Henning Tolle

Input-dependent stability of joint torque control of tendon-driven robot hands

The input-dependent stability observed during torque control experiments using the first joint of the Darmstadt-HAND is discussed. Friction and compliance existing in tendon-sheath drive systems introduce a hysteresis nonlinearity between the joint torque output and the actuator displacement. Although this transmission characteristic is close to the well-known backlash behavior of the gears situated between a motor and a load shift, this hysteresis loop exhibits input-dependent characteristics in the backlash region of the transmission system, with springlike behavior within a portion of the backlash region. Experiments confirmed that there is a close relationship between the input-dependent backlash characteristics and the input-dependent stability. Based on these experiments, the authors describe the transmission characteristic using a simple model and explore the system stability using sinusoidal-input-describing-functions (SIDF). A nondimensional stability-criterion-map that successfully predicts the experimental results is presented. >

Motion planning with many degrees of freedom-random reflections at C-space obstacles

This paper describes a motion planner that automatically avoids collisions with obstacles. A transformation of obstacles into configuration space (C-space) is not necessary. Given a polyhedral description of robot, load and environment the algorithm first computes (off-line) a graph which roughly represents the skeleton of the freespace. Due to the way of its computation this graph does not preserve connectivity, although the freespace is a connected set, which is assumed throughout this paper. Reflecting randomly at C-space obstacles a connection between subgraphs is generated. In a second step (online) a connection between a start and a goal configuration to the graph is searched for. The impact of this algorithm concerning movable obstacles is discussed. This method is general and four examples are presented to demonstrate its feasibility by applying it to different kinematic structures with 3, 6 and 12 degrees of freedom.<<ETX>>

Learning control with interpolating memories—general ideas, design lay-out, theoretical approaches and practical applications

The paper discusses the importance of interpolating memories for different lay-outs of learning control loops, design considerations for such memories and the convergence of the learning process for one such memory. Finally, applications of learning control loops and future research in this field are considered.

Object manipulation by a multifingered gripper: on the transition from precision to power grasp

Whereas the emphasis of precision grasping only using the fingertips is placed on dexterity and sensitivity, power grasps guarantee high security and stability. But even if the task requires a power grasp, in the initial configuration the complete enveloping by the fingers is often impossible due to the position of the object in the workspace and only a fingertip grasp can be applied. Then a transition from precision to power grasping has to be performed. Based on an experimental study this paper discusses several possibilities to realize this transition by using a three-fingered dextrous hand mounted on a 6DOF robot. In particular it is distinguished between strategies requiring a coordination of robot and hand and strategies performing the transition only by the fingers like humans do.

A self-adjusting active compliance controller for multiple robots handling an object

Abstract This paper presents the concept and experimental validation of a self-adjusting active compliance controller for n robots handling its compliant behaviour concerning partly unknown flexible object. The control strategy is based on the decomposition of the 6 n -dimensional position/force space and includes a feedforward and feedback level. The feedforward level contains motion coordination, force distribution of external forces, creation of internal forces, and an additional loop adding the elastic displacements due to the applied forces to the planned robot positions. The feedback level is organized in the form of an active compliance control law. For adjusting the controller to the, in general, unknown flexible behaviour, which in practice is the main problem of the controller design, a quasi-static model of the system is derived for different contact cases of the object and a procedure is presented, which by use of this model is capable of determining the compliance of the considered system and therefore of adjusting the controller. Experiments with two puma-type robots have been conducted to show the applicability of the self-adjusting control strategy. The task has been to grasp and move an unconstrained object. It is shown, that the system can adjust the control parameters to the unknown system compliance and that the control performance is improved considerably.

Input-dependent stability on joint torque control of robot hand

Input-dependent stability was observed during torque control experiments using the first joint of the Darmstadt-HAND. The friction and compliance existing in tendon-sheath driving systems bring a hysteresis characteristic into the dependence of joint torque output on actuator displacement. While this transmission characteristic is close to the well-known backlash behavior appearing for the gears situated between a motor and load shaft, input-dependent characteristics were found to exist in the backlash region of the transmission system in the case of the use of a practical tendon length. The observed characteristic shows springlike behavior even in the backlash region and it also depends on input. Through experiments, it was confirmed that there are close relationships between the input-dependent backlash characteristics and the input-dependent stability. Based on the experiments, a description is given of the transmission characteristic in simple model equations, and the system stability is explored by using the sinusoidal-input-describing-function technique. A nondimensional stability-criterion map explaining the experimental results is shown.<<ETX>>

Integration of Expert Systems and Neural Networks for the Control of Fermentation Processes

Abstract Expert systems and neural networks are new tools for the control of fermentation processes. With expert systems the fermentation plant and the process itself is modelled via a generalized, qualitative system description based on the experience of human experts. On the other hand neural networks and interpolating associative memories can learn the process behaviour directly by process observation. The paper at hand reports, how both control techniques can be combined for purposes like process supervision, modelling and optimization of biological plants.

Capturing pyramidal-like objects

This paper discusses a grasp scheme for a pyramidal-like object placed on a table under the gravitational field. We assume that the contact friction is small enough to ensure that any direct grasp fails in achieving an equilibrium grasp. For an idealized 2D triangle object placed vertically on a table, we first show that a planar two-fingered hand can always achieve an equilibrium grasp under frictional contact. We also extend the basic idea to a polyhedron whose face is triangle and show two possible procedures leading to equilibrium grasps. Finally, we verify our idea experimentally for the polyhedra.

BioX++: new results and conceptions concerning the intelligent control of biotechnological processes

Abstract The article at hand presents new results and conceptions concerning the intelligent and autonomous control of biotechnological processes by integrating conventional, knowledge-based and learning methods. The extended system BioX++ facilitates the transparent generation of process control strategies and sequences based on automatically self-organized structured process models. Experimental results showing the increased product yield and the discussion of approach-specific problems are part of this paper as well as the new approaches actually examined.

Force-Controlled Robotic Deburring

Abstract In this paper a three-step active deburring strategy is proposed based on force feedback control. Strategies for automatic contour following are developed to identify unknown workpiece contours, to automatically generate desired robot motions, and to detect unknown burr sizes. Under the active deburring strategy burr size variations are identified through contour following and different sizes of burrs are removed with a specification of a variable desired deburring force. Implementation of and experiments with the proposed strategies are described.