Valdemir Carrara

Estimating Friction Parameters in Reaction Wheels for Attitude Control

The ever-increasing use of artificial satellites in both the study of terrestrial and space phenomena demands a search for increasingly accurate and reliable pointing systems. It is common nowadays to employ reaction wheels for attitude control that provide wide range of torque magnitude, high reliability, and little power consumption. However, the bearing friction causes the response of wheel to be nonlinear, which may compromise the stability and precision of the control system as a whole. This work presents a characterization of a typical reaction wheel of 0.65 Nms maximum angular momentum storage, in order to estimate their friction parameters. It used a friction model that takes into account the Coulomb friction, viscous friction, and static friction, according to the Stribeck formulation. The parameters were estimated by means of a nonlinear batch least squares procedure, from data raised experimentally. The results have shown wide agreement with the experimental data and were also close to a deterministic model, previously obtained for this wheel. This model was then employed in a Dynamic Model Compensator (DMC) control, which successfully reduced the attitude steady state error of an instrumented one-axis air-bearing table.

Attitude Determination with Magnetometers and Accelerometers to Use in Satellite Simulator

Attitude control of artificial satellites is dependent on information provided by its attitude determination process. This paper presents the implementation and tests of a fully self-contained algorithm for the attitude determination using magnetometers and accelerometers, for application on a satellite simulator based on frictionless air bearing tables. However, it is known that magnetometers and accelerometers need to be calibrated so as to allow that measurements are used to their ultimate accuracy. A calibration method is implemented which proves to be essential for improving attitude determination accuracy. For the stepwise real-time attitude determination, it was used the well-known QUEST algorithm which yields quick response with reduced computer resources. The algorithms are tested and qualified with actual data collected on the streets under controlled situations. For such street runaways, the experiment employs a solid-state magnetoresistive magnetometer and an IMU navigation block consisting of triads of accelerometers and gyros, with MEMS technology. A GPS receiver is used to record positional information. The collected measurements are processed through the developed algorithms, and comparisons are made for attitude determination using calibrated and noncalibrated data. The results show that the attitude accuracy reaches the requirements for real-time operation for satellite simulator platforms.

Current and Speed Control Operating Modes of a Reaction Wheel

This paper presents some approaches to the design and some experimental results for current and speed control loops of a reaction wheel (RW). Reaction wheels are largely employed in satellite attitude control due to its large range of torque capability, small power consumption and high reliability. However, to achieve such performance the RW design shall deal with several restrictions, such as to support the space environment hazards (radiation, vacuum, high and fast temperature variation), and launch requirements (vibration, noise and choke). In this work some experimental results of an air-bearing table attitude control equipped with a Fiber Optics Gyro (FOG), a reaction wheel and a small fan will be presented. The RW is controlled by speed reference, and a second speed mode control similar to the first one was implemented in an external computer. Both were then compared by means of the air-bearing attitude control performance during the wheel zero-speed crossing. The results showed that the controllers have similar performance, as expected, and the maximum attitude pointing error remained below 0.08 degrees, which complies with the attitude requirements of Earth pointing satellites.

Estimation and Attitude Control in CONASAT's Nominal Operation Mode: An Approach for SDRE Filter and PID Control

CubeSat attracted interest of international community with numerous studies being developed in some universities, schools or even space enthusiastic. Miniaturization of components and use of conventional electronics reduced costs with space projects, that previously restricted access to space to only a few nations at costs of several million dollars. So, governments and space agencies, especially in developing nations, has begun to invest in small satellite projects, because now they can have space missions at low cost. Thus, the National Institute for Space Research (INPE), in Brazil, has planning some CubeSat missions as an academic tool to spread the space technology across the country to universities and researches. One of the missions in development is the Nanosatellite Constellation for Environmental Data Collection (CONASAT). This project intends to launch at least two small satellites to replace the SCD1 and SCD2 satellites of the Brazilians environmental data collection system. The main objective of this paper is to propose state estimation technique, known as State-Dependent Riccati Equation (SDRE), together with a PID controller based on attitude error given by the Euler angle and axis for nominal mode attitude control of CONASAT. Simulations were performed using an open computational tool for attitude and orbit simulation.

Modelo matemático de roda de reação à baixa velocidade, estimação de parâmetros e controle de atitude de mesa de mancal a ar com roda

Neste trabalho e realizado uma caracterizacao, estimacao de parâmetros e do modelo matematico de uma roda de reacao de uma mesa de mancal a ar. A dinâmica nao linear da roda sujeita a baixas velocidades, e analisada e metodos de compensacao de corrente e velocidade angular sao propostos a fim de linearizar o modelo da roda. Um controle classico PID e proposto e projetado tanto manualmente como atraves de sintonizacao por Ziegler-Nichols e suas performances sao comparadas.

Projeto, implementação e controle de uma plataforma aerostática com jatos de ar

Este trabalho tem por objetivo o projeto, implementacao e controle de uma plataforma sustentada por mancal aerostatico usando jatos de ar comprimido, a fim de simular o controle de atitude de satelites artificiais empregando atuadores de jatos de gas. Para simular o ambiente espacial, o sistema de controle foi implantado numa mesa (plataforma) de mancal aerostatico, a qual permite simular um ambiente de baixo atrito, semelhante aquele encontrado nas orbitas de satelites. Neste trabalho sao descritos os procedimentos de projeto, os requisitos, a plataforma aerostatica, o esquema de controle e comunicacao, o balanceamento estatico, o modelamento matematico, bem como se discorre sobre simulacoes realizadas de controle tipo bang-bang. Este projeto visa conseguir um ambiente experimental no qual diversas leis de controle baseadas em jatos de gas e utilizando diversos sensores possam ser testadas, tanto para servir como prototipo a um sistema real embarcado em satelites quanto para experimentacoes de novas tecnicas de controle, alem de uso academico.