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REVIEW 4 major objections 6 minor 2 cited by

High-Performance Reinforcement Learning on Spot: Optimizing Simulation Parameters with Distributional Measures

T0 review · 4 major / 6 minor · reviewed 2026-08-16 · deepseek-v4-flash

Pith's one-line read A distributional sim-to-real gap score, optimized with CMA-ES over eight actuator parameters, lets a fully learned policy drive Spot at over 5.2 m/s with a flight-phase gallop, slippery-ground robustness, and disturbance rejection.

desk verdict A credible engineering advance—first learned end-to-end policy on Spot—with real but addressable weaknesses in the quantitative evidence and the noise handling. read the letter →

arxiv 2504.17857 v3 pith:VZPSOQM4 submitted 2025-04-24 cs.LG cs.RO

classification cs.LGcs.RO
keywords reinforcementlearningsim-to-realtransferquadrupedlocomotiondistributionalgapmeasureWassersteindistancemaximummeandiscrepancyCMA-ESactuatorparameteroptimization
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper sets out to show that a gray-box modeling loop can close the sim-to-real gap for Spot enough to run a fully learned end-to-end control policy at speeds and gaits the stock controller cannot reach. The authors score the difference between hardware and simulation by comparing distributions of joint positions, joint velocities, and policy actions with Wasserstein distance and maximum mean discrepancy, then use CMA-ES to tune eight actuator parameters (joint friction and torque-speed limits) that are hard to measure directly. A single retraining pass with the optimized model yields a policy that sustains 5.2 m/s, more than triple Spot's default 1.6 m/s limit, gallops with a flight phase, and stays upright on soap-wet plexiglass. If correct, this is the first end-to-end RL policy deployed on Spot with public training code, and it offers a reusable recipe for high-performance learned control on a commercial quadruped.

What carries the argument

The load-bearing mechanism is a distributional gap score built from Wasserstein distance and maximum mean discrepancy, computed over scaled joint positions, joint velocities, and policy actions from hardware and simulated rollouts of the same command sequences. The score plays the role of a GAN-style evaluation: the simulated policy is the generator and the hardware data is the target, so the two rollouts do not need to start from matched states. CMA-ES treats that score as the objective and searches an eight-parameter gray-box actuator model (two friction coefficients plus a six-parameter torque-speed curve), and the optimized values are used to retrain the policy.

What would settle it

Train a second policy with a substantially different gait or reward budget using the same eight optimized parameters without retuning, then deploy it; if the distributional gap score shrinks but hardware speed and robustness do not improve, the optimization was overfitting to the original policy rather than recovering the true actuator model.

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Extended reading notes

Core claim

The paper's central claim is that the sim-to-real gap for Spot is best treated as a distribution-matching problem rather than a trajectory-matching one. Hardware rollouts are collected under several scripted command sequences, the same commands are replayed in simulation, and the gap is scored as the Wasserstein and MMD distance between the scaled joint positions, joint velocities, and policy actions of the two data sets. CMA-ES then minimizes that score over eight parameters describing joint friction and torque-speed limits, and a single retraining pass with the optimized simulator produces policies that gallop with a flight phase, reject disturbances, and reach 5.2 m/s sustained speed — more than triple the stock controller's 1.6 m/s maximum and well above the roughly 3.7 m/s baseline policy trained without the optimization.

Load-bearing premise

The method assumes that matching the distribution of joint positions, joint velocities, and actions between simulation and hardware is enough to identify the true actuator mismatches, so the eight optimized parameters are the real cause of the gap rather than an overfit to one policy and its command sequences.

Editorial extensions

If this is right

  • An end-to-end learned controller can push Spot to a sustained 5.2 m/s flying-trot gallop, a capability the stock controller does not offer.
  • The distributional scoring procedure makes simulator tuning possible without joint torque measurements or matched-state resets, relying only on onboard state and action logs.
  • One iteration of optimization plus retraining is enough for a substantial transfer gain, raising sustained speed from roughly 3.7 m/s to 5.2 m/s.
  • The released training and deployment code provides a publicly reproducible baseline for future low-level RL control on Spot.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A stress test the paper does not run: keep the eight optimized parameters fixed while training a policy with a different gait or reward budget; if speed and robustness gains persist, the parameters are true actuator properties rather than compensations for one policy's failure modes.
  • The scoring features could be extended beyond joint positions, velocities, and actions to exteroceptive signals such as body height or contact forces, which may expose mismatches the current joint-space score cannot see.
  • The optimization loop couples system identification with policy retraining; a variant that optimizes parameters against a frozen policy and only then retrains would isolate how much of the gain comes from each step.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 6 minor

Summary. The paper presents a workflow for sim-to-real transfer of a learned locomotion policy on a Boston Dynamics Spot robot. The authors quantify the sim-to-real gap by comparing distributions of joint positions, joint velocities, and policy actions between hardware rollouts and simulation rollouts, using Wasserstein distance and Maximum Mean Discrepancy. These measures serve as the objective for CMA-ES, which optimizes eight actuator-related parameters: hip and knee friction coefficients, torque limits, speed limits, and torque-speed intersection points. After optimization, a PPO policy is retrained in NVIDIA Isaac Lab and deployed through the Spot RL Researcher Development Kit. The paper claims the deployed policy achieves over 5.2 m/s forward speed, more than triple Spot's default controller maximum, and shows robustness to slippery surfaces and a flight phase gait. The authors state that training code is publicly available and that this is the first end-to-end RL policy deployed on Spot with open training code.

Significance. If the quantitative results hold, this would be a notable empirical contribution: a publicly documented end-to-end RL pipeline on a commercial quadruped, with a concrete method for tuning simulation parameters from distributional data. The idea of using Wasserstein/MMD scores as a CMA-ES objective is a reasonable and potentially reusable approach for gray-box actuator identification, and the hardware demonstrations of a flying trot at 5.2 m/s would be impressive. However, the evidence supporting the central claims is currently incomplete: the headline speed results lack trial counts and error bars, the reward weights are omitted, the code link is missing, and the scoring procedure excludes sensor noise from simulation while hardware data necessarily includes it. These gaps make the paper's central methodological and performance claims plausible but not yet fully supported.

major comments (4)
  1. [IV.C] Section IV.C reports that the policy 'reached sustained running speeds of 5.2m/s in a flying trot gait' and compares this to the baseline policy's '3.7m/s sustained maximum,' but the manuscript gives no number of hardware trials, no run-to-run variability, and no measurement protocol for either number. The headline claim of a more-than-tripled maximum speed is load-bearing and currently rests on single-point estimates. Please report repeated trials with mean and standard deviation (or all individual runs) for both the baseline and the CMA-ES-optimized policy under identical command protocols, including how the maximum speed was determined from each trial.
  2. [IV.A] Section IV.A states, 'Importantly, simulated data does not include sensor noise,' while the hardware rollouts used as the optimization target are necessarily corrupted by state-estimation noise. Section III.A describes a Gaussian observation-noise model, but that model is not applied to the simulated data used in scoring. The distributional distance therefore contains a noise-induced component that no actuator parameter can physically eliminate, so CMA-ES may reduce the score by shifting simulated joint distributions to match the noise spread. The optimized friction and torque-speed values in Table I may thereby partly absorb sensor-noise mismatch rather than physical actuator effects. A concrete test is to corrupt the simulated observations with the Section III.A noise model before scoring, or to denoise the hardware estimates, and to report whether the optimized parameters change.
  3. [III.D / IV.A] Section III.D lists reward components but gives no weights, and Sections III.C and IV.A state that 'some fine-tuning of the reward functions may be required' and that 'only minor weight value adjustments were made.' Because the speed comparison is between a baseline policy and a policy retrained after both parameter optimization and reward modifications, the reported improvement from 3.8 m/s to 5.2 m/s cannot be cleanly attributed to the CMA-ES parameter search alone; it could also come from reward fine-tuning or from PPO stochasticity across random seeds. Please report the exact reward weights before and after the optimization, run multiple training seeds, and include an ablation that keeps rewards fixed while changing only the optimized simulation parameters.
  4. [IV.B] Section IV.B explicitly states that 'both policies struggle to produce negative torque during swing leg recirculation... due to power distribution limits not being included in our modeling.' This is an admitted unmodeled physical effect that appears in exactly the high-performance regime where the distributional score is computed. The eight optimized parameters cannot represent this mismatch, so the optimized values and the residual distributional gap may be dominated by the missing power-limit model rather than by the identified actuator parameters. Please either include a power-distribution-limit model in the simulator, or add an experiment that isolates whether the optimized parameters capture the observed hardware-simulation difference beyond this unmodeled effect.
minor comments (6)
  1. [Abstract / Conclusion] The abstract and conclusion promise publicly available training code, but no repository URL appears anywhere in the manuscript; add a link, ideally in a footnote in Section I or III.
  2. [III.B] The scoring procedure is described qualitatively, but the manuscript does not state which Wasserstein variant is used (1D, sliced, or joint product), which MMD kernel and bandwidth are chosen, how the two measures are combined into the reported weighted average, or how many simulated rollouts and samples are used per CMA-ES candidate. An appendix with these details would make the method reproducible.
  3. [III.A] The sentence 'Delaying action application the total delay is identical to delaying both the observations and actions' is grammatically incomplete, and the choice of a 5 ms buffer is not justified against the measured delay distribution in Fig. 2.
  4. [III.D] The reward components are listed by name but no weights are given; please include the full reward table, since the policy and its reward tuning are central to the performance comparisons.
  5. [IV.A vs IV.C] The baseline maximum speed is reported as 3.8 m/s in Section IV.A and 3.7 m/s in Section IV.C; please reconcile these numbers and use one consistent value.
  6. [Figures 2 and 6] Figure 2's axis labels and tick marks are unclear (the x-axis appears to show only 2-6), and the term 'Occurences' is misspelled; Figure 6 lacks units and trial-count annotations. Please improve caption clarity.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the optimized parameters are fitted to external hardware rollouts, and the headline speed is a newly measured outcome under a retrained policy, not a fitted constant.

full rationale

The paper's derivation chain is a standard system-identification loop rather than a self-justifying construction. First, the authors collect hardware rollouts under an initial policy (Section III.C: 'we deploy this policy on hardware and log the real data'). Second, they define a distributional distance between hardware and simulated rollouts in joint position, joint velocity, and action space, and minimize it with CMA-ES over eight actuator parameters (friction and torque-speed limits). The hardware data are external ground truth, not derived from the simulator or from the parameters being optimized. Third, they retrain a new policy with the optimized parameters and measure its hardware speed: 'we found a single iteration of the algorithm sufficient to markedly improve performance and increase maximum linear velocity from 3.8m/s to 5.2m/s on hardware' (Section IV.A). This speed is an outcome of a subsequent hardware deployment, not a quantity fitted by the optimization; nothing in the equations forces the speed to equal the optimized parameters. The reported distributional gap is the optimization objective itself, but using a gap metric as a cost function is inherent to system identification, not a circular prediction. The concern that simulated data exclude sensor noise (Section IV.A: 'Importantly, simulated data does not include sensor noise') is an identifiability or confounding issue: unmodeled noise could be absorbed into the fitted actuator values. That weakens causal interpretation of the optimized parameters, but it is not a definitional reduction, because the score is not constructed from the parameters and the parameters are not defined in terms of the final speed. No load-bearing self-citation appears: the only author-overlapping reference ([11], an MPC paper) is contextual related work, and the procedure does not invoke a uniqueness theorem or an ansatz smuggled in via citation. The comparison between baseline and optimized policies on hardware (Section IV.B) is an external benchmark, and the robustness and speed demonstrations are empirical results with independent content. Therefore no circular step can be exhibited from the paper's own equations or citations.

Assumptions & free parameters 12 free parameters · 8 assumptions · 0 invented entities

The central claim rests on a gray-box system identification loop: eight actuator parameters are fitted to hardware distributional data, plus a delay estimate, observation noise model, and hand-set control gains and reward weights. No new physical entities are introduced. The main burden is whether the simplified parameterization captures the true sim-to-real gap rather than absorbing sensor noise and unmodeled power limits.

free parameters (12)
  • Hip friction coefficient = 0.008 Nm
    Optimized by CMA-ES to minimize distributional sim-to-real gap; Table I.
  • Knee friction coefficient = 0.180 Nm
    Optimized by CMA-ES; remotized lead-screw joint has higher friction; Table I.
  • Torque limit max = 97.00 Nm
    Optimized torque-speed curve parameter; Table I.
  • Torque limit min = -108.79 Nm
    Optimized torque-speed curve parameter; Table I.
  • Speed limit max = 25.03 rad/s
    Optimized torque-speed curve parameter; Table I.
  • Speed limit min = -22.22 rad/s
    Optimized torque-speed curve parameter; Table I.
  • Torque-speed intersect max = 9.48 rad/s
    Optimized parameter for quadrant 1 torque-speed trade-off; Table I.
  • Torque-speed intersect min = -8.32 rad/s
    Optimized parameter for quadrant 3 torque-speed trade-off; Table I.
  • Communication delay buffer = 5 ms
    Estimated from measured gRPC delay histogram (Fig. 2) and baked into simulator as an action buffer; chosen from data rather than optimized.
  • Observation noise sigma per feature = Not reported
    Estimated from an FFT of state estimator data during walking; values used in training but not listed in the paper.
  • Reward weights = Not reported
    Reward terms are listed in Section III.D but the weights are omitted; Section IV.A mentions 'minor weight value adjustments' without specifics.
  • PD gains kp and kd = kp = 60.0 Nm/rad, kd = 1.5 Nm s/rad
    Hand-set control-law constants for all joints; central to the action-to-torque mapping in Eq. 1.
assumptions (8)
  • domain assumption Isaac Sim physics engine with rigid-body contact models is an adequate representation of Spot dynamics for policy training.
    Used throughout Section III.D; no validation of the physics engine against hardware other than the distributional scores.
  • domain assumption Torque-sensing feedback in Spot's motor controllers makes commanded torque accurately achieved, so only friction, delay, and limits need modeling.
    Section III.A states this to justify ignoring gear ratios and unreliable actuators.
  • domain assumption Wasserstein distance and MMD on joint position, velocity, and action features are sufficient measures of sim-to-real gap for parameter optimization.
    Section III.B chooses these features and measures; no ablation or comparison with other feature sets is provided.
  • domain assumption CMA-ES with 100 iterations and population size 10 finds a good minimum of the distributional score.
    Section III.C; no convergence analysis or repeated optimization runs are reported.
  • domain assumption A fixed 5 ms action buffer captures the effect of gRPC communication delay.
    Section III.A and Fig. 2; a measured distribution of delays is collapsed to a constant.
  • domain assumption The FFT-based normalization estimates zero-mean Gaussian observation noise per feature.
    Section III.A; noise is assumed white and Gaussian, with sigma computed from the right half of the frequency spectrum.
  • domain assumption Power distribution limits can be neglected for the main claims; the paper admits they are not modeled.
    Section IV.B states swing-leg negative torque struggles are due to unmodeled power distribution limits.
  • domain assumption The default Spot controller maximum speed is 1.6 m/s as given by the specification sheet.
    Used in Section IV.C as the baseline for the tripling claim.

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Pith. "Pith review of High-Performance Reinforcement Learning on Spot: Optimizing Simulation Parameters with Distributional Measures." pith.science (2026). https://pith.science/paper/VZPSOQM4

@misc{pith2026250417857,
  author       = {Pith},
  title        = {Pith review of: High-Performance Reinforcement Learning on Spot: Optimizing Simulation Parameters with Distributional Measures},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/VZPSOQM4}},
  note         = {Machine review of arXiv:2504.17857}
}
read the original abstract

This work presents an overview of the technical details behind a high performance reinforcement learning policy deployment with the Spot RL Researcher Development Kit for low level motor access on Boston Dynamics Spot. This represents the first public demonstration of an end to end end reinforcement learning policy deployed on Spot hardware with training code publicly available through Nvidia IsaacLab and deployment code available through Boston Dynamics. We utilize Wasserstein Distance and Maximum Mean Discrepancy to quantify the distributional dissimilarity of data collected on hardware and in simulation to measure our sim2real gap. We use these measures as a scoring function for the Covariance Matrix Adaptation Evolution Strategy to optimize simulated parameters that are unknown or difficult to measure from Spot. Our procedure for modeling and training produces high quality reinforcement learning policies capable of multiple gaits, including a flight phase. We deploy policies capable of over 5.2ms locomotion, more than triple Spots default controller maximum speed, robustness to slippery surfaces, disturbance rejection, and overall agility previously unseen on Spot. We detail our method and release our code to support future work on Spot with the low level API.

Figures

Figures reproduced from arXiv: 2504.17857 by the authors.

Figure 1
Figure 1. Spot galloping in flight on a flat track with our policy at 5.2m/s. [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Histogram plot of the measured communication delay of Google [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗
Figure 3
Figure 3. Torque-position (left) and torque-speed (right) limits of the [PITH_FULL_IMAGE:figures/full_fig_p002_3.png] view at source ↗
Figures from the paper (3 more)
Figure 4
Figure 4. Figure 4: Command sequence used to generate test data for evaluation of [PITH_FULL_IMAGE:figures/full_fig_p003_4.png]
Figure 5
Figure 5. Figure 5: Visual overview of the training and deployment pipeline. On the left, is an approximation of how the policy is trained in simulation, and on the [PITH_FULL_IMAGE:figures/full_fig_p005_5.png]
Figure 6
Figure 6. Figure 6: Comparison of knee joint action and desired torque during a [PITH_FULL_IMAGE:figures/full_fig_p006_6.png]

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Forward citations

Cited by 2 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Differentiable Weightless Controllers: Learning Logic Circuits for Continuous Control

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  2. Towards bridging the gap: Systematic sim-to-real transfer for diverse legged robots

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Pith tools

Reviewed August 16, 2026 · model on record in the stance chip above.