Kristina Marković

Optimized cross-spring pivot configurations with minimized parasitic shifts and stiffness variations investigated via nonlinear FEA

ABSTRACT Compliant mechanisms are nowadays a well-established means of achieving ultra-high precision, albeit at the expense of complex kinematics with the presence of parasitic motions. Diverse design configurations of compliant rotational joints called cross-spring pivots are hence studied in this work by applying various analytical and numerical approaches. Depending on the required precision and loading conditions, the limits of applicability of the available analysis tools, validated with nonlinear finite element calculations tuned with experimental data reported in literature, are established. The variation of design parameters allows, in turn, establishing design configurations of the studied mechanism that allow attaining minimized parasitic shifts and slight variations of its rotational stiffness, even when a broad range of rotations and varying transversal loads are considered, creating thus the preconditions for their application in high-precision micropositioning applications.

Influence of Tip Relief Profile Modification on Involute Spur Gear Stress

This paper deals with the effects of linear tip profile modification on tooth root and tooth flank stress of involute spur gears. The increase in tooth flank stress due to tip profile modification is potentially hazardous due to increased risk of micro pitting initiation. The amount of required tip relief profile modification depends on the amount of tooth elastic deformation that needs to be compensated. In order to study the effects of linear tip profile modification on gear stress, two matching finite element models of involute spur gear pairs have been created, and tip relief profile modification has been applied to one pair. Tooth root and tooth flank stresses for both gear pairs have been compared in order to establish the effect of tip relief profile modification on gear stress.

Material behavior simulation of 42CrMo4 steel

It is becoming increasingly important to make possible constitutive modelling and simulation of material behaviour for the prediction of possible failures in material. This can allow to the optimization of design of highly loaded engineering components. In order to achieve that goal, material parameters should be accurately determined for the chosen material model. The major step in material parameters identification is material behaviour simulation. The procedure of material behaviour simulation is based on the results of the fatigue testing on the materials samples. The paper presents the procedures required for the material behaviour simulation of 42CrMo4 steel, starting from the fatigue testing, through numerical procedures related to complex material model, which results in material parameters identification, to the validation of described procedures by comparison of the simulated and real materials response in cyclic loading conditions.

Procedure for modelling of soft tissues behavior

The procedural steps for the efficient modelling and simulation of material behavior of human cervical spine ligaments have been presented in the paper. They are based on the mechanical principles incorporated in the material model, suitable for the description of soft tissues behavior. The material parameters set which have to be identified for the presented model has been additionally expanded here to make possible better calibration of the model by the application of genetic algorithm. The genetic algorithm has been recognized in this investigation as an efficient tool to overlap the bridge among the non-linearity of material behavior and material parameters of the chosen model and thus make possible material behavior modelling which follows materials response as accurately as possible. The basic process steps have been developed here for the efficient optimization of soft tissues behavior modelling.

Autonomous solutions for powering wireless sensor nodes in rivers

There is an evident need for monitoring pollutants and/or other conditions in river flows via wireless sensor networks. In a typical wireless sensor network topography, a series of sensor nodes is to be deployed in the environment, all wirelessly connected to each other and/or their gateways. Each sensor node is composed of active electronic devices that have to be constantly powered. In general, batteries can be used for this purpose, but problems may occur when they have to be replaced. In the case of large networks, when sensor nodes can be placed in hardly accessible locations, energy harvesting can thus be a viable powering solution. The possibility to use three different small-scale river flow energy harvesting principles is hence thoroughly studied in this work: a miniaturized underwater turbine, a so-called ‘piezoelectric eel’ and a hybrid turbine solution coupled with a rigid piezoelectric beam. The first two concepts are then validated experimentally in laboratory as well as in real river conditions. The concept of the miniaturised hydro-generator is finally embedded into the actual wireless sensor node system and its functionality is confirmed.

Characterization of cross-spring pivots for micropositioning applications

Compliant mechanisms gain at least part of their mobility from the deflection of flexible member. They are characterised by high precision, as well as no backlash and wear. Several analytical and numerical methods are used in this work to characterise the behaviour of compliant rotational mechanisms, known as cross-spring pivots, aimed at micropositioning applications. When ultra-high precision is required, the limits of applicability of approximated calculation algorithms have to be determined. The results obtained by employing these methods are thus compared with results obtained by using nonlinear finite element calculations tuned with experimental data reported in literature. The finite element model allows also considering the influence of lateral loads and of non-symmetrical pivot configurations where the angle or point of intersection of the leaf springs, or even the initial curvature of the springs, can be varied. The aim of this part of the work is to determine the influence of the cited design parameters on the minimisation of the parasitic shifts of the geometric centre of the pivot as well as on the minimisation of the variability of the rotational stiffness of the pivot so as to ensure its stability. The obtained results allow therefore determining design solutions applicable in ultra-high precision micropositioning applications, e.g. in the field of production or of handling and assembly of MEMS.