Signal segmentation into states ( Stop/mov)
It seems like you already got all the right components. Your goal is to detect segments where speed!=0 in the data. Since speed is the derivative of position, you could transform the data into the first difference series - simply subtract the preceding point from each data point, i.e. $x_t - x_{t-1}$, and you'll have a crude estimate of the derivative.
The series of differences then can be segmented with any standard method for peak or changepoint detection. In particular, since you know the background value (zero), and the speed appears roughly constant during each movement episode, it seems that epidemic detection of changes in mean should be able to solve this nicely. I am aware of a couple packages for this in R, but it's also relatively easy to implement it yourself if it's not available for your programming language (just search for &quot;epidemic changepoint detection&quot;). Multivariate detectors in the same framework are also available, if you were interested in incorporating several sensors to detect movement, for example.