Development of Simulation Testbed for Autonomous On-Orbit Servicing Technology Yong Chen, Yiyong Huang, and Xiaoqian Chen
Abstract—While autonomous On-Orbit Servicing(OOS) capabilities are prerequisites for the success of the envisioned sample return and human missions to the bodies of the Solar System, the mishap of previous mission indicates the mastery of autonomous OOS technologies is still an open challenge. On the other hand, ground test is a relatively low-risk, low-cost and high-return method for validating autonomous OOS technologies. An experimental testbed which consists of several spacecraft simulators floating on a flat marble table has been constructed to systematically demonstrate various key technologies for autonomous OOS mission. Ongoing research is to demonstrate the scientific problems in on-orbit refueling especially in modularized integrated docking/refueling mechanism analysis and design, miniaturized modularized fluid management mechanism design, and on-orbit refueling imitation.
I. INTRODUCTION
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N-ORBIT servicing(OOS)[1, 2], as a potential new paradigm in space mission design to reduce life-cycle costs, increase payload sensor availability, extend spacecraft orbital lifetime, enhance spacecraft capabilities, augment mission flexibility and operational readiness, is the process of improving a space-based capability through a combination of on-orbit activities including inspection, rendezvous and docking, and value-added modifications to a satellite’s position, orientation, and operational status. Considering manned OOS on Hubble Space Telescope (HST) and International Space Station (ISS) are extremely expensive and cannot be adopted as a future routine On-Orbit servicing structure, autonomous vehicle technology has been developed to make OOS affordable to most space missions[3]. An unmanned satellite, called servicer, would rendezvous and dock with satellites to be serviced, then perform autonomous servicing such as repairing, refueling and upgrading, thus provide potential cost savings, extenuate vulnerabilities to on-orbit spacecraft, and finally increase capacity to react to uncertainty of space missions traditionally restricted by unapproachable spacecraft[4]. While the autonomous OOS capabilities are prerequisites for the success of the envisioned sample return and human missions to the bodies of the Solar System, the mishap of the NASA’s Demonstration of Autonomous Rendezvous
Manuscript received March 30, 2011. This work was supported by the National Natural Science Foundation of China (NO. 50975280) and Program for New Century Excellent Talents from Ministry of Education of China (NCET-08-149). The Authors are with Department of Aerospace and Material Engineering, National University of Defense Technology, Changsha, 410073, P. R. China (e-mail:
[email protected]).
c 2011 IEEE 978-1-61284-250-9/11/$26.00
Technology(DART) mission in April 2005 indicates that the mastery of autonomous OOS technologies is still an open challenge[5, 6]. On the other hand, ground-test experimentation[7] is a relatively low-risk, low-cost and potentially high-return method for validating technologies such as mechanical structure analysis and design, GNC (guidance, navigation and control), and analytical developments and numeric simulations. While the MIT Space Laboratory(SSL) has developed the Synchronized Position Hold Engage and Reorient Experimental Satellites (SPHERES) facility for the testing of formation flight in a six degree of freedom(DOF) environment and autonomous docking algorithm and visual navigation inside the ISS, in NASA’s reduced gravity aircraft and in a 1-g laboratory environment[8, 9], the Lockheed Martin Controls and Automation Laboratory in Palo Alto, California presents a unique environment for the development and testing of technologies associated with automated rendezvous, docking, and self-assembly tasks between a group of modular robotic spacecraft emulators[10]. Moreover, to validation of GN&C methods for the proximity navigation and docking between two small spacecraft of similar mass, the Spacecraft Robotics Laboratory of Naval Postgraduate School is conducting Autonomous Docking and Spacecraft Servicing Simulator (AUDASS)[7, 11]. At the same time, Roberta M. Ewart and Jie Z. Jacquot[12] established Algorithm Development Lab to provide neutral environment for generating, implementing, testing, and assessing the performance of ground algorithms for data from the type of Wide Field-of-View (WFOV) sensors. On the other hand, the Micro-satellite Dynamic Test Facility [13](MDTF) at the University of Southern California enables real-world testing of full-scale rendezvous and proximity operations in an affordable environment. Furthermore, the Hawaii Space Flight Laboratory[14] (HSFL) was established at the University of Hawaii at Manna to educate students and help prepare them to enter the technical workforce, and to help establish a viable space industry that will benefit the State of Hawaii. In addition, a hardware-in-the-loop (HWIL) simulation testbed[15], invested by SAIC company, is built for autonomous rendezvous and docking program of the NASA Marshall Space Flight Center, as well as flight-like processor hardware for GN&C functions and optical sensor-in-the-loop capability. Development of modularized hardware and software components and interfaces will be instrumental towards reducing system costs, complexity and development time. In addition, modularized components potentially allow for
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spacecraft repair and maintenance by replacing faulty parts and greatly enhance the capabilities and prolonging mission life. To systematically observe the difficulties associated with autonomous OOS technologies, ongoing research at the new established Spacecraft Robotics Laboratory is to explore various challenges especially in autonomous rendezvous and docking, modularized mechanics analysis and design, human-in-the-loop teleoperation, as well as on-orbit refueling imitation. Unlike SPHERE in MIT, ongoing research is to demonstrate the scientific problems in on-orbit refueling especially in modularized integrated docking/refueling mechanism analysis and design, miniaturized modularized fluid management mechanism design. The paper is organized as follows: Section II details the design of the experimental test bed. Section III introduces the methods used for the autonomous on-orbit refueling imitation. Finally, Section IV concludes the paper. II. TESTBED OVERVIEW The main objective of the testbed is to explore various challenges associated with OOS servicing technology; however, ongoing research is to demonstrate the scientific problems in on-orbit refueling operation especially in modularized integrated docking/refueling mechanism analysis and design, miniaturized modularized fluid management mechanism design. From Fig.1, it can be seen that the testbed consists of three main subsystems which are a floating surface, some spacecraft simulators, as well as flight software design (FSW). In addition, an off board monitor computer is used to verify human-in-the-loop teleoperation, upload software, reconfigure the experimental tests, and deal with the emergent accidents. Moreover, the whole testbed has been designed modularly and extendedly to have the option of convenient component addition, repairing and upgrading throughout the life of the project. The main subsystems of the testbed are briefly described below.
Fig.1. Overall of the test bed
A. Weightlessness and Frictionless Environment Construction As can be seen from Fig.1, a 10m × 6m wide marble table constitutes the base of the floatation of the spacecraft simulators. Specifically, the high evenness and the leveling of the marble table guarantee that the gravitational disturbing
force is small. Moreover, due to an small average residual slope angle for the floating surface, the average value of the residual gravity acceleration, which is measured by analyzing the free motion of the spacecraft and affects the dynamics of floating vehicles, is two orders of magnitude lower than the nominal amplitude of the acceleration fluctuation obtained during the reduced gravity phase of flights. Thus, by floating a spacecraft simulator via air-pads on the flat marble table, it is possible to reproduce the kinematics and vehicle dynamics for three degrees-of-freedom (two horizontal translations and the rotation about the vertical axis) with respected weightlessness and frictionless conditions. In addition, as the Hill-Clohessy-Wiltshire (HCW) dynamics can be considered a disturbance to be compensated for by the spacecraft navigation and control system during the final docking phase, the use of such marble testbed can capture many of the critical aspects of autonomous docking maneuver. B. Spacecraft Simulator The spacecraft simulator, which consists of three modular decks mounted on top of the other, is completely autonomous during the experimental tests. In particular, the chaser spacecraft simulator does not require any external reference for its navigation, besides the four Light Emitting Diodes (LEDs) mounted on the target vehicle simulator. Furthermore, by using six gas thrusters and a reaction wheel, the chaser spacecraft simulator is capable of independent translation and attitude control. Moreover the reaction wheel allows for significant propellant conservation and extends the time endurance of single experimental runs. The lowest modular deck houses the floatation and thrust subsystem which is called the Transport Platform. The Transport Platform consists of four tanks that feed compressed air through two independent pressure regulators to six thrusters and three air pads. Additionally, a pressure surge chamber is installed along the thrust air supply line between the regulator and the solenoid valves in order to stabilize pressure oscillations during the thruster's operation. Specifically, each of the six thrusters consists of a convergent nozzle and is activated by a normally closed on-off solenoid valve. To test the on-orbit refueling mission, the second deck of the simulator hosts the prototype which consists of integrated modularized docking/refueling interface and miniaturized modular fluid management mechanism. Concretely, the active portion of the docking/refueling mechanism mounted on the chaser spacecraft consists of a motor-driven leading screw that actuates a linkage which engages the passive portion of the capture system mounted on the target spacecraft simulator, by retracting, seats another three short columns mounted on the active portion into three fixed hole on the passive portion to stabilize the attitude of the spacecraft, thus establishes a rigid interface and locks the refueling couplings for gas and liquid resupply. Moreover, the Integrated Docking/Refueling interface was designed to have a relatively large envelope for possible misalignment, to execute the mate and de-mate operations easily, and to carry a
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suite of pipeline connectors and electrical connectors to execute cooperative missions. Furthermore, miniaturized modular fluid management mechanism integrates various types of valves, gauges and pipelines in one plate, each of which is placed on the target and chaser spacecraft and managed by a refueling operation DSP-based Control Board. The top floor contains the electronics subsystem, navigation subsystem, power management subsystem, and a reaction flywheel. Concretely, the reaction flywheel is used for the attitude control of the spacecraft to provide the distinct advantage of propellant conservation and significantly extend the time endurance of single experimental run. At the core of the power management subsystem there is a lithium ion battery with a 24 VDC bus tension and some DC-DC voltage converters. Likewise, an On-Board Computer with PC104 platform constitutes the core of the electronics subsystem. Specifically, the computer manages the real-time mission procedure, monitors the whole system, processes data from other subsystems and commands the actuators with real-time calculated relative navigation information. Additionally a MEMS-based Inertial Measurement Unit (IMU) and Complementary Metal Oxide Semiconductor (CMOS) camera are installed on the floor constituting the navigation
subsystem. Finally, wireless Ethernet is installed which enables the Off-Board monitor computer and the spacecraft simulators communicate with each other during the corporative missions. C. Flight Software The flight software autonomously monitors and maintains the state of the payloads, controls the operation of the spacecraft, monitors and reconfigures the schedule of mission, and performs the calculations for the integrated IMU/CCD navigation subsystem used by other subsystems. Furthermore, via wireless Ethernet, It can be reloaded or modified from the Off-Board monitor. On the other hand, the Off-Board monitor system is able to command and supervise the spacecraft over a private wireless network. On experiment, each simulator can receive, parse and respond to plain-text commands using a pre-defined coding format. In addition, spacecraft's real-time information and videos are displayed at the Off-Board system and if required, can be projected onto a screen at the end of the laboratory. Fig. 3 shows the data stream of the spacecraft simulators in the on-orbit refueling mission
Fig.2. Diagram of data stream of the spacecraft simulators in on-orbit refueling mission system
To provide a stable basis for various autonomous OOS operation, component autonomy enhancement and transparent and global quality of service (Qos) assurance, the architecture of the flight software is designed to be future-proof, re-useable, robust, scalable, persistent, and platform-independent to support runtime replacement, component integration and evolutionary upgrade.
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III. APPLICATION TO AUTONOMOUS ON-ORBIT REFUELING On-Orbit refueling [16]is a subject of great interest to many spacefarers. Future missions such as human deep exploration of the universe and space based laser missile defense systems require refueling with large quantities of propellants in microgravity environment. Although on-orbit propellant
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A. Guidance, Navigation and Control As described in section II, by processing data from CMOS camera, on-board computer measures with good accuracy the relative position and orientation of the chaser with respect to the target. Nevertheless, these measurements, available asynchronously with a nominal frequency of about 3 Hertz, are affected by the computer performance and a time-varying delay due to the image-processing and pose estimation. In order to compensate for the limitation, On-board IMU has been added to the chaser spacecraft to estimate the translation and orientation. However, the gyro and accelerometers data are affected by noise and drift rates. Therefore, Kalman filters are employed to fuse the data from the Camera and the IMU. Particularly, Extended Kalman filter (EKF) is introduced to deals with the single DOF attitude motion while Discrete-Time Linear Kalman filter is implemented to handle the Two DOF translational motion. To complete the autonomous rendezvous, a second-order sliding mode controller installed on the chaser spacecraft simulator was designed. Specifically, the controller calculates the control law with respect to the guidance information from IMU/CCD integrated navigation system, then commands the reaction wheel and thruster through CAN-Bus, finally precisely control the translational motion and attitude. Fig. 3 demonstrates the controller performance in the autonomous Rendezvous and Docking mission
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resupply has been successfully performed numerous times by the progress resupply vehicle, routine servicing of satellites has not been budgeted with the lack of standardized servicing hardware. Yet, the cancellation of plans to resupply the U.S. Propulsion Module indicates the level of risk and uncertainty inherent in On-Orbit Refueling Mission[17]. Therefore, a lot of deep researches have to be conducted in this challenge area. This section reports details on the autonomous on-orbit refueling mission including procedure of autonomous rendezvous and docking, propellant resupply process. To validate on-orbit refueling mission, preliminary research focuses on the CCD/IMU integrated navigation and flight control, integrated modularized docking/refueling interface mechanism design, and miniaturized modular fluid management mechanism design.
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mission. The interface consists of an active portion installed on the chaser spacecraft simulator, and a passive portion mounted on the target spacecraft. Fig. 4 shows the whole interface.
It can be seen that the whole mechanism consists of active part which was installed in the chaser spacecraft and passive part which was placed in the target chaser spacecraft. Moreover, the mechanism integrates various types of valves, gauges and pipelines in one plate and is controlled by a refueling operation DSP-based Control Board which communicates with on-board computer via CAN-Bus.
Fig. 4. The modularized docking/refueling interface. Top: active part. Bottom: passive part
After the end of the soft-docking, the mechanism mates the fluid coupling by a screw actuator driven by a monument motor. The mating operation begins when collision bursts out between the passive portion and an extended shaft of the active portion. Once the shaft establishes a structural connection between the two portions by interlocking with passive portion after shaking, rotation of the motor in the opposite direction draws the two portions together and three auto-alignment load-bearing posts mechanically adjusts the relative attitude. Then a fluid coupling creates an interface seal tightly and opens the flow path across the interface. Furthermore, to improve the success of missions, large tolerance for docking and modularization are considered in the design of the interface. C. Miniaturized modularized fluid management mechanism To well manage the flow in the propellant resupply processing, it’s important that various types of valves, gauges, and pipelines must be well monitored and controlled by the operation center. Thus, a modularized fluid management mechanism is introduced to manage various valves, gauges and pipelines related to propellant resupply task, which is designed to be miniaturized to meet the light mass and small size requirement. Fig. 5 shows the structure of the fluid management mechanism. 152
Fig. 5. Structure of miniaturized modular fluid management mechanism. Top: active part. Bottom passive part
IV. CONCLUSION This paper introduces a new planar experimental test bed for the simulation of the OOS missions. In current research, two spacecraft simulators floating via air pads on a flat marble table are used to verify the autonomous rendezvous and docking operation as well as on-orbit refueling mission. The flat marble table provides a better level of gravity reduction than parabolic flights and offers a low-risk and relatively low-cost intermediate validation step between analytical-numerical simulations and outer space missions. Data from CCD and IMU are fused to support the GN&C system. Moreover, the modularized spacecraft mechanism design represent one possible way to get large payloads and upgrade on-orbit satellites’ components in a cost-effective
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manner. Experimentally, the work presented herein demonstrates in a laboratory setting a number of rudimentary capabilities necessary for OOS activities including autonomous rendezvous and docking operation and propellant resupply operation. Furthermore, the GN&C algorithm proposed in the paper can be extended to three dimensional applications in principle. In the future, the modularized docking/refueling interface and miniaturized modular fluid management mechanism may be tested in the microgravity environment to validate its feasibility. Finally, based on the achieved results, next research will focus on the multidiscipline design and optimization of the modularized docking/refueling interface and the miniaturized modular fluid management mechanism, high accurate measurement gauges with respect to propellant resupply operation, and collision avoidance in autonomous rendezvous operation.
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ACKNOWLEDGMENT The authors gratefully acknowledge those who contributed material to this paper. REFERENCES [1]
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