Systemic disturbances in the design of a Reaction Wheel System

[RicardoPereiraGomes]

I'm working on an university project to developed a reaction wheel for a cubesat.

After defining that the minimum generated torque for the RWS is 8.8e-6 Nm and that the reaction wheels shall be able to hold an angular momentum of 4.82e-4 Nms (around the Z axis), I start computing the systemic disturbances due to the axial shaft play of the motor and the manufacturing error of the flywheel. After making some calculation I reach the conclusion that this perturbations are in the same order of magnitude as the required torque. Thus, this I've created the following upper and lower bounds in the M-n graph (Fig1) for each reaction wheel. Nevertheless, they are periodic and well defined (torque generated in the X - Y direction - Fig2), but since i`m dealing with perturbations in the same order of magnitude it will be difficult to implement a controller to deal with? If I double integrate the systemic disturbance torque during half a period I conclude that the satellite will oscillate around 4.6e-6 deg (which for our mission can be neglected). Otherwise, the sensors we are going to use have a white noise some orders of magnitude smaller than 4.5e-6 deg, thus this error due to the axial shaft play can not be neglected? Moreover, if the sampling frequency of the sensors are 1/2 smaller than frequency of the disturbance this can also be neglected, correct? I do not know yet the sensors characteristics. I really would like to check if I'm being logic and if you have any ideas about how can I start defining the controller for the motor.


Thanks,
Ricardo

 

[Sparweb]

I'm sorry Ricardo, but posting problems as a student is not allowed, here.

Follow this link to read the site rules, please:

http://www.eng-tips.com/market.cfm

I do, however, support cubesat projects and find them interesting. As a result, I also know that they are frequently designed by students, who, if they are lucky, and create good designs, can go on to build the real thing and see it launched. You should find an adviser or a professor familiar with mechanics to review your design ideas and/or check your analysis.

Without red-flagging your posting, yet, I will limit myself to saying that you probably haven't selected a strict enough manufacturing and mounting system for the reaction wheel, if the alignment errors and wheel imbalances are equal to or greater than the desired reaction impulses.

STF