Modeling and optimization of ultrasonic linear motors

Ultrasonic motors have received much attention these last years, in particular with regard to their modeling and their design principle. Their operating principle is based on piezoelectric ceramics that convert electrical energy into mechanical energy in the form of vibrations of an elastic body whose surface points perform an elliptic motion with a frequency in the ultrasonic range (≥ 20 kHz). The moving part, which is pressed against the vibrating body by a prestressing force, can move thanks to the friction forces presented at the interface between the stator (resonator) and the rotor (slider). Their specific properties make ultrasonic motors a very attractive solution for a direct transmission for different applications like precise positioning devices. Indeed, they present the possibility to obtain unlimited motions, high resolution and excellent dynamics of positioning. Then, it is obvious that ultrasonic motors could be used in new application fields, in particular to replace conventional electromagnetic motors. However, they have to overcome two principal difficulties: their efficiency is rather poor and they are often too expensive. Moreover, their use in the car industry or for the positioning of axes in machine tools for example requires driving forces and velocities higher than those which they currently present. Analytical modeling of such motors is not obvious and assumptions that are made are often too restrictive. This is why the use of a numerical modeling (3-D) is necessary to model the behavior of this type of motors. Thus, finite element simulations are used but they often require high computing times. To avoid it, the number of simulations can be decreased by choosing the input parameters (dimensions, materials, boundary conditions,...) more judiciously according to their influence on the output parameters. Thus, one can obtain the sensitivity of an input parameter on the value of the output parameter. With this intention, the application of design of experiments has been adopted in this thesis work. This methodology, applied to finite element simulations, is an innovative technique in the field of theoretical modeling of such motors. This methodology is particularly interesting in sight of predicting the results but also to find out an optimal set of input parameters for the motor. According to the results obtained and presented in this thesis work, the use of design of experiments in the field of ultrasonic motors modeling proves to be very promising and demonstrates to be a powerful tool. The application of the proposed methodology for the optimization of an ultrasonic linear motor used for the auto-focus function of the lens of an optical system also made it possible to show the validity and the potential of this optimization method.

Sensorless Position Control of Piezoelectric Ultrasonic Motors

Markus Flückiger

This dissertation considers mechatronic systems driven by piezoelectric ultrasonic motors (PUM). The focus is set on optimal system design and sensorless position control. Mechatronic industry faces the challenge to deliver ever more efficient and reliable products while being confronted to increasingly short time to market demands and economic constraints driven by competition. Although optimal design strategies are applied to master this challenge, they do not entirely respond to the given circumstances, as often only local criteria are optimised. In order to obtain a globally optimal solution, the many subfunctions of a mechatronic system and their models must be interrelated and evaluated concurrently from the very beginning of the design process. In this context PUM have been used increasingly during the last decade for various positioning applications in the field of mechatronic systems, laboratory equipment, and consumer electronics where their performances are superior to conventional electromechanical drive systems based on DC or BLDC motors. The position of the mobile component must be controlled. In some cases open-loop control is a solution, but more often than not sensors are used as feedback device in closed-loop control. Sensors are expensive, large in size and add fragile hardware to the device that compromises its reliability. Thus, not only the superior performance is not fully exploited but also the economical feasibility of the PUM drive system is jeopardised. Replacing sensors by advanced control techniques is an approach to these problems that is well established in the field of BLDC motors. Those sensorless control strategies are not directly transferrable, because of the fundamentally different working principles of PUM. Hence, the research of sensorless closed-loop position control techniques applicable to PUM and their validation with digitally controlled functional models is the very topic of this thesis. We propose a dedicated design methodology to this statement of the problem. A core model of the mechatronic system is conceived as general and simple as possible. It then develops for the different interrelated views reflecting the mechanical, electromechanical, drive electronic, sensorial and digital control functions of the global system. Each one becoming more specific and detailed in this process, the different views still enable mutual constraint adjustments and the dynamic integration of results from the other views during the design process. Starting with the stator of the PUM, a view describes the mechanical displacement. An electric equivalent model is written such that power input from the drive electronics is related to the mechanical energy transmitted to the mechanics. The resulting differential equations are solved by the finite element method (FEM). Position feedback configurations in the mobile part of the PUM are modelled analytically in order to be implemented in digital control and their electrical implications are updated to the stator model. In this way, sensors do not necessarily materialise physically any more, but are distributed among the mechanical configuration, the drive electronics and the digital controller. With respect to the sensor data, the controller is not simply receiving finalised data on the measured system parameter, but rather implements the sensor itself in software. Finally, the position detection performance obtained with the aforementioned design methodology was evaluated with the example of mechatronic locking devices actuated by custom-made as well as OEM motors. Functional models of motors, electronics and digital controllers were used to identify the limits of the proposed methods, and suggestions for further research were deduced. These results contribute to the development of robust sensorless position controllers for PUM.

EPFL2010

Analytical and numerical modeling of an ultrasonic stepping motor using standing waves

José Fernandez Lopez, Yves Perriard

Piezoelectric motors (usually called ultrasonic) show interesting characteristics. Not only they deliver high torques at low speeds but their structure confers to them a braking torque without energy supply. This paper firstly present a list of possible solutions for this type of motors. Thereafter, the most promising solution is chosen and developed more in detail. Finite elements simulations of the vibration modes of the motor are carried out with the aim of determining the resonance frequency of the system for the selected mode. A theoretical modeling of the operating principle of the motor is established in order to be able to characterize the position, speed and torque according to the selected parameters. Lastly, the feasibility and the characteristics of such a motor are discussed. © 2004 IEEE.

2004

Internal electric fields and electrode effects in ferroelectric thin films for piezoelectric energy harvesting

Robin Nigon

The continuing decrease of power requirements of electronic circuits offers the potential to deploy wireless systems as embedded sensors for cars or industrial tools, and implanted medical devices. Harvesting ambient vibrations by an appropriate energy harvesting (EH) device allows to avoid an undesirable battery replacement. At the scale of micro-electromechanical systems, where severe size constraints must be met, microfabricated EH devices with piezoelectric thin films offer the best energy density. Ferroelectric lead zirconate titanate (PZT) thin films with interdigitated electrodes (IDE) appear as the most promising device design for this purpose. Accurate characterization of the thin film response is necessary to determine the PZT composition and doping, the electrode geometry, and the stack design for maximum EH efficiency. Unfortunately, there is no rigorous description of the physical behavior of the IDE system to date. One goal of this thesis was thus to provide a better understanding of the experimental observations made by previous researchers. In addition, only a limited PZT composition and doping range has been investigated in this configuration. It was the second goal of this thesis to widen this range in order to determine the combination that yields the best EH efficiency. Finally, the risk of partial or total depoling over the device lifetime is always present in ferroelectric materials. The phenomena of aging and self-poling are of great interest to ensure proper retention of the poled state and, thus, the reliability of the harvesting device. Neither of the two are well understood. It was the third goal of this thesis to investigate the aging behavior and methods to promote self-poling. In this thesis work, we have have proposed a description of the physical behavior of the IDE system, and we have developed an analytical model for extracting the effective material properties from standard characterization measurements, which is well supported by both finite element (FE) simulations and experimental data. We found that if the substrate is conductive enough, a parasitic capacitance is present in parallel to the material response. We have provided a method to subtract the contribution of the parasitic capacitance, which has an accuracy of better than 4% in a wide range of IDE geometries as determined by FE simulations. We have investigated the performances of doped PZT thin films with IDE for several combinations of dopant and composition. We have improved an existing fabrication route to obtain textured PZT films on an insulating MgO layer. We found that dopants systematically reduced the piezoelectric response and retention capability, and increased the dielectric constant. All three are detrimental for EH. Undoped compositions should be chosen. We have studied methods to improve the stability of the poled state through aging and self-poling. Introducing the latter into the IDE configuration did not provide sufficiently strong effects to be of practical interest. On the contrary, the aging process may allow to tune the extrinsic contributions to the dielectric and piezoelectric response. It is likely caused by polarization discontinuities at grain boundaries. Further work is needed for fully optimizing this phenomenon. Finally, from the previous investigations, we could deduce and propose golden rules for the design of IDE structures, and discuss typical applications where they are advantageous.