Temperature compensation for transparent force sensors

• US 9,851,845 B2
• Filed: 07/08/2015
• Issued: 12/26/2017
• Est. Priority Date: 08/12/2014
• Status: Expired due to Fees

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Smith, John S., Agarwal, Manu
• Primary Examiner(s): Lubit, Ryan A
• Application Number:
  US14/793,987
• Publication Number:
  US 20160048266A1
• Time in Patent Office: 902 Days
• CPC Class Codes: G01L 1/18 using properties of piezo-resistive materials, i.e. materials of which the ohmic resistance varies according to changes in magnitude or direction of force applied to the material
• CPC Class Codes: G01L 1/2281 for temperature variations
• CPC Class Codes: G01L 1/2293 of the semi-conductor type semi-conductor devices controllable by variations of applied mechanical force H01L29/84
• CPC Class Codes: G01L 25/00 Testing or calibrating of apparatus for measuring force, torque, work, mechanical power, or mechanical efficiency
• CPC Class Codes: G06F 3/04144 using an array of force sensing means position sensing using the local deformation of sensor cells G06F3/0447
• CPC Class Codes: G06F 3/0418 for error correction or compensation, e.g. based on parallax, calibration or alignment

Abstract

An optically transparent force sensor element compares a force reading from a first strain-sensitive film element with a second strain-sensitive film element, having a compliant and thermally conductive intermediate layer positioned therebetween to compensate for temperature changes. While in the idle state, the optically transparent force sensor can be periodically calibrated to account for additional changes in temperature.

Claims

1. An electronic device comprising:

an input surface comprising;

a substrate;

a first sensor coupled to the substrate and formed from a material with a first strain-sensitive electrical property; and

a second sensor positioned below and aligned with the first sensor, coupled to the substrate, and separated from the first sensor by a selected distance, the second sensor formed from a material with a second strain-sensitive electrical property; and

a controller configured to obtain a measurement representing a difference between the first strain-sensitive electrical property and the second strain-sensitive electrical property.

2. The electronic device of claim 1, wherein the controller is configured to obtain a first single-ended measurement of the first strain-sensitive electrical property and, separately, a second single-ended measurement of the second strain-sensitive electrical property.

3. The electronic device of claim 1, wherein the first strain-sensitive electrical property and the second strain-sensitive electrical property comprises electrical resistance.

4. The electronic device of claim 1, wherein the first strain-sensitive electrical property and the second strain-sensitive electrical property comprises an induced charge.

5. The electronic device of claim 1, wherein the first and second sensors are formed from an indium-tin oxide.

6. The electronic device of claim 1, wherein the first and second sensors are formed, at least in part, of one of the group consisting of carbon nanotubes and graphene.

7. The electronic device of claim 1, the substrate further comprising a top surface;

wherein;

the first sensor is positioned a first distance from the top surface; and

the second sensor is positioned a second distance from the top surface.

8. An electronic device comprising:

an input surface comprising;

a substrate having a top surface;

a strain sensor coupled to the substrate and comprising;

a first and second strain-sensitive resistors arranged as a voltage divider, the first strain-sensitive resistor having a first strain-sensitive electrical property and being separated from the top surface by a first distance and the second strain-sensitive resistor having a second strain-sensitive electrical property and being separated from the top surface by a second distance; and

a common node coupled to both the first and second strain-sensitive resistor; and

a controller configured to perform the operations of;

injecting a selected current into the common node; and

measuring a voltage between the common node and ground, the voltage representing a difference between the first strain-sensitive electrical property and the second strain-sensitive electrical property.

9. The electronic device of claim 8, wherein the first and second strain-sensitive resistors are formed from an indium-tin oxide.

10. The electronic device of claim 8, wherein the first and second strain-sensitive resistors are formed, at least in part, of one of the group consisting of carbon nanotubes and graphene.

11. The electronic device of claim 8 wherein the substrate is formed from glass.

12. The electronic device of claim 8, wherein the first and second strain-sensitive resistors are formed from materials having substantially identical temperature coefficients of resistance.

13. The electronic device of claim 8, wherein the first and second strain-sensitive resistors are formed from materials having substantially identical coefficients of thermal expansion.