Tobias Mahl’s research while affiliated with University of Stuttgart and other places

In the previous decade, multiple useful approaches for kinematic models of continuum manipulators were successfully developed. However, dynamic modeling approaches needed for fast simulations and the development of model-based controller design are not powerful enough yet—especially for spatial manipulators with multiple sections. Therefore, a practicable lumped mass model similar to common dynamic models of rigid-link manipulators is needed, which can be used for simulations and model-based control design. The model incorporates mechanical interconnections of parallel and serially connected bellows and uses constant curvature kinematics and its analytical derivatives to balance forces and energies in a global reference frame. The parameters of the resulting model are identified with measurements before the simulation results are experimentally validated. The obtained dynamic model can be used to both simulate the manipulator dynamics and calculate the inverse dynamics needed for model-based controller design or path planning.

Dynamic models of continuum manipulators tend to become very complex, especially for spatial manipulators with multiple sections. Therefore a practicable model is needed that can be used for simulations and model-based control design. Neglecting rotational energies and assuming a continuum manipulator that consists of a single concentrated mass per section, dynamic equations for each actuator state are derived using the Euler-Lagrange formalism. Forces, positions and velocities are transformed to a global reference system using the homogeneous transformation based on constant curvature robot kinematics and its derivatives. Measurements of an example manipulator verify the resulting dynamic model that can be used to both simulate the dynamics and calculate the inverted robot dynamics needed for model-based controller design.

We present a new variable curvature continuum kinematics for multisection continuum robots with arbitrarily shaped backbone curves assembled from sections with three degrees of freedom (DoFs) (spatial bending and extension, no torsion). For these robots, the forward kinematics and the differential forward kinematics are derived. The proposed model approach is capable of reproducing both the constant and variable backbone curvature in a closed form. It describes the deformation of a single section with a finite number of serially connected circular arcs. This yields a section model with piecewise constant and, thus, a variable section curvature. Model accuracy and its suitability for kinematic real-time control applications are demonstrated with simulations and experimental data. To solve the redundant inverse kinematics problem, a local resolution of redundancy at the velocity level through the use of the robot’s Jacobian matrix is presented. The Jacobian is derived analytically, including a concept for regularization in singular configurations. Experimental data are recorded with Festo’s Bionic Handling Assistant. This continuum robot is chosen for experimental validation, as it consists of a variable backbone curvature because of its conically tapering shape.

Continuum manipulators are continuously bending robots consisting of an infinite number of kinematic degrees of freedom (DOF). To reduce the number of actuators, the manipulators are designed in a way to build several serially connected groups of mechanically coupled DOF. These groups are called sections. For real-time motion control, a kinematic model capable to describe the manipulator's deflection is necessary. A common way to model the manipulator kinematics is to describe the deformation of a single section by a curve with constant curvature. This assumption constitutes an intense constrain with respect to manipulator design or model accuracy. Thus, a new kinematic modeling approach capable to describe the kinematics of continuum manipulators with variable section curvature is proposed in the present work. It subdivides a single section in a finite number of virtual units with piecewise constant curvature. This provides the possibility to shape the modeled section curvature closely to the deformation of any arbitrarily bending continuum manipulator. To demonstrate that this modeling approach is well suited for real-time control applications, simulation results of a Jacobian based feed-forward pose control are presented that are applied to the common class of three actuator continuum manipulators.

The Bionic Handling Assistant is a compliant, pneumatically actuated continuum manipulator designed to be used for cooperative manipulation. In 2010, it won the German Federal Presidents prize for achievements in technology and innovation, called Deutscher Zukunftspreis. Unlike most manipulators, its flexible structure is link and actuator at the same time, copying an elephant's trunk. For this robot arm, the forward kinematics is derived and validated by test bench measurements. The forward kinematics is an analytical description of the manipulator's backbone curvature. It describes the manipulator's tool center point pose (position and orientation) dependent of the actuator expansions. The kinematic model is assembled of several in series connected three degrees of freedom parallel mechanisms of type 3UPS-1PU.

During offshore installations in harsh sea conditions, the involved crane system must satisfy rigorous requirements in terms of safety and efficiency. The forces resulting from the vertical motion of the vessel have an extensive effect on the overall crane structure and its lifetime. Moreover, vessel motion handicaps the operator during fine positioning of the payload. Hence, an active compensation system for the vertical vessel motion is proposed. An important point to consider for such systems is the time delay between the sensors and actuators, which diminishes performance. To compensate the dead times in the system, a prediction algorithm for the vertical motion of the vessel is proposed in the first part. In the second part, an inversion-based control strategy for the hydraulic-driven winch is formulated that considers the dynamic behavior of the drive system. A feedforward controller compensates the vertical-motion disturbance using the predicted motion. The proposed controller together with the prediction algorithm decouple the motion of the rope-suspended payload from the vessel's motion. The active compensation approach is evaluated with simulation and measurement results.

Accurate knowledge of the instantaneous friction torque of an automotive clutch is a key claim while achieving comfortable, automated gearshifts, improving fuel economy or reducing wear. Nevertheless, torque sensors are not commonly used in automotive drive trains because of their additional size and costs. Thus, to estimate the clutch torque, a detailed dynamic model of each component integrated into a clutch system is needed. In the following, a nonlinear dynamic model of a Dual Mass Flywheel (DMF) as autonomous part of a clutch system is presented and verified by test bench data. A DMF is used to reduce the cyclic irregularity of the torque generated by a combustion engine. It is typically assembled between crankshaft and clutch. The level of detail of the modelled DMF dynamics is chosen in a way that a real-time simulation on a car’s control unit is feasible. Using the engine speed and the clutch torque as model inputs, the proposed model has the ability to simulate the DMF deflection and therefore the clutch rotational speed. Resulting torques acting on the DMF’s primary and secondary mass are reconstructed, too. If both rotation speeds of the DMF masses (the engine and clutch speed) can be measured, this model can also be used to reconstruct the clutch torque.

The dual mass flywheel (DMF) is primarily used for dampening of oscillations in automotive powertrains and to prevent gearbox rattling. TWs paper explains the DMF mechanics along with its application and components. Afterwards a detailed ab-initio model of the DMF dynamics is presented. This mainly includes a model for the two arc springs in the DMF and their friction behavior. Both centrifugal effects and redirection forces act radially on the arc spring which induces friction. A numerical simulation of the DMF model is compared to measurements for model validation. Finally the observability of the engine torque using the DMF is discussed. For this purpose a linear torque observer is proposed and evaluated.

Offshore installations during harsh sea conditions results in rigorous requirements in terms of safety and efficiency for the involved crane system. Hence a heave compensation system based on heave motion prediction and an inversion based control strategy is proposed. The control objective is to let the rope suspended payload track a desired reference trajectory in an earth fixed frame without being influenced by the heave motion of the ship or vessel. Therefor a combination of a trajectory tracking disturbance decoupling controller and a prediction algorithm is presented and evaluated with simulation and measurement results.

To increase the effectiveness of the cargo transshipment process is one of the most important objectives for the automation of cranes. Therefore new control strategies are applied. This paper presents a nonlinear controller in order to solve the trajectory tracking and disturbance rejection problem for a boom crane. A model of the crane dynamics in radial direction is derived considering the dominant nonlinearities such as the actuator kinematics. Further on, the coupling of a slewing and luffing motion is taken into account. This coupling is caused by the centrifugal acceleration of the load in radial direction during a slewing motion. Based on the nonlinear model a linearizing and disturbance decoupling control law is derived and discussed. Measurement results from the boom crane validate the good performance of the nonlinear controller.