| ??? 07/28/09 15:57 Modified: 07/28/09 16:07 Read: times |
#167967 - Upward and downward... Responding to: ???'s previous message |
Farshid said:
Ahh, My bad! By the 'g' above the numbers I meant the unit not the number power 'g'. (I had a teacher which used to write the units above the number and I got used to it). Well, the whole world once learned from you the "zero", so it might be over-hasty to say anything bad about this notation... To your schemes: 
Take care with the words "upward acceleration" and "downward acceleration". It's correct, if you mean the additional acceleration vector also acting on the reference mass. But the method to make the reference mass feel this acceleration is quite opposite! To make the reference mass feel a downward acceleration of 2g you must accelerate the rocket upwards by 1g! This comes from the effect, that not the reference mass is feeling this additional acceleration directly, but the frame where the spring is mounted. According to Newton's third law of inertia the reference mass feels not the "action" but the "reaction" of this acceleration, which is of opposite sign. The gravitational g, on the other hand, is acting directly on the reference mass. Farshid said:
I don't understand the third way that you suggested as a solution above.
- If I want to place them symmetrically then I can solder one on the top layer and one right below that one on the bottom layer. This way I have both of the 'X' axes pointing to one direction, but 'y' axes will be pointing in opposing directions (left and right)! - Its possible to place them on top of each other so 'x' axes point to one direction and 'y' axes to one other direction. But this no longer would be symmetrical. Well, I didn't propose this way actually. I tried to find out whether there's an arrangement at all that would profit from two acceleration sensors in series, in the hope to find out what the guys in your link were doing. I meant an arrangement like this: 
There are two green acceleration sensors measuring the roll angle. Each individual sensor would not only measure the corresponding component of g vector to find out the roll angle against horizon, but also centrifugal acceleration acting on the reference mass, when the plane is rolling arround the rolling axis. As this centrifugal acceleration is of opposite sign in the two green sensors, summing up their signals would cancel centrifugal acceleration. If you want to subtract the signals of two corresponding ADXL202 acceleration sensors you could use a scheme like this: 
To get the additional centrifugal acceleration, you would need to also read out at least one individual ADXL202 by the micro, though. Well, additionally measuring the centrifugal acceleration isn't probably of much interest, so take only one ADXL202, put it at the center of mass, read out the x and y signal and compare with the g vector to find out the roll and pitch tilt angles. The ADXL202 mounted at the center of gravity is immune against centrifugal accelerations anyway. Why the authors have used two ADXL202 they don't even know either probably... Kai |
