기전 시스템의 압전-구조 연성과 전자기-구조 연성 해석

Abstract

Electromechanical systems are energy transformation devices that convert electrical energy to mechanical energy or vice versa. Piezoelectric devices and electromagnetic devices are included in electromechanical systems, and are used in various applications such as actuators, sensors, switches, and motors. They have been developed to have high efficiency and low noise and vibration through mathematical and finite-element methods. However, as new applications such as smart devices and electric vehicles have appeared in recent years, the use of conventional electromechanical system mechanisms have experienced problems such as low outputs, undesired noise, and vibration due to different operation circumstances. In the case of piezoelectric devices, piezoelectric actuators have recently been highlighted as a promising candidate for smart device vibration motors that need output displacements of hundreds of microns at a low haptic frequency because piezoelectric materials do not generate magnetic flux leakage. The displacement-generation mechanisms of conventional piezoelectric actuators are mainly categorized into three groups: internally leveraged mechanisms, externally leveraged mechanisms, and frequency-leveraged mechanisms. Frequency-leveraged mechanisms can only generate a displacement of hundreds of microns, but their operating frequency is much higher than their haptic frequency. In the case of electromagnetic devices, electric vehicles that use electric motors instead of gasoline engines have recently become popular, and undesired noise and vibration have occurred due to external impacts and magnetic forces. Many researchers have investigated the magnetic excitation of the electromechanical systems used in conventional applications such as washing machines and air conditioners, but they did not consider the changes in magnetic field that are caused by structural deformations. Thus, the latest electromechanical systems used in electric vehicles need accurate magnetic excitation analysis that considers structural deformations, because they can generate larger deformations than conventional systems due to external impacts or strong magnetic fields. Coupled analyses based on the structural finite element analysis were performed in this dissertation to increase outputs from excitations by means of novel piezoelectric mechanisms, and to analyze the excitation characteristics accurately that include structural coupling effects for the electromechanical systems used in the latest applications. First, the background of the dissertation is presented, which covers the displacement-generation mechanisms of conventional piezoelectric actuators and the characteristics of magnetic excitations in conventional electric motors. Second, this dissertation investigates two novel piezoelectric actuator mechanisms within the volume of 10 mm diameter and 3.4 mm thickness that can meet the requirements of the vibration motors used in smart devices such as displacements of hundreds of microns and an operating frequency of low haptic frequency, through piezoelectric-structural coupled analyses. The resonant piezoelectric actuator (RPA) is developed by applying the first novel mechanism in which the piezoelectric unimorph base excites the mass-spring system at the haptic natural frequency, and the externally leveraged resonant piezoelectric actuator (ELRPA) is developed by applying the second novel mechanism in which the bending deformation of a piezoelectric unimorph is transformed into a large axial displacement of the springs. The output displacement, natural frequency, and response time of the proposed models are calculated via finite element analyses and verified experimentally. The RPA generates an output displacement of 427 µm at its haptic natural frequency of 188.8 Hz. The ELRPA generates an output displacement of 290 µm at its haptic natural frequency of 242 Hz, and in particular, this has the fastest response time of 14 ms (approximately one-fifth of the RPA) for vibration motors of equivalent size. Finally, this dissertation investigates the magnetic-structural two-way coupled analysis of a C-core switch and an IPM motor with 4 poles and 24 slots by considering magnetic and structural interactions to accurately identify magnetic excitation. In the two-way coupled analysis method, magnetic force calculated using the Maxwell stress tensor was applied to the structural finite element model to determine its elastic deformation, and the magnetic finite element model was rearranged by means of the moving mesh method to represent the structural deformation. In the C-core switch, both the 40 Hz component (twice the input current frequency) and the 80 Hz component (four times the input current frequency) of the magnetic force were generated due to structural vibrations in the upper core, and this was verified using a magnetic circuit method and vibration experiments. In the IPM motor, the global magnetic force acting on the rotor had a component of 667 Hz (the first natural frequency of the rotor, corresponding to the translational mode) as well as components of the 1st, 11th, 13th, 23th, and 25th harmonics owing to the vibration of the rotor when the IPM motor experienced rotor eccentricity.