Wave-Based Control (WBC)
WBC is an attitude control experiment or “software payload” onboard EIRSAT-1.
Attitude is another name for the orientation of the satellite in space (the direction it is pointing). This is usually expressed either as three angles (roll, pitch and yaw) or as a special type of number called a quaternion*.
The Attitude Determination and Control System of the satellite (or ADCS for short) has two functions:
Using onboard sensors, to calculate the spin rates and current attitude of the satellite (the determination part).
With this information, to drive the satellite’s actuators, so that it points in the desired direction (the control part).
EIRSAT-1 has several sensors that assist with attitude determination including 5 coarse sun sensors, 1 fine sun sensor, magnetometers, rate gyroscopes and a GPS receiver. Data from these sensors is periodically collected by the ADCS motherboard (computer). These are then passed through a control algorithm to calculate the output for the satellite actuators.
*The use of quaternions to represent the attitude of EIRSAT-1 is particularly fitting as they were invented by William Rowan Hamilton, an Irish mathematician, astronomer and physicist. In 1843 as he walked along the Royal Canal, the concept was taking shape in his mind and he carved the formula for the quaternions into the stone of Brougham Bridge in Cabra, Co. Dublin.
Attitude Determination and Control
What is “Magnetic Attitude Control”?
EIRSAT-1 uses magnetic actuation. By passing controlled electric currents through several coils onboard the satellite a magnetic dipole can be created. In orbit, this magnetic dipole creates a torque that rotates the satellite to align with the Earth’s magnetic field, similar to the torque that rotates the needle of a compass. By changing the onboard dipole, this interaction with the Earth’s magnetic field can be used to control how the satellite rotates.
Magnetic attitude control is challenging as the torques created are quite small and the directions of those torques are limited by the direction of the local magnetic field. However, when fast manoeuvres aren’t required, magnetic actuation can provide effective attitude control using low power and mass with zero moving parts.
What is 'Wave-Based Control'?
For several years, the Dynamics and Control Group in the UCD School of Mechanical and Materials Engineering, led by Asst. Prof. David McKeown and Assoc. Prof. Willi O'Connor, has been researching novel control algorithms for both position and attitude control of underactuated and flexible systems. In particular a method called Wave-Based Control or WBC has been applied to flexible robot arms, cranes and large space structures. Most recently it has been applied in simulation to launch vehicles (rockets) with flexible structures and onboard fuel sloshing. This work was part of a contract on ESA’s Future Launchers Preparatory Programme (FLPP). The WBC payload, taking its name from this algorithm, will serve as a testbed where novel magnetic attitude control algorithms can be uploaded and tested on orbit. To test the ADCS system of EIRSAT-1 and the WBC payload, a new test facility has been built in a clean room in UCD Engineering. This includes a Helmholtz cage for simulation of the Earth’s magnetic field, a sun simulator lamp and a turntable to simulate the rotation of the satellite.