High-accuracy surface position sensor based on a two-lens system

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Abstract

Two optical lenses are used to compose a surface position measuring system. A measurand is positioned near the front focal point of the first lens. At the output end of the other lens, a beam splitter is used to split the beam into two paths. One photodetector is positioned before the focal point of the lens in one path and another after the focal point in the other so that the intensities falling onto the two detectors can be adjusted to be equal when the first lens is in focus. This device provides a compact and high-accuracy surface sensor. In this paper, the design and experimental study of the sensor system is described. It is shown that such a sensor embodiment can lead to a resolution of 1 μm.

Introduction

In microsterolithographic rapid prototyping [1], and many other applications, an accurate position of a liquid surface needs to be monitored. To measure the absolute distance, interference-based measurement devices are usually too complicated and sensitive to environmental perturbations. However, using an intensity-based two-lens system to build a compact, yet highly accurate sensor system is very suitable in many application. We have developed a very sensitive measuring system by optimising the optical elements in the system. A numerical simulation is carried out with the commercial computer package Zemax.1 Experiments using a liquid resin surface, which has a low reflection coefficient compared with a mirror, have been conducted. The system is easy to mount onto the microsterolithographic rapid prototyping machine in our laboratory.

Section snippets

Theoretical principle

The measuring system consists of two lenses, two 50/50 beam splitters, and two photodetectors with a pinhole attached on the surface of each as shown in Fig. 1. As the measurand moves up and down near the focal point of lens L0, the reflected light will be parallel, converging or diverging after it passes through lens L0 according to the position of the measurand as shown in Fig. 2. The positions of the pinholes and their diameters in Fig. 1 are chosen so that the intensity received by the two

Experimental results

To verify the above simulation results, experiments are conducted with the system layout being basically the same as in Fig. 1. A 5 mW He–Ne laser light beam is expanded so that a relatively uniform beam is achieved within a diameter of 5 mm. The output signals are detected by a photodiode system, and read out by a voltmeter. The measurand is a resin surface. A single experimental result for two different F# cases is shown in Fig. 6. The average sensitivity within the range of ±0.25 mm is 2.41 mm−1

Concluding remarks

A two-lens surface position monitoring system has been computer modelled and experimentally demonstrated. This shows that the F# of lens L0 has a large effect on the sensitivity and linear measuring range of the system. The sensitivity of the system is inversely proportional to the F# of lens L0. The average sensitivity within the sensing range of ±0.25 mm for the 1L case (F#=3.2) is 1.79 mm−1 higher (by addition) than the 3L case (F#=7) in the simulation and 2.05 mm−1 higher in the experiment.

References (1)

  • M. Farsari et al.

    Micro-fabrication using a spatial light modulator in the UVexperimental results

    Opt Lett

    (1999)

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