N. Frohleke

Simulation model for ultrasonic motors powered by resonant converters

A rotary traveling wave type ultrasonic motor powered by a resonant converter is modeled to optimize the overall drive performance by simulation and to develop suitable control strategies for the drive. Based on mechanical approaches in modeling the stator under consideration of unsymmetries and reflecting the nonlinear stator/rotor-contact by an elastic contact model, an extended contact model of the stator/rotor interaction with closed form solutions for nonideal traveling waves is derived referring to a two-mode approximation of the stator and piezoceramic. By incorporation of a converter model into the mechanical subsystem, a proper simulation model for ultrasonic drives is presented, whereby some interesting phenomena are verified by simulations and explained by strong graphical means.

Analysis and design of improved isolated full-bridge bidirectional DC-DC converter

A novel isolated full-bridge DC-DC converter with bidirectional power flow is proposed in this paper. By adding auxiliary active clamping circuits to both bridges, zero-voltage and zero-current-switching are achieved to improve the performance of the bidirectional PWM converter. The switches are controlled by phase shifted PWM signals with a variable duty cycle. The principle of operation is analyzed and simulated. Experimental results of a 3 kW 50 kHz prototype are presented.

Improved analytical modeling of conductive losses in gapped high-frequency inductors

An improved method to predict conductive losses in gapped high frequency inductors is presented and used for parametrization of an equivalent small signal circuit model. The method is based on the superposition of power losses resulting from the well known one-dimensional field calculation and losses due to eddy currents caused by the fringing field of airgaps determined from new analytical 2D-field calculations. Losses due to reactive currents in the windings caused by the self capacitance are also considered An accuracy improvement of at least 50% percent compared to the known methods is proved by measurements.

Adaptive digital slope compensation for peak current mode control

Peak current mode control as well as digital control offers a number of benefits. Therefore it is an interesting approach to combine these two techniques in one control structure. Based on microcontrollers with on-chip comparators, this combination is realizable with very low effort. In order to eliminate the drawbacks of peak current mode control, a slope compensation has to be added. This paper presents such a slope compensation implemented on a microcontroller not using an analog ramp signal but instead pre-calculating the desired comparator switch-off threshold. In contrast to conventional analog control, adaptive algorithms can be used to maintain optimal slope compensation over a wide operating range. Problems that occur in practice due to reverse recovery current spike and computing time can be handled with simple measures. The effectiveness of the proposed digital slope compensation is verified by experimental results.

Harmonic reduction techniques for single-switch three-phase boost rectifiers

Techniques for reducing harmonic distortion in single-switch three-phase boost rectifiers are presented. Optimal pulse-width modulation methods for the main switch to minimize input harmonic current distortion or to maximize the converter power range are described first. Effects of the voltage control loop on the input current waveform are then investigated by using the harmonic balance method. It is found that through proper design of the voltage controller, the input current harmonics in the closed-loop controlled system can be reduced to almost the same level as when the optimized PWM is applied. Simulation and experimental results are included to validate the analysis.

Enhanced control scheme for three-phase three-level rectifiers at partial load

A prominent boost-type three-level topology (VIENNA Rectifier I), which proved to represent a cost-effective and highly efficient solution for switched-mode rectifiers is inspected toward its operation at discontinuous conduction mode (DCM). This mode of operation occurs not only at high input voltage in conjunction with low load currents but even at medium loading in the vicinity of mains voltage zero crossings. When this circuit is operated in DCM, additional measures are required for improved behavior to avoid conflicts with requirements on total harmonic distortion and regulations as well as safe operation in terms of voltage balancing and overvoltage protection. A detailed analysis of DCM and associated states is performed enabling determination and location of error voltages. Basic rules for the location of error voltages can be found. This leads to a novel optimized modulation and control scheme, facilitating designs without additional inductance. Selected simulation and measurement results prove the enhanced modulation scheme.

Optimized size design of integrated magnetic components using area product approach

Substantial reductions in size and costs of magnetic components and power supplies can be achieved by an integration of magnetic components on one core. So far, no adequate method is known for computerized design of integrated magnetics. In this contribution, an optimized size design approach of integrated magnetic components using area product formulation is presented. In a first step, optimal flux density and area product values are determined for discrete multiple-winding inductors and transformers. Using these results in a second step, optimal area products are calculated for integrated magnetic components, consisting of e.g. transformer and either resonant or filter inductor

Modeling and Control Design for a Very Low-Frequency High-Voltage Test System

With the rapid growth of energy generation by regenerative decentralized sources and the continuous replacement of overhead power lines by subterraneous distribution grids, the amount of power supply cables is increasing. In consequence, a large demand for mobile high-voltage cable test systems is expected within the coming years. This contribution deals with modeling and control design of a novel high-voltage test system based on a zero-voltage switching series-parallel resonant converter and a three-stage Cockcroft-Walton voltage multiplier rectifier, generating a true sinus test voltage of 85 kV (rms) at 0.1 Hz. Due to the inherent high dynamics of the power supply as compared to the slowly varying output test voltage, a cascaded control scheme relying on a fast inner current control loop combined with an outer voltage control is established. In order to derive the relevant waveforms, the complex rectifier was significantly simplified to a one-stage voltage doubler rectifier. By applying a suitable generalized averaging method in combination with an extended describing function, a steady-state solution along with the corresponding small signal model can be established, which is utilized for the current control design. The cascaded control strategy using an observer enables substitution of voluminous and costly high-voltage current sensors. The total high-voltage test system was implemented and validated by experimental tests.

Identification of nonlinear two-mass systems for self-commissioning speed control of electrical drives

A self-commissioning system for high performance speed and position control of electrical drives requires identification of the mechanical model structure and parameters. This substantial task within the self-commissioning scheme is carried out by a step-by-step procedure which includes different test signals and identification methods collected in a toolbox. Methods based on discrete time models, on frequency responses, on the extraction of characteristic features of the system responses and basis function networks integrated into Kalman filters are combined to an effective scheme. The working principle is demonstrated on an adjustable mechanical load set-up driven by an industrial inverter (servo controller).

Self-Optimization as a Framework for Advanced Control Systems

Optimization is a usual step of control design. To do so, clear design goals have to be defined and sufficient system information must be given as prerequisites. However, data are often inaccurate or missing and design goals may change during operation. That is why a concept of self-optimizing systems is proposed, which is able to optimize the system even during operation. The proposed concept should be understood as a framework to incorporate various control and optimization methods. A key element of the proposal is the operator controller module, which consists of a cognitive part for planning tasks with lower real-time requirements and a reflective part for the execution level. A particular focus is given to how to handle multi-objective optimization with on-line adaptation of the objectives depending on internal and external design goals. Examples how to employ the concept in practice are given