Give Your Phone a Pat on the Back

With billions of people worldwide now using smartphones on a daily basis, there is no doubting that the touchscreen is here to stay. And with good reason — it is an interface that is always right where it needs to be, and it doesn’t get much simpler than touching what you want to interact with. But maybe it is time that we take a step back and reevaluate this ubiquitous interface.

That is what engineers at Columbia University and Snap Inc. think, anyway. Noting an inherent problem with the technology — that users’ fingers block the very display that they are interacting with — they have developed a new hybrid touch interface, called BackTrack, that also utilizes the normally uninstrumented back of the device as an additional input surface.

BackTrack requires no power supply, wireless connection, or modifications of the operating system. The system accomplishes this by extending the capacitive sensing grid present on the front of the device to the back. When a finger comes into contact with the front capacitive sensors, the electric field will be distorted, and this can be interpreted as a touch. The insight here is that by attaching conductors to the sensor grid, and wrapping them around the back of the device to create an electrode grid, finger interactions with the conductors on the back will also be made to register on the capacitive sensor on the front.

Easy, right? Not so fast. To get high resolution sensing, you would need to have a dense grid of conductors that would have to be routed to the back without intersecting. Further, these traces need to be well separated — any traces that are placed too close together would introduce additional capacitance that would interfere with touch sensing.

Instead, the team took a clever approach that takes advantage of the fact that smartphone screens are not only touchscreens, but multitouch screens. By placing only two rows of electrodes near the edge of the front screen, it is possible to encode x and y coordinates that specify any position on the screen. The electrodes on the rear trackpad are arranged such that any area touched will register with both an x and y electrode. The front electrodes are made of indium tin oxide coated glass, which makes the surface transparent, much like a screen protector. This front covering does not interfere with normal operation. A switch on the side of the device can turn off the trackpad to prevent unintentional touches.

A BackTrack prototype was built into a custom case for a Google Pixel 2 with a 28mm by 28mm rear trackpad. The interface was evaluated by a group of twelve participants that had no previous exposure to it. They were each asked to perform a series of tapping and swiping actions. After only three minutes of practice, accuracies of 99.16% and 100% were achieved in tapping and swiping, respectively. No participants noted any disruptions to normal operation of the front touchscreen.

One thing to note about BackTrack is that while it makes use of the existing touchscreen, it does so in a new way (e.g. x and y coordinates inferred from a 2D array of electrodes), so each app that makes use of the rear trackpad must be developed specifically for it. For demonstration purposes, the team built a web browser, a camera app, a game, and a file manager app. Also at present, the prototype trackpad is not capable of multitouch interactions. Despite these limitations, BackTrack is an easy to use, unobtrusive addition to a smartphone and may prove to be a very useful add on.