Soğuk gaz itki sistemlerinde tribolojik iyileştirme ile hassas itki kontrolu
Fine thrust control in cold gas propulsion systems through tribological improvements
- Tez No: 1019322
- Danışmanlar: PROF. DR. İLKER MURAT KOÇ, DR. BÜLENT SÜMER
- Tez Türü: Doktora
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2026
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Lisansüstü Eğitim Enstitüsü
- Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Makine Mühendisliği Bilim Dalı
- Sayfa Sayısı: Belirtilmemiş.
Özet
Uzay araçlarında yönelim kontrolü, hassas yörünge düzeltmeleri ve otonom manevraların gerçekleştirilebilmesi, düşük seviyeli ve yüksek doğruluklu itki üretimine olan ihtiyacı artırmaktadır. Özellikle küçük uydu ve mikro uydu platformlarının yaygınlaşmasıyla birlikte, operasyonel güvenilirliği yüksek, yapısal olarak sade ve görev güvenliği açısından avantajlı tahrik sistemleri ön plana çıkmıştır. Bu bağlamda soğuk gaz itki sistemleri, kimyasal reaksiyon içermeyen çalışma prensipleri, düşük operasyonel riskleri ve sistem yapısındaki sadelik nedeniyle uzay görevlerinde yaygın olarak kullanılan tahrik çözümlerinden biri haline gelmiştir. Bununla birlikte, bu sistemlerde itkinin hassas biçimde ayarlanabilmesi, gaz akışının geniş bir çalışma aralığında yüksek doğrulukla kontrol edilmesini gerektirmektedir. Bu kontrol görevi çoğunlukla oransal valf aracılığıyla gerçekleştirilmekte olup, valf içerisindeki temas yüzeylerinde ortaya çıkan tribolojik etkiler sistem performansını doğrudan belirleyen temel unsurlar arasında yer almaktadır. Yüksek basınç altında çalışan oransal iğne valf yapılarında konik iğne ucu ile valf yuvası arasındaki temas bölgesinde oluşan yüksek normal yükler, sürtünme kuvvetlerinin belirgin biçimde artmasına ve hareket başlangıcında statik sürtünmenin hâkim olduğu bir kalkış eşiğinin ortaya çıkmasına neden olmaktadır. Kalkış eşiği (breakaway) olarak tanımlanan bu davranış, özellikle düşük hız ve küçük açıklık rejimlerinde yapışma--kayma (stick--slip) karakterli hareketlere yol açarak valf konumlandırma doğruluğunu sınırlamakta ve itki modülasyonunda düzensizliklere neden olabilmektedir. Bu nedenle yüksek basınçlı oransal valf sistemlerinde tribolojik temas davranışının anlaşılması ve sürtünme kaynaklı doğrusal olmayanlıkların kontrol performansı üzerindeki etkilerinin ortaya konması, hassas itki sistemi tasarımında belirleyici bir mühendislik problemi oluşturmaktadır. Bu tez çalışmasında, yüksek basınçlı soğuk gaz itki sistemlerinde kullanılan oransal iğne valflerin tribolojik davranışı ve bu davranışın sistem kontrol performansı üzerindeki etkileri deneysel ve analitik yöntemlerle incelenmiştir. Çalışmanın ilk aşamasında, konik temas geometrisine sahip iğne ucu--yuva çiftlerinde farklı malzeme kombinasyonlarının sürtünme davranışı deneysel olarak değerlendirilmiş; açma--kapama torku, kalkış torku, tork dalgalanmaları ve çevrimsel aşınma eğilimleri karşılaştırmalı olarak analiz edilmiştir. Bu analizler sonucunda malzeme çiftinin yalnızca ortalama sürtünme seviyesini değil, aynı zamanda kalkış eşiğinin büyüklüğünü, tork kararlılığını ve eyleyici yükünü belirleyen kritik bir tasarım parametresi olduğu gösterilmiştir. Tezin ikinci aşamasında, valflerde gözlenen sürtünme davranışı dinamik sürtünme modelleme yaklaşımları kapsamında ele alınmış ve LuGre sürtünme modeli kullanılarak temsil edilmiştir. Model parametreleri deneysel ölçümlerden elde edilen zaman serileri kullanılarak tanımlanmış ve modelin özellikle düşük hız rejiminde gözlenen yapışma--kayma davranışını temsil etme kabiliyeti değerlendirilmiştir. Bu model, tribolojik davranışın kontrol hesabına eklenmesine olanak vermektedir. Çalışmanın son aşamasında ise sürtünme kaynaklı doğrusal olmayanlıkların valf kontrol performansı üzerindeki etkilerini azaltmak amacıyla farklı aktif kontrol yöntemleri uygulanmış ve deneysel olarak karşılaştırılmıştır. Bu kapsamda klasik PID tabanlı kontrol yaklaşımı, mikro salınım (dither) tabanlı sürtünme telafisi ve LuGre modeline dayalı ileri beslemeli kontrol yöntemi değerlendirilmiştir. Regülatörsüz basınç düşümlü besleme koşullarında gerçekleştirilen deneyler sonucunda mikro salınım tabanlı yaklaşımın statik sürtünme bariyerini azaltarak kalkış davranışını iyileştirdiği ve izleme doğruluğunu artırdığı; model temelli ileri besleme yaklaşımının ise belirli çalışma rejimlerinde sürtünme kaynaklı enerji kayıplarını azaltarak kontrol performansına katkı sağlayabildiği gösterilmiştir. Elde edilen bulgular, yüksek basınçlı soğuk gaz itki sistemlerinde valf tasarımının yalnızca akış performansı açısından değil, aynı zamanda tribolojik temas davranışı ve kontrol teknolojisi ile birlikte ele alınması gerektiğini ortaya koymaktadır. Bu tez kapsamında geliştirilen deneysel yöntemler, sürtünme modelleme yaklaşımı ve kontrol teknolojileri, hassas itki modülasyonu gerektiren uzay uygulamalarında kullanılabilecek oransal valf sistemlerinin tasarımı için temas, sürtünme ve kontrol değişkenlerini aynı deney hattında birleştiren tasarım yaklaşımı sunmaktadır.
Özet (Çeviri)
Attitude control, small orbital corrections, precision pointing, and autonomous maneuvering requirements in space missions increase the need for propulsion systems capable of producing low, repeatable, and accurately adjustable thrust. With the increasing use of small satellites and CubeSat-class platforms, the expected performance of propulsion systems is no longer limited to maximum thrust capability. Low mass, mechanical simplicity, operational safety, low power consumption, and long-term reliability have become equally important design requirements. In this context, cold-gas propulsion systems remain an attractive solution for small spacecraft because they operate without chemical reaction, have relatively simple system structures, and provide comparatively safe operation. In a cold-gas propulsion system, thrust is generated by releasing high-pressure gas through a control valve and directing it toward a nozzle. The valve is therefore not a secondary component that merely opens or closes the flow path. It determines how the pressurized gas is released, how the system responds to command inputs, and how repeatable the low-thrust output can be. Conventional solenoid valves are practical for pulsed operation and fast switching. Nevertheless, for high-pressure systems requiring fine and continuous thrust adjustment, they may become limited by their on-off operating nature, flow restriction, actuation force requirements, and power consumption. These limitations motivate the use of proportional valves, where the opening can be continuously adjusted over a specified operating range. In a proportional needle valve, the axial position of a conical needle modifies the effective flow area. This allows the gas passage to be regulated continuously rather than only through discrete on-off states. However, the use of such valves under high pressure introduces a critical tribological limitation. In the closed position, the conical needle tip must be seated firmly against the valve seat to ensure sealing. This requirement is beneficial for leakage prevention, but it produces high normal load, elevated contact pressure, and a static-friction-dominated breakaway threshold at the needle-seat interface. When the valve is commanded to open, the actuator must first overcome this static adhesion barrier. Once this threshold is exceeded, the friction regime may change abruptly, and stick-slip motion can occur in the low-velocity regime. The main objective of this thesis is to investigate the tribological behavior of proportional conical needle valves used in high-pressure cold-gas propulsion systems and to determine how this behavior affects opening and closing torque, actuator effort, position tracking accuracy, and overall system performance. The study further aims to reduce these effects through a combined passive and active engineering approach. The passive part focuses on needle-tip material selection and contact behavior, whereas the active part focuses on friction-aware control strategies. The thesis does not treat valve geometry, material selection, friction modeling, and control design as isolated problems. Instead, it combines contact mechanics, material-pair testing, surface degradation analysis, dynamic friction modeling, and control evaluation on the same experimental basis. In the first stage of the study, the mechanical basis of the conical needle tip-seat contact was investigated. Hertzian contact theory was used to estimate the contact radius, equivalent elastic modulus, and maximum contact pressure for different material combinations. These analytical calculations were supported by finite element analyses to examine stress distribution and elastic deformation in the contact zone. The results clarified the fundamental trade-off between sealing and actuation torque. Metallic contacts with high elastic modulus produce smaller contact areas and higher local contact pressures. Polymer-tipped configurations, on the other hand, produce larger contact areas and more compliant contact behavior under the same loading condition. This contact compliance affects not only sealing behavior but also the static friction threshold and the torque required to initiate valve motion. In the second stage, the tribological response of different needle-tip materials paired with a steel valve seat was experimentally compared while preserving the same valve structure. Steel-steel, aluminum-steel, POM-steel, and PA66-steel material pairs were evaluated in terms of opening torque, closing torque, torque fluctuation, motor current, and surface roughness change after repeated operation. The experiments were conducted on a high-pressure cold-gas thrust test setup. Torque, pressure, thrust force, temperature, and servo telemetry were recorded synchronously to allow consistent interpretation of mechanical and tribological quantities. The material pairs were tested under different pressure conditions for short-cycle torque characterization. In addition, a 1000-cycle durability and wear test was performed to quantify the evolution of surface topography. The experimental results showed that the needle-seat material pair is a primary design variable for proportional valve performance. The steel-steel pair produced the highest opening torque due to high stiffness and elevated contact pressure. It also showed larger torque fluctuations, which is disadvantageous for precision control. The aluminum-steel pair reduced torque relative to steel-steel, but its cyclic stability and surface degradation behavior were less favorable. POM-steel and PA66-steel pairs produced lower torque levels and a more manageable torque envelope compared with metallic contacts. In particular, the POM-steel configuration provided a favorable balance between low actuation torque and surface stability. These findings indicate that the benefit of polymer tips cannot be explained solely by a lower friction coefficient. Their lower modulus and higher contact compliance modify the stress distribution in the contact band and shift the breakaway process toward a lower-energy transition. However, material selection alone is not a complete solution. The contact load required for sealing cannot be removed entirely, and a static adhesion effect remains when the valve starts to move from the closed position. Therefore, passive tribological improvement is necessary for reducing torque demand and limiting torque fluctuation, but it is not sufficient to fully suppress stick-slip behavior in the low-velocity regime. For this reason, the passive material-based approach was complemented with active control strategies in this thesis. In the third stage, the friction behavior of the valve mechanism was represented using the LuGre dynamic friction model. Simple friction descriptions based only on Coulomb, viscous, and static friction terms are not sufficient to reproduce pre-sliding hysteresis, the Stribeck transition, and stick-slip behavior near zero velocity. The LuGre model was therefore selected because it represents the elastic deformation of microscopic surface asperities through an internal state variable. This model allows static friction, dynamic friction, pre-sliding behavior, and velocity-dependent friction transition to be expressed within a single dynamic framework. The LuGre model parameters were identified using a combination of constant-speed tests and small-amplitude dynamic excitation tests. The constant-speed experiments were used to estimate the Coulomb friction level, static friction level, Stribeck velocity, and viscous component. The dynamic tests were used to identify pre-sliding stiffness, micro-damping, and equivalent inertia. This hybrid identification procedure enabled the low-speed, direction-dependent, and breakaway-related friction behavior of the valve to be represented within a unified dynamic modeling framework. Thus, friction was treated not only as an experimentally observed limitation, but also as a dynamic effect that could be incorporated into controller design. In the final stage of the study, several control strategies were experimentally compared to reduce the influence of friction-induced nonlinearities on valve position control. The tested strategies included open-loop DC motor operation as a reference drive, classical PID control, PID with micro-oscillation input, and PID with LuGre-based feedforward compensation. All strategies were evaluated under the same open-hold-close task profile. The experiments were conducted under unregulated blow-down supply conditions, where the line pressure decreases during the operating cycle. This condition is important because the load acting on the valve evolves throughout the experiment. The performance assessment was not reduced to a single criterion. Instead, breakaway current, breakaway torque, integral absolute error, RMS tracking error, velocity power spectral density, and current-position hysteresis were evaluated together. The results showed that classical PID control is a necessary baseline but not a sufficient solution for this class of tribological control problem. Since PID does not predict or physically weaken the static friction barrier, it tends to accumulate control effort during the stuck phase. This accumulated effort can lead to higher torque peaks when the breakaway threshold is finally exceeded. The PID with micro-oscillation strategy reduced the effective static adhesion by introducing a small periodic excitation during the initial opening phase. This approach made the breakaway transition more manageable and produced strong improvements in both breakaway behavior and tracking accuracy. Compared with PID, it reduced the breakaway current and breakaway torque and improved IAE and RMS tracking errors. The LuGre-based feedforward strategy aimed to reduce friction-induced resistance by adding a model-based compensation term to the control signal. When the model remained representative of the actual friction behavior, this approach contributed positively to control performance. However, under unregulated blow-down conditions, the contact and load state of the valve changes over time. Therefore, a fixed-parameter friction model cannot be expected to provide the same level of accuracy in all operating regions. This finding indicates that model-based feedforward compensation has significant potential, but its robustness depends on whether the model parameters can remain valid under changing pressure, temperature, wear, and cyclic operating conditions. The findings of this thesis show that the design of proportional needle valves for high-pressure cold-gas propulsion systems cannot be evaluated only in terms of geometric opening or actuator capacity. The sealing load, contact material pair, surface topography, breakaway torque, stick-slip tendency, and control strategy are strongly coupled. For reliable thrust modulation, passive tribological improvement and active friction compensation must therefore be considered together. The experimental procedures, friction modeling framework, and multi-criteria control comparison developed in this thesis provide an integrated engineering basis for designing and operating proportional valve systems intended for precise control under high-pressure operating conditions. In conclusion, this thesis demonstrates that precision valve control in high-pressure cold-gas propulsion systems is not merely a control problem. It is a multidisciplinary mechanical engineering problem involving contact mechanics, tribology, material selection, friction modeling, and control technology. The results show that material-control interaction must be placed at the center of proportional needle valve design when low-torque, repeatable, and precise valve motion is required.
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