REVIEW 3 major objections 6 minor 28 references
Hamiltonian-based neural networks for systems under nonholonomic constraints
T0 review · 3 major / 6 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read A single physics-informed loss trains three networks to learn the Hamiltonian, constraints, and constraint forces of nonholonomic systems from data alone.
desk verdict A clean three-network architecture for learning constrained dynamics from data, but the simultaneous-recovery claim overreaches; the identifiability gap is real and the supplement only closes it for one low-dimensional example. 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 central object is the pseudo-Hamiltonian formulation of nonholonomic mechanics: $\dot q = \partial H/\partial p$, $\dot p = -\partial H/\partial q + A^T(q) \lambda$, subject to the linear constraint $A(q) \dot q = b(q)$. The architecture factors this identity into three networks, one for each learned quantity, and uses a single loss whose three terms are the squared residuals of the two Hamilton equations and of the constraint equation. A supplementary uniqueness argument shows that for a fixed constraint matrix $A$, the split between $H$ and $\lambda$ is identifiable whenever $\nabla \times (A^T \lambda^*) = 0$ admits no nonzero solution $\lambda^*$; the paper verifies this condition for its point-mass example.
What would settle it
Train the three-network architecture on a nonholonomic system whose constraint matrix admits a nonzero perturbation $\lambda^*$ satisfying $\nabla \times (A^T \lambda^*)=0$, the condition the paper itself identifies as the only obstruction to uniqueness. If the trajectory predictions remain accurate but the learned constraint force $\hat A^T \hat\lambda$ differs from the analytical force $A^T \lambda$ along the same trajectory, then the decomposition is not identifiable and the simultaneous recovery claim fails.
Extended reading notes
Core claim
The paper's central claim is that the constrained Hamilton equations $\dot q = \partial H/\partial p$ and $\dot p = -\partial H/\partial q + A^T \lambda$, together with the constraint equation $A \dot q = b$, can serve as a single training objective. Three parallel multilayer perceptrons represent the Hamiltonian $H(q,p)$, the constraint matrix $A(q)$ with offset $b(q)$, and the multiplier vector $\lambda(q,p)$; automatic differentiation yields the required derivatives of $H$, and the loss sums squared residuals of both Hamilton equations and of the constraint equation. In clean data, the learned energy and constraint forces match ground truth with mean normalized force errors around $4 \times 10^{-4}$; under 2% multiplicative noise, force errors stay near $10^{-2}$ although long-time trajectory predictions diverge. The paper concludes that this extends Hamiltonian-based learning from conservative systems to systems with holonomic and nonholonomic constraints, producing models of the constraints and their forces without any labels for $H$, $A$, $b$, or $\lambda$.
Load-bearing premise
The approach assumes that the training data let the networks tell the Hamiltonian apart from the constraint forces, meaning that the learned constraint matrix $A$ converges to the true one and that $H$ does not secretly absorb part of the constraint force, even though the loss only pins down the combined term $\partial H/\partial q - A^T \lambda$.
Editorial extensions
If this is right
- A single training run produces a model of the unconstrained dynamics through $H$, of the constraints through $A$ and $b$, and of the constraint forces through $A^T \lambda$, using only phase-space data.
- The method handles acatastatic constraints, where $b$ is nonzero and energy is not conserved, as demonstrated by the ball on a spinning table.
- Adding 2% multiplicative noise to the training data increases trajectory divergence over long horizons but keeps normalized constraint-force errors low, around 1-2%, suggesting that the learned constraint structure is relatively robust to imperfect data.
- The architecture is not restricted to holonomic constraints; it treats any linear constraint of the form $A \dot q = b$, integrable or not.
- If the learned Hamiltonian and constraints are correct, the network can be used to predict energy and constraint forces for applications such as noise prediction, dissipation detection, and robot control.
Reading between the lines
- Because the loss only pins down the combined term $\partial H/\partial q - A^T \lambda$, any practical deployment should validate learned constraint forces against an analytical force law before relying on them for control; the paper's uniqueness analysis covers only a fixed constraint matrix $A$.
- The same architecture could serve as a data-driven way to discover nonholonomic constraint equations from raw robot trajectories, since the constraint network directly outputs a matrix $A(q)$ and vector $b(q)$ that can be read off after training.
- For systems where measurements provide positions and velocities but not momenta, the method could be chained to a learned momentum map, though the paper lists such extensions as future work.
- Because gauge transformations such as rescaling or linearly combining constraints leave the dynamics invariant, learned representations of $A$ and $\lambda$ may differ between runs; quantifying this gauge freedom would tell users how much of the learned structure is physical.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a three-network Hamiltonian-based architecture for learning the dynamics of constrained mechanical systems from phase-space data. One network learns the Hamiltonian H(q,p), a second learns the constraint matrix A(q) and vector b(q), and a third learns the Lagrange multiplier vector λ(q,p). The networks are trained with the single loss in Eq. (7), which enforces the constrained Hamilton equations and the linear constraint A qdot − b = 0. The method is tested on a rolling disk and on solid and hollow balls on a spinning table, with and without 2% multiplicative noise, and a supplementary experiment on a 3D nonholonomically constrained particle is included. Reported results include trajectory predictions, constraint residuals, mean-shifted energy MAE, normalized constraint-force error, and Lyapunov divergence times. The supplementary material also contains a proof intended to establish uniqueness of H and λ for a fixed constraint matrix A.
Significance. If the central identifiability claim were established, the paper would make a useful contribution to data-driven nonholonomic mechanics: recovering H, A, b, and λ simultaneously from unlabelled phase-space data would enable energy and constraint-force estimation in robotics and control. The paper is clearly written, the architecture and training loss are fully specified, and the trajectory predictions in Figs. 3, 5, and 6 are accurate with small constraint residuals, both clean and noisy. The supplementary uniqueness attempt is a genuinely constructive step beyond purely empirical benchmarking. However, as detailed below, the identifiability proof covers only one toy example, an exact symmetry invalidates the claim for the stated holonomic scope, and the ball-on-table experiments omit a necessary holonomic constraint on the Euler-Rodrigues parameters. The paper therefore currently demonstrates trajectory and constraint-residual fitting rather than the certified recovery of H and λ.
major comments (3)
- [Supplementary §2; main §2.2, Eq. (7)] The uniqueness proof is not sufficient to support the claim that H, A, b, and λ are simultaneously learned. The proof is carried out only for the point-mass constraint matrix A = [−y, 0, 1], and the rolling-disk and spinning-table matrices (Eqs. 10 and 15) are never checked against condition Eq. (9). Moreover, the proof fixes A, whereas in training A is a learned quantity. Because the loss Eq. (7) constrains only the combined term ∂H/∂q − A^T λ and the residual A qdot − b, any alternative decomposition that leaves these two combinations unchanged is invisible to the training objective. Please either prove that no nonzero λ* satisfies ∇ × (A^T λ*) = 0 for each experimental constraint matrix, including the gauge transformations of Eqs. (5)–(6), or explicitly introduce an identifiability assumption/regularization and state the resulting scope of the claim.
- [§2.1, §2.2, Eq. (7)] The stated scope includes holonomic constraints, and there the central claim is exactly false. For a holonomic constraint g(q) = 0 with A = ∂g/∂q in the time-differentiated form, any smooth F gives H' = H + F(g) and λ' = λ + F'(g), and one verifies that ∂H'/∂p = ∂H/∂p, ∂H'/∂q − A^T λ' = ∂H/∂q − A^T λ, and A qdot − b = dg/dt = 0, so the loss Eq. (7) is unchanged exactly. Thus infinitely many (H, λ) pairs with different reported energies and constraint forces fit the training objective identically, and the mean-shifted energy MAE in Tables 5–6 cannot distinguish them. The authors should either narrow the claim to the nonholonomic setting or add a mechanism that breaks this symmetry, such as an additional energy-conservation residual or auxiliary information about H.
- [§3.2, Eqs. (13)–(15)] The ball-on-spinning-table experiments omit the holonomic unit-norm constraint on the Euler-Rodrigues parameters. The Hamiltonian in Eq. (13) and the momenta in Eq. (14) are only valid on the unit sphere ||e||^2 = 1, and with e_dot = p_e/(4I) the unconstrained flow leaves that sphere unless the additional velocity constraint e · e_dot = 0 is imposed. The learned A network outputs only two constraint rows, those in Eq. (15), so the architecture has no explicit mechanism to preserve ||e|| = 1. Please include the quaternion normalization as an additional constraint row (or justify that it is automatically satisfied), and report the e-norm error along the learned trajectories, particularly in the noisy experiments.
minor comments (6)
- [§3.1, Table 1] The learning-rate entry for the λ and A networks is marked with an asterisk, but the table has no footnote; the text mentions reductions at 1.5e5 and 3e5 epochs, so the table should state the schedule explicitly.
- [Throughout] The word 'asses' is used repeatedly where 'assess' is intended; please run a spelling pass over the manuscript.
- [Reference [11]] The in-text citation appears as 'Alltawaitan et al.' while the reference list spells the first author 'Altawaitan'; please unify the spelling.
- [§3.4] The Lyapunov-exponent calculation is described only briefly: initial 30% of the error vector is removed and the remainder is normalized, but the fitting window and the number of trajectories used are not stated. Please specify these details for reproducibility.
- [§3.1, Eq. (11)] The expression for phi_dot is ambiguous because the parentheses are unbalanced in the printed form; please rewrite it with unambiguous notation.
- [General] No statement of code or data availability is included; providing the trained-model code would improve reproducibility and allow readers to test the identifiability issue directly.
Circularity Check
No significant circularity: the loss is an inverse-problem residual and all learned quantities are benchmarked against independently defined ground-truth functions.
full rationale
The proposed method does not derive H, A, b, or λ from the loss by definition; it trains three networks to minimize the residual of Eqs. (2)-(3), which are standard mechanical equations taken from the literature, and then checks the learned functions against analytical Hamiltonians (Eqs. 8 and 13), analytical constraints (Eqs. 10 and 15), and ground-truth trajectories generated from the same equations. This is a physics-prior inverse problem, not a tautology: the true H, A, λ are never used as labels, and a network could in principle fit the residuals without matching them, which is why the supplementary material attempts a uniqueness proof. That proof is an independent mathematical argument (differences of solutions reduce to ∂H*/∂q = A^T λ* and the curl condition), not a restatement of the loss. The paper contains no load-bearing self-citations: the cited pseudo-Hamiltonian formulation [18]-[20] is external prior work. The identifiability gap noted in the supplement (Eq. 9 is verified only for the point-mass A, not for the rolling-disk or spinning-table A, and holonomic constraints admit exact loss-invariant transformations H→H+F(g), λ→λ+F'(g)) is a correctness and generalization concern, not circularity; it does not make the reported trajectory or energy benchmarks coincide with the training objective by construction.
Assumptions & free parameters
free parameters (3)
- Learning-rate schedule for A and λ networks (rolling disk) =
5e-5, reduced to 2e-5 after 1.5e5 epochs, then 1e-5 after 3e5 epochs
- Energy comparison offset =
mean of predicted energy shifted to mean of ground truth
- Network size hyperparameters =
H/A/λ hidden neurons: 200/400/300 for disk, 200/50/200 for ball
assumptions (5)
- domain assumption The constrained Hamilton equations ˙q = ∂H/∂p and ˙p = -∂H/∂q + A^T λ hold for the systems considered.
- domain assumption Constraints can be written as linear in velocities, A(q) ˙q = b(q), with A and b independent of ˙q.
- domain assumption A is smooth enough that differentiating A ˙q = b yields λ as a function of q and p.
- ad hoc to paper The multilayer perceptrons can represent H, A, b, and λ accurately enough, and gradient descent converges to a good minimum.
- domain assumption Synthetic data generated from known analytical models are representative of real measurements.
Cite this review
Pith. "Pith review of Hamiltonian-based neural networks for systems under nonholonomic constraints." pith.science (2026). https://pith.science/paper/3KAXLO65
@misc{pith2026241203018,
author = {Pith},
title = {Pith review of: Hamiltonian-based neural networks for systems under nonholonomic constraints},
year = {2026},
howpublished = {\url{https://pith.science/paper/3KAXLO65}},
note = {Machine review of arXiv:2412.03018}
}
read the original abstract
There has been increasing interest in methodologies that incorporate physics priors into neural network architectures to enhance their modeling capabilities. A family of these methodologies that has gained traction are Hamiltonian neural networks (HNN) and their variations. These architectures explicitly encode Hamiltonian mechanics both in their structure and loss function. Although Hamiltonian systems under nonholonomic constraints are in general not Hamiltonian, it is possible to formulate them in pseudo-Hamiltonian form, equipped with a Lie bracket which is almost Poisson. This opens the possibility of using some principles of HNNs in systems under nonholonomic constraints. The goal of the present work is to develop a modified Hamiltonian neural network architecture capable of modeling Hamiltonian systems under holonomic and nonholonomic constraints. A three-network parallel architecture is proposed to simultaneously learn the Hamiltonian of the system, the constraints, and their associated multipliers. A rolling disk and a ball on a spinning table are considered as canonical examples to assess the performance of the proposed Hamiltonian architecture. The experiments are then repeated with a noisy training set to study modeling performance under more realistic conditions.
Figures
Figures from the paper (11 more)
Reference graph
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Both the learned and ground truth constraints deviations are close to zero, with the former being lower
It can be seen that both the trajectory and the constraint forces match the ground truth with high precision. Both the learned and ground truth constraints deviations are close to zero, with the former being lower. This 2 is probably due to the fact that the loss function in e...
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Uniqueness of ˆH and ˆλ A relevant question is whether the three networks will learn exactly and only the quantity they are assigned to. In principle, the networks could learn in such a way that both ˙q and ˙p are properly modeled, but the terms ˆH and ˆA T ˆλ do not correspon...
Reviewed August 11, 2026 · model on record in the stance chip above.
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