Instrumentation & Measurement Magazine 25-9 - 31

simple and intuitive way to study and compare the profound
variances in how the joint angles change as geckos move on
horizontal and vertical substrates.
Motion of Gecko's Feet and Coordination
Strategy
Trajectories of feet are crucial to reveal the intrinsic mechanism
and coordination strategy of an animal during locomotion. It is
also an important part for most of the robot locomotion control.
Trajectories [24] of a gecko's feet relative to hip joints when trotting
on a horizontal substrate are shown in Fig. 4a. Dots show
unprocessed data, whereas solid curves represent smoothed
data. The diagonal trajectories of feet are represented by the
twisted pair of curves. FL, FR, RL and RR represent the front left
leg, front right leg, rear left leg and rear right leg, respectively.
Fig. 4b is the proposed curve inspired by the trajectories of the
animal's foot, which will be described later.
The feet move backward and around the hips in a nearly
straight line during the stance phase. Three steps can be used
to characterize the motion in the swing phase. In the initial
phase, the foot ascends and maintains its prior speed along a
traveling direction. Later, the foot slows to a stop. The foot alters
its direction and picks up speed in the second stage.
The speed reduces to zero once more as it approaches the
forward-facing point, moving from the back to the front. The
foot switches directions and picks up speed to move backward
in the final phase. The foot will then reach motion speed during
the swing phase before switching to the stance phase. The
other set of diagonal feet go through the identical process. The
front feet's attitudes are generally higher than the back feet's
in leading positions. Even though they are practically identical
in attitude and phase in the backmost position, the front
feet change direction a little bit earlier in the foremost position
than the back feet.
Inter-limbs Coordination
The front and rear feet on diagonal swing in air as an open kinematic
chain with different ranges due to the diverse length,
and the major goal of the foot trajectory investigation during
the stance phase is to examine inter-coordination. From several
experimental campaigns, it was possible to assess that the
trajectories of the front right foot and rear left foot on stance
phase have a correlation coefficient of 0.9105, whereas the
trajectories of the front left foot and front right foot have a correlation
coefficient of 0.9302 each. These findings demonstrate
the strong correlation between the data of feet on a diagonal
line. According to the duty cycle of the front right foot, curves
are separated into four segments (S1, S2, S3, and S4) (Fig. 4a)
[24]. The four discrete plot pairings in the stance phase are split
from the entire cycle, and the solid lines represent the front
feet's linear regression findings (Fig. 4a). Each graphic reveals
that the two sets of data change in a manner that is close to a
linear trend.
The high correlations between the curves indicate that they
are often trending in the same direction. The segments from
top to bottom depict the motion of the feet during the stance
phase in relation to the hip joint; the coincidence reveals that
their speeds are nearly the same during locomotion, despite
the fact that the body is soft and swinging. This result solves
the inter-limbs coordinate requirement for a robot with a stiff
body to alleviate the force constraint between stance feet.
Intra-limb Coordination
For a single leg, a motion cycle comprises of the swing and
stance phases in the trajectory of feet (Fig. 4a). In the swing
phase, the motion direction switches twice. Between the first
and second stages is where the first process takes place. After
reaching its peak during the stance phase, the speed drops
to zero and then reverses course. The second process, which
Fig. 4. Trajectories of gecko's feet and the proposed robot's foot trajectory. (a) Original curves of gecko's feet; (b) Proposed trajectory of robot's foot inspired from
animal in up-down and moving direction. (Adapted from [24] with permission ©2017 Springer.)
December 2022
IEEE Instrumentation & Measurement Magazine
31

Instrumentation & Measurement Magazine 25-9

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