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Srinivasa Salapaka

Publications and source records attributed to Srinivasa Salapaka.

16 recordsLinked to original sources

Autonomy-Aware Clustering: When Local Decisions Supersede Global Prescriptions

Clustering arises in a wide range of problem formulations, yet most existing approaches assume that the entities under clustering are passive and strictly conform to their assigned groups. In reality, entities often exhibit local autonomy, overriding prescribed associations in ways not fully captured by feature representations. Such autonomy can substantially reshape clustering outcomes -- altering cluster compositions, geometry, and cardinality -- with significant downstream effects on inference and decision-making. We introduce autonomy-aware clustering, a reinforcement learning (RL) framework that learns and accounts for the influence of local autonomy without requiring prior knowledge of its form. Our approach integrates RL with a Deterministic Annealing (DA) procedure, where, to determine underlying clusters, DA naturally promotes exploration in early stages of annealing and transitions to exploitation later. We also show that the annealing procedure exhibits phase transitions that enable design of efficient annealing schedules. To further enhance adaptability, we propose the Adaptive Distance Estimation Network (ADEN), a transformer-based attention model that learns dependencies between entities and cluster representatives within the RL loop, accommodates variable-sized inputs and outputs, and enables knowledge transfer across diverse problem instances. Empirical results show that our framework closely aligns with underlying data dynamics: even without explicit autonomy models, it achieves solutions close to the ground truth (gap ~3-4%), whereas ignoring autonomy leads to substantially larger gaps (~35-40%). The code and data are publicly available at https://github.com/salar96/AutonomyAwareClustering.

cs.LG↗

Inverter Output Impedance Estimation in Power Networks: A Variable Direction Forgetting Recursive-Least-Square Algorithm Based Approach

As inverter-based loads and energy sources become increasingly prevalent, accurate estimation of line impedance between inverters and the grid is essential for optimizing performance and enhancing control strategies. This paper presents a non-invasive method for estimating output-line impedance using measurements local to the inverter. It provides a specific method for signal conditioning of signals measured at the inverter, which makes the measured data better suited to estimation algorithms. An algorithm based on the Variable Direction Forgetting Recursive Least Squares (VDF-RLS) method is introduced, which leverages these conditioned signals for precise impedance estimation. The signal conditioning process transforms measurements into the direct-quadrature (dq) coordinate frame, where the rotating frame frequency is determined to facilitate a simpler and more accurate estimation. This frequency is implemented using a secondary Phase-Locked Loop (PLL) to attenuate grid voltage measurement variations. By isolating the variation-sensitive q-axis and relying solely on the less sensitive d-axis, the method further minimizes the impact of variations. The VDF-RLS estimation method achieves rapid adaptation while ensuring stability in the absence of persistent excitation by selectively discarding outdated data during updates. Proposed conditioning and estimation methods are non-invasive; estimations are solely done using measured outputs, and no signal is injected into the power network. Simulation results demonstrate a significant improvement in impedance estimation stability, particularly in low-excitation conditions, where the VDF-RLS method achieves more than three time lower error compared to existing approaches such as constant forgetting RLS and the Kalman filter.

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Orthogonal Nonnegative Matrix Factorization with Sparsity Constraints

This article presents a novel approach to solving the sparsity-constrained Orthogonal Nonnegative Matrix Factorization (SCONMF) problem, which requires decomposing a non-negative data matrix into the product of two lower-rank non-negative matrices, X=WH, where the mixing matrix H has orthogonal rows HH^T=I, while also satisfying an upper bound on the number of nonzero elements in each row. By reformulating SCONMF as a capacity-constrained facility-location problem (CCFLP), the proposed method naturally integrates non-negativity, orthogonality, and sparsity constraints. Specifically, our approach integrates control-barrier function (CBF) based framework used for dynamic optimal control design problems with maximum-entropy-principle-based framework used for facility location problems to enforce these constraints while ensuring robust factorization. Additionally, this work introduces a quantitative approach for determining the ``true" rank of W or H, equivalent to the number of ``true" features - a critical aspect in ONMF applications where the number of features is unknown. Simulations on various datasets demonstrate significantly improved factorizations with low reconstruction errors (as small as by 150 times) while strictly satisfying all constraints, outperforming existing methods that struggle with balancing accuracy and constraint adherence.

cs.DS↗

A Control Barrier Function Approach to Constrained Resource Allocation Problems in a Maximum Entropy Principle Framework

This paper presents a novel approach to solve capacitated facility location problems (FLP) that encompass various resource allocation problems. FLPs are a class of NP-hard combinatorial optimization problems, involving optimal placement and assignment of a small number of facilities over a large number of demand points, with each facility subject to upper and lower bounds on its resource utilization (e.g., the number of demand points it can serve). To address the challenges posed by inequality constraints and the combinatorial nature of the solution space, we reformulate the problem as a dynamic control design problem, enabling structured constraint handling and enhanced solution efficiency. Our method integrates a Control Barrier Function (CBF) and Control Lyapunov Function (CLF)-based framework with a maximum-entropy principle-based framework to ensure feasibility, optimality, and improved exploration of solutions. Numerical experiments demonstrate that this approach significantly enhances computational efficiency, yielding better solutions and showing negligible growth in computation time with problem size as compared to existing solvers. These results highlight the potential of control-theoretic and entropy-based methods for large-scale facility location problems.

math.OC↗

Towards Enabling Learning for Time-Varying finite horizon Sequential Decision-Making Problems*

Parameterized Sequential Decision Making (Para-SDM) framework models a wide array of network design applications spanning supply-chain, transportation, and sensor networks. These problems entail sequential multi-stage optimization characterized by states, control actions, and cost functions dependent on designable parameters. The challenge is to determine both the sequential decision policy and parameters simultaneously to minimize cumulative stagewise costs. Many Para-SDM problems are NP-hard and often necessitate time-varying policies. Existing algorithms tackling finite-horizon time-varying Para-SDM problems struggle with scalability when faced with a large number of states. Conversely, the sole algorithm addressing infinite-horizon Para-SDM assumes time (stage)-invariance, yielding stationary policies. However, this approach proves scalable for time-invariant problems by leveraging deep neural networks to learn optimal stage-invariant state-action value functions, enabling handling of large-scale scenarios. This article proposes a novel approach that reinterprets finite-horizon, time-varying Para-SDM problems as equivalent time-invariant problems through topography lifting. Our method achieves nearly identical results to the time-varying solution while exhibiting improved performance times in various simulations, notably in the small cell network problem. This fresh perspective on Para-SDM problems expands the scope of addressable issues and holds promise for future scalability through the integration of learning methods.

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Multi-Mode Inverters: A Unified Control Design for Grid-Forming, Grid-Following, and Beyond

We present a novel, integrated control framework designed to achieve seamless transitions among a spectrum of inverter operation modes. The operation spectrum includes grid-forming (GFM), grid-following (GFL), static synchronous compensator (STATCOM), energy storage system (ESS), and voltage source inverter (VSI). The proposed control architecture offers guarantees of stability, robustness, and performance regardless of the specific mode. The core concept involves establishing a unified algebraic structure for the feedback control system, where different modes are defined by the magnitude of closed-loop signals. As we demonstrate, this approach results in a two-dimensional continuum of operation modes and enables transition trajectories between operation modes by dynamically adjusting closed-loop variables towards corresponding setpoints. Stability, robustness, and fundamental limitation analyses are provided for the closed-loop system across any mode, as well as during transitions between modes. This design facilitates stable and enhanced on-grid integration, even during GFM operation and weak grid conditions. Ultimately, we demonstrate the key attributes of the proposed framework through simulations and experiments, showcasing its seamless transition in on-grid operation, functionality in islanded mode, and robustness to line impedance uncertainty.

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One-shot Generative Distribution Matching for Augmented RF-based UAV Identification

This work addresses the challenge of identifying Unmanned Aerial Vehicles (UAV) using radiofrequency (RF) fingerprinting in limited RF environments. The complexity and variability of RF signals, influenced by environmental interference and hardware imperfections, often render traditional RF-based identification methods ineffective. To address these complications, the study introduces the rigorous use of one-shot generative methods for augmenting transformed RF signals, offering a significant improvement in UAV identification. This approach shows promise in low-data regimes, outperforming deep generative methods like conditional generative adversarial networks (GANs) and variational auto-encoders (VAEs). The paper provides a theoretical guarantee for the effectiveness of one-shot generative models in augmenting limited data, setting a precedent for their application in limited RF environments. This research contributes to learning techniques in low-data regime scenarios, which may include atypical complex sequences beyond images and videos. The code and links to datasets used in this study are available at https://github.com/amir-kazemi/uav-rf-id.

cs.LG↗

Control Designs for Critical-Continegency Responsible Grid-Following Inverters and Seamless Transitions To and From Grid-Forming Modes

This article introduces two control frameworks: one for Grid-Following (GFL) inverters aiding Grid-Forming (GFM) inverters in voltage regulation during large contingency events and optimizing power transactions under normal conditions; and another for seamless transitions between grid-tied and grid-isolated setups, managing voltage transient characteristics. In microgrids, GFM inverters regulate voltage, while GFL inverters handle power transactions. The proposed GFL control detects abrupt load/generation changes, adjusting power transactions using local storage to support GFM inverters during contingencies. Additionally, a transition control ensures smooth GFL-GFM shifts, reducing power and voltage fluctuations. Simulation results validate improved voltage regulation during contingencies and enhanced power tracking during slow changes, alongside minimized transient overshoot.

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Enhanced Grid Following Inverter (E-GFL): A Unified Control Framework for Stiff and Weak Grids

This paper presents an extensive framework focused on the control design, along with stability and performance analysis, of Grid-Following Inverters (GFL). It aims to ensure their effective operation under both stiff and weak grid conditions. The proposed framework leverages the coupled algebraic structure of the transmission line dynamics in the $dq$ frame to express and then mitigate the effect of coupled dynamics on the GFL inverter's stability and performance. Additionally, we simplify the coupled multi-input, multi-output (MIMO) closed-loop system of the GFL into two separate single-input, single-output (2-SISO) closed-loops for easier analysis and control design. We present the stability, robust stability, and performance of the original GFL MIMO closed-loop system through our proposed 2-SISO closed-loop framework. This approach simplifies both the control design and its analysis. Our framework effectively achieves grid synchronization and active damping of filter resonance via feedback control. This eliminates the need for separate phase-locked loop (PLL) and virtual impedance subsystems. We also utilize the Bode sensitivity integral to define the limits of GFL closed-loop stability margin and performance. These fundamental limits reveal the necessary trade-offs between various performance goals, including reference tracking, closed-loop bandwidth, robust synchronization, and the ability to withstand grid disturbances. Finally, we demonstrate the merits of our proposed framework through detailed simulations and experiments. These showcase its effectiveness in handling challenging scenarios, such as asymmetric grid faults, low voltage operation, and the balance between harmonic rejection and resonance suppression.

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Sparse Linear Regression with Constraints: A Flexible Entropy-based Framework

This work presents a new approach to solve the sparse linear regression problem, i.e., to determine a k-sparse vector w in R^d that minimizes the cost ||y - Aw||^2_2. In contrast to the existing methods, our proposed approach splits this k-sparse vector into two parts -- (a) a column stochastic binary matrix V, and (b) a vector x in R^k. Here, the binary matrix V encodes the location of the k non-zero entries in w. Equivalently, it encodes the subset of k columns in the matrix A that map w to y. We demonstrate that this enables modeling several non-trivial application-specific structural constraints on w as constraints on V. The vector x comprises of the actual non-zero values in w. We use Maximum Entropy Principle (MEP) to solve the resulting optimization problem. In particular, we ascribe a probability distribution to the set of all feasible binary matrices V, and iteratively determine this distribution and the vector x such that the associated Shannon entropy gets minimized, and the regression cost attains a pre-specified value. The resulting algorithm employs homotopy from the convex entropy function to the non-convex cost function to avoid poor local minimum. We demonstrate the efficacy and flexibility of our proposed approach in incorporating a variety of practical constraints, that are otherwise difficult to model using the existing benchmark methods.

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Towards Efficient Modularity in Industrial Drying: A Combinatorial Optimization Viewpoint

The industrial drying process consumes approximately 12% of the total energy used in manufacturing, with the potential for a 40% reduction in energy usage through improved process controls and the development of new drying technologies. To achieve cost-efficient and high-performing drying, multiple drying technologies can be combined in a modular fashion with optimal sequencing and control parameters for each. This paper presents a mathematical formulation of this optimization problem and proposes a framework based on the Maximum Entropy Principle (MEP) to simultaneously solve for both optimal values of control parameters and optimal sequence. The proposed algorithm addresses the combinatorial optimization problem with a non-convex cost function riddled with multiple poor local minima. Simulation results on drying distillers dried grain (DDG) products show up to 12% improvement in energy consumption compared to the most efficient single-stage drying process. The proposed algorithm converges to local minima and is designed heuristically to reach the global minimum.

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Control Design for Inverters: Beyond Steady-State Droop Laws

This paper presents a novel control structure and control synthesis method for regulating the output voltage/frequency and power injection of DC-AC inverters. The traditional droop method offers attractive solution to achieve compromise between clashing power and voltage/frequency regulation objectives. However, it relies on use of nonlinear power variables through slow outer control loop. In this paper we formulate the traditional droop method as a feedback control problem based on static power-flow equations and show how neglecting the dynamics of inverter and transmission line restricts the attainable closed-loop bandwidth and stability and robustness margin. Then we introduce a mapping between power variables and current in $dq$ frame under given PLL condition, allowing for replacing the fast acting current variables as a proxy for power. Consequently, we present a novel control structure and control synthesis method based on disturbance rejection framework, and demonstrate inherent droop like characteristics in underlying dynamics for special cases of resistive and inductive line. Moreover, we generalize the proposed control synthesis procedure to include a generalized complex line dynamical model and introduce concept of hybrid-sourced-intverter. Finally, we validate higher bandwidth and better transient performance of our proposed design through experimental validation.

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Isochronous Architecture-Based Voltage-Active Power Droop for Multi-Inverter Systems

Advanced microgrids consisting of distributed energy resources interfaced with multi-inverter systems are becoming more common. Consequently, the effectiveness of voltage and frequency regulation in microgrids using conventional droop-based methodologies is challenged by uncertainty in the sizeand schedule of loads. This article proposes an isochronous architecture of parallel inverters with only voltage-active power droop (VP-D) control for improving active power sharing as well as plug-and-play of multi-inverter based distributed energyresources (DERs). In spite of not employing explicit control for frequency regulation, this architecture allows even sharing of reactive power while maintaining reduced circulating currents between inverters. The performance is achieved even when there are mismatches between commanded reference and power demanded from the actual load in the network. The isochronous architecture is implemented by employing a global positioning system (GPS) to disseminate timing signals that enable the microgrid to maintain nominal system frequency in the entire network. This enables direct control of active power through voltage source inverter (VSI) output voltage regulation, even in the presence of system disturbances. A small signal eigenvalue analysis of a multi-inverter system near the steady-state operating point is presented to evaluate the stability of the multi-inverter system with the proposed VP-D control. Simulation studies and hardware experiments on an 1.2 kVA prototype are conducted. The effectiveness of the proposed architecture towards active and reactive power sharing between inverters with load scenarios are demonstrated. Results of the hardware experiments corroborate the viability of the proposed VP-D control architecture.

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A Clustering Approach to Edge Controller Placement in Software Defined Networks with Cost Balancing

In this work we introduce two novel deterministic annealing based clustering algorithms to address the problem of Edge Controller Placement (ECP) in wireless edge networks. These networks lie at the core of the fifth generation (5G) wireless systems and beyond. These algorithms, ECP-LL and ECP-LB, address the dominant leader-less and leader-based controller placement topologies and have linear computational complexity in terms of network size, maximum number of clusters and dimensionality of data. Each algorithm tries to place controllers close to edge node clusters and not far away from other controllers to maintain a reasonable balance between synchronization and delay costs. While the ECP problem can be conveniently expressed as a multi-objective mixed integer non-linear program (MINLP), our algorithms outperform state of art MINLP solver, BARON both in terms of accuracy and speed. Our proposed algorithms have the competitive edge of avoiding poor local minima through a Shannon entropy term in the clustering objective function. Most ECP algorithms are highly susceptible to poor local minima and greatly depend on initialization.

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Multiway k-Cut in Static and Dynamic Graphs: A Maximum Entropy Principle Approach

This work presents a maximum entropy principle based algorithm for solving minimum multiway $k$-cut problem defined over static and dynamic {\em digraphs}. A multiway $k$-cut problem requires partitioning the set of nodes in a graph into $k$ subsets, such that each subset contains one prespecified node, and the corresponding total cut weight is minimized. These problems arise in many applications and are computationally complex (NP-hard). In the static setting this article presents an approach that uses a relaxed multiway $k$-cut cost function; we show that the resulting algorithm converges to a local minimum. This iterative algorithm is designed to avoid poor local minima with its run-time complexity as $\sim O(kIN^3)$, where $N$ is the number of vertices and $I$ is the number of iterations. In the dynamic setting, the edge-weight matrix has an associated dynamics with some of the edges in the graph capable of being influenced by an external input. The objective is to design the dynamics of the controllable edges so that multiway $k$-cut value remains small (or decreases) as the graph evolves under the dynamics. Also it is required to determine the time-varying partition that defines the minimum multiway $k$-cut value. Our approach is to choose a relaxation of multiway $k$-cut value, derived using maximum entropy principle, and treat it as a control Lyapunov function to design control laws that affect the weight dynamics. Simulations on practical examples of interactive foreground-background segmentation, minimum multiway $k$-cut optimization for non-planar graphs and dynamically evolving graphs that demonstrate the efficacy of the algorithm, are presented.

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On the Persistence of Clustering Solutions and True Number of Clusters in a Dataset

Typically clustering algorithms provide clustering solutions with prespecified number of clusters. The lack of a priori knowledge on the true number of underlying clusters in the dataset makes it important to have a metric to compare the clustering solutions with different number of clusters. This article quantifies a notion of persistence of clustering solutions that enables comparing solutions with different number of clusters. The persistence relates to the range of data-resolution scales over which a clustering solution persists; it is quantified in terms of the maximum over two-norms of all the associated cluster-covariance matrices. Thus we associate a persistence value for each element in a set of clustering solutions with different number of clusters. We show that the datasets where natural clusters are a priori known, the clustering solutions that identify the natural clusters are most persistent - in this way, this notion can be used to identify solutions with true number of clusters. Detailed experiments on a variety of standard and synthetic datasets demonstrate that the proposed persistence-based indicator outperforms the existing approaches, such as, gap-statistic method, $X$-means, $G$-means, $PG$-means, dip-means algorithms and information-theoretic method, in accurately identifying the clustering solutions with true number of clusters. Interestingly, our method can be explained in terms of the phase-transition phenomenon in the deterministic annealing algorithm, where the number of distinct cluster centers changes (bifurcates) with respect to an annealing parameter.

cs.LG↗