Single Sided Capacitive Force Sensor for Electronic Devices

• US 20100024573A1
• Filed: 07/29/2008
• Published: 02/04/2010
• Est. Priority Date: 07/29/2008
• Status: Active Grant

• Current Assignee: Google Technology Holdings LLC (Alphabet Inc.)
• Original Assignee: Motorola Mobility, Inc. (Lenovo Group Ltd.)
• Inventors: Daverman, Dodge, Olley, Michael, Cranfill, David
• Granted Patent: US 7,784,366 B2
• Time in Patent Office: Days
• US Class Current: 73/862.626
• CPC Class Codes: G01L 1/142 using capacitors
• CPC Class Codes: G01L 1/146 for measuring force distributions, e.g. using force arrays G01L1/148 takes precedence
• CPC Class Codes: G06F 3/0443 using a single layer of sensing electrodes

Abstract

A capacitive force sensor (100) includes a substrate (101) having at least one electrode pair (102,103) defining a capacitance disposed thereon. The substrate (101) is fixed relative to a first plate (106). A drive circuit (104) is configured to apply a voltage relative to a circuit ground (105) to the electrode pair (102,103). The first plate (106) is separated from a second plate (107) that is coupled to circuit ground (105) by a compliance member (108,109). The compliance member (108,109) is configured to oppose a compression force (110) while allowing the first plate (106) to physically move relative to the second plate (107). A capacitive detection circuit (111) is then configured to detect a change the capacitance when the compliance member (108,109) is compressed. The compression force (110) is then determined from the change in capacitance and the spring constant of the compliance member (108,109).

Claims

1. A capacitive force sensor, comprising:

a substrate having at least one electrode pair defining a capacitance disposed thereon;

a drive circuit configured to apply a voltage relative to a circuit ground to the at least one electrode pair;

a first plate, fixed relative to the substrate;

a second plate electrically coupled to the circuit ground;

a compliance member disposed between the first plate and the second plate, configured to mechanically separate the first plate and the second plate and to oppose a compression force while allowing the first plate to physically move relative to the second plate in response to the compression force; and

a capacitance detection circuit configured to detect a change the capacitance when the compliance member is compressed, wherein the capacitance is dependent upon a distance between the first plate and the second plate.

2. The capacitive force sensor of claim 1, wherein the at least one electrode pair comprises a first electrode and a second electrode, wherein both the first electrode and the second electrode are disposed on a single substrate, on a same side of an air gap existing between the substrate and the second plate.

3. The capacitive force sensor of claim 1, wherein the compliance member comprises a shaped elastomer material.

4. The capacitive force sensor of claim 1, wherein the compliance member comprises one of a metal or a plastic spring mechanism.

5. The capacitive force sensor of claim 1, wherein the first plate is manufactured from one of glass, plastic, metal, or fiberglass.

6. The capacitive force sensor of claim 5, further comprising an electrical shield configured to electrically shield the at least one electrode pair from capacitively coupling with objects proximately located with the first plate.

7. The capacitive force sensor of claim 6, wherein the first plate comprises the electrical shield, wherein the electrical shield comprises an electrically conductive material that is coupled to the circuit ground or comprises electrically driven circuit coupled layer.

8. The capacitive force sensor of claim 1, further comprising a capacitive touch sensor configured to detect a proximate position of an object.

9. The capacitive force sensor of claim 1, wherein the at least one electrode pair comprises a first electrode and a second electrode disposed on a first side of the substrate, further comprising an electrode mesh disposed along a second side of the substrate above the at least one electrode pair, wherein the electrode mesh is electrically coupled to one of the first electrode or the second electrode.

10. A electronic device having a touch sensitive user interface, the touch sensitive user interface comprising:

a fascia member configured to receive touch input, the fascia member spanning at least a portion of a face of the electronic device;

a substrate, disposed beneath the fascia member and being fixed relative to the fascia member, having disposed thereon at least one electrode pair;

an electrically conductive plate coupled to a ground node, disposed within the electronic device and physically separated from the substrate by an air gap;

a compliance member disposed between the fascia member and the electrically conductive plate; and

a capacitance sensing circuit, operable with the at least one electrode pair, configured to determine a capacitive coupling of the at least one electrode pair when the compliance member is compressed.

11. The electronic device of claim 10, further comprising a processing circuit, operable with the capacitance sensing circuit, and configured to determine a contact force associated with the touch input, wherein the contact force depends upon an amount of compression of the compliance member.

12. The electronic device of claim 11, wherein the processing circuit is further configured to provide a device actuation signal where the contact force exceeds a predetermined contact force threshold.

13. The electronic device of claim 11, wherein the electronic device further comprises a display disposed beneath the fascia member, wherein the substrate is disposed beneath the display.

14. The electronic device of claim 11, further comprising a touch sensor layer disposed beneath at least a section of the fascia member, the capacitive touch sensor layer being configured to detect a position along the at least the section of the fascia member corresponding to the touch input.

15. The electronic device of claim 11, wherein the at least one electrode pair comprises a plurality of electrode pairs, wherein portions of the plurality of electrode pairs are disposed in an array along the substrate.

16. The electronic device of claim 11, wherein the fascia member comprises a rigid fascia member having electrically conductive material, wherein the electrically conductive material is coupled to the ground node.

17. The electronic device of claim 11, wherein the substrate comprises a flexible substrate having the at least one electrode pair disposed on one side of the flexible substrate, wherein the at least one electrode pair comprises a first electrode and a second electrode, wherein the first electrode is interlaced with the second electrode.

18. A method of determining a contact force being applied to a surface, the method comprising the steps of:

providing a touch sensitive apparatus comprising the surface and an electrically conductive, grounded plate, separated by a compliance member, further comprising at least one electrode pair disposed on a substrate retained in a fixed relationship with the surface;

providing a capacitive sensing circuit, coupled to an operable with the at least one electrode pair;

obtaining a reference capacitance associated with the at least one electrode pair when the compliance member is in a rest state;

determining, from the capacitive sensing circuit, a compression capacitance when the contact force is applied to the surface, thereby compressing the compliance member; and

determining a magnitude of the contact force based upon a change between the compression capacitance and the reference capacitance.

19. The method of claim 18, wherein the step of determining the magnitude of the contact force comprises the steps of:

obtaining a spring factor associated with the compliance member;

determining an amount of compression of the compliance member based upon the change between the compression capacitance and the reference capacitance; and

employing the amount of compression and the spring factor to determine the magnitude of the contact force.

20. The method of claim 19, wherein the step of determining the magnitude of the contact force comprises the step of applying an age compensation factor to the spring factor.