Chikara Murakami

On orbit control using gravity gradient effects

Abstract Possibility of orbit control using gravity gradient (GG) effects without any mass expulsion is discussed. For simplicity, a dumb-bell type satellite and circular orbits are mainly considered. It is shown that the GG effects can be applied to convert attitude torques into orbital torques and vice versa. In central gravitational force fields, maximum orbital torques or thrusts are available from the GG force when roll or pitch angle is ± π 4 provided that the attitude angle is null when the dumb-bell axis coincides with the local vertical. Such external torques as geomagnetic or solar wind pressure can be utilized to maintain the ± π 4 attitude, then the orbital torques are available forever. In non-central gravitational fields, without any external torque, the orbital radii of circular orbits can be increased by controlling the satellite attitude using electric energy. The use of the Earths oblateness effects and the exterior Lunar potential is discussed.

Active nutation control of spacecraft using gimballed momentum wheel

Abstract In this paper, mainly a control method using both spacecraft attitude information and wheel gimbal signals is described. At first, system formation and its equations of motion are described, where interaxis cross coupling is positively utilized. And then described are approximate analytical solutions of the equations, where the system is linearized and complex variables are used. If, sensor signals of both roll and yaw attitude angles of spacecraft are available in gimbal servo electronics, nutational motion of the spacecraft can be decayed very quickly. And the system eigen values can be solved in very simple forms. It is shown that these spacecraft attitude signals bring on by far effective results of the nutation damping compared to the open loop case where only gimbal-axis sensor signals are used. These results are discussed and it is pointed out that outer-rotor type momentum wheels using spiral grooved bearing or ball bearing may cause nutational motion of the spacecraft, i.e. negative nutational damping. Finally, the authors recommend inner-rotor type momentum wheel, in case of spiral grooved or ball bearing, from a stand point of spacecraft nutational motion. Computer simulation results are presented which verify the above-metioned analytical solutions.

A new type of magnetic gimballed momentum wheel and its application to attitude control in space

Abstract A new type of magnetically suspended gimbal momentum wheel utilizing permanent magnets is described. The bearing was composed of four independent thrust actuators which control the rotor thrust position and gimbal angles cooperatively, so that the bearing comes to have a simple mechanism with high reliability and light weight. The high speed instability problem due to the internal damping was easily overcome by introducing anisotropic radial stiffness. A momentum flywheel with the 3-axis controlled magnetic bearing displays good performance for attitude control of satellite with biased momentum.

A Suppression Method of Conical Motion of an Axi-Symmetrical Spinning Rigid Rotor Suspended by Magnetic Bearings

New definitions of nutation and precession are proposed, which are usuful not only for physical understanding but also for intuitive interpretation of the optimal control of axi-symmetrical spinning rigid bodies. At first it is shown that the spin axis does coning motion about the instantaneous angular momentum vector with constant rotational speed (nutation angular velocity) regardless of its coning angle.

Vibration and Control in Outer Rotor Type Magnetic Bearings

The magnetic bearings described here has been developed for high speed rotating machinery. The manufactured experimental model is 4-axis-active type with simple structure. Since the model is so called outer-rotor type, resonance frequency of the rotor could set higher than rotating frequency, and only first resonance frequency of the stator shaft is in the rotating region. In this paper, we describe the analysis and experimental results to obtain stable high speed rotation. Firstly, stability of the gyroscopic motion of rigid mode is examined at low speed rotation. Subsequently, influence of flexibility of the stator shaft on stability is discussed. Then we indicate that nonlinearity of the power amplifier plays an important role in stability of the system. Finally, a suppression method using notch-filters and inter-axis cross coupling feedback at relatively high speed rotation is discussed.

Damping characteristics of a passivelly stabilized rotor suspended by magnetic bearing.

This paper describes the damping characteristics of rotor in two radial axis, which are passively stabilized by magnetic bearing. The equations of rotor dynamics are introduced considering the effect of anisotropic stiffness of bearing. From the solution of the equations obtained by applying the method of averaging, the damping characteristics variation of the angular velocity is related to the anisotropy of the stiffness. The limit speed at which the damping factor is equal to zero increases with the difference of bearing stiffness in two orthogonal axes in the plane perpendicular to the axis of the rotation. These relations are examined in an experiment on a one-axis-controlled magnetic bearing.

Inertial Force Field Patterns Due to Nutational Motion of Spinning Satellites

The motion of nutation dampers on spin-stabilized asymmetric spacecraft (including dual-spin satellites) is analyzed in general form. Up to now, many papers on this problem have been presented, but as far as the authors know, there is no paper that treats the effects of mounting position and direction of the damper motion.