Fluidic FlowBots: Intelligence embodied in the characteristics of recirculating fluid flow

TL;DR

The paper introduces FlowBots, soft robots that embed control functionality directly into their bodies by exploiting closed-loop recirculating fluid flow. By leveraging viscous losses, flow impedance, and programmable series/parallel connections, FlowBots achieve analogue, bidirectional actuation with a minimal set of control inputs, and can be monolithically 3D-printed as single parts. Through three demonstrations—a bidirectional actuator, a gripper, and a quadruped swimmer—the work shows simplified control architectures, compatibility with water or air, and rapid on-site prototyping, all while enabling a broader design space. The authors advocate a design methodology combining CFD/FEA for exploration and open-source CAD to empower broader adoption, highlighting the potential for robust, sustainable, and versatile soft robotic systems built around recirculating flows.

Abstract

The one-to-one mapping of control inputs to actuator outputs results in elaborate routing architectures that limit how complex fluidic soft robot behaviours can currently become. Embodied intelligence can be used as a tool to counteract this phenomenon. Control functionality can be embedded directly into actuators by leveraging the characteristics of fluid flow phenomena. Whilst prior soft robotics work has focused exclusively on actuators operating in a state of transient/no flow (constant pressure), or pulsatile/alternating flow, our work begins to explore the possibilities granted by operating in the closed-loop flow recirculation regime. Here we introduce the concept of FlowBots: soft robots that utilise the characteristics of continuous fluid flow to enable the embodiment of complex control functionality directly into the structure of the robot. FlowBots have robust, integrated, no-moving-part control systems, and these architectures enable: monolithic additive manufacturing methods, rapid prototyping, greater sustainability, and an expansive range of applications. Based on three FlowBot examples: a bidirectional actuator, a gripper, and a quadruped swimmer - we demonstrate how the characteristics of flow recirculation contribute to simplifications in fluidic analogue control architectures. We conclude by outlining our design and rapid prototyping methodology to empower others in the field to explore this new, emerging design field, and design their own FlowBots.

Figures (7)

• Figure 1: Soft fluidic robots utilising a) "steady state" b) "pulsatile/cyclic" c) "recirculating" flow, as defined in this paper.
• Figure 2: Inspiration for exploiting fluid flow phenomena as a form of embodied intelligence can be found in nature. The geometry of the shark intestine (c) resembles that of a Tesla-valve (a, b). Flow circulation induced by the spiral column geometry results in a diode-like behaviour, making it harder for fluid to flow in one direction (left-to-right as pictured on the figure) than the other (right-to-left). This type of fluidic diode has no moving components; the added functionality is embodied into the structure of the device. Figure (c) has been adapted from the CT scans found in leigh_shark_2021.
• Figure 3: The principle of operation of the bidirectional actuator. By varying the outlet vent constrictions, the direction, and magnitude of the flow, as well as the pressure difference across the actuator can be controlled with an analogue characteristic. Reversing the flow direction causes the actuator to deform symmetrically in the other direction due to the mirroring of the pressure asymmetry. The percentage value of the constriction is an approximation; we estimated it indirectly by measuring the pressure difference across the actuator using two pressure gauge probes. Note that the flow path from the pressure source to the atmospheric vent is a closed, recirculatory path; this is more intuitive in the hydraulic domain, in which the reservoir, from which the flow/pressure source (PD/centrifugal pump) draws fluid, is only finitely sized.
• Figure 4: The principle of operation of a gripper consisting of two bidirectional actuators. By switching between a parallel ((a) to (g)) and series ((h) and (i)) connection of the actuators, we were able to achieve fully independent analogue control of each of the fingers using only three control inputs rather than four. Note that in the series configurations the actuators deform slightly less, due to the pressure difference across each one being effectively half of that in the parallel configuration.
• Figure 5: Operation of a quadruped FlowBot with a swimming gait that builds upon the control architecture used for the gripper described by Section \ref{['sec:gripper']}. Due to the embodiment of control into the body of the robot, as well as the characteristics of recirculating flow, FlowBots of increasing complexity can still be manufactured as single components.