A force-sensing capacitor is a material whose capacitance changes when a force, pressure or mechanical stress is applied. They are also known as "force-sensitive capacitors". They can provide improved sensitivity and repeatability compared to force-sensitive resistors[1] but traditionally required more complicated electronics.[2]
Typical force-sensitive capacitors are examples of parallel plate capacitors. For small deflections, there is a linear relationship between applied force and change in capacitance, which can be shown as follows:
The capacitance, , equals , where is permeability, is the area of the sensor and is the distance between parallel plates. If the material is linearly elastic (so follows Hooks Law), then the displacement, due to an applied force , is , where is the spring constant. Combining these equations gives the capacitance after an applied force as:
This can be rearranged to:
Assuming that , which is true for small deformations where , 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.[8]