Toshio Kashiwase

Shape control of flexible structures

A shape control algorithm for a large flexible structure is proposed: (a) a reduced order model, which represents the precise shape of a structure; (b) a shape esti mation filter, which reduces the estimation error caused not only by the white noises of sensor measurements but also by the variation of the D.C. offsets of sensors; (c) a simple and robust controller to regulate many state variables by using many sensors and actuators. The controller is realized by a zero proportional, integral, derivative controller (zero P-ID controller) with the concept of low authority control/high authority control (LAC/HAC). The proposed shape control algorithm is illustrated through applications to a numerical model of an experimental setup.

Design and Performance of Tip-Tilt Mirror System for Solar Telescope

The tip-tilt mirror (TTM) system was developed for the extreme ultraviolet (XUV) Cassegrain telescope aboard the Institute of Space and Astronautical Science sounding rocket. The spatial resolution of the telescope is about 5 arcsec, whereas the rocket pointing is only controlled to be within ±0.5 deg of the target (sun) without additional stability control. To stabilize the XUV image within about 5 arcsec on the focal plane, the TTM system controls the tilt of the secondary mirror with two-axis fixed-coil magnetic actuators. The TTM system has a wide tilt angle and can drive the large secondary mirror at high frequency. The two position-sensitive detectors, one placed in the telescope and the other in the TTM mechanical structure, are used for closed-loop control of the TTM. The closed-loop control system, which has command and telemetry, is executed by the flight software on the digital signal processor. The TTM has a launch-lock mechanism to protect against launch vibrations up to about 16G. The sounding rocket was launched from the Kagoshima Space Center on 31 January 1998. The TTM worked perfectly during the flight and achieved better than the expected 5-arcsec stability on the focal plane during CCD charge-coupled device exposures.

Development of the tip-tilt mirror system for the solar XUV telescope

This paper describes the design and prelaunch performance of the tip-tilt mirror (TTM) system developed for the XUV Cassegrain telescope aboard the ISAS sounding rocket experiment. The spatial resolution of the telescope is about 5 arcsec, whereas the rocket pointing is only controlled to be within +/- 0.5 degree around the target without stability control. The TTM is utilized to stabilize the XUV image on the focal planes by tilting the secondary mirror with two-axes fixed-coil type actuators. The two position- sensitive detectors in the telescope optics and in the TTM mechanical structure from the normal and local closed-loop modes. The TTM has four grain modes with automatic transition among the modes. The low gain mode is used in the initial acquisition, and in case the TTM loses the tracking. The high gain mode is used in the normal tracking mode. This arrangement provides us with the wide initial acquisition angle with single TTM system as well as the high pointing accuracy once the tracking is established. The TTM has a launch-lock mechanism against the launch vibration of 16G. The closed-loop control with command and telemetry interface is done by the flight software against the launch vibration of 16G. The closed-loop control with command and telemetry interface is done by the flight software on the DSP processor. The use of the fast processor brings in the significant reduction in the weight and size of the control- electronics, more flexible control system, and shorter design and testing period.

On-Orbit Experiments of Frequency-Shaped H.INF. Control for Flexible Space Structure.

Flexible control experiments have been performed as one type of on-orbit experiment for the Engineering Test Satellite VI (ETS-VI) which was launched in August, 1994. The aim of the flexible control experiments is the feasibility study of new attitude control schemes for future flexible space structures. We have designed two types of controllers. One is a controller designed according to the Low Authority Control (LAC)/High Authority Control (HAC) scheme. The purpose of LAC is damping augmentation of structural vibration modes, while that of HAC is achievement of high control performance. The other controller is a HAC-only controller: this HAC controller is designed for high-performance attitude control of a rigid body, as well as for damping augmentation of the vibration modes. The HAC controllers in both cases are designed using H∞ control theory with the frequency-shaping technique. These control schemes have been tested on orbit by ETS-VI. The experimental results have been analyzed in both time and frequency domains. It is shown from the analysis that both control schemes can be accurately realized on orbit and perform well with respect to damping augmentation and attitude control.

Simplified gimbal design for LEO satellites tracking to a GEO satellite

The design concept of simplified gimbals for the purpose of geostationary satellite tracking from low earth orbits is discussed and the preliminary results of a trial fabrication of a tracking system with a 19-cm-diameter telescope and an acquisition/tracking/pointing control system are shown. A new method is introduced for producing light-weight and small equipment by a self-alignment mechanism and its implementation into the optics design is described.