  The stiffness modulation mechanism for soft robotics has gained considerable
attention to improve deformability, controllability, and stability. However,
for the existing stiffness soft actuator, high lateral stiffness and a wide
range of bending stiffness are hard to be provided at the same time. This paper
presents a bioinspired bidirectional stiffening soft actuator (BISA) combining
the air-tendon hybrid actuation (ATA) and a bone-like structure (BLS). The ATA
is the main actuation of the BISA, and the bending stiffness can be modulated
with a maximum stiffness of about 0.7 N/mm and a maximum magnification of 3
times when the bending angle is 45 deg. Inspired by the morphological structure
of the phalanx, the lateral stiffness can be modulated by changing the pulling
force of the BLS. The lateral stiffness can be modulated by changing the
pulling force to it. The actuator with BLSs can improve the lateral stiffness
about 3.9 times compared to the one without BLSs. The maximum lateral stiffness
can reach 0.46 N/mm. And the lateral stiffness can be modulated decoupling
about 1.3 times (e.g., from 0.35 N/mm to 0.46 when the bending angle is 45
deg). The test results show the influence of the rigid structures on bending is
small with about 1.5 mm maximum position errors of the distal point of actuator
bending in different pulling forces. The advantages brought by the proposed
method enable a soft four-finger gripper to operate in three modes: normal
grasping, inverse grasping, and horizontal lifting. The performance of this
gripper is further characterized and versatile grasping on various objects is
conducted, proving the robust performance and potential application of the
proposed design method.
