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Ultrasonic touch detection could help iPhone screens work better in the rain

A future version of the iPhone could work better in the rain or the bath by detecting only touches of the display by the user, with Apple coming up with a way to tell the difference between finger tips and raindrops using ultrasonic acoustic sensing.

Over the years, Apple has made its mobile devices more water resistant, to the level that an Apple Watch could be used to monitor the wearer while they are swimming. While devices are becoming more capable of withstanding moisture and the elements in general, the ability to use the touchscreen-equipped iPhone and other products in wet environments is still an issue that has no solution.

Current capacitive touchscreen systems operate by detecting changes in voltage and electrical flow across the display, such as when a person touches the glass. The technique is effective in most situations, but as water is capable of making similar voltage changes, this can mean a device could detect rain or water droplets as unwanted touches, or for complete immersion, can prevent the display from being used at all.

In a patent application published by the US Patent and Trademark office on Thursday, Apple proposes the use of an "Ultrasonic Polarizer" to act as a touch-sensing system that can filter out fake touches from water, leaving only the user's own to be interpreted by the device.

The core of the idea focuses on an acoustic touch sensing system using one or more transducers coupled to the device's surface, such as piezoelectric transducers, which can transmit ultrasonic waves along the surface to another set of transducers.

An 'exemplary configuration' of an acoustic touch sensing circuit

Objects touching the display, such as the user's finger, interrupts the wave's progress, changing how they travel across the surface. By detecting these reflections, delayed waves, or an absence of waves, the display can determine the physical touch point, and use that for the input.

More importantly, the patent application proposes this style of device could be improved further, as such a system would still have to deal with anything that comes into contact with the display. In short, the basic form would not prevent water drops from being detected as physical touches.

A solution to this is the use of the titular ultrasonic polarizer, which can be fitted between the transducers and the touch surface. In this case, the polarizer would be able to filter out compressional waves relating to light touches, like water, over firmer shear wave touches from a finger, improving water rejection.

The polarizer could be made using multiple layers of a piezoelectric material with a poling direction different to the transducer, or a magnetic material that can generate eddy currents, with both able to cancel out the unwanted water-based compressional waves. A multi-layer structure using varying materials and layer thicknesses is also proposed as a way to filter out compressional waves from shear waves.

A simple flowchart of two sets of transducers sending and receiving acoustic energy, filtering for water throughout
A simple flowchart of two sets of transducers sending and receiving acoustic energy, filtering for water throughout

Apple files numerous patent applications with the USPTO on a weekly basis, but while the existence of a filing may not guarantee the appearance of the concept in a future product or service, it does give an indication of areas of interest for Apple's research efforts.

This is not the only idea Apple has come up with to make water-based usage a reality. In October 2018, two patent applications for "Finger tracking in wet environment" suggested the use of filtering to determine whether a detected touch is intended by the user or not, by analyzing the number of touch nodes interacted with in each touch event.

Acoustic touch has also been explored in a similar area, namely Touch ID. Multiple filings have suggested employing transducers and ultrasonic monitoring of the display to perform acoustic fingerprint imaging, bringing not just the possibility of using Touch ID back to the iPhone, but one that would work across the entire display, rather than using a designated fingerprint sensor.