Monolithic Units: Actuation, Sensing, and Simulation for Integrated Soft Robot Design

TL;DR

The paper addresses the challenge of integrating actuation, structure, and sensing in soft robots by introducing the Monolithic Unit (MU), a co-designed actuator–lattice–sensor block whose geometry ties lattice unit cell size $U$ to actuator features such as bladder diameter $d=2U$. A simulation-informed workflow uses lattice homogenization and SOFA-based finite-element models to exhaustively evaluate candidate embedded optical waveguide paths, selecting the path that minimizes deviation from baseline actuation. Experimental validation across three scales and a two-finger gripper demonstrates that sensing can be embedded with minimal perturbation to mechanical response, preserving actuation while enabling optical readouts. The MU framework offers reproducible design rules and scalable, simulation-guided integration that pave the way for multi-unit, proprioceptive soft robotic systems.

Abstract

This work introduces the Monolithic Unit (MU), an actuator-lattice-sensor building block for soft robotics. The MU integrates pneumatic actuation, a compliant lattice envelope, and candidate sites for optical waveguide sensing into a single printed body. In order to study reproducibility and scalability, a parametric design framework establishes deterministic rules linking actuator chamber dimensions to lattice unit cell size. Experimental homogenization of lattice specimens provides effective material properties for finite element simulation. Within this simulation environment, sensor placement is treated as a discrete optimization problem, where a finite set of candidate waveguide paths derived from lattice nodes is evaluated by introducing local stiffening, and the configuration minimizing deviation from baseline mechanical response is selected. Optimized models are fabricated and experimentally characterized, validating the preservation of mechanical performance while enabling embedded sensing. The workflow is further extended to scaled units and a two-finger gripper, demonstrating generality of the MU concept. This approach advances monolithic soft robotic design by combining reproducible co-design rules with simulation-informed sensor integration.

Figures (6)

• Figure 1: Overall workflow for the Monolithic Unit (MU). Input: a lattice unit cell and actuator geometry define the base structure. Initialization: candidate sensor points are extracted from lattice nodes and constrained by manufacturable sensor geometry. Design: simulation of the baseline (Model A) and sensorized (Model $\alpha$) configurations identifies the path minimizing curvature deviation. Output: printed MU incorporating the selected sensor layout. Application: demonstration of the sensorized MU integrated into a simple gripper.
• Figure 2: Design and fabrication of the Monolithic Unit. (a) Co-designed actuator and lattice geometry showing the body-centered lattice (green) and pneumatic cavities (gray). (b) Microscopy images of internal lattice and cross-section of the membrane junctions (outlined in red) confirming print fidelity. (c) Fabricated MUs at scales $0.75\times$, $1.00\times$, and $1.50\times$
• Figure 3: Simulation of the MU. (a) Broken-out view of components used in simulation: envelope, lattice, membrane, cavity, and sensor domains. (b) Pressure actuation profile used to drive deformation during bending cycles. (c) Model A (no sensors) and Models $\alpha_1$–$\alpha_{10}$ (sensorized configurations) showing different simulated sensor lengths and positions.
• Figure 4: Evaluation of simulated sensing configurations. Average deviation along actuator length (left) and actuation time (middle) for each sensor configuration $\alpha$ at three scales: $0.75\times$ (red), $1.0\times$ (blue), and $1.5\times$ (green). Fabricated models (right) highlight sensors $\alpha_4$ and $\alpha_2$, which exhibit minimal deviation from the baseline Model A.
• Figure 5: Experimental characterization of the Monolithic Units. (a) Sequential images showing bending of the printed MU under pneumatic actuation. (b) Bending angle as a function of pressure for the three scales ($0.75\times$, $1.0\times$, $1.5\times$). (c) Normalized optical response ($\Delta V/V_f$) versus bending angle, showing reduced sensitivity at small angles for the $1.5\times$ MU due to limited curvature.