Ultrasonic and strain dual mode sensor for contact switch

Patent metadata

Field	Value
Publication number	US-9698776-B2
Application number	US-201514639964-A
Country	US
Kind code	B2
Filing date	Mar 5, 2015
Priority date	Mar 5, 2014
Publication date	Jul 4, 2017
Grant date	Jul 4, 2017

Abstract

A system and method for ultrasonic touch switch combined with piezoelectric touch mode is described. The sensor structure may be embodied as a substrate with a piezoelectric element at the back surface of the substrate, with the front surface being a touch sensitive surface. Both operational modes are possible by use of the same sensor element with filtering of the signals so that different advantages of two modes are combined, while disadvantages of either mode are mitigated. The tolerance of substrate thickness of ultrasonic touch mode is improved by use of a wide range of frequency deviation of drive signal by filtering out the fundamental modulation frequency from the output signal and wherein only sharp pulses corresponding to abrupt impedance changes of the piezoelectric element are extracted. The amplitude of the sharp pulses decreases with touching the front surface of substrate. At the same time pressure force applied to the front surface causes the substrate to undergo bending displacement and causes piezoelectric element expansion strain to generate voltage. These two modes are combined and used to control switch of appliance or equipment.

First claim

What is claimed is:

1. A device for dual mode operation of ultrasonic touch switch mode in combination with piezoelectric touch switch mode which detects a touch at a touch sensitive front surface of a substrate and having a piezoelectric element at a back surface, said device comprising
a drive circuit coupled to said piezoelectric element for applying a frequency modulated drive signal thereto to detect resonance at the substrate,
wherein said piezoelectric element is connected to a terminal end of a first capacitor at a first node, the first capacitor disposed between said drive circuit and said piezoelectric element; and wherein said piezoelectric element is connected at said first node to a terminal end of a second capacitor downstream of said first capacitor, and
a detector arrangement electrically coupled to the piezoelectric element configured to detected and filter signals indicative of abrupt changes in impedance corresponding to a sensed ultrasonic mode signal indicative of a touch or no touch condition using said detected resonance, and a sensed piezo strain signal indicative of a touch or no touch condition, wherein said detector arrangement comprises:
a peak detector circuit for detecting amplitude variations of the frequency modulated signal from said piezoelectric element and generating a pulse signal having a repetition rate corresponding to the modulation signal;
a first communication path including a first band pass filter circuit responsive to the output of the peak detector circuit for detecting a high frequency signal pulse indicative of a sensed ultrasonic signal, and generating a first output signal indicative thereof; and
a second communication path including a first low pass filter circuit resistively coupled to said first capacitor at said first node for detecting a low frequency signal indicative of a sensed strain of said piezoelectric element, and generating a second output signal indicative thereof.

2. The device of claim 1 , wherein in the ultrasonic mode, frequency deviation of the frequency modulated (FM) drive signal is sufficiently wide to detect an abrupt change of impedance representing a frequency change, thereby ensuring a given tolerance of thickness of the substrate.

3. The device of claim 1 , wherein the detector arrangement further comprises an acoustic gain controller (AGC) for maintaining amplitudes of the signal pulses to a substantially constant value to increase tolerance of sensitivity for different piezoelectric elements.

4. The device of claim 3 , wherein the amplitudes of the signal pulses are reduced by finger touch at the surface of the substrate.

5. The device of claim 1 , wherein the detector arrangement electrically coupled to the piezoelectric element is configured to detect the piezoelectric strain signal induced by planar expansion of the piezoelectric element due to pressure from a finger or stylus.

6. The device of claim 1 , wherein signals from both the ultrasonic mode and the piezoelectric touch mode are output to a decision circuit which discriminates between noise and a detected touch condition, and provides an output to a switch controlling a state of an appliance.

7. The device of claim 1 , further comprising a decision circuit for receiving at least the first and second output signals, the decision circuit including a processor for identifying a finger touch condition or no-touch condition according to the first and second output signals and weighting the first and second output signals based on one or more rules associated with operation of said device.

8. The device of claim 7 , wherein the one or more rules associated with operation of said device include weighting based on one or more environmental conditions.

9. The device of claim 8 , wherein the one or more environmental conditions includes one or more of: external vibration applied on said device; and a surface contamination material on said device substrate.

10. The device of claim 7 , wherein the detector arrangement further comprises:
a third communication path including a second low pass filter circuit coupled to the output of the peak detector for detecting a low frequency signal lower than that of said first low pass circuit and indicative of thermal energy from a finger impinging upon the substrate, and generating a third output signal for receipt by said decision circuit.

11. The device of claim 10 , wherein the detector arrangement further comprises:
a fourth communication path including a third low pass filter circuit coupled to the output of the peak detector for capacitively detecting a low frequency signal and providing a unity gain output signal indicative of a steady variation in the output of the peak detector for receipt by said decision circuit.

12. The device of claim 1 , wherein the first low pass filter circuit has a passband of about 10 Hz to 100 Hz; and the first band pass filter circuit has a passband of about 5 KHz to 25 KHz.

13. The device of claim 1 , further comprising a time duration detector for comparing the amplitude of the detected first output signal indicative of a sensed ultrasonic signal as a function of time with a threshold value to determine whether the detected first output signal is indicative of a touch detection or a false activation.

14. A device for dual mode operation of ultrasonic touch switch mode in combination with piezoelectric touch switch mode which detects a touch at the surface of a substrate, the device comprising:
a piezoelectric element positioned at a back surface of the substrate to be touched;
a drive circuit coupled to said piezoelectric element for applying a frequency modulated drive signal thereto to detect resonance at the substrate;
a first detector arrangement coupled to the piezoelectric element for converting the frequency modulated signal to detect amplitude variations and generate a first output signal indicative of a sensed touch or no touch condition according to changes in the detected amplitudes based on resonance at the substrate;
a second detector arrangement coupled to the piezoelectric element and configured to generate a second output signal indicative of a sensed strain on said piezoelectric element and representative of a sensed touch or no touch condition; and
a decision circuit including a processor responsive to the outputs of the first and second detector arrangements for identifying a touch condition or no-touch condition according to the amplitudes of the first and second detector output signals and weighting of the first and second detector output signals based on one or more rules associated with operation of said device.

15. The device of claim 14 , wherein the first detector arrangement includes a peak detector and a band pass amplifier responsive to said peak detector for generating said first output signal representing a sensed touch or no touch condition according to changes in pulse amplitude and duration based on resonance at the substrate; and wherein the second detector arrangement includes a low pass amplifier for removing high frequency components for generating said second output signal representing a sensed touch or no touch condition based on detected strain of said piezoelectric element.

16. The device of claim 15 , further comprising a low pass filter amplifier coupled to the output of the peak detector for detecting a low frequency signal indicative of thermal energy from a finger impinging upon the substrate, and generating a third output signal for receipt by said decision circuit.

17. The device of claim 14 , further comprising a time dura

Assignees

• Measurement Spec Inc

Inventors

• Toda Minoru
• Park Kyung Tae

Classifications

• H03K17/964 Primary

  Piezoelectric touch switches · CPC title

• H03K2217/96007

  by reflection · CPC title

• H03K2217/94026

  Automatic threshold calibration; e.g. threshold automatically adapts to ambient conditions or follows variation of input · CPC title

• H03K2217/94036

  Multiple detection, i.e. where different switching signals are generated after operation of the user is detected at different time instants at different locations during the actuation movement by two or more sensors of the same or different kinds · CPC title