How to find inflection point in graph when dx is very small
Hello,
I'm trying to sort through position data from a feedback pot to calculate linearity of movement as part of a testing procedure. An image of the data is attached. I want to delete all data in the 'flat' part, in order to do a linear fit and R^2 of the sloped line.
This is made trickier by the fact that the number of samples, the start and end points, and slope of the line can all vary considerably. Therefore I need some way to find the 'knee' in the graph and remove all subsequent samples from the data.
The most obvious method I tried was to use a derivative to find the inflection point, but since I have so many data points, the dx is very small
(~0.05) when using the built-in labview derivatives. This means that I can't distinguish the inflection point from any other change in the values due to noise or change in velocity. I made my own derivative function, which was a newton quotient that looked at (xi+N)-(xi-N) instead of (xi+1) - (xi-1), but this still did not give good results. My next idea is to just look at the difference of every N points, and arbitrarily decide a threshold to indicate when it has levelled off. Any other ideas would be really helpful though.
Thanks,
-SK-
Attachments:
position.PNG 17 KB
Lets see if I can answer without having to write a book.
Fisrt I'd run the data through a zero-phase shift low pass filter. Then look at the 2nd derivative to find the knee. Using a zero-pahse shift fileter I was able to detect when a value was jumping up when it started to jump rather than after. THis thread may be interesting.
Ben
Ben Rayner
I am currently active on.. MainStream Preppers
Rayner's Ridge is under construction
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