Investigation on Physical Meaning of Three-loop Autopilot
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ISSN:1598-6446 eISSN:2005-4092 http://www.springer.com/12555
Investigation on Physical Meaning of Three-loop Autopilot Chang-Hun Lee*, Shaoming He, and Ju-Hyeon Hong Abstract: This paper aims to reveal the hidden physical meaning or the working principle of the three-loop autopilot. First, the minimal control structure that ensures the desired dynamic characteristics is analyzed. The results show that the three-loop autopilot has a redundant feedback loop from the minimal control structure standpoint. Based on this observation, its physical meaning is interpreted by utilizing the closed-loop tracking error dynamics in the presence of aerodynamic uncertainties. It turns out that the three-loop autopilot has the minimal control structure with an additional command which is based on the instantaneous direct model reference adaptive control. A clear mechanism of how the aerodynamic uncertainties are compensated is provided in the three-loop autopilot. Finally, numerical simulations are performed to validate our findings. The analysis results in this paper show the beauty of the three-loop autopilot: it can be considered as the most concise form to achieve desired dynamic characteristics and counteract model uncertainties. The potential importance of the results obtained is that they allow us to properly design the three-loop autopilot by reflecting its physical meaning and have potential directions to improve the performance of the three-loop autopilot as well as the adaptive control. Keywords: Adaptive control, missile autopilot, physical meaning, three-loop autopilot.
1.
INTRODUCTION
The classical three-loop autopilot [1] has been extensively employed for acceleration controls of various missiles: homing missiles [2], tactical missiles [3], spinning missiles [4], bank-to-turn missiles [5], and agile missiles [6]. The key features of the classical three-loop autopilot that led to these successes are its simplicity, practicality, and effectiveness in the implementation. It has a simple structure as two feedback loops of pitch rate and one feedback loop of acceleration. With this architecture, the classical three-loop autopilot is realized using the linear control theory [7] with an appropriate gain-scheduling method. In recent years, there have been extensive studies on the three-loop autopilot in various directions to maximize its capabilities. The first attempt is to apply the advanced control methodologies to the three-loop autopilot, such as feedback linearization control [3, 8–10], optimal control [5, 11], and asymptotic output tracking control [12]. Another effort is to combine the three-loop autopilot and adaptive control in order to improve the robustness against aerodynamic uncertainties [9, 13]. Also, there have been some studies on improving the gain-
scheduling method [14] or gain design method [15] to satisfy a given open-loop crossover frequency constraint. Some researches have been conducted to improve the performance by modifying the three-loop autopilot structure [11, 16, 17]. Additionally, some e
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