1) Researchers developed a reduced-scale prototype of the MantisBot wearable robot to enable testing of stepping algorithms without human trials or a full-scale system. 2) The prototype was designed to have one-third the joint torques and limb lengths of the full-scale robot while maintaining full-scale weight. 3) Testing found the prototype's radial stiffness matched expectations but axial stiffness was less than predicted, providing data to refine the stiffness models for the limb design.

1. Building Reduced-Scale Supernumerary Robotic Limbs (SRLs)
Jairo Y. Maldonado-Contreras
" , Phillip H. Daniel
# , Harry H. Asada
#
1. Department of Mechanical and Aerospace Engineering, California State University - Long Beach
2. Department of Mechanical Engineering, Massachusetts Institute of Technology
Background
Wearable robots may prove useful in reducing the physical
strain that workers experience within industrial settings.
The MantisBot, a wearable robot, aims to provide support
to the wearer’s upper body during ground-level work with
two Supernumerary Robotic Limbs (SRLs).
This research focuses on the development of a reduced-
scale prototype of the MantisBot that would enable the
testing of step-panning algorithms on the design without
the need of human trials, high-cost testing, and the
complexity of the full-scale system.
Figure 1. MantisBot design concept.
Design Concept
Goal:
Design a quadrupedal, ten-degree-of-freedom, reduced-
scale MantisBot prototype capable of human-like
movement and step execution. The prototype's joint
torques and limb lengths must be scaled down to one-
third, but maintain the full-scale weight.
Design Constraints:
Reduced-scale Joint Torques:
• Hip: 26 Nm (y-axis) and 10 Nm (x-axis)
• Shoulder: 44Nm (y-axis) and 21 Nm (x-axis)
Reduced-scale Limb Lengths:
• Thigh: 0.2193 m
• SRL: 0.24 - 0.27m
Full-scale Weight: 92.3 kg
Design Models/Calculations:
• The axial and radial stiffnesses of the design were
modeled via Macaulay's method of deflection analysis
𝐾%&'%( = 0.47
.
/0
& 𝐾1%2'%( = 11.48
.
/0
• Expected angular backlash of the system was 1.5 deg.
Prototype Design
The prototype consists of four limbs that simulate two
thighs (2-DOF each) and two SRLs (3-DOF each). I
focused on the design of the two 2-DOF limbs (Figure 3).
Results
Conclusion and Future Work
Acknowledgments
110 mm
110 mm
65.5mm56mm
86 mm
76 mm
Figure 3. Top- and bottom-half of the 2-DOF limb.
1. Kurek, D. A., & Asada, H. H. (2017). The MantisBot: Design and impedance control of
supernumerary robotic limbs for near-ground work. 2017 IEEE International Conference on
Robotics and Automation (ICRA). doi:10.1109/icra.2017.7989700
2. Daniel, P. H. & Asada, H. H. (2017). A potential function approach to human guidance and
warning in active Mantis-Bot control. Progress Report Submitted to Sumitomo Heavy Industries,
Ltd.
References
I would like to thank Phillip Daniel for his mentorship and guidance during the development of the
prototype. Also, a thank you to Dr. Harry Asada for welcoming me into his lab Lastly, thank you MIT
MSRP staff for your support and for giving me the opportunity to conduct research at the highest
level in the most exciting of labs. This research project is supported by Sumitomo Heavy Industries,
Ltd.
The prototype was designed to have a similar range of
motion to that of a human thigh during ground-level work.
It was also required to meet the design constraints without
plastic deformation or failure. Near zero backlash was
desired to improve position control accuracy.
The actual radial stiffness of the 2-DOF limb (9.23 N/um)
was found to be within 20% of the expected radial stiffness.
The actual axial stiffness (0.19 N/um) was an not within
20% of the expected axial stiffness. Thus, our stiffness
models were representative of the limb’s radial stiffness,
but not of its axial stiffness. The next step is to build a
connecting body and begin the testing of step-planning
algorithms on the system.
Properties:
• Material: Aluminum
• Weight: 4.8 kg
• Range: 180 deg./DOF
• Backlash: 1.5 deg.
Body
2-DOF
Limb
3-DOF
Limb
Figure 5. Axial and radial stiffness data for the 2-DOF limb.
Figure 6. Cross-section view of complete assembly of the prototype.
Figure 4. 2-DOF limb complete assembly.
Figure 2.
Coordinate
system