| Rapid development of high-tech manufacturing leads to increasing demand for high accuracy fabrication of Computer numerical control(CNC) machine. The tools for linear displacement detection and control are changing from angle encoders with ball screws to linear encoders with linear shaft motors. However, hitherto no domestic linear encoder can meet the criteria of closed-loop CNC machine. The absolute linear encoder is a new type of linear encoder product launched in 2005. Compared with incremental linear encoder, the absolute linear encoder has significant improvement in both function and performance. It has been widely used in the medium to high-end CNC system. Therefore, research on measuring technique of the high accuracy absolute linear encoder is of great importance.This thesis studies the working principle of absolute linear encoder and proposes the research scheme, based on the domestic and foreign references. By intensive study on key technologies including the single track absolute position coding, dedicated photoelectric device and serial communication protocol, the research on absolute linear encoder is finished. The design results are shown here.This thesis studies the method of generating absolute position coding through linear feedback shift register. It illustrates the method of getting primitive polynomial of feedback polynomial. Then it gives 10 stages primitive polynomials, of which the coefficients are valued in two elements field. In order to increase the reliability for decoding, the Manchester coding has been studied in the absolute position coding application. By using the methods proposed above, the design for the absolute position coding is accomplished.The thesis discusses the detection method of absolute linear encoder’s photoelectric signal, and introduces the application of CMOS image sensor. For the dedicated photoelectric device in absolute linear encoder, the scheme and schematic design are elaborated in detail with design results. It then analyzes the relationship between frequency response and maximum measurement speed, as well as relationship between signal to noise ratio and resolution. Then it gives the simulation and test results.The thesis introduces a filtering method to improve the quality of Moiré Fringe. In which each field of scanning reticle is divided into multiple windows, containing different phases with certain relationship. This method suppresses the high harmonic generations and improves the sinusoidal of the Moire Fringe.The thesis builds an error measurement system for absolute linear encoder. Based on the performance test results, it proposes an error compensation method in the thesis which improves accuracy significantly.The thesis introduces a method of interpolation deviation compensation for absolute linear encoder. The Moiré Fringe signal models are set up by applying the Fourier analysis to two channels of Moire Fringe from absolute linear encoder. Then by comparing the interpolation position between the practical and ideal signal models, it obtains the interpolation error. By compensating the interpolation position based on the obtained error, the interpolation accuracy can be improved remarkably.With the methods presented in this thesis, the design of the JC09 absolute linear encoder is accomplished. Its main specs matches up with foreign competitors. This is the first domestic linear encoder on CNC. Experiment results show that the accuracy of JC09 is ±8μm/m, the resolution is 0.01μm, and maximum operating speed is up to 180m/min, maximum acceleration is 3g. By applying the build-in compensation method in this thesis, the accuracy of the JC09 can achieve ±1.5μm/m. |
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