How can an incremental linear encoder achieve nanometer-level displacement measurement using a grating ruler?
Release Time : 2025-10-30
The core of incremental linear encoders for nanometer-level displacement measurement lies in the synergistic effect of optical diffraction, moiré fringe effect, and electronic subdivision technology of the grating ruler. Its working principle begins with the precise grating structure of the grating ruler—the main grating and the indicator grating are periodically arranged at micrometer-level intervals. When illuminated by a laser, the light wave diffracts at the edges of the grating, forming multiple coherent beams. These light waves, through interference and superposition, generate alternating bright and dark moiré fringes with a fringe spacing much larger than the grating's own grating spacing, achieving initial optical amplification of the displacement. For example, when the two gratings overlap at a small angle, the fringe spacing is inversely proportional to the grating period, converting nanometer-level displacement into micrometer-level fringe movement, providing a captureable signal for subsequent photoelectric detection.
The photodetector array is the key component for the encoder to achieve high-precision measurement. When the grating ruler and the reading head move relative to each other, the changes in the brightness of the moiré fringes are captured in real time by the photodiode array and converted into a sinusoidal electrical signal. Due to the difference in grating density between the main grating and the indicator grating, the signal received by the detector contains rich phase information. At this point, the A and B phase quadrature signals (90° phase difference) distinguish the direction of motion through the direction-sensing circuit, providing a basis for electronic subdivision. The Z-phase reference signal marks the absolute zero position of the grating ruler, ensuring the consistency of the measurement reference after each power-on. This multi-signal collaborative mechanism allows the encoder to not only detect displacement but also determine the direction of motion through phase relationships, providing complete information for high-precision control.
Electronic subdivision technology is the core means to overcome the physical resolution limitations of gratings. Through interpolation algorithms and high-precision AD conversion, the encoder further subdivides a single moiré fringe period. For example, a grating with a 1μm period combined with a 4096x subdivision circuit can improve the resolution to 0.24nm. This process relies on the signal processing chip's precise sampling and fitting of the sinusoidal signal, calculating waveform zero-crossing points, peaks, and other characteristic points to achieve sub-pixel-level displacement resolution. The application of differential signal transmission technology (such as A+/A-, B+/B-) effectively suppresses common-mode noise, ensuring signal stability during long-distance transmission, maintaining repeatability within ±1nm even in the face of electromagnetic interference.
Innovations in materials and processes have provided the physical basis for nanoscale measurements. Modern optical encoders employ ultra-precision machining techniques to etch high-density, low-error lines onto glass or metal substrates; the uniformity of the line spacing directly affects measurement accuracy. The optical system inside the readhead is optimized through collimating lenses and beam splitters to ensure the laser beam illuminates the grating at an ideal angle, maximizing the interference effect. Sealed protective designs prevent the intrusion of oil, dust, and other impurities, avoiding signal attenuation or increased noise due to contamination. For example, some high-end encoders employ temperature control modules, using PID algorithms to maintain stable operation of optical components within ±0.1℃, eliminating the impact of temperature drift on measurement results.
In the semiconductor manufacturing field, the nanoscale precision of incremental linear encoders directly drives process breakthroughs. The lithography machine stage needs to achieve 0.1nm-level positioning to accurately project mask patterns onto silicon wafers. The encoder, through real-time feedback of stage displacement, works in conjunction with a laser interferometer for fine-tuning, ensuring subatomic-level precision in the wafer exposure process. Similarly, chip mounting machines rely on high-resolution encoders to precisely mount tiny chips onto substrates, requiring repeatability and positioning accuracy controlled at the nanometer level to meet the packaging demands of advanced processes. In these applications, encoder stability and response speed are equally critical—tens of thousands of pulses per second support the high-speed dynamic adjustments of the equipment.
In industrial automation, encoder requirements emphasize reliability and environmental adaptability. In CNC machine tools, linear encoders directly measure the actual position of the worktable, eliminating errors caused by lead screw thermal expansion and wear, ensuring tool trajectory errors in five-axis machining are controlled within ±0.001mm. Automated logistics sorting systems use encoders to monitor conveyor belt displacement, optimizing material handling efficiency with a positioning accuracy of 99.99% and a throughput exceeding 120,000 pieces per hour. New energy vehicle motor testing platforms require encoders to acquire displacement signals within 0.01 milliseconds, maintaining zero signal distortion even with 100-meter-long wiring, ensuring data accuracy during high-speed rotor rotation.
The development of incremental linear encoders will further integrate intelligent and integrated technologies. By integrating vibration sensors and temperature monitoring modules, encoders can upload equipment health data in real time, using cloud algorithms to predict maintenance needs and avoid unplanned downtime. Hybrid encoders combine incremental and absolute encoding functions, balancing dynamic response and power-off memory capabilities, expanding long-stroke application scenarios. The application of new materials such as metasurface gratings is expected to improve measurement accuracy to the picometer level, driving the development of cutting-edge fields such as quantum computing and gravitational wave detection. These innovations not only extend the technological lifespan of incremental encoders but also make them the "nerve center" of precision manufacturing in the Industry 4.0 era.