An Ultra-Precision Absolute-Type Multi-Degree-of-Freedom Grating Encoder

Area of Science:

• Optics and Photonics
• Metrology
• Mechanical Engineering

Background:

• Accurate multi-axis measurement is crucial for advanced applications.
• Existing encoders often lack simultaneous four-DOF absolute measurement capabilities.
• High-precision positioning is essential for large-scale optical systems.

Purpose of the Study:

• To demonstrate the first absolute-type four-degree-of-freedom (four-DOF) grating encoder.
• To achieve simultaneous measurement of three-axis pose (θx, θy, θz) and one-axis out-of-plane position (Z).
• To validate the encoder's performance with a prototype.

Main Methods:

• Utilized a grating encoder with a stationary reading head and movable grating reflector.
• Employed three diffracted beams (0th, +1st, -1st order) and three quadrant photodetectors (QPDs).
• Developed modeling and decoupling algorithms for independent four-DOF absolute position calculation.

Main Results:

• Successfully demonstrated an absolute-type four-DOF grating encoder.
• Achieved simultaneous measurement of θx, θy, θz, and Z with sub-arcsecond and sub-micrometer accuracy.
• Verified the theoretical decoupling and independent calculation of four DOFs.

Conclusions:

• The developed grating encoder is the first to achieve absolute simultaneous measurements of four-DOF position and pose over a large range.
• This technology offers significant benefits for multi-DOF positioning applications.
• Potential applications include large-scale synthetic aperture optics (SAO), such as stitching off-axis parabolic mirrors and pulse compression gratings.