High-precision3-dimensional Displacement Measurement System Based On2-dimensional Grating

In the21st century, the precise displacement measurement technologies arerequired in the field of the semiconductor manufacturing technology and theultra-precision machining. The grating displacement encoder, which is low price,has the property of high resolution, high precision and insensitive to theenvironment. The grating displacement encoder, which has been widely used inprecision displacement measurement, gets lots of research interests. The technologyof grating encoder for measuring linear and planar displacement has been welldeveloped. However, there are few studies on grating encoders in three-dimensionaldisplacement measurement. Meanwhile, the measurement range ofthree-dimensional grating encoders for the out-of-plane displacement is finite.Therefore, a three-dimensional grating displacement measurement system which hasenough measurement range in three directions, is essential for three-axialdisplacement measurement.A high-precision three-dimensional displacement measurement system basedon two-dimensional grating is proposed. The gratings and other optical elements areautocollimatic. The measurement range of the system is greatly improved in z-axialdisplacement. The measurement range isn’t influenced by the diameter of thediffraction spot.The diffraction characteristics of linear and planar gratings are analyzed b yusing Fourier optical theory. The condition of the autocollimatic configuration isalso investigated. The measurement principle is explored according to theinterference theory of the diffraction light and the Doppler Effect.The three-dimensional displacement measurement system is experimentallyconstructed and the optical system is designed. The proposed measurement system iscomposed of a scale planar grating with the period of8μm and an optical readingsystem. A reference planar grating, which has the same periodic structures as thoseof the scale planar grating, is applied in the optical reading system. The system canmeasure three-axial displacement of Δx, Δy and Δz simultaneously. Furthermore, dueto the autocollimatic configuration of the optical elements, the interference zone ofthe measurement signal is constant when the scale planar grating is moving along z.Therefore, the measurement range in z-axial displacement is greatly extended. Whilethe wavelength of the laser is632.8nm and the scale planar grating is100mm×100mm, the maximum z-directional displacement that the proposed measurementsystem can measure is1263mm theoretically.The structure of the planar gratings in the system is optimized and analyzed. The features of used reflective mirrors, prisms and lens worked as optical deflectionelements (ODEs) in the measurement system are discussed. Meanwhile, a new ODElike linear gratings is designed. The proposed ODE is convenient to install and thestructure of the proposed measurement system is simplified when the ODE isintroduced.The primary error sources and experimental results of the measurement systemare analyzed. According to the experimental results, the system is proved that x-, y-and z-directional displacements can be measured simultaneously. When the scalegrating moves20mm along z axis, the measurement signals are also detected. Whenthe interpolation is applied to the measured data, the measurement resolution and thestandard deviation in z axis is5nm, while the measurement resolution in x or y axisis100nm.