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+--- Thread: Change of the sampling rate of the experiment (/showthread.php?tid=403)
Change of the sampling rate of the experiment - xsherlock - 09-27-2019
Hi,
I' would like to use the inclinometer experiment (upright tab) to measure the sway of the mast/tree in heavy wind.
However it only updates @ 2Hz.
Is there a way to change that to be more responsive , if I see raw data from the sensor they arrive at 18 Hz.
TIA
Maciej
RE: Change of the sampling rate of the experiment - Sebastian Staacks - 09-27-2019
You can open the experiment in our editor (https://phyphox.org/editor) and change it to your needs. In this case, hit "Load" in the bottom right corner and select the inclination experiment. On the main tab, you need to change the value for "sleep" to the desired interval (for example 0.1s for 10 Hz) and on the input tab, you need to change the sensor rate accordingly (for example 10 Hz). Then go to the download tab, generate an online QR code and scan it with the app.
However, you should be aware of how the inclination is determined. This uses the accelerometer and just calculates the angle of earth's acceleration. Any actual acceleration (of a swaying tree or from vibrations) is indistinguishable from earth's acceleration and will lead to different angles. Just shake the phone without rotating it to see this. I am not saying that this is not usable, but you should be careful when interpreting the data as this method to determine an angle is only 100% correct if the phone is resting.
RE: Change of the sampling rate of the experiment - xsherlock - 09-27-2019
Many thanks I got that running in no time @ 10hz.!!!!
And I have to admit that I'm not sure if I understand your precaution as when I firmly attach a phone to the tree then the only movement of the phone comes from the sway of the tree, is that not a valid andle to find a maximum angle change?
There is no up/down movement of the tree. just a left right swing as i try to mount it perpendicular to the wind direction that is very predictable in that location
If we make such a an assumption would that give any meaningful results?
RE: Change of the sampling rate of the experiment - Sebastian Staacks - 09-30-2019
I have to admit that I do not have a feel for the acceleration of the tree doing a swaying motion, so I don't have an estimate of how bad it is. But the inclination is calculated from all axes of the acceleration. Let me give an example:
Let's assume that the y axis is pointing upwards along the tree (that's probably the most natural way to mount a phone, but any other orientation would lead to the same) and that the wind is blowing along the z direction (perpendicular to the screen). Then the tree would sway around the x axis.
When the tree is perfectly upright, phyphox should see an acceleration of 9.81m/s² along y and nothing along x or z. If the tree is bent to an angle of 10°, we will recognize this because earth's acceleration is no longer pointing perfectly along y, but there is also a small component along z as well. The ratio of the contribution along y over the contribution along z the tangent of the angle, so phyphox will calculate atan(y/z) to get that angle. That is 1.7m/s² along z and 9.66m/s² along y to get atan(y/z) = 10°.
Now, if the tree is not simply bent that way, but actually swaying (and the phone is swaying with it), there is an acceleration at the reversal points of the swaying motion, which also contributes to the measured acceleration along z. So, if there is an acceleration of only 0.5m/s² (again, I don't have a feel for the numbers here, so I am not sure if this is an appropriate estimate), phyphox will see 2.2m/s² along z instead and will get atan(y/z) = 13°.
How bad this error gets, depends on how much acceleration you get from the swaying motion. You can get a good estimate if you assume a harmonic oscillation and put in the swaying period and the distance over which the tree sways and if it is significant, this info can also be used for a correction. Alternatively, it might be a good idea, to simply try it. Capture the swaying on camera while measuring it in the wind and then bent the tree to the same orientation while holding it there to measure again without swaying (well, of course it depends on the kind of tree if this is possible - I'm obviously thinking of small ones here).