Design and development of a large range linear encoder with subnanometer resolution

This article presents the design and development of a large range linear encoder system, in which a displacement transducer is controlled to achieve a stable oscillation over the moving reference grating so as to modulate the motion being measured. Utilizing a special demodulating signal and according to linear control theory, a phase-locked feedback loop is designed to extract motion from the transducer's measurement signal. The measurement dynamics, limited by the sinusoidal nonlinearity in the loop, is analyzed. A method that compensates the effect of the reference grating's geometric error and that improves the measurement accuracy is also proposed. A prototype system, using an optical pickup unit as the displacement transducer and a holographic grating as the reference, is implemented. The performance of the system is tested to verify its resolution, bandwidth, and accuracy. A measurement resolution of 0.06 nm rms with 140 Hz bandwidth is achieved, and the measurement range is only limited by the size of the reference grating employed. All the experimental results confirm the theoretical analysis. Calibration of the system shows that after compensation the measurement accuracy is in the nanometer order