Piezoelectric Force Sensing

• US 20090146533A1
• Filed: 12/11/2007
• Published: 06/11/2009
• Est. Priority Date: 12/11/2007
• Status: Active Grant

• Current Assignee: Nokia Technologies Oy (Nokia Corporation)
• Original Assignee: Nokia Corporation
• Inventors: Keski-Jaskari, Turo, Jalkanen, Jukka, Leskinen, Miikka
• Granted Patent: US 7,816,838 B2
• Time in Patent Office: Days
• US Class Current: 310/338
• CPC Class Codes: G01L 1/16 using properties of piezoelectric devices
• CPC Class Codes: G06F 3/016 Input arrangements with force or tactile feedback as computer generated output to the user
• CPC Class Codes: H03K 17/964 Piezoelectric touch switches
• CPC Class Codes: H03K 17/975 using a capacitive movable element
• CPC Class Codes: H03K 2217/96062 with tactile or haptic feedback
• CPC Class Codes: H10N 30/302 Sensors
• CPC Class Codes: H10N 30/802 Circuitry or processes for operating piezoelectric or electrostrictive devices not otherwise provided for, e.g. drive circuits

Abstract

A system and method for measuring absolute pressure applied to a piezoelectric element by measuring a transition time of a voltage pulse applied to a piezoelectric element. The transition time (such as the rise time and/or fall time of the voltage pulse) is affected by the capacitance of the piezoelectric element, which in turn is affected by the amount of pressure currently being applied to the piezoelectric element. The system may also provide haptic feedback via the same piezoelectric element.

Claims

1. An apparatus, comprising a sensor configured to generate a signal that depends upon an absolute pressure applied to a piezoelectric element.

2. The apparatus of claim 1, wherein the sensor has an input electrically coupled to the piezoelectric element and is configured to compare a voltage at the input with a reference voltage, the sensor being configured to generate the signal based on the comparison.

3. The apparatus of claim 2, wherein the sensor is further configured to determine a time from a starting time until the voltage at the input exceeds the reference voltage, the sensor being configured to generate the signal based on the time.

4. The apparatus of claim 2, wherein the sensor is further configured to determine a time from a starting time until the voltage at the input falls below the reference voltage, the sensor being configured to generate the signal based on the time.

5. The apparatus of claim 2, further comprising a signal source electrically coupled to the piezoelectric element and the sensor, and configured to send a pulsed voltage signal to the piezoelectric element, wherein the pulsed voltage signal has a peak voltage exceeding the reference voltage.

6. The apparatus of claim 1, further comprising:

a first operational amplifier having an output coupled to a first electrode of the piezoelectric element and to the sensor; and

a second operational amplifier having an output coupled to a second electrode of the piezoelectric element.

7. The apparatus of claim 1, wherein the sensor comprises an operational amplifier having a first input electrically coupled to the piezoelectric element and a second input electrically coupled to a reference potential.

8. The apparatus of claim 1, wherein the apparatus is a handheld electronic device comprising a user input device, wherein the user input device comprises the sensor.

9. The apparatus of claim 1, wherein the sensor is configured to feed a measurement pulse to the piezoelectric element, measure a rise time of the measurement pulse, and determine the absolute pressure based on the rise time.

10. The apparatus of claim 1, wherein the sensor is configured to feed a measurement pulse to the piezoelectric element, measure a fall time of the measurement pulse, and determine the absolute pressure based on the fall time.

11. An apparatus, comprising:

a piezoelectric element;

a first operational amplifier having an output electrically coupled to a first electrode of the piezoelectric element;

a second operational amplifier having an output electrically coupled to a second electrode of the piezoelectric element; and

a third operational amplifier having a first input electrically coupled to the first electrode of the piezoelectric element.

12. The apparatus of claim 11, further comprising:

a current path extending from the first electrode of the piezoelectric element to a fixed potential other than the output of the first operational amplifier.

13. The apparatus of claim 12, wherein the current path comprises a resistor.

14. The apparatus of claim 11, wherein the third operational amplifier has a second input electrically coupled to a fixed potential.

15. The apparatus of claim 11, wherein the third operational amplifier has a second input electrically coupled to a reference potential.

16. The apparatus of claim 11, wherein the output of the first operational amplifier is electrically coupled to an input of the first operational amplifier.

17. The apparatus of claim 11, further comprising a circuit configured to generate a first signal that depends upon a time at which a second signal output from the third operational amplifier changes in value.

18. The apparatus of claim 17, wherein the first operational amplifier has an input configured to receive a third signal, and wherein the circuit is further configured to generate the first signal to depend upon the time at which the second signal changes in value relative to a time at which the third signal changes in value.

19. A method, comprising measuring a transition time of a voltage at a node that is electrically coupled to a piezoelectric element.

20. The method of claim 19, wherein measuring comprises comparing the voltage with a reference voltage.

21. The method of claim 20, further comprising generating a signal that depends upon a time that begins from a starting time until a time that the voltage exceeds the reference voltage.

22. The method of claim 20, further comprising generating a signal that depends upon a time that begins from a starting time until a time that the voltage falls below the reference voltage.

23. The method of claim 20, further comprising generating a pulsed voltage signal at the node, wherein the pulsed voltage signal has a peak voltage exceeding the reference voltage.

24. A method, comprising:

feeding a first series of pulses into a first electrode of a piezoelectric element;

measuring a transition time of each of the first pulses;

feeding a second series of pulses into the first electrode of the piezoelectric element; and

feeding a third series of pulses into a second electrode of the piezoelectric element,

wherein the second and third series of pulses have a peak-to-peak voltage differential greater than a peak-to-peak voltage differential of the first series of pulses.

25. The method of claim 24, wherein feeding the second and third series of pulses comprises feeding the second and third series of pulses in an alternating manner.

26. The method of claim 24, wherein the first series of pulses has a peak-to-peak voltage differential of less than ten volts and the second and third series of pulses each has a peak-to-peak voltage differential of more than thirty volts.

27. The method of claim 24, wherein the first series of pulses are pulsed at a frequency of less than five Hertz.

28. An apparatus, comprising:

means for feeding measurement pulses into a piezoelectric element; and

means for measuring a transition time of the measurement pulses.

29. The apparatus of claim 28, further comprising means for feeding haptic feedback pulses into the piezoelectric element.

30. A method, comprising:

applying a plurality of measurement pulses to a first electrode of a piezoelectric element; and

measuring a transition time of the measurement pulses.

31. The method of claim 30, further comprising applying a plurality of haptic feedback pulses into both the first electrode and an opposing second electrode of the piezoelectric element.

32. The method of claim 31, wherein applying the plurality of measurement pulses is performed while the second electrode is maintained at a constant potential.

33. The method of claim 31, wherein the plurality of measurement pulses are less than five volts in amplitude and the plurality of haptic feedback pulses are greater than thirty volts in amplitude.

34. An apparatus, comprising:

a piezoelectric element; and

a circuit coupled to the piezoelectric element and configured to drive the piezoelectric element with a plurality of measurement pulses and to measure a transition time of each of the measurement pulses.

35. The apparatus of claim 34, wherein the circuit is further configured to drive the piezoelectric element with a plurality of haptic feedback pulses.

36. The apparatus of claim 34, wherein the circuit is configured to drive the measurement pulses from only a first side of the piezoelectric element and the haptic feedback pulses are driven from both the first side and also an opposing second side of the piezoelectric element.