MEMS Scanner Resonant Frequency Tracking With Digital Phase Lock
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Solution Overview
Problem
MEMS scanning mirrors in pico-projection systems face challenges in identifying and operating on the correct resonant mode frequency due to multiple resonant modes, leading to suboptimal operation and reduced effectiveness.
Innovation Solution
A digital phase-locked loop system with a drive signal generator that uses direct digital synthesis and phase comparison to accurately match the horizontal drive signal to the correct resonant mode frequency, tracking it over temperature and time while avoiding adjacent modes, and employing a phase correct accumulator to adjust the frequency with high precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If peak search algorithms are used to find resonant modes, then the MEMS scanner can operate at resonant frequency, but the system becomes susceptible to local minima/maxima and requires complex search inquiries far from current operating point
Solution Approach 1:
The patent implements a feedback mechanism where the system continuously monitors the resonant frequency by measuring the phase and amplitude response of the MEMS scanner. The controller adjusts the drive frequency based on feedback from the scanner's actual response, enabling automatic tracking of the resonant mode without complex peak search algorithms. This feedback loop replaces the need for far-reaching search inquiries by making small, incremental adjustments based on real-time measurements.
Solution Approach 2:
The system performs preliminary characterization of the MEMS scanner's frequency response during manufacturing or initial setup, storing the resonant frequency information for later use. This preliminary action eliminates the need for complex runtime peak searching, as the system can directly initialize operation at the known resonant frequency and make minor adjustments as needed.
2Measurement precision
If high resolution ADC is used to detect low sensor signal size far from resonant mode, then frequency detection precision improves, but system cost and complexity increase
Solution Approach 1:
Instead of using a high-resolution ADC to detect very small signals far from resonance, the system applies partial action by operating close to the resonant frequency where the signal amplitude is naturally larger. This approach uses moderate ADC resolution combined with small frequency excursions from the resonant point, achieving sufficient measurement precision without requiring expensive high-resolution converters.
Solution Approach 2:
The system exploits the mechanical vibration characteristics of the MEMS scanner at resonant frequency, where the natural oscillation amplifies the sensor signal. By driving the scanner at or near its resonant frequency, the system generates sufficient signal amplitude that can be detected by standard-resolution ADCs, eliminating the need for high-resolution converters.
3Measurement precision
If two measurements are taken for every frequency move to determine direction, then accurate resonant mode tracking is achieved, but operation time increases
Solution Approach 1:
The system maintains continuous operation by using a seamless frequency sweeping mechanism that continuously adjusts the drive frequency based on real-time measurements. Instead of stopping to take discrete pairs of measurements, the system continuously sweeps through frequencies and automatically identifies the resonant point, eliminating idle time between measurements and maintaining uninterrupted operation.
Solution Approach 2:
The patent implements periodic frequency sweeping where the system rapidly cycles through a small frequency range around the expected resonant point. This periodic action allows the system to quickly identify the resonant mode through repeated measurements over short intervals, achieving accurate direction determination without significant time loss. The periodic nature enables efficient use of measurement cycles.
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
Enables efficient operation of the MEMS scanner on the intended resonant mode with improved beam deflection efficiency and reduced susceptibility to local minima/maxima, enhancing overall performance and effectiveness.
Implementation Method 1
The horizontal sweep is typically done at one of the resonant mode frequencies of the scanning mirror that is on the order of 18 kHz. Operating on a resonant mode allows maximum beam deflection with minimal input energy.
Implementation Method 2
The phase comparator is capable of receiving the horizontal sensor signal from the MEMS scanner, as well as the horizontal drive signal, and comparing the signals to determine phase errors for each horizontal cycle
Implementation Method 3
A digital phase-locked loop system with a drive signal generator that uses direct digital synthesis and phase comparison to accurately match the horizontal drive signal to the correct resonant mode frequency
Data Source
AI summary
A method for operating a micro-electro-mechanical system (MEMS) scanner on a resonant mode frequency is provided. The method includes generating a drive signal for a MEMS scanner. A sensor signal is received from the MEMS scanner. The drive signal is compared to the sensor signal. An accumulated correction signal is generated based on the comparison of the drive signal and the sensor signal. The drive signal for the MEMS scanner is then adjusted based on the accumulated correction signal.