Measurement And Research Of Dynamic Surface Of Active Stressed Lap

In the fabrication of large aspherical mirrors, active stressed lap has been proved to be a efficient solution with high accuracy and high intelligence. Using active stressed lap, a serial of large aspherical mirrors was finished with high performance. Different from traditional rigid laps, active stressed lap could deform in real time under the cooperation of actuators on the lap to fit the curve of the mirror during moving and rotating on the lap, which improves the efficiency and reduces the middle-high frequency error. Before the lap is used on mirrors, the calibration of lap deformation is essential to ensure the lap accuracy in grinding and polishing. The common calibration method is based on contact displacement sensors, which means to locate the sensor array in a special arrangement, measure lap surface before and after deforming, the difference between two surfaces is the deformation of the lap. This method only records the static surface changes of stressed lap, which is helpless for presenting dynamic lap surface change and accuracy. This paper aims at the dynamic surface measurement and research of active stressed lap and some achievements is as following:(1) The measurement error analysis of contact sensor array method. Themain error in the measurement using sensor array contains ball-probeerror as system error and sensor location error as stochastic error.Ball-probe error could be compensated and sensor location error has littleimpact on the measurement. And the sensor number could meet the needin the analysis using Zernike polynomial.(2) Using the lap surface measurement system, experimental verification isput forward and the RMS of the measured lap surface is within 2μm inpractice. A general improvement in lap surface accuracy using theZernike polynomial is shown. After compensating the calculation errors,the lap surface accuracy is improved by 8%–23%, and achievesRMS<1.5μm, which is appropriate for practical grinding and polishing.(3) The removal model of active stressed lap in different surface accuracy isintroduced respectively, which shows that lap surface accuracy has greatimpact on the removal efficiency and mirror finish. And a method of lapsurface representation based on structure function is introduced.Different from the representation based on PV and RMS, structurefunction could describe the average variance over all regions on the lapin a given separation, which is more accurate to represent lap surface.(4) A dynamic surface acquisition system of active stressed lap using FPGAand USB communication is introduced. This system containshigh-precision micro-displacement sensor array, acquisition circuit board,host computer. During measuring, the system could create simulationencoder signal to lap deformation control system to control the lapdeforming as current location needs, the system also gathers the sensordata together with location information and transfer them via USBinterface to the host computer. Experimental results show that this systemis capable of measuring dynamic lap surface accurately.(5) Experiments of dynamic lap surface measurement in different lapdeforming velocity are carried out, and dynamic lap surface accuracy isanalyzed. A suggestion of lap rotation velocity in grinding and polishingis introduced based on the results of dynamic lap surface accuracyanalysis.In this paper, the active stressed lap surface accuracy for the fabrication of 4m class mirror is calibrated and testified. The further conclusion of dynamic surface accuracy indicates that lap surface accuracy decreases little when lap rotation velocity increases, which means that the active stressed lap could work in a higher rotation velocity than current velocity without breaking down lap surface and mirror finish. So the optimization of fabrication strategy is feasible by choosing different lap velocities for different processing periods.