TAKEOFF OF A FLYING MICROROBOT WITH COTS SENSOR PAYLOAD USING ELECTROHYDRODYNAMIC THRUST PRODUCED BY SUB-MILLIMETER CORONA DISCHARGE

This paper demonstrates the first flying microrobot using electrohydrodynamic thrusters, or ionocraft, to successfully take off while carrying an onboard commercial sensor package. The 13.6mg, 1.8cm by 1.8cm ionocraft is shown to take off while carrying a 40mg Flex PCB with 9-axis IMU and associated passives while tethered to a power supply. A new emitter electrode design has decreased corona onset voltage by over 30% and takeoff voltage by over 20% from previous efforts. Thrust density scaling with increasing numbers of emitter wires, continued geometric scaling for decreased operating voltage, device lifetime improvement via thin film deposition, and new assembly techniques are all explored. INTRODUCTION AND RELATED WORK Pico air vehicles, flying microrobots with mass under 100mg and characteristic length under 5cm, are being developed by a large number of research groups [1]. The majority of efforts focus on biomimetic flapping wing designs, typically using either piezoelectric or electromagnetic actuators. This work is unique in that it uses a microfabricated corona discharge based electrohydrodynamic actuator (Fig. 1) to produce thrust almost silently and with no moving parts, creating the highest thrust-toweight ratio pico air vehicle capable of takeoff (Fig. 2) to date [2]. Figure 1: Thrust is produced when ions, drifting in the applied electric field, collide with neutral air molecules and impart momentum. Bipolar ions are generated in the corona plasma region localized at the sharp tip of the emitter electrode, but only positive ions (mainly N2+) will drift towards the collector grid. A one-dimensional model for electrohydrodynamic thrust, based on the electrostatic force on a volume of ions, leads to an expression for force in terms of ion drift current, I, distance the ions travel d, and the ion mobility, μ, which is about 2cm2/Vs for N2+ ions in air [3]: FF = /μμ (1) Figure 2: Photograph of the assembled ionocraft. With four individual addressable thrusters, it is about 1.8cm by 1.8cm and masses 13.6mg. It is comprised of 13 individual components connected by a combination of mechanical slots and UV-curable epoxy. The design includes various tabs for handling and connection to power tethers. The dimensionless term β represents deviation from the ideal force caused by losses in the system due to factors like aerodynamic drag, bipolar plasma radius to drift distance ratio, and undesired ion path directions (e.g. horizontal velocity components “lost” as vertical thrust in collisions) [4]. Ions are generated using corona discharge, an atmosphericpressure, low-temperature DC plasma mechanism. In this process, an applied voltage between two asymmetric electrodes (e.g. a wire “emitter” and a plate “collector") yields a locally enhanced electric field around one of the two; if the potential is high enough, electrons will gain enough energy to initiate Townsend avalanche breakdown in the neutral air around the electrode, producing a bipolar plasma. Corona discharge is the most common mechanism of ion production for EHD force, with implementations using electrode gaps ranging from the 10s of centimeters [5] to the sub-millimeter [6]. Although models for corona discharge in all but a few simple geometries are largely empirically derived, resultant ion current versus applied voltage generally follows the relationship: ββ = CCCC(CC − CC0) (2) Where C is an empirical parameter that is a strong function of electrode geometry and V0 is the corona onset voltage. The combination of these two governing equations yields an expression for EHD force that depends on two empirical factors, C and β, that can be extracted from experimental data: FF = ββCCCC(CC − CC0) ββ/μμ (3) Designs for EHD thrusters can be assessed by attempting to maximize C and β while minimizing V0. Designs can further be 5mm Corona discharge