Teilinstitut Dynamik/Mechatronik - Forschung
Institut für Technische Mechanik

Adaptive Friction Dampers

Fidlin Fig. 1
Fidlin, Alexander, and Mauricio Lobos. "On the limiting of vibration amplitudes by a sequential friction-spring element." Journal of Sound and Vibration 333.23 (2014): 5970-5979
Fidlin Fig. 5
Fidlin, Alexander, and Mauricio Lobos. "On the limiting of vibration amplitudes by a sequential friction-spring element." Journal of Sound and Vibration 333.23 (2014): 5970-5979

Constantly increasing energy costs and restrictive legal regulations make it necessary to consistently increase the efficiency in all types of machines and systems. As a result, damping influences are systematically reduced. In addition, mechanical structures are increasingly being designed in terms of lightweight construction, which further increases their sensitivity to vibration excitation. That is why it is urgently necessary to reduce vibrations of mechanical structures effectively and in a focused manner without significantly influencing the efficiency of the machine as a whole. The nonlinearities of the damping forces, which are often neglected in the design, open up great potential for realizing situation-dependent behavior without having to resort to active control and external energy supply.

In particular, dry friction with the inherent stick-slide transitions enables the realization of mechanical switching elements that can serve as basic building blocks for adaptable dissipative devices. Above all, the aim is to investigate how novel devices based on dry friction can be used for the targeted reduction of externally excited, parametrically excited, and self-excited vibrations. In addition to the analysis of the dissipative device itself, methods for determining an optimal configuration and spatial placement of these devices are also proposed. In order to achieve these goals, various devices based on four basic elements (elasticity, play, friction with possibly modulated normal force and spring with distributed friction) and their combinations are compared in terms of their effectiveness and their self-adaptive properties. In addition, analytical methods are being developed in order to be able to reliably evaluate the efficiency of these devices. Prototypes for the most promising concepts are manufactured and experimentally tested.

Contact: Prof. A. Fidlin, J. Aramendiz

Contact Mechanics and Friction induced Vibrations

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Rough Surface
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Real contact area for two bodies with rough surfaces

Systems with friction are widespread in all kinds of applications. Unfortunatelly the precise simulative prediction of the resulting friction force in frictional contacts is still an unsolved task which has led to the developement of many empirical friction laws. Nevertheless, a deeper insight into the actual contact situation and the corresponding friction force with its dependency is necessary to improve technical systems e.g. with regard to energy efficiency and wear.

Early works on this topic by Greenwood & Williamson and Archard for the pure elastic and Bowden & Tabor for the pure plastic deformation case have at least led to a justification for Coulombs friction law. Their results indicated that the real contact area is almost proportional to the normal contact force whereat this relation can be attributed to surface roughness in both cases. Coulombs friction law can then be justified by the additional assumption that the friction force is proportional to the real contact area.

Further influences on contact forces, real contact area and friction coefficient besides surface roughness are investigated by contact simulations on microscopic scale which include for example temperature evolution and thermoelastic distortions due to frictional heat. Keeping in mind that a specific friction coefficient is always closely connected to the actual macroscopic system bevhaviour and vice versa investigations on the macroscopic system behaviour are performed as well.

Contact: Prof. C. Proppe, L. Oestringer

Design of bipedal robots with optimized energy efficiency in varying environments

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five link bipedal robot
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mean energy consumption per distance over different climbing inclination angles
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energy consumption per distance over different walking speeds

In the development of biped robots, energy efficient locomotion via walking or running is a major research priority. Due to the limited energy storage (battery), energy efficiency significantly determines the walking distance that can be covered. Energy efficiency depends not only on the controller used for stabilizing the motion, but also on the structural design and its mechanical model parameters. Hence, the purpose of this research is to develop and apply a method to systematically optimize the structure of a bipedal robot to maximize energy efficiency in different environments.

In the first step we consider an underactuated robot model which consists of five segments. Its rigid segments are additionally connected by elastic couplings such as torsion springs. Knowing that the robot’s periodic walking or running gaits can be analyzed as limit cycles of the controlled mechanical system, its resonance frequency can be adjusted to match the current step frequency by modifying the elastic couplings. A systematic approach to achieve this matching consists in the simultaneous optimization of the elastic couplings and the controlled motion. Consequently, the robot exploits the mechanical system's natural dynamics instead of wasting energy on its suppression.

Since the optimization of the parameters essentially depends on the conditions of the environment and the gait, the focus is on how the adaptation of the robot to the current operating state can be realized. Unlike the motion, which can be continuously updated by the controller based on the measured state variables, the elastic couplings need to be optimized to achieve the best characteristics by using "compliant smart mechanics" (COSM). Between the stance and swing phases of the robot’s legs, and especially during switching processes of the movement, an optimal (force-displacement and/or force-velocity) characteristic of COSM leads to an overall high energy efficiency for a wide range of walking scenarios.

This project is being carried out jointly by two research groups: The Compliant Systems Group (FG NSYS) from the Ilmenau University of Technology (TU Ilmenau) and the Institute of Engineering Mechanics (ITM) from the Karlsruhe Institute of Technology (KIT). While the team at TU Ilmenau investigates the compliant mechanical systems and their design and implementation, the team at KIT simulates and optimizes bipedal robots with those COSM mechanisms in different environments. It is financially supported by the German Research Foundation (DFG), grant FI 1761/4-1 | ZE 714/16-1.

 

Project partner: Prof. L. Zentner External Link, M. Zirkel External Link.

Contact: Prof. A. Fidlin, Dr. U. Römer, Y. Luo

Dynamics and Control of Hydraulic Systems

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Pressure-controled variable displacement vane pump
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Identified volume flow of a pressure relief valve

Hydraulic valves are known to show interesting dynamic behavior. Nevertheless they have not yet been investigated extensively from the viewpoint of nonlinear dynamics and are not suffciently understood. An elementary hydraulic pressure control valve can be described as a system of third order with a non-smooth nonlinearity.

The transition from an ideally impermeable valve to a valve allowing for leakage flow uncovers an instability mechanism for for certain valve geometries. Leakage changes the character of the equilibrium position from a set-valued equilibrium position to a unique one. A loss of stability of the equilibrium position and the birth of a limit cycle due to leakage can be shown when increasing leakage flow or the working point pressure of the system. A bifurcation analysis reveals the different solution types for the system under external forcing, yielding evidence of period-doubling phenomena up to quasi-periodic solutions.

Building on the findings for the dynamics of the foundational element in hydraulics - the valve - a variable displacement vane pump is currently investigated. This type of pump is frequently used in automotive engineering in order to provide the required pressure for the actuation of a clutch mechanism. A subfunction of the pump is to provide the volume flow required by a cooling unit. The system under investigation shows many aspects which are characteristic for hydraulic systems. In steady state, the valve regulating the system pressure is lapped critically, therefore giving rise to non-smooth dynamics. Apart from analyzing the stability behavior of the pump system in a first step, in a second step control strategies shall be devised that result in a change of the working point of the system. Drawing on the control strategies identified, the task then is to synthesize hydraulic elements and their topology such that the control strategies can be implemented by means of hydraulic action.

The hydraulic consumer provided with volume flow from the variable displacement vane pump is the third field of interest in this research project. As pointed out, it is a clutch actuation mechanism. In order to simulate the dynamics of clutch systems adequately, reasonable estimates of the system parameters have to be known. By means of Kalman filtering, important parameters of the clutch actuation mechanism can be identified. To do so, the clutch actuation system is subjected to transient volume flow excitation. Measurements of the system responses to the transient excitation are then synchronized with a slave model of the consumer, resulting in good estimates of the true parameters to be identified.

Contact: Prof. A. Fidlin, S. Schröders

Dynamics of Piezo Actuated Journal Bearings

Rotor-bearing system
Rotor-bearing system
Piezo actuation
Piezo actuated shaping

The dynamic behaviour of rotor-bearing systems represents an ongoing field of research.

By increasing the rotational speed of the rotor an instability can be detected which is often referred to as "oil-whirl" or "half-frequency-whirling" in literature. As the frequency of this "whirling" instability meets an eigenfrequency of the associated elastic rotor, its oscillation amplitudes increase tremendously which is also known as "oil-whip". These "oil-whirl" and "oil-whip" effects can be rated as rather critical and should be avoided during the operation of the rotor-bearing system.
Various modifications (compared to the "classical" cylindrical bearing) have been proposed in literature in order to suppress or at least to decrease these unwanted effects. By modifying the shape of the bearing sleeves improvements of the rotor's dynamic behaviour are expected. Starting from an initially circular shape, the bearing sleeve is elastically deformed by piezoelectric actuators which leads to a complex fluid-solid-interaction.

The effects of the piezo actuated shaping on the rotor-bearing system are studied by means of systematic stability and bifurcation analysis, focusing on time-efficient modelling methods.


Contact: Prof. W. Seemann, A. Bitner

Dynamics of Systems with Multiple Friction Contacts

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Two-Masses-Oscillator on a moving belt
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Stability Map
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Effective Friction Characteristic

Dry friction is present in many technical systems and is the reason for a variety of undesired phenomena. The property of non-smoothness and a negative slope at low relative velocities of the friction force may cause friction induced oscillations or stick-slip motion. One attempt to quench these oscillations are superposed high-frequency vibrations. Thus, the effective friction characteristic is smoothed and undesired fricion induced oscillations can be quenched. For systems with one degree of freedom, this method is investigated by simulations and experminents, which show good accordance. Systems with multiple degrees of freedom and mutliple friction contacts however show a largely richer dynamic behavior. Also, the influence of high-frequency ecxitation is not inverstigated yet. To improve the comprehension of such systems, numerical and analytical approaches are inquired.

Applied procedures are the excitation of single masses and investigating the effect on the whole system. Furthermore, the influence of the direction of the excitation and the importance of which masses are excited is examined.

Contact: Prof. W. Seemann, S. Keller

Friction Induced Vibrations in Shift Gearboxes

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Components of a shift gearbox: Pressure plate (PP), clutch disc (CD), gears (G1,G2), shafts
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Shift gearbox (elastic lamella)

A shift gearbox is a commonly used element in automotive transmissions, which is needed for two reasons. Firstly, it transmits the driving torque of the motor. Secondly, because a combustion engine works best at its nominal speed, it changes the gears. Experimental data reveal that undesired vibrations can arise during the clutch engagement process. The aim of this research is to explain the emergence of such vibrations in order to understand how to successfully prevent them.

Depending on the manifestation of the vibrations, several model approaches are thinkeable to explain the effect. A basic idea is to investigate the stability of the stationary behaviour and the origination of friction-induced vibrations, because the vibrations only occur while the disc contact is sliding. The physical modelling of the system is a flexible multibody system approach. Components taken into account are the clutch disc, gears, shafts and the actuation.

Well-known reasons for friction induced vibrations are clutch judder at low frequencies and the wobbling disc instability. They both cause vibrations perceiveable by customers, and which can be measured e.g. acoustically.

Another instability at medium frequencies arises because of the gear coupling, where the amplification of clutch friction forces induces an instability. In this case, strong translational vibrations of shafts occur with opening contacts in clutch and gears and stick-slip-transitions. Because of the translational movement, such vibrations can be measured in the actuation signal of the clutch.

Yet further instabilities can arise because of the interaction of elastic disc modes with rigid-body modes. The frequency of such instable behaviour is higher than the previous ones. Here too, translational vibrations occur in combination with elastic plate oscillations.

Contact: Prof. A. Fidlin

Gas Bearings in Rotordynamic Applications

Bearing model
Bearing model
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Rotor model
self-excited vibrations
Animation of self-excited vibrations

The two major advantages of gas bearings compared to other lubricated bearings are inherently low viscosities and their chemical stability over a wide temperature range. Furthermore, the usage of gas lubrication can lead to auxiliary benefits, depending on the application. For example, a simpler lubrication supply, a contamination free environment, less maintenance and less noise generation. All these reason could additionally lead to reduced manufacturing- and maintenance costs.

Unfortunately, in gas bearing mounted rotors are prone for instabilities. Reasons are, for example, the low viscosity of the fluid film, resulting in a low damping characteristic and self-excited vibrations, caused by the bearing reacting forces. Consequently, a proper designing- and dimensioning process of the rotor-bearing system can be very extensive.

In order to identify the physical effects, derivate design guidelines for rotor-bearing-systems and detect the limitations of gas bearing technology, gas bearing – rotor interactions are studied. Currently, the focus is on time-efficient nonlinear bearing modelling for rotor dynamic simulation, as well as stability- and bifurcation analysis of the rotor.

Contact: Prof. W. Seemann, T. Leister

Influence of Fast Motions on Systems with Dry Friction

Smoothing dry friction
1-DoF-friction oscillator with high-frequency excitation
Dynamic friction models
Accountig for contact compliance: Asperity model of two surfaces in contact

Undesired phenomena in systems with dry friction are well known. Especially for low relative velocities, non-smooth properties and velocity dependence of dry friction may lead to friction induced vibrations or stick-slip motion.

By imposing high-frequency vibrations to a system, the effective friction characteristics can be smoothed and, consequently, undesired friction induced phenomena can be quenched.

Considering for example revolute joints in positioning applications, high-frequency excitation may be provided using piezoelectric transducers. Smoothing the friction characteristics can reduce break-away forces significantly and thus improve accuracy.

In order to get deeper insight in the mechanism of smoothing dry friction, several aspects such as the direction, amplitude and frequency of excitation, pre-stress or material properties have to be studied. Focusing on different frictional materials, contact compliance and therefore dynamic properties of friction may become important. Dynamic friction models can be used in order to improve modeling and thus achieve better agreement with experimental results.

Contact: Prof. W. Seemann

Influence of the Microstructure of Metal Foams on Macroscale

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Volume element of metalic foam

Metal foams and their applications are of increasing interest in the area of research and development. High stiffness in conjunction with low density, high percentage of free surface within the volume, capacity of high energy absorbation and good damping properties of metal foams are relevant for light weight construction, chemical reactions, crash elements and vibration damping. Metal foams are used for example as car body elements, crash absorbers, fast moving vibrating devices, heat exchangers, catalysts, heat shieldings.

In dynamic applications it is necessary to describe eigenfrequencies and damping properties of metal foams. Due to the strongly developed inhomogenities of the microstructure of metal foams the material parameters and properties fluctuate.

The influence of the inhomogenities at the micro level to the macroscopic behavior is being researched. Two aspects are of main interest: The size effect and the standard deviation of the material parameters. The aim is to develop a simulation model that depicts the influence of the microstructural inhomogenities on the macroscopic behavior.

Contact: Prof. C. Proppe

Optimized wave propagation based on the example of percussion drilling

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Experimental results of an impact recorded with a high speed camera

Impacting rods are used in various devices in practical life but also for scientific experiments. Examples are rock drilling and piling machinery, while the Hopkinson split bar is used for testing materials. The typical arrangement is a primary rod with a tool at the end contacting the process material and a free tip, onto which a piston rod is hitting with a given kinetic energy. The hit is followed by a complicated sequence of phenomena. A stress wave, whose length is double the physical length of the piston rod, starts to propagate along the primary rod to finally reach the tool-process material interface. In case of rock drilling machine, a nonlinear penetration of the bit into the rock takes place and a reflecting wave component is generated. In the Hopkinson apparatus the test specimen experiences axial deformations and the stress wave is split into reflecting wave in the primary rod and into transmitted wave in the secondary rod beyond the test specimen. In the long history of rock drilling, fundamental mechanisms contributing to the effective drilling process are still unknown. To produce maximal tool penetration for each hit, rules have to be derived which specify the dimensioning of the piston and drill rod. Recent investigations, that cover the issue of optimization, are revealing that the efficiency of the drilling process strongly depends on the shape of the longitudinal wave transmitted through the drill rod to the drill bit. A well-known fact is that the cross-sectional profile of the piston is shaping the stress wave profile. Therefore, the main object of the research project is to adjust the geometrical form of the impacting piston for an optimized shape of the stress wave from which a maximal penetration follows.



 

Research Topic: Self-balancing of the planetary moving rotor

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Unbalanced rotor on the rigid carrier

The phenomenon of self-balancing of rigid rotors is well known and investigated for rotors with fixed bearings. However, in some technical devices the rotor performs complex motions. An example of such system is a computed tomography scanner. Its anode rotates very fast in the housing of the X-ray tube. At the same time the X-ray tube itself rotates rather slowly around the patient’s body. It is very important for CT scanner to keep the minimal possible level of vibrations in order to obtain good image quality. The objective is to investigate how and to which extent the self-balancing devices can be used for reducing vibrations in a planetary moving rotor.

The model to consieder consists of the rotor of mass M, which is fixed on the end of the rigid carrier. The other end of the carrier is elastically suspended with radial spring-dampers of a certain stiffness c and damping b. The carrier rotates around it’s point of suspension with a constant velocity . At the same time the rotor rotates around its symmetry axis with a given velocity ω. Its centre of mass has an offset relative to the rotation axis. Two pendulum balancers of mass m, moment of inertia J and length r are placed on the rotation axis of the rotor.

The stationary solutions of the system and the passage through the resonanceare are investigated analytically using averaging technique for the strongly damped systems. Analytic results match very well with numeric simulations when the velocity of the planetary motion is sufficiently small.

Contact: Prof. A. Fidlin, O.Drozdetskaya

Stochastic Analysis of Geometric Mistuning in Radial Compressor

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radial compressor
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Histogram of the maximum blade response amplitude obtained by Monte Carlo simulation
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Sector of an academic bladed disk with random geometry modifications

Radial compressor in turbochargers is often considered in theory as periodic system, but in fact it features inevitable small imperfections caused by material defects and manufacturing tolerances which break the cyclic periodicity. This is called mistuning. The loss of periodicity changes drastically the dynamic behavior of the compressor. Typically the forced response level of the mistuned bladed disk is larger than the tuned design. Because of the random nature of mistuning, the determination of the largest resonant response at any frequency has to be considered as a stochastic problem.

Mistuning receives significant attention from the research community since the late 1960s. Models using coupled lumped mass oscillators have allowed the fundamental phenomena of mistuning to be understood. More recently finite element model are used to explore with a better precision the behavior of mistuned compressor. To minimize the computational costs the finite element model has to be reduced first before performing a Monte Carlo Analysis. In the last decade several model reduction methods were developed. The way in which mistuning is implemented depends on the used reduction method and the vast part of them accounts only for a frequency mistuning model or a proportional mistuning model, in which the ideal tuned configuration is not modified.

Today a key topic is to combine this reduction methods with a more realistic introduction of mistuning, such as geometric mistuning. In other words, it will be tried to take directly random geometry modifications into consideration in a stochastic problem.

Contact: Prof. C. Proppe, M. Koebele

Surrogate models for uncertainty quantification for the forecast of the West African Monsoon

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Hourly rainfall [in mm] as a result of model simulation runs
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Cuts through the hyperplane of the surrogate model as a tool to visualize the quantity of interest (vertical axis) and confidence intervals with respect to each input parameter (horizontal axis)

Surrogate modeling is a method that can be applied if quantities of interest cannot be easily directly measured or simulated, e.g. if a simulation run or experiment is very expensive. In this case, a surrogate model for the outcome is obtained and used instead. Due to increasing complexity of models, surrogate modeling is playing an increasing role in various engineering, but also other scientific disciplines. In this work, a meteorological problem is analyzed in collaboration with the Institute of Meteorology and Climate Research - Department Troposphere Research at KIT.


The West African monsoon is a major wind system that affects regions between latitudes 9° and 20° N. The monsoon is the result of the seasonal shifts of the ITCZ (Intertropical Convergence Zone) and seasonal temperature and humidity differences between the Sahara and the equatorial Atlantic Ocean. The forecast of this monsoon has shown to suffer from remarkable uncertainties in several quantities. Some of these quantities are the local rainfall and the north-south shift which have a great impact on the inhabitants, particularly on the agriculture. In order to quantify the uncertainty in the forecast of the West African monsoon, a sensitivity analysis for a range of uncertain parameters in the weather model is conducted.


For this study the ICON model which is operationally used by the Deutscher Wetterdienst (DWD) is applied to carry out weather simulations. Since for the analysis a steady state for the monsoon quantities over simulated days is intended, the simulated time and thus the computational cost for one simulation run is very high. Therefore, surrogate models seem to be a promising opportunity. In this work, Gaussian Process Surrogates are used to achieve a relation between input parameters and monsoon quantities. The surrogate model is then used to carry out a global sensitivity analysis given defined ranges and probability density functions for all parameters. The results can offer an indication which parameter definitions should be specified more detailed by conducting further studies in order to reduce the uncertainty in forecasted monsoon quantities. Furthermore, the surrogate model can serve as a basis for parameter identification studies.

Contact: Prof. C. Proppe, M. Fischer

Transient Behaviour of Coupled Exciters

Two coupled exciters
Two coupled exciters

Application of several unbalanced exciters instead of one is a widely used design for vibratory machines. This may have the benefit of distributing the excitation along the machine or decreasing the load of the exciter’s bearing, if several low power exciters are used instead of one powerful exciter. It also offers the possibility of coordinating rotors dynamics without kinematic connections between them. The self-synchronization phenomenon, which allows the automatic coordination of exciter rotations, is utilized in many vibratory machines for generating the required excitation force with constant or another required (for example elliptically changing) direction.

Even though stationary solutions of many self-synchronising systems, their condition of existence and stability has been extensively investigated, little is known about the transient behaviour about such systems. Attraction domains of already known stationray solutions and behaviour during passage through resonance are the two main topics of the investigation. Asymptotic methods, which are used to analyse systems with a single unbalanced exciter, will also be applied here.

Contact: Prof. A. Fidlin, T. Yüzbasioglu