János Zierath

HiL simulation in biomechanics: A new approach for testing total joint replacements

Instability of artificial joints is still one of the most prevalent reasons for revision surgery caused by various influencing factors. In order to investigate instability mechanisms such as dislocation under reproducible, physiologically realistic boundary conditions, a novel test approach is introduced by means of a hardware-in-the-loop (HiL) simulation involving a highly flexible mechatronic test system. In this work, the underlying concept and implementation of all required units is presented enabling comparable investigations of different total hip and knee replacements, respectively. The HiL joint simulator consists of two units: a physical setup composed of a six-axes industrial robot and a numerical multibody model running in real-time. Within the multibody model, the anatomical environment of the considered joint is represented such that the soft tissue response is accounted for during an instability event. Hence, the robot loads and moves the real implant components according to the information provided by the multibody model while transferring back the position and resisting moment recorded. Functionality of the simulator is proved by testing the underlying control principles, and verified by reproducing the dislocation process of a standard total hip replacement. HiL simulations provide a new biomechanical testing tool for analyzing different joint replacement systems with respect to their instability behavior under realistic movements and physiological load conditions.

Load Calculation on Wind Turbines: Validation of Flex5, Alaska/Wind, MSC.Adams and SIMPACK by Means of Field Tests

Load calculations on wind turbines are an essential part of its development. In the preliminary design phase simplified multibody models are used for the estimation of the interface loads. The interface loads are used within an iterative development loop to design the components of the wind turbine such as gearbox, blades, tower and so on. Due to the early application of load calculations within the development process, the quality of the simulation results has a great influence on the wind turbine design.In this contribution the simulation results of the multibody codes alaska/Wind, MSC.Adams and SIMPACK are compared with measurements obtained from a prototype of a 2.05 MW wind turbine developed by W2e Wind to Energy. Furthermore, simulation results of the special wind turbine design code Flex5, developed at the Technical University of Denmark Copenhagen, are taken into account. A statistical and dynamical evaluation of the simulation and measurement results has been done. Due to the use of the same controller procedures as used on the physical wind turbine, the wind turbine models show almost the same behaviour (electrical power, pitch angle, rotor speed) as the wind turbine in the field. Differences occur during the evaluation of the interface loads due to the different kinds of wind turbine modelling.Copyright

Robot-Based HiL Test of Joint Endoprostheses

To simulate the dislocation behavior of total hip endoprostheses in their anatomical environment a novel Hardware-in-the-Loop (HiL) simulator is built up. It couples a real endoprosthesis with a numerical simulation of its environment by means of an industrial robot as actuator system. The simulation model describes the dynamics of the biomechanical motions including the tissue and muscle forces. The motion and joint constraint forces are calculated by the simulation model and applied to the endoprosthesis by the robot under hybrid position/force control. The actual position of the endoprosthesis in the constrained directions and torques in the unconstrained directions are measured and fed back into the simulation model closing the control loop. To demonstrate the functional principle of the HiL simulator the dynamic behavior of a test setup is numerically simulated.

Comparison and Field Test Validation of Various Multibody Codes for Wind Turbine Modelling

The decrease of fossil energy sources leads to an increased use of renewable energy sources like wind energy. The design of the mechanical components of a wind turbine is considerably governed by their fatigue behaviour over the product life cycle. Therefore, reliable estimations of the interface loads on the components, by means of appropriate multibody models, are necessary. While simplified wind turbine design codes, such as Flex5 or GH Bladed, have been mainly used for previous wind turbine developments, general purpose multibody simulation environments like MSC.Adams, SIMPACK, or alaska/Wind in combination with specific aerodynamic simulation packages are now applied. Here, the components of a wind turbine, such as the drive train with gear pair contacts, a flexible main frame, or a lattice tower, can be modelled in much more detail and specific manner compared to previous simulation models. For example, the geometric nonlinear behaviour of the blades can be taken into account which is essential for the simulation of long slim blade designs. In the present contribution, different multibody packages for wind turbine modelling are compared on the basis of simulation models of an existing wind turbine. Beside of the special wind turbine design code Flex5, developed at the Technical University of Denmark Copenhagen (DTU), the commercial multibody simulation packages MSC.Adams and SIMPACK are used. For the validation of the simulations, extensive measurements on a wind turbine prototype have been evaluated comprising measurement data over a period of more than 1.5 years. To compare measurements and simulations, statistical and dynamical evaluations of the results have been done.

Contact Modelling in Multibody Systems by Means of a Boundary Element Co-simulation and a Dirichlet-to-Neumann Algorithm

The present contribution introduces the modelling of elastic contacts by coupled multibody an boundary element systems. Compared to contacts modelled by impact laws, physically more accurate results can be obtained. Due to the use of boundary element systems, the contact stresses are obtained within the contact calculation.

A Novel Approach for Dynamic Testing of Total Hip Dislocation under Physiological Conditions

Constant high rates of dislocation-related complications of total hip replacements (THRs) show that contributing factors like implant position and design, soft tissue condition and dynamics of physiological motions have not yet been fully understood. As in vivo measurements of excessive motions are not possible due to ethical objections, a comprehensive approach is proposed which is capable of testing THR stability under dynamic, reproducible and physiological conditions. The approach is based on a hardware-in-the-loop (HiL) simulation where a robotic physical setup interacts with a computational musculoskeletal model based on inverse dynamics. A major objective of this work was the validation of the HiL test system against in vivo data derived from patients with instrumented THRs. Moreover, the impact of certain test conditions, such as joint lubrication, implant position, load level in terms of body mass and removal of muscle structures, was evaluated within several HiL simulations. The outcomes for a normal sitting down and standing up maneuver revealed good agreement in trend and magnitude compared with in vivo measured hip joint forces. For a deep maneuver with femoral adduction, lubrication was shown to cause less friction torques than under dry conditions. Similarly, it could be demonstrated that less cup anteversion and inclination lead to earlier impingement in flexion motion including pelvic tilt for selected combinations of cup and stem positions. Reducing body mass did not influence impingement-free range of motion and dislocation behavior; however, higher resisting torques were observed under higher loads. Muscle removal emulating a posterior surgical approach indicated alterations in THR loading and the instability process in contrast to a reference case with intact musculature. Based on the presented data, it can be concluded that the HiL test system is able to reproduce comparable joint dynamics as present in THR patients.

Robot-Based Testing of Total Joint Replacements

Instabilities of artificial joints are prevalent complications in total joint arthroplasty. In order to investigate failure mechanisms like dislocation of total hip replacements or instability of total knee replacements, a novel test approach is introduced by means of a hardware-in-the-loop (HiL) simulation combining the advantages of an experimental with a numerical approach. The HiL simulation is based on a six-axes industrial robot and a musculoskeletal multibody model. Within the multibody model, the anatomical environment of the correspondent joint is represented such that the soft tissue response is considered during an instability event. Hence, the robot loads and moves the real implant components according to the data provided by the multibody model while transferring back the relative displacement of the implant components and the resisting moments recorded. HiL simulations provide a new biomechanical testing tool which enables comparable and reproducible investigations of various joint replacement systems with respect to their instability behaviour under realistic movements and physiological load conditions.

Optimal sensor placement for modal testing on wind turbines

The mechanical design of wind turbines requires a profound understanding of the dynamic behaviour. Even though highly detailed simulation models are already in use to support wind turbine design, modal testing on a real prototype is irreplaceable to identify site-specific conditions such as the stiffness of the tower foundation. Correct identification of the mode shapes of a complex mechanical structure much depends on the placement of the sensors. For operational modal analysis of a 3 MW wind turbine with a 120 m rotor on a 100 m tower developed by W2E Wind to Energy, algorithms for optimal placement of acceleration sensors are applied. The mode shapes used for the optimisation are calculated by means of a detailed flexible multibody model of the wind turbine. Among the three algorithms in this study, the genetic algorithm with weighted off-diagonal criterion yields the sensor configuration with the highest quality. The ongoing measurements on the prototype will be the basis for the development of optimised wind turbine designs.

Development of a Three-Dimensional Musculoskeletal Model for the Hardware-in-the-Loop Joint Simulation

Dislocation of artificial joints causes serious complications after total hip arthroplasty. There are various factors influencing the dislocation process related to implant component parameters as well as physiological conditions. Because in vivo measurements of the dislocation process are not possible, a novel, mechatronic hardware-in-the-loop joint simulator was recently introduced connecting a physical test setup with a computer simulation running in real-time. The purpose of this work is the development of the required multibody model providing the musculoskeletal structure of the lower limb. The objective of this model is the prediction of hip joint reaction forces tested for a standing-up motion.