A high performance antenna pointing control algorithm for an inter-orbit communication satellite

Abstract

A new high-performance pointing controller for optical antennas on inter-orbit communication satellites is proposed on the basis of two-degrees-of-freedom (2-DOF) control methodology. A model following controller and a disturbance compensator were developed to satisfy the device requirements, i.e., quick acquisition of the target satellite, high tracking accuracy, and disturbance rejection. A new technique was introduced in designing the disturbance compensator, which allows the disturbance compensator bandwidth to be designated so that the H=° norm of the control system becomes smaller. The proposed approach makes it easy to derive a comparatively low-order controller. In an example of optical antenna-pointing control, the order of a derived controller becomes six for each axis. It appears that implementing this algorithm to on-board pointing control will not be especially difficult. The proposed controller has been evaluated by simulation. The results obtained show that its performance is significantly better than that of controllers developed using a Copyright © 1995 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. calssical-basd approach. Introduction Test flights for inter-orbit communications are being planned for the coming decade in order to develop such technologies as global data acquisition from geostationary satellites, and communications between geostationary and low-earth-orbit satellites. One of the most important issues in inter-orbit satellite communication is the development of a precise antenna-pointing control. A number of control methods involving structural dynamics have been discussed in recent years '. In these methods, the settling time required to make a connection between satellites was within five seconds; hence, the pointing control systems were conservatively designed using classical methods. Optical inter-orbit communication between satellites has also been considered as a means of increasing the network bandwidth. There are two important problems in this type of pointing control: achieving a settling time of less than 250 msec, and achieving sufficient pointing accuracy. To meet these requirements, the control system bandwidth needs to be designed broadly, in the classical approach. This means that the internal stability margin of the control system will be American Institute of Aeronautics and Astronautics reduced. A robust controller design approach called H<=°///-synthesis has also been discussed as part of a flexible structure control 3 mechanism. With this approach, structural mode vibration can be suppressed effectively. However, the derived controller generally becomes a highorder one; consequently, applying it to a practical system is not easy. This paper presents a new pointing-control algorithm for optical antennas on inter-orbit communication satellites. The algorithm can derive a low-order controller, and achieve both quick settling (under 250 msec) and accurate antenna pointing (closer than 0.01 degrees). In our previous work', we showed that a disturbance compensator can suppress the influence of both disturbances and parameter changes due to the space environment and structural flexibility. A redesigned disturbance compensator is also used in the new control system design. The two-degrees-of-freedom (2-DOF) control methodology plays an important role in the problem of obtaining optical-antenna pointing control. With this in mind, to meet the control requirements, we introduced a feed-forward control with a reference model which depends on a simplified controlled object. The effectiveness of the control system was demonstrated through simulations. The results obtained showed that the disturbance compensator can also absorb the simplified controlled object's parameter uncertainties. The remainder of this paper is organized as follows. In Section 2, we describe the dynamics of the optical antenna and the desired control design goals. The controller design approach is shown in Section 3. Simulation results are presented and discussed in Section 4, in which we also show that the proposed control design approach can lead to the development of a low-order controller. Model of the Optical Antenna Optical antenna dynamics The optical antenna (Fig. 1) consists of a 2DOF mechanism, whose motion is divided into elevation axis (EL axis) motion and azimuth axis (AZ axis) motion. The ranges of motion are 360 deg for the AZ axis and 242 deg for the EL. The structural parameters are given in Table 1. The controlled object block diagram and frequency response are shown in Figs.2 and 3 for the EL axis, and in Fig.4 for the AZ axis. Here, km represents the motor amplifier gain, kt is the motor torque coefficient, kb is the back e.m.f. coefficient of the motor, Lm is the motor armature inductance, and Rm is the resistance of the motor armature.