A device for detecting a touch at a surface of a substrate comprises a piezoelectric element positioned at a back surface of the substrate to be touched. A drive circuit is 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 is configured to generate a first output signal indicative of a sensed touch 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 the piezoelectric element is configured to generate a second output signal indicative of a sensed touch or no touch condition according to a sensed strain on said piezoelectric element. A decision circuit including a processor is configured to identify a touch condition or no touch condition at the surface of the substrate according to the first and second output signals.

A device for detecting a touch at a surface of a substrate comprises a piezoelectric element positioned at a back surface of the substrate to be touched. A drive circuit is 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 is configured to generate a first output signal indicative of a sensed touch 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 the piezoelectric element is configured to generate a second output signal indicative of a sensed touch or no touch condition according to a sensed strain on said piezoelectric element. A decision circuit including a processor is configured to identify a touch condition or no touch condition at the surface of the substrate according to the first and second output signals.

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.