IEEE Robotics & Automation Magazine - June 2021 - 38

such as interaction forces, stiffness and compliance, dexterity,
and the number of degrees of freedom (DoF) [1].
Conventional rigid robotic hands for industrial applications
are generally able to provide high positional accuracy,
thanks to their sophisticated actuation and sensing mechanisms.
However, it is hard to control the contact force
between the hand and objects, as the rigid hand structure
driven by electrical motors commonly generates large contact
forces. In complex real scenarios, robots may often
need to manipulate objects with varying shapes, sizes, and
poses in uncertain environments [2]. Moreover, when the
targeted objects are fragile or delicate, large contact forces
can deform or damage them. Further drawbacks of these
rigid hands lie in their heavy weight and high cost. Thus,
soft robotic hands with passive compliance have attracted
attention as inherently safe and adaptive alternatives. Not
only can soft robotic hands easily adapt to objects of various
shapes and sizes, but they can also achieve contact
smoothly, without the need of sophisticated control as
required by rigid hands. Furthermore, their soft nature
helps to minimize the damage to the manipulated objects.
Soft bending actuators, used as fingers, are the main
component of soft robotic hands and grippers. These actuators
can be sorted into various types, such as fluidic elastomer
actuators (FEAs), cable-driven actuators, shape
memory alloys, and electromagnetic/magnetic actuators.
Among these, FEAs have played a particularly remarkable
role in pushing the development of compliant hands. FEAs
are mainly made of silicone rubber and driven by pneumatics
or hydraulics. The pneumatic types of FEAs are
known as soft pneumatic actuators (SPAs). The most popular
SPAs are the pneumatic networks (pneu-nets), which
are a class of bending actuators described in [3], and the
fiber-reinforced actuators, described in [4]. Pneu-nets are
bonded by two layers: a silicone-based top layer containing
a network of numerous connected chambers and an inextensible
bottom layer. When the actuator is inflated, the
top layer extends, and the actuator achieves a bending
motion. The fiber-reinforced actuator comprises an extensible
chamber, an inextensible layer, and fibers. Its bending
mechanism is similar to that of the pneu-nets actuator.
The fiber reinforcement is used to limit the chamber in
axial extension instead of useless radial expansion. Both
types of actuators are simple in design, effective, and easy
to fabricate.
In the literature, there are different designs and
applications for soft robotic hands based on pneu-nets
bending-actuator [5]-[8] and fiber-reinforced actuator
prototypes [1], [9]-[12]. However, compared with
each other, the pneu-nets actuator has a lower capacity
of input pressure because of its total soft top layer under
the same wall
thickness, which limits its maximal
grasping force. In addition, the fiber-reinforced one has
a lower bending efficiency, which limits its bending
angle under the same pressure. To overcome their shortcomings,
in this article, we discuss the design of a novel
HBSF in which we integrate the inner-chamber network
structure inspired by pneu-nets with the fiber-reinforcement
method.
Recently, there have been rapid developments in soft
robotic hands and grippers. However, these improvements
have focused mostly on the study of soft fingers
and overlooked the importance of the palm, with the
fingers usually being assembled together and fixed to a
rigid palm or base. However, the palm plays a considerable
role in grasping functionality, and the fixed finger
position greatly limits the variety of grasp types and
poses of the hands.
The ability to perform dexterous manipulation
Figure 1. The new soft humanoid hand and the grasped objects
used in this work.
38 * IEEE ROBOTICS & AUTOMATION MAGAZINE * JUNE 2021
depends on the postural variability of a hand: the higher
this variability, the more dexterous the hand (examples of
grasping postures as in the Feix taxonomy are given in the
article by Feix et al. [13]). A robotic hand with a changeable
palm can adjust the position and orientation of its
fingers, which can significantly improve the postural flexibility
of the hand in terms of the sizes and shapes of
objects that can be grasped. Sun et al. [7] presented a
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