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

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.

Multi-level high-voltage power supply for dea application

Morgan Almanza, Yoan René Cyrille Civet, Lucas Depreux, Yves Perriard, Lucas Pniak

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.

2019

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

High voltage metal oxide thin film transistors to drive arrays of dielectric elastomer actuators

Alexis Pierre Henri Marette

This thesis advances the field of high-voltage thin film transistors (HVTFTs) and dielectric elastomer actuators (DEAs) by demonstrating a strategy for low-voltage addressing of an array of high voltage soft actuators suspended on a flexible substrate. First, I present the first HVTFTs operating at 1 kV drain-source voltage, switching with an on-off ratio of 20 at 80 V gate-source voltage. The HVTFTs can operate at high voltage thanks to geometrical features increasing the breakdown voltage: a thick gate dielectric composed of a bilayer of alumina (100 nm) and Parylene-C (1 um), a long semiconducting channel (500 um), and a 150 &mlong non-gated region between the drain and the gate electrode called the offset gate. The use of an amorphous oxide semiconductor (AOS), zinc tin oxide (ZTO), enables a high on-currents of 0.1 mA. The ZTO was synthesized by a sol-gel process after spin-coating on a flexible polyimide substrate, previously passivated with alumina. I optimized the HVTFT switching properties by doping the ZTO layer with yttrium (5%). It improved the on-off ratio up to 1000 at 500 V operation voltage by decreasing the leakage current down to 100 nA. Then, I show the first integration of HVTFTs with DEAs. My ZTO HVTFTs switch DEAs on and off with only 30 V gate voltage under a bias voltage of 1.4 kV. The system time response in 50 ms. The demonstrator is a 4x4 array of diaphragm DEAs. A layer of 4x4 DEAs is suspended over a layer of 4x4 HVTFTs built on flexible polyimide. The DEAs and the HVTFTs were interconnected thanks to a flexible PCB in a resistive load inverter circuit architecture. A flexible 3D printed chamber was constantly biasing the DEA diaphragms with a back-pressure. The DEAs were made of PDMS and the active region is defined by overlapping carbon-PDMS electrodes. The device operates down to a 5mm radius of curvature. Finally, I demonstrate latching of the HVTFT and the DEA by using triboelectric sensors. Under a constant 500 V circuit bias, the control of the HVTFT gate with triboelectric generators enabled 4s latching of the inverter output voltage at 470 V for the off-state and at 120 V for the on-state. The latching of the DEAs with the HVTFT circuit finally proves that this approach can lead to a bistable control of DEAs. This PhD thesis results show that my HVTFTs are versatile components usable not only to address DEAs but also to interface low voltage sensors with high voltage actuators.

EPFL2018

High voltage metal oxide thin film transistors to drive arrays of dielectric elastomer actuators

Alexis Pierre Henri Marette

EPFL2018