Inter-axis Coupling Research Articles

Automatic gas-jet parameter estimation for high precision spacecraft attitude control systems

A simple and easily implemented method is presented here for automatic on-board estimation of a critical parameter of the control jet (thruster) impulse transfer characteristic. This facilitates optimal control with regard to thruster lifetime, fucel consumption and pointing accuracy, despite uncertainties in the thruster performance, The new estimator facilitates automatic, in-orbit adaptation of the attitude control system to slow changes in the control jet characteristics, thereby maintaining optimal performance. The parameter estimator is intended for microprocessor implementation and assumes the use of a state estimator. Attention is restricted to a single, isolated axis. No difficulty is envisaged in developing the technique further to cater for three-axis stabilised spacecraft with significant inter-axis coupling The parameters to be determined are the intercepts on the characteristics of each control jet relating the actual jet firing time to that demanded. A mismatch in these parameters gives rise to a state estimator transient following a jet firing. This provides the stimulus to the parameter estimator, which is basically a resettable pure integrator. The intercept estimate is updated by an amount proportional to the integral of the state estimator error taken over a finite period, commencing at the beginning of the jet firing. Suitable choice of the gain of proportionality yields dead-beat parameter estimation. Besides compensating for jet firing time mismatch, the control system is shown to be capable also of compensating for uncertainties in the thrust level for short turn-on times.

A gyro drift and dynamics state estimator for satellite attitude control systems utilizing a star mappert†

A recursive estimator is presented which, when given attitude measurements from a star mapper and rate integrating gyro, together with control signals, provides filtered estimates of gyro drift, disturbing angular acceleration, body angular velocity and attitude. Attention is restricted at present to satellites with substantially rigid body dynamics and negligible inter-axis coupling. A single axis attitude control system is considered, this being sufficient for demonstration of the principles. The estimator is based on the Kalman filter with the following developments ; (a) A low level fictitious covariance matrix element is introduced to facilitate estimation of the disturbing angular acceleration, and (b) the gain matrix is modified to enable the real-time model iterations and gyro sampling to be carried out at a much higher rate than the star mapper sampling. Furthermore, the part of the gain matrix relevant to the star mapper output is adapted to enable the correction to the model to be applied continuously, thereby avoiding stepping of the state estimate following new star mapper measurements. A form of the estimator is presented in which it is unnecessary to know the time to the next star mapper reading. Digital simulation results are given for a complete control system in which two control loops are completed by state-variable feedback, comprising a linear gyro-drift control law and a non-linear gas jet control law.

A Balloon-Borne Three Axis Stabilised Platform for Large Astronomy Experiments

A prototype 3-axis stabilised balloon platform has been developed by the Appleton Laboratory of the Science Research Council for the use of UK astronomy groups. The platform is designed to carry experiments weighing up to 500 kg at altitudes of about 40 km and point them with an accuracy of better than one minute of arc. It was successfully flown in September and October 1978.The main features of the platform are outlined and the position and stabilising sensors which use both terrestrial and celestial frames of reference are described.The paper discusses the problems specific to controlling a platform of this type when attached to a large balloon by some 100 m of flexible suspension. The difficulty of predicting or simulating the behaviour of such a suspension system has led to a design in which many of the control loop parameters may be adjusted by command from the ground. The design measures used to overcome such problems as suspension resonance, pendulum motion, bearing friction and inter-axis coupling are described.Preliminary results from ground tests and from the two flights which took place in Texas are presented and discussed and the paper concludes with a summary of achievements to date and suggestions for further improvements to the control system.

Interaxis coupling with reaction-wheel satellite attitude control

A nondimensional parameter is described that allows preliminary design estimates of interaxis coupling effects.

Observer-based Attitude Control In The Sliding Mode

With the increasing interest in small, relatively inexpensive satellites, envisaged to be produced on a large scale, there is a need to minimise development costs by embodying as many common elements as possible. The research presented here is aimed at the production of a 'universal' attitude controller in partial fulfilment of this need. Continuous momentum exchange torque actuators are assumed at present. Such a controller must maintain a specified dynamic response to changing attitude demands through a wide range of spacecraft dynamics parameters and in the presence of external disturbance torques. The sliding mode approach is pursued in view of its relative simplicity and effectiveness, especially when dealing with the continuous non-linearities of a) the kinematic differential equations for large angle slewing manoeuvres and b) the spacecraft dynamics when the angular velocities are sufficiently high for gyroscopic inter-axis coupling to be significant. Since the pointing accuracy of any attitude control system is ultimately limited by the accuracy of the attitude measurement system, a minimal requirement is on-board instrumentation for high precision measurement of attitude angles. To avoid the necessity of additional instrumentation for the measurement of other state variables such as angular accelerations, the control system is observer based. The sliding mode controller embodies two novel features: a) control smoothing integrators to eliminate actuator drive chatter and b) block control principle to facilitate straightforward design despite the non-linearities. The control system may be designed simply by eigenvalue assignment to yield a specified closed-loop dynamic performance. Three possible special observers Transactions on the Built Environment vol 19, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509 640 Structures in Space are investigated which are intended to be insensitive to spacecraft dynamics modelling errors, to be compatible with the control law The control system performance is demonstrated by simulation, including sensitivity to measurement noise.

On the synthesis of suboptimal, inertia-wheel attitude control systems

Two techniques are presented for the synthesis of suboptimal systems using motor-driven inertia wheels as the source of torque for three-dimensional attitude control. These techniques approximately minimize the integral of a quadratic function of system error and control effort, and both procedures compensate for non-linear inter-axis coupling. The techniques developed in this paper are applied to the design of attitude control systems for two typical artificial satellites. The resulting control laws are in feedback form. In a computer simulation, systems designed on the basis of the procedures developed are shown to respond faster and more accurately than those designed by optimization procedures based on linearized approximations of the equations of motion or by conventional transform methods.

Gyroscopic Coupling in Space Vehicle Attitude Control Systems

The problem of controlling the attitude of a space vehicle is unusual in several respects. While the required precision may be extreme—less than 0.1 second of arc in certain cases—the required response time may often be very slow, measured in minutes or hours. Vehicles weighing tons may have to be controlled by inch-ounces of torque, and control energy is at an extreme premium. The present paper discusses the effects, on performance, of inter-axis coupling due to internal spinning parts. A de-coupling computer to nullify gyroscopic torque is described, and its utility is evaluated. The computer is found to improve precision, but to reduce energy consumption only in certain cases. It is shown that by postulating such a computer the performance of a given system may be accurately evaluated on the basis of much simpler single-axis relations, even though strong coupling is present. Specifically: (1) It is shown that the best available performance is established by postulating de-coupling control; and (2) a method is given for determining the amount by which a conventional system will fail to achieve that performance.