Challenge
The AOCS must continuously adjust the Solar Orbiter spacecraft’s attitude so that the solar shield provides maximum protection as the spacecraft passes close to the sun. For safety reasons, the AOCS cannot allow the spacecraft to depoint more than 6.5 degrees from the Sun at any time, even after a failure. During scientific observations, pointing stability must be within a few tenths of an arcsecond.
In addition to meeting these requirements, the AOCS had to account for disturbance torques from solar radiation pressure, gravity gradient, and aerodynamic forces.
The spacecraft’s physical structure compounded the AOCS design challenge. The solar shield contributed to an unusual mass distribution that made stability a challenge. In addition, multiple flexible appendages—including solar arrays—made the entire structure susceptible to resonance.
Tessella engineers would need to design control and estimation algorithms that accounted for all these factors while meeting the performance requirements and ECSS-stated common values for stability margins, as well as accommodating changes to hardware specifications and requirements
Solution
Tessella engineers used Model-Based Design to design, model, simulate, and perform preliminary tuning of the algorithms, and prove their suitability for formal coding and verification.
Working in Simulink, the team modeled the spacecraft’s actuation systems, including its four reaction wheels and chemical propulsion thrusters. To provide fine-grained control of the thrusters, the team developed and modeled an actuator commanding algorithm using pulse-width modulation.
The engineers used Optimization Toolbox™ to optimize this thruster commanding algorithm and minimize propellant consumption while operating within the physical constraints of the thrusters.
They modeled the spacecraft dynamics and its approximately 40 flexible modes as a mass-spring-damper system. Using Control System Toolbox™, they created a state-space model that enabled them to quickly determine the frequency response of the system and avoid resonances.
The engineers used Signal Processing Toolbox™ to analyze the frequency spectrum of the spacecraft rates. They used the analysis results to avoid frequencies that could excite resonant modes and to design low-pass filters that limited the energy fed into these frequencies.
The spacecraft sensors, including the sun sensor, star tracker, and gyroscopes, were modeled in Simulink. The team designed and modeled estimators that calculate the spacecraft’s attitude and angular rates based on the noisy data obtained from these sensors.
Continuing to work in Simulink, the team designed and modeled the main AOCS control algorithms. These algorithms generate demands for spacecraft accelerations based on input received from the estimators and the spacecraft’s guidance system while accounting for solar radiation pressure and other disturbances.
Using Simulink Check™ and Simulink Coverage™, the team checked compliance with modeling standards and measured model coverage.
The engineers used a continuous integration server to automate an extensive suite of unit tests, long-duration simulations, and parameter-sweep analyses in Simulink and MATLAB.
Results from the simulations and analyses were used to automatically update data tables and figures in Microsoft® Word® documents that served as the formal AOCS software specification.
The implementation of the AOCS flight software is currently undergoing testing, and is on track to meet the Solar Orbiter mission’s target launch date.
Result
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ECSS compliance demonstrated. “MathWorks tools helped us greatly in demonstrating compliance with the stability margin requirements of ECSS standards, including ECSS-E-60A,” says Pollard. “I don’t know of any other platform that offers the features we needed.”
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Complex analysis completed on schedule. “The level of analysis required on the AOCS project and the number of challenges that we had to address were much higher than on any previous mission we worked on,” Pollard says. “MATLAB and Simulink were key to enabling us to complete it all on a compressed time scale.”
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Models reused on follow-on projects, cutting design effort by up to 80%. “We are reusing parts of the Simulink model we developed for the Solar Orbiter mission on a project to model complex spacecraft dynamics around small solar system bodies for a UK Space Agency study, again working in collaboration with Airbus,” says Pollard. “Without model reuse, the project would have taken at least five times more effort, and we probably would not have been able to perform the work within the constraints of the project.”
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