Saša Zelenika

Analytical and experimental characterisation of high-precision flexural pivots subjected to lateral loads

Abstract This work addresses the parasitic motion of high-precision rotation mechanisms based on flexural pivots subjected to lateral loads. This case has great importance from the mechanical design point of view, since generally flexural pivots support mechanical elements of considerable weight and their rotation is obtained by loading the pivot with a force instead of a pure couple. From an analytical point of view, the problem is approached by studying the large deflections of an elastic frame. The equilibrium equations are considered and a solution based on the Newton–Raphson method is proposed. This approach is compared with other theoretical approaches. An experimental assessment performed by using laser interferometric techniques is presented. It is shown that the proposed solution allows the influence of lateral loads to be clearly established and proves to be adequate when the most common cases of limited lateral loads and rotations are considered.

A generalized elastica-type approach to the analysis of large displacements of spring-strips

Abstract An elastica-type analytical solution to the problem of large deflections of slightly curved spring-strips, fixed at one end and loaded at the other with couples and with forces of various directions, is obtained in this work. The main methods of calculation of elliptic integrals are studied, and the limits of their applicability are established as functions of the required degrees of accuracy and of the loading conditions of the spring-strips. The results obtained with the proposed method are then compared to particular cases already developed in the literature with different approaches. It is shown that in all the cases considered the method permits accurate results to be obtained.

Analytical characterization and experimental validation of performances of piezoelectric vibration energy scavengers

One of the main requirements in wireless sensor operation is the availability of autonomous power sources sufficiently compact to be embedded in the same housing and, when the application involves living people, wearable. A possible technological solution satisfying these needs is energy harvesting from the environment. Vibration energy scavenging is one of the most studied approaches in this frame. In this work the conversion of kinetic into electric energy via piezoelectric coupling in resonant beams is studied. Various design approaches are analyzed and relevant parameters are identified. Numerical methods are applied to stress and strain analyses as well as to evaluate the voltage and charge generated by electromechanical coupling. The aim of the work is increasing the specific power generated per unit of scavenger volume by optimizing its shape. Besides the conventional rectangular geometry proposed in literature, two trapezoidal shapes, namely the direct and the reversed trapezoidal configuration, are analyzed. They are modeled to predict their dynamic behavior and energy conversion performance. Analytical and FEM models are compared and resulting figures of merit are drawn. Results of a preliminary experimental validation are also given. A systematic validation of characteristic specimens via an experimental campaign is ongoing.

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.

CHARACTERIZATION OF HIGH PRECISION PARALLEL SPRING TRANSLATORS

This work deals with the characterization of parallel spring translators for high precision applications; both the cases of the simple and of the compensated elastic suspensions are considered. Analytical methods for the evaluation of “parasitic” deflections are compared with the aim to determine the relative limits of applicability depending on the required degree of accuracy. These methods allow to improve the mechanical design minimizing the departures from rectilinearity. Experimental verifications employing an interferometric technique were performed confirming the analytical results. A large bibliographical reference is given with the aim to help the reader to get a broader insight on the subject.

Nano-positioning using an adaptive pulse width approach:

Abstract Macro- and micro-dynamic mechanical non-linearities limiting the precision of conventional DC motor-driven positioning systems based on sliding and rolling elements have been characterized experimentally via a laser interferometric system. The obtained results confirm recent tribological models. In particular, the parameters describing the non-linear elastic and plastic phenomena related to pre-sliding displacement have been identified and used to develop an integrated system model. It was therefore possible to prove that, in the range of displacements corresponding to the pre-sliding phase, there is a quadratic dependence between the duration of an actuating impulsive force and the resulting displacement. According to a pulse width scheme, a control law has been implemented whose adaptive structure compensates for the position and time variability of the observed non-linear effects. The application of the proposed approach to both short and long travel ranges and the reached nanometric accuracies confirm the applicability of the proposed control scheme to compensate concurrently the macro- and the micro-dynamic effects.

Load optimised piezoelectric generator for powering battery-less TPMS

The design of a piezoelectric device aimed at harvesting the kinetic energy of random vibrations on a vehicle’s wheel is presented. The harvester is optimised for powering a Tire Pressure Monitoring System (TPMS). On-road experiments are performed in order to measure the frequencies and amplitudes of wheels’ vibrations. It is hence determined that the highest amplitudes occur in an unperiodic manner. Initial tests of the battery-less TPMS are performed in laboratory conditions where tuning and system set-up optimization is achieved. The energy obtained from the piezoelectric bimorph is managed by employing the control electronics which converts AC voltage to DC and conditions the output voltage to make it compatible with the load (i.e. sensor electronics and transmitter). The control electronics also manages the sleep/measure/transmit cycles so that the harvested energy is efficiently used. The system is finally tested in real on-road conditions successfully powering the pressure sensor and transmitting the data to a receiver in the car cockpit.

Design of Compliant Micromechanisms

A broad overview of the topics related to the mechanical design of compliant micromechanisms is presented. Design methodologies to be used in the design of devices based on leaf springs, flexural notches and continuum structures with distributed compliance are given, and a critical presentation of the peculiarities of these solutions is provided. The extensive bibliographical list is given as means to extend further the study to details of each of the treated topics.

Nanometric positioning accuracy in the presence of presliding and sliding friction: Modelling, identification and compensation

ABSTRACT Presliding and sliding frictional effects, limiting the performances of ultrahigh precision mechatronics devices, are studied in this work. The state-of-the-art related to frictional behavior in both motion regimes is, hence, considered, and the generalized Maxwell-slip (GMS) friction model is adopted to characterize frictional disturbances present in a micromanipulation device. All the parameters of the model are identified via experimental set-ups and included in the overall MATLAB/SIMULINK model. With the aim of compensating frictional effects, the modelled response of the system is thus compared to experimental results when using proportional-integral-derivative (PID) control, feed-forward model-based compensation and a self-tuning adaptive regulator. The adaptive regulator proves to be the most efficient and is, hence, used in the final repetitive point-to-point positioning tests allowing to achieve nanometric precision and accuracy.

Comparison of different DC motor positioning control algorithms

A comparison between different DC motor positioning control algorithms is performed in this work. Transient responses while employing a PID controller, a cascade controller and a state-space controller are considered. LabVIEW programming environment with a suitable acquisition card and a miniature DC motor with an integrated encoder are used for experimental assessment. Calculations and control system simulations are made using Matlab. The PID controller is implemented via the predefined PID block in LabVIEW. In turn, the state-space controller is modelled by using Matlab while the accuracy of the results is confirmed experimentally using LabVIEW. The cascade controller is developed as a series of two Proportional-Integral (PI) controllers, one representing the positioning and the other the velocity loop. The obtained results allow establishing that positioning control via the state-space controller has the fastest response and the lowest settling times.