Constructing a capacitive sensor using springs 1

Capacitive sensing serves as a robust alternative to traditional mechanical switches. However, certain applications cannot accommodate printed circuit boards directly beneath a cover layer or connect them to the device housing. These applications include household appliances such as stoves, washing machines, refrigerators, and microwaves, as well as radio tuners, control panels, and occupant detection systems in seats (Seat electronic devices).

The metal spring sensor (spring sensor) can replace the solid conductive sensor, allowing the PCB position to be moved away from the cover layer and providing reliable sensor connectivity under conditions of intense vibration or drastic temperature changes. Additionally, the spring can also serve other functions such as backlight illumination and the integration of mechanical and capacitive buttons.  diode

Spring electric field simulation

To compare the behavior between the spring-type sensor and the solid-state conductive sensor, it is necessary to model the electric fields of the relevant objects. Figures 1 and 2 show the electric field distributions of the solid-state conductive sensor and the spring-type sensor, created using Comsol Multiphysics ver.3.2.

When modeling solid-state conductive sensors and spring sensors, we assume the following conditions:

The cover layer is made of inorganic glass: relative permittivity e = 4.2; thickness = 4 mm.

Model the finger as a metallic ellipsoid: height = 20 mm; diameter = 10 mm.

The solid-state conductive sensor is a solid-state metal ring: diameter = 10 mm; thickness = 0.1 mm.

The spring sensor is modeled as a hollow metal cylinder: height = 20 mm; diameter = 10 mm; metal thickness = 1 mm.   diode