Willem H. Steyn

Attitude control for small satellites using control moment gyros

Abstract In this paper a new Attitude Control System is proposed, based on Control Moment Gyroscopes (CMG). These actuators can provide unique torque, angular momentum and slew rate capabilities to small satellites without any increase in power, mass or volume. This will help small satellites become more agile. Agility considerably increases the operational envelope and efficiency of spacecraft and substantially increases the return of earth and science mission data. In the proceeding sections the fundamental features of CMGs are presented. A low cost, miniature SGCMG designed for an enhanced microsatellite is analysed. Sizing of a proposed SGCMG indicates the advantages of using CMGs. The SGCMG is able to produce a torque of 9.82 mNm and this is confirmed through experiments performed on an air-bearing table. A comparison between the SGCMG and a RW demonstrates the mass and power savings that can be gained by using CMGs.

A new unified approach to power quality management

A new control algorithm for a power converter-based device that is capable of alleviating the problems of harmonic interference and voltage regulation on radial distribution lines is introduced. It comprises a relocatable converter in series with a passive filter and is known as the Power Quality Manager. Simulation results based on an existing 88-kV line are presented. Experimental results are presented, based on a three-phase 200-VA scaled model of the existing 88-kV system. It is a cost-effective and flexible solution to improving power quality.

Design and testing of a control moment gyroscope cluster for small satellites

Experimental results on the performance of a control moment gyroscope cluster are presented. The goal is to design and evaluate a control moment gyroscope cluster for three-axis control for agile small satellites. The experimental data are compared with simulation (theoretical) results and both are used to verify the principles, advantages, and performance specifications of a control moment gyroscope cluster for a small satellite, in a practical way. Control moment gyroscope systems are considered in the literature to be more efficient devices, from an electrical power point of view, than current actuators such as reaction/momentum wheels. Experimental measurements are presented and then compared to two reaction wheels of different size. Control moment gyroscopes are shown to have a potential performance advantage over reaction/momentum wheels for spacecraft with agile requirements.

Near-Minimum-Time Eigenaxis Rotation Maneuvers Using Reaction Wheels

A near-minimum-t ime (near-maximum-acceleration/deceleration) control technique is presented to rotate a spacecraft around its eigenaxis (shortest angular path). Practical control constraints such as wheel torque and wheel speed are also taken into account. In this paper a nominally nadir pointing satellite in a circular low Earth orbit under passive gravity gradient stabilization and active reaction wheel control will be used as an example. A standard linear feedback quaternion regulator will be used to three-axis stabilize the spacecraft before and after each large angular rotation. Robustness against inertia modeling errors is ensured by tracking the reference maneuver. The algorithm presented can easily be adapted to be used on inertially stabilized spacecraft.

Comparison of low-earth-orbit satellite attitude controllers submitted to controllability constraints

A rule-based fuzzy controller is presented and compared with an adaptive MIMO LQR controller in a low-Earth-orbit small satellite attitude control system. The attitude is passively gravity gradient stabilized and actively three-axis magnetorquer controlled. This method insures an Earth-pointing satellite making use of a nondepletable and nonmechanical means of control. A realistic simulation environment, using a nonlinear satellite dynamic model with linear attitude estimators plus sensor measurement noise and external disturbance torques, was used to evaluate the different control techniques.

Fuzzy control for a non-linear MIMO plant subject to control constraints

A rule based fuzzy controller is presented for attitude stabilization of a small satellite in low earth orbit. Active control Is done by using 3-axis magneto-torquing. A preferred torque vector can not always be generated from the magnetic coils in certain regions of the orbit, due to an unfavourable direction of the geomagnetic field vector. The main objective of an acceptable control algorithm will then be to optimize the control effort: minimize the desired influence on the controlled axis and minimize the undesired disturbances to the other axes. It was found that a fuzzy controller can achieve these goals in a computationally effective way, compared to other more involved MIMO algorithms, while performing just as well or better under similar conditions. This fuzzy controller solves the control constraint problem by choosing the best magneto-torquer, the polarity and switching instances. >

Robust defocus blur identification in the context of blind image quality assessment

A defocus blur metric for use in blind image quality assessment is proposed. Blind image deconvolution methods are used to determine the metric. Existing direct deconvolution methods based on the cepstrum, bicepstrum and on a spectral subtraction technique are compared across 210 images. A variation of the spectral subtraction method, based on a power spectrum surface of revolution, is proposed and is found to compare favourably with existing direct deconvolution methods for defocus blur identification. The method is found to be especially useful when distinguishing between in-focus and out-of-focus images.

An optimal image transform for threshold-based cloud detection using heteroscedastic discriminant analysis

We present a simple image transform that optimally combines four image channels into a greyscale image for threshold-based cloud detection. These image channels, namely blue, green, red and near infrared, are present on many low Earth-orbit resource satellites. Applying a single threshold to a greyscale image is a computationally efficient method suitable for onboard implementation. We used heteroscedastic discriminant analysis (HDA), which is a generalization of the popular dimension-reducing linear discriminant analysis, to transform the image. Comparative tests between HDA, existing transforms from the remote-sensing literature (the haze optimized and D transforms), as well as the single red and blue image channels were conducted. Although thin clouds remain challenging for global threshold-based techniques, the HDA transform consistently gave the best average segmentation errors across the test dataset. This dataset consisted of 32 1 megapixel Quickbird and Landsat images. HDA has not previously been applied to remote-sensing data.

GROUND-TARGET TRACKING CONTROL OF EARTH-POINTING SATELLITES

Many on-board payloads for monitoring, communication and Earth imaging have extensive requirements for target tracking maneuvers in many space missions. A quaternion-bas ed PID feedback control for ground-target tracking of a three-axis stabilized Earth-pointing satellite is systematically analyzed. Based upon the orbital measurement of the satellite position and velocity by an accurate GPS receiver, we present a general method to compute the quaternion error and its integral error with respect to the commanded ground target for any selected pointing axis of the satellite. In addition, an optimal way to derive the desired angular velocity reference for tracking is discussed in detail. Three-axis reaction wheels are employed to demonstrate the feasibility of this control algorithm on a imaginary low-Eartheccentric-orbit satellite. A special tracking scheme for the pointing axis along the body z-axis of the satellite was also investigated. This scenario can be readily applied in real-time for practical target tracking with high accuracy.

Arcminute Attitude Estimation for CubeSats with a Novel Nano Star Tracker

Abstract The next generation of CubeSats will require accurate attitude knowledge throughout orbit for advanced science payloads and high gain antennas. A star tracker can provide the required performance, but star trackers have traditionally been too large, expensive and power hungry to be included on a CubeSat. This paper briefly surveys the state of current CubeSat ADCS systems and goes on to describe the development of a CubeSat-compatible star tracker. Algorithms for star detection, matching and attitude determination were investigated and implemented on a CubeSat-compatible embedded system. The resultant star tracker, named CubeStar, is one of the smallest star trackers in existence. It can operate fully autonomously, outputting attitude estimates at a rate of 1 Hz. An engineering model was completed and demonstrated an accuracy of better than 0.01 degrees during night sky tests.