Study On The Methods To Improve The Resolution And The Accuracy Of Small Photoelectric Encoder

With the rapid development of airborne and spaceborne applications, portablemilitary investigation, positioning, command weapon system, higher demands wererequired for the angular accuracy and resolution of small photoelectric encoders.Since the quality of Moiré fringe photoelectric signals constrains high-resolutioninterpolation of small photoelectric encoder, and the interpolation error and thelong-period error are main factors that affect the accuracy of photoelectric encoders,it is very important to carry out research on the Moiréfringe signals calibration, newinterpolation methods, and long-period error correction to improve the resolutionand accuracy of small photoelectric encoders and thus follow the internationaladvanced level.Based on large reference at home and abroad, starting from the principle ofMoiré fringe photoelectric signals generation, the factors which affected theaccuracy of photoelectric signals were analyzed thoroughly, including the Moiréfringe signal’s quality for high-resolution interpolation, reasons of interpolation errorgeneration, and long-period errors for the accuracy of photoelectric encoders. Mucheffort was paid on the methods of improving the resolution and accuracy of smallphotoelectric encoders.The error calibration method was proposed for Moiré fringe photoelectric signals in the form of approximate triangular waves. The parameter equation ofMoiréfringe photoelectric signal waveform was built firstly, which contained directcurrent errors, amplitude errors and waveform distortions, and then Foruiertransform was applied to solve the waveform parameters. The high spatial harmonicsin the signal waveform were transformed into higher order components using themultiple angle formula, and photoelectric signals were calibrated to standard sineand cosine ones using the Newton iteration method. After that, the phase errorsbetween signals were calibrated by the least squares fitting method. The calibrationfrom two channels sample signals to the standard orthogonal sine and cosine signalswas realized, which improved the interpolation accuracy of small photoelectricencoders.A novel high-resolution interpolation method based on CORDIC algorithm wasproposed. Simple shifting and addition operations were used to resolve theinterpolation phases of Moiré fringe photoelectric signals, which could get rid ofconventional computed interpolation error cased by the deviation of samplesignals from the ideal ones. For the contradiction of calculation speed and accuracyexisting in CORDIC algorithm, two sorts of improvement were proposed includingapproximation of rotation angles and prediction of rotation direction. The resolutionof small photoelectric encoder was thus improved.The Fourier neural network model to correct long-period error of smallphotoelectric encoder was established. Output values of a high accuracy benchmarkencoder were set as the learning reference, and orthogonal trigonometric basisfunctions served as activation function of nodes in intermediate hidden layers. Thedifferential evaluation algorithm combined with simulated annealing strategy wasapplied for training to solve the error correction parameters. The correction of thelong-period error improved the accuracy of small photoelectric encoder effectively.The methods proposed above were applied to a small photoelectric encoderwhich was made by CIOMP (Changchun Institute of Optics, Fine Mechanics andPhysics, Chinese Academy of Sciences). After actual measurement, the resolution ofthe photoelectric encoder was improved from16-bit to18-bit and the root mean square error was reduced from60″to20″. The experiment result shows that, themethods could effectively improve the resolution and accuracy of smallphotoelectric encoders, and it is beneficial for development of miniaturized,high-resolution and high-accuracy small photoelectric encoders.