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, $C$ , equals $ε A / d$ , where $ε$ is permittivity, $A$ is the area of the sensor and $d$ is the distance between parallel plates. If the material is linearly elastic (so follows Hooks Law), then the displacement, due to an applied force $F$ , is $x = F / k$ , where $k$ is the spring constant. Combining these equations gives the capacitance after an applied force as:

C = ε A / (d_{n o m i n a l} - F / k)

, where

d_{n o m i n a l}

is the separation between parallel plates when no force is applied.

This can be rearranged to:

C = (ε A d_{n o m i n a l} + ε A F / k) / (d_{n o m i n a l}^{2} - F^{2} / k^{2})

Assuming that $d_{n o m i n a l}^{2} > > F^{2} / k^{2}$ , which is true for small deformations where $d_{n o m i n a l} > > x$ , we can simplify this to:

C

≃ (ε A d_{n o m i n a l} + ε A F / k) / (d_{n o m i n a l}^{2})

It follows that:

C

≃ C_{n o m i n a l} + ε A F / k d_{n o m i n a l}^{2}

C

≃ C_{n o m i n a l} + B F

where

B = ϵ A / k d^{2}

, which is constant for a given sensor.

We can express the change in capacitance $Δ C$ as:

Δ C = B F

Production

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. The 8mm SingleTact has a nominal capacitance of 75 pF, which increases by 2.2 pF 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 esophagus and to image breast and prostate cancer.

References

↑ 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. S2CID 28558376.
↑ Bentley, John P. (1995). Principles of measurement systems (3rd ed.). Harlow [England]: Longman Scientific & Technical. ISBN 0470234458. OCLC 30781109.
1 2 "SingleTact Datasheet" (PDF). SingleTact.
↑ US Grant US10961981, Pakrs, Thomas, "High resolution solid state pressure sensor", published 2015-07-14, assigned to Sierra Scientific Instruments Inc
↑ "Using Capacitive Force Sensors in Next-Gen Medical Products". Medical Design Technology. 2016-03-01. Retrieved 2018-06-21.
↑ Egorov, V.; Sarvazyan, A.P. (2008-09-01). "Mechanical Imaging of the Breast". IEEE Transactions on Medical Imaging. 27 (9): 1275–1287. Bibcode:2008ITMI...27.1275E. doi:10.1109/tmi.2008.922192. ISSN 0278-0062. PMC 2581459. PMID 18753043.
↑ "SureTouch". SureTouch. Retrieved 2018-06-21.
↑ "Artann Labs". www.artannlabs.com. Retrieved 2018-06-21.

[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. S2CID 28558376.

[2] Bentley, John P. (1995). Principles of measurement systems (3rd ed.). Harlow [England]: Longman Scientific & Technical. ISBN 0470234458. OCLC 30781109.

[:0-3] 1 2 "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. Bibcode:2008ITMI...27.1275E. 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.