Rainer Krenn

DLR's torque-controlled light weight robot III-are we reaching the technological limits now?

A third generation of torque-controlled light weight robots has been developed in DLRs robotics and mechatronics lab which is based on all the experiences that have been had with the first two generations. It aims at reaching the limits of what seems achievable with present day technologies not only with respect to light-weight, but also with respect to minimal power consumption and losses. One of the main gaps we tried to close in version III was the development of a new, robot-dedicated high energy motor designed with the best available techniques of concurrent engineering, and the renewed efforts to save weight in the links by using ultralight carbon fibres.

Planetary rover mobility simulation on soft and uneven terrain

Rovers on Mars or the Moon for planetary exploration are obtaining increased importance within the spaceflight nations. To achieve full mission success, drivability and mobility in all kinds of complex motion scenarios have to be guaranteed. Here, proper modelling and understanding of the complex wheel–soil interaction, i.e. the terramechanics for flexible and rigid wheels interacting with hard, soft and loose soil, are a major driver for supporting reliable rover design and assisting in testing of the flight model. This paper deals with the terramechanical characteristics for wheel–soil contact dynamics modelling and simulation and its experimental validation on the basis of the future European Mars rover mission ExoMars. The physical contact models are integrated by a multi-body system approach and the performance of the rover mobility will be shown for various driving scenarios on hard and soft soil.

Analytical and Experimental Stability Investigation of a Hardware-in-the-Loop Satellite Docking Simulator

The European Proximity Operations Simulator of the DLR-German Aerospace Center is a robotics-based simulator that aims at validating and verifying a satellite docking phase. The generic concept features a robotics tracking system working in closed loop with a force/torque feedback signal. Inherent delays in the tracking system combined with typical high stiffness at contact challenge the stability of the closed-loop system. The proposed concept of operations is hybrid: the feedback signal is a superposition of a measured value and of a virtual value that can be tuned in order to guarantee a desired behavior. This paper is concerned with an analytical study of the system’s closed-loop stability, and with an experimental validation of the hybrid concept of operations in one dimension. The robotics simulator is modeled as a second-order loop-delay system and closed-form expressions for the critical delay and associated frequency are derived as a function of the satellites’ mass and the contact dynamics stiffness and damping parameters. A numerical illustration sheds light on the impact of the parameters on the stability regions. A first-order Pade approximation provides additional means of stability investigation. Experiments were performed and test results are described for varying values of the mass and the damping coefficients. The empirical determination of instability is based on the coefficient of restitution and on the observed energy. There is a very good agreement between the critical damping values predicted by the analysis and observed during the tests. The contact duration shows also a very good fit between analysis and experiment. In addition, results from a one-dimensional contact experiment carried on an air-floating testbed are successfully emulated using the proposed hybrid docking simulator. This illustrates the flexibility of the hybrid simulator, where various contact dynamics can be emulated without changing any hardware elements.

Soft Soil Contact Modeling Technique for Multi-Body System Simulation

In the context of planetary exploration with mobile robots a soil contact model (SCM) for prediction and assessment of locomotion performance in soft uneven terrain has been developed. The SCM approach provides a link between the classical, semi-empirical terramechanics theory of Bekker and the capabilities of multi-body system (MBS) simulation technique for general, full 3D simulations of soil contact dynamics problems. Beyond the computation of contact forces and torques SCM keeps track of the plastic soil deformation during simulation. For this purpose it comprises features such as generation of ruts and displacement of soil material that allow computing typical terramechanical contact phenomena like bulldozing, multi-pass effects and drawbar-pull–slippage relations. Unlike volumetric, Finite Element/Discrete Element Method-like approaches SCM applies exclusively surface oriented algorithms with relatively small complexity constants. Moreover, most of the algorithms are of linear complexity. Therefore, the computational efficiency is quite high and adequate for MBS simulation requirements.

Parameter Identification of a Planetary Rover Wheel–Soil Contact Model via a Bayesian Approach

Soft soil contact models developed for planetary exploration rovers play an important role in the study of rover mobility. Nowadays, most of the existing contact models are based on Bekker theory, which requires the evaluation of several soil parameters usually measured via bevameter tests. However, substantial differences existing between the plate-soil contact scenario and the wheel-soil contact scenario, along with large variability associated with the bevameter experiments, give rise to large uncertainty in the choice of the model parameter values. In this paper, a Bayesian procedure is proposed to deal effectively with the presence of uncertainty. In the proposed approach, model parameters are random variables with prior distributions derived from bevameter measurements. The prior distributions are then enhanced to posterior distributions through single wheel test data. At the end, the procedure identifies a set of possible model parameter configurations that result in high experimental-numerical matching

Modellbasierte Regelungsansätze für überaktuierte planetare Rover und robotische Elektromobile

Zusammenfassung Der Beitrag beschreibt modellbasierte Regelungsstrategien für planetare Rover und innovative Elektromobile. Trotz prinzipiell gleichen Aufbaus ihrer Fahrwerke werden doch unterschiedliche Reglerlösungen bevorzugt, deren Bandbreite in diesem Beitrag vorgestellt wird. Beide Systeme sind überaktuierte robotische Fahrzeuge, deren Regler die jeweils unabhängig ansteuerbaren Rad- und Lenkantriebe koordinieren. Im Fall des planetaren Rovers wird ein Fahrzeugmodell mit Rad-Sand-Kontaktdynamik verwendet, das innerhalb eines modellprädiktiven Reglers (MPC) zum Einsatz kommt. Für das wesentlich dynamischere Straßenfahrzeug wird der MPC-Ansatz erweitert. Zur Reduzierung der Steuerungsdimension wird ein invertiertes Modell der Fahrdynamik eingeführt und das Optimierungsproblem auf das Bestimmen von Hilfsgrößen für die verbleibenden Freiheitsgrade reduziert. Abstract The paper introduces model based control strategies for planetary rovers and novel electric vehicles. Despite the same fundamental structure of their chassis different types of controllers are preferred. The bandwidth of potential solutions is shown in the paper. Both systems are overactuated robotic vehicles with controllers for coordinating the individually controlable wheel and steering drives. For planetary rovers a model predictive control (MPC) approach is implemented that considers the specific wheel-soil contact dynamics. The solution for the dynamic road vehicles goes beyond the MPC approach. For minimizing the number of controls an inverted vehicle dynamics model is introduced such that the optimization problem is reduced on computing auxiliary variables only, which are representing the remaining degrees of freedom of the system.