Research On Orbital And Attitude Control For Proximity Motion Of Small Body Spacecraft

Small celestial bodies, in addition to the planets and moons, refer to the millions ofasteroids and comets which have a mean diameter of less than100km. So there is anincreasing interest in small body exploration, which is of great significance in theory andpractical value in engineering. The dynamics environment nearby is very complicated dueto their remoteness, small size, irregular shape, gravity properties of small body.Spacecraft proximity operations near small bodies require G&C (Guidance and Control)methods that are capable of handling significant sources of uncertainty in gravity profiles,terrain geometry, and other relevant space disturbing force. Spacecraft planetary and lunaroperations typically have better-known parameter profiles due to long-term observation ofthese bodies, so ground-based G&C schemes are the accepted norm for proximityoperations. The ability to utilize ground-based G&C is less with small-body missionswhich have long light-time delays and there are many problems need to be resolved, suchas how to describe irregular gravitational field, orbital control for proximity motion ofspacecraft, attitude control for proximity motion of spacecraft, and integrated translationand rotation control. Space disturbance and uncertainty make the dynamics and G&Cmore complex, so robust and adaptive control strategies become the key technology tomake spacecraft soft landing on the surface of small body successfully. From the currentsituation of research at home and abroad for the system with small body spacecraftproximity motion, many researches study the system analysis and orbital control designbased on the relative motion model, however, the research on spacecraft attitude control,integrated translation and rotation control, and the robustness and adaptability is ofimportance and significance.With the support of National Basic Research Program of China (973Program)‘research on navigation, guidance and control for spacecraft precise landing on the surfaceof the planet’, this paper is concerned with control for proximity motion of small bodyspacecraft. The research mainly covers the orbital control scheme, attitude control scheme,integrated translation and rotation control scheme based on disturbance during spacecraftorbiting, falling and landing on the surface of small body. The main content andinnovation are as follows.1. The research background and significance have been presented first, thenoverview the research status of proximity motion of small body spacecraft, as well as thekey problems of the research.2. Orbital dynamic model in the body-fixed coordinate system while spacecraftfalling down to small body is deduced based on Newton’ law and relative differentialprinciple. Then orbital dynamic model in the landing-site coordinate system whilespacecraft soft landing on the surface of small body is formulated based on coordinatetransformation. According to compound movement and Newton-Euler algorithm, the attitude dynamics of a rigid spacecraft near small body are deduced. Finally, due to thedifferent actuator layout, two kinds of coupled relative attitude and orbit dynamics ofspacecraft are formulated for proximity missions.3. Firstly, the desired descent trajectories are planned as three power polynomialforms on the basis of Apollo mission in order to decrease the fuel consumption. Then, theterminal sliding mode control scheme with compensation term is proposed usingLyapunov method based on the trajectory tracking control theory consideringthe uncertainties and exterior disturbances. The unknown upper bounds of disturbanceare estimated through adaptive scheme, and the spacecraft can reach to the desiredtrajectory in the finite time robustly. However, some difficulties such as control parametertuning, practical engineering applications and singularity appear. Hereby, robust trackingcontrol scheme is proposed based on dynamic surface and back-stepping ideas, so theposition and velocity of spacecraft can reach the desired polynomial trajectories and arriveat the preset site under the condition of uncertainty and external disturbance.4. The small body shape and physical parameters will be observed and determinedwhile spacecraft orbiting small bodies, but the effects of uncertainty and disturbance maydeteriorate the attitude performance significantly, which leading to unstable attitudemotion and thereby failure of the space mission. Attitude dynamics and control of aspacecraft orbiting a small body are necessary. Three-dimensional attitude motion of thespacecraft is examined based on a simplified attitude model. Additionally, a detailed studyis undertaken to examine the effects of various system parameters on the system response.Finally, the robust adaptive attitude control laws are developed to stabilize the system.5. To reduce the chattering phenomenon in the traditional sliding mode control andadjust the attitude of spacecraft down to small body, a dynamic sliding mode controlscheme is proposed. Firstly, the dynamic sliding mode is defined, while the design stepsand basic ideas are given for the dynamic sliding mode. The second-order dynamic slidingmode controller for the outer control loop and the first-order dynamic sliding modecontrol for the inner control loop are designed. The control scheme could reduce thechattering phenomenon by embedding discontinuous functions into the differential ofcontroller. The upper boundary of the compound disturbance is estimated online via anadaptive law. Thereafter, the estimated value is utilized to design the compensation item.Then, two dynamic sliding mode control schemes based on disturbance observer areproposed to overcome severely external disturbance. A robust term is added intocontroller based on the upper boundary of the observer error by the adaptive law, whichmake the system strong robustness.6. To obtain the accuracy and safety for spacecraft soft landing on the surface ofsmall body, the position and the attitude of spacecraft are often required to simultaneouslyachieve the desired states with high maneuverability and control accuracy. A6DOFrobust adaptive fuzzy control scheme is developed based on back-stepping ideas. Uncertainty and disturbance parts are estimated through fuzzy system, and the optimalapproximation parameters in fuzzy system are updated online by adaptive law. Only onebig thrust engine is configured on the spacecraft for fast orbit maneuver. According to theabove actuator configuration scheme, the robust control strategy is developed based onback-stepping ideas considering outer disturbance. The position and attitude of spacecraftcan reach the desired value, and the system was proved to be stable.Based on the research on small body autonomous guidance and control, robustcontrol and the adaptive control, the conclusion and the perspective of future research aregiven at the end of the paper.