Control, Sensor Calibration, and Parasitic Torque Cancellation of a Dual-Rotor Haptic Actuator

Vibrotactile haptic sensations are commonly achieved by a Linear Resonant Actuator (LRA) or an Eccentric Rotating Mass (ERM). ERMs dominated the early years of the vibrotactile haptic market in large part due to their simplicity and low cost. An ERM produces output proportional to the square of its angular velocity. To achieve a desired force output, the device must accelerate to the associated velocity. In typical open-loop methods of ERM control, this acceleration takes some finite time, a limit on responsiveness. This limitation has led to increased emphasis on LRAs as manufacturers seek improved responsiveness in devices such as smart phones, game controllers, and interfaces for virtual reality systems. Another limitation of ERMs is that they cannot decouple the amplitude of output from the frequency of vibration, despite both being relevant to haptic perception. This work addresses these two deficiencies by presenting a novel closed-loop system comprised of two ERMs. Output is controlled by cascaded velocity and phase control loops–the velocities of the motors controls the output frequency, and the relative phase controls the output amplitude, decoupling the two outputs. Also, changes in relative phase can be performed faster than starting up the motor from rest, improving response time. In this implementation, the switching time from a cancellation state to maximum output is faster than thirty milliseconds. Also introduced are novel counter-balances to cancel parasitic torque, and learning algorithms that leverage symmetry and accelerometer measurements to calibrate the relative alignment of the eccentric masses with sensors.