This approach was adopted from S. Hwang, A. Bianchi, M. Ahn, and K.Y. Wohn and utilizes the magnetic sensor present in common smart phones for the measurement of changes in the magnetic field around the smart phone.
In our implementation, the magnetic sensor provides us with the strength of the magnetic field for all three dimensions in µT. These three values are used to calculate the absolute magnetic field force. The resulting value is next normalized and then categorized into levels. With this approach, the phsyical pressure is not applied to the smart phone itself but rather to an additional input device. Initially, we used a prototypically prepared pen that contained a magnet. Later we adopted a hole puncher because the pen posed the problem that it could not be held well within our target group.
A major drawback we identified with this approach is that the magnetic field that surrounds the recording device (smart phone) changes whenever the situation changes (temporal or local). Therefore the device has to be calibrated before each use to recognize the correct values as lowest pressure and highest pressure.