TiltX

Feature extraction from mobile phone motion sensor data

This repository implements a method which extracts some of the key features from mobile phone motion sensor data.

At t=0 a visual stimulus appears on the screen. As the response, a participant (person holding the phone) is required to tilt the phone in a specific direction (left, right, up, or down). Once the roll or picth angle reaches 0.5 radians (t=T>0), the movement is completed. T is the event time registered by the app. The rotaion along the third axis (yaw) does not affect the outcome of the task, hence it is neglected.

The features extracted by TiltX are: reaction time, L1 norms, maximal deviation, number of turning points, and sample entropy.

Usage

These steps introduce how to use TiltX.

Input data

The DataGenerator is a utility class that generates sample datasets.

from tiltx.data_generator import DataGenerator
t, alpha, beta = DataGenerator.example(1)

Note: t, alpha, and beta need to be equal length arrays.

Extract features

The FeatureExtractor extracts key features from the data.

from tiltx.feature_extractor import FeatureExtractor
features = FeatureExtractor(t,alpha,beta,RT_method='cumsum',correct='up')

Note: cumsum is a method for reaction time detection and correct is the direction in which the phone should be tilted in order to successfully complete the task.

print('Reaction time: %.3f'%(features.RT))
print('L1-norm of the triggering axis: %.3f'%(features.L1_x))
print('L1-norm of the remaining (non-triggering) axis: %.3f'%(features.L1_y))

Reaction time: 0.669
L1-norm of the triggering axis: 0.492
L1-norm of the remaining (non-triggering) axis: 0.006

Visualize

By calling plot() function in FeatureExtractor, we plot the roll and pitch angles from the moment the stimulus appears till the moment when the app registers the move as completed.

features.plot()

• x is a triggering angle (the one which reaches 0.5 radians first)
• y is the remaining (non-triggering) angle
• the shaded regions are proportional to L1 norms
• the black dotted line marks the begining of the reaction

Documentation

For a tutorial and a brief note on signal features, check out the notebooks folder.