Transparent force sensor with strain relief

Abstract

A transparent force sensor for detecting an applied force on a surface of a device. The transparent force sensor includes a transparent force-sensitive film having an array of strain-relief features oriented along a first direction. The transparent force-sensitive film is formed from a transparent piezoelectric material that exhibits a substantially reduced net charge when strained along a primary direction. The force sensor also includes a display element disposed on one side of the transparent force-sensitive film.

We claim: 
     
         1 . A transparent force sensor for detecting a force of a touch on a surface of a device, the transparent force sensor comprising:
 a transparent force-sensitive film having an array of slit features oriented along a first direction, wherein the transparent force-sensitive film is formed from a piezoelectric material that exhibits a substantially reduced net charge when strained along a primary direction.   
     
     
         2 . The transparent force sensor of  claim 1 , further comprising a display element disposed on one side of the transparent force-sensitive film. 
     
     
         3 . The transparent force sensor of  claim 1 , wherein the piezoelectric material exhibits a strain-direction dependent charge polarity. 
     
     
         4 . The transparent force sensor of  claim 1 , wherein the piezoelectric material exhibits a positive charge when bent upward along a first strain direction and exhibits a negative charge when bent upward along a second strain direction that is substantially perpendicular to the first strain direction. 
     
     
         5 . The transparent force sensor of  claim 1 , wherein the piezoelectric material is a poly-L-lactide (PLLA) or poly-D-lactide (PDLA) polymer film material. 
     
     
         6 . The transparent force sensor of  claim 1 , further comprising:
 a first electrode disposed above the transparent force-sensitive film;   a second electrode disposed below the transparent force-sensitive film; and   sense circuitry electrically coupled to the first and second electrodes, wherein the sense circuitry is configured to detect a change in an electrical property of the transparent force-sensitive film due to the force on the device.   
     
     
         7 . The transparent force sensor of  claim 6 , wherein the electrical property is an electrical charge. 
     
     
         8 . A transparent force sensor for detecting a force applied to a device, the transparent force sensor comprising:
 a cover forming a portion of an exterior surface of the device;   a transparent force-sensitive film disposed below the cover and having an array of strain-relief features oriented along a first direction, wherein the transparent force-sensitive film is formed from a piezoelectric material that exhibits a substantially reduced net charge when strained along a primary direction, and   a display element disposed below the transparent force-sensitive film.   
     
     
         9 . The transparent force sensor of  claim 8 , wherein the strain-relief features are slit features oriented along the first direction. 
     
     
         10 . The transparent force sensor of  claim 8 , wherein the strain-relief features are recessed channel features oriented along the first direction. 
     
     
         11 . The transparent force sensor of  claim 8 , wherein the strain-relief features are perforated features that are arranged along the first direction. 
     
     
         12 . A transparent force sensor for detecting a force of a touch on a device, the transparent force sensor comprising:
 a first transparent force-sensitive film having an array of slit features oriented along a first direction, wherein the transparent force-sensitive film is formed from a piezoelectric material that exhibits a substantially reduced net charge when strained along a first primary direction; and   a second transparent force-sensitive film having an array of slit features oriented along a second direction, wherein the second transparent force-sensitive film is formed from a piezoelectric material that exhibits a substantially reduced net charge when strained along a second primary direction.   
     
     
         13 . The transparent force sensor of  claim 12 , wherein the first direction is substantially perpendicular to the second direction. 
     
     
         14 . The transparent force sensor of  claim 12 , wherein the first primary direction is substantially perpendicular to the second primary direction. 
     
     
         15 . The transparent force sensor of  claim 12 , wherein the first direction is approximately 45 degrees from the first primary direction and the second direction is approximately 45 degrees from the second primary direction. 
     
     
         16 . The transparent force sensor of  claim 12 , further comprising:
 a cover disposed above the first transparent force-sensitive film; and   a display element disposed below the second transparent force-sensitive film, wherein the second transparent force-sensitive film is disposed below the first transparent force-sensitive film.   
     
     
         17 . The transparent force sensor of  claim 12 , further comprising:
 a top electrode disposed above the first transparent force-sensitive film;   a middle electrode disposed below the first transparent force-sensitive film;   a bottom electrode disposed below the second transparent force-sensitive film; and   sense circuitry electrically coupled to the top, middle, and bottom electrodes, wherein the sense circuitry is configured to detect a change in an electrical property of the first and second transparent force-sensitive films due to the force on the device.   
     
     
         18 . The transparent force sensor of  claim 17 , wherein the first transparent force-sensitive film is configured to produce a charge when strained perpendicular to the first direction, and the second transparent force-sensitive film is configured to produce a charge when strained perpendicular to the second direction. 
     
     
         19 . The transparent force sensor of  claim 17 , wherein the sense circuitry is configured to measure a magnitude of the touch. 
     
     
         20 . The transparent force sensor of  claim 17 , further comprising:
 a first optically-clear adhesive disposed between the top electrode and the first transparent force-sensitive film; and   a second optically-clear adhesive disposed between the middle electrode and the first transparent force-sensitive film.