pegasixi:
- Using a strain gauge/piezoelectric sensor on either edge of the foil to detect if one side is starting to receive more force than the other
- Concern: Foil is so thin that I'm not sure the sensors will detect any meaningful output from the contact
- Using a infrared diode + receiver on either edge of foil. Measure if foil is starting to cover more light on one receiver
- Concern: Not certain how well this can be calibrated/ how easy this will be...
- Pray to the gods.
- Concern: I'm not religious...
Of the three ideas you present, IMHO the second is the best to pursue. You are correct that for such a thin material any weight based system (at least any that is under multiple thousands of dollars) is going to be at best unreliable. Although pylon's suggestion is even better, especially if the extra information is useful for you.
With either a laser or IR based system the detection of the edge will depend a lot on how much light is transmitted through material at the given wavelength being sensed. If it's completely opaque to the give wavelength, it's as easy as distiguishing between HIGH and LOW on a digital I/O pin. If it's translucent, well then you might have to use an analog input and experimentally determine the proper threshold. Also on the subject of wavelengths, it could be possible to do this with visible light LEDs as well. You should also know that most LEDs are capable of acting as passable detectors for their output wavelengths when reversed biased.