Peter Lugner

Systemdynamik und Regelung von Fahrzeugen

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Recent advances in tyre models and testing procedures

Owing to the advances in the simulation techniques of vehicle development and design, the modelling of the tyre is of special importance. Thereby, not only the reliability of quantitative results but also the extension to higher frequency ranges and the possibility to account for local road surface structures smaller than the tyre patch is becoming a necessity. A description of some tyre models and the required features for these last two aspects are presented. Reliable modelling of tyre characteristics based on measurements and efficient application with multi-body system (MBS) programs for vehicle dynamics simulation are checked with the ‘Tyre Model Performance Test (TMPT)’. With the TMPT handling and high frequency behaviour of tyre models are tested with a virtual test rig implemented in three different MBS software programs. Also, the validation of measurements is combined with specific capability tests to show the range of application of the tyre models. After an outline of the test conditions, the participating tyre models are introduced. A selection of results offers the possibility to compare the tyre models, the tyre model—MBS software combinations and simulation results with measurements. It becomes obvious that a thorough check of the tyre model and the interaction with the MBS software is essential to be aware of the range of application of simulations and the quality of the results.

Test bed with force-measuring crank for static and dynamic investigations on cycling by means of functional electrical stimulation

Cycling by means of functional electrical stimulation (FES) is an attractive training method for individuals with paraplegia. The physiological benefits of FES are combined with the psychological incentive of independent locomotion. In addition, cycling has the advantage in that the generated muscle forces are converted into drive power with relatively high efficiency compared to other means of locomotion, e.g., walking. For the design of an appropriate cycling device and the development of optimal stimulation patterns, it has to be investigated how the geometry for FES cycling, influenced by individual parameters of the FES-generated drive torques and the magnitude of variations among subjects with paraplegia, can be optimized. This study shows the design of a freely adjustable test bed with additional motor drive which allows static and dynamic measurements of force components and drive torque at the crank. Furthermore, the influence of geometry and various individual parameters on FES pedaling can be tested for each subject individually. A pedal path realized by a three-bar linkage that was optimized according to preliminary simulations further increases leg cycling efficiency. Safety precautions avoid injuries in case of excessive forces, e.g., spasms. Test results illustrate the application of the test bed and measurement routines. A test series with four paraplegic test persons showed that the presented static and dynamic measurement routines allow to provide optimal stimulation patterns for individual paraplegic subjects. While pedaling with these optimal stimulation patterns only negligible negative active drive torques, due to active muscle forces, were applied to the crank and sufficient drive power was generated to power a cycle independently.

Dynamic simulation of FES-cycling: influence of individual parameters

Cycling by means of functional electrical simulation (FES) is an attractive training method for spinal cord injured (SCI) subjects. FES-cycling performance is influenced by a number of parameters like seating position, physiological parameters, conditions of surface stimulation, and pedaling rate. The objective of this paper was the determination of the influence of the most important parameters on optimal muscle stimulation patterns and power output of FES-cycling on a noncircular pedal path. The rider-cycle system was modeled as a planar articulated rigid body linkage on which the muscle forces are applied via joint moments and implemented into a forward dynamic simulation of FES-cycling. For model validation, the generated drive torques that are predicted by the simulation were compared to measurements with an individual paraplegic subject. Then, a sensitivity analysis was carried out to determine the influences of the most important parameters for surface stimulation of gluteus maximus, quadriceps, hamstrings, and peroneus reflex. The results show how optimal stimulation patterns and the expected mean active power output can be estimated based on measured individual parameters and adjusted geometry and stimulation parameters for a particular SCI-subject. This can considerably improve FES-cycling performance and relieve the patients by shortening the time that is necessary for experimental adaptation of the stimulation patterns.

Comfort Enhancement By an Active Vibration Reduction System for a Flexible Railway Car Body

In order to improve the ride comfort of lightweight railway vehicles, an active vibration reduction system using piezo-stack actuators is proposed and studied in simulations. The system consists of actuators and sensors mounted on the vehicle car body. Via a feedback control loop, the output signals of the sensors which are measuring the flexible deformation of the car body generate a bending moment, which is directly applied to the car body by the actuators. This bending moment reduces the structural vibration of the vehicle car body. Simulations have shown that a significant reduction in the vibration level is achieved.

Tyre model performance test: first experiences and results

The tyre model performance test (TMPT) provides information about the efficiency and problems of the simulation sequence from tyre measurements (tyre modelling and parametrization) to multibody-system (MBS) software integration. Based on test rig tyre measurements with respect to vehicle-handling behaviour and higher-frequency-range tests (running over a cleat; vibration modes), tyre models have to be provided in a way that they can be connected with the standard Tyre interface (STI) or a modified tyre interface to at least one of the three MBS programs ADAMS, DADS and SIMPACK. The test conditions and the simulation test rig were defined in advance by an international group of experts. Four tyre model providers participated for the handling (low-frequency) manoeuvres and four for the low- and higher-frequency-ranges. Since a planned feedback from the MBS simulation results (they were done by the MBS-program providers) to the tyre model providers was not completely finished, the results shown are preliminary only. For the steady-state behaviour, all tyre models show good agreement with measurements for lateral and longitudinal tyre forces while, for the self-aligning torque, larger differences appear. For ‘crossing of a cleat’, all high-frequency-range models show good agreement with measurements as well but, for some tests (e.g. sinusoidal sweeps), qualitative differences even between MBS programs for the same tyre model can be noticed. This indicates that the application of tyre models within MBS programs requires some preliminary testing and maybe additional checks before using them for vehicle dynamics analysis.

STABILITY CONTROL OF A PASSENGER CAR BY COMBINED ADDITIONAL STEERING AND UNILATERAL BRAKING

SUMMARY To investigate the combined control of the side slip angle and yaw velocity of a vehicle in critical driving situations a complex vehicle model (developed in ADAMS) with ABS and adequate tyre characteristics are used. The control loop includes an observer to estimate the side slip angle of the vehicle and a ‘sliding mode controller’ based on a simplified 2-wheel and 4-wheel vehicle model respectively. The output of the controller is one of the three combinations containing two of the possible actuator actions: additional steering at the front, at the rear and in combination with unilateral braking, an additional yaw moment. This yaw moment has to be transformed via corresponding longitudinal slip values into braking moments. Simulation results for a severe steering step input and braking during cornering very well illustrate the advantages of all three investigated control strategies. Only for heavy braking during cornering (for more than 0.7g longitudinal deceleration on a dry surface) the combination unilateral braking with additional steering of the front wheels is less favourable as a result of the control limitations due to the dominant ABS.

Mechanical factors responsible for the obstruction of the gliding mechanism of a dynamic hip screw for stabilizing pertrochanteric femoral fractures.

BACKGROUND In treatment of pertrochanteric femoral fractures with dynamic hip screws (DHSs) (135-degree, Synthes, Bettlach, Switzerland), damage was observed in removed lag screws, leading to the conclusion that the gliding mechanism must have been obstructed as a result of either inappropriate position of the implant or insufficient medial support in the fracture zone. METHODS The forces and moments transmitted in the screw socket are calculated using a mathematical model to find the optimal position of the implant. RESULTS The forces and moments depend on the position and orientation of the lag screw as well as on the position of the contact point between the two main fragments. By changing the point of contact, a better decrease of the load to the DHS can be achieved than by changing the position and orientation of the screw. For a low contact point, the model shows the lowest values for the forces in the socket. CONCLUSION Complete agreement was found between the results of the presented calculations and our own clinical experience in removed DHSs.

Modeling of the Ski-Snow Contact for a Carved Turn

Carved turns with alpine skis are investigated. During the movement of a ski, snow is loaded and unloaded. Compacted snow is not elastic, i.e. deformations remain. Such effects are modeled by a hypoplastic constitutive equation. During a turn the shovel digs into the snow and the tail maintains nearly the same penetration depth as the part under maximum load. This results in a higher resistance against shearing for the afterbody of the ski. In the present work we investigated the benefits of the hypoplastic against the elastic forcepenetration relationship. Simulation results for a sledge on two skis are compared to experimental track data.

MODELING OF ARTIFICIALLY ACTIVATED MUSCLE AND APPLICATION TO FES CYCLING

Functional Electrical Stimulation (FES) enables paraplegics to move their paralyzed limbs; the skeletal muscles are artificially activated. The purpose of this study is to establish a mechanical muscle model for an artificially activated muscle, based on a Hill-type muscle model. In comparison to modeling a physiologically activated muscle, for the artificially activated muscle, a number of additional parameters and their influence on the force generation has to be considered. The model was implemented into a forward dynamic simulation of paraplegic cycling. The stimulation patterns were optimized for surface stimulation of gluteus maximus, quadriceps, hamstrings, and peronaeus reflex. A simulation of a startup with 50% of maximum activation in the optimized stimulation intervals analyses drive torques and mean power per cycle and the resulting riding performance of the rider-cycle system. For validation of the simulation, the results were compared to measurements of the forces applied to the crank during steady-state cycling of a paraplegic test person.