NUTS - NTNU Test Satellite, a Norwegian CubeSat Project

Developing a pointing mechanism for NUTS

Written by: √ėyvind Rein
Published: 16.10.13

The scientific goal of the NUTS project is to take images of Earth with an infrared camera. In order to do that it is necessary to have some mechanism that can change the attitude of the satellite, e.g. the direction it is pointing. A system like that is often referred to as an Altitude Determination and Control Subsystem, or ADCS for short. 

Development of such a system requires knowledge in several topics. Ranging from building circuits and programing control algorithms to modeling air drag, solar wind, gravity gradients and other disturbances that are present in the harsh space environment.  Another important role of the ADCS is to make sure that none of the satellite’s sides are overheated. If this will be solved by a slow rotation around an axis, or by a different more systematic approach, is yet to be decided, just like many other aspects of the NUTS project.

The ADCS consists of some key parts:

-          Magnetorquers: Three coils that interacts with Earth’s magnetic field and create torque.

-          Sun sensors: Determine the Suns direction relative to the satellite body.

-          3D Compass sensor: Get a feeling of where Earth is by measuring the magnetic field

-          Gyroscopes: Useful in absence of either sun sensor or compass sensor, which will happen at certain times.

-          Microcontroller: Maintain desired attitude using estimation and control algorithms.

-          ADCS module: A printed circuit board (PBC) that hosts the microcontroller, compass and gyroscope as well       as an interface to the sun sensors and a driver for the coils

hw_diagram

A lot of people have been doing research on the ADCS up to this point, either through master theses, projects or as volunteers. Estimation methods have been investigated, different control algorithms have been suggested and simulated, magnetorquers have been demonstrated, and much more.

During this fall we hope to, with the previous work as guidance, create a complete prototype of the ADCS hardware which includes all the parts mentioned above. So far the specifications for the magnetorquers have been revised with respect to wire max-current, physical volume and power consumption. The resulting coils are more balanced. The small coil (z-axis) can now produce almost the same amount of torque that the large ones can (x and y-axis).

The following weeks we will be doing some testing of different photodiodes and figure out how accurate they can measure an angle of a single light source and if they can work as a sun sensor. The testing will be conducted in one of NTNU’s acoustic halls that has a motor driven rotating rig. If the results are unsatisfactory we must go back to the drawing board and investigate other ways of measuring the Sun angle.

 
fotodiode-krets 
Early prototype of the Sun Sensor Driver

Since all parts, except the sun sensors, have been decided for the prototype, the next step will be to start designing the ADCS module. There still remains some issues related to design of the magnetorque driver that needs to be solved. Specifically how to deal with the time delay in a coil when using pulse width modulation.

In the end, a simple piece of software should be written in order to test the new hardware and confirm that it is working as expected.