Hirohisa Kojima

Control Moment Gyro Singularity-Avoidance Steering Control Based on Singular-Surface Cost Function

Although control moment gyros have the advantage of producing large output torques, their singularities hamper their use in spacecraft. Many studies have investigated steering laws to avoid singularities. Some of these steering laws are capable only of local avoidance. On the other hand, global steering laws require complex calculations that have to be performed offline. In this study, to establish a semiglobal singularity-avoidance method that does not require a large amount of offline calculation, a geometrical singularity-avoidance steering law is proposed by introducing a cost function. This function is termed the surface cost, and it calculates the relation between singular surfaces and the given command torque. The steering law keeps the cost function low by adding perturbation torques to the angular momentum trajectories of control moment gyros. The effectiveness of the proposed steering law is validated numerically and experimentally.

Adaptive Deflection-Limiting Control for Slewing Flexible Space Structures

This paper proposes an adaptive deflection-limiting input shaping technique for slewing flexible space structures. The bending moment at the root of the flexible appendage is measured and used to adjust the control parameters. This measurement is more easily obtained than the tip acceleration on a space structure and it is very useful for avoiding damage to the flexible structure. The control timing of the input shaping is automatically tuned during the slewing motion to limit the induced bending moment and to eliminate the residual vibration. The effectiveness of the adaptive deflection-limiting input shaping control is verified experimentally and compared with the traditional input shaping control methods.

Nonlinear control of librational motion of tethered satellites in elliptic orbits

A method to control the librational motion of a tethered satellite system in an elliptic orbit is presented. To simplify the analysis, gravity is treated as the only external force affecting the tethered satellite system, only in-plane motion is considered, and the flexibility and mass of tethers are neglected. The tethered satellite system treated in this paper consists of two subsatellites and a mother satellite, such as the space shuttle, connected together in series via massless tethers. This type of tethered satellite system has very important applications in Earth observation, space observation, communications, and satellite constellations. The librational motion of the tethered satellite system in an elliptic orbit is known to be chaotic and can be stabilized to undergo periodic motion by the delayed feedback control method. The periodic motion of a tethered satellite in a circular orbit is employed as the reference trajectory for tracking by the actual tethered satellite, which is in an elliptic orbit. The decoupling and model tracking control methods, based on differential geometric control theory are combined with the delayed feedback control method in a new approach to controlling the librational motion of the tethered satellite system in elliptic orbits. The results of numerical simulations show that the proposed control scheme has good performance in controlling the librational motion of a tethered satellite system in an elliptic orbit.

Nonlinear Control of a Double Pendulum Electrodynamic Tether System

T HE electrodynamic tether (EDT) system uses the Lorentz force as a thruster generated by the interference between the Earth’s magnetic field and the electric current along the tether. The main advantage of this system is that its required electric current can be produced by solar power and electric particles such as Xenon, the mass of which is much smaller than those of traditional chemical propellants. As a result, this system is expected to be employed as a new space technology that will be useful in many types of space applications [1] such as reboosting of the international space station [2]; deorbiting of dysfunction satellites [3] (space debris removal system); the momentum exchange electrodynamic reboost (MXER) [4] system, which could be used to boost payloads from a low Earth orbit (LEO) to a geosynchronous transfer orbit (GTO); and scientific missions that could include the observation of meteors, the highaltitude atmosphere, and the aurora. The motion of the EDT system consists of librations that are similar to themotion of a pendulum and vibrations of the tether that are similar to the vibrations of a string. The librational motion of the tether system in an elliptic orbit is known to be chaotic [5,6]. The librational and vibrational motion of the tethered satellite system must be stabilized to successfully perform space operations such as observation of the atmospheric environment at high altitude. For the purpose of stabilization, a great number of control schemes have been proposed for various aspects of control [7], such as stationkeeping, deployment and retrieval control, generation of optimal trajectories [8], suppression of tether vibrations by electric current variation [9], periodic motion of the electrodynamic tether systemon inclined orbits [10], and effect of the electrodynamic force on orbit [11]. However, to the best knowledge of the authors, except delayed feedback controls, application of nonlinear control schemes to the multipendulum type of electrodynamic tether systems for achieving constellation missions has not yet been widely studied. The tether system treated in this Note consists of two subsatellites and a mother satellite, such as the space shuttle, connected in series via conductive massless tethers. Although this model seems simple, because the mass and flexibility of the tether are ignored, it can be used to investigate the behavior of the in-plane motion of an electrodynamic tether system in an elliptic orbit without significant computational effort and to investigate the applicability of the nonlinear controllers to the simple model. In this study, two nonlinear control methods to stabilize the inplane librational motion of an electrodynamic tether system in an elliptic orbit are investigated. The nonlinear controllers treated in this Note are a decoupling control method [6,12] and amodel-followingdecoupling control method [6,12]. The decoupling control can be used to control each tether attitude independently, and this independent motion is suitable for achieving satellite constellations. If this motion is realized for a three-mass tethered satellite system, various scientific missions will be possible, including the observation of planets with a magnetic field and the observation of the aurora from more than two directions simultaneously. The model-following-decoupling control method is introduced to achieve a periodic motion by which the Earth atmosphere at the specific altitude can be studied periodically. In this study, the periodic in-plane motion of a tethered satellite system in a circular orbit is employed as the reference trajectory for tracking by the actual tethered satellite system in an elliptic orbit. The results of numerical simulations show that the nonlinear control schemes considered herein can stabilize the in-plane librational motion of a double pendulum electrodynamic tether system in an elliptic orbit.

Space Demonstration of Bare Electrodynamic Tape-Tether Technology on the Sounding Rocket S520-25

A spaceflight validation of bare electro dynamic tape tether technology was conducted. A S520-25 sounding rocket was launched successfully at 05:00am on 31 August 2010 and successfully deployed 132.6m of tape tether over 120 seconds in a ballistic flight. The electrodynamic performance of the bare tape tether employed as an atmospheric probe was measured. Flight results are introduced through the present progressive report of the demonstration and the results of flight experiment are examined as the premier report of the international cooperation between Japan, Europe, USA and Australia. Future plans for maturing space tether technology, which will play an important role for future space activities, are also discussed.

Experimental verification of periodic libration of tethered satellite system in elliptic orbit

Tethered satellite systems are often considered as revolu- tionary new technologies capable of solving complex tasks for challenging future space missions such as construction of large space structures, momentum exchange [1], debris elimination [2], artificial gravity generation by rotational motion, and so on [3]. To make the application of tethered satellite systems feasible, their dynamics and control must be comprehensively understood and properly addressed.

An Application of Input Shaping For Electrodynamic Tether System

** In the present study, Input Shaping method is applied to reduce the initial vibrations of electrodynamic tether system. In the initial phase of the propulsion, the electrodynamic force induces vibrations on the tether. Libration and string vibration are undesirable for any missions of the system. These are caused by the flexibility or orbital motion of the system. The different shapers are applied for the vibrations and the input command is designed by multiplying the both shapers respectively. As examples, two cases of de-orbit missions of space debris are analyzed numerically. The results of this study show significant performances of suppressing by employing the Input Shaping applications for tether system.

Slew Maneuver of a Flexible Space Structure with Constraint on Bending Moment

A control scheme is proposed for a rest-to-rest slew maneuver of a e exible space structure with constraints on the maximum value of the bending moment of the e exible structure. The e exible space structure treated is a rigid body equipped with a e exible appendage. The slew maneuver is a rest-to-rest maneuver with a constraint on the bending moment at the root of the e exible appendage, and a control scheme is studied to minimize the bending moment due to the structural vibration inevitably excited by the slew maneuver of the e exible structure. The optimal control proe le is studied both analytically and experimentally to solve the control algorithm and also to verify the validity of the implementation. Results of the numerical and experimental analyses of the present control are compared with those of the well-known time-optimal control and the robust time-optimal control to demonstrate the effectiveness of the present control scheme.

Experimental Study on Delayed Feedback Control for Libration of Tethered Satellite System

Experimental study on delayed feedback control for libration of tethered satellite system was conducted. To investigate the effect of gravity on the motion of the tethered satellite system, the elasticity of the tether and the aerodynamic drag force affecting the system were assumed to be negligible. It is necessary to assign practical values to the nondimensional force affecting the tethered subsatellite to emulate its in-plane motion in elliptic orbits. These variations in tether length occur once per seven orbits in the numerical simulations, but once per three orbits in the experiments. The results obtained reveal that DFC using tether length rate control can stabilize chaotic librational motion of a tethered satellite system in an elliptic orbit to a periodic motion. A tethered satellite system whose libration is stabilized to a periodic motion can serve as a sky hook, or space-born transportation system, to periodically pick up launched payloads or capture and release space debris.

Estimation of Flyable Regions for Planetary Airships

Scientific observations of Venus were conducted by two balloons. Future plans now being considered by ISAS/ JAXA and NASA include a Mars airplane and other planetary balloons. This study deals with the use of remotely piloted airships as platforms for planetary observation. An airship can be placed in the target planets atmosphere. The planets surface topography, gravitational field, magnetic field, and atmospheric layer can be observed over an extended period of time with little effort or fuel expenditure. Taking into consideration the temperature and pressure on Mars and Venus, as well as basic limitations of airships, it was determined that a planetary airship should fly just below the cloud level on Venus, whereas on Mars there was no suitable region in which to station a planetary airship.