Boylamsal uçuş kontrolünde frekans alanı tabanlı döngü şekillendirme
Frequency-domain loop shaping for longitudinal flight control
- Tez No: 1024237
- Danışmanlar: DR. ÖĞR. ÜYESİ ABDURRAHMAN YILMAZ
- Tez Türü: Yüksek Lisans
- Konular: Havacılık ve Uzay Mühendisliği, Bilgisayar Mühendisliği Bilimleri-Bilgisayar ve Kontrol, Aeronautical Engineering, Computer Engineering and Computer Science and Control
- Anahtar Kelimeler: Frekans analizi, Kontrol mühendisliği, Loop shaping tekniği, Uçuş denetim sistemleri, Frequency analysis, Control engineering, Loop shapping technique, Flight control systems
- Yıl: 2026
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Lisansüstü Eğitim Enstitüsü
- Ana Bilim Dalı: Kontrol ve Otomasyon Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Kontrol ve Otomasyon Mühendisliği Bilim Dalı
- Sayfa Sayısı: Belirtilmemiş.
Özet
Bu tez çalışmasında, F-16 savaş uçağının boylamsal uçuş kontrolü için frekans alanına dayalı döngü şekillendirme yaklaşımı ile düşük mertebeli bir kontrolör tasarımı gerçekleştirilmiştir. Modern savaş uçaklarında yüksek manevra kabiliyeti ve performans gereksinimleri, uçuş kontrol sistemlerinin hem kararlı hem de gürbüz olacak şekilde tasarlanmasını zorunlu kılmaktadır. Özellikle eyleyici dinamikleri ve zaman gecikmeleri gibi pratik etkiler, kontrol sisteminin frekans alanı davranışını bozarak istenilen performansın elde edilmesini zorlaştırmaktadır. Çalışma kapsamında öncelikle F-16 hava aracının doğrusal olmayan modeli doğrusallaştırılarak durum uzayı formunda ifade edilmiştir. Boylamsal dinamikler, hız, hücum açısı, yunuslama açısal hızı ve yunuslama açısından oluşan indirgenmiş bir model ile temsil edilmiştir. Kontrol tasarım sürecinde kısa periyot dinamiklerinin stabilizasyonuna odaklanan bir Stabilite Artırma Sistemi (SAS) ile yunuslama açısı takibini gerçekleştiren PI tabanlı bir kontrol yapısı oluşturulmuştur. Elde edilen kontrol yapısının zaman ve frekans alanındaki performansı incelendiğinde, referans takibinin sağlanmasına rağmen eyleyici dinamikleri ve zaman gecikmelerinin açık çevrim karakteristiği üzerinde önemli etkiler oluşturduğu gözlemlenmiştir. Özellikle kesişim bölgesinde genlik çökmesi ve eğim bozulmaları meydana gelmiş, bu durum duyarlılık fonksiyonunda artışa ve sistem gürbüzlüğünde azalmaya neden olmuştur. Yapılan frekans alanı analizleri sonucunda, mevcut kontrol yapısının açık çevrim karakteristiğinin iyileştirilebileceği ve daha uygun gürbüzlük özelliklerinin elde edilebileceği belirlenmiştir. Elde edilen sonuçlar, önerilen yöntemin kesişim bölgesinde düzgün bir eğim sağladığını, duyarlılık tepe değerini azalttığını ve faz payını iyileştirdiğini göstermektedir. Zaman alanı analizlerinde ise klasik yönteme kıyasla daha düşük aşım ve daha dengeli bir geçici rejim davranışı elde edilmiştir. Ayrıca tasarlanan kontrolör, doğrusal olmayan model üzerinde yeniden ayar yapılmaksızın test edilmiş ve performansın korunduğu gözlemlenmiştir. Sonuç olarak, bu çalışma frekans alanında açık çevrim karakteristiğini doğrudan hedefleyen ve optimizasyon ile desteklenen bir döngü şekillendirme yaklaşımının, düşük mertebeli uçuş kontrol sistemlerinde gürbüzlük ve performans arasında dengeli bir çözüm sunduğunu göstermektedir.
Özet (Çeviri)
Flight control system design for high-performance fighter aircraft requires a careful balance between stability, yunuslama açısı performance, disturbance rejection, actuator limitations, and robustness. Modern fighter aircraft operate over a wide flight envelope and are expected to maintain satisfactory handling qualities under varying operating conditions. In addition, the control system must preserve stability and acceptable performance in the presence of practical implementation effects such as actuator dynamics, sensor dynamics, measurement delays, transport delays, and model uncertainties. These requirements make flight control system design a challenging problem, especially for aircraft with highly coupled and nonlinear dynamics. Classical flight control architectures are still widely used in aerospace applications due to their structural simplicity, interpretability, and ease of implementation. In particular, proportional-integral-derivative based controllers and stability augmentation systems are commonly preferred in practical flight control applications. These controllers are generally low-order and can be implemented with limited computational burden. However, classical tuning procedures are often based primarily on time-domain performance measures such as rise time, settling time, overshoot, and steady-state error. Although these criteria are important, they do not directly guarantee desirable frequency-domain robustness properties. A controller that provides an acceptable step response under simplified assumptions may exhibit poor robustness once actuator dynamics and time delays are introduced. Frequency-domain control design provides a more direct way to evaluate and shape robustness properties. In particular, the open-loop transfer function determines the behavior of the sensitivity and complementary sensitivity functions. Therefore, shaping the open-loop response around the crossover frequency is a key step in achieving a desired balance between disturbance rejection, noise attenuation, and robustness margins. A smooth crossover region with an appropriate slope is generally desired in order to avoid excessive sensitivity peaking and to preserve adequate phase margin. However, practical implementation effects may distort the intended loop shape. Actuator dynamics and transport delays introduce additional phase lag, especially near the crossover region, which may lead to magnitude depression, slope distortion, reduced phase margin, and increased sensitivity peak. As a result, the actual implemented loop may deviate significantly from the ideal loop shape considered during the design stage. The main motivation of this thesis is to address this practical gap by developing an optimization-aided frequency-domain loop-shaping approach for the longitudinal control of an F-16 aircraft. The objective is not to introduce a completely new control theory, but rather to propose a systematic refinement framework that improves the practical consistency of a low-order control architecture under actuator and delay effects. The proposed method aims to preserve the advantages of classical low-order control structures while explicitly shaping the open-loop frequency response to improve robustness and closed-loop performance. In this study, the F-16 aircraft is considered as the main application platform. The nonlinear aircraft model is first trimmed around a steady flight condition and then linearized to obtain a state-space model suitable for control design. Since the focus of the thesis is longitudinal control, the longitudinal dynamics are extracted from the full aircraft model. The main longitudinal states considered in the reduced model are forward velocity, angle of attack, pitch rate, and pitch angle. This reduced representation captures the essential longitudinal motion required for pitch-axis control while keeping the controller design process manageable The initial design stage investigates modern state-feedback methods. A full-state Linear Quadratic Regulator controller is first designed using the four-state longitudinal model. This approach provides a systematic method for selecting feedback gains and shaping the closed-loop poles. Although the LQR design achieves closed-loop stability, satisfactory pitch-angle yunuslama açısı cannot be obtained. The main reason is associated with the limited low-frequency authority of the elevator over pitch angle. In other words, directly regulating all longitudinal states through full-state feedback does not automatically produce the desired pitch-yunuslama açısı behavior. To improve yunuslama açısı performance, additional structures such as integral augmentation and reference scaling are investigated. However, these methods either produce excessively slow responses or increase transient amplification. Therefore, the full-state LQR-based pitch yunuslama açısı approach is not selected as the final control structure. Considering the natural modal structure of aircraft longitudinal dynamics, the design focus is shifted toward the short-period dynamics. The short-period mode is mainly characterized by angle of attack and pitch rate and is dominant in the faster longitudinal response of the aircraft. Therefore, a reduced-order model consisting of angle of attack and pitch rate is used for stability augmentation design. An LQR-based feedback law is obtained for these dominant short-period states. The resulting stability augmentation system improves the damping of the short-period dynamics and provides a more suitable basis for pitch-angle yunuslama açısı. After the reduced-order gains are obtained, they are embedded back into the full-order longitudinal model so that the controller is evaluated in the complete system environment rather than only in the reduced design model. After short-period stabilization, pitch-angle is introduced using a proportional-integral controller. The PI controller processes the pitch-angle yunuslama açısı error and generates an elevator command contribution. This command is combined with the stability augmentation feedback based on angle of attack and pitch rate. Thus, the baseline controller consists of a low-order PI plus SAS architecture. This structure is practical, interpretable, and compatible with conventional flight control design philosophy. Under simplified linear conditions, the baseline controller provides acceptable pitch angle behavior. However, when actuator dynamics and transport delays are included, the open-loop frequency response of the system deteriorates significantly. The frequency-domain analysis of the baseline PI plus SAS controller shows that actuator and delay effects distort the intended open-loop shape near the crossover frequency. In particular, the loop response exhibits a magnitude depression and an irregular slope around the crossover region. This behavior is undesirable because the crossover region strongly influences phase margin, sensitivity peak, and closed-loop robustness. The distortion indicates that the controller tuned in a simplified setting does not preserve the intended frequency-domain behavior when realistic implementation effects are considered. This observation forms the basis of the proposed loop-shaping refinement. To overcome this problem, an ideal open-loop frequency template is defined. This template represents the desired engineering characteristics of the loop response. It includes moderate low-frequency gain for reference yunuslama açısı and disturbance rejection, an approximately smooth crossover behavior, and bounded high-frequency magnitude to avoid excessive noise amplification. The actual open-loop response of the implemented controller is then compared with this ideal template. Instead of relying only on classical gain retuning, the deviation between the actual and ideal loop shapes is treated as a design objective. A notch-type shaping filter is introduced into the control architecture in order to compensate for local frequency-domain distortions. The filter is selected because the dominant distortion appears around a limited frequency region near the crossover. By adjusting the filter parameters, the open-loop magnitude can be reshaped without unnecessarily increasing controller order. However, manually tuning the filter parameters may be inefficient and may not provide a repeatable design process. Therefore, an optimization-aided loop-shaping procedure is developed. The optimization problem is formulated in the frequency domain. A selected frequency grid is defined around the shaping bandwidth, with emphasis on the crossover region and its neighborhood. The cost function is constructed as the squared deviation between the actual open-loop magnitude response and the ideal open-loop magnitude template. The optimization algorithm adjusts the selected controller and filter parameters to minimize this deviation. In this way, the actual loop response is systematically aligned with the desired loop shape. The proposed approach directly penalizes the mismatch in the open-loop frequency response and therefore provides a more transparent connection between design objectives and robustness characteristics. The refined controller is then evaluated using frequency-domain and time-domain analyses. In the frequency domain, the proposed loop-shaped controller provides a smoother crossover region compared to the baseline PI plus SAS design. The magnitude depression observed in the baseline case is reduced, and the open-loop slope becomes more consistent with the ideal template. As a result, the sensitivity peak is reduced and the phase margin is improved. These results indicate that the proposed refinement improves robustness without requiring a high-order controller or aggressive bandwidth increase. Time-domain simulations are also performed to examine the effect of the proposed method on transient performance. The loop-shaped controller is compared with the classical PI plus SAS controller under the same operating conditions. The results show that the classical controller may provide slightly faster settling in some cases; however, it also produces higher overshoot and less favorable robustness characteristics. The proposed loop-shaped controller provides a more balanced transient response with reduced overshoot and improved frequency-domain robustness. This result is consistent with the main design objective of the thesis, which is not to maximize speed alone but to obtain a balanced robustness-performance trade-off. The final controller is also tested on the nonlinear F-16 model without retuning. This step is important because a controller designed only on a linear model may not preserve its performance when implemented on the nonlinear aircraft dynamics. The nonlinear validation is performed around the selected trim condition using reference yunuslama açısı and disturbance rejection scenarios. Different delay values are considered to assess the sensitivity of the controller to implementation uncertainty. The results indicate that the proposed loop-shaped controller maintains consistent performance in the nonlinear simulations. In disturbance rejection cases, the loop-shaped design provides lower pitch-angle error levels and more robust behavior compared to the baseline design. These findings support the practical relevance of explicitly considering actuator and delay effects during the frequency-domain design stage. Overall, this thesis demonstrates that actuator dynamics and transport delays can significantly alter the open-loop characteristics of low-order flight controllers. If these effects are not explicitly considered, the controller may lose the robustness properties expected from the initial design. The proposed optimization-aided loop-shaping method provides a systematic way to correct these implementation-induced distortions. By aligning the actual open-loop response with an ideal template, the method improves sensitivity behavior, robustness margins, and transient response quality. The main contribution of this thesis is the development of a practical frequency-domain refinement framework for a low-order F-16 longitudinal flight controller. The proposed approach combines reduced-order short-period stabilization, PI-based pitch tracking, notch-filter-based loop shaping, and frequency-domain optimization. The resulting controller preserves the simplicity and implementability of classical flight control structures while improving robustness under actuator and delay effects. Therefore, the proposed methodology can be considered a useful design framework for low-order flight control systems where both practical implementation constraints and frequency-domain robustness requirements must be taken into account.
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