Force sensor with dilatant fluid stop
A force sensor particularly suited for use in an electronic stylus that senses the contact force on its nib for recording pen strokes and handwriting recognition. The sensor has a housing for a load bearing member for receiving an input force to be sensed and associated circuitry for converting the input force into an output signal indicative of the input force. A coupling transmits the input force to the load bearing member and, a compressible reservoir containing dilatant fluid mounted between the housing and the coupling restricts the input force to the load bearing member caused by shock loading to the coupling. This protects the force sensor from damage by sharp impact loads such as dropping the stylus on its nib.
1. A force sensor assembly comprising:
a housing;
a load bearing member for receiving an input force to be sensed, said load bearing member having a first protrusion with round end for engagement with the coupling;
circuitry for converting the input force into an output signal indicative of the input force;
a coupling for transmitting the input force to the load bearing member, said coupling having a second protrusion extending centrally from a flange such that the distal end of the second protrusion engages the rounded end of the first protrusion from the load bearing member;
a compressible reservoir containing dilatant fluid mounted between the housing and the flange to restrict the input force to the load bearing member caused by shock loading to the coupling,
wherein the housing defines a recess for the circuitry, the rounded end of the first protrusion from the load bearing member extends proud of the recess for engaging the coupling.
2. A force sensor assembly according to claim 1 wherein the compressible reservoir of dilatant fluid maintains a gap between the load bearing member and the coupling when the input force is not applied, and the compressible reservoir compresses to allow the coupling to directly engage the load bearing member with a steady application of the input force.
3. A force sensor assembly according to claim 2 wherein the compressible reservoir is secured to the housing and the coupling, and the coupling is biased away from the housing to maintain the gap between the coupling and load bearing member when the input force is not applied.
4. A force sensor assembly according to claim 1 wherein the compressible reservoir has an annular shape and is positioned around the opening to the recess and around the second protrusion from the flange of the coupling.
5. A force sensor assembly according to claim 1 wherein the output signal from the circuitry support a hand writing recognition facility.
6. A force sensor assembly according to claim 1 wherein the load bearing member can move up to 100 microns relative to the housing.
7. An electronic stylus comprising:
an elongate body;
a nib extending from one end of the elongate body; and,
a load bearing member mounted to the elongate body for receiving an input force caused by contact on the nib, said load bearing member having a first protrusion with round end for engagement with the coupling;
circuitry for converting the input force into an output signal indicative of the input force;
a coupling for transmitting the input force to the load bearing member, said coupling having a second protrusion extending centrally from a flange such that the distal end of the second protrusion engages the rounded end of the first protrusion from the load bearing member; and,
a compressible reservoir containing dilatant fluid mounted between the housing and the flange to restrict the input force to the load bearing member caused by shock loading to the coupling,
wherein the housing defines a recess for the circuitry, the rounded end of the first protrusion from the load bearing member extends proud of the recess for engaging the coupling.
8. An electronic stylus according to claim 7 wherein the compressible reservoir has an annular shape and is positioned around the opening to the recess and around the second protrusion from the flange of the coupling.
9. An electronic stylus according to claim 7 wherein the compressible reservoir of dilatant fluid maintains a gap between the load bearing member and the coupling when the input force is not applied, and the compressible reservoir compresses to allow the coupling to directly engage the load bearing member with a steady application of the input force.
10. An electronic stylus according to claim 9 wherein the compressible reservoir is secured to the housing and the coupling, and the coupling is biased away from the housing to maintain the gap between the coupling and load bearing member when the input force is not applied.
11. An electronic stylus according to claim 7 wherein the nib of the electronic stylus is a ball point writing nib with a tubular ink cartridge extending from the nib toward the load bearing member such that the coupling is a detachable boot that fits over the end of the cartridge opposite the nib.
12. An electronic stylus according to claim 11 wherein the input force can be up to 5 Newtons.
13. An electronic stylus according to claim 7 wherein the circuitry is a piezoresistive bridge circuit.
14. An electronic stylus according to claim 7 wherein the load bearing member can move up to 100 microns relative to the housing.
15. An electronic stylus according to claim 7 wherein the output signal from the circuitry support a hand writing recognition facility.