WO2014095209A1 - Position detector - Google Patents

Position detector

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Publication number
WO2014095209A1
WO2014095209A1 PCT/EP2013/074193 EP2013074193W WO2014095209A1 WO 2014095209 A1 WO2014095209 A1 WO 2014095209A1 EP 2013074193 W EP2013074193 W EP 2013074193W WO 2014095209 A1 WO2014095209 A1 WO 2014095209A1
Authority
WO
Grant status
Application
Patent type
Prior art keywords
detector
reference
stage
phase
measurement
Prior art date
Application number
PCT/EP2013/074193
Other languages
French (fr)
Inventor
Kieran O'mahony
Original Assignee
Waterford Institute Of Technology
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date

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Classifications

    • GPHYSICS
    • G01MEASURING; TESTING
    • G01DMEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infra-red, visible, or ultra-violet light
    • G01D5/266Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infra-red, visible, or ultra-violet light by interferometric means

Abstract

A position detector comprises an interferometer operable to split a reference beam of coherent light from an input arm between a pair of output arms, each output arm having a respective output port for emitting light to produce a pattern of interference fringes across a path of travel of a moving object. A reference detector is mounted on the moving object, and is arranged to sequentially sample a sinusoidally changing pattern of light from the interference fringes as the object moves across the path of travel. A controller is operably connected to the reference detector and is arranged to provide an accumulated phase value of the sinusoidally changing pattern of light sampled by the reference detector as the moving object translates across the path of travel. The controller is calibrated to map the accumulated phase value to a respective displacement value of the moving object across the path as a function of a nominal phase value for the reference detector as it moves across the path of travel.

Description

Position Detector

Field

The present invention relates to a position detector and a method for detecting the position of a moving object. Background

It is often desirable to determine the location of a moving object for example a motor driven translation stage. One reason this needs to be measured, rather than assumed based on drive signalling, is that such stages may not move uniformly in accordance with their drive mechanism and some feedback for a control system is required to know exactly where the translation stage lies.

Detector arrays measure light in a wide variety of applications. However, these arrays, typically used in machine vision applications and spectrometers, are expensive and in for Near-lnfraRed and Mid-Infrared vision, they suffer from low yield rates. Other than the ability to take a single snapshot of a stage, the advantage of detector arrays is that the position of the pixels in the array is known with absolute certainty, and when compared to a single element detector mounted on a translation stage to cover the same measurement range, there are no moving parts and therefore there is no measurement degradation due to non uniform velocity of the translation stage as it is scanned across its path of movement. The cost of detection systems can be dramatically reduced when a single element is used to replace a detector array. However, in order to compete with the detector arrays, the position of the single detector element needs to be known with more certainty that can be currently achieved, due to the non-uniform scanning velocity of the translation stage. The problem of measurement degradation due to non-uniform scanning velocities is well known in interferometric applications, where typically one arm of the

interferometer is scanned to generate an interference pattern, and this is typically overcome using either: phase locked loop control of the translation stage, or zero- crossing detection circuits. Figure 1 shows one such conventional system based on a Michelson interferometer. As the mirror is scanned by the translation stage, the detector sees a series of bright and dark fringes. As indicated, to compensate for non-uniform velocity of the translation stage, two approaches are conventionally used: Phase locked loop control of the motor (not shown) seeks to control the velocity of the stage so that there is little to no compression or stretching of the fringes. It is expensive to implement.

Zero-crossing detection circuits seek to sample both the reference and measurement interference patterns on the zero-crossings of the reference i.e. only once per wavelength. It can also be used to determine position of the stage, but only at the points where the zero-crossings occur and so provides relatively low resolution. The circuit is non-trivial and also expensive to implement. The zero-crossing method also requires that in interferometric applications the light generating the measurement interference pattern has to have a lower frequency (longer wavelength) than the reference, so that it is sampled above the Nyquist criterion, e.g. a 632nm laser reference could only be used to measure light with a wavelength greater than

1264nm.

The same limitations are true for the Mach Zehnder interferometer. It is therefore an object of the present invention to overcome these problems. Summary

According to one aspect of the present invention there is provided a position detector according to claim 1 .

According to a second aspect, there is provided a method for detecting the position of a moving object according to claim 9. This invention allows the replacement of detector arrays with a single discrete detector element by using a reference high coherence interference pattern to remove measurement uncertainties due to the non-uniform scanning velocity of the translation stage. An added advantage of using an interference pattern based on a high coherence reference light source is that measurement degradation due to effects that are common to measurement and reference detectors can be eliminated, e.g. variations in sampling times caused by clock errors, etc.

The invention differs from the prior art in that a spatial interference pattern is spread across the scanning area. The frequency of the fringes in the interference pattern can be altered and the interference pattern can be sampled as frequently as possible (depends on scanning speeds and sampling speeds) to provide a higher resolution and therefore more accurate determination of position, than for example, the zero- crossing approach.

The measurement capability of the position detector can be enhanced when compared to detector arrays by increasing the sampling frequency so that more than a single measurement is taken over the dimensions of the pixel. Referring to Figure 3, a translation stage 20 with a reference detector 22 translates across the

interference pattern produced by an interferometer (not shown in figure 3) - the light level sensed by the detector 22 can be sensed at about 5 times over the dimensions of the detector element 22. On the other hand, for a conventional type detector array of pixels 30, the same movement of a sensed element 24' would only provide two discrete measurements. In addition, the measurement range for the present invention can be readily extended as the only limit is on the extent of movement of the translation stage. In order to extend the range of a detector array, additional pixels have to be added, which is a much more costly process.

Brief Description of the Drawings

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a conventional Michelson Interferometer. Figure 2 shows schematically a position detector according to an embodiment of the invention.

Figure 3 illustrates the potential to increase sample density with the present invention by taking more measurements with a reference detector over the same range as where only two could be taken using a traditional detector array. Figure 4 shows a spectrum of a high coherence interference pattern sensed by a reference detector mounted on a translation stage and showing the effects of the non-uniform scanning of the translation stage.

Figure 5 shows an unwrapped temporal phase vector based on the spectrum of Figure 4.

Figure 6 shows a residual of a linear fit to the unwrapped phase of Figure 5 showing the extent of noise generated by the non-uniform translation stage.

Figure 7 illustrates an exemplary sampling database for use in an embodiment of the present invention. Description of the Preferred Embodiment

Figure 2 shows a position detector according to an embodiment of the present invention. The detector comprises an interferometer 1 for example of the type described in PCT/EP2012/057826, comprising a splitter/coupler including first and second input arms 10, 12 and first and second output arms 14, 16, each arm 10-16 comprising a respective fibre-optic waveguide. A 2x2 splitter 18 splits the light from each input arm 10, 12 between the output arms 14, 16. (In certain embodiments the input arm 12 can be omitted, in which case the splitter 18 may be replaced by either a 1 x2 splitter or a wavelength independent y-junction.) In use, a reference beam of light is coupled into the input arm 10 or 12 and split, independently of wavelength, equally between the two output arms 14, 16 by the splitter 18. Each output arm 14, 16 has a respective exit port 20, 22 facing a translation stage 20, so that light from the exit ports is diffracted onto the surface of the stage to form interference fringes produced by overlapping divergent wavefronts from the exit ports.

The stage includes a single element reference detector 22 formed on the surface of the stage facing the interferometer. A measurement detector element 24 (only one shown) is mounted elsewhere on the stage and in this case on the surface opposite the interferometer. In such an application, the stage 20 might be scanned until the element 24 detects an event, such as a photodiode detector 24 detecting a change in light level indicating an edge to be detected, and then a processor (not shown) coupled to the reference and measurement detectors determines the location of the stage at that time.

Nonetheless, it will be appreciated that different measurement detector(s) 24 can be used for a wide range of applications from machine vision to spectroscopy to simple photography. For example a mid-IR measurement requires a Mercury Cadmium Telleride detector element 24 or a pyroelectric detector element, Near Infrared may require an InGaAs photodiode, visible light could require a silicon photodiode, whereas photography might require RGB reading capability (an example involving more than a single detector 24 element). According to the present invention, accurate measurement of pixel position is made by first calibrating for the non-uniform velocity of the translation (scanning) stage 20 using a reference (known wavelength) interference pattern generated from a high coherence source. This high coherence source is launched into the interferometer 1 where the splitting ratio is optimally 50/50 at the wavelength of the reference beam in order to maximise the visibility of the interference fringes. The light entering the splitter is coupled into two output arms acting as a wavefront splitting interferometer or a Young's slits type apparatus. At the outputs of the splitter, the light from both outputs is projected across the scanning range of the translation stage where an interference pattern is formed. The reference photodiode 22 captures the series of bright and dark fringes as it is moved across the measurement range by the translation stage.

In this apparatus, both the measurement 24 and reference photodiodes 22 are scanned at the same time, therefore both suffer from the same non-uniform scanning velocities. The effects of this non-uniform velocity can be seen in figure 4 where the Fourier transform of the reference high coherence interference pattern sensed by the reference photodiode 22, instead of a sharp spike in the spectrum, comprises a broadened series of spikes.

To calibrate for the degradation in measurement position due to non-uniform velocity of the scanning stage, the Fourier transform of the reference interference pattern shown in Figure 4 is taken, and the DC and negative values are removed in the frequency domain. An inverse Fourier transform is then performed on the result to get an analytic signal. The analytic signal acts as a phasor from which temporal phase values can be obtained. These phase values are then unwrapped to provide a temporal phase vector i.e. a table of phase values, illustrated in Figure 5. The plot of Figure 5 appears generally straight is due to the number of points in the plot and the amplitude of the variation in velocity of the motor, i.e. the variation is +1- 60 radians (noise due to the stage velocity) by comparison to a total excursion shown in Figure 4 of the order of 2.5 x106 radians. Figure 6 shows the extent of the noise in the phase vector by plotting the residual of a straight line fit to the temporal phase vector shown in Figure 5. This straight line corresponds to the actual position of the stage for each sampled point acquired by the reference detector and as such enables any sensed phase value for the reference detector to be mapped to a calibrated position of the translation stage. (It will be appreciated that other fits including cubic or spline fits can also be used.) Thus, the phase vector information illustrated in Figure 5 contains information on the components that make up the interference pattern, i.e. the high coherence source and the effects of non-uniform velocity of the scanning stage which results in a nonuniform sampling of the optical delay. As the source which generates the reference interference pattern is known, and it is generated from a stationary (no-moving parts) interferometer, the non-uniform sampling of optical delay can be attributed to the non-uniform velocity of the stage.

Therefore, a sampling database can be generated based on true optical delay values (essentially this is a measure of the distance that the reference light has travelled). Interpolation of database entries based on other optical signals subjected to the same non-uniform velocity, for example, events detected by the detector 24, removes the effect of the non-uniform velocity, as it is essentially common-mode noise.

The sampling database is generated by mapping the optical delay of the known reference source at each sampling point by using the measured phase values of Figure 5, to a location of the translation stage based on the calibration information shown in Figure 6. Therefore irrespective of the velocity of the scanning stage, at each point of the reference interference pattern sampled, the optical delay value for this point is known from the reference phase values.

Thus, in use, as the stage is being translated, the phase for the detector 22 is measured and sequentially mapped to sampling points 1 ...N. For any time at which the measurement detector 24 detects an event, corresponding reference detector measurements from the sampling database can be interpolated to give an accurate measurement of the position of the measurement detector 24, therefore giving an accurate determination of the position at which the event has been detected. Referring to Figure 7, at any given instant, Td, at which an event is detected, the measurement detector 24 will lie between two given sample points m, m+1 extracted from the table of Figure 5. Each entry of the table of Figure 5 is mapped to

respective translation stage locations D1 ...Dn based on the adjustment performed from the difference data shown in Figure 6. The processor (not shown) connected to each of the reference and measurement detectors can therefore interpolate distances Dm and Dm+1 to provide an accurate measurement of the displacement of the translation stage, based on the sampled phase of the interference pattern acquired by reference detector 22.

As both the reference 22 and the measurement 24 photodiodes are sampled in tandem, the position of the detector element for any measurement, neglecting clock errors in the sampling hardware, can easily be determined based on the

measurement of the optical path length of the scan using the high coherence reference interference pattern.

Claims

Claims:
1 . A position detector comprising: an interferometer operable to split a reference beam of coherent light from an input arm between a pair of output arms, each output arm having a respective output port for emitting light to produce a pattern of interference fringes across a path of travel of a moving object; a reference detector mounted on said moving object, said reference detector being arranged to sequentially sample a sinusoidally changing pattern of light from said interference fringes as said object moves across said path of travel; a controller operably connected to said reference detector and arranged to provide an accumulated phase value of said sinusoidally changing pattern of light sampled by said reference detector as said moving object translates across said path of travel; said controller being calibrated to map said accumulated phase value to a respective displacement value of said moving object across said path as a function of a nominal phase value for said reference detector as it moves across said path of travel.
2. A position detector as claimed in claim 1 wherein said controller is arranged to perform a Fourier analysis of said sampled pattern of light for retrieval of said accumulated phase value.
3. A position detector as claimed in claim 1 wherein said position detector further comprises a measurement detector mounted on said moving object, and wherein said controller is responsive to a measurement event detected by said measurement detector at an event time to determine a displacement of said moving object at said event time according to an accumulated phase value provided from said reference detector for said event time.
4. A position detector as claimed in claim 3 wherein said controller is responsive to said event time being between sampling times of said reference detector for interpolating displacement values for accumulated phase values sampled immediately before and after said event time to determine a displacement value of said moving object at said event time.
5. A position detector as claimed in claim 1 in which said nominal phase values are determined by fitting reference accumulated phase values to sampled accumulated phase values.
6. A position detector as claimed in claim 5 wherein said fitting comprises one of: linear, spline or cubic spline fitting.
7. A position detector according to claim 3 in which said measurement detector comprises one or more optical detectors arranged to detect an optical event.
8. A position detector according to claim 1 in which said moving object comprises a motor driven translation stage.
9. A method for detecting the position of a moving object comprising: splitting a reference beam of coherent light to produce a pattern of interference fringes across a path of travel of a moving object; mounting a reference detector on said moving object; sequentially sampling through said reference detector a sinusoidally changing pattern of light from said interference fringes as said object moves across said path of travel; providing an accumulated phase value of said sinusoidally changing pattern of light sampled by said reference detector as said moving object translates across said path of travel; calibrating said accumulated phase value to map said accumulated phase value to a respective displacement value of said moving object across said path as a function of a nominal phase value for said reference detector as it moves across said path of travel; and responsive to an event at a time corresponding to a given accumulated phase value, providing a corresponding displacement value.
PCT/EP2013/074193 2012-12-17 2013-11-19 Position detector WO2014095209A1 (en)

Priority Applications (2)

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IES2012/0540 2012-12-17
IES20120540 2012-12-17

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WO2014095209A1 true true WO2014095209A1 (en) 2014-06-26

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0342016A2 (en) * 1988-05-10 1989-11-15 THE GENERAL ELECTRIC COMPANY, p.l.c. Optical position measurement
EP0694764A2 (en) * 1994-07-06 1996-01-31 Hewlett-Packard Company Detector array for use in interferomic metrology systems
US5699158A (en) * 1993-10-27 1997-12-16 Canon Kabushiki Kaisha Apparatus for accurately detecting rectilinear motion of a moving object using a divided beam of laser light
US5900936A (en) * 1996-03-18 1999-05-04 Massachusetts Institute Of Technology Method and apparatus for detecting relative displacement using a light source
DE10242749A1 (en) * 2002-09-13 2004-04-08 Bergische Universität Wuppertal 3D interferometric position measurement system, e.g. for use in locating the probe of a raster scanning microscope, whereby a measurement area is created by the interference of at least three coherent waves

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0342016A2 (en) * 1988-05-10 1989-11-15 THE GENERAL ELECTRIC COMPANY, p.l.c. Optical position measurement
US5699158A (en) * 1993-10-27 1997-12-16 Canon Kabushiki Kaisha Apparatus for accurately detecting rectilinear motion of a moving object using a divided beam of laser light
EP0694764A2 (en) * 1994-07-06 1996-01-31 Hewlett-Packard Company Detector array for use in interferomic metrology systems
US5900936A (en) * 1996-03-18 1999-05-04 Massachusetts Institute Of Technology Method and apparatus for detecting relative displacement using a light source
DE10242749A1 (en) * 2002-09-13 2004-04-08 Bergische Universität Wuppertal 3D interferometric position measurement system, e.g. for use in locating the probe of a raster scanning microscope, whereby a measurement area is created by the interference of at least three coherent waves

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