Haptics with Input: Back-EMF in Linear Resonant Actuators to Enable Touch, Pressure and Environmental Awareness

Today's wearable and mobile devices typically use separate hardware components for sensing and actuation. In this work, we introduce new opportunities for the Linear Resonant Actuator (LRA), which is ubiquitous in such devices due to its capability for providing rich haptic feedback. By leveraging strategies to enable active and passive sensing capabilities with LRAs, we demonstrate their benefits and potential as self-contained I/O devices. Specifically, we use the back-EMF voltage to classify if the LRA is tapped, touched, as well as how much pressure is being applied. The back-EMF sensing is already integrated into many motor and LRA drivers. We developed a passive low-power tap sensing method that uses just 37.7 uA. Furthermore, we developed active touch and pressure sensing, which is low-power, quiet (2 dB), and minimizes vibration. The sensing method works with many types of LRAs. We show applications, such as pressure-sensing side-buttons on a mobile phone. We have also implemented our technique directly on an existing mobile phone's LRA to detect if the phone is handheld or placed on a soft or hard surface. Finally, we show that this method can be used for haptic devices to determine if the LRA makes good contact with the skin. Our approach can add rich sensing capabilities to the ubiquitous LRA actuators without requiring additional sensors or hardware.