CubeSat developments at KU Leuven (Part 2)
In a previous blog post, we explained that we are developing an attitude determination and control system. This is the system that controls the orientation of the CubeSat, so that it's payload - such as a camera e.g. - can be pointed to a point of interest. The special thing about our system is that it will increase the accuracy and agility with which the orientation can typically be controlled. Let us tell you how we do that.
To point your CubeSat, you need to be able to do two main things:
- Find out what the current orientation of the CubeSat is. (Determination)
- Change the orientation of the CubeSat. (Control)
Determining the orientation of the CubeSat can be done using a wide variety of sensors. These all measure 'something' and try to determine the attitude of the spacecraft using that information. The most intuitive one might be a magnetometer. This sensor measures the magnetic field that surrounds the Earth and this way tells us what the direction of that field is. In essence, this is very simple to a device we all know: a compass. A second sensor, called the sun sensor, is sensitive to light and gives us the direction towards the sun. Because we have models that tell us where the sun is located at specific times in space, we can get a second direction out of this information. These are the typical sensors that are used in a CubeSat. They consume little power, are simple and cheap.
We add a very powerful sensor to this combination: the Star Tracker. This is a camera that takes a picture of the stars and has algorithms on board to identify those stars. Using the knowledge of the star positions, we can then very accurately determine the orientation of the CubeSat. This sensor is more complex and expensive, but it can increase the accuracy of the spacecraft attitude determination with a factor of 10! At KU Leuven, we have developed new and efficient algorithms which make the star tracker less complex and expensive, while keeping that great accuracy!
Controlling the orientation of the CubeSat is typically done using magnetorquers. These are magnets that can be turned on or off. By actuating them in the right sequence and on the correct times, these magnets can interact with the magnetic field of the Earth to change the orientation. This kind of control does not give a lot of flexibility and does not let you change your orientation accurately or with high agility.
That is why we add three actuators which do offer that agility and accuracy. We have developed reaction wheels that can control the attitude of the spacecraft. These reaction wheels contain a flywheel that can be rotated using an electric motor. When one of the wheels is spun up, the satellite starts spinning the other direction. This is a principle called conservation of angular momentum. With three wheels, the CubeSat attitude can be controlled in three axis.
The combination of these actuators and sensors will allow us to control the attitude of the spacecraft with high accuracy and agility. This will lead to better scientific measurements! CubeSats will from then on also be able to support more demanding missions!