Linear Resonant Actuator Duty Cycle Control Using Back-EMF
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Solution Overview
Problem
Conventional methods for controlling linear resonant actuators (LRAs) face challenges such as limited power delivery, slow response times, and susceptibility to auto-resonance errors due to varying resonant frequencies and lack of mechanical dampers, which affect the ability to provide crisp haptic feedback.
Innovation Solution
Dynamically adapting the drive duty cycle of LRAs by adjusting the high-impedance time to maximize power application and minimize detection time for resonant frequency, using a method that includes detecting back electromotive force (BEMF) to optimize the duty cycle based on current operating conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the driver is forced into high-Z mode for extended periods to measure BEMF and detect resonant frequency, then measurement accuracy is improved, but power delivery to the LRA is reduced and response time increases
Solution Approach 1:
The patent applies dynamics by making the high-Z measurement window adaptive rather than fixed. The controller dynamically adjusts the duration and timing of high-Z states based on detected resonant frequency and operating conditions. This allows the system to minimize high-Z time while still accurately capturing BEMF zero-crossings, thereby reducing the trade-off between measurement accuracy and response time.
Solution Approach 2:
The patent uses feedback by continuously monitoring BEMF signals and using the detected resonant frequency information to adjust subsequent drive waveforms. The system measures BEMF during high-Z states, detects zero-crossings to determine resonant frequency, then feeds this information back to optimize future measurement and drive timing, creating a closed-loop system that improves both accuracy and efficiency over time.
2Power
If the drive duty cycle is increased to deliver more power to the LRA, then vibration amplitude is improved, but the time available for BEMF measurement and resonant frequency detection is reduced
Solution Approach 1:
The patent applies periodic action by implementing regular high-Z measurement windows within the drive cycle. Instead of continuous driving, the system periodically inserts high-Z states at strategically timed intervals to capture BEMF signals. This periodic measurement approach ensures sufficient power delivery during drive phases while maintaining accurate resonant frequency detection through regular BEMF sampling.
Solution Approach 2:
The system dynamically adjusts the drive duty cycle based on detected resonant frequency and operational requirements. By making the duty cycle adaptive rather than fixed, the system can optimize power delivery while ensuring adequate high-Z measurement time is allocated, resolving the contradiction between power delivery and measurement precision.
3Adaptability or versatility
If the resonant frequency varies due to temperature changes and spring elasticity loss, then adaptability is improved, but measurement and control complexity increases
Solution Approach 1:
The patent applies self-service by enabling the LRA system to automatically track and adapt to its own changing resonant frequency without external intervention. The built-in BEMF measurement and zero-crossing detection circuitry continuously monitors frequency shifts caused by temperature or aging, and the controller automatically adjusts drive parameters accordingly. This self-adapting mechanism provides robust frequency tracking while minimizing additional control complexity.
Solution Approach 2:
The system uses feedback to continuously monitor BEMF signals and detect resonant frequency variations in real-time. This feedback loop allows the system to automatically compensate for frequency drift due to temperature changes or spring elasticity loss, maintaining optimal performance without requiring complex external calibration or adjustment mechanisms.
4Device complexity
If conventional quarter-wave drive is used to simplify control, then device complexity is reduced, but power delivery efficiency is limited to about half the possible power
Solution Approach 1:
The patent applies dynamics by transitioning from a fixed quarter-wave drive pattern to an adaptive drive scheme. The controller dynamically adjusts the drive waveform characteristics based on detected resonant frequency and operational conditions. This dynamic adaptation allows the system to maintain simple control architecture while significantly improving power delivery efficiency by optimizing the timing and duration of drive phases.
Solution Approach 2:
The system changes drive parameters such as duty cycle, frequency, and phase timing based on detected resonant frequency. By dynamically adjusting these parameters rather than using a fixed quarter-wave pattern, the system achieves higher power delivery efficiency while maintaining relatively simple control implementation through parameter optimization rather than architectural complexity.
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 allows for faster acceleration, higher G-forces, and reduced auto-resonance errors, enabling LRAs to provide crisp haptic feedback comparable to mechanical buttons.
Implementation Method 1
Back EMF (BEMF) is an EMF that is induced by a coil turning inside a magnetic field, which, for ERM devices, acts against the applied voltage that is causing the motor to spin
Implementation Method 2
An LRA has an inherent resonant frequency, and when LRA is driven with a frequency other than its resonant frequency, the performance and efficiency is dramatically reduced
Data Source
Figure 1(A)~1(B)
Figure 2
Figure 3(A)~3(B)
AI summary
Disclosed are various techniques operating a linear resonant actuator (LRA). In some aspects, a method for operating an LRA includes generating an LRA control signal having a period, the period having an active portion and a high-Z portion according to a duty cycle; detecting, during the high-Z portion of the period, a back electromotive force (BEMF) threshold voltage crossing time and zero voltage crossing time; calculating a period; calculating a BEMF measurement window; calculating a target duty cycle based on the period, the BEMF measurement window, and a margin time; and adjusting the duty cycle of the LRA control signal towards the target duty cycle.