Ultrasonic and strain dual mode sensor for contact switch

A method comprising:

• applying a frequency modulated drive signal to a piezoelectric element positioned at a back surface of a substrate to be touched to detect resonance at the substrate;
• detecting voltage changes of the piezoelectric element when said frequency modulated drive signal is applied and generating a first output signal indicative of a sensed touch or no touch condition according to said detected voltage changes;
• detecting a voltage signal based on a sensed strain on said piezoelectric element and generating a second output signal indicative of a sensed touch condition or no touch condition in response thereto;
• and determining a touch condition or no touch condition at the surface of the substrate according to the first and second output signals, wherein the second output signal from the sensed strain on the piezoelectric element determines the accuracy or validation of the touch/no touch condition.

A device for detecting a touch at a surface of a substrate, comprising:

• a piezoelectric element positioned at a back surface of the substrate to be touched;
• a drive circuit configured to apply a frequency modulated drive signal to the piezoelectric element to detect resonance at the substrate;
• a first detector arrangement coupled to the piezoelectric element and configured to generate a first output signal indicative of a sensed touch condition or no touch condition according to detected voltage changes and responsive to the frequency modulated drive signal applied to the piezoelectric element;
• a second detector arrangement coupled to said piezoelectric element and configured to generate a second output signal indicative of a sensed touch condition or no touch condition according to a sensed strain on said piezoelectric element;
• and
• a decision circuit including a processor configured to identify a touch condition or no touch condition at the surface of the substrate according to the first and second output signals,
• 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.