Action and Reaction
Hej! Ready to learn something new from B2D2? Well, prepare yourselves for this fascinating and mind spinning post.
One of the B2D2 mission requirements is to de-tumble the FFU and point the cameras away from the sun so they can nicely record the boom deployment and we can share a high quality video with you. But the FFU lacks thrusters or propellers. So how can it achieve this? The answer: Wheels. But not the wheels that you can find in a car or a bicycle, but a special type called Reaction Wheels.
The Reaction wheels work by utilizing the principle of the conservation of angular momentum, so by spinning the wheels the momentum gets transferred to from the FFU to the reaction wheels. This makes sure that the total angular momentum of the system remains constant.
B2D2’s reaction wheels are made out from brass, with a thickness of 4 mm and 65 mm diameter. A brushless DC motor spins them. The wheels can spin up to around 1200 RPM and deliver a torque of 10 mN-m. The motor shaft is connected with a beveled 4:1 gearbox which is assembled to the reaction wheel. This system is placed inside an aluminum casing to provide mechanical stability.
Since we want to control the three FFU axes, three reaction wheels are required. Each with its spin axis aligned with a FFU body axis. The three wheels have their own motor which is controlled by a motor driver PCB. Except for the aluminum housing, the gears and the motor all the components are manufactured at KTH student workshop.
The command to the motors is given by the attitude control system, which takes the FFU attitude information from the sensors in the attitude determination system. The controller receives this data and compares it with a target reference, defined by the sun position. Then, the error is calculated and by means of a nonlinear control law, it is driven to zero. The controller outputs the required torque to drive such error to zero and sends it to the motor driver as a voltage input. The motors apply the corresponding torque and spin the wheels. The motion of the FFU is modified and a new attitude and rate information is gathered by the sensors and the process is repeated. This closed loop process continues until a desired tumbling rate and attitude is achieved. Currently, we are in the process of manufacturing and testing the sun sensors and the reaction wheel assembly-
That is it from us this week. To know more about the B2D2 experiment make sure to visit our website www.b2d2.se, also don’t forget to take action and leave a reaction in our Facebook and Instagram pages, as we constantly keep the wheel spinning by uploading interesting information about the experiment.