Bruno Strah

Power-optimized stiffness and nonlinear position control of an actuator with Variable Torsion Stiffness

Introducing compliant actuation to robotic joints is an approach to ensure safety in closer human-machine interaction. Further, the possibility to adjust stiffness can be beneficial considering energy storage and the power consumption required to track certain trajectories. The subject of this paper is the stiffness and position control of the Variable Torsion Stiffness (VTS) actuator for application in compliant robotic joints. For the realization of a variable rotational stiffness, the active length of a torsional elastic element in serial configuration between drive and link is adjusted in VTS. After the deduction of an extended drive train model, this paper gives an advanced power analysis clarifying power-optimal settings from previous basic models and identifying additional settings that allow for a more versatile operation. Based on these results that can be generalized to other variable elastic actuator concepts, an optimized strategy for setting stiffness is determined considering the whole system dynamics including natural frequencies as well as antiresonance effects. For position control of VTS in a prototypical implementation, a nonlinear position controller is designed by means of feedback linearization. Although the system is modified significantly by changing drive train stiffness, the stiffness adaptation of the controller ensures the required tracking performance.

Working space analysis and optimization of the main positioning system of FAST cabin suspension

This paper devotes to the working space analysis of the main positioning system of FAST cabin suspension, a flexible-cable-driven parallel manipulator. The problem formulation is deduced through equilibrium analysis of the cabin platform and suspension cables, which changes subsequently into a nonlinear constrained optimization intending a uniform allocation of the six cable tension force. The analysis verifies the accessibility of focus cabin to the whole focus surface. The optimization investigates the orientation of the focus cabin under equilibrium and the optimal cable forces, as well as elaborates their importance in the finite element modeling of the cable-cabin system and the respective layout designs of the rotator, Stewart stabilizer and capstan motors. In the end, the influences of the tower height and the position of mass center of the focus cabin on the optimization results are discussed.

Experimental comparison of nonlinear motion control methods for a variable stiffness actuator

Variable compliant actuators play a key role in the development of efficient biomechatronic systems since energy can be stored in the compliant element thus leading to consumption reduction. In this paper, experimental results comparing passivity-based control (PBC) and feedback linearization (FL) for motion control of an actuator with variable torsional stiffness (VTS) aiming at applications like prosthetic knee joints are presented. The concept of VTS and the experimental setup are described and a mathematical model of the latter one is derived. Based on this, a control architecture consisting of an extended Kalman filter (EKF) to estimate the velocities, a friction compensation as well as the mentioned controller types is developed. Both control methods are analyzed in terms of accuracy, dynamics and their control torque. FL and PBC lead to a stable control with high performance whereas the robustness is low by reason of the model-based control design. FL is superior to the PBC in terms of accuracy and control torque, which is mainly due to the high sensitivity of PBC regarding the discrete position signals. In addition, it is shown that FL can be applied for stable operation near the second natural frequency for different stiffness values.

High dynamics tilt estimation using simple observer

A tilt angle estimation was done using an sensor data fusion of inclinometer and gyroscope. The fusion was achieved using an observer structure with a simple feedback, where just advantageous sensor characteristics are extracted for the fusion signal. Therefore, a precise model of the inclinometer mechanics was derived. The observer was analyzed and designed to achieve high dynamic estimation of the real tilt angle. The fusion signal noise was analyzed due to gyroscope and inclinometer. The method was tested in a real test environment.

Autonomous Stair Climbing of a Wheeled Double Inverted Pendulum

Abstract The described wheeled double inverted pendulum was built to serve as a stair-climbing device (SCD). It can negotiate steps autonomously. The overall SCD model is represented by a hybrid automaton. It consists of nonlinear situation-changing continuous-time properties. Depending on the situation, the SCD is either fully actuated or under-actuated. Furthermore, discontinuous phenomena exist due to wheel-to-ground unilateral constraints. Feedback linearization is used as a basis for the control design. Due to a different situation-changing relative degree a full-state linearization or a partial linearization is applied. The state transition “settling” is developed within the virtual constraints framework.

Mobile inverse pendulum modeling and control

Modeling using Lagrange equation of 1st kind and state-space control of a real inverse pendulum is considered. The mathematical model of the pendulum is derived and compared with measurements of the real system. The negative influence of the dry friction was considered and the oscillation was successfully reduced.

Hybrid dynamic model of a wheeled double inverted pendulum

The presented stair climbing device (SCD) allows consideration of applications like stair-climbing wheelchairs and assistive robots. It can be considered as a wheeled double inverted pendulum which goes through different contact situations during the stair-climbing. The derived hybrid dynamic model covers the whole stair-climbing functionality. A model analysis is shown from the control point of view whereat the state transition “lift-off” is described in detail considering conditions and limits for successful transition. Presented simulation results give an insight into system properties. The “lift-off” simulation is compared with measurements of the real SCD prototype.

Proposed design concepts of the FAST focus cabin suspension

The National Astronomical Observatories of China (NAOC) plan to build a 500m radio telescope in southern China [1]. The telescope has a fixed but active main reflector, and large sky coverage is achieved by moving the receivers on a focus surface 160m above the main reflector. The paper describes recommended design concepts for the cable system, the drives and the cabin mechanisms, which position and point the receiver platform. The simulation study, which is basis of the presented results, was executed by engineers of the Technical University Darmstadt under a contract of NAOC in cooperation with two visiting engineers of NAOC and lead by the author [2]. The analysis results and end-toend simulations itself are described in more detail in two other contributions to this conference [3], [4].

Mechatronic System Integration Potential for Different Applications

Starting with some general and historical perspective, mechatronics is placed in the technological field map as a multidisciplinary discipline. The considered advantages of mechatronic systems are exposed from the human perspective, which enables the consideration of the social impact of this technological discipline. Due to the multidisciplinary character of mechatronics, the integration issues including synergetic effects consider both geometrical and functional aspects. The same reason provides a very large variety of different solutions to fulfill a certain functionality. Choosing an appropriate solution for a specific application is done using systematic methods which shape this problem to an optimization problem. General statements are illustrated with different application examples.

Trajectory control of cable suspended FAST telescope focus cabin

Operation of the Five-Hundred-Meter Aperture Spherical Telescope (FAST) requires accurate positioning and movement of the receiver platform on a spherical workspace with a radius of 160 m. Supported above the 500 m diameter main reflector it has to be positioned with an accuracy of several millimeters. To achieve this, the receiver is located in the receiver cabin that is suspended on six cables. The cables are attached to six towers located on the circumference of the main reflector and can be actuated via six capstans. In this paper a control concept for the cable-system is presented. Using a detailed mathematical model of the system the performance of the control and the sensitivity to wind and other disturbances is evaluated via simulation. The mechanics are modeled via FEM, the capstan-drives as lumped-mass elements including nonlinear effects like friction and backlash. The control scheme presented consists of position control loops for the capstans and numerically optimized PID-controllers for the positioning of the cabin platform.