KEYWORDS: Computer programming, Error analysis, Sensors, Diffraction, Digital signal processing, Photodetectors, Calibration, Analog electronics, Far-field diffraction, Detection and tracking algorithms
Pointing and tracking applications usually require relative gimbal angles to be measured for reporting and controlling the
line-of-sight angular position. Depending on the application, angular resolution and/or accuracy might jointly or
independently determine the angle transducer requirements. In the past decade, encoders have been increasingly taking
the place of inductive devices where the measurement of angles over a wide range is required. This is primarily due to
the fact that encoders are now achieving very high resolution in smaller sizes than was previously possible. These
advances in resolution are primarily due to improved encoder disk and detector technology along with developments in
interpolation techniques. Measurement accuracy, on the other hand, is primarily determined by mounting and bearing
eccentricity as it is with all angular measurement devices. For very demanding accuracy requirements, some type of
calibration of the assembled system may be the only solution, in which case transducer repeatability is paramount. This
paper describes a unique encoder-to-digital tracking converter concept for improving interpolation of optical encoders.
The new method relies on Fraunhofer diffraction models to correct the non-ideal sin/cos outputs of the encoder
detectors. Diffraction model concepts are used in the interpolation filters to predict the phase of non-ideal sin and cosine
encoder outputs. The new method also minimizes many of the open loop pre-processing requirements and assumptions
that limit interpolation accuracy and rate loop noise performance in ratiometric tracking converter designs.
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