Master's Thesis : Simulation of a programmable RLC impedance (analog and digital implementation)

Abstract

Different kinds of electronic load simulators for testing of power supply have been developed and commercialised. A programmable RLC impedance with adjustable R, L and C can be useful to simulate the load of a generator like a motor for example. Some companies may be interested in testing their voltage/current generators under some conditions and study their behaviours. Both inductive and capacitive loads can be simulated by adjusting the phase difference between the voltage and current waveforms.

However, there exist many drawbacks, for example, it can provide only 4 modes including constant resistance (pure resistive load), constant power, constant current and constant voltage to simulate simple dc load. But more often, they cannot simulate a RL or RC load with a good dynamic response. The main objective of this master thesis is to design, manufacture and test a programmable RLC impedance. This circuit must cover the entire inductive-resistive-capacitive range (voltage/current phase shift between -90° and +90°). The RLC load must be able to dissipate up to 5 W. Two different implementations are suggested: the analog and digital RLC impedance with their pros and cons. The first step is to design an analog impedance. A gyrator is used in order to simulate an non ideal inductor with parallel redundant resistor. The main property of the gyrator is to invert the current-voltage characteristic of an electrical component. This way, a capacitor can be used to get a non-ideal variable inductor. The inductance can be adjusted with simple potentiometers. The second step consists in removing the parallel resistors in order to simulate an ideal inductor and reach the 90° phase-shift. This objective can be achieved by connecting the Negative Impedance Converter (N.I.C) in parallel to the gyrator. After that, a more detailed study of the output current and output voltage across the op amps allows to highlight the main limitations of such circuits. The voltage and current saturation of the op amps limit the available range of inductance/capacitance values. Several simulations with a real precision op amp (AD744) are performed. Finally, those circuits have been implemented on a breadboard in the CSL laboratory1 with commercial op amps such as the famous LM324. The results in terms of output current are compared with the simulations. The second step consists in designing a digital RLC impedance. By contrast to the analog impedance, the filtering process is done digitally using the microcontroller ATMEGA2560. A software feedback control approach is employed to adjust the current amplitude and the power factor. The objective is to maintain the impedance value constant regardless of the variation of the source voltage amplitude. Many parameters such as the sampling frequency and the resolution of the ADC/DAC may affect the quality of the output current. The voltage is converted to current using a Howland current source. Several simulations have been performed to visualise the performance of the digital RLC filter.

Finally, the analog and digital programmable RLC impedances are compared in terms of their maximum input voltage, inductance/capacitance range, frequency, ... It will be shown that the digital implementation offers more flexibility to the range of the inductance/capacitance values than the analog impedance.