Table 2 List of extracted features from each sensor modality

Mean (µ)

\(\overline{s} = \frac{1}{N}\sum\nolimits_{i = \,1}^{N} {s_{i} }\)

Root mean square (\(R_{ms}\))

\(rms = \sqrt {\frac{1}{n}} \sum\nolimits_{i = 1}^{N} {\left( {s_{i} } \right)}^{2}\)

Median (\(M_{e}\))

\(median_{i} \left( {s_{i} } \right)\)

Peak amplitude (\(\,P_{a}\))

\({ \hbox{max} }(s_{i} ) - { \hbox{min} }(s_{i} )\)

Maximum (\(\,M_{a}\))

\({ \hbox{max} }_{i} \left( {s_{i} } \right)\)

Pitch angle (\(\,P_{k}\))

\(\arctan \left( {\frac{{x_{i} }}{{\sqrt {y^{2} + x_{i}^{2} } }}} \right)\)

Minimum (\(\,M_{i}\))

\({ \hbox{min} }_{i} \left( {s_{i} } \right)\)

Signal power (\(\,S_{p}\))

\(\sum\nolimits_{i = 1}^{N} {s_{i}^{2} }\)

Harmonic mean (\(H_{m}\))

\(\frac{1}{N}\sum\nolimits_{i = 1}^{n} {\frac{1}{{s_{i} }}}\)

Kurtosis (\(\,K_{r}\))

\(E\left[ {\left( {s_{i} - \overline{s} } \right)^{4} } \right]/E\left[ {\left( {s_{i} - \overline{s} } \right)^{2} } \right]^{2}\)

Standard deviation (\(\,\sigma\))

\(\sigma \, = \,\sqrt {\frac{1}{N}} \sum\nolimits_{i = 1}^{N} {\left( {s_{i} - \mathop s\limits^{\_} } \right)}^{2}\)

Skewness (\(\,S_{k}\))

\(E\left[ {\left( {\frac{{s_{i} - \overline{s} }}{\sigma }} \right)^{3} } \right]\)

Variance (\(\,\sigma^{2}\))

\(\sigma^{2} \, = \,\frac{{\sum\nolimits_{{}}^{{}} {\left( {s_{i} - \overline{s} } \right)^{2} } }}{N}\)

Energy (\(\,E\))

\(\frac{{\sum\nolimits_{i = 1}^{N} {\left[ {s_{i} } \right]^{2} } }}{{length(s_{i} )}}\)

Coefficient of variation (\(\,C_{v}\))

\(\,\frac{{\sigma_{si} }}{{\mu_{si} }}\)

Entropy (\(\,H\))

\(\frac{{ - \sum\nolimits_{i = 1}^{N} {\left[ {S_{i} } \right]} \log \left[ {S_{i} } \right]}}{{length(S_{i} )}}\)

Interquartile range (\(\,I_{r}\))

\(Q_{3} (s_{i} ) - Q_{1} (s_{i} )\)

Mean frequency (µF)

\({{\sum\nolimits_{i = 1}^{N} {\left( {is_{i} (F)} \right)} } \mathord{\left/ {\vphantom {{\sum\nolimits_{i = 1}^{N} {\left( {is_{i} (F)} \right)} } {\sum\nolimits_{j = 1}^{N} {s_{j} } }}} \right. \kern-0pt} {\sum\nolimits_{j = 1}^{N} {s_{j} } }}(F)\)