LRA Resonance Tracking Using Staircase Drive and Back-EMF Feedback
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Solution Overview
Problem
Existing methods for determining the resonance frequency of linear resonant actuators (LRAs) in electronic devices are inaccurate and complex, leading to undesirable audible noise and vibration issues due to deviations from the actual resonant frequency during haptic events, and require separate measurements that disturb the user.
Innovation Solution
A method involving a drive signal with at least two intervals of nearly-constant non-zero current, allowing for real-time estimation of the resonance frequency by determining the phase difference between the back electromotive force (BEMF) and current signal using a feedback control loop, which adjusts the drive signal frequency to converge to the resonant frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If separate measurements are performed to determine resonance frequency, then measurement precision is improved, but device complexity and user disturbance increase
Solution Approach 1:
The patent combines the resonance frequency measurement function with the existing haptic driver chip, integrating sensing blocks and processing logic into the same device that drives the LRA. This eliminates the need for separate external measurement equipment and reduces system complexity while maintaining measurement precision.
Solution Approach 2:
The haptic driver chip performs self-diagnosis by monitoring its own driving signals and the LRA's response during normal haptic operations. The sensing blocks within the driver chip automatically detect resonance frequency without requiring external intervention or separate measurement procedures, enabling the system to serve itself.
2Device complexity
If resonance frequency is not accurately tracked, then device complexity is reduced, but audible noise and vibration issues increase
Solution Approach 1:
The patent implements a feedback control loop where the sensing blocks continuously monitor the LRA's vibration response during haptic events, and the processing logic adjusts the driving frequency in real-time to maintain resonance. This closed-loop feedback system automatically tracks resonance frequency variations without complex external control mechanisms, reducing audible noise while keeping the system relatively simple.
Solution Approach 2:
The system dynamically adjusts the driving frequency based on real-time resonance conditions detected during haptic operations. Rather than using a fixed frequency or complex pre-calibration procedures, the driver chip continuously adapts the frequency to match the LRA's current resonant state, effectively reducing audible noise through dynamic tracking.
3Use of energy by moving object
If resonance frequency deviates during haptic events, then power transmission efficiency is reduced, but measurement and control complexity increases
Solution Approach 1:
The patent enables continuous resonance frequency tracking during haptic events by integrating sensing and processing blocks that operate throughout the entire haptic event duration. This continuous monitoring and adjustment ensures the driving frequency remains optimized for power transmission efficiency without requiring complex intermittent measurement procedures or system interruptions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach provides accurate and robust resonance frequency tracking during haptic events, reducing audible noise and simplifying device design by eliminating the need for separate measurements, ensuring precise vibration control and efficient power transmission.
Implementation Method 1
An LRA is basically a Voice Coil Motor (VCM) where electrical current running in a stationary coil actuates a moveable magnet back and forth, in a linear manner
Implementation Method 2
The magnet is held by a set of springs and there is usually damping oil surrounding the moving part to increase working bandwidth. Thus, from the mechanical point of view, this system can be described as a Mass-Spring-Damper (MSD) system. Since the MSD system is in fact a resonator characterized by a resonant frequency f0
Implementation Method 3
determining a back electromotive force (BEMF) of the LRA
Data Source
Figure 1~2B
Figure 3A~3B
Figure 4A~4B
AI summary
The invention relates to a method for driving a linear resonant actuator (LRA), and a processing system in a driver chip for driving the LRA using such method. The method comprises providing a drive signal to the LRA, wherein the drive signal is configured such that each period of a current signal over the LRA has at least two intervals with nearly-constant non-zero current and substantially shorter duration than the period of the current signal, wherein the drive signal is an oscillating voltage or current drive signal. Furthermore, the invention relates to a control circuit configured to drive a LRA.