Detect opening mouth

i know absurd question, know how detect opening mouth , home in device  (say, automatic spoon) it?

my sister severely disabled , feeding no simple task. can eat grinded (baby) food , can take bites size of pill. takes ~2 hours feed meal. feeding 4 times day, 24/7, not fun.

while weak, mentally capable , making robotic hand her. able press buttons,but, make device that:

- can pick baby food bowl using special designed spoon (by ceramic 3d printing)
- recognize , home in spoon mouth
- release food, rinse , repeat

i know suggest emg controlled arm, elbow cannot bend. if can make robotic hand not able bend arms , use spoon.

now have trouble in sensing opening mouth (so spoon not poke nose). seems ultrasonic, ir sensor, photoresistors, etc not suitable. best best facial recognition beyond capacity of arduino. have ideas in detecting opening mouth?

if mouth in same position, can distance measurement. example ultrasonic.
can use water baby food , feed through tube ?
i have seen free computer software recognizes blink of eye (for mouse click), don't know name of program anymore, complex arduino far.

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