A thermal deformation monitoring system was developed in this study by applying the thermocouple sensors and capacitive displacement sensors, along with a Long Short Term Memory (LSTM) Network Model classifier, for the alignment turning system (ATS). An ATS can simultaneously provide the functions of measuring the centration error and dimensions of the lens cell in-line, and machining the lens barrel housing with reference to the lens optical axis. The ATS can manufacture precise lens cells, applied for optical metrology, high numerical aperture objective lenses, and lithography projection lenses. While rising temperature, the thermal error would occur on hydrostatic spindle which build in ATS. Therefore, the predetermined machining point would offset, thereby resulting in the machining error. In order to acquire the oil temperature of rotor and the relative thermal displacement between hydrostatic spindle and turret, the thermocouple sensors and capacitive displacement sensors were assembling on ATS. According to the measurement of oil temperature and relative displacement, the thermal deformation monitoring system of ATS hydrostatic spindle was established. Cause of the high resolution of capacitive displacement sensors, the more precise measurement values could be obtain so that the monitoring system would have higher accuracy. LSTM is a variant of Recurrent Neural Network (RNN) and could remember longer information changes than traditional RNN. The thermal deformation monitoring system with LSTM could be applied to compensate the thermal error to improve the workpiece quality in real-time, and also could save time and money of warming up centering machines in the future. Results shows that the mean square error (MSE) and RScore of forecasting thermal error is less than 0.0002 and higher than 0.997, which is highly accurate forecasting.
We present a portable non-contact displacement sensor (NCDS) based on astigmatic method for micron displacement measurement. The NCDS are composed of a collimated laser, a polarized beam splitter, a 1/4 wave plate, an aspheric objective lens, an astigmatic lens and a four-quadrant photodiode. A visible laser source is adopted for easier alignment and usage. The dimension of the sensor is limited to 115 mm x 36 mm x 56 mm, and a control box is used for dealing with signal and power control between the sensor and computer. The NCDS performs micron-accuracy with ±30 μm working range and the working distance is constrained in few millimeters. We also demonstrate the application of the NCDS for lens centration error measurement, which is similar to the total indicator runout (TIR) or edge thickness difference (ETD) of a lens measurement using contact dial indicator. This application has advantage for measuring lens made in soft materials that would be starched by using contact dial indicator.
In general, the drop-in and cell-mounted assembly are used for standard and high performance optical system respectively. The optical performance is limited by the residual centration error and position accuracy of the conventional assembly. Recently, the poker chip assembly with high precision lens barrels that can overcome the limitation of conventional assembly is widely applied to ultra-high performance optical system. ITRC also develops the poker chip assembly solution for high numerical aperture objective lenses and lithography projection lenses. In order to achieve high precision lens cell for poker chip assembly, an alignment turning system (ATS) is developed. The ATS includes measurement, alignment and turning modules. The measurement module including a non-contact displacement sensor and an autocollimator can measure centration errors of the top and the bottom surface of a lens respectively. The alignment module comprising tilt and translation stages can align the optical axis of the lens to the rotating axis of the vertical lathe. The key specifications of the ATS are maximum lens diameter, 400mm, and radial and axial runout of the rotary table < 2 μm. The cutting performances of the ATS are surface roughness Ra < 1 μm, flatness < 2 μm, and parallelism < 5 μm. After measurement, alignment and turning processes on our ATS, the centration error of a lens cell with 200mm in diameter can be controlled in 10 arcsec. This paper also presents the thermal expansion of the hydrostatic rotating table. A poker chip assembly lens cell with three sub-cells is accomplished with average transmission centration error in 12.45 arcsec by fresh technicians. The results show that ATS can achieve high assembly efficiency for precision optical systems.