IEEE Robotics & Automation Magazine - December 2020 - 57

promising characteristics such as adaptability, light weight,
less required assembly, and low cost [1]. The intrinsic deformable structure of soft robots encourages scientists to engage
different technologies for their dynamization. One of the
most widely used actuating technologies for soft robotics is
the fluidic elastomer actuator (FEA), powered by a pressurized fluid (gas or liquid) [2]. Due to their many advantages
including easy fabrication, production high forces and large
strokes, and low-cost elastomer materials [3], FEAs have been
used in numerous configurations for various purposes, such
as locomotion [4], manipulation [5], medical applications [6],
and wearable devices [7]. These actuators can generate distributed forces that are proportional to the operating pressure
of the fluid and the surface area on which the pressure is
applied [8]. Even though there is a large diversity of applications for FEAs, many challenges remain in this field, including stiffness control and shape configuration. Researchers
have increased the performance of these kinds of actuators by
integrating them with other types of methods that help FEAs
in terms of shape control and variable stiffness. These lateral
technologies are mainly based on using variable-stiffness
materials, including shape memory polymers (SMPs) [9],
combinations of SMPs with thermoplastic polyurethane [10],
and low melting point alloys (LMPAs) [11]. The main drawbacks of SMPs are a high hysteresis and a low actuation speed
that differs from 5 to 60 s, regarding the size of the actuator
[3]. Using LMPAs is another suggested method for changing
the bending point and shape configuration in FEAs. Applying
an electric current to the alloy and heating, the structure
phase-changes locally from rigid to soft, and, thus, variable
stiffness can be achieved [12]. Like SMPs, the transition time
is the main issue in LMPAs. Depending on size and geometry,
the melting time for LMPAs differs from 1 to 30 s, while cooling takes more than 60 s [13].
In this article, we introduce a novel type of soft finger
based on bending point control and variable stiffness. The
proposed finger achieved is more flexible than previous solutions in terms of the attainable 3D space and applicable contact forces at the fingertip, by changing the position of its joint
and, thus, the bending point. The design consists of one elastomer tube as the soft link and one movable soft joint as the
actuator. Applying air pressure to the joint, it and the link will
bend concurrently. Two stepper motors are responsible for
moving the joint longitudinally along the link as well as rotating it around its axis. The joint can thus change the effective
length of the finger and the bending direction. Unlike previously proposed integrating methods with FEAs based on
SMPs or LMPAs, the position of the bending point is movable
along the length of the link, which makes the finger more
dexterous and reconfigurable.
Due to the nonlinear behavior of FEAs, their performance
strongly depends on the geometry and dimensions of the
actuator. Elsayed et al. [14] showed the effects of the position
and shape configuration of the chamber on the bending
direction and angle value; they deployed an FEM to study and
optimize these design parameters. Decroly et al. [15]

conducted an optimization study using a numerical model to
miniaturize FEAs to be applicable in minimally invasive surgery. In our work, developing an optimization procedure is
also essential to achieve our operating objectives: reconfigurability and variable stiffness. The NSGA-II algorithm is chosen as the optimization method due to its fast, nondominated
sorting approach; fast crowded distance-estimation procedure; and simple crowded-comparison operator [16]. We use
these capabilities for maximizing the bending angle up to 90°
and simultaneously minimizing the length and diameter of
the joint while dealing with a variety of design parameters.
Moreover, we investigate the sensitivity of each parameter to
reduce the computational cost and, thus, increase the convergence speed of the design procedure.
Operating Principles and Design
The schematic of the proposed soft finger is illustrated in Figure 1. The finger is composed of a pneumatically actuated
joint (the blue cylinder) and a soft link (the gray cylinder).
A longitudinal channel, embedded inside the joint, inflates
by supplying the air pressure (P1) and leads to the bending
of the joint and, consequently, the link [Figure 1(a)]. The
bending location can be longitudinally changed by sliding
the joint along the link [Figure 1(b)]. The joint can also
rotate around its main axis while the link remains steady
due to its fixed connection to the base. This causes the finger to bend in any direction in 3D space [Figure 1(c)].

Pressure Input 1 (P1)
Link

Connected to Electric
Motor (Translation
and Rotation)

Actuator Joint
(a)
Connector Rods

(b)
Pressure Input 2 (P2)

(c)
Figure 1. (a) A schematic view of the proposed finger. (b) The
sliding of the joint along the link changes the bending point and
effective length of the finger. (c) The rotation of the joint along its
axis results in changing the bending direction in 3D space.

DECEMBER 2020

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IEEE ROBOTICS & AUTOMATION MAGAZINE

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IEEE Robotics & Automation Magazine - December 2020

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