Scanning Mirror Control Using LMS Tone Adders
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Controlling the angular motion of mirrors in scanned light beam displays is challenging due to mechanical vibrations at resonant frequencies, which distort the raster scan pattern and affect the quality of the display.
Innovation Solution
A feedback loop system that modifies scanning mirror drive signals using a Least Mean Square (LMS) harmonic controller with a harmonic coefficient weighting array, adjusting the amplitude and phase of harmonic signals to counteract high mechanical gain and non-linear characteristics, and employing a bridged-T compensator to stabilize the control loop.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the scanning mirror is operated at resonant frequency to achieve high-speed scanning, then scanning speed is improved, but mechanical vibrations are generated that distort the raster scan pattern
Solution Approach 1:
The patent implements a feedback control system using a phase-locked loop (PLL) that continuously monitors the mirror's resonant frequency and adjusts the drive signal accordingly. Position sensors detect the mirror's actual position, and this information is fed back to the control algorithm, which modifies the drive waveform to maintain accurate raster scanning despite resonant vibrations. This closed-loop feedback mechanism allows the system to operate at high speeds while compensating for distortion in real-time.
Solution Approach 2:
The patent dynamically adjusts the drive signal parameters (amplitude, frequency, phase) based on the mirror's resonant characteristics. The control algorithm modifies the drive waveform parameters to counteract the effects of resonant vibrations, transforming the drive signal characteristics in response to measured mirror behavior. This parameter adaptation enables maintenance of scan accuracy while operating at resonant frequencies for high-speed performance.
2Area of stationary object
If the mirror deflection is increased to cover the desired field of view, then the field of view is improved, but the mechanical vibrations and distortion are amplified
Solution Approach 1:
The patent applies pre-compensation to the drive signal by characterizing the mirror's resonant behavior in advance and incorporating compensation algorithms before actual scanning operation. The system measures the mirror's transfer function and resonant frequencies during calibration, then uses this pre-acquired information to pre-distort the drive signal in the opposite direction of expected vibrations. This preliminary action reduces the need for high-gain feedback and minimizes distortion amplification during high-deflection scanning.
3Manufacturing precision
If a high gain is used in the control loop to reduce distortion, then scanning accuracy is improved, but the control loop becomes unstable due to resonant frequencies
Solution Approach 1:
The patent employs adaptive control where the controller's characteristics dynamically change based on operating conditions. The phase-locked loop continuously tracks the mirror's resonant frequency, and the control algorithm adjusts its parameters in real-time according to the measured system response. This dynamic adaptation allows the system to maintain stability margins while achieving high accuracy, as the controller automatically adjusts to prevent instability rather than relying on fixed high gain settings.
Data Source
Figure 1
Figure 2
Figure 3
AI summary
A scanning beam projection system (100) includes a scanning mirror (116) having a fast-scan axis and a slow-scan axis. Movement on the slow-scan axis is controlled by a slow-scan scanning mirror control system (130). The control system receives position information describing angular displacement of the mirror. An outer loop of the control system includes least mean square (LMS) tone adders that determine harmonically related signals that when combined produce a scanning mirror drive signal. An inner loop of the control system compensates for a scanning mirror resonant vibration mode at a frequency within the frequency band occupied by the harmonically related signals.