REVIEW 4 major objections 5 minor 59 references
Robust control for multi-legged elongate robots in noisy environments
T0 review · 4 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read Multi-legged elongate robots have an emergent stride ceiling near four times leg length—about 33 cm per cycle—no matter how many legs are added.
desk verdict A serious, experimentally grounded robotics paper with a clean geometric speed bound; the MIMO 'approaching the bound' claim is slightly overstated due to a footnote they acknowledge but don't correct. 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
The load-bearing object is the 'bac'—basic active contact, one leg-ground contact treated as a bit in a communication channel—together with the analogy that maps mechanical intelligence (passive gravity redistribution of ground reaction forces) to a majority-vote decoder, simple repetition/spatial synchronization to repetition coding, and feedback control (computational intelligence) to automatic repeat request. Gait design is carried by geometric mechanics: the body shape space with local connection matrix A(w), whose curl (the forward height function) gives the net stride from a closed gait path via Stokes' theorem. A vertical body wave modulates contact timing to spread thrust uniformly in time, which sets the 'majority vote window size' and is the tuning knob for the speed-robustness trade-off. The claimed 4l upper bound is derived from two elementary models—non-slip kinematics and steady slipping with Coulomb friction—so the bound is a geometric consequence of leg length rather than of the friction model.
What would settle it
A robot with more legs than the saturation point that achieves a flat-ground stride strictly greater than four times its leg length (4l) would falsify the claimed bound. Separately, a rugose terrain whose height variations are strongly correlated—one obstacle lifting several legs at once—on which open-loop redundancy loses its predicted reliability would falsify the independent-bit noise model.
Extended reading notes
Core claim
For an elongate robot whose legs all do the same synchronized thrust, additional legs add reliability but not speed; the paper's central discovery is that this speed ceiling is not an engineering accident but a geometric bound. With a lateral undulation wave and one leg pair per module, geometric mechanics predicts that stride length is maximized at a body amplitude set by self-collision constraints, that the optimal amplitude falls as leg count rises, and that the resulting maximum stride converges to 4l (four times leg length, 33.6 cm for l = 8.4 cm). The saturation occurs near N = 7 leg pairs for the tested geometry. The same 4l limit is recovered from a non-slip kinematic model and from a steady-state slipping model with isotropic Coulomb friction, so it does not depend on the no-slip assumption. The authors then show the limit is approachable on rough terrain: a reinforcement-learned MIMO controller, which adapts body undulation, leg amplitude, and vertical wave in response to measured bac loss, reaches 0.40 ± 0.03 BL/cycle against the predicted 0.43, and point-foot robots with C-shaped legs reach comparable speed in open loop because the distributed foot contact makes bac loss rare.
Load-bearing premise
The load-bearing premise is that terrain-induced failures of individual leg-ground contacts behave like independent random bit errors, so that redundancy and coding arguments (majority vote, forward error correction, retransmission) apply quantitatively to locomotion.
Editorial extensions
If this is right
- Adding legs beyond roughly seven pairs does not increase flat-ground stride; robots should be designed with fewer legs plus body undulation or feedback.
- A simple single-input single-output controller can give a 12-legged robot reliability comparable to a 16-legged open-loop robot, showing that sensor complexity can substitute for leg count.
- C-shaped legs let an MER traverse flat and rugose terrains with statistically indistinguishable open-loop speeds, approaching the four-times-leg-length bound.
- The speed-robustness trade-off is controlled by encoding choices: less spatial redundancy or a smaller vertical wave raises smooth-terrain speed but costs robustness.
Reading between the lines
- Beyond the paper: terrain noise with correlated bac losses (one obstacle lifting several legs at once) should break the independent-bit redundancy guarantee; a terrain characterized by bac-loss correlation length would test this.
- Beyond the paper: the 4l result bounds stride per cycle, not velocity; the natural next bound to seek is on cycle frequency, since absolute speed is stride times frequency.
- Beyond the paper: the stated equivalence between computational and design complexity implies an iso-performance surface; measuring wall-clock or energy cost of a many-leg open-loop robot versus a few-leg feedback robot on identical terrain would quantify the exchange rate.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a communication-theoretic framework for multi-legged elongate robots (MERs), in which each leg-ground contact is a 'bac' analogous to a bit, passive gravity-driven force redistribution is mechanical intelligence (MI) analogous to a majority-vote decoder, and active feedback is computational intelligence (CI) analogous to ARQ. Using geometric mechanics, the authors derive an emergent upper bound on absolute speed of approximately 4l (about 33 cm/cycle for l = 8.4 cm), predict that speed saturates near N = 7 leg pairs, and validate these predictions numerically and experimentally on flat and rugose terrains. They then introduce vertical-wave contact modulation, a SISO adaptive controller, an RL-trained MIMO controller, and C-shaped legs, reporting speeds that approach the predicted bound on noisy terrain. The communication-theory analogy is used throughout to justify redundancy-based reliability and speed-robustness trade-offs.
Significance. If the results hold, the paper provides a rare predictive design principle for legged locomotion: a simple kinematic speed bound 4l that is independent of leg count, together with a quantitative account of when adding legs ceases to improve speed. The geometric-mechanics predictions match the flat-ground experiments, the N≈7 saturation is tested both in simulation and in hardware, and the C-leg experiments demonstrate terrain-agnostic open-loop performance. These are concrete strengths. The communication analogy is stimulating but not yet fully load-bearing: the reliability guarantees rest on an untested independence assumption for bac errors, several key proofs are deferred to the SI, and the MIMO comparison to the upper bound uses an inconsistent reference leg length. The paper also does not state data/code availability, which limits reproducibility of the simulation and RL components. With targeted revisions, this could be a strong contribution to the field.
major comments (4)
- [Approach the upper bound of speed on noisy landscapes; footnote 4] The claim that MIMO CI at 0.40±0.03 BL/cycle 'approaches the theoretical upper bound of 0.43 BL/cycle' is internally inconsistent with footnote 4, which states that the contact sensors effectively increase leg length and thereby raise the upper bound. Since the predicted bound is 4l, the comparison must use the effective leg length of the sensor-equipped robot. As a concrete illustration, increasing l from 8.4 cm to 9.4 cm changes the bound to approximately 0.48 BL/cycle, making the achieved speed about 83% of the bound rather than 93%. Please recompute the comparison with the actual effective leg length or rephrase the claim so that it does not overstate proximity to the bound.
- [Temporal and spatial synchronization for simple repetition] The main text twice asserts that a constant thrust profile minimizes the coefficient of variation of cycle-averaged velocity and that a more uniform thrust distribution lowers Cv, with proofs deferred to the SI. The SI is not supplied with the manuscript as provided, so these load-bearing claims are not verifiable from the submission. Please include the proofs or state the exact modeling assumptions under which they hold, and confirm that the vertical-wave robustness results do not depend on an unstated circularity between the thrust-uniformity measure and the performance metric.
- [Gravity as an MI control scheme; Table 1] The reliability framework assumes that terrain-induced bac losses behave like independent bit errors in a communication channel. The experiments use random-height step fields, but the paper does not measure or model correlated bac losses, such as a single obstacle lifting several legs at once or a body tilt causing systematic contact loss. Under correlated noise, the majority-vote and redundancy guarantees need not hold. Please either add experiments or analysis with controlled correlation lengths of terrain failures, or explicitly limit the reliability claims to the tested class of terrains.
- [Embedding MI in bac through foot morphology design] The C-leg result is compared to '4×leg length, 0.44 BL/cycle,' but the effective leg length of the C-shaped leg is not defined. If the C-leg's effective length differs from the point-leg value l = 8.4 cm used for the earlier bound, the reference bound changes and the 'approaching the upper bound' claim is weakened. Please specify the leg length used for the C-leg comparison and recompute the bound accordingly, or report the ratio of achieved speed to the appropriate 4l value for the C-leg geometry.
minor comments (5)
- [Emergent upper bound of absolute speed with additional legs] The text contains the typo 'self-collusion' where 'self-collision' is meant; please correct it.
- [Multi-input-multi-output CI via reinforcement learning] The phrase 'MIMI CI' appears in the text and should be 'MIMO CI'.
- [Table 1] The table entry 'Automatic-Repeat Qequest' contains a typo; it should read 'Automatic Repeat Request'.
- [Figure 7.A.2] The figure labels the non-MIMO controller 'SIMO CI,' while the text refers to it as 'SISO CI'; please unify the terminology.
- [Single-input-single-output CI] The SISO controller gain p = 3.2 is chosen from empirical experiments; a sensitivity analysis or cross-validation would help establish that the reported improvement is not specific to this particular value.
Circularity Check
The 4l speed bound is derived independently, but the N=7 saturation is an experimentally noticed crossover relabeled as a theory prediction, and the MIMO 'approaching bound' comparison uses a stale, lower bound after the paper admits the robot's leg length increased.
-
fitted input called prediction
[Section 'Emergent upper bound of absolute speed with additional legs', Eq. (6) and Fig. 4.B.1]
"In our robot experiments, we notice that A_SC < A*_b when N<7; and A_SC > A*_b when N>7. Based on the observation, we hypothesized that increasing the number of legs can lead to substantially longer stride when N<7; however, above N=7, further increasing the number of legs will not lead to longer stride. ... the saturation N in both numerical and experimental analysis is reasonably close to 7, as predicted in theory."
The predicted threshold value N=7 is read off the empirical crossover of A_SC and A*_b that was noticed in the robot experiments, and Eq. (6) then uses that same crossover to select the body undulation amplitude. Calling the resulting saturation 'predicted in theory' relabels an empirical observation as a first-principles prediction, and the experimental verification on the same MER platforms is not an independent test. The 4l upper bound itself is not involved in this circularity.
full rationale
The paper's headline upper bound on absolute speed (4l = 33.6 cm/cycle) is derived from a parameter-free kinematic model with non-slip and slipping Coulomb-friction arguments, not from fitting speed data, so that core derivation is self-contained and non-circular. The MI/FEC and CI/ARQ analogy is taken from the authors' prior published work, but it is used as an organizing framework rather than as an unverified uniqueness theorem, and the geometric-mechanics height-function calculations are independent computations. The N=7 saturation claim is the main partial-circularity point: the threshold was first noticed in the same robot experiments later cited as verification, although the numerical simulation provides some independent support. Separately, footnote 4 near the MIMO CI claim explicitly admits that the sensor-equipped robot has a longer effective leg length, which raises the applicable 4l_eff bound above the quoted 0.43 BL/cycle; comparing the measured 0.40 BL/cycle against the stale 0.43 bound is therefore not established by the paper's own equations. That is a load-bearing comparison error rather than a definitional circularity, and it weakens the 'approaching the upper bound' narrative without invalidating the geometric bound itself.
Assumptions & free parameters
free parameters (4)
- SISO controller gain p =
3.2 rad
- Desired duty factor d =
not reported
- MLP policy weights (MIMO CI) =
not reported (trained via RL)
- Self-collision threshold =
1 cm foot separation
assumptions (5)
- domain assumption The robot operates in a quasi-static, thrust-dominated regime (coasting number C ≈ 0.01), so inertia is negligible and force balance (Eq. 2) holds at every instant.
- domain assumption Foot-ground contact follows isotropic Coulomb dry friction, with ground reaction force of fixed magnitude opposite to the slip direction.
- domain assumption Terrain-induced bac losses behave like independent bit errors in a communication channel, so redundancy (FEC/MI) and retransmission (ARQ/CI) analogies apply.
- domain assumption For the 4l speed bound, the non-slip model assumes no foot slip and one contralateral leg always in stance; the slipping model assumes steady-state equal forward/backward slip (d1 = d2).
- ad hoc to paper The useful gait space is parameterized by two Fourier modes (Eq. 1) and circular gait paths in shape space, with all other shape coordinates prescribed by prior lower-level controllers.
invented entities (1)
-
Bac (basic active contact)
Cite this review
Pith. "Pith review of Robust control for multi-legged elongate robots in noisy environments." pith.science (2026). https://pith.science/paper/DTL5VTOF
@misc{pith2026250615788,
author = {Pith},
title = {Pith review of: Robust control for multi-legged elongate robots in noisy environments},
year = {2026},
howpublished = {\url{https://pith.science/paper/DTL5VTOF}},
note = {Machine review of arXiv:2506.15788}
}
read the original abstract
Modern two and four legged robots exhibit impressive mobility on complex terrain, largely attributed to advancement in learning algorithms. However, these systems often rely on high-bandwidth sensing and onboard computation to perceive/respond to terrain uncertainties. Further, current locomotion strategies typically require extensive robot-specific training, limiting their generalizability across platforms. Building on our prior research connecting robot-environment interaction and communication theory, we develop a new paradigm to construct robust and simply controlled multi-legged elongate robots (MERs) capable of operating effectively in cluttered, unstructured environments. In this framework, each leg-ground contact is thought of as a basic active contact (bac), akin to bits in signal transmission. Reliable locomotion can be achieved in open-loop on "noisy" landscapes via sufficient redundancy in bacs. In such situations, robustness is achieved through passive mechanical responses. We term such processes as those displaying mechanical intelligence (MI) and analogize these processes to forward error correction (FEC) in signal transmission. To augment MI, we develop feedback control schemes, which we refer to as computational intelligence (CI) and such processes analogize automatic repeat request (ARQ) in signal transmission. Integration of these analogies between locomotion and communication theory allow analysis, design, and prediction of embodied intelligence control schemes (integrating MI and CI) in MERs, showing effective and reliable performance (approximately half body lengths per cycle) on complex landscapes with terrain "noise" over twice the robot's height. Our work provides a foundation for systematic development of MER control, paving the way for terrain-agnostic, agile, and resilient robotic systems capable of operating in extreme environments.
Figures
Figures from the paper (7 more)
Reference graph
Works this paper leans on
-
[1]
R. M. Alexander,Principles of animal locomotion(Princeton University Press, 2003)
work page 2003
-
[2]
S. Childress, A. Hosoi, W. W. Schultz, J. Wang,Natural locomotion in fluids and on sur- faces: swimming, flying, and sliding, vol. 155 (Springer, 2012)
work page 2012
-
[3]
N. Gravish, G. V . Lauder,Journal of Experimental Biology221, jeb138438 (2018)
work page 2018
-
[4]
S. Kim, C. Laschi, B. Trimmer,Trends in biotechnology31, 287 (2013)
work page 2013
-
[5]
Aguilar,et al.,Reports on Progress in Physics79, 1 (2016)
J. Aguilar,et al.,Reports on Progress in Physics79, 1 (2016)
work page 2016
- [6]
-
[7]
J. Lee, J. Hwangbo, L. Wellhausen, V . Koltun, M. Hutter,Science robotics5, eabc5986 (2020)
work page 2020
-
[8]
M. H. Raibert,Legged robots that balance(MIT press, 1986)
work page 1986
Show all 59 references
-
[9]
Tan,et al.,arXiv preprint arXiv:1804.10332(2018)
J. Tan,et al.,arXiv preprint arXiv:1804.10332(2018)
2018 arXiv
-
[10]
Raibert, K
M. Raibert, K. Blankespoor, G. Nelson, R. Playter,IFAC Proceedings Volumes41, 10822 (2008)
2008
-
[11]
Hutter,et al.,2016 IEEE/RSJ international conference on intelligent robots and systems (IROS)(IEEE, 2016), pp
M. Hutter,et al.,2016 IEEE/RSJ international conference on intelligent robots and systems (IROS)(IEEE, 2016), pp. 38–44
2016
-
[12]
Rudin, D
N. Rudin, D. Hoeller, P. Reist, M. Hutter,Conference on Robot Learning(PMLR, 2022), pp. 91–100
2022
-
[13]
Huang, J
J.-K. Huang, J. W. Grizzle,IEEE Transactions on Robotics39, 2093 (2023). 34
2023
-
[14]
Sitti,Extreme Mechanics Letters46, 101340 (2021)
M. Sitti,Extreme Mechanics Letters46, 101340 (2021)
2021
-
[15]
Wang,et al.,Science Robotics8, eadi2243 (2023)
T. Wang,et al.,Science Robotics8, eadi2243 (2023)
2023
-
[16]
Saranli, M
U. Saranli, M. Buehler, D. E. Koditschek,The International Journal of Robotics Research 20, 616 (2001)
2001
-
[17]
Ozkan-Aydin, B
Y . Ozkan-Aydin, B. Chong, E. Aydin, D. I. Goldman,2020 3rd IEEE International Confer- ence on Soft Robotics (RoboSoft)(IEEE, 2020), pp. 156–163
2020
-
[18]
Chong,et al.,Bioinspiration & Biomimetics17, 046015 (2022)
B. Chong,et al.,Bioinspiration & Biomimetics17, 046015 (2022)
2022
-
[19]
Chong,et al.,Proceedings of the National Academy of Sciences120, e2213698120 (2023)
B. Chong,et al.,Proceedings of the National Academy of Sciences120, e2213698120 (2023)
2023
-
[20]
Chong,et al.,Science380, 509 (2023)
B. Chong,et al.,Science380, 509 (2023)
2023
-
[21]
J. R. Pierce,An introduction to information theory: symbols, signals and noise(Courier Corporation, 2012)
2012
-
[22]
Chong,et al.,Bulletin of the American Physical Society(2023)
B. Chong,et al.,Bulletin of the American Physical Society(2023)
2023
-
[24]
J. Y . Jun, D. Haldane, J. E. Clark,Experimental Robotics: The 12th International Sympo- sium on Experimental Robotics(Springer, 2014), pp. 759–773
2014
-
[25]
Wilczek, A
F. Wilczek, A. Shapere,Geometric phases in physics, vol. 5 (World Scientific, 1989)
1989
-
[26]
Shapere, F
A. Shapere, F. Wilczek,Am. J. Phys57, 514 (1989)
1989
-
[27]
Shapere, F
A. Shapere, F. Wilczek,Physical Review Letters58, 2051 (1987)
1987
-
[28]
Berry,Physics Today43, 34 (1990)
M. Berry,Physics Today43, 34 (1990). 35
1990
-
[29]
Montgomery,Comm
R. Montgomery,Comm. Math. Phys.128, 565 (1990)
1990
-
[30]
J. E. Marsden, R. Montgomery, T. S. Rat,iu,Reduction, symmetry, and phases in mechanics, vol. 436 (American Mathematical Soc., 1990)
1990
-
[31]
P. S. Krishnaprasad, D. P. Tsakiris,Proceedings of 1994 33rd IEEE Conference on Decision and Control(1994), vol. 3, pp. 2955–2960 vol.3
1994
-
[32]
S. D. Kelly, R. M. Murray,Journal of Robotic Systems12, 417 (1995)
1995
-
[33]
Ostrowski, J
J. Ostrowski, J. Burdick,The international journal of robotics research17, 683 (1998)
1998
-
[34]
A. D. Lewis,IEEE Transactions on Automatic Control45, 1420 (2000)
2000
-
[35]
A. M. Bloch,Nonholonomic Mechanics(Springer New York, New York, NY , 2003), chap. 5, pp. 207–276
2003
-
[36]
J. B. Melli, C. W. Rowley, D. S. Rufat,SIAM Journal on Applied Dynamical Systems5, 650 (2006)
2006
-
[37]
C. E. Shannon,The Bell system technical journal27, 379 (1948)
1948
-
[38]
E. F. Moore, C. E. Shannon,Journal of the Franklin Institute262, 191 (1956)
1956
-
[39]
J. M. Rieser,et al.,Proceedings of the National Academy of Sciences121, e2320517121 (2024)
2024
-
[40]
R. J. Full, D. E. Koditschek,Journal of Experimental Biology202, 3325 (1999)
1999
-
[41]
J. M. Rieser,et al.,arXiv preprint arXiv:1906.11374(2019)
2019 arXiv
-
[42]
Chong, T
B. Chong, T. Wang, E. Erickson, P. J. Bergmann, D. I. Goldman,Proceedings of the Na- tional Academy of Sciences119, e2118456119 (2022). 36
2022
-
[43]
R. M. Murray, Z. Li, S. S. Sastry, S. S. Sastry,A mathematical introduction to robotic manipulation(CRC press, 1994)
1994
-
[44]
R. L. Hatton, H. Choset,The European Physical Journal Special Topics224, 3141 (2015)
2015
-
[45]
D. H. Wolpert, W. G. Macready,IEEE transactions on evolutionary computation1, 67 (1997)
1997
-
[46]
A. S. Tanenbaum,ACM Computing Surveys (CSUR)13, 453 (1981)
1981
-
[47]
J. He, B. Chong, Z. Xu, S. Ha, D. I. Goldman,arXiv preprint arXiv:2409.09473(2024)
2024 arXiv
-
[48]
J. C. Spagna, D. I. Goldman, P.-C. Lin, D. E. Koditschek, R. J. Full,Bioinspiration & biomimetics2, 9 (2007)
2007
-
[49]
Altendorfer,et al.,Autonomous Robots11, 207 (2001)
R. Altendorfer,et al.,Autonomous Robots11, 207 (2001)
2001
-
[50]
He,et al.,IEEE Transactions on Robotics (submitted)(2024)
J. He,et al.,IEEE Transactions on Robotics (submitted)(2024)
2024
-
[51]
Flores, B
E. Flores, B. Chong, D. Soto, D. I. Goldman,arXiv preprint arXiv:2410.01050(2024)
2024 arXiv
-
[52]
Iaschi,et al.,arXiv preprint arXiv:2410.01046(2024)
M. Iaschi,et al.,arXiv preprint arXiv:2410.01046(2024)
2024 arXiv
-
[53]
Accessed: 2025-05-29
Icra 2025 quadruped robot challenges,https:// quadruped-robot-challenges.notion.site/ ICRA-2025-Quadruped-Robot-Challenges-698cbe44ec434512a0064d3799f18c85. Accessed: 2025-05-29
2025
-
[54]
D. E. Koditschek, R. J. Full, M. Buehler,Arthropod structure & development33, 251 (2004)
2004
-
[55]
Butler, K
Z. Butler, K. Kotay, D. Rus, K. Tomita,Proceedings 2002 IEEE International Conference on Robotics and Automation (Cat. No. 02CH37292)(IEEE, 2002), vol. 1, pp. 809–816. 37
2002
-
[56]
Yasui,et al.,Scientific reports9, 1 (2019)
K. Yasui,et al.,Scientific reports9, 1 (2019)
2019
-
[57]
Gilbert,Journal of Comparative Physiology A181, 217 (1997)
C. Gilbert,Journal of Comparative Physiology A181, 217 (1997)
1997
-
[58]
A. J. Ijspeert,Annual Review Of Control, Robotics, And Autonomous Systems, Vol 3, 2020 3, 173 (2020)
2020
-
[59]
Sponberg,Physics Today70, 34 (2017)
S. Sponberg,Physics Today70, 34 (2017)
2017
-
[60]
Baines,et al.,Nature610, 283 (2022)
R. Baines,et al.,Nature610, 283 (2022). 38
2022
Reviewed August 15, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.