A force-sensing capacitor is a material whose capacitance changes when a force, pressure or mechanical stress is applied.
They can provide improved sensitivity and repeatability compared to force-sensitive resistors[1] but traditionally required more complicated electronics.
For small deflections, there is a linear relationship between applied force and change in capacitance, which can be shown as follows: The capacitance,
If the material is linearly elastic (so follows Hooks Law), then the displacement, due to an applied force
Combining these equations gives the capacitance after an applied force as: This can be rearranged to: Assuming that
, we can simplify this to: It follows that: We can express the change in capacitance
as: SingleTact makes force-sensitive capacitors using moulded silicon between two layers of polyimide to construct a 0.35 mm thick sensor, with force ranges from 1 N to 450 N.[3] The 8mm SingleTact has a nominal capacitance of 75 pF, which increases by 2.2 pF when the rated force is applied.
[3] It can be mounted on many surfaces for direct force measurement.
Force-sensing capacitors can be used to create low-profile force-sensitive buttons.
They have been used in medical imaging to map pressures in the esophagus[4][5] and to image breast[6][7] and prostate cancer.