| The high processing speed and positioning accuracy of Computer Numerical Control(CNC)machine tools are increasingly required.In terms of linear displacement measurement,the traditional incremental sensors gradually cannot be satisfied with the requirements of precise positioning and closed-loop control for linear displacement,and begin to transform to absolute displacement sensors.At present,the absolute optical grating displacement sensor is the most widely used one.However,due to the harsh working environment of CNC machine tools(mechanical vibration,dust contamination,etc.),the absolute grating measurement accuracy is weakened and its cost is added.Meanwhile,the measuring method of optical grating and the manufacturing of grating line are restricted by the optical diffraction limit and the precision processing technology,respectively,which makes it difficult to further improve its performance.The time-grating proposed in recent years,taking advantage of the three orders of magnitude advantage that the time measurement accuracy is over the spacial survey precision,adopts the high frequency clock interpolation technology to realize the high precision measurement of the absolute space displacement through the measurement of relative time difference and avoids the limitation of precision processing technology.On this basis,an absolute linear displacement sensor with time-grating that employs a vernier-type structure based on alternating electric field was proposed in this paper,and its main work is as follows:(1)According to the measurement principle of capacitive linear time-grating displacement sensor,the absolute linear time-grating structure employing a a vernier-type structure was designed.The sensor is composed of two single-row sensing structure: upper single-row sensing structure and lower single-row sensing structure.The upper single-row sensing structure is composed of n periods in the whole length of the sensor,used for precise measurement to ensure measurement accuracy.The lower single-row sensing structure is composed of n-1 periods in the same length of the sensor,which is used to obtain the phase difference of two output signal between the upper single-row sensing structure and the lower single-row sensing structure for absolute positioning.(2)The error mechanism and error source of absolute linear time-grating employing vernier-type structure is studied.The error mechanism is mainly studied from three aspects: amplitude,phase and harmonic components.The error sources are mainly analyzed from electrodes edge manufacturing,installation,and signal crosstalk.(3)An experimental platform was set up for the experimental study of sensor accuracy.The harmonic analysis method based on fast Fourier transform is used to analyze the frequency spectrum of the measurement error of a period.According to the error mechanism and error source of the sensor,combined with its theoretical analysis and experimental research,the analysis shows that the first harmonic error was caused by the signal crosstalk and the excessive influence of the electrodes on both sides of the sensor,and the fourth harmonic error was affected by the proportion of electrode width to interval width.(4)According to the causes of harmonic errors,a series of optimization and analysis were carried on the sensor structure.The first harmonic error was reduced by adding shielding layer,and designing the differential sensing structure.The optimum proportion of electrode width to interval width has been chose to weaken the fourth harmonic error.Finally,the experiment results demonstrate that the measurement error of the optimized sensor prototype is ±1.375?m over the full 550 mm range and ±0.3?m over a single period. |
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