Jens Reinecke

Online in-hand object localization

Robotic hands are a key component of humanoids. Initially more fragile and larger than their human counterparts, the technology has evolved and the latest generation is close to the human hand in size and robustness. However, it is still disappointing to see how little robotic hands are able to do once the grasp is acquired due to the difficulty to obtain a reliable pose of the object within the palm. This paper presents a novel method based on a particle filter used to estimate online the object pose. It is shown that the method is robust, accurate and handles many realistic scenario without hand crafted rules. It combines an efficient collision checker with a few very simple ideas, that require only a basic knowledge of the geometry of the objects. It is shown, by experiments and simulations, that the algorithm is able to deal with inaccurate finger position measurements and can integrate tactile measurements. The method greatly enhances the performance of common manipulation operations, such as a pick and place tasks, and boosts the sensing capabilities of the robot.

Impedance control of a non-linearly coupled tendon driven thumb

A large workspace and proper force capabilities of a robotic thumb can be obtained using a tensegrity structure for the actuation, similar to the human thumb base muscles. Using nonlinear stiffness elements and an antagonistic architecture, the joint stiffness can be adjusted by variation of the tendon pre-tension. However, the highly nonlinear actuation creates new control challenges and in particular the nonlinear tendon kinematics must be accounted for. Despite the challenges, the nonlinear structure is required to achieve the desired torques. In this paper, the dynamic equations of a tendon driven thumb are established. An efficient formulation is proposed to generate the pretension forces in order to preserve the torques and approximate the stiffness matrix. A cascaded structure is used for the controller. The equations for the inner tendon force control loop and the outer impedance control loop are presented. Because of the absence of link side position sensors, an iterative estimation algorithm is proposed and implemented in real-time. It is shown that, using the mechanical joint flexibility, the controller impedance gain can be adjusted to improve the steady-state effective impedance. The search algorithm robustness is evaluated through a set of simulations. Finally, experimental results and equivalent simulations demonstrate the effectiveness of our controller.

Guiding Effects and Friction Modeling for Tendon Driven Systems

This paper discusses tendon friction effects regarding guiding and material selection. In order to extract valuable information for designers of tendon driven systems, several experiments are conducted to investigate e. g. the intrinsic friction or sliding effect. The results are used to build an experimental friction model and to derive a set of guidelines. The mechanical designer can use the proposed models to anticipate the friction for a given tendon path. Additionally, the guidelines help the mechanical designer to systematically verify the numerous constraints involved in the design process of a tendon driven system.

Experimental comparison of slip detection strategies by tactile sensing with the BioTac ® on the DLR hand arm system

Dexterous manipulation of everyday objects requires a precise tactile sense. Slip detection is mandatory to overcome uncertainty and compensate for external disturbances. We compare three different approaches for detecting slip. The methods are model-based slip detection via friction cones, vibration-based detection via bandpass filtering, and a common learning algorithm. They are implemented and tested on a tendon-driven two-finger setup equipped with two tactile BioTac® sensors. Several experiments are conducted to evaluate each approach. The characteristics of the methods are discussed and compared.

Derivation and verification of synergy coordinates for the DLR hand arm system

Dexterous robotic hands are still not used in production lines, mainly because of the complexity to operate such hands. Most often a robotic hand is used to grasp and hold the object of interest while the object motion is performed by the arm. From studies on human grasping it is known that most of the grasping task can be reduced to two independent degree of freedom which drive the joints of the hand. Based on this idea, we analyzed a robotic grasp database to find suitable robotic “synergy coordinates”. 77% of these grasps can be represented by two coordinates that were originally defined by the 19 joint variables of the DLR Hand Arm System. In this work we also discuss the importance of the grasp approach configurations, which have significant influence on the results. The synergy coordinates were used to extend the joint level impedance controller and to provide a complexity reduced interface to an user or a planning algorithm, which leads to simplified operation of the hand and to reduced teach-in-times. From a different perspective this concept allows to imitate the behavior of a synergistic, respectively underactuated, hand by means of programming. The controller was evaluated successfully on the DLR Hand Arm System by realizing several different grasps covering a simplified grasping taxonomy. The DLR Hand Arm System has additional degrees of freedom to adjust the mechanical joint stiffness, which was used to modify the grasp force at a given pinch grasp configuration.

FAS A flexible antagonistic spring element for a high performance over actuated hand

In robotic hands design tendon driven systems have been considered for years. The main advantage is a small end effector inertia e.g. a light, small hand with high dynamics due to remote actuators. To protect the actuators from impact in unknown environments a compliant mechanism can be used. It absorbs energy during an impact or saves energy to enhance the joint dynamics. In this paper an antagonistic tendon mechanism is presented. It fits 38 times in the DLR Hand Arm System forearm and enables is adapted to the different finger joints and different tendon lengths. A magnetic sensor was developed for the force measurement of the tendons. Finally, the calibration and the robustness are demonstrated through a set of experiments.

A structurally flexible humanoid spine based on a tendon-driven elastic continuum

When working in unknown environments, it is beneficial for robots to be mechanically robust to impacts. To obtain such robustness properties and maintain human-like performance, strength, workspace and size especially in the upper body, an elastic backbone approach is presented and discussed. Compared to the human spine stabilized by ligaments, intervertebral discs and muscles, we use a continuum mechanism based on silicone and tendons for actuation. The work presents the development of such a mechanism, that could be either used as neck or torso, but concentrates on the cervical part (neck). To prove functionality of the proposed concept, a planar setup was designed and experimental data regarding motion capabilities, robustness and dynamics are presented. With that knowledge, a modular multiple DOF prototype is built which can easily be equipped with different tendon routing and elastic continuum shapes. The results of that final setup will help the mechanical designer to choose the suitable solution for the robotic spine and provides a test bed to develop control strategies for such types of mechanisms.

The DLR C-runner: Concept, design and experiments

Legged locomotion requires highly dynamic and efficient actuation as well as robust environment interaction. In the past years soft robots with elastic actuation have been investigated and their fitness for cyclic tasks and safe and robust environment interaction has been shown. To evaluate the benefits and drawbacks of series elastic actuation, variable impedance actuation as well as multi-articular elastic coupling in legged locomotion, we developed a two legged human size test-bed. These modular robotic legs give the possibility to evaluate and directly compare different elastic actuation concepts on a single system. Hopping and bipedal modal motion experiments where performed to proof the concept.

FAS A flexible antagonistic spring element for a high performance over

In robotic hands design tendon driven systems have been considered for years. The main advantage is a small end effector inertia e.g. a light, small hand with high dynamics due to remote actuators. To protect the actuators from impact in unknown environments a compliant mechanism can be used. It absorbs energy during an impact or saves energy to enhance the joint dynamics. In this paper an antagonistic tendon mechanism is presented. It fits 38 times in the DLR Hand Arm System forearm and enables is adapted to the different finger joints and different tendon lengths. A magnetic sensor was developed for the force measurement of the tendons. Finally, the calibration and the robustness are demonstrated through a set of experiments.

FRCEF: The new friction reduced and coupling enhanced finger for the Awiwi hand

The combination of tendon driven robotic fingers and variable impedance actuation in the DLR hand arm system brings benefits in robustness and dynamics by enabling energy storage. Since the force measurement and motors are in the forearm the tendon path should have low friction for accurate movements and precise finger control. In this paper an enhanced generation of the Awiwi hand finger design is presented. It reduces the friction in the actuation system about 20 percent and increases the maximum fingertip force about 33 percent. A test finger was designed to evaluate different tendon couplings and to test a magnetic sensor to measure the joint position. In a next step a new finger design for DLR hand arm system has been developed. Finally, the low friction and the robustness are proven using several experiments.