Spacecraft Fine Tracking Pointing Using Control

Paper number

IAC-07-C1.1.01

Author

Prof. Brij Agrawal, Naval Postgraduate School, United States

Year

2007

Abstract

For several space missions, fine tracking pointing in micro-radians is required. In these missions, the desired trajectory is known.  Currently, feed forward and feedback control is used to achieve fine tracking pointing.  The problem in feed forward control is the uncertainty in the estimated spacecraft inertia, resulting in error in pointing.  To reduce pointing error, on orbit system identification of inertia is proposed on some programs. However, in some cases, because of multiple bodies, spacecraft inertia will be changing during tracking.  This paper presents adaptive control technique where the feed forward control is based on a reference trajectory that is modified from desired trajectory based on pointing errors.  This paper presents several improvements to the similar approaches in the literature.  The proposed adaptive control approach has been validated by experiments using three axis spacecraft simulator at the Spacecraft Research and Design Center.

	In the proposed approach, the parameters for feed forward and feedback are independent.  This provides flexibility to optimize feedback and feed forward control independently.  The paper presents   analytical and experimental simulations results using this adaptive control approach and comparison with the other approaches in the literature.  The results show significant improvement in tracking pointing using this approach.  The results also indicate that using this approach is effective even with large error in estimating spacecraft inertia. Therefore on-orbit system identification of inertia is not necessary.  The stability using adaptive control approach has been derived using Lyapunov function. 

	The spacecraft three –axis simulator is supported on a spherical air bearing and also has automatic mass balancing to minimize gravitation torque.  It has four variable speed control moment gyros (CMGs), Northrop Grumman Litton LN-200 IMU consisting of three fiber optics rate gyroscopes, sun sensors, magnetometers, inclinometer, fine sensor, batteries, power switching and control electronics, and automatic balancing system.    The power is supplied at 12V and 24V.   The CMGs are mounted such that its beta angle can be changed. One CMG has a maximum angular momentum of 22.5 N-m-s and maximum torque capability of 12Nm.  The rate bias for IMU is 1 deg/hr to 10 deg/hr. The flexibility of the spacecraft is simulated by torsion spring and a disk along each axis.   

Abstract document

IAC-07-C1.1.01.pdf

Manuscript document

IAC-07-C1.1.01.pdf (🔒 authorized access only).

To get the manuscript, please contact IAF Secretariat.