ADCS testing part 1
ADCS: what is it?
ADCS stands for Attitude Determination and Control System. It is one of the most crucial subsystems in a spacecraft. Its role is to accurately compute the heading and other angles associated with the attitude, as well as direct them to the desired values. An example of its use would be in the Hubble Telescope, where the ADCS is in charge of pointing the spacecraft to the desired attitude where the star that it is to be studied is located.
What is its goal?
In our case the main goal of the ADCS is to determine and control the attitude of the Free Flying Unit (FFU), so that once it is ejected from the rocket, it will point in the opposite direction of the Sun, with a tolerance of around 30°. Afterwards, the boom will be deployed and the camera will have good lighting to record the events.
Figure 1: Diagram of the FFU showing the boom, cameras and RWs.
Reaction Wheel (RWs) and how do they work?
In order to change the attitude of the FFU, the ADCS counts three Reaction Wheels, one for each axis. The RWs are brass disks that have a high density. By accelerating and decelerating their rotations, it is possible to change the orientation of the FFU thanks to the principle of conservation of angular momentum.
Figure 2: Assembly of four RWs with their DC motors.
Figure 3: One of the first brass RWs.
Recently, new RWs were created so the first test of the ADCS was to make sure that the RWs were able to modify the angles of the FFU. For that, a basic debugger program was created that accelerates the RWs clockwise and counter-clockwise at different intervals. As it can be seen in the video below, the ADCS has a quick response and the RWs behave adequately. The first test was considered a success.
Second test and new problems
For the second test, the whole ADCS algorithm was to be studied. The video below shows the FFU rotating in the opposite direction of the light (that is, the +Z side, the one without sun sensors, pointing towards the left of the video). This test was not fully successful since some problems occurred with the sun sensors. This made the FFU rotate around the desired orientation somewhat erratically, instead of stopping once the condition was met.
We noticed then that the board used resistors that did not have the same values as the previous ones, and therefore the conversion of the values received by the sun sensors was not accurate. This will be solved in future testings after a new board is developed using the old resistors. Additionally, some sensors gave inadequate readings and therefore they will be changed and the calibration procedure will be redone.
To continue then…