Matti Vilkko

Displacement Control of Piezoelectric Actuators Using Current and Voltage

This paper introduces a feedforward charge control scheme, which controls the velocity of a piezoelectric actuator by the current fed to the actuator. The velocity-current relation, however, is not linear over the entire motion range, and therefore, information about the actuator voltage is also utilized. The required current is estimated using an actuator model, which is separated in two phases combining individual models for a motion phase and a static phase. The method is verified by a series of experiments, where a piezoelectric actuator is moved with variable velocities and displacements. During the experiments, the current is fed to the actuator using a current driver consisting of a voltage amplifier, a precise current meter, and a controller. The results show a significant improvement in comparison to open-loop voltage control; the hysteresis is less than 2% and the drift approximately 1%. This indicates that the motion of piezoactuators is a function of current and voltage, and does not depend considerably on the motion history. Therefore, a sensorless control scheme to overcome the hysteresis and drift would be feasible.

Online Grid Impedance Measurement Using Discrete-Interval Binary Sequence Injection

Grid impedance affects the stability and control performance of grid-connected power electronic devices, such as inverters used to integrate wind and solar energy. Adaptive control of such inverters, to guarantee stability under different grid conditions, requires online measurement of the grid impedance performed in real time. Such online measurement can be performed by injecting a current perturbation from the inverter into the grid and by reading the grid voltage responses. To minimize the impact on the inverter operation, the injection must be kept as small as possible while producing enough voltage perturbation that can be reliably measured and processed to extract its various frequency components. This paper proposes the use of a discrete-interval binary sequence (DIBS) for this application to minimize the injection. The DIBS is a computer-optimized binary sequence, where the energy is maximized at specified harmonic frequencies based on the expected grid-impedance characteristics. Experimental results based on a three-phase grid-connected inverter are presented to demonstrate the effectiveness of the proposed methods.

A short-term scheduling for a hydropower plant chain

An optimal control of a hydropower plant chain is introduced. The aim is to meet a predefined power demand and in the same time provide sufficient control capabilities. At first a state-space model of the river chain is presented. Then an optimal feedback control method is developed by introducing Hamiltonian for the system. The constraints are considered using Lagrangian multipliers. As an example a river bed with eight hydro plants is studied. The results show the suitability of the method to production planning and to analysis of the hydro system behaviour. It can also be used for on-line scheduling.

Fast Loop Gain Measurement of a Switched-Mode Converter Using a Binary Signal With a Specified Fourier Amplitude Spectrum

Switched-mode power converters are extensively used to power various electronic loads and processes. The use of low-cost components with high parameter variation together with the requirement of short time to market has set new challenges for the designers such as the necessity to characterize a large number of converters before production ramp-up. It has been shown that the frequency-domain characterization yields most useful information on the quality of a converter. The conventional frequency response analyzers can be used for measuring the responses, but the time required for one measurement is too long for characterizing hundreds or thousands of converters. Recent studies show that the time for single measurement can be drastically reduced by using broadband excitation signals such as the maximum length pseudorandom binary sequence (MLBS). The MLBS excitation signal is not well suited for measuring the voltage or current loop gains because of their sensitivity to the disturbance signals. This paper proposes a new method based on the use of MLBS but with a specified Fourier amplitude spectrum in order to avoid the said problem. The method is verified both with simulation and practical measurements, and the results are compared to the conventional MLBS technique.

Broadband methods for online grid impedance measurement

Grid impedance is an important parameter for the operation and control of grid-connected inverters used for the integration of solar, wind, and other distributed generation resources. Since the grid impedance usually varies over time and with grid operation conditions, online measurement is required for adaptive control of grid-connected inverters. Existing online measurement methods based on impulse perturbation and Fourier analysis require large current or voltage injection that may interfere with normal operation of the inverter. This paper proposes the use of maximum-length binary sequence (MLBS) injection and averaging Fourier techniques to overcome the drawbacks of impulse injection. Experimental results based on a three-phase grid-connected inverter are presented and used to demonstrate the effectiveness of the proposed methods.

Self heating of piezoelectric actuators: measurement and compensation

This paper introduces the effect of self heating on the displacement of piezoelectric actuators and a novel method to quantify self heating. Issues influencing self heating were discussed which include; the frequency and the amplitude of the driving voltage, and the size, or more specifically the volume-area ratio of the actuator. The effect of a load on the heat generation is also studied. According to the experiments, the peak-to-peak value of the consumed current is a good indication of the temperature rise of the actuator. This can be used for the protection of the actuator from overheating, or as the authors will propose in the paper, it can be used to compensate for the changes in the displacement induced by the self heating. The displacement error of the heated actuator reduces in average down to one part in three when the proposed compensation is used.

Corrected Mathematical Model of Quadruple Tank Process

A quadruple tank apparatus has been developed in many universities for use in undergraduate chemical engineering laboratories. The control experiment illustrates the performance limitations for multivariable systems posed by ill-conditioning, right half plane transmission zeros, and model uncertainties. The experiment is suitable for teaching how to select among multiloop, decoupling, and fully multivariable control structures. A number of these reports are, however, based on erroneous mathematical modeling and thus resulting incorrect results. Obviously all these reports refer originally to the one and same paper which includes this incorrect part of modeling. The error is significant if the pumps used in the experiment are not identical. If they are identical the error is, however, negligible. Mathematical derivation and simulation results are provided to give a corrected model and illustrate the effect of the widespread incorrect modeling.

Online grid impedance measurement using discrete-interval binary sequence injection

Grid impedance affects the stability and control performance of grid-connected power electronic devices, such as inverters used to integrate wind and solar energy. Adaptive control of such inverters, to guarantee stability under different grid conditions, requires online measurement of the grid impedance performed in real time. Such online measurement can be performed by injecting a current perturbation from the inverter into the grid and by reading the grid voltage responses. To minimize the impact on the inverter operation, the injection must be kept as small as possible while producing enough voltage perturbation that can be reliably measured and processed to extract its various frequency components. This paper proposes the use of a discrete-interval binary sequence (DIBS) for this application to minimize the injection. The DIBS is a computer-optimized binary sequence, where the energy is maximized at specified harmonic frequencies based on the expected grid-impedance characteristics. Experimental results based on a three-phase grid-connected inverter are presented to demonstrate the effectiveness of the proposed methods.

On Frequency-Response Measurements of Power-Electronic Systems Applying MIMO Identification Techniques

Switched-mode converters are often the key to operating various electronic systems in everyday life. This means that the reliable operation of the converters is of prime importance and that the functioning must be verified during both the design phase and production. Recent studies have shown that the converters can be fully characterized by a set of frequency responses that can be efficiently used to validate the operation of the converters and analyze the related systems. Several methods have been proposed for quickly and accurately measuring the frequency responses, but surprisingly, the use of multiple-input multiple-output (MIMO) identification techniques has not been considered. Applying MIMO identification techniques, the operating conditions of a converter can be kept constant during the frequency-response measurements, and the overall measurement time can be further shortened. This paper reviews the techniques and proposes an implementation setup. The presented techniques provide efficient means to characterize the switched-mode converters during one measurement cycle. The methods can be used for example in field-programmable gate-array-based controller implementation and in fast online analysis. Experimental measurements are shown from a high-frequency switched-mode converter.

Dynamical profile of a switched-mode converter - reality or imagination

An electrical device such as a switched-mode converter has unique internal dynamical profile, which mainly dictates, how the device as a subsystem in an interconnected system would behave and how it will affect the other subsystems within the constellation. The paper defines the vital representative parameters by means of which the stability and dynamics of the individual electrical device as well as the overall interconnected system can be analyzed and predicted. The very existence of such a dynamical priflle is proved based on extensive experimental evidence.