Andreas Pott

IPAnema: A family of Cable-Driven Parallel Robots for Industrial Applications

Nowadays there are very little robot systems in operation in the field of medium to large-scale handling and assembly mostly due to lack of repetitive processes or shortcomings in programming and configuring such robots. In this paper we introduce a family of cable-driven parallel robot called IPAnema that are designed for industrial processes. We address the system architecture, key components such as winches and controller, as well as design tools. Furthermore, some experimental data from the evaluation are presented to illustrate the performance of cable robots.

Closed-form Force Distribution for Parallel Wire Robots

This paper presents an algorithm to determine feasible force distributions for parallel wire robots in closed-form. The force distributions are continuous along trajectories and differentiable at most of the points. The computational efforts are strictly bounded and small even for large numbers of wires. The algorithm is compared to other approaches for calculation of force distribution in terms of the numerical effort and their applicability for control purposes.

Control of an pseudo-omnidirectional, non-holonomic, mobile robot based on an ICM representation in spherical coordinates

For mobile platforms with steerable standard wheels it is necessary to precisely coordinate rotation and steering angle of their wheels. Especially for redundantly actuated platforms the misalignments of a single wheel directly leads to invalid configurations which may cause degraded motion of the platform and high internal forces. An established approach to deal with this problem is to represent the current state of motion in form of the instantaneous centre of motion (ICM) and to derive a valid trajectory for this point. However, this representation bears severe numerical drawbacks. To remedy those numerical problems an alternative ICM representation based on spherical coordinates is proposed in this work. Furthermore, the relations between ICM and generalized robot velocities are addressed. It is shown, that one receives a basis of a subspace within the kinematical constraints¿ nullspace by decomposing the generalized velocity vector in spherical coordinates. Finally the proposed ICM-based control is particularized and simulative analyzed w.r.t. the Care-O-bot 3 demonstrator.

Forward Kinematics and Workspace Determination of a Wire Robot for Industrial Applications

This paper presents the recent results from a newly designed parallel wire robot which is currently under construction. Firstly, an overview of the system architecture is given and technically relevant requirements for the realization are identified. A technique to compute and transfer an estimation of the workspace to CAD tools is presented. Furthermore, tools to solve the forward kinematics of some special configuration under real-time requirements are explored. Simulation results show the feasibility of the presented algorithms.

Cable-Driven Parallel Robots

Welcome.- Motion Planning.- Force Distribution.- Application and Protoypes.- Design and Components.- Kinematics and Interval Methods.- Calibration und Identification.- Control.- Dynamics Modelling.

An Elastic Cable Model for Cable-Driven Parallel Robots Including Hysteresis Effects

Experimental results indicate that time invariant linear elastic models for cable-driven parallel robots show a significant error in the force prediction during operation. This paper proposes the use of an extended model for polymer cables which allows to regard the hysteresis effects depending on the excitation amplitude, frequency, and initial tension level. The experimental design as well as the parameter identification are regarded.

Addressing input saturation and kinematic constraints of overactuated undercarriages by predictive potential fields

Currently, pseudo-omnidirectional, wheeled mobile robots with independently steered and driven wheels seem to provide a solid compromise between complexity, flexibility and robustness. Yet, such undercarriages are imposed to the risk of actuator fighting and suffer from singular regions within their configuration space. To address these problems we expand a previously developed potential field (PF) based approach by expanding it with a predictive horizon. The proposed method is based on a model predictive control (MPC) approach, incorporating a gradient descent optimization step via the Pontryagin minimum principle. To enforce adherence to the constraints during optimization, we modify the Lagrange-multipliers within the backpropagation of the costates. The proposed approach is evaluated simulatively w.r.t. the undercarriage of the Care-O-bot ® 3 mobile robot and is compared to the potential field based and a model predictive control approach.

System identification and cable force control for a cable-driven parallel robot with industrial servo drives

In a cable-driven parallel robot, elastic cables are used to manipulate the end effector in the workspace. In this paper we present a dynamic analysis and system identification for the complete actuator unit of a cable robot including servo controller, winch, cable, cable force sensor and field bus communication. We establish a second-order system with dead time as an analagous model. Based on this investigation, we propose the design and stability analysis of a cable force controller. We present the implementation of feed-forward and integral controllers based on a stiffness model of the cables. As the platform position is not observable the challenge is to control the cable force while maintaining the positional accuracy. Experimental evaluation of the force controller shows, that the absolute positional accuracy is even improved.

Load identification and compensation for a Cable-Driven parallel robot

In Cable-Driven parallel robots, elastic cables are used to control the movement of a platform in the workspace. The accuracy of a robot system is one of the most important comparison criteria for various applications. In this paper, we present a new approach to increase the relative accuracy of a cable robot under changing payload, by compensating the error induced by elasticity. In the first step, the load is identified based on cable force sensors and the acceleration of the platform. Second, for the actual pose the stiffness matrix is computed and the expected displacement of the platform in Cartesian space is determined. Lastly, the displacement is compensated by adapting the cable lengths. Furthermore, the influence of the calibration of cable force sensors and their parameters inaccuracies are discussed. The algorithms are implemented and evaluated on a 6-DOF cable robot. We could experimentally prove an improvement of over 50% in the relative accuracy under changing load.

Implementing Extended Kinematics of a Cable-Driven Parallel Robot in Real-Time

This paper describes the implementation of extended pulley kinematics for parallel cable robots. An algorithm for the extended kinematics taking into account cable pulleys is discussed and implemented in real-time. This solution uses an iterative solver which can be computationally costly, depending on convergence. The convergence was tested for a specific geometry and successfully implemented on the cable robot IPAnema. Accuracy of both the standard and extended kinematics were tested according to the ISO 9283 standard. The Absolute accuracy was measured to be 22.32 mm for the standard and 17.50 mm for the extended kinematics which shows some improvement. A method for testing accuracy of orientations is also introduced.