Multi-level high-voltage power supply for dea application

In order to reap the benefits of Dielectric Elastomer Actuator (DEA) high energy density, the electronics powering the actuator should be as small as possible. Knowing that only a small fraction of the energy delivered to DEAs is effectively converted into mechanical energy, the power supply has to be highly efficient and bidirectional in order to recover the unused energy, which requires more volume. Furthermore, DEA applications call for high voltage (up to 18kV for a 200um film), but as the voltage increases, so does the volume. In addition, there are no MOS transistors rated above 4.5kV, which limits our options. Since these goals are contradictory, we need to demonstrate the feasibility of DEA integration and find its limits. Therefore we present a prototype of Multi-Level Converter designed for DEA, which is a bidirectional structure that has proven to be especially effective in the power electronics field. A Multi-Level Converter overcomes the voltage limitation by equally distributing the desired voltage among levels; because the voltage of each level is sufficiently low, smaller available parts can now be used. Our prototype includes 20 levels of 1kV each. A numerical study of this topology has demonstrated that the efficiency is above 90%. Upcoming experimental works will discuss the advantages and the drawbacks of such Multi-Level Converters. Finally, the volume of the DEA will be compared with the volume of electronics.

Control Strategy for the Discharge Phase of an Ultra-High Voltage (>7kV) Bi-Directional Flyback Converter Driving Capacitive Actuators

Alexis Boegli, Raphaël Johannes Charles Mottet, Yves Perriard

Dielectric Elastomer Actuators (DEAs) require ultra high voltages in the order of several kilo volts to operate. To supply such voltages, a DC-DC flyback converter topology was selected. One of the main advantage of using such a structure is the possibility to modify it to work bidirectionally. Meaning that electrical energy can not only flow from the power supply to the actuator, but also from the actuator back to the power supply. This characteristic is of significant importance because of the need to recuperate the electrical energy stored in DEAs to improve the overall efficiency of the system instead of dissipating it as heat. Indeed, due to their capacitive nature, DEAs can store an important amount of energy when fully deformed which needs to be removed to bring the actuator back to its original shape. This paper focuses on the control aspect to remove the stored energy when using a bi-directional flyback converter capable to supply a voltage of up to 7 kV to the load from a 12V power supply. After a brief presentation of the working principle of the discharge procedure, the implemented control strategy is detailed in depth with the presentation of the hardware used and the control algorithm put in place. Experimental results then show that the control strategy works well with an output voltage of up to 7 kV but is nonetheless close to its limits because of increasingly small measuring time spans going down to several micro seconds.

2020

Ultra-high voltage, low power and energy recovering electronics for dielectric elastomer actuators

Raphaël Johannes Charles Mottet

Heart failure is a cardiovascular disease affecting between 1 and 2% of the population of developed countries alone - more than 10 million people - with this number bound to increase due to the ageing of the population. This condition starts to develop itself when the heart is not capable to properly pump the blood in the circulatory system anymore, and it can be due to other cardiovascular diseases, hypertension, heart attacks, etc. When cases become very severe, surgical procedures are then necessary to install cardiac assist devices to help reduce the stress experienced by the heart and allow it to potentially heal itself. However, the devices currently used are bulky and very invasive with risks of infections and rejections.As such, the Center for Artificial Muscles (CAM) was tasked to investigate and develop a new kind of cardiac assist devices which would be much less invasive to install. To do so, the idea envisioned consisted of working with Dielectric Elastomer Actuators (DEAs) which are often nicknamed artificial muscles due to their close resemblance with the natural ones as they, amongst other things, light, thin and can deform themselves significantly. One of the main challenges of working with this technology comes from the fact that DEAs have very uncommon needs concerning the driving electronics. Indeed, to operate to the maximum of their abilities, DEAs require voltages of several thousands of volts - between 5 and 20 kV depending on the actuator - to be applied to them. Currently, these actuators are powered using extremely large and impractical laboratory power supplies which is unacceptable for portable and medical applications. It was therefore critical to investigate and uncover a solution for the electronics so that the DEAs' needs could be met with a system as compact and efficient as possible.Thus, this thesis presents the investigating process followed to obtain in the end an electronics system capable of supplying at least 8 kV to a DEA and recover unused electrical energy stored in the actuator. Process which consisted at first of an investigation of power electronics topologies capable of amplifying a given input voltage to a higher level in order to determine the most suitable one. It was concluded that the DC/DC flyback converter was the best candidate for the job.This was followed by an in depth analysis of the working principle of this converter topology to determine the limiting factors regarding the supply of high voltages. This analysis was confirmed through the development of an analytical model depicting the behavior of the converter when it charges the actuator to the required voltage. Thanks to this a list of solutions to overcome these factors could be given.Then, due to the voltage levels worked with and the necessity to recover the unused electrical energy stored in the actuator, it was revealed that commercially available solutions were scarce to nonexistent to properly control the electronics. As such, control strategies and custom devices were ultimately conceived to resolve this problem.In the end, the final ultra-high voltage bidirectional flyback converter prototype was used to operate a DEA pump placed in a test bench recreating the pressure conditions and blood flow which are expected to be found in a living body. These tests showed that the electronics performed very well and managed to operate without major issues the new cardiac assist device created by the CAM.

EPFL2022

Aero Engine Flow Control by Surface Discharge Plasma

Sami Goekce, Christoph Hollenstein, Pénélope Leyland, Peter Ott, Philip Peschke

Experimental investigations and numerical modeling with the objective to study the effects that surface discharge plasmas can provoke on aerodynamic flow situations, including high speed flows present in aero-engines, are undertaken at EPFL. The actuator’s potential to influence the flow under realistic flight and engine operating conditions, largely depends on its geometric configuration and the power supply. Initially, a long-life (more than 12 hours) surface discharge actuator was designed at EPFL and proven to survive transonic flow speeds on typical compressor blades. This was the first time that such a long-term, continuously operating actuator was applied successfully. However, with AC voltage applied to the electrodes, the effects on the shock waves did not appear to be significant [1, 2]. Recent experiments conducted at EPFL focused on fast rise voltage pulse driven dielectric barrier discharge (DBD) actuators and the development of a power supply that could provide the required voltage pulses. A generic DBD actuator with two electrodes in asymmetric configuration, separated by a dielectric was chosen. High speed imaging was applied to investigate the spatial and temporal discharge development. In order to study the influence of voltage and current on the plasma, several power supply configurations were examined by varying voltage rise and fall time, maximum voltage, peak current and pulse width. Two discharge periods were observed on the positive and the negative edge of the voltage pulse. It was observed that the characteristics of the plasma differ considerably between these two periods. Furthermore, the light emission intensity and the propagation speed of the plasma along the surface of the dielectric increased with the maximum voltage and peak current. A similar effect was observed when the voltage rise time was decreased. In a next step, a technique similar to synthetic schlieren was applied to examine the interaction of the DBD actuator with air under atmospheric pressure. A micro shock wave that propagates from the electrode edge normal to the surface was observed, which agrees well with numerical results gained during a parallel investigation. In addition, the impact on the flow structure of maximum voltage and frequency is currently examined. First results suggest that the intensity of the micro shock wave increases with the voltage whereas the frequency primarily provokes electrode heating. With the objective to investigate the modelling of the fast rise pulse driven DBD discharge, a numerical tool was developed that applies emission spectroscopy to determine the electron energy distribution function (EEDF) by comparing the relative intensity of bands, as described by several authors [3-5]. The EEDF is then used to infer populations present in the gas through electron impact ionization, excitation or dissociation cross sections data [6]. Contrary to previous studies, this approach employs two naturally occurring transitions in atmospheric plasmas in conjunction with recent data on collisional quenching [7], which makes the method more reliable and does not require the adjunction of other species such as Helium. References: 1. Pavon S., Dorier J.L., Hollenstein C., Ott P., Leyland P., “Effects of high-speed airflows on a surface dielectric barrier discharge”, J. Phys. D: Appl. Phys. 40. No 6.2 1733-1741, 2007. 2. Pavon S., Sublet A., Dorier J.L., Hollenstein C., Ott P., Leyland P., “Long Lifetime system for the generation of Surface Plasmas”, International Patent no: P1883PC00/13-71, PCT/IB2009/050489, 2008. 3. Bibinov N. K., Kokh D.B., Kolokolov N.B., Kostenko V.A., Meyer D., Vinogradov I.P., Wiesemann K., “A comparative study of the electron dis-tribution function in the positive columns in N2 and N2/ He dc glow dis-charges by optical spectroscopy and probes”, Plasma Sources Science and Technology, 298-309,1998. 4. Behringer K., Fantz U., “Spectroscopic diagnostics of glow discharge plasmas with non-maxwellian electron energy distributions”, Journal of Applied Physics D, 2128-2135, 1994. 5. Isola L.M., Gómez B.J., Guerra V., “Determination of the electron tem-perature and density in the negative glow of a nitrogen pulsed discharge using optical emission spectroscopy”, Journal of Applied Physics D, 015202, 2010. 6. Itikawa Y., “Cross sections for electron collisions with nitrogen mole-cules”, Journal of Physical and Chemical Reference Data, 31-53, 2006. 7. Dilecce G., Ambrico P.F., Benedictis S., “On the collision quenching of N2plus by N2 and O2 and its influence on the measurement of E over N by intensity ratio of nitrogen spectral bands”, Journal of Applied Physics D, 195201, 2010.