Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Progress Towards Submersible Microrobots: A Novel 13-mg Low-Power SMA-Based Actuator for Underwater Propulsion

Cody R. Longwell, Conor K. Trygstad, Francisco M. F. R. Goncalves, Ke Xu, Nestor O. Perez-Arancibia

TL;DR

This paper tackles the challenge of powering underwater SMA-based microrobots, where conventional dry SMA actuators incur large energy penalties due to the high heat-transfer coefficient of water. By systematically comparing actuator performance in air and water, the authors quantify power requirements: average power rises from about 40 mW in air to about 0.9 W in water (with peak powers up to 10.7 W at 1 Hz). To address this, they propose a Kapton-based insulating air chamber around the SMA wires to passively reduce heat transfer, enabling similar power needs in water as in air. A first-generation 13 mg prototype achieves 1 Hz operation with roughly 150 mW average power and 1.6 W peak power, demonstrating a viable path toward autonomous underwater microswimmers and guiding future evolution of the VLEIBot++ with bioinspired propulsion mechanisms.

Abstract

We introduce a new low-power 13-mg microactuator driven by shape-memory alloy (SMA) wires for underwater operation. The development of this device was motivated by the recent creation of microswimmers such as the FRISHBot, WaterStrider, VLEIBot, VLEIBot+, and VLEIBot++. The first four of these robots, ranging from 30 to 90 mg, function tethered to an electrical power supply while the last platform is an 810-mg fully autonomous system. These five robots are driven by dry SMA-based microactuators first developed for microrobotic crawlers such as the SMALLBug and SMARTI. As shown in this abstract, dry SMA-based actuators do not operate efficiently under water due to high heat-transfer rates in this medium; for example, the actuators that drive the VLEIBot++ require about 40 mW of average power at 1 Hz in dry air while requiring about 900 mW of average power at 1 Hz in water. In contrast, the microactuator presented in this abstract consumes about 150 mW of average power at 1 Hz in both dry air and water; additionally, it can be excited directly using an onboard battery through simple power electronics implemented on a custom-built printed circuit board (PCB). This technological breakthrough was enabled by the integration of a soft structure that encapsulates the SMA wires that drive the actuator in order to passively control the rates of heat transfer. The results presented here represent preliminary, yet compelling, experimental evidence that the proposed actuation approach will enable the development of fully autonomous and controllable submersible microswimmers. To accomplish this objective, we will evolve the current version of the VLEIBot++ and introduce new bioinspired underwater propulsion mechanisms.

Progress Towards Submersible Microrobots: A Novel 13-mg Low-Power SMA-Based Actuator for Underwater Propulsion

TL;DR

This paper tackles the challenge of powering underwater SMA-based microrobots, where conventional dry SMA actuators incur large energy penalties due to the high heat-transfer coefficient of water. By systematically comparing actuator performance in air and water, the authors quantify power requirements: average power rises from about 40 mW in air to about 0.9 W in water (with peak powers up to 10.7 W at 1 Hz). To address this, they propose a Kapton-based insulating air chamber around the SMA wires to passively reduce heat transfer, enabling similar power needs in water as in air. A first-generation 13 mg prototype achieves 1 Hz operation with roughly 150 mW average power and 1.6 W peak power, demonstrating a viable path toward autonomous underwater microswimmers and guiding future evolution of the VLEIBot++ with bioinspired propulsion mechanisms.

Abstract

We introduce a new low-power 13-mg microactuator driven by shape-memory alloy (SMA) wires for underwater operation. The development of this device was motivated by the recent creation of microswimmers such as the FRISHBot, WaterStrider, VLEIBot, VLEIBot+, and VLEIBot++. The first four of these robots, ranging from 30 to 90 mg, function tethered to an electrical power supply while the last platform is an 810-mg fully autonomous system. These five robots are driven by dry SMA-based microactuators first developed for microrobotic crawlers such as the SMALLBug and SMARTI. As shown in this abstract, dry SMA-based actuators do not operate efficiently under water due to high heat-transfer rates in this medium; for example, the actuators that drive the VLEIBot++ require about 40 mW of average power at 1 Hz in dry air while requiring about 900 mW of average power at 1 Hz in water. In contrast, the microactuator presented in this abstract consumes about 150 mW of average power at 1 Hz in both dry air and water; additionally, it can be excited directly using an onboard battery through simple power electronics implemented on a custom-built printed circuit board (PCB). This technological breakthrough was enabled by the integration of a soft structure that encapsulates the SMA wires that drive the actuator in order to passively control the rates of heat transfer. The results presented here represent preliminary, yet compelling, experimental evidence that the proposed actuation approach will enable the development of fully autonomous and controllable submersible microswimmers. To accomplish this objective, we will evolve the current version of the VLEIBot++ and introduce new bioinspired underwater propulsion mechanisms.
Paper Structure (3 sections, 2 figures)

This paper contains 3 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Dry Actuation and Power-Consumption Characterization in Air and Water
  3. Novel Low-Power SMA-Based Actuation for Underwater Propulsion

Figures (2)

  • Figure 1: Photograph of the VLEIBot++. This microrobot is an $810$-mg surface swimmer driven by a dry SMA-based actuator that can function autonomously from both the power and control perspectives (see accompanying supplementary movie).
  • Figure 2: Experimental measurement of the power consumed by SMA-based microactuators during operation in air and water.(a) Dry SMA-based microactuator developed to drive VLEIBot-like swimmers and characterized through the power experiments discussed in this abstract (top), and conceptual design of a low-power underwater SMA-based microactuator (bottom). (b) Experimental setup used to measure displacement and power consumption of the tested SMA-based microactuator in both air and water. The output displacement of the actuator is measured using a Keyence LK $031$ laser sensor and the current used to compute power is measured using an Adafruit INA260 sensor. (c) Hardware configuration used to collect the data for identifying the system that maps the true actuation output to the measurement distorted by the path of the sensing laser (water and acrylic). (d) Identified model of the sensing system described in (c). (e) Mean and experimental standard deviation (ESD) of the average ($P_{\text{a}}$) and peak ($P_{\text{p}}$) power consumption of dry SMA-based actuator tested in air and water. (f) Actuator response and power consumption in air and water for a $1$-Hz PWM excitation.