REVIEW 4 major objections 5 minor 1 cited by
Optimal Trajectory Planning in a Vertically Undulating Snake Locomotion using Contact-implicit Optimization
T0 review · 4 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read A reduced-order, contact-implicit model based on Moreau's stepping-forward approach can predict the vertical undulation gait of the COBRA snake robot, matching trajectory shape, head velocity, and joint torques across simulations and…
desk verdict Useful contact-implicit ROM for vertical snake undulation, but accuracy claims rest on qualitative comparison and hand-set contacts; title overreaches. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
Moreau–Jean time-stepping with contact constraints expressed as a differential inclusion, solved as a second-order cone program. At each step, an unconstrained velocity $\tilde{v}$ is predicted, then corrected by contact impulses $f_{c,i}$ subject to complementarity conditions $g_i \ge 0$, $f^n_{c,i} \ge 0$, $g_i f^n_{c,i} = 0$, a friction cone $|f^t_{c,i}| \le \mu_i |f^n_{c,i}|$, and a semi-implicit position update $q_{n+1} = q_n + \Delta t v_{n+1}$. The optimization over the next state and contact forces replaces any hand-scheduled contact pattern; contact forces and gaits emerge from the constraints.
What would settle it
Run the same vertical undulation gait on the physical COBRA robot over two surfaces with clearly different friction properties (for instance, polished vinyl and rubber mat) while keeping joint commands fixed; if the SOCP model with a single $\mu$ does not reproduce the change in head velocity, travel distance, or slip direction seen on hardware, the point-contact Coulomb assumption is falsified for this gait. A more direct check would place a force plate under the robot and compare measured ground reaction forces with the model's predicted short-duration force spikes.
Extended reading notes
Core claim
The central claim is that the SOCP-based reduced-order model (Eqs. 13-19) reproduces the vertical undulation of COBRA: over a ten-second gait the model and hardware both travel roughly one meter forward, with matching movement direction, trajectory shape, head-module velocity, and actuation torque profiles. The model's predictions are not identical to hardware; the paper reports that joint backlash attenuates the physical robot's joint motion and shortens its travel, while the analytical model shows larger oscillations, shorter travel, and sharp torque spikes from numerical integration instabilities. The authors interpret the comparison as validation of the model's ability to capture the overall dynamics, with discrepancies attributed to ground reaction force estimation and tunable contact hyperparameters rather than to a wrong model structure.
Load-bearing premise
The model's predictions rest on representing the ground as flat with thirteen point contacts under Coulomb friction with a fixed friction coefficient ($\mu = 0.5$) and on hand-set spring-damper parameters in the comparison simulations; if real ground contact is distributed, compliant, or direction-dependent, the agreement shown may not carry to other surfaces.
Editorial extensions
If this is right
- The same SOCP can be used directly as a planner: joint torques, contact forces, and base motions are decision variables, so a vertical undulation trajectory needs no predefined contact schedule.
- Because the model is a small convex program with a dozen states, it can be evaluated quickly enough for iterative gait design on the physical robot, where a 500 Hz controller already runs in the head module.
- If the model generalizes to other gaits, the contact-implicit formulation transfers to three-dimensional contact patterns and loco-manipulation, as the conclusion sketches.
- The torque profiles match hardware up to actuation limits, so the model can be used to check feasibility of planned motions within the robot's 10 Nm saturation.
Reading between the lines
- A direct test of the load-bearing assumption would vary the ground surface under COBRA (e.g., low-friction vinyl vs. high-friction rubber) while keeping the gait fixed; the point-contact Coulomb model with a single $\mu$ predicts specific changes in slip and travel that could be checked against hardware.
- The same contact-implicit program could be extended to uneven terrain by substituting measured depth maps into the gap function $g_i$, which the model already updates from $n_i^T J_{c,i} v$, turning the planner into a closed-loop reactive controller.
- One could embed the SOCP in a receding-horizon model predictive controller by treating the measured joint state as the initial condition and using the predicted contact forces as feedforward; the paper does not claim this, but the structure of Eqs. (13)-(19) is compatible with it.
- The observed torque spikes from numerical instabilities suggest a regularization or smoothing modification to the objective $\Phi(v_{n+1})$ that would make the planner's outputs directly executable without filtering; this is an engineering refinement, not a change to the model's core claim.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces a reduced-order dynamic model of a 12-link snake robot moving on flat ground, in which each module is represented by a virtual convex sphere, ground contacts are governed by Coulomb friction, and the state trajectory is resolved through a Moreau-Jean time-stepping scheme formulated as an optimization problem (Eqs. 13-19). The model is evaluated on a prescribed vertical undulation gait by comparing its predictions against a Simscape reduced-order model, a CAD-based high-fidelity Simulink model, and hardware experiments on the COBRA robot. The central claim is that the analytical model accurately captures the head-module trajectory shape, movement direction, and velocity, with a close match in actuation torques. The manuscript frames this work as enabling contact-implicit optimal trajectory and acyclic contact planning for snake locomotion.
Significance. If the model-accuracy claim is substantiated, the work is a useful step toward applying differential-inclusion/Moreau contact-implicit formulations to a multi-contact snake platform: it offers a planar reduced-order model that is computationally efficient, it is validated against independent hardware data rather than only against itself, and the authors are transparent about observed discrepancies such as joint backlash, torque spikes, and shorter travel distance. These strengths make the central approach plausible and worth pursuing. However, the validation is currently qualitative: no error metrics, no sensitivity analysis, and no repeated-trial statistics are provided, and the most load-bearing claim of 'accurate' trajectory and velocity prediction is accompanied by the paper's own admission of larger oscillations and shorter travel. The contribution would meet the journal bar only after the accuracy claim is quantified and the optimization formulation is clarified.
major comments (4)
- [Section IV, Figure 7] The central accuracy claim, stated as 'the analytical model accurately captures the overall trajectory shape and movement direction, with a similar agreement observed in the predicted velocity of the head module,' is supported only by qualitative visual comparison. No error metric (e.g., RMSE, normalized trajectory error, or travel-distance error), no repeated experimental trials, and no confidence intervals are reported, and the same paragraph admits that the analytical model 'predicts larger oscillations with shorter travel distances.' Because the friction coefficient (mu = 0.5) and the Simulink spring-damper parameters (10^4 and 10^3) are hand-set, the observed agreement could be an artifact of parameter tuning. Please add quantitative trajectory and velocity errors, a friction-coefficient sensitivity study, and uncertainty bounds over experimental trials.
- [Section III, Eq. (16)] The complementarity constraint g_i f_i = 0 is bilinear and nonconvex, so the optimization problem in Eqs. (13)-(19) is not a second-order cone program as claimed, unless some convex relaxation or regularization is applied. The manuscript does not state how this constraint is handled in practice (e.g., complementarity smoothing, penalty relaxation, active-set iteration, or an MPCC solver) nor which numerical solver is used. Since the paper's stated computational benefit rests on the optimization being a tractable SOCP, this is a load-bearing algorithmic detail that must be clarified.
- [Section IV, first paragraph] The experiments validate a prescribed joint-position trajectory, not an optimized trajectory: the text states that 'this gait was defined by a prescribed joint trajectory, from which joint torques, contact forces, and base accelerations were computed.' The title and introduction claim optimal trajectory planning, but the optimization in Eqs. (13)-(19) includes only a regularization cost u^T u, and no planning scenario, objective, or optimized gait is presented. Please specify how the prescribed joint trajectory is enforced as a constraint or as an input to the optimization, and, if trajectory planning is a claimed contribution, include a demonstration of it.
- [Section IV, Figure 8] The contact-force comparison reveals qualitatively different behavior between the analytical/ROM point-contact models and the high-fidelity model: the point-contact models produce shorter, higher-magnitude impulses, while the high-fidelity model distributes contact over longer durations. Because contact modeling is the core of the proposed approach, the paper needs to quantify this discrepancy (e.g., contact duration, peak force, impulse) and discuss whether the point-contact model remains adequate for the intended planning use despite this qualitative mismatch. Without such a quantitative contact-level assessment, the central modeling claim is only partially verified.
minor comments (5)
- [Section III] There is a typo in 'optimiztion' before Eq. (13), and the Moreau time-stepping derivation is repeated in Eqs. (4)-(6) and (7)-(12); the duplication could be streamlined to improve readability.
- [Section III, Eqs. (2), (14)] The sign convention for the normal contact force is inconsistent: Eq. (2) states f_n <= 0 for compressive force, while Eq. (14) imposes f_n >= 0. Please clarify the relationship between these definitions and the contact normal n_i = [0,1].
- [Section III, Eq. (1)] The state dimension q in R^12 should be reconciled with the stated platform of 12 links and 11 actuated joints, and the dimensions of J_{c,i} and the contact normal n_i should be defined explicitly for the 13 contacts.
- [Section IV] Figures 7-9 would benefit from axis labels, units, and legend details; in particular, Figure 9 should state whether the hardware torque data are from joint current sensing and over how many trials, and Figure 8 should specify which representative link and which time window are shown.
- [Section III] For reproducibility, please report the time step used in the Moreau scheme, the convergence tolerance, the solver used for the optimization, and the runtime of the 10-second simulation window.
Circularity Check
No significant circularity: the contact-implicit SOCP model is validated against independent hardware and Simulink data, and self-citations concern the hardware platform rather than the claimed derivation.
full rationale
The derivation chain is self-contained: the claimed prediction is the SOCP contact-implicit model (Eqs. 13-19) built on Moreau's stepping-forward approach (Eqs. 4-12), which is introduced from external nonsmooth mechanics literature rather than from the authors' prior results. No predicted quantity (head trajectory, velocity, torques, contact forces) is defined in terms of the experimental data it is compared against: the prescribed joint trajectory is the same input to all models and the hardware, and the Section IV comparison is an external validation against independent measurements. The friction coefficient mu = 0.5 and the Simulink spring-damper coefficients (1e4 and 1e3) are stated model parameters, not fitted outputs, and the paper's own concession that the analytical model 'predicts larger oscillations with shorter travel distances, likely due to differences in ground reaction force estimation stemming from optimization constraints and tunable hyperparameters' is a limitation on accuracy, not a circular reduction. The COBRA self-citations concern the robot hardware platform and prior locomotion studies; they do not supply the mathematical result being claimed, and no uniqueness theorem or ansatz is imported from the authors' own work. No self-definitional, fitted-input-as-prediction, or imported-uniqueness circularity is present.
Assumptions & free parameters
free parameters (5)
- Ground friction coefficient mu =
0.5
- Ground spring stiffness (Simulink) =
1e4
- Ground damping coefficient (Simulink) =
1e3
- Ground restitution coefficient =
not reported
- Virtual contact sphere radius =
not reported
assumptions (4)
- domain assumption The robot is a rigid-body system on flat ground with Coulomb friction and complementarity contact conditions.
- ad hoc to paper Each module's contact can be represented by a convex sphere with a gap function.
- standard math Moreau-Jean time-stepping with a small regularization term yields correct contact resolution for this system.
- domain assumption The same prescribed joint trajectory is executed by all simulated models and the physical robot.
invented entities (1)
-
Virtual convex contact spheres surrounding each robot module
Cite this review
Pith. "Pith review of Optimal Trajectory Planning in a Vertically Undulating Snake Locomotion using Contact-implicit Optimization." pith.science (2026). https://pith.science/paper/7M53DZAT
@misc{pith2026250802953,
author = {Pith},
title = {Pith review of: Optimal Trajectory Planning in a Vertically Undulating Snake Locomotion using Contact-implicit Optimization},
year = {2026},
howpublished = {\url{https://pith.science/paper/7M53DZAT}},
note = {Machine review of arXiv:2508.02953}
}
read the original abstract
Contact-rich problems, such as snake robot locomotion, offer unexplored yet rich opportunities for optimization-based trajectory and acyclic contact planning. So far, a substantial body of control research has focused on emulating snake locomotion and replicating its distinctive movement patterns using shape functions that either ignore the complexity of interactions or focus on complex interactions with matter (e.g., burrowing movements). However, models and control frameworks that lie in between these two paradigms and are based on simple, fundamental rigid body dynamics, which alleviate the challenging contact and control allocation problems in snake locomotion, remain absent. This work makes meaningful contributions, substantiated by simulations and experiments, in the following directions: 1) introducing a reduced-order model based on Moreau's stepping-forward approach from differential inclusion mathematics, 2) verifying model accuracy, 3) experimental validation.
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Forward citations
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Reference graph
Works this paper leans on
-
[1]
Contact-Aware Non- Prehensile Manipulation for Object Retrieval in Clut- tered Environments,
Y . Jiang, Y . Jia, and X. Li, “Contact-Aware Non- Prehensile Manipulation for Object Retrieval in Clut- tered Environments,” in 2023 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) , ISSN: 2153-0866, Oct. 2023, pp. 10 604–10 611
work page 2023
-
[2]
Planning and control for dynamic, nonprehensile, and hybrid ma- nipulation tasks,
J. Z. Woodruff and K. M. Lynch, “Planning and control for dynamic, nonprehensile, and hybrid ma- nipulation tasks,” in 2017 IEEE International Confer- ence on Robotics and Automation (ICRA) , May 2017, pp. 4066–4073
work page 2017
-
[3]
Stable Prehen- sile Pushing: In-Hand Manipulation with Alternating Sticking Contacts,
N. Chavan-Dafle and A. Rodriguez, “Stable Prehen- sile Pushing: In-Hand Manipulation with Alternating Sticking Contacts,” in 2018 IEEE International Con- ference on Robotics and Automation (ICRA) , ISSN: 2577-087X, May 2018, pp. 254–261
work page 2018
-
[4]
One-Finger Manipu- lation of 3D Objects by Planning Start-to-Push Points and Pushing Forces,
M. Xiao, Y . Ding, and S. Fan, “One-Finger Manipu- lation of 3D Objects by Planning Start-to-Push Points and Pushing Forces,” IEEE Robotics and Automation Letters, vol. 9, no. 3, pp. 2694–2701, Mar. 2024, Conference Name: IEEE Robotics and Automation Letters
work page 2024
-
[5]
A Unified Motion Modeling Approach for Snake Robot’s Gaits Generated With Backbone Curve Method,
W. Huang, Y . Fang, X. Guo, H. Liu, and L. Liu, “A Unified Motion Modeling Approach for Snake Robot’s Gaits Generated With Backbone Curve Method,”IEEE Transactions on Robotics, pp. 1–15, 2024, Conference Name: IEEE Transactions on Robotics
work page 2024
-
[6]
Modular snake robot velocity for side- winding gaits,
K. Melo, “Modular snake robot velocity for side- winding gaits,” in 2015 IEEE International Confer- ence on Robotics and Automation (ICRA) , ISSN: 1050-4729, May 2015, pp. 3716–3722
work page 2015
-
[7]
Virtual Chassis for Snake Robots: Definition and Applica- tions,
D. Rollinson, A. Buchan, and H. Choset, “Virtual Chassis for Snake Robots: Definition and Applica- tions,” Advanced Robotics , vol. 26, no. 17, pp. 2043– 2064, Dec. 2012, Publisher: Taylor & Francis eprint: https://doi.org/10.1080/01691864.2012.728695
arXiv 2012
-
[8]
Economic model predictive control for snake robot locomotion,
M. Nonhoff, P. N. K ¨ohler, A. M. Kohl, K. Y . Pettersen, and F. Allg ¨ower, “Economic model predictive control for snake robot locomotion,” in 2019 IEEE 58th Conference on Decision and Control (CDC) , ISSN: 2576-2370, Dec. 2019, pp. 8329–8334
work page 2019
Show all 22 references
-
[9]
Automatic Snake Gait Gener- ation Using Model Predictive Control,
E. Hannigan, B. Song, G. Khandate, M. Haas-Heger, J. Yin, and M. Ciocarlie, “Automatic Snake Gait Gener- ation Using Model Predictive Control,” in 2020 IEEE International Conference on Robotics and Automation (ICRA), ISSN: 2577-087X, May 2020, pp. 5101–5107
2020
-
[10]
Study on the 3D shape of active cord mechanism,
H. Yamada and S. Hirose, “Study on the 3D shape of active cord mechanism,” in Proceedings 2006 IEEE International Conference on Robotics and Automation,
2006
-
[11]
Unilateral Contact and Dry Friction in Finite Freedom Dynamics,
J. J. Moreau, “Unilateral Contact and Dry Friction in Finite Freedom Dynamics,” en, in Nonsmooth Me- chanics and Applications, J. J. Moreau and P. D. Pana- giotopoulos, Eds., Vienna: Springer, 1988, pp. 1–82
1988
-
[12]
Pitroda, E
S. Pitroda, E. Sihite, K. V . Krishnamurthy, et al., Quadratic Programming Optimization for Bio- Inspired Thruster-Assisted Bipedal Locomotion on In- clined Slopes , arXiv:2411.12968 [cs], Nov. 2024
2024 arXiv
-
[13]
K. V . Krishnamurthy, C. Wang, S. Pitroda, et al. , Thruster-Assisted Incline Walking , arXiv:2406.13118 [cs, eess], Jun. 2024
2024 arXiv
-
[14]
Posture manipula- tion of thruster-enhanced bipedal robot performing dy- namic wall-jumping using model predictive control,
E. Sihite, S. Pitroda, T. Liu, et al., “Posture manipula- tion of thruster-enhanced bipedal robot performing dy- namic wall-jumping using model predictive control,” in 2024 IEEE-RAS 23rd International Conference on Humanoid Robots (Humanoids) , ISSN: 2164-0580, Nov. 2024, pp. 491–496
2024
-
[15]
Salagame, K
A. Salagame, K. Gangaraju, H. K. Nallaguntla, E. Sihite, G. Schirner, and A. Ramezani, Loco- Manipulation with Nonimpulsive Contact-Implicit Planning in a Slithering Robot , arXiv:2404.08174 [cs, eess], Apr. 2024
2024 arXiv
-
[16]
Dynamic Posture Manipulation During Tumbling for Closed-Loop Heading Angle Control,
A. Salagame, E. Sihite, G. Schirner, and A. Ramezani, “Dynamic Posture Manipulation During Tumbling for Closed-Loop Heading Angle Control,” in 2024 IEEE International Conference on Advanced Intel- ligent Mechatronics (AIM) , ISSN: 2159-6255, Jul. 2024, pp. 64–69
2024
-
[17]
Hierarchical RL-Guided Large-scale Navigation of a Snake Robot,
S. Jiang, A. Salagame, A. Ramezani, and L. L. Wong, “Hierarchical RL-Guided Large-scale Navigation of a Snake Robot,” in 2024 IEEE International Conference on Advanced Intelligent Mechatronics (AIM) , ISSN: 2159-6255, Jul. 2024, pp. 1347–1352
2024
-
[18]
How Strong a Kick Should be to Topple Northeastern’s Tumbling Robot?
A. Salagame, N. Bhattachan, A. Caetano, et al., “How Strong a Kick Should be to Topple Northeastern’s Tumbling Robot?” In 2024 IEEE International Con- ference on Advanced Intelligent Mechatronics (AIM) , ISSN: 2159-6255, Jul. 2024, pp. 76–81
2024
-
[19]
Non-impulsive Contact-Implicit Motion Planning for Morpho-functional Loco-manipulation,
A. Salagame, K. Gangaraju, H. K. Nallaguntla, et al., “Non-impulsive Contact-Implicit Motion Planning for Morpho-functional Loco-manipulation,” in 2024 IEEE International Conference on Advanced Intel- ligent Mechatronics (AIM) , ISSN: 2159-6255, Jul. 2024, pp. 309–314
2024
-
[20]
Heading Control for Obstacle Avoidance using Dynamic Posture Manipulation dur- ing Tumbling Locomotion,
A. Salagame, K. Gangaraju, E. Sihite, G. Schirner, and A. Ramezani, “Heading Control for Obstacle Avoidance using Dynamic Posture Manipulation dur- ing Tumbling Locomotion,” in 2024 IEEE/RSJ Inter- national Conference on Intelligent Robots and Systems (IROS), ISSN: 2153-0866, ...
2024
-
[21]
Snake Robot with Tactile Perception Navi- gates on Large-scale Challenging Terrain,
S. Jiang, A. Salagame, A. Ramezani, and L. L. S. Wong, “Snake Robot with Tactile Perception Navi- gates on Large-scale Challenging Terrain,” in 2024 IEEE International Conference on Robotics and Au- tomation (ICRA) , May 2024, pp. 5090–5096
2024
-
[2006]
, ISSN: 1050-4729, May 2006, pp
ICRA 2006. , ISSN: 1050-4729, May 2006, pp. 2890–2895
2006
Reviewed August 6, 2026 · model on record in the stance chip above.
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