Material whose capacitance changes when a force is applied
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]
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, , 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:
, where is the separation between parallel plates when no force is applied.
This can be rearranged to:
Assuming that , which is true for small deformations where , we can simplify this to:
C
It follows that:
C
C where , which is constant for a given sensor.
We can express the change in capacitance as:
Production
SingleTact makes force-sensitive capacitors using moulded
silicon between two layers of
polyimide to construct a 0.35mm thick sensor, with force ranges from 1N to 450N.[3] The 8mm SingleTact has a nominal capacitance of 75pF, which increases by 2.2pF when the rated force is applied.[3] 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 esophagus[4][5] and to image breast[6][7] and prostate cancer.[8]
^US Grant US10961981, Pakrs, Thomas, "High resolution solid state pressure sensor", published 2015-07-14, assigned to Sierra Scientific Instruments Inc