Estimating Children Engagement Interacting with Robots in Special Education Using Machine Learning

Table 2

Facial action coding system (FACS).

Emotion	Action units (AU)	Description
Happiness/joy	6 + 12	Cheek raiser and lip corner puller
Sadness	1 + 4 + 15	Inner brow raiser, brow lowerer, and lip corner depressor
Surprise	1 + 2 + 5 + 26	Inner brow raiser, outer brow raiser, upper lid raiser, and jaw drop
Fear	1 + 2 + 4 + 5 + 7 + 20 + 26	Inner brow raiser, outer brow raiser, brow lowerer, upper lid raiser, lid tightener, lip stretcher, and jaw drop
Anger	4 + 5 + 7 + 23	Brow lowerer, upper lid raiser, lid tightener, and lip tightener
Disgust	9 + 15	Nose wrinkler, lip corner depressor, and lower lip depressor
Contempt	12 + 14	Lip corner puller and dimpler