New Advances of Chinese Spacecraft Control Technologies*

19th IFAC Symposium on Automatic Control in Aerospace September 2-6, 2013. Würzburg, Germany

New Advances of Chinese Spacecraft Control Technologies  Duzhou Zhang ∗ ∗

Science and Technology on Space Intelligent Control Laboratory, Beijing Institute of Control Engineering, Beijing 100190, China.

Abstract: As an institute that focuses on spacecraft control, Beijing Institute of Control Engineering (BICE) undertakes most of the control and propulsion system developments for satellites and manned spacecrafts, e.g., the first man-made satellite of China, “Shenzhou” manned spacecraft, and “ChangE” lunar explorers. The paper, incorporating the recent achievements in Beijing Institute of Control Engineering, reviews state of the art on spacecraft control in China, with emphasis on new results on spacecraft control theory for earth satellites, manned spacecraft and deep-space exploration. Finally, we discuss the possible future research directions of spacecraft control in China. Keywords: attitude control; manned spacecraft; deep-space exploration; intelligent control; autonomous control. 1. INTRODUCTION Human being conducts a series of space activities for exploring, developing and using space. In particular, extensive activities and remarkable achievements occur in three major fields, i.e., earth satellite, manned astronautic activity and space exploration [1]. On October 4, 1957, the first man-made satellite of human being was sent by Soviet. On April 12, 1961, Yury Alekseyevich Gagarin in Soviet made a journey in space for 108 minutes. Neil Alden Armstrong, from the United States, landed on lunar surface for the first time on July 20, 1969. The landing on Venus was achieved by Soviet in August, 1970. Up to now, the number of various spacecraft including common satellites, manned spacecraft, space station and deep-space explorers has reached more than 6000. From 1970, when the first Chinese earth satellite was sent, to present, over 130 spacecraft have been sent, making a substantial contribution to the human space development. Spacecraft control is one of the key technologies that ensure the achievement of the given tasks. As an institute that focuses on spacecraft control, Beijing Institute of Control Engineering undertakes most of the control and propulsion system developments for satellites and manned spacecraft, e.g., the first man-made satellite of China, “Shenzhou” manned spacecraft, and “ChangE” lunar explorers. Up to now, it has made six categories of control systems including recoverable three-axis stabilized, geostationary orbit dual spinning, geostationary three-axis stabilized, and Sun synchronous orbit threeaxis stabilized control systems, manned spacecraft, and small satellite control system, and it also made cold gas propulsion system, monopropellant unified propulsion system, and bipropellant unified propulsion system [2]. In addition, our institute has developed attitude sensors such  This work is supported by the National Key Basic Research and Development Program (973) of China under Grant 2013CB733100.

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as star sensors, lunar ultraviolet sensors and CCD optical imaging sensor, on-board controllers (e.g., double-CPU three-modular fault-tolerant computer, RISC computer, SoC slice system), as well as various momentum wheels, magnetic torquers, control moment gyroscopes, solar-panel and antenna drive mechanisms. The technologies grasped and the hardware developed by BICE have made it play a leading role in the field of spacecraft control in China. This paper reviews state of the art of spacecraft control in China from three aspects, i.e., earth satellite control, manned spacecraft control and deep-space explorer control, with emphasis on the recent results on spacecraft control theory and approaches, and concludes the presentation by discussing the possible future research directions. 2. EARTH-ORBIT SATELLITE CONTROL The earth-orbit satellites can be categorized as middleearth-orbit, low-earth-orbit, and high-earth-orbit satellites. The performance of the satellites is reflected in the following aspects: the accuracy of the attitude determination and attitude control, the degree of attitude stabilization, attitude jitter, the ability of the attitude maneuver. Accompanied by the progress of satellite technologies and the expansion of the satellite application field is the higher and higher requirement on the satellite performance. Although there is a great gap in control accuracy between the technological levels of the most advanced countries and that of China, however, our country has conducted effective research on the theory and approaches in high-accuracy attitude determination, high-frequency oscillation measurement, high-accuracy attitude stabilization and attitude maneuvering control. Table 1 shows the comparison between typical high-performance satellites. Details of the abbreviation: ADA—Attitude determination accuracy 10.3182/20130902-5-DE-2040.00064

2013 IFAC ACA September 2-6, 2013. Würzburg, Germany

Table 1. Comparison between typical highperformance satellites satellites

ADA

GEOEYE-1(USA)

0.4”

ACA

DAS

75” 0.007”/s

AJ

AM

-

-

also conducts the ground simulation study. The results demonstrate that the linear accelerator based attitude measurement approach is feasible, which can achieve the attitude measurement resolution 10−6 degree. 2.2 Highly accurate attitude stabilization and maneuver

In order to realize satellite super high accuracy of attitude control, comprehensive analysis of satellite dynamical features is needed, such as changes of structure and parameters caused by multi-body movements and fuel ALOS(Japan) 1.08” 342” 0.684”/5s consumption, electronic performance and parameters variation, frictions and gitter of sensors and actuators. In order to solve the control problem with high order, timeWorldView-2(USA) 3.5 deg/s varying, nonlinear, strong coupling and uncertainty plant emerged under super high accuracy control requirements, Characteristic model based adaptive control theory and ZY-3 (China) 36” 108” 1.8”/s method, which was proposed by Academician Hongxin Wu, is used in theoretical analysis, simulation and experACA—Attitude control accuracy imental demonstration. By using this method a low order control model can be obtained by compressing unmodeled DAS—The degree of attitude stabilization dynamics and high frequency information of satellite dynamics into the characteristic model parameters [11,12]. AJ—Attitude jitter According to the characteristic model, an satellite intelAM—Attitude maneuver ligent adaptive controller is designed by inducing logical derivative control and logical integral control. It is demonHST—Hubble Space Telescope strated by theoretical analysis that the closed-loop system ALOS—Advanced Land Observing Satellite composed of this controller is stable[13,14]. And the high accuracy control quality of the proposed method has been 2.1 Highly precise attitude determination and high-frequency demonstrated by gas bearing simulation experiments. oscillation measurement Some studies on the key technologies of agile maneuver are undergoing. And some results have been achieved. When the accuracy of satellite attitude determination For example, large control momentum gyro has been deis required to be better than the unit—arc second, the veloped, and its singularity avoidance method has been otherwise negligible minor factors that influence the ac- studied. Furthermore, maneuver trajectory optimality, atcuracy of the attitude determination become prominent, titude fast damping and stable tracking control for flexible e.g., system error and low-frequency error of star sensors satellites are also under study. [5, 6]. For this, our institute has performed the study on the development of star sensors with super-high accuracy and super-highly accurate attitude determination. For the 2.3 Constellation Control problem of low-frequency errors of the star sensors, based on the results in [7] [8] [9], relying on the engineering practice, we established the low-frequency error model of For fulfilling the demands of the client, besides the singlethe star sensor and proposed an EKF based low-frequency- satellite control technologies, constellation control techerror correction approach. Our correction approach bears nologies are needed. Up to December, 2012, all satellites of the characteristics such as low amount of computation, the second-generation navigation constellation in the firstintuitively understandable principle, and easy on-orbit re- stage engineering are in orbit, and the navigation system is built. The second-generation constellation includes GEO alization. satellites, IGSO satellites and MEO satellites, and the conThe high-frequency vibration of the satellite attitude is a troller is designed based on the respective attitude/orbit key factor that influences the image quality of the satellite control requirements and the perturbation characteristics for remote sensing of the earth, and therefore, the measure- of the three categories of satellites, which can be states ment of the attitude high-frequency vibration has received as follows: 1) adopting magnetic unloading approach so more and more attention. The usual attitude information that the influence of the jet thrust unloading technique can be measured by many approaches, e.g., gyroscopes, on the orbit be eliminated, 2) adopting effective controller star sensors, and Sun/Earth sensors, yet, their measure- compensation to eliminate the influence of the disturbance ment frequency is relatively low and dynamic range is torques (e.g., solar pressure) to the satellite attitude, 3) limited, which is far from the requirement of the measure- performing active measuring and control for the satellite ment of the high-frequency vibration. The measurement yaw angle and solar-panel rotational angle to guarantee accuracy and frequency range of translational accelerators the power supply of the satellite, and 4) for the whole is acceptable and relatively mature. Therefore, our insti- constellation, designing shape keeping control such that tute examines an approach that combines several linear good GDOP, covering range and overlapping index are accelerators to measure the high-frequency vibration, and maintained. HST(USA)

Milli arcsecs 0.01”

-

0.007”

-

360

2013 IFAC ACA September 2-6, 2013. Würzburg, Germany

3. MANNED SPACECRAFT CONTROL Chinese manned spacecraft mainly includes manned spacecraft, Tiangong 1 target vehicle used for rendezvous and docking experiments and space station technology demonstration, cargo spacecraft and space station which are under development. Manned spacecraft control mainly includes return and reentry control, rendezvous and docking control and space station control. 3.1 Return and Reentry Control of Manned Spacecraft In order to solve the reentry control problem of manned spacecraft, control parameters were designed by using analytical guidance method, matching working tables in US, Soviet Union and other countries. According to the all-coefficient adaptive control theory and method, engineering design essentials and experience, an adaptive control scheme, which estimates lift-drag ratio of the return capsule of manned spacecraft, is designed in China. By using this control scheme, the control accuracies at the time of opening parachute of eight Shenzhou spacecraft are better than requirement, around 10km. Shenzhou spacecraft landed stably, and the landing accuracy reaches the international advanced level [1].

Fig. 1. Manual rendezvous and docking between Shenzhou 9 and Tiangong 1—astronaut operation

3.2 Control of Rendezvous and Docking The guidance, navigation and control (GNC) system design of Shenzhou spacecraft is implemented with the concept that “automatic control is major and manual control is backup”. On 3 November 2011 the first Chinese automatic rendezvous and docking was successfully completed between Shenzhou 8 and Tiangong 1. After that, on 24 June 2012 the first Chinese manual RVD was implemented between Shenzhou 9 and Tiangong 1 by three astronauts on Shenzhou 9 with another great success. The main characters of Shenzhou spacecraft automatic rendezvous and docking are as follows: 1) an automatic control scheme is designed considering both the capability of rendezvous and that of withdrawing from the target for the requirement of safety, 2) The phase-plane adaptive control method based on characteristic model is proposed according to characteristic model based intelligent adaptive control theory, which successfully solved the control problems of systems with large flexible vibration, serious thruster plume disturbances, cross-coupling of attitude and orbit control, and large time-delay. It guaranteed the stability and high accuracy of rendezvous and docking control system, and 3) to solve the problem of the cross-coupling between attitude control and position control of thrusters, a new real time thruster control allocation algorithm based on optimal thruster combination table was proposed, inspired by the concept of thruster combination design in ATV, which improved the accuracy of attitude and position control and deduced fuel consumption. With manual RVD design experiences of other countries for reference, semi-automatic control strategy for attitude control, and fully manual control strategy for position control is designed in the manual measurement and control system of the Shenzhou spacecraft. In Fig. 1 and Fig. 2, images of Shenzhou-9 and Tiangong-1 RVD process are displayed. 361

Fig. 2. Manual rendezvous and docking between Shenzhou 9 and Tiangong 1—image of the TV image 3.3 Control of Space Station During the process of space station assembly, structure change distinctly, flexible mode frequencies are low and uncertain, gas torque and gravity gradient torque cannot be omitted. These result in the difficulty of high accuracy control of space station. Therefore high accuracy robust adaptive control methods are under study with respect to large flexible structure under structure variation and dynamical uncertainty. 4. DEEP-SPACE EXPLORER CONTROL The space exploration program that has already been started in China is the lunar exploration engineering, and the other one that has been currently discussed is the Mars exploration program. Lunar exploration is divided into three phases, in the first of which, it mainly aims to realize lunar surrounding and exploration, in the second it aims to realize lunar soft landing and automatic patrol and exploration, and in the third, it aims to realize automatic sampling and return [15]. 4.1 Control of lunar-surrounding explorer Satellite ChangE 1, after launched into the orbit, was again acted on by several accurate orbital maneuvers, and finally entered the lunar-surrounding working orbit in November,

2013 IFAC ACA September 2-6, 2013. Würzburg, Germany

scheme is proposed for handling the unclear texture of the lunar surface terrain and the lower solar elevation. 4.3 Lunar sampling re-entry control Lunar sampling re-entry control mainly includes the rendezvous and docking in lunar orbit and the re-entry control of the spacecraft returning from the lunar to the earth. For this, on the basis of the manned spacecraft rendezvous and docking technology, we are currently studying the highly accurate rendezvous and docking control approach in lunar orbit. Meanwhile, in order to fulfill the demand of the accurate re-entry under the second cosmic speed, we also performed research on the re-entry control such as direct atmospheric re-entry and skip re-entry. 4.4 Autonomous control of Mars explorer

Fig. 3. The trajectory of ChangE 2 during its rendezvous with asteroid Toutatis 2007. The main characteristics of the satellite controller are as follows: 1) autonomous attitude determination with the attitude determination algorithm relying on the combination of star sensors and gyros, 2) attitude and orbital maneuver realized by Euler parameters based phase plane control algorithm, 3) attitude maintaining during the orbital control using the algorithms of Euler parameters based “PID plus structural filter” and digital pulse-width modulation, and 4) autonomous orbit transferring control scheme with autonomous fault inspection and rapid orbit control recovering [16]. Satellite “ChangE 2”, after accomplishing the lunar exploration, performed two further experiments. On August 25, 2011, it accurately entered the second Lagrange point. After accurately transferred several times, it passed by the asteroid Tutatis, and the smallest rendezvous distance is merely 3.2 km. Fig. 3 plots the trajectory of “ChangE 2” from its starting point to the rendezvous position with asteroid Toutatis. 4.2 Control of Lunar soft landing and lunar surface patrol Satellite “ChangE 3” includes the lander and the rover, which will be launched in 2013. Taking the process of soft landing of ChangE 3’s lander as an example, we investigated new fault diagnosis and control approach for dealing with the possible large variation of the attitude so that safer landing control can be achieved. In order to adapt to the terrain/physiognomy, illumination and mechanical property of the inspected area, we have solved the following control problems for ChangE 3’s rover: 1) new attitude/position determination algorithms are proposed to deal with the problem of attitude/position determination in a varying terrain, 2) new motion control approach for wheel-based mobile system is proposed such that coordinated motion performance of the rover in the varying terrain is ensured and the slide and side-slide are reduced, and 3) autonomous obstacle-avoidance planning 362

We performed the following studies for the control problems of Mars explorer: 1) due to the long unmeasurable time, large TC/TM time delay, and the remarkably deteriorated orbit determination with the increase of the distance, we studied the autonomous navigation approach using the satellite-mounted Mars sensor and star sensor, 2) we studied autonomous fault diagnosis and system reconfiguration approach for the problem of Sun transmit outage of the Mars explorer, and 3) we studied the autonomous task execution for the problem that the Mars exploration task cannot be realized under the ground supervision. 5. FUTURE RESEARCH AND DIRECTIONS The earth-orbit satellite control is demanded from high accuracy to super-high accuracy, space rendezvous and docking develops from operator control and automatic control to autonomous control, and the deep-space explorer demands autonomous control. These applications suggest, in certain sense, that spacecraft intelligent autonomous control has already become a major direction of the future spacecraft control. Early in 1995, Academician Jiachi Yang suggested that China should develop space intelligent and autonomous control. To realize spacecraft intelligent and autonomous control, from the perspective of control system design, besides handling the problems of spacecraft autonomous navigation, autonomous fault diagnosis and reconfiguration, and the autonomous management of the information, we also need to examine, from a theoretical perspective, the problems of modeling of the intelligent control system, controller design and system stability analysis. Most of the traditional spacecraft employ the standard PID control and phase plane control. Some of the present on-orbit high-accuracy satellites attempted to use modern control algorithms, e.g., H∞ control, robust control and adaptive control, yet, all of them encounter various problems and cannot systematically solve the problem of spacecraft intelligent and autonomous control. The characteristic model based intelligent adaptive control achieves numerous theoretic results on system modeling, controller design and stability analysis of the closed-loop system, and in addition, successfully applied to manned spacecraft reentry problem and space rendezvous and docking, which,

2013 IFAC ACA September 2-6, 2013. Würzburg, Germany

thus, may possible become an effective approach to realize spacecraft intelligent and autonomous control. REFERENCES [1] [2]

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