Alexander Hildebrandt

Cascaded control concept of a robot with two degrees of freedom driven by four artificial pneumatic muscle actuators

Pneumatic muscles are interesting in their use as actuators in robotics, since they have a high power/weight ratio, a high-tension force and a long durability. This paper presents a two-axis planar articulated robot, which is driven by four pneumatic muscles. Every actuator is supplied by one electronic servo valve in 3/3-way function. Part of this work is the derivation of the model description, which describes a high nonlinear dynamic behavior of the robot. Main focus is the physical model for the pneumatic muscle and a detailed model description for the servo valves. The aim is to control the tool center point (TCP) of the manipulator, which bases here on a fast subsidiary torque regulator of the drive system compensating the nonlinear effects. As the robot represents a MIMO system, a second control objective is defined, which corresponds here to the average pressure of each muscle-pair. An optimisation-strategy is presented to meet the maximum stiffness of the controlled drive system. As the torque controller assures a fast linear input/output behavior, a standardized controller is implemented which bases here on the Computed Torque Method to track the TCP. Measurement results show the efficiency of the presented cascaded control concept.

A Variable Curvature Continuum Kinematics for Kinematic Control of the Bionic Handling Assistant

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 robots Jacobian matrix is presented. The Jacobian is derived analytically, including a concept for regularization in singular configurations. Experimental data are recorded with Festos 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.

A flatness based design for tracking control of pneumatic muscle actuators

Because of their high power/weight ratio pneumatic actuators, especially the so called pneumatic muscles, are very interesting for the use as actuators in robotics. But, in fact the physical model is highly nonlinear, in the following a flatness based position controller for the pneumatic artificial muscles is presented. The considered pneumatic muscle is produced by the manufacturer Festo and possesses a high pulling force to 4000N and a very long lifetime at least to 10 million switching cycles. The control objective is to track the payload along a specified reference path including an active attenuation. Since a model based control approach is pursued, a physical model is presented for an experimental setup. The model is very nonlinear making nonlinear flatness based control desirable. Experimental results are included and demonstrate the efficiency of the control.

Optimal System Design of SISO-Servopneumatic Positioning Drives

Servopneumatic actuators are very attractive for automated handling tasks or robot operations. They have many advantages such as high speed, high robustness in rough manufacturing environment or high power-to-weight ratio. The considered actuator system is a standard configuration in pneumatics consisting of a double acting pneumatic cylinder controlled by a proportional directional control valve. For the set-up a detailed mathematical model is derived. In order to guarantee an accurate tracking behaviour, a model-based nonlinear controller is presented. Model based approaches for the control design have several advantages. Tuning of the controller can be reached in a systematic way even in the case of a large variety of different configurations. But not only the control design itself can be treated. The model offers the possibility to optimize the size of components for demanded automation tasks. In most cases, this is solved based on steady state assumptions. In this contribution, a method for a design procedure for the pneumatic actuator system based on the dynamic equation is presented. With the representation of the system, an optimization procedure for the components is introduced. The optimization criteria consist of the minimization of the air consumption and investment costs.

Anti-sway system for boom cranes based on an optimal control approach

For the operation of boom cranes a significant high level of experience is needed to achieve a large amount of handled containers. Therefore, automation systems with anti-sway abilities for boom cranes were developed to increase the handling rate especially for inexperienced crane operators. In the paper, an optimal control approach is presented as an alternative to feedback control systems. After deriving the nonlinear dynamic model, the anti-sway problem is formulated as a nonlinear constrained optimal control problem. The numerical solution method for this optimal control problem is presented. The results are illustrated at measurements for the harbour mobile crane LIEBHERR LHM400.

A variable curvature modeling approach for kinematic control of continuum manipulators

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 manipulators 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.

Model-based feedforward position control of constant curvature continuum robots using feedback linearization

Fast and exact motions of continuum robots are hardly seen so far. Partly this is caused by physical constraints, e.g. small available actuation forces. Another reason is the dynamic coupling between the actuators that cannot be neglected during fast motions. Therefore, a model-based MIMO controller in actuator space was developed, that is based on a spatial dynamic model with one mass point per section. Using feedback linearization, the actuators can be decoupled and feedforward control in combination with linear controllers can be applied. Measurements of an example manipulator show the good tracking result of pure feedforward action with feedback linearization. Adding a linear PD-controller increases the robustness against disturbances without reducing the possibility of fast motions.

Dynamic modeling of constant curvature continuum robots using the Euler-Lagrange formalism

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.

Control design for the rotation of crane loads for boom cranes

This paper handles the control of flexible link robot systems. Since in the considered case a manipulator for grabbing containers is suspended on two ropes, torsional oscillation occurs changing the position of the manipulator. A control strategy, which consists of feedforward and disturbance observer based feedback control implemented on boom cranes to assure an accurate tracking of the manipulator along a reference path is shown. The system parameters like rope length, moment of inertia of the load and its mass are changed frequently during the crane operation. Therefore the controller is fully adaptive due to the varying system parameters. Measurement results show the efficiency of the presented control strategy implemented on a harbor mobile crane LIEBHERR LHM 400.

Forward kinematics of a compliant pneumatically actuated redundant manipulator

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 elephants 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 manipulators backbone curvature. It describes the manipulators 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.