Low Voltage Electrohydraulic Actuators for Untethered Robotics

Abstract

Rigid robots can be precise in repetitive tasks, but struggle in unstructured environments. Nature's versatility in such environments inspires researchers to develop biomimetic robots that incorporate compliant and contracting artificial muscles. Among the recently proposed artificial muscle technologies, electrohydraulic actuators are promising since they offer performance comparable to that of mammalian muscles in terms of speed and power density. However, they require high driving voltages and have safety concerns due to exposed electrodes. These high voltages lead to either bulky or inefficient driving electronics that make untethered, high-degree-of-freedom bio-inspired robots difficult to realize. Here, we present hydraulically amplified low voltage electrostatic (HALVE) actuators that match mammalian skeletal muscles in average power density (50.5 W kg-1) and peak strain rate (971 % s-1) at a driving voltage of just 1100 V. This driving voltage is approx. 5-7 times lower compared to other electrohydraulic actuators using paraelectric dielectrics. Furthermore, HALVE actuators are safe to touch, waterproof, and self-clearing, which makes them easy to implement in wearables and robotics. We characterize, model, and physically validate key performance metrics of the actuator and compare its performance to state-of-the-art electrohydraulic designs. Finally, we demonstrate the utility of our actuators on two muscle-based electrohydraulic robots: an untethered soft robotic swimmer and a robotic gripper. We foresee that HALVE actuators can become a key building block for future highly-biomimetic untethered robots and wearables with many independent artificial muscles such as biomimetic hands, faces, or exoskeletons.

Figures (19)

• Figure 1: A low voltage muscle system for untethered electrostatic robots. (A) General structure of a HALVE actuator and working mechanism. The electrodes zipp-in upon application of voltage potential and displace the dielectric liquid into a progressively more cylindrical shape. The transition from relaxed to fully zipped can be seen with $0V<V_1<V_2$. The structural shell carries the load on the actuator and insulates the electrodes. (B) Average specific power of a HALVE actuator (5 PVDF-TrFE-CTFE) versus a Peano-HASEL (15 BoPET) as the dielectric shell with a 300 load. The horizontal line indicates the typical specific power of mammalian skeletal muscle at 50Madden2004MuscleHuman. (C) (i) Untethered gripper that can be lifted from the floor. (ii) Fully integrated demonstrator artificial fish floating in the water. The artificial fish measures approx. 28 in length.
• Figure 2: Effective dielectric constant for model predictions of HALVE actuators force/strain behavior and model validation. (A) (i) Depiction of the dielectric measurement setup used to measure the data points for Fig. \ref{['fig:hasel_model']}B. (ii) Actuator design specifications for which the actuator energy densities are calculated in \ref{['fig:hasel_model']}D. (B) Discharge D-E curves for BoPET, PVDF-HFP, and P(VDF-TrFE-CTFE), the highlighted gray area corresponds to the energy density integral of PVDF-HFP at 300. (C) Effective permittivities calculated using Eq. \ref{['eq.effper']} using the experimental data shown in \ref{['fig:hasel_model']}B. (D) Prediction for the resulting actuator energy densities for different dielectrics. (E) Force/strain measurements and model prediction. (i) Model is plotted with a constant dielectric constant of 40 as suggested in kellaris2019analytical. (ii) Model is plotted with effective dielectric constant values from Fig. \ref{['fig:hasel_model']}C.
• Figure 3: Performance characterization of the HALVE actuator. (A) HALVE actuator at rest and contracted during actuation. (B) Strain-force curves of a 5 P(VDF-TrFE-CTFE) actuator and a 15 BoPET actuator, in pairs of actuation voltages which lead to the same theoretical Maxwell-stress term. (C) Specific work plotted against actuation voltage of a HALVE actuator with 5 P(VDF-TrFE-CTFE) as solid dielectric layer and traditional HASEL actuator with 15 BoPET as solid dielectric layer. (D) Comparison between strain responses to voltage step inputs of different voltage amplitudes of a P(VDF-TrFE-CTFE) HALVE actuator while lifting a 22 weight. (E) Left axis: relation between actuation strain rate and actuation voltage of a P(VDF-TrFE-CTFE) HALVE actuator at a constant force of 0.22. Right axis: relation between actuation strain rate and force at a constant voltage of 1100.
• Figure 4: Implementation features of the HALVE actuator system. (A) A HALVE actuator is touched on the high voltage side in the zipped state at 800. (B) Three pouch HALVE actuator partially submerged in a laboratory beaker filled with tap water lifting a 20 weight. (C) Five pouch HALVE actuator with chrome/gold electrodes lifting its power supply (). (D) Self-clearing capabilities of the HALVE actuator. (i) Strain curve of a single-pouch HALVE actuator lifting a 22 weight at a low frequency of 0.1. The actuator drops slightly in strain after the dielectric breakdown event and recovers its full strain over the next few seconds. (ii) Picture of the same actuator after $\sim 30$ dielectric breakdown events, which is still fully functional.
• Figure 5: Untethered gripper demonstrator powered by HALVE actuator. (A) Schematic view of the untethered gripper. Shown is the tendon system which connects a HALVE actuator muscle pack to each gripper finger, as well as the return spring visible in green and yellow respectively. Arrows of the same color mark the direction in which the tendon is pulled and the direction of the resulting torque on the finger's joint. (B) Front, and side views of the untethered gripper. (C) Size comparison of the HALVE actuator gripper to a standard-sized Rubiks Cube ($5.6 \times 5.6 \times 5.6$)