REVIEW 3 major objections 5 minor 32 references
Subspace Identification of Temperature Dynamics
T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Temperature dynamics of a heated aluminum bar can be captured by low-order linear state-space models identified from short data records.
desk verdict Careful experimental study, but the identification step cannot work as written—the regressor matrix can't be full row rank with these data sizes—and the validation results are selected, not independent. 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 predictor-based subspace identification (PBSID) pipeline, specifically the one-step-ahead VARX predictor $y_k \approx M_{p-1}z_{k-p,k-1}+e_k$, whose Markov-parameter matrix $M_{p-1}$ is estimated by least squares. That matrix is rearranged into $Q_{p-1}$, and a singular value decomposition of $\hat{Q}_{p-1}Z_{0,p-1}^{(l)}$ yields the state sequence; the system matrices $A$, $B$, $C$, and the Kalman gain $K$ then follow from two further least-squares problems. The machinery also includes an AIC-based choice of the past window $p$, a grid search over state order and future window using validation error, and a white-noise residual test based on autocorrelation bounds $\pm 2/\sqrt{N_1}$.
What would settle it
On the published data set, form the regressor $Z_{0,p-1}^{(l)}$ for $N=180$ and $p=24$ and compute its rank; it will be at most 157, which is less than the 264 rows, so equation (20) is not executable as written. The decisive check is whether a correctly implemented estimator—using a smaller $p$ or a pseudoinverse—reproduces the claimed validation errors of about 1 to 6 percent; if it does, the empirical claim stands but the derivation needs revision, and if it does not, the central claim is refuted.
Extended reading notes
Core claim
The central claim is that a seven-output, four-input MIMO temperature process—physically described by the heat equation with nonlinear boundary conditions—can be represented by a linear Kalman innovation state-space model identified from only 180 input-output samples, and that the best model order depends on how the model will be used. For System 1 (96-second sampling), open-loop simulation of the state-space model selects $n=34$ states with a validation error of 3.34 percent; open-loop simulation of the innovation model selects $n=12$ with 5.83 percent; and closed-loop simulation driven by measured outputs selects $n=6$ with 1.18 percent. For System 2 (208-second sampling), the open-loop result is 4.73 percent with $n=13$. The paper interprets these results as evidence that low-order linear models are adequate for temperature prediction, and that using real measurements during simulation reduces the required model order.
Load-bearing premise
The load-bearing premise is that the data matrix $Z_{0,p-1}^{(l)}$ in equation (20) has full row rank, so the least-squares inverse written there exists; with $N=180$ identification samples and $p=24$, that matrix has 264 rows and at most 157 columns, so the inverse cannot exist as stated.
Editorial extensions
If this is right
- Temperature control and observer design for this class of mechatronic systems can proceed with low-order linear state-space models rather than high-order PDE discretizations.
- Short identification experiments (about 180 samples at 96- to 208-second sampling) are enough to obtain validation errors below 6 percent.
- When measured outputs are available during operation, the innovation/observer form requires far fewer states (six for System 1) than a pure open-loop simulator (34 states).
- The quadratic actuator nonlinearity can be removed by defining inputs as squared voltages, making a linear model appropriate despite the physically nonlinear heat transfer.
- Residual correlations (e.g., channel 2 at short lags) indicate remaining unmodeled dynamics, so the identified models are approximate rather than exact representations.
Reading between the lines
- The stated full-row-rank condition on $Z_{0,p-1}^{(l)}$ in equation (20) cannot hold for the reported $N=180$ and $p=24$: the regressor has 264 rows and at most 157 columns, so the estimates in the paper must have come from a numerically regularized or otherwise repaired version of the equation, and the written algorithm needs correction even if the empirical conclusions survive.
- Because the closed-loop model (Method C) uses measured outputs to drive the state, its low order and low error are partly an observer property; a fairer benchmark for open-loop control design is the 34-state open-loop model.
- A direct robustness test would be to re-estimate with a smaller past window where full row rank is possible, and to check whether the validation errors stay in the reported range; if they do, the low-order conclusion is not an artifact of the algebraic issue.
- The residual correlation in output 2 suggests that a nonlinear or spatially refined model of the local heat transfer near that thermocouple could reduce the remaining error.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper describes an experimental study in which a long aluminum bar is heated by four band heaters and sensed by seven thermocouples, with the goal of identifying a low-order MIMO state-space model of the temperature dynamics. The author combines predictor-based subspace identification (PBSID) with AIC-based order selection and evaluates the resulting models under three validation scenarios: open-loop simulation, simulation driven by measured outputs, and closed-loop simulation. The data and code are publicly available. The paper reports relative validation errors below about 6% for several identified models and concludes that low-order linear state-space representations capture the temperature dynamics with reasonable accuracy.
Significance. If the claims were supported, the paper would provide a useful experimental benchmark for applying PBSID-type methods to slow, spatially distributed thermal systems with limited data, and the public code and data would be a practical asset for the system-identification community. The experimental sections, especially the noise analysis, step-response characterization, and nonlinearity compensation via squared voltage inputs, are thoughtful and well documented. However, the central estimation step is not well posed as written, and the validation protocol makes the reported error rates fitted quantities rather than independent predictive assessments. These issues undermine the main quantitative claims of the paper.
major comments (3)
- [§III.B, Eq. (20), and §IV.A/IV.B] This is a load-bearing issue: the reported identification results and validation errors are not supported by the derivation as written.
- [§III.C1 and Algorithm 1, Step 3] This concern affects all three model-selection methods A, B, and C and the headline error figures.
- [§IV.C, paragraphs reporting System 2 results] For System 2, the reported best result (e=4.73%) is obtained with p=26, f=18, and n=13, but the paper gives no detailed prediction plots or eigenvalue analysis for this model. Given that System 2 is used to support the claim that the approach works at a second sampling period, the absence of comparable validation details weakens the empirical support, although this is secondary to the issues above.
minor comments (5)
- [§III.A, after Eq. (12)] The symbol K_{p-1} appears where L_{p-1} is intended: the text says 'where Kp−1∈ R^{n×11p} and Of−1∈ R^{7f×n}', but the preceding definition uses L_{p-1}.
- [Footnote 1] The repository URL contains a typo: 'Temperture' should be 'Temperature'.
- [Figure 9(b)] The vertical axis label 'e' should explicitly refer to the relative error defined in Eq. (39), since the same symbol is used for the innovation process in Eq. (8).
- [Throughout] The notation 'V ARX' and 'VARX' are used inconsistently; one spelling should be adopted throughout.
- [§IV.A] The definition of identification and validation sequence lengths is slightly ambiguous: the text says '180 and 120 discrete-time samples', while the formal sets I and V use indices k=0,...,N and k=0,...,N_1, so the relationship between N and 180 is not explicit.
Circularity Check
Validation data is used both to select the model and to score it, so the reported relative errors are partly fit statistics rather than independent predictions.
-
fitted input called prediction
[Section III-C1 (Methods A-C, Eqs. (35)-(39)); results in Section IV.C]
"“We compute the model error (39) for all the pairs of state orders and future windows in the set S. We select the pair for which e has the smallest value. This model selection method is referred to as Method A.” ... “For Method B, the final model relative error is 5.83% that is obtained for n = 12, f = 11. For Method C, the final model relative error is 1.18% that is obtained for n = 6, f = 1.”"
The validation sequence V is used both to select the hyperparameters (state order n, future window f) by minimizing the relative error e in Eq. (39), and then to compute the reported 'final model relative error' for the selected model. Since the same data determines both the model choice and the reported score, the published errors (3.34%, 4.73%, 5.83%, 1.18%) are the minimum of the selection criterion over the candidate set S on that same data, not independent out-of-sample predictions. The central accuracy claim is therefore partly a fitted quantity: the 'validation' error is an optimistically biased in-sample statistic for model choice, not a prediction of unseen data. This is pattern 2 (fitted input called prediction) rather than a mathematical identity.
full rationale
The PBSID estimation chain in Section III is internally consistent as a derivation: the VARX parameters in Eq. (20), the past-window AIC step in Eq. (21), the SVD state extraction in Eq. (26), and the least-squares system matrices in Eqs. (32)-(34) do not by themselves reduce to their inputs by definition, and the system matrices are fitted to the identification data, not to the validation data. The one load-bearing circular step is the model-order/future-window selection in Section III-C1: Methods A, B, and C each choose (n, f) from S by minimizing the validation error e, and Section IV.C then reports that same minimized e as the 'final model relative error.' This is a fitted prediction rather than an independent held-out estimate. Method C additionally feeds measured validation outputs y_k into z_k in the state recursion, so its 1.18% error is an observer-based reconstruction; the authors disclose this, and it is a scenario choice, not a circularity. The full-row-rank condition for Eq. (20) is a separate mathematical correctness issue, not a circularity: with N=180 and p=24, Z_{0,p-1}^{(l)} is 264 by at most 157, so the stated inverse cannot exist; that is a well-posedness defect in the derivation as written. The citations to prior work by the same author are descriptive and not load-bearing; no uniqueness theorem or ansatz is imported via self-citation. Score 5 reflects one significant fitted-prediction circularity while the core estimation procedure retains independent content.
Assumptions & free parameters
free parameters (5)
- Past window p =
24 (System 1), 26 (System 2)
- State order n =
34 (System 1 Method A), 12 (Method B), 6 (Method C), 13 (System 2)
- Future window f =
5 (System 1 Method A), 11 (Method B), 1 (Method C), 18 (System 2)
- Butterworth filter cutoff frequency =
0.2 Hz
- Sampling periods =
96 s (System 1), 208 s (System 2)
assumptions (4)
- domain assumption The temperature dynamics can be adequately approximated by a linear time-invariant state-space model (5)-(6) with additive white noise.
- domain assumption The past window p satisfies ~A^s approximately zero for all s >= p (equation 14).
- domain assumption The regressor matrix Z_{0,p-1}^{(l)} has full row rank (persistency of excitation).
- standard math AIC can be used to select the VARX order with the given penalty term.
Cite this review
Pith. "Pith review of Subspace Identification of Temperature Dynamics." pith.science (2026). https://pith.science/paper/KF7WA5NF
@misc{pith2026190802379,
author = {Pith},
title = {Pith review of: Subspace Identification of Temperature Dynamics},
year = {2026},
howpublished = {\url{https://pith.science/paper/KF7WA5NF}},
note = {Machine review of arXiv:1908.02379}
}
read the original abstract
Data-driven modeling and control of temperature dynamics in mechatronics systems and industrial processes are challenging control engineering problems. This is mainly because the temperature dynamics is inherently infinite-dimensional, nonlinear, spatially distributed, and coupled with other physical processes. Furthermore, the dominant time constants are usually long, implying that in practice due to various economic and time constraints, we can only collect a relatively small number of data samples that can be used for data-driven modeling. Finally, since sensing and actuation of temperature dynamics are often spatially discrete, special attention needs to be given to sensor (actuator) placement and identifiability problems. Motivated by these challenges, in this manuscript, we consider the problem of data-driven modeling and validation of temperature dynamics. We have developed an experimental setup consisting of a long aluminum bar whose temperature dynamics is influenced by spatially distributed heat actuators and whose temperature is sensed by spatially distributed thermocouples. We address the noise reduction problem and perform step response and nonlinearity analyses. We combine predictor based subspace identification methods with time series analysis methods to identify a multiple-input multiple-output system model. We provide detailed treatments of model structure selection, validation, and residual analysis problems under different modeling and prediction scenarios. Our extensive experimental results show that the temperature dynamics of the experimental setup can be relatively accurately estimated by low-order models.
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Works this paper leans on
- [1]
-
[2]
Predictive control of thermally induced wavefront aberrations,
A. Haber, A. Polo, I. Maj, S. F. Pereira, H. P. Urbach, and M. Verhaegen, “Predictive control of thermally induced wavefront aberrations,” Optics Express, vol. 21, no. 18, pp. 21 530–21 541, 2013
work page 2013
-
[3]
Identification of a dynamical model of a thermally actuated deformable mirror,
A. Haber, A. Polo, S. Ravensbergen, H. P. Urbach, and M. Verhaegen, “Identification of a dynamical model of a thermally actuated deformable mirror,” Optics Letters, vol. 38, no. 16, pp. 3061–3064, 2013
work page 2013
-
[4]
Deformation control of a thermal active mirror,
R. Saathof, M. V . Wansink, E. C. Hooijkamp, J. W. Spronck, and R. H. M. Schmidt, “Deformation control of a thermal active mirror,” Mechatronics, vol. 39, pp. 12–27, 2016
work page 2016
-
[5]
Linear phase retrieval for real-time adaptive optics,
A. Polo, A. Haber, S. F. Pereira, M. Verhaegen, and H. P. Urbach, “Linear phase retrieval for real-time adaptive optics,” Journal of the European Optical Society-Rapid publications , vol. 8, 2013
work page 2013
-
[6]
S. K. Ravensbergen, P. C. J. N. Rosielle, and M. Steinbuch, “De- formable mirrors with thermo-mechanical actuators for extreme ultra- violet lithography: design, realization and validation,” Precision Engi- neering, vol. 37, no. 2, pp. 353–363, 2013
work page 2013
-
[7]
Iterative learning control of a membrane deformable mirror for optimal wavefront correction,
A. Haber, A. Polo, C. S. Smith, S. F. Pereira, P. Urbach, and M. Ver- haegen, “Iterative learning control of a membrane deformable mirror for optimal wavefront correction,” Applied optics , vol. 52, no. 11, pp. 2363–2373, 2013
work page 2013
-
[8]
Sparsity preserving optimal control of discretized pde systems,
A. Haber and M. Verhaegen, “Sparsity preserving optimal control of discretized pde systems,” Computer Methods in Applied Mechanics and Engineering, vol. 335, pp. 610–630, 2018
work page 2018
Show all 32 references
-
[9]
Online parameterization of lumped thermal dynamics in cylindrical lithium ion batteries for core temperature estimation and health monitoring,
X. Lin, H. E. Perez, J. B. Siegel, A. G. Stefanopoulou, Y . Li, R. D. Anderson, Y . Ding, and M. P. Castanier, “Online parameterization of lumped thermal dynamics in cylindrical lithium ion batteries for core temperature estimation and health monitoring,” IEEE Transactions on ...
2013
-
[10]
Identification of distributed-parameter systems from sparse measurements,
Z. Hidayat, R. Babu ˇska, A. Nunez, and B. De Schutter, “Identification of distributed-parameter systems from sparse measurements,” Applied Mathematical Modelling, vol. 51, pp. 605–625, 2017
2017
-
[11]
Identification of a linear continuous time stochastic model of the heat dynamics of a greenhouse,
B. Nielsen and H. Madsen, “Identification of a linear continuous time stochastic model of the heat dynamics of a greenhouse,” Journal of Agricultural Engineering Research , vol. 71, no. 3, pp. 249–256, 1998
1998
-
[12]
Model predictive control for the operation of building cooling systems,
Y . Ma, F. Borrelli, B. Hencey, B. Coffey, S. Bengea, and P. Haves, “Model predictive control for the operation of building cooling systems,” IEEE Transactions on Control Systems Technology , vol. 20, no. 3, pp. 796–803, 2012
2012
-
[13]
Predictive control for energy efficient buildings with thermal storage: Modeling, stimulation, and experiments,
Y . Ma, A. Kelman, A. Daly, and F. Borrelli, “Predictive control for energy efficient buildings with thermal storage: Modeling, stimulation, and experiments,” IEEE Control Systems Magazine , vol. 32, no. 1, pp. 44–64, 2012
2012
-
[14]
Parametric system identification using deep convolutional neural networks,
S. Genc, “Parametric system identification using deep convolutional neural networks,” in 2017 International Joint Conference on Neural Networks (IJCNN). IEEE, 2017, pp. 2112–2119
2017
-
[15]
D. E. Seborg, D. A. Mellichamp, T. F. Edgar, and F. J. Doyle III, Process Dynamics and Control . John Wiley & Sons, 2010
2010
-
[16]
Performance of a thermally deformable mirror for correction of low-order aberrations in laser beams,
M. Kasprzack, B. Canuel, F. Cavalier, R. Day, E. Genin, J. Marque, D. Sentenac, and G. Vajente, “Performance of a thermally deformable mirror for correction of low-order aberrations in laser beams,” Applied Optics, vol. 52, no. 12, pp. 2909–2916, 2013
2013
-
[17]
Ljung, System Identification
L. Ljung, System Identification. Springer, 1998
1998
-
[18]
Verhaegen and V
M. Verhaegen and V . Verdult, Filtering and System Identification: A Least Squares Approach. Cambridge University Press, 2007
2007
-
[19]
Incropera and D
F. Incropera and D. DeWitt, Introduction to Heat Transfer , 1985
1985
-
[20]
Development of an analytical mirror model addressing the problem of thermoelastic deformation,
C. J. Myers and R. C. Allen, “Development of an analytical mirror model addressing the problem of thermoelastic deformation,” Applied optics, vol. 24, no. 13, pp. 1933–1939, 1985
1933
-
[21]
State observation and sensor selection for nonlinear networks,
A. Haber, F. Molnar, and A. E. Motter, “State observation and sensor selection for nonlinear networks,” IEEE transactions on control of network systems, vol. 5, no. 2, pp. 694–708, 2017
2017
-
[22]
Subspace identification of large-scale interconnected systems,
A. Haber and M. Verhaegen, “Subspace identification of large-scale interconnected systems,” IEEE Transactions on Automatic Control , vol. 59, no. 10, pp. 2754–2759, 2014
2014
-
[23]
Moving horizon estimation for large-scale interconnected sys- tems,
——, “Moving horizon estimation for large-scale interconnected sys- tems,” IEEE Transactions on Automatic Control , vol. 58, no. 11, pp. 2834–2847, 2013
2013
-
[24]
Van Overschee and B
P. Van Overschee and B. L. De Moor, Subspace identification for linear systems: Theory-Implementation-Applications . Springer Science & Business Media, 2012
2012
-
[25]
Identification of spatially distributed discrete-time state-space models,
A. Haber and M. Verhaegen, “Identification of spatially distributed discrete-time state-space models,” IFAC Proceedings Volumes, vol. 45, no. 16, pp. 410–415, 2012
2012
-
[26]
V ARMAX- based closed-loop subspace model identification,
I. Houtzager, J.-W. van Wingerden, and M. Verhaegen, “V ARMAX- based closed-loop subspace model identification,” in Proceedings of the 48h IEEE Conference on Decision and Control (CDC) held jointly with 2009 28th Chinese Control Conference . IEEE, 2009, pp. 3370–3375
2009
-
[27]
The role of vector autoregressive modeling in predictor- based subspace identification,
A. Chiuso, “The role of vector autoregressive modeling in predictor- based subspace identification,” Automatica, vol. 43, no. 6, pp. 1034– 1048, 2007
2007
-
[28]
Recursive predictor-based subspace identification with application to the real-time closed-loop tracking of flutter,
I. Houtzager, J.-W. van Wingerden, and M. Verhaegen, “Recursive predictor-based subspace identification with application to the real-time closed-loop tracking of flutter,” IEEE Transactions on Control Systems Technology, vol. 20, no. 4, pp. 934–949, 2012
2012
-
[29]
L ¨utkepohl, New Introduction to Multiple Time Series Analysis
H. L ¨utkepohl, New Introduction to Multiple Time Series Analysis . Springer Science & Business Media, 2005
2005
-
[30]
Identi- fication of temperature dynamics using subspace and machine learning techniques,
A. Haber, F. Pecora, M. U. Chowdhury, and M. Summerville, “Identi- fication of temperature dynamics using subspace and machine learning techniques,” in ASME 2019 Dynamic Systems and Control Conference . ASME, 2019
2019
-
[31]
Two ways to measure temperature using thermo- couples feature simplicity, accuracy, and flexibility,
M. Duff and J. Towey, “Two ways to measure temperature using thermo- couples feature simplicity, accuracy, and flexibility,” Analog Dialogue, vol. 44, no. 10, pp. 1–6, 2010
2010
-
[32]
Nonlinearity tests for dynamic processes,
R. Haber, “Nonlinearity tests for dynamic processes,” IFAC Proceedings Volumes, vol. 18, no. 5, pp. 409–414, 1985
1985
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