IEEE Robotics & Automation Magazine - June 2021 - 46

transmitted to, and measured by, the scale. The weight was
estimated via F = mg; e.g., the generated grasping force by
applying 65-kPa air pressure equally to both chambers of
S11 is 249.40 × 10-3 × 9.8 = 2.44 N.
According to the test results, the bending angle increases
nonlinearly with the applied air pressure, with a relatively
slow increase in the low-pressure region and a rapid
increase beyond 20 kPa. It is obvious that the variation
between the force and bending angle has a close positive
correlation. However, it is also noted that, under the same
air pressure, fingers with more segments generate larger
bending angles and forces when pressurizing both left and
right chambers, but they generate smaller angles and forces
when pressurizing only a single chamber. This phenomenon
actually reflects the different mechanisms of the bending
and deflection motions. The greater number of
segments means more grooves, which mutually swell during
inflation, working as pneu-nets actuators and fewer
small sections, which extend during inflation, working as
fiber-reinforcement actuators. In addition, as shown in Figure
7(j), the generated grasping force of an S1 finger
becomes larger than for those with multiple segments when
the applied air pressure goes from 20 to 50 kPa.
Figure 7(h) shows the deflection displacement in the sideto-side
axis. The S11 finger has significantly better performance
for deflections up to 20 mm [see Figure 7(e) under
" 25 kPa " ]. Combining the experimental results of bending
angle and force, the S11 finger is the second most easily driven
under double-chamber actuation, and it presents the minimum
bending angle under single-chamber actuation. Both
characteristics are conducive to enhancing the deflection
region of the HBSFs. It is also notable that the finger first
wiggles outward in the initial stage of the actuation with
lower air pressure and then turns back and wiggles inwards
when the bending angle of the HBSFs becomes large at higher
air pressure. This results in a loss of efficacy of the deflection
motion, even to the point of achieving exactly the
opposite displacement. To get the maximum deflection displacement
in the side-to-side axis, we selected the key structure
parameters of the HBSFs of finger S11 in our new hand.
The grasping force of S11 is about 1.23 N and 0.82 N under
50-kPa air pressure applied to both chambers and one single
chamber, respectively.
Analysis of the Palm
Figure 8 presents the experimental results for the soft
palm. In contrast to the more typical fixed palm, almost
all of the parts of our soft palm are deformable, which
greatly increases the diversity of hand postures. The
measuring method for the palm is the same as that for
the HBSF.
Figure 8(e) shows the relationship between the deformation
performance and the air pressure of PP-A. It indicates that the
splaying angle and force increase slowly when the applied air
pressure increases from 10 to 60 kPa and then achieve a rapid
increase beyond 60 kPa. Figure 8(a) shows the posture of the
hand under 90-kPa air pressure with a splaying angle of 50°. The
air pressure should not be more than 100 kPa to avoid collapse.
The influence of gravity on palm bending can be observed
in Figure 8(b) and (c). The bending angles of the pronated
palm are obviously larger than those of the supinated palm.
The maximum angle of the palm-bending motion is 68°, and
that of the thumb abduction is about 90°. These two bending
ranges can meet most common grasping needs. Without the
fiber reinforcement, the two actuators of PP-B should be driven
under a safe range of air pressure: lower than 40 kPa.
(a)
(b)
80
40
50
20
02060
Air Pressure (kPa)
(e)
100
3
4
2
1
02040
Air Pressure (kPa)
(f)
02040
Air Pressure (kPa)
(g)
Figure 8. The validation of palm performance on bending angle and output force for (a) and (e) palm splaying; (b), (c), and (f) palm
bending with palm down and palm up; and (d) and (g) thumb abduction. The red plots correspond to the red axes.
46 * IEEE ROBOTICS & AUTOMATION MAGAZINE * JUNE 2021
40
Palm Down
Palm Up
Force
(c)
4
3
2
1
50
70
90
(d)
3
2
1
Splaying Angle (°)
Force (N)
Splaying Angle (°)
Force (N)
Splaying Angle (°)
Force (N)
IEEE Robotics & Automation Magazine - June 2021

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