The timely detection of ice formation on exposed surfaces represents a critical problem in both the wind energy sector, where blade accretion reduces aerodynamic efficiency and may cause unbalanced rotor loads, and in aviation, where alterations of the wing profile directly compromise flight safety. Conventional detection systems, based on optical, thermal, or ultrasonic technologies, present significant limitations in terms of power consumption, sensitivity to dirt, and vibrational noise. Planar capacitive sensors represent an attractive alternative due to their constructive simplicity and low cost, but when interrogated at a single frequency they generate a measurement ambiguity between liquid water and ice, since both media exhibit high and comparable relative permittivity values at low frequencies. This thesis proposes a single-element planar capacitive measurement system, interrogated at two distinct frequencies, capable of discriminating the physical state of the medium covering the sensor by exploiting the frequency dependence of the dielectric constant described by the Debye relaxation model. While liquid water maintains a high permittivity across the entire interrogation spectrum employed, ice exhibits a marked collapse of its dielectric constant as frequency increases, due to the freezing of the mobility of the lattice defects responsible for orientational polarization. The comparison between the capacitances measured at a low and a high frequency thus enables reliable discrimination between air, liquid water, and ice, without resorting to shielded reference capacitors. The experimental work involved the adaptation and optimization of a pre-existing measurement system, based on an interdigitated electrode sensor and a conditioning electronic stage connected to an STM32 microcontroller. Hardware modifications were introduced to improve the robustness of the system under thawing conditions, including the application of a protective conformal coating on the analog board, together with firmware modifications implementing a twelve-point multi-frequency sweep. The experimental characterization was carried out on samples of clear ice and rime ice, synthesized in laboratory under controlled conditions, over a thermal range between −30 °C and +25 °C. During the experimental tests, an initially unexpected disturbance phenomenon emerged: at frequencies below 1 kHz, capacitive measurements were systematically distorted by a parasitic contribution attributable to Maxwell-Wagner interfacial polarization, induced by humidity adsorbed on the sensor substrate surface. The experimental investigation conducted to identify and characterize this phenomenon allowed precise determination of the frequency threshold above which the system is immune to environmental disturbance, while simultaneously providing a quantitative validation of the 1 kHz / 100 kHz operating frequency pair, identified as optimal for maximizing the dielectric contrast between the states of interest while preserving measurement robustness. The results obtained demonstrate that the system is able to reliably and reproducibly classify the states of air, liquid water, and ice across the entire thermal range considered, laying the groundwork for a low-cost, scalable implementation over extended aerodynamic surfaces.
La rilevazione tempestiva della formazione di ghiaccio su superfici esposte rappresenta un problema critico sia nel settore eolico, dove l'accumulo sulle pale riduce l'efficienza aerodinamica e può causare carichi sbilanciati sui rotori, sia in quello aeronautico, dove l'alterazione del profilo alare compromette direttamente la sicurezza del volo. I sistemi di rilevamento convenzionali, basati su tecnologie ottiche, termiche o a ultrasuoni, presentano limiti significativi in termini di consumo energetico, sensibilità allo sporco e rumore vibrazionale. I sensori capacitivi planari rappresentano un'alternativa interessante per la loro semplicità costruttiva e il basso costo, ma se interrogati a singola frequenza generano un'ambiguità di misura tra acqua liquida e ghiaccio, poiché entrambi i mezzi presentano valori elevati e comparabili di permittività relativa alle basse frequenze. Il presente lavoro di tesi propone un sistema di misura capacitivo planare a singolo elemento sensibile, interrogato a due frequenze distinte, in grado di discriminare lo stato fisico del mezzo sovrastante sfruttando la dipendenza in frequenza della costante dielettrica descritta dal modello di rilassamento di Debye. Mentre l'acqua liquida mantiene una permittività elevata su tutto lo spettro di interrogazione impiegato, il ghiaccio presenta un crollo marcato della propria costante dielettrica al crescere della frequenza, dovuto al congelamento della mobilità dei difetti reticolari responsabili della polarizzazione orientazionale. Il confronto tra le capacità misurate a una frequenza bassa e una alta consente quindi di distinguere in modo affidabile aria, acqua liquida e ghiaccio, senza ricorrere a capacità di riferimento schermate. Il lavoro sperimentale ha previsto l'adattamento e l'ottimizzazione di un sistema di misura preesistente, basato su un sensore a elettrodi interdigitati e un'elettronica di condizionamento collegata a un microcontrollore STM32. Sono state introdotte modifiche hardware per migliorare la robustezza del sistema in condizioni di scongelamento, tra cui l'applicazione di un rivestimento protettivo (conformal coating) sulla scheda analogica, e modifiche firmware per implementare una scansione multifrequenza a dodici punti spettrali. La caratterizzazione sperimentale è stata condotta su campioni di ghiaccio vetroso (clear ice) e brina di ghiaccio (rime ice), sintetizzati in laboratorio in condizioni controllate, su un intervallo termico compreso tra −30 °C e +25 °C. Nel corso delle prove sperimentali è emerso un fenomeno di disturbo inizialmente inatteso: alle frequenze inferiori al kHz, le misure capacitive risultavano sistematicamente alterate da un contributo parassita riconducibile alla polarizzazione interfacciale di Maxwell-Wagner, indotta dall'umidità adsorbita sulla superficie del substrato del sensore. L'indagine sperimentale condotta per identificare e caratterizzare questo fenomeno ha permesso di stabilire con precisione la soglia di frequenza al di sopra della quale il sistema risulta immune al disturbo ambientale, fornendo al contempo una validazione quantitativa della coppia di frequenze operative di 1 kHz e 100 kHz, individuata come ottimale per massimizzare il contrasto dielettrico tra gli stati di interesse mantenendo la robustezza della misura. I risultati ottenuti dimostrano che il sistema è in grado di classificare in modo affidabile e riproducibile gli stati di aria, acqua liquida e ghiaccio sull'intero intervallo termico considerato, ponendo le basi per un'implementazione a basso costo e scalabile su superfici aerodinamiche estese.
Ottimizzazione di un sensore capacitivo a doppia frequenza per la rilevazione di ghiaccio
Invernizzi, Giacomo
2025/2026
Abstract
The timely detection of ice formation on exposed surfaces represents a critical problem in both the wind energy sector, where blade accretion reduces aerodynamic efficiency and may cause unbalanced rotor loads, and in aviation, where alterations of the wing profile directly compromise flight safety. Conventional detection systems, based on optical, thermal, or ultrasonic technologies, present significant limitations in terms of power consumption, sensitivity to dirt, and vibrational noise. Planar capacitive sensors represent an attractive alternative due to their constructive simplicity and low cost, but when interrogated at a single frequency they generate a measurement ambiguity between liquid water and ice, since both media exhibit high and comparable relative permittivity values at low frequencies. This thesis proposes a single-element planar capacitive measurement system, interrogated at two distinct frequencies, capable of discriminating the physical state of the medium covering the sensor by exploiting the frequency dependence of the dielectric constant described by the Debye relaxation model. While liquid water maintains a high permittivity across the entire interrogation spectrum employed, ice exhibits a marked collapse of its dielectric constant as frequency increases, due to the freezing of the mobility of the lattice defects responsible for orientational polarization. The comparison between the capacitances measured at a low and a high frequency thus enables reliable discrimination between air, liquid water, and ice, without resorting to shielded reference capacitors. The experimental work involved the adaptation and optimization of a pre-existing measurement system, based on an interdigitated electrode sensor and a conditioning electronic stage connected to an STM32 microcontroller. Hardware modifications were introduced to improve the robustness of the system under thawing conditions, including the application of a protective conformal coating on the analog board, together with firmware modifications implementing a twelve-point multi-frequency sweep. The experimental characterization was carried out on samples of clear ice and rime ice, synthesized in laboratory under controlled conditions, over a thermal range between −30 °C and +25 °C. During the experimental tests, an initially unexpected disturbance phenomenon emerged: at frequencies below 1 kHz, capacitive measurements were systematically distorted by a parasitic contribution attributable to Maxwell-Wagner interfacial polarization, induced by humidity adsorbed on the sensor substrate surface. The experimental investigation conducted to identify and characterize this phenomenon allowed precise determination of the frequency threshold above which the system is immune to environmental disturbance, while simultaneously providing a quantitative validation of the 1 kHz / 100 kHz operating frequency pair, identified as optimal for maximizing the dielectric contrast between the states of interest while preserving measurement robustness. The results obtained demonstrate that the system is able to reliably and reproducibly classify the states of air, liquid water, and ice across the entire thermal range considered, laying the groundwork for a low-cost, scalable implementation over extended aerodynamic surfaces.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/260759