory, provided by an offline motion planner and adapted online to account for modeling uncertainties, is tracked with a visual servo for the approach phas

Workspace Fixation for Free-Floating Space Robot Operations

When a space robot accidentally or voluntarily comes in contact with a target object, a workspace shift happens due to exchange of momentum between the objects. The problem of workspace adjustment is addressed herein. A novel controller is derived to simultaneously adjust the workspace and control t…

Eigenmodes of Nonlinear Dynamics: Definition, Existence, and Embodiment into Legged Robots With Elastic Elements

Pogo-stick bouncing or the spring loaded inverted pendulum represents fundamental dynamics models for hopping and running in legged locomotion. However, these conceptual models are in general of lower order than the elastic multibody dynamics of versatile segmented legs. The question how to embody t

Legged Elastic Multibody Systems: Adjusting Limit Cycles to Close-to-Optimal Energy Efficiency

Compliant elements in robotic systems can strongly increase the energy efficiency of highly dynamic periodic motions with large energy consumption such as jumping. Their control is a challenging task for multijoint systems. Typical control algorithms are model-based and thus fail to adjust to unexpe

Passive Hierarchical Impedance Control Via Energy Tanks

Modern robotic systems with a large number of actuated degrees of freedom can be utilized to perform several tasks at the same time while following a given order of priority. The most frequently used method is to apply null space projections to realize such a strict hierarchy, where lower priority t

Repetition sampling for efficiently planning similar constrained manipulation tasks

We present repetition sampling, a new adaptive strategy for sampling based planning, which extracts information from previous solutions to focus the search for a similar task on relevant configuration space. We show how to generate distributions for repetition sampling by learning Gaussian Mixture M…

A passivity-based approach for trajectory tracking and link-side damping of compliantly actuated robots

This paper presents a control method to implement trajectory tracking and disturbance rejection characteristics for the link-side dynamics of compliantly actuated robots with nonlinear spring characteristics. This is achieved by introducing new motor coordinates reflecting the damping and feedforwar

KONTUR-2: Force-feedback teleoperation from the international space station

This paper presents a new robot controller for space telerobotics missions specially designed to meet the requirements of KONTUR-2, a German & Russian telerobotics mission that addressed scientific and technological questions for future planetary explorations. In KONTUR-2, Earth and ISS have been us

Modal Matching: An Approach to Natural Compliant Jumping Control

This letter derives the basic concept of modal matching-an approach to natural motion control. Modal matching exploits the nonlinearity of the rigid multi-body dynamics (and the variability of the elastic transmissions) as degree of freedom to fit the natural plant dynamics to the desired dynamics o

Optical-inertial tracking of an input device for real-time robot control

Minimally invasive robotic surgery systems are usually controlled by input devices, that are mechanically linked to the environment. These input devices often have a limited workspace, which makes intuitive operation difficult. This paper presents a tracking algorithm of a handheld input device, whi

Path-Accurate Online Trajectory Generation for Jerk-Limited Industrial Robots

Standard industrial controllers of robot arms define constraints on the velocity, the acceleration, and the jerk and abort execution in case any of them is violated. In addition to satisfying these constraints, the presented method tries to generate a trajectory that is path-accurate, i.e., that exa

Robotic simulation of on orbit servicing including hard impacts

Industrial robots are often used for the simulation of satellites during on orbit servicing. In order to cover also the docking phase, both robots are equipped with force-torque sensors, and the measured forces and torques are taken to compute the desired motion of the position controlled robots. Si…

Classifying compliant manipulation tasks for automated planning in robotics

Many household chores and industrial manufacturing tasks require a certain compliant behavior to make deliberate physical contact with the environment. This compliant behavior can be implemented by modern robotic manipulators. However, in order to plan the task execution, a robot requires generic pr…

Robotic agents capable of natural and safe physical interaction with human co-workers

Many future application scenarios of robotics envision robotic agents to be in close physical interaction with humans: On the factory floor, robotic agents shall support their human co-workers with the dull and health threatening parts of their jobs. In their homes, robotic agents shall enable peopl…

Targeted jumping of compliantly actuated hoppers based on discrete planning and switching control

We address the operation of robotic legs with intrinsic elasticity in hopping cycles determined by the mechanical resonant properties of the system. This ensures energy efficiency and high jumping velocity and distance. Recently, we have shown in simulation that a simple, biologically inspired bang-…