Water droplet rejection for capacitive micromachined ultrasonic transducer (CMUT) touch sensing based on spatial distribution of CMUT receivers

An ultrasonic touch sensor, comprising:

• a housing having an ultrasound chamber;
• a touch structure comprising a touch surface configured to receive a touch, wherein the touch structure is coupled to the housing and arranged over the ultrasound chamber, and wherein the touch structure comprises a touch interface at the touch surface;
• a first signal generator configured to:
• generate a first excitation signal for producing a first ultrasonic transmit wave, and
• generate a second excitation signal for producing a second ultrasonic transmit wave;
• a first ultrasonic transmitter, arranged within the ultrasound chamber, configured to:
• receive the first excitation signal, and
• transmit the first ultrasonic transmit wave toward the touch structure based on the first excitation signal;
• a second ultrasonic transmitter, arranged within the ultrasound chamber and different from the first ultrasonic transmitter, configured to:
• receive the second excitation signal, and
• transmit the second ultrasonic transmit wave toward the touch structure based on the second excitation signal;
• a first ultrasonic receiver, arranged within the ultrasound chamber, configured to:
• receive a first ultrasonic reflected wave produced by a reflection of the first ultrasonic transmit wave, and
• generate a first measurement signal representative of the first ultrasonic reflected wave;
• a second ultrasonic receiver, arranged within the ultrasound chamber and different from the first ultrasonic receiver, configured to:
• receive a second ultrasonic reflected wave produced by a reflection of the second ultrasonic transmit wave, and
• generate a second measurement signal representative of the second ultrasonic reflected wave;
• and
• a measurement circuit, coupled to the first ultrasonic receiver and the second ultrasonic receiver, configured to:
• measure a difference between the first measurement signal and the second measurement signal,
• perform a comparison based on the difference between the first measurement signal and the second measurement signal and a first detection threshold, and
• determine whether a no-touch event or a touch event has occurred at the touch surface based on whether the difference between the first measurement signal and the second measurement signal satisfies the first detection threshold.