Thorsten Brandt

Wire Robots Part I: Kinematics, Analysis & Design

One drawback of classical parallel robots is their limited workspace, mainly due to the limitation of the stroke of linear actuators. Parallel wire robots (also known as Tendon-based Steward platforms or cable robots) face this problem through substitution of the actuators by wires (or tendons, cables, . . .). Tendon-based Steward platforms have been proposed in (Landsberger & Sheridan, 1985). Although these robots share the basic concepts of classical parallel robots, there are some major differences:

Wire Robots Part II Dynamics, Control & Application

In (Bruckmann et al., 2008) the kinematics, analysis and design of wire robots were presented. This chapter focuses on control and applications of wire robots. Wire robots are a very recent area of research. Nevertheless, they are well studied and already in application (see section 5). Due to their possible lightweight structure, wire robots can operate at very high velocities. Hence, as can be seen by experiment, only positioning control using the inverse kinematics is not sufficient. In particular, slackness in the wires can be observed at highly dynamic motions. To overcome this problem, force control can be employed. In section 4 different control schemes are proposed. The required dynamical model is obtained in section 2, while for the calculation of feasible wire force distributions are proposed in section 3. Since wire robots are kinematically redundant the latter is not straightforward, but requires advanced approaches. The same holds for the control schemes, since a CRPM as well as a RRPM is a non-linear, coupled, redundant system (Ming & Higuchi, 1994).

A Novel Tensed Mechanism for Simulation of Maneuvers in Wind Tunnels

Wind tunnels are a standard tool to evaluate the air flow properties of aerodynamical vehicles in model scale. This is widely used to optimize the design of aircrafts and aircraft components. Additionally, the hydrodynamic properties of marine components like ship hulls or propulsion systems can be predicted. It is desirable to guide the models along defined trajectories during the tests to vary the angle of attack. Parallel wire robots were successfully used to perform airplane maneuvers in wind tunnels due to their good aerodynamical and mechanical properties. Compared to aircraft design, marine models are very heavy (up to 500kg). Thus, the positioning system must be very stiff to avoid vibrations. Additionally, fast maneuvers require powerful drives. Nevertheless, the positioning system should not influence the air flow. In this contribution, a novel design is presented. Additionally, a new realtime capable force distribution calculation method for parallel tensed systems is presented.Copyright

Design Approaches for Wire Robots

Wire robots consist of a movable end-effector which is connected to the machine frame by motor driven wires. Since wires can transmit only tension, positive wire forces have to be ensured. During workspace analysis, the wires forces need to be calculated. Discrete methods do not produce satisfying results, since intermediate points on the discrete calculation grids are neglected. Using intervals instead of points leads to reliable results. Formulating the analysis problem as a Constraint-Satisfaction-Problem (CSP) allows convenient transition to the synthesis problem, i.e. to find suitable designs for practical applications. In this paper, two synthesis approaches are employed: Design-to-Workspace (i.e. calculation of an optimal robot layout for a given workspace) and an extension called Design-to-Task (i.e. calculation of the optimal robot for a specific task). To solve these optimization problems, the paper presents approaches to combine the reliability and robustness of interval-based computations with the effectiveness of available optimizer implementations.Copyright

Symbolic Model Reduction for Interval-Valued Scenarios

In many cases, the quantitative relevance of physical effects for a given technical problem is not known a priori. This holds especially for the analysis of the dynamics. Adopted from nonanalog circuit design, in the last years symbolic model reduction techniques found their way towards mechatronic system modeling. Given a scenario (system inputs, initial values, parameters) and an error bound, symbolic model reduction reduces the detailed model to a less complex model, which is guaranteed to stay within predefined error bounds. However, presently symbolic reduction techniques deliver reduced models, which are only verified for a single scenario. For example a reduced vehicle model emerging from the reduction of a complex multibody vehicle model for a cornering maneuver with a small constant steering angle, is not verified to stay inside the error bounds for any other maneuver. In this contribution this drawback is addressed by the use of interval-valued scenarios.© 2009 ASME

Real-Time Vehicle Dynamics Using Equation-Based Reduction Techniques

Due to the increased computing power in the last decade, more and more complex vehicle models were developed. Nowadays even complex multibody models can be generated via graphical user interfaces in object-oriented simulation tools like Dymola or SimulationX. On the other hand, the available computing power in electronic control units is still limited, mostly by the cost pressure in the automotive industry. Hence, it is not possible to generate a complex model by drag and drop via a graphical user interface and run it in real-time within a desired time cycle on an ECU inside the vehicle. The same holds for HIL-testbeds and driving simulators, where the model must run in real-time as well. Thus, generally the model is adjusted in an iterative process until the model can be integrated in real-time on the particular ECU. In other words, a model has to be generated that is on the one hand complex enough to reproduce the desired physical effects and on the other hand simple enough to fulfill the real-time requirements. As it is easy to generate a complex model nowadays, an algorithm for the automated reduction of the model is required. Equation-based reduction techniques are a tool for the automated reduction of a given DAE-system for a defined error bound. This approach was already adopted and extended to generate vehicle models with an adjustable accuracy. In this contribution, equation-based reduction techniques are extended to generate models, which are guaranteed to run in real-time on a given real-time target within a given real-time cycle.

On four legs towards flexible and fast locomotion

The breathtaking development of electronic components is one key factor in the fast evolution of mobile robots. Depending on the mission, robots might operate autonomously or in cooperation with a human operator. In either case the locomotion of the robot is a key element. In general, legged systems are more flexible compared to wheel based systems and can be deployed in impractical terrain. Quadruped locomotion has the particular advantage to feature static as well as dynamic walking. Compared to n > 4 legs quadruped motion can be very agile, which is impressively demonstrated by cheetahs being the fastest mammal runners. On the other hand, quadrupeds feature static walking, which is beneficial for large scale walking machines such as legged excavators. In this paper a static free-gait pattern for quadrupeds allowing interaction with a human operator is discussed. Since experiments with large scale walking machines are very expensive, a scaled prototype is presented. However, the scaled prototype differs from the original system in many details. To guarantee the portability of the free-gait algorithm, the scaled workspace of the feet of the walking machine is proposed as criterion.