IEEE Robotics & Automation Magazine - March 2023 - 52

the length of the released bandage, while the remain six joints'
SEAs are used to sense the direction of the end effector's force
interacting with the human limb. Combining the length of the
released bandage and the direction of the force, the movement
of the human limb then can be estimated. Finally, according to
the estimation, the trajectory of the end effector will be adjusted
to follow the human limb's movement.
This article comprises six sections. The " Hardware Design "
section shows the hardware specifications and the premise
parameters of the bandaging task. In the " Method " section,
trajectory planning and posture adjustment strategies as well
as detailed force-position control strategy methods, including
compliant control, force control, prediction of the swaying arm
direction, and motion strategies for the robot arm, are carried
out. The analysis of the predicted results, actual experiments,
and evaluation using the prototype robot are investigated in
the " Experiment " section. The discussion and conclusion are
presented in the final sections.
HARDWARE DESIGN
The design and dimensions of the hardware are shown in Figure
2. The weight of the robot arm is 2.74 kg. The main materiTABLE
1. A comparative study of operating
soft objects.
RELATED TASKS
Dressing task [7], [8], [9], [10]
Sewing task [11], [12], [13], [14]
Soft tissue manipulation [15], [16]
Taping task [17], [18], [19], [20]
SCT
×
×
×
:
Bandaging task (proposed method) :
CFC
×
:
:
×
:
TTM
:
×
:
×
:
CFC: continuous force control; SCT: specific circling trajectory; TTM:
tracking target movement.
al used is acrylonitrile butadiene styrene plastic, which is
printed using a 3D printer. There are seven joints in the robot
arm, including three degrees of freedom (DoF) for the shoulder
joint, 1 DoF for the elbow joint, 2 DoF for the wrist joint,
and 1 DoF for the end effector. In addition, a bending structure
is designed for the elbow joint, the fourth joint, to expand
the working space of the robot arm. There are two types of
motor used in this robot arm, both of which are Dynamixel
drives supplied by Robotis Co., Ltd., Korea. The first through
fifth joints are equipped with XM540-W270-R motors, and
the sixth and seventh joints are equipped with XH430W210-R
motors.
All joints are designed with an SEA for inherent compliance.
As shown in Figure 2, the output of the motor and
the output of the shell of the joint are connected to the input
flange and output flange of the SEA, respectively, rather than
being directly solidly connected. Instead, the input and output
flanges of the SEA are connected to each other by springs.
The spring is mounted in the cavity formed by the input
flange and the output flange. The spring is compressed when
the two are rotated relative to each other. With this design,
in the event of a collision, the spring is compressed first to
absorb the impact. Conversely, the spring is also compressed
first when the motor does external work. This means that the
output torque of the joint can be obtained by measuring the
amount of compression of the spring and based on the spring
deformation relationship.
A magnetic encoder (AS5048, AMS, Inc.) is used to measure
the relative rotation angle si between the output and input
flange discussed earlier. The elastic force fs of the compressed
spring and the torque sx produced on the joint can be obtained
from the following:
fakssi=
xss= fa
(1)
(2)
where a is the center distance of the spring module, and k is
the stiffness of the spring module. The parameters of the
compliant joints are listed in Table 2, and they are set according
to the required torque range of the joints during the bandaging
task.
An active end effector is proposed based on the demand
FIGURE 1. The proposed method for a compliant robot to perform
the bandaging task on a swaying arm. (For details, please refer
to the experiment described in the " Bandaging Task With the
Proposed Method " section.)
52 IEEE ROBOTICS & AUTOMATION MAGAZINE MARCH 2023
for force control. To avoid tangling of the sensor wires of
the SEA during multiple rotations, the sensor is mounted on
another shaft, and the measurement of the spring compression
is achieved by gear transmission. As shown in Figure 2, it
includes the motor, input flange, output shaft 1, input gear,
output shaft 2, and magnetic encoder. The motor is connected
directly to the input flange, which transmits the torque to the
input gear through a spring in the groove, which then delivers
the power to output shafts 1 and 2. The magnetic encoder is
installed outside the bottom of output shaft 2, and the positioning
magnet is installed in the cylindrical groove at the bottom
of output shaft 2. The output is a bandage roller, and the end
effector can actively release the bandage to maintain a constant
torque while feeding back the length of the released bandage.
IEEE Robotics & Automation Magazine - March 2023

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