by afernan » Fri Mar 14, 2014 7:56 am
@Kisssys:
- Rotational stiffness: you´r right. One mode of vibration of arm will be torsion around the tube axis. Usually this number is higher compared with main bending mode. It will depend on the diameter and thickness of the tube, the cog of the motor wrt axis (the lower the better), and the mass of the motor. Section of the arm must be "closed" (circular, square, etc). Those plastic machined arms are very bad in torsion.
- rectangular section. This section has lower frequency in one transversal direction than the other, but not too different. For instance, an aluminum rectangular tube 12x10x1mm (I used a lot) has 37Hz and 41 respectively )for a 300mm arm, 100g motor): not a dramatic difference
- Q factor is so important like the frequency itself. CF has much higher Q than alu. A cantilever CF tube well clamped can have Q=25-35. Typically alu has Q=15. In the other side we want Q high to get narrow peaks. So it´s a compromise. Each bolted attachement decrease about 3dB the transmission. Anyway most of this factors are design drived so we can´t change. For instance, the motor bracket 3D printed I have in my "monoblock" should have high damping due to the load transmission is done thru a large friction surface.
-Attachments to plates. All affects to the frequency of the arm, in particular the clamping. I´ve tried to simplify the calculations suposing a "infinitely" rigid clamping, which is not the case. Reality will be a bit more flexible, depending of the blocks, CF plates, etc.The simplest way to design is to have motor-arm frequencies as "separated as posible" between them. Other designing way complicates innecesarely the situation.
- adding weight to the boom. Again, try to be simple! and choose very clear solutions. Yes if you add mass in the center of the boom will change the frequency of the arm, but also the "mode" wich is also very important. There are very effitient methods to reduce vibrations along the tube like introducing "bags of sand" or similar inside. We´ve used that solutiion in a real satelite with incredible results.
-outer ring. You said already: it complicates a lot. In fact only reduce(change) the vibration in the plane of the arms. Vertical vibration remain the same. I don´t like that.
- "advanced frame". Yes there is: thick (40mm) sanwich CF skins and Roacell (or any damping foam) as core. Skins can be CF very thin (0.5mm or so) and the whole thing, arms + central plates all in one single piece. Vibration damping is very good. I have one design (on paper) like that. The problem is to manufacture well and cheap, but some day I´ll try one!.
Hope I´ve answer your questions. Please, consider that this is never an absolute truth, and I can be wrong as any mortal!.
Angel