# The bionic Pohod15Leg leg: an exoskeletal ant-inspired leg for load transportation

Julien R. Serres, Ilya Brodoline, Stéphane Viollet

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Julien R. Serres, Ilya Brodoline, Stéphane Viollet. The bionic Pohod15Leg leg: an exoskeletal ant-inspired leg for load transportation. Multidisciplinary Biomechanics Journal, 2025, 50th congress of the., 10.46298/mbj.16143. hal-05191695v2

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# The bionic *Pohod15Leg* leg: an exoskeletal ant-inspired leg for load transportation

Julien R. Serres<sup>a, b*</sup>, Ilya Brodoline<sup>a</sup>, Stéphane Viollet<sup>a</sup>

<sup>a</sup> Aix Marseille Univ, CNRS, ISM, Marseille, France

<sup>b</sup> Institut Universitaire de France, IUF, Paris, France

* Corresponding author: julien.serres@univ-amu.fr

Received date: 31/03/2025

Accepted date: 27/06/2025

Publication date: 27/10/2025

**Keywords:** locomotion, bio-inspired robotics, legged robotics, hexapod, bionics, biorobotics

© 2025 The Authors
Licence CC-BY 4.0
Published by Société de Biomécanique

## 1. Introduction

Every year, numerous bio-inspired hexapod prototypes are built in various fields of research, such as navigation, social behavior and locomotion. Staying within these three fields, several hexapod robots developed over the last two decades are presented in this review (Manoonpong et al. 2021). The most common category corresponds to hexapods with a mass 0.5~10kg. All these robots are driven by low-power electric servomotors (each consuming an electric power of around 30W). Servomotor-based hexapods are frequently used because of their low manufacturing cost 0.5~3k€. Moreover, electric servomotors are easy to drive, as there are usually many prefabricated development boards integrating the control system.

To better understand how animals work, for example, how ants run so fast or carry such a heavy load? This requires to estimate the gap between the limits of the animal model and the bio-inspired hexapod. It is therefore relevant to proceed with the hexapod design step by step, considering the current technological limits constraining the robot design process.

In this study, we will compare the level of performance of the first exoskeletal ant-inspired leg for load transportation, called *Pohod15Leg*, designed and fabricated to be compared to the current state-of-the-art in servomotor-based legs, i.e., a bionic leg actuated by 3 Dynamixel AX-18A servomotors, called *AXLeg*. These two bionic legs will be evaluated in similar experimental conditions with the same test bench, called MiMiC-ANT

test bench (Brodoline et al. 2022). More information about the test bench structure and components is available on the following GitHub link: https://github.com/IlyaBrod/MiMiC-ANT-testbench.

## 2. Methods

### 2.1. Scaling the leg design

To sum up this part and compare the performance of a bionic *Pohod15Leg* leg with conventional legs of other hexapods (Manoonpong et al. 2021), we retain a typical scale of 1:30 allowing us to insert actuators and sensors into the exoskeleton. The *Pohod15Leg*'s tibia is equal to 30 times the size of the ant's tibia and tarsus, to compensate for the loss of body height. For the same reasons, the manufactured leg has three degrees of freedom, as do the fused coxa-trochanter and tibia-tarsus segments as often found in hexapod robotics (Manoonpong et al. 2021). Various stages are required in designing a hexapod robot inspired by the desert ant *Cataglyphis fortis*. The aim is to create an easily replicable design, in the form of a construction kit (Fig. 1) see also the GitHub project: https://github.com/IlyaBrod/Pohod15. As current hexapod robots are made up of six identical legs, the design of a single leg *Pohod15Leg* was built and evaluated with the MiMiC-ANT test bench.

### 2.2. Manufacture of the Pohod15Leg leg

Currently, in the category of electrically powered bio-inspired hexapods, with a wingspan of up to 70cm and
a maximum mass of 3kg, all parts are manufactured by means of fused filament fabrication (FFF) (Manoonpong et al. 2021). Despite the many shortcomings of this technology, it is still the most accessible and cheapest (Dey et al. 2021) The ant-inspired exoskeleton leg *Pohod15Leg* (Fig. 1) was printed on the Volumic STREAM30 ULTRA printer (Volumic 3D, Nice, France) using the Volumic ULTRA material. For this first prototyping, we did not seek to optimize the distribution of material in the exoskeleton, we only respected the scale ratio.

## 2.3. DC motors in joints

The motorization chosen for all three of the *Pohod15Leg* leg's joints was based on three DC motors. Each motor is attached to a worm gear-nut mechanism with a reduction stage. The two spur gears double the torque of the motors to reach a maximum theoretical torque of 11N.m, three times greater than the torque of Dynamixel AX-18A servomotors fitted to conventional robots. In terms of speed, the helical linkage with a pitch of 0.7mm greatly reduces the motor's rotational speed. As the no-load speed of MABUCHI RK-370CC-14230 motors is 20,700rpm, the maximum angular speed of a leg joint was estimated at 56rpm. With this rotation speed, the hexapod could walk up to an estimated maximal speed of 94cm/s. The design of each joint was carried out and optimized manually using a custom-made design software (see the GitHub project: https://github.com/IlyaBrod/Pohod15).

## 3. Results and discussion

Unlike the AntBot hexapod (Brodoline et al. 2024), which is based on a skeleton motorized by electric servomotors. The prototype *Pohod15Leg* leg is an exoskeletal ant-inspired leg (Fig. 1) and its geometry is based on *Cataglyphis* desert ant legs. The *Pohod15Leg* leg weighs 417g, which would lead to the construction of a complete hexapod robot weighing 2.5kg (the same weight as AntBot, Brodoline et al. 2024) and capable of carrying a load of 700g. The *Pohod15Leg* and *AXLeg* legs were compared using the MiMiC-ANT bench (Brodoline et al. 2022) specially built in the laboratory to compare different robotic legs with a walking frequency of 0.3 Hz (i.e. a 3-second walking cycle). This walking frequency is close to that of the *Barbarus* ant when carrying loads of up to 15 times its body weight (whereas a weightlifter can lift around twice its body weight). *Barbarus* is a Herculean ant, whereas the *Cataglyphis* desert ant is a

sprinter with a gait cycle as short as 25 milliseconds (or 40Hz). By way of comparison, Olympic sprinters reach gait cycles of 250 milliseconds (i.e. 4 steps per second, or 4Hz). To summarize, this study demonstrated that an electric servomotor-based leg was three times more energy-efficient than the *Pohod15Leg* leg, but that the *Pohod15Leg* leg heated up 34% slower than an electric servomotor-based leg (Tab. 1). After a 10-minute walk in room equipped with an air conditioning regulating the temperature at 23°C, a Dynamixel AX-18A servomotor heated up to nearly 50°C, a value close to its maximum operating temperature, i.e., 70°C. Above this temperature, the servomotors no longer function (see the video: https://www.youtube.com/watch?v=kTL-rTLoUXU). In the design of the *Pohod15Leg* leg, the DC motors are in direct contact with the air without a housing, which facilitates ventilation. In addition, the energy consumption of an electric servomotor-based leg when stationary was 33% of its energy consumption when running (Table 1), whereas the *Pohod15Leg* leg consumed nothing when stationary (Table 1), because *Pohod15Leg* leg uses worm gears in its joints. Finding a compromise between energy efficiency and heat generation using the heat-conducting structure of the leg could improve the energy performance and endurance of hexapods.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Photo of the ant-inspired Pohod15Leg leg installed on a MiMiC-ANT evaluation treadmill. Credit: Ilya Brodoline and Cyril Frésillon (2022), Institut des Sciences du Mouvement - Étienne-Jules Marey (CNRS/AMU, ISM UMR7287).</figcaption></figure>

## 4. Conclusion

These results show that ants still have a lot to teach us when it comes to undermining their locomotion in
extreme conditions. The robotic embodiment of an ant-inspired leg mimicking its structure, actuation, and coordination remains a big challenge. Our methodology will be relevant to evaluate any other bionic legs and lays the groundwork for more powerful versions of bionic legs.

<table><caption>**Table 1.** Comparing the performance indicators of ant-inspired legs to an ant (Wöhrl et al. 2017).</caption><thead><tr><th>Legs</th><th>Cataglyphis fortis</th><th>AXLeg</th><th>Pohod15Leg</th></tr></thead><tbody><tr><td>Scaling factor α</td><td>1:1</td><td>1:58</td><td>1:30</td></tr><tr><td>Body mass of the full hexapod [g]</td><td>25m</td><td>1.9k</td><td>2.5k</td></tr><tr><td>one-sixth body mass [g]</td><td>4.1m</td><td>317</td><td>417</td></tr><tr><td>Speed [cm/s]</td><td>8.7</td><td>7.8</td><td>8.1</td></tr><tr><td>Step length [mm]</td><td>9.2</td><td>106</td><td>122</td></tr><tr><td>Stride frequency [Hz]</td><td>9.1</td><td>0.37</td><td>0.33</td></tr><tr><td>Mean electric consumption [W]</td><td>-</td><td>28</td><td>76</td></tr><tr><td>Stationary electric consumption [W]</td><td>-</td><td>9</td><td>0</td></tr><tr><td>Temperature rise [°C/min]</td><td>-</td><td>3.2</td><td>2.1</td></tr></tbody></table>

## References

Brodoline, I., Moretto, P., Viollet, S., Fourcassié, V., & Serres, J. R. (2022). Test Benches for comparing the performance of an ant's leg with a hexapod robot's Leg. *IEEE Instrumentation & Measurement Magazine*, 25(9), 36–41. https://doi.org/10.1109/MIM.2022.9955461

Brodoline, I., Sauvageot, E., Viollet, S., & Serres, J. R. (2024). Shaping the energy curves of a servomotor-based hexapod robot. *Scientific Reports*, 14(1), 11675. https://doi.org/10.1038/s41598-024-62184-y

Manoonpong, P., Patanè, L., Xiong, X., Brodoline, I., Dupeyroux, J., Viollet, S., Arena, P., & Serres, J. R. (2021). Insect-inspired robots: bridging biological and artificial systems. *Sensors*, 21(22), 7609. https://doi.org/10.3390/s21227609

Wöhrl, T., Reinhardt, L., & Blickhan, R. (2017). Propulsion in hexapod locomotion: how do desert ants traverse slopes? *Journal of Experimental Biology*, 220(9), 1618–1625. https://doi.org/10.1242/jeb.137505

Dey, A., Roan Eagle, I. N., & Yodo, N. (2021). A review on filament materials for fused filament fabrication. *Journal of manufacturing and materials processing*, 5(3), 69. https://doi.org/10.3390/jmmp5030069

## Conflict of Interest Statement

None.

## Funding

This research was supported by the French Agence de l'innovation de défense (Grant DGA-2018-60-0047), the CNRS (PEPS MiMiC-ANT Program), Aix Marseille University. This work has been partially supported by ROBOTEX 2.0 (Grants ROBOTEX ANR-10-EQPX-44-01).

## Contributor Roles

JRS and SV: Conceptualization, Investigation, Methodology, Resources, Funding acquisition, Supervision, Validation, Writing original draft; IB: Methodology, Investigation, Software, Data curation, Validation, Writing-review & editing.