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[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 polymide 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]

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]

References

  1. Martinelli, L; Hurschler, C; Rosenbaum, D (2006-06-01). "Comparison of Capacitive versus Resistive Joint Contact Stress Sensors". Clinical Orthopaedics and Related Research. 447: 214–220. doi:10.1097/01.blo.0000218730.59838.6a. ISSN 0009-921X. PMID 16672899.
  2. 1943-, Bentley, John P. (1995). Principles of measurement systems (3rd ed.). Harlow [England]: Longman Scientific & Technical. ISBN 0470234458. OCLC 30781109.CS1 maint: numeric names: authors list (link)
  3. "SingleTact Datasheet" (PDF). SingleTact.
  4. US Grant US10961981, Pakrs, Thomas, "High resolution solid state pressure sensor", published 2015-07-14, assigned to Sierra Scientific Instruments Inc
  5. "Using Capacitive Force Sensors in Next-Gen Medical Products". Medical Design Technology. 2016-03-01. Retrieved 2018-06-21.
  6. Egorov, V.; Sarvazyan, A.P. (2008-09-01). "Mechanical Imaging of the Breast". IEEE Transactions on Medical Imaging. 27 (9): 1275–1287. doi:10.1109/tmi.2008.922192. ISSN 0278-0062. PMC 2581459. PMID 18753043.
  7. "SureTouch". SureTouch. Retrieved 2018-06-21.
  8. "Artann Labs". www.artannlabs.com. Retrieved 2018-06-21.
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