Pushing the Boundaries of Buttons
HaptiCoil is a new take on the physical button that uses a hydraulically-coupled speaker to both provide haptic feedback and sense touches.
Buttons may be the workhorses of the user interface, but with the proliferation of touchscreens in recent years, they are now starting to feel quite dated. We have grown accustomed to dynamic, digital buttons that can adapt to the needs of different applications or provide the user with feedback. But a touchscreen is just not the right tool for every job. Sometimes a physical button is the only thing that makes sense.
Perhaps we can at least spice those clunky old buttons up a bit. That is the goal of a trio of engineers at the University of Illinois Urbana-Champaign, at least. They have developed a unique system called HaptiCoil that turns physical buttons into dynamic canvases that can provide their users with tactile feedback. They also have the ability to detect when they are being pressed, which is probably a good feature to include in any button design.
The HaptiCoil platform is an embedded system specifically designed for rapid prototyping of soft, compact, and highly customizable haptic buttons. It has the ability to offer a wide tactile bandwidth (1–500 Hz), enabling a broader range of physical sensations than typical solutions like the vibration motors found in smartphones or gaming controllers.
The device is equipped with a mass-produced, waterproof planar micro-speaker. Instead of simply playing sounds, these speakers are adapted to deliver mechanical feedback directly to a user’s skin through a novel hydraulic coupling mechanism. When the speaker's voice coil moves, it transfers that motion through an incompressible fluid (in this case, regular tap water) to a soft membrane that the user feels, creating a dynamic and responsive tactile experience. The actuation and sensing are controlled by a Teensy 4.0 development board.
It is by measuring changes in self-inductance of the speaker’s coil that the system can detect when a user presses on the button. As the user presses, the deformation of the membrane moves the speaker coil, altering its inductance in a detectable way. This dual function of sensing and feedback happens with only two electrical connections, keeping the electronics simple.
To make a HaptiCoil button, the team 3D-printed a small fluid chamber (slightly larger than the speaker itself) using clear resin for easy inspection. The speaker is then bonded to the chamber using cyanoacrylate adhesive to prevent leaks. A thin, flexible silicone membrane — the part the user touches — is carefully attached over the chamber. Two main techniques ensure good adhesion — a plasma surface treatment followed by the addition of pressure-sensitive adhesive sheets for clean builds, or a more cost-effective but messier direct gluing method.
Once the assembly is complete, the chamber is filled with tap water via a small side port, and any remaining air bubbles are carefully evacuated to ensure consistent hydraulic performance. The port is then sealed with room-temperature vulcanizing silicone. The result is a compact, durable, and waterproof button that can easily integrate into a variety of devices.
The engineers envision HaptiCoil being used in many applications, ranging from spatial computing and digital inking devices to remote controls. Importantly, the modular design of HaptiCoil allows researchers and developers to easily customize button size, shape, and feedback characteristics, encouraging rapid experimentation in haptic experience design.
