Instrumentation & Measurement Magazine 25-9 - 38

positions and joint angles and their derivative as a function of
time, i.e., the linear and angular velocities and accelerations
(Fig. 2) [6].
Walking Performance of a CentimeterScale
Hexapod Robot
In this study, we focused on relatively small hexapod robots
that weigh about 2-3 kg and have a maximal body span
of one meter. These dimensions are constrained by the size
and performance of commercial electric servomotors, such as
the Dynamixel AX-18A presented here. At this scale, the conventional
structure of a robotic leg is presented in Fig. 3. To
facilitate construction, reduce energy expenditure and simplify
control, the robotic leg is composed of three segments
compared to the five main segments of a real insect leg (Fig.
3) [1].
The characterization of a robotic leg (or a hexapod robot)
lacks a proxy, and thus the quantification of each level
of performance is not well defined within the robotic community.
However, such proxies deserve to be rigorously
defined for future comparisons between robotic designs.
Few test benches for hexapod robots have been built [7]. This
test bench (Fig. 4) includes the measurement techniques of
insect performance to define a unique experimental setup
that provides access to meaningful indicators for legged robotics.
Thus, this universal setup provides a complete set of
data, appropriate for estimating any characteristics of a chosen
robotic leg.
The test bench is composed of four modules:
◗ A vertical one-axis moving stage with an instrumented
treadmill: these two components simulate the robot
walking movement. Since we study only a single leg, the
movement of the ground relative to the robot's body is
represented by the freewheel treadmill and the vertical
Fig. 4. Dedicated test bench to evaluate any robotic leg. The bench allows the
leg to " walk " on a treadmill simulating the terrain displacement. Photography
by Tifenn Ripoll-VOST Collectif / Institut Carnot STAR (2021).
body oscillations [8] by the moving stage. To limit vertical
oscillations, a mechanical stop is implemented to prevent
the leg from hitting the ground during the swing phase.
The contact force of the leg's tip is measured by a 2-axis
force sensor.
◗ A three-dimensional motion capture system (Qualisys™,
Sweden. https://www.qualisys.com/) provides the
kinematics of the leg joints and segments during a walk
phase.
◗ An electrical current sensor and tangential and
normal treadmill force sensors are used to estimate the
consumption of both electrical and mechanical energy.
It is noteworthy that the normal force measurement is
provided by four force sensors covered by an aluminum
plate, the bearing surface.
Tarsus
Tibia
Femur
Trochanter, Coxa
◗ A thermal camera provides an overview of the leg's structural
weaknesses, the efficiency of thermal dissipation
and energy loss. Furthermore, ambient temperature is
measured by this camera, providing experimental repeatability
and highlighting temperature dependent leg
performance indicators. The leg can be loaded by a mass
m, and the room is maintained at 25 °C (Fig. 4).
Typically, the tested leg walks on a treadmill for a fixed
amount of time (15 min), with the sensors capturing data
which, at the end of the session, are processed to extract desired
characteristics (Table 1).
1.58
30 mm
Fig. 3. Conventional leg of a hexapod robot equipped with 3 degrees of
freedom, inspired by the Messor barbarus ant (scale 1:58). Angle α corresponds
to the thorax-coxa joint position, angle β corresponds to the trochanter-femur
joint (thus far, robotic designs have often fused the coxa-trochanter joint), and
angle γ represents the femur-tibia orientation.
38
Crossing Multiscale Data, Comparison
Between Animal and Robotic
Measurements
Table 1 presents the results obtained from measurements carried
out on a Messor barbarus ant and the hexapod robot leg
(Fig. 3). The values grouped in the table form a standard set of
indicators used for the comparison of the robotic leg with its
animal counterpart or that of another robot. A scale factor has
been added to relevant indicators, which allowed us to compare
two agents of different body scales [9]. To compare an
ant's performance indicator with that of the robot, we have to
IEEE Instrumentation & Measurement Magazine
December 2022
https://www.qualisys.com/

Instrumentation & Measurement Magazine 25-9

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