Robust touch sensing via ultra-sonic sensors

A touch sensor, comprising:

• a housing having a recess;
• a touch structure coupled to the housing and arranged over the recess such that the recess forms an enclosed interior volume, wherein the touch structure comprises a touch surface and an inner surface arranged opposite to the touch surface, wherein the inner surface faces towards the enclosed interior volume;
• a temperature sensor arranged inside the enclosed interior volume and configured to measure a temperature;
• a memory configured to store a plurality of sets of reference samples that each corresponds to a different temperature, of a plurality of temperatures, or a different range of temperatures of a plurality of ranges of temperatures;
• a transmitter arranged inside the enclosed interior volume, wherein the transmitter is configured to transmit an ultra-sonic transmit signal towards the inner surface;
• a receiver arranged inside the enclosed interior volume and coupled to the transmitter via a propagation path, wherein the receiver is configured to receive an ultra-sonic reflected signal produced from the ultra-sonic transmit signal and the propagation path;
• and
• a sensor circuit, arranged inside the enclosed interior volume, configured to:
• receive the temperature from the temperature sensor,
• select, based on the temperature, a set of reference samples, of the plurality of sets of reference samples, to be used as a first plurality of reference samples,
• receive the ultra-sonic reflected signal from the receiver,
• acquire a plurality of digital samples, from the ultra-sonic reflected signal, within an observation window that starts at an expected time-of-flight of the ultra-sonic reflected signal plus a duration of the ultra-sonic transmit signal,
• calculate a difference of each digital sample, of the plurality of digital samples, from a corresponding reference sample, of the first plurality of reference samples,
• calculate a sum of the difference of each digital sample, the plurality of digital samples, from the corresponding reference sample, of the first plurality of reference samples,
• determine a first Euclidean distance of the plurality of digital samples to the first plurality of reference samples based on the sum, and
• determine whether a no-touch event or a touch event has occurred at the touch surface based on the first Euclidean distance.