IEEE Robotics & Automation Magazine - December 2020 - 71

larger forces during depressurization, as observed in the previous experiment [Figure 4(c)]. This effect was observable at all
loads but might cause significant issues for control at lower
payloads, as the actuator then requires very low pressures to
lower its joint angle.
The encoder was then used to realize proportional-integral-derivative (PID) control of the joint (P: 1.200, I: 0.010,
D: 0.001) and made to follow a sinusoidal wave with an
amplitude of 80°, from 20 to 100°, with a period of 20 s and
a load of 800 g [Figure 4(d)]. Although the joint can follow
the input reasonably well for most of the motion, the inflation speed is too slow to follow the desired input at high
pressures, and the deflation speed is too slow at low pressures. This could be due to the pneumatic hardware but
appears to be mainly due to reduced airflow between pouch
motors as the channel between them gets compressed. The
hysteresis of the joint also causes the actuator to stay at
100°, while the pressure is reduced sufficiently to overcome
the hysteresis. These experiments show that the joint is
controllable but that its inflation and deflation speed could
be an issue if high speeds are required and that position
sensing with feedback control is necessary for precise control. Higher airflow, during both inflation and deflation,
could also help overcome the hysteresis. Such a soft robotic
joint, coupled with a precise sensor, could be used to realize
precise motions with safe human-robot interaction due to
its innate compliance and low inertia.

The stiffness of the joint at different pressures was tested
by setting the pressure in both antagonistic chambers to
equal values while the arm was unloaded and by adding a
payload at the tip. The payload was then increased in increments of 200 g until 1,000 g and then decreased back to 0 g
in decrements of 200 g while measuring the bending angle
of the joint [Figure 4(e)]. The results show that an increase
in pressure of the actuators of the joint significantly increases its stiffness and that adjusting the antagonistic actuator
can be used to modify the behavior of the joint for different
applications. The hysteresis of the joint can be seen in this
experiment by the joint not returning to its original angle
after unloading.
Applications
Soft Robotic Arm
A 3-DoF soft robotic arm with a three-fingered gripper was
built by assembling three joints with designs identical to the
previously presented joint using rigid elements [Figure 5(a)].
The first joint is connected directly to the second, an inflatable
member is used to distance the second and third, and a second
inflatable member is used to distance the third and the gripper.
The total length of the arm from its base to the tip of the gripper is 80 cm [Figure 5(b)]. The first joint is used to produce a
horizontal rotary motion at the base of the arm and produces a
range of motion of 140°, while the second joint is used to

Second
Joint
Gripper

First
Joint

y
z
First Joint Frame

Second Joint Frame

Third Joint Frame

Third Joint

Gripper Frame

(a)

(b)

y
z

y
x

(c)

(d)

(e)

Figure 5. (a) The rigid parts for assembly of the robotic arm. (b) The assembled robotic arm with three joints and three fingers. (c)
The horizontal motion of the first joint. (d) The combined motion of the first and second joint. (e) The combined range of motion of
all three joints.

DECEMBER 2020

IEEE ROBOTICS & AUTOMATION MAGAZINE

IEEE Robotics & Automation Magazine - December 2020

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