IEEE Robotics & Automation Magazine - December 2020 - 63

tipping point of the finger is measured as a function of different parameters, including the longitudinal position of the
joint and the stiffening pressure (P2). All of the tests are conducted under the actuation pressure of P1 = 14 kPa. The
results are presented in Figure 10(b). As illustrated, by changing the position of the joint toward the tip of the finger, the

(a)

(b)

(c)

100
90
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60
50
40
30
20
10
0

Conclusions
In this article, an innovative variable-stiffness soft finger with a
fluid-actuated movable joint was introduced and optimized in
terms of its primary characteristics. The finger consists mainly of
one soft sliding and rotating joint as the bending actuator and a
soft link as the body. Applying pressurized air into the joint's
chamber, the joint and, consequently, the link both bend in a
specific direction. The location and direction of the bending
can be changed by sliding the joint longitudinally along the
link and rotating it around its main axis using two electric
motors. The variable length of the finger with the capability
of bending in different directions results in a large diversity
of configurations. The workspace analysis exhibited the
advantage of this reconfigurability by extending the available workspace of the fingertip, in contrast to conventional
FEAs. The local sensitivity of the design parameters
involved in the problem was analyzed. Optimization over
the important parameters was performed to minimize the
joint dimensions and maximize the bending angle of the
finger. The model included a large number of design parameters with nonlinear relations, which made prediction of the
deformation difficult, i.e., small changes in each one could
lead to large deviations in the final results. Hence, implementing the optimizing process is necessary to investigate
the acceptable and manufacturable range of these parameters. The optimal geometrical parameters were used for fabricating a prototype that validates the numerical model.
Another experiment was designed to study the amount of

700

Numerical Results
Experimental Results

600
Force (mN)

Bending Angle (°)

Figure 9. (a) The overall view of the assembled prototype.
(b) The test bench for measuring the joint angle. (c) The test
bench for measuring the fingertip force.

applied force increases by almost three times. Furthermore,
applying the pressurized air into the link results in higher
stiffness and thus a higher amount of force, up to 650 mN,
which is twice the initial value.

P2 = 0

500

P2 = 130 kPa

400
300
200
100

8
10
P1 (kPa)
(a)

6
7
8
Position of Joint Center (cm)
(b)

Figure 10. (a) The bending angles of the finger under different applied pressures: a comparison between the ANSYS FEM numerical
simulation and the experimental results. (b) The fingertip force test results as a function of stiffening pressure (P2) and joint longitudinal
position.

DECEMBER 2020

IEEE ROBOTICS & AUTOMATION MAGAZINE

IEEE Robotics & Automation Magazine - December 2020

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