Investigating the Performance of Soft Robotic Adaptive Feet with Longitudinal and Transverse Arches

TL;DR

The paper addresses the problem of ground adaptation for biped robots with flat, rigid feet by extending the SoftFoot design to include a flexible transverse arch, enabling frontal-plane adaptability in addition to sagittal-plane compliance. It systematically narrows the design space to five five-module configurations (KKF, KKK, KRF, KRK, KRR) and benchmarks them against a prior SoftFoot and a fully rigid foot using benchtop tests with a robotic arm interacting with obstacle-filled plates. Results show that configurations with an elastic frontal arch and strategic rear/heel connections (notably KRK and KRF) offer superior stability and a larger stability region, especially for forefoot obstacle contact, outperforming baselines. The findings suggest two-arch soft feet can enhance robotic locomotion and prosthetic foot performance on unstructured terrains, with future work focusing on stiffness tuning and validation on real walking robots and prosthetic devices.

Abstract

Biped robots usually adopt feet with a rigid structure that simplifies walking on flat grounds and yet hinders ground adaptation in unstructured environments, thus jeopardizing stability. We recently explored in the SoftFoot the idea of adapting a robotic foot to ground irregularities along the sagittal plane. Building on the previous results, we propose in this paper a novel robotic foot able to adapt both in the sagittal and frontal planes, similarly to the human foot. It features five parallel modules with intrinsic longitudinal adaptability that can be combined in many possible designs through optional rigid or elastic connections. By following a methodological design approach, we narrow down the design space to five candidate foot designs and implement them on a modular system. Prototypes are tested experimentally via controlled application of force, through a robotic arm, onto a sensorized plate endowed with different obstacles. Their performance is compared, using also a rigid foot and the previous SoftFoot as a baseline. Analysis of footprint stability shows that the introduction of the transverse arch, by elastically connecting the five parallel modules, is advantageous for obstacle negotiation, especially when obstacles are located under the forefoot. In addition to biped robots' locomotion, this finding might also benefit lower-limb prostheses design.

Figures (11)

• Figure 1: (A) The human foot. (B) The SoftFoot described by piazza2016piazza2024. (C) The SoftFoot proposed in this paper. The longitudinal arch is highlighted in red, the transverse arch in yellow, and the plantar aponeurosis in green.
• Figure 2: (A) The components of the basic 2D module are: (1) adaptive sole; (2) frontal arch; (3) rear arch; (4) heel; (5) coil spring. In the adaptive sole, the elastic bands are shown in light blue, and the tendon routing in red. (B) Five basic 2D modules placed in parallel form the SoftFoot 3D. Note that the rear arch of the central module is longer than that of the other modules to allow the connection of the foot to the user and, thus, forces transmission.
• Figure 3: (A) Schematic representation of the six components (referred to as links) and the five joints of each basic module of the SofFoot 3D. The coil spring connecting the rearmost small rigid body of the adaptive sole to the rear arch is also shown, together with the two elastic elements at the toes representing the passive elastic joints connecting the three phalanges in series. The smaller images on the right show the behaviour of the adaptive sole when the foot is loaded (B) on even ground, or (C)-(E) on obstacles.
• Figure 4: (A)-(E) The five selected configurations of the SoftFoot 3D and (F) the SoftFoot described in piazza2016piazza2024, labeled according to the connection types across respectively the frontal arches, the rear arches, and the heel links. The connection types are displayed in green if free (F), orange if elastic (K), and red if rigid (R).
• Figure 5: Solid models of (A)-(E) the five configurations of the SoftFoot 3D, (F) the SoftFoot described in piazza2016piazza2024, and (G) a completely rigid foot. The constraints to connect the five basic modules are implemented: cylindrical steel pins are used for the rigid connection; a sheet of nitric rubber fixed to the links through bolts is used for the elastic connection.