Force-sensing capacitor

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 but traditionally required more complicated electronics.
Operation principle.
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, formula_1, equals formula_2, where formula_3 is permeability, formula_4 is the area of the sensor and formula_5 is the distance between parallel plates. If the material is linearly elastic (so follows Hooks Law), then the displacement, due to an applied force formula_6, is formula_7, where formula_8 is the spring constant. Combining these equations gives the capacitance after an applied force as:
This can be rearranged to:
Assuming that formula_12, which is true for small deformations where formula_13, we can simplify this to:
It follows that:
We can express the change in capacitance formula_18 as:
Production.
SingleTact makes force-sensitive capacitors using moulded silicon between two layers of polymide to construct a 0.35mm thick sensor, with force ranges from 1N to 450N. The 8mm SingleTact has a nominal capacitance of 75pF, which increases by 2.2pF when the rated force is applied. It can be mounted on many surfaces for direct force measurement.
Uses.
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 esophagusand to image breast and prostate cancer. 