Power-Efficient Actuation for Insect-Scale Autonomous Underwater Vehicles

TL;DR

This work tackles the power challenge of insect-scale underwater actuation by first demonstrating a subgram autonomous surface swimmer (VLEIBot++) powered by two bare SMA actuators, and then systematically analyzing the stark power penalty of operating SMA actuators underwater. Through heat-transfer modeling and experimental testing, the authors develop a 13 mg encapsulated SMA actuator featuring an air-filled Kapton capsule that passively reduces the local heat-transfer coefficient, achieving average actuation power near air levels (~70–80 mW) in both air and water. The combination yields a drastic underwater power reduction (≈91% relative to bare SMA) while preserving actuation performance, enabling ~20 minutes of onboard operation for the 900 mg VLEIBot++ and moving toward practical insect-scale autonomous underwater vehicles. The work introduces a design paradigm based on passive heat-transfer control to realize energy-efficient underwater MEMS/SMA actuation, with clear implications for swarms of insect-scale AUVs and real-time onboard autonomy.

Abstract

We present a new evolution of the Very Little Eel-Inspired roBot, the VLEIBot++, a 900-mg swimmer driven by two 10-mg bare high-work density (HWD) actuators, whose functionality is based on the use of shape-memory alloy (SMA) wires. An actuator of this type consumes an average power of about 40 mW during in-air operation. We integrated onboard power and computation into the VLEIBot++ using a custom-built printed circuit board (PCB) and an 11-mAh 3.7-V 507-mg single-cell lithium-ion (Li-Ion) battery, which in conjunction enable autonomous swimming for about 20 min on a single charge. This robot can swim at speeds of up to 18.7 mm/s (0.46 Bl/s) and is the first subgram microswimmer with onboard power, actuation, and computation developed to date. Unfortunately, the approach employed to actuate VLEIBot++ prototypes is infeasible for underwater applications because a typical 10-mg bare SMA-based microactuator requires an average power on the order of 800 mW when operating underwater. To address this issue, we introduce a new 13-mg power-efficient high-performance SMA-based microactuator that can function with similar power requirements (approx. 80 mW on average) and actuation performance (approx. 3 mm at low frequencies) in air and water. This design is based on the use of a sealed flexible air-capsule that encloses the SMA wires that drive the microactuator with the purpose of passively controlling the heat-transfer rate of the thermal system. Furthermore, this new power-efficient encapsulated actuator requires low voltages of excitation (3 to 4 V) and simple power electronics to function. The breakthroughs presented in this paper represent a path towards the creation of insect-scale autonomous underwater vehicles (AUVs).

Figures (7)

• Figure 1: A swimmer and an actuator. The VLEIBot++ (left), a $900$-mg autonomous surface swimmer driven by two $10$-mg bare SMA-based actuators; and, a new low-power $13$-mg encapsulated HWD SMA-based actuator for underwater operation (right). This new actuator has a length of $15.25\,\text{mm}$, a volume without the capsule of $2.37\,\text{mm}^3$, and a volume with the capsule of $33.02\,\text{mm}^3$. The actuators that drive the VLEIBot++ have a length of $12\,\text{mm}$ and volume of $1.89\,\text{mm}^3$ each.
• Figure 2: Design and fabrication of the VLEIBot++ .(a) Fabrication process of the robot's custom-designed PCB. In Step $1$, sheets of CuFR4 are etched using laser rasterization in order to remove areas of the Cu-coating and thus create the patterns designed for each side of the PCB. In Step $2$, the two sides of the PCB are pin-aligned and adhered together with a sheet of Pyralux adhesive by applying pressure and heat inside a curing oven. In Step $3$, the off-the-shelf elements composing the interconnected power, actuation, and computation circuits are installed on the two sides of the PCB using conductive silver epoxy (MG-Chemicals $8331$D) and cured inside an oven. (b) Top and bottom of the robot's PCB. The top side of the PCB supports an MCU for computation and an IMU for sensing. The bottom side of the PCB supports two $\text{N}$-channel MOSFETs and a voltage regulator. The two sides of the PCB are connected through vias. Decoupling capacitors and pull-up/down resistors are necessary to ensure the functionality of the circuits. (c) Exploded view of the VLEIBot++'s design. This robot is composed of six main types of components: (i) two bare SMA-based microactuators of the type described in VLEIBot_2024 with passive hinges installed at their ends; (ii) two soft propulsors that propel the microswimmer forward; (iii) two transmissions that amplify the output displacements generated by the robot's actuators into the two significantly larger stroke angles required to excite the two soft propulsors of the swimmer; (iv) a convex hull made of CF that provides a buoyancy force to support the microswimmer on water; (v) an 11-mAh $3.7$-V $507$-mg single-cell Li-Ion battery (Powerstream~GM$301014$H); and, (vi)~the custom-built PCB described in~(a). (d) Swimming mode of the VLEIBot++ . The two bare SMA-based microactuators undulate the tails of the swimmer to produce the hydrodynamic forces required for forward propulsion.
• Figure 3: Open-loop swimming experiments of the VLEIBot++ .(a) Photographic composite of frames taken at $6$-s intervals from video footage of open-loop swimming Test $1$. (b)$2$D trajectories of the VLEIBot++ , $\left\{x,y\right\}$, during open-loop swimming Tests $1$, $2$, and $3$. In this plot, we placed the beginning of the trajectories at $\left\{0,0\right\}$ for the purpose of comparison. (c) Swimming speed, $v$, during Test $1$. In this test, we measured average and maximum speeds of $11.9\,\text{mm/s}$ and $18.7\,\text{mm/s}$, respectively. Video footage of these swimming experiments can be seen in the accompanying supplementary movie. This movie is also available at https://wsuamsl.com/resources/ISRR2024movie.mp4.
• Figure 4: Power-consumption characterization of bare SMA-based actuator.(a) Experimental setup used in the characterization experiments. A Mathworks Simulink Real-Time host--target system, equipped with a National Instruments PCI-$6229$ AD/DA board, is used to generate the PWM signal that triggers the MOSFET-based circuit (four-channel YYNMOS-$4$) that excites the tested actuator; a current sensor (Adafruit INA$260$ in conjunction with an Arduino UNO) measures and records the current that flows through the SMA wire that drives the actuator; a laser displacement sensor (Keyence LK-$031$) measures the instantaneous actuation output and the corresponding data are recorded using the host--target system at a sampling rate of $5$ kHz. We performed the in-air characterization first; then, we performed the underwater characterization using an acrylic pool filled with water. In~(b) and (c), respectively corresponding to operation in air and water, each blue data point indicates the mean of five $P_{\text{p}}$ values, $\bar{P}_{\text{p}}$, and associated ESD corresponding to five back-to-back experiments, for the five different PWM pairs, $\left\{f,\text{DC}\right\}$, considered here; and, each red data point indicates the mean of five $P_{\text{a}}$ values, $\bar{P}_{\text{a}}$, and associated ESD corresponding to the same five back-to-back experiments. Operating in air at $f = 1$ Hz and $\text{DC} = 7\,\%$, we measured average and peak power consumptions on the orders of $40$ mW and $0.5$ W, respectively. Operating underwater at $f = 1$ Hz and $\text{DC} = 7\,\%$, we measured average and peak power consumptions on the orders of $800$ mW and $10.6$ W, respectively. (d)--(e) Show, in red, the responses of the actuator to a $1$-Hz $7$-percent PWM voltage while operating in air and water, respectively; and, in blue, the corresponding instantaneous power consumptions. Video footage of the tested bare SMA-based actuator operating in both air and water can be viewed in the accompanying supplementary movie. This movie is also available at https://wsuamsl.com/resources/ISRR2024movie.mp4.
• Figure 5: Proposed solution for underwater actuation.(a) Design of encapsulated SMA-based microactuator capable of underwater operation. (b) One-dimensional heat-transfer model of an SMA wire surrounded by an air pocket contained by a Kapton-made capsule. $R_{\text{cond},\text{k}}$ is the thermal resistance for conduction through the Kapton membrane, $R_{\text{conv},\text{a}}$ and $R_{\text{conv},\text{f}}$ are the thermal resistances for convection through the air and external fluid (air or water), respectively. (c) Numerical-simulation results for the temperatures over time of both the driving SMA wire, $T_{\text{sma}}$, and air cavity, $T_{\text{air}}$, during a $1$-Hz $7$-percent PWM-excited actuation cycle of the encapsulated microactuator during underwater operation.