Rafal Wisniewski

Linear Time Varying Approach to Satellite Attitude Control Using only Electromagnetic Actuation

Recently, small satellite missions have gained considerable interest due to low-cost launch opportunities and technological improvement ofmicroelectronics. Required pointing accuracy of small, inexpensive satellites isoften relatively loose, within a couple of degrees. Application of cheap, lightweight, and power efe cient actuators is therefore crucial and viable. Linear attitude control strategies for a low-Earth-orbit satellite actuated by a set of mutually perpendicular electromagnetic coils are discussed. The principle is to use the interaction between the Earth’ s magnetic e eld and the magnetic e eld generated by the coils. A key challenge is that the mechanical torque can only be produced in a plane perpendicular to the local geomagnetic e eld vector; therefore, the satellite is not controllable when considered at e xed time. Availability of design methods for time-varying systems is limited; nevertheless, a solution of theperiodicRiccati equation gives an excellent framework fordevelopment and analysis of magnetic attitude control algorithms. An observation that thegeomagnetic e eld changesapproximately periodically when a satellite is on a near-polar orbit is used. Three types of attitude controllers are proposed: an ine nite horizon, a e nite horizon, and a constant gain controller. Their performance is evaluated and compared in the simulation study of the realistic environment.

Optimal magnetic attitude control

Abstract Magnetic torquing is attractive as means of control for small satellites. The actuation principle is to use the interaction between the earths magnetic field and a magnetic field generated by a coil set in the satellite. This control principle is inherently time-varying, and difficult to use because control torques can only be generated perpendicular to the local geomagnetic field vector. This has been a serious obstacle for using magnetorquer based control for three-axis attitude control. This paper deals with three-axis stabilization of a low earth orbit satellite. The problem of controlling the spacecraft attitude using only magnetic torquing is realized in the form of a infinite horizon periodic controller

Autonomous attitude determination and control system for the Orsted satellite

The Orsted satellite mission imposes comparatively high requirements on autonomy of the attitude control system. Cost requirements, on the other hand, impose simple hardware and cheap actuators in form of magnetorquer coils. These conflicting requirements are fulfilled through development of novel attitude and control algorithms and wide on-board autonomy. The entire control and attitude determination system has the ability to reconfigure in real time, based on mission phase and contingency operation requirements. Attitude determination embraces three different strategies, dependent on the availability of attitude sensors. Possible sensor faults are detected and a control system supervisor autonomously reconfigures attitude determination. Estimated satellite attitude and angular velocity are used in the attitude controller. Control tasks vary with the mission phase. Initially, after release from the launch vehicle, the angular velocity is controlled. In subsequent mission phases, the satellite is three-axis stabilized. The main contributions are development of novel algorithms for attitude control applying magnetic torquing, attitude determination schemes based on the geomagnetic field measurements, and integration into a supervisory control architecture. The salient feature of this system is fault tolerant autonomous operation with a minimum of hardware redundancy.

Periodic H-2 synthesis for spacecraft attitude control with magnetorquers

A control synthesis for a spacecraft equipped with a set of magnetorquer coils is addressed. The electromagnetic actuation is particularly attractive for small low-cost spacecraft missions, due to their relatively low price, high reliability, light weight, and low power consumption. The interaction between the Earth’s magnetic field and an artificial magnetic field generated by the coils produces a control torque. The magnetic attitude control is intrinsically periodic due to cyclic variation of the geomagnetic field in orbit. The control performance is specified by the generalized H2 operator norm. A linear matrix inequality-based algorithm is proposed for attitude control synthesis. Simulation results are provided, showing the prospect of the concept for onboard implementation. Nomenclature

Output Regulation of Large-Scale Hydraulic Networks

The problem of output regulation for a class of hydraulic networks found in district heating systems is addressed in this brief. The results show that global asymptotic and semiglobal exponential output regulation is achievable using a set of decentralized proportional-integral controllers. The fact that the result is global and independent of the number of end users has the consequence that structural changes such as end-user addition and removal can be made in the network while maintaining the stability properties of the system. Furthermore, the decentralized nature of the control architecture eases the implementation of structural changes in the network.

Gain-scheduled model predictive control of wind turbines using Laguerre functions

This paper presents a systematic approach to design gain-scheduled predictive controllers for wind turbines. The predictive control law is based on Laguerre functions to parameterize control signals and a parameter-dependent cost function that is analytically determined from turbine data. These properties facilitate the design of speed controllers by placement of the closed-loop poles (when constraints are not active) and systematic adaptation towards changes in the operating point. Vibration control of undamped modes is achieved by imposing a certain degree of stability to the closed-loop system. The approach can be utilized to the design of new controllers and to represent existing gain-scheduled controllers as predictive controllers. The numerical example and simulations illustrate the design of a speed controller augmented with active damping of the tower fore-aft displacement.

On Propagating Requirements and Selecting Fuels for a Benson Boiler

In this paper, the problem of optimal choice of sensors and actuators is addressed. Given a functional encapsulating information of the desired performance and production economy the objective is to choose a control instrumentation from a given set to comply with its minimum. The objective of the work is twofold: reformulation of the business objectives into mathematical terms and providing solution to the given optimization. Commonly, there exist overall business objectives which dictate how a plant should be instrumented and operated either directly or indirectly. The work shows how to propagate a global objective to local subsystems. Particular focus is on a boiler in a power plant operated by DONG Energy - a danish energy supplier. The business objectives have been propagated to the actuator level to allow for selection of an actuator configuration.

Rotational motion control of a spacecraft

This note describes a systematic procedure for the control synthesis of a rigid spacecraft using the energy shaping method. The geometric concept of a mechanical system in a coordinate-independent form is used to derive a control algorithm for the Euler-Poincare equations. The main result of this note is a specialization of the method on the unit quaternion group. This note is concluded with the examples of the potential functions and implementation for the three-axis attitude control problem.

Synchronization and desynchronizing control schemes for supermarket refrigeration systems

A supermarket refrigeration system is a hybrid system with switched nonlinear dynamics and discrete-valued input variables such as opening/closing of valves and start/stop of compressors. Practical and simulation studies have shown that the use of distributed hysteresis controllers to operate the valves leads to synchronization, meaning that the opening and closing actions of the valves coincide. Consequently, the compressor periodically has to work hard resulting in low efficiency, inferior control performance and a high wear on the compressor. In this paper we propose two control schemes of low complexity for desynchronizing the valve operations while improving performance. Simulation results indicate the potential increase in efficiency and reduction in wear comparing with traditional control schemes.

Model Reduction by Nice Selections for Linear Switched Systems

A moment-matching method for model reduction of linear switched systems (LSSs) is presented. The method can be seen as a non-trivial extension of the Krylov subspace methods for linear time-invariant (LTI) systems. The procedure is based on the so called “nice selections,” which represent a choice of basis in the reachability or observability space of the LSS. The framework can also be used for exact matching of the input-output behavior of an LSS with a reduced order LSS for a specific switching sequence. Conditions for applicability of the method for model reduction are stated and finally the results are illustrated on numerical examples.