High-Frequency Capacitive Sensing for Electrohydraulic Soft Actuators

Abstract

The need for compliant and proprioceptive actuators has grown more evident in pursuing more adaptable and versatile robotic systems. Hydraulically Amplified Self-Healing Electrostatic (HASEL) actuators offer distinctive advantages with their inherent softness and flexibility, making them promising candidates for various robotic tasks, including delicate interactions with humans and animals, biomimetic locomotion, prosthetics, and exoskeletons. This has resulted in a growing interest in the capacitive self-sensing capabilities of HASEL actuators to create miniature displacement estimation circuitry that does not require external sensors. However, achieving HASEL self-sensing for actuation frequencies above 1 Hz and with miniature high-voltage power supplies has remained limited. In this paper, we introduce the F-HASEL actuator, which adds an additional electrode pair used exclusively for capacitive sensing to a Peano-HASEL actuator. We demonstrate displacement estimation of the F-HASEL during high-frequency actuation up to 20 Hz and during external loading using miniaturized circuitry comprised of low-cost off-the-shelf components and a miniature high-voltage power supply. Finally, we propose a circuitry to estimate the displacement of multiple F-HASELs and demonstrate it in a wearable application to track joint rotations of a virtual reality user in real-time.

Figures (11)

• Figure 1: (A) Wearable VR application to track the users knee and hip joint rotation in a 2D-plane (B) Proposed F-HASEL, characterized by an additional electrode used exclusively for sensing. (C) Visualization of the methodology behind the wearable VR application, composed of the schematic of the sensing circuitry which enables quasi-simultaneous displacement estimation of multiple F-HASEL actuators as well as the subsequent data processing. (D) Virtual avatar to which the users tracked joint rotations are transmitted.
• Figure 2: The traditional Peano-HASEL and F-HASEL actuators both undergo distinct stages of actuation with increasing driving voltages, but F-HASELs also integrate sensing electrodes for improved control and feedback. (A) Traditional Peano-HASEL actuator at different stages of actuation caused by the driving voltages $V_0$, $V_{D1}$, and $V_{D2}$ of increasing magnitude applied to the HV electrodes. (B) F-HASEL actuator, characterized by the LV sensing electrodes, at different stages of actuation caused by the driving voltages $V_0$, $V_{D1}$, and $V_{D2}$ of increasing magnitude applied to the HV electrodes.
• Figure 3: Schematic of the sensing circuitry to enable the displacement estimation of a single F-HASEL actuator using two different methods: (A) the voltage-method, and (B) the impedance-method.
• Figure 4: Experimental displacement estimation against ground truth across various actuation cycles, while the sensing circuitry facilitates the estimation for a single actuator. (A) Experimental setup to compare the displacement estimation to the ground truth for a variety of actuation cycles. (B) PCB of the sensing circuitry to enable displacement estimation of a single F-HASEL actuator.
• Figure 5: The experiments illustrate noise reduction in differential voltage measurements. (A) Experimental setup to show the difference in noise present on the differential voltage measurements during a constant driving voltage of magnitude 4.8kV in the proposed method compared to a previous work by Ly et al. ly2021miniaturized. (B-D) Recorded differential voltage measurements $V_K$, $V_C$, and $V_H$ over a time frame of 0.5ms during a constant driving voltage of magnitude 4.8kV.