  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.
