TABLE OF CONTENTS It should be emphasized that the arbitrary classifications presented in Table 1.1 and the ideal elements listed in Table 1.2 are used only as aids in system modeling. Real systems usually exhibit nonideal characteristics and/or combinations of characteristics that depart somewhat from the ideal models used here. However, analysis of a system represented by nonlinear, partial differential equations with time-varying coefficients is extremely difficult and requires extraordinary computational resources to perform complex, iterative solution procedures, an effort that can rarely be justified by the purpose of the analysis. Naturally, theref ore, simplified descriptions of the actual system behavior are used whenever possible. In particular, nonlinear system characteristics are approximated by linear models for which many powerful methods of analysis and design, involving noniterative solution procedures, are available.
Consider for example a linear variable differential transformer (LVDT), which is commonly used to measure displacement in high-performance applications. A simplified schematic diagram of a LVDT is show in Fig. 2.18. In this simple device, a ferromagnetic core is placed inside three coils. The center coil is energized by an external ac power source, and the two end coils pick up the voltage induced by a magnetic field established by the center excitation coil. When the core is in the center of the coils, the amplitudes of the voltages induced in the end coils are the same and because the end coils are connected in opposite phase, the output voltage is zero. When the core is displaced left or...
