On this week I did some research on how could I measure the forces on a pedal. There are two systems, one using Force Sensing Resistors (FSR) and other using Force Load Cells.
The FSR are flat sensors that have variable resistances between their 2 pins depending upon the force applied. When the force applied increases the resistance goes down. These sensors are easy to use and also relatively inexpensive, however it has some disadvantages such as drift and poor accuracy, errors up to 25%.
Depending on how long and how hard it's pressed, the FSR value will come back to approximately 95% of its initial value almost instantly, and than drift the final 5% over a few seconds. In conclusion these sensors aren't the best choice when it's required accuracy and repeatability.
The Force Load Cells are transducers that convert force into an electrical signal. The applied force deforms a strain gauge and that strain gauge measures the deformation as an electrical signal. The output signal is typically very small so it's required to amplify it by using an instrumentation amplifier. The load cells have higher dimensions than FSR, however they have high accuracy and repeatability, which, in my opinion, are very important aspects when building a system like mine.