Capacitive force sensor
In one embodiment, a method includes detecting movement of a spring structure of a force sensor coupled to a center shaft of an active stylus; measuring a change of capacitance between electrodes of the force sensor; and determining an amount of force applied to the center shaft based on the change of capacitance.
1. A method comprising:
detecting movement of a spring structure of a force sensor coupled to a center shaft of an active stylus, the spring structure being formed from at least a portion of a printed circuit board (PCB) of the active stylus such that the at least a portion of the PCB forms at least a portion of the spring structure, the spring structure comprising a plurality of cuts in the PCB of the active stylus such that the at least a portion of the PCB from which the spring structure is formed comprises a continuous portion of the PCB;
measuring a change of capacitance between a plurality of electrodes of the force sensor, the electrodes comprising a moving electrode and a stationary electrode, the moving electrode and the stationary electrode formed from the continuous portion of the PCB; and
determining an amount of force applied to the center shaft based on the change of capacitance.
2. The method of claim 1 , wherein the plurality of cuts in the PCB of the active stylus comprises a plurality of substantially serpentine cuts in the PCB of the active stylus, the electrodes comprising the moving electrode and the stationary electrode formed in the substantially serpentine cuts from a metal layer of the PCB.
3. The method of claim 2 , wherein the substantially serpentine cuts are located at an end of the PCB proximate to the center shaft.
4. The method of claim 2 , wherein the substantially serpentine cuts are substantially symmetric with respect to a longitudinal axis of the active stylus.
5. The method of claim 2 , wherein the spring structure comprises a cutout portion in the PCB separating one portion of substantially serpentine cuts from another portion of substantially serpentine cuts.
6. The method of claim 1 , wherein the force sensor comprises an air gap separating the moving electrode from the stationary electrode.
7. The method of claim 1 , wherein a tilt component on the measured force is compensated by averaging a common-mode signal or determining a ratio of signals having a multi-spatial wavelength.
8. One or more computer-readable non-transitory storage media embodying logic configured when executed to:
detect movement of a spring structure of a force sensor coupled to a center shaft of an active stylus, the spring structure being formed from at least a portion of a printed circuit board (PCB) of the active stylus such that the at least a portion of the PCB forms at least a portion of the spring structure, the spring structure comprising a plurality of cuts in the PCB of the active stylus such that the at least a portion of the PCB from which the spring structure is formed comprises a continuous portion of the PCB;
measure a change of capacitance between a plurality of electrodes of the force sensor, the electrodes comprising a moving electrode and a stationary electrode, the moving electrode and the stationary electrode formed from the continuous portion of the PCB; and
determine an amount of force applied to the center shaft based on the change of capacitance.
9. The media of claim 8 , wherein the plurality of cuts in the PCB of the active stylus comprises a plurality of substantially serpentine cuts in PCB of the active stylus, the electrodes comprising the moving electrode and the stationary electrode formed in the substantially serpentine cuts from a metal layer of the PCB.
10. The media of claim 9 , wherein the substantially serpentine cuts are located at an end of the PCB proximate to the center shaft.
11. The media of claim 9 , wherein the substantially serpentine cuts are substantially symmetric with respect to a longitudinal axis of the active stylus.
12. The media of claim 9 , wherein the spring structure comprises a cutout portion in the PCB separating one portion of substantially serpentine cuts from another portion of substantially serpentine cuts.
13. The media of claim 8 , wherein a tilt component on the measured force is compensated by averaging a common-mode signal or determining a ratio of signals having a multi-spatial wavelength.
14. A device comprising:
a force sensor comprising:
a spring structure of a force sensor coupled to a center shaft of an active stylus, the spring structure being formed from at least a portion of a printed circuit board (PCB) of the active stylus such that the at least a portion of the PCB forms at least a portion of the spring structure, the spring structure comprising a plurality of cuts in the PCB of the active stylus such that the at least a portion of the PCB from which the spring structure is formed comprises a continuous portion of the PCB; and
a plurality of electrodes, the electrodes comprising a moving electrode and a stationary electrode, the moving electrode and the stationary electrode formed from the continuous portion of the PCB; and
a controller configured to receive a signal from the force sensor indicative of a change of capacitance between the electrodes and determining an amount of force applied to the center shaft based on the change of capacitance.
15. The device of claim 14 , wherein the plurality of cuts in the PCB of the active stylus comprises a plurality of substantially serpentine cuts in the PCB of the active stylus, the electrodes comprising the moving electrode and the stationary electrode formed in the cuts from a metal layer of the PCB.
16. The device of claim 15 , wherein the substantially serpentine cuts are located at an end of the PCB proximate to the center shaft.
17. The device of claim 15 , wherein the substantially serpentine cuts are substantially symmetric with respect to a longitudinal axis of the active stylus.
18. The device of claim 15 , wherein the spring structure comprises a cutout portion in the PCB separating one portion of substantially serpentine cuts from another portion of substantially serpentine cuts.
19. The device of claim 15 , wherein the force sensor comprises an air gap separating the moving electrode from the stationary electrode.
20. The device of claim 14 , wherein a tilt component on the measured force is compensated by averaging a common-mode signal or determining a ratio of signals having a multi-spatial wavelength.