Back-EMF Sensing of Linear Resonant Actuators for Detection of Tap-Induced Vibration

Abstract

Abstract This paper examines the dynamics, detection, and interpretation of back electromotive force (EMF) in linear resonant actuators (LRAs). LRAs are small motors that are commonly used to generate haptic signals in a wide variety of devices including phones, smartwatches, and video game controllers. LRAs use a coil attached to the motor chassis to generate a magnetic field. This magnetic field causes displacement of a magnetic mass that is attached to the LRA chassis using a spring. The spring acts to return the mass to its equilibrium position, causing it to vibrate. The physical design of an LRA produces dynamics that are similar to a mass-spring-damper system. Haptic signals are generated by moving the internal mass at frequencies that can be felt by the user. A possible application of sensing back-EMF in LRAs is detecting user taps on consumer electronic devices such as smartphones and smart watches. Active sensing, which requires driving the LRA with an alternating current, can be used for pressure and environment sensing but requires careful consideration of the frequency of the driving signal and sampling rate. Passive sensing such as tap, swipe, and motion sensing can be realized with limited current draw, but are difficult to utilize when the LRA is actively being used for generating haptic effects. This paper applies passive back-EMF sensing to detect taps and explores the classification of different tap intensities.