CLIAMS:We Claim:
1. A method for multiple motion measurement of an object, the method comprising:
identifying a light source;
obtaining a calibrated reflected beam from the
identified source;
directing the calibrated reflected beam to at least one distinct surface on the object to obtain a deflection specific to the surface;
detecting each of the deflected beam to obtain a signal; and
analyzing the signal to obtain measurement of the motion of the object
wherein the measurement is atleast one of linear
displacement, an angular displacement or a combination
thereof, wherein the displacement is about all available
axes with respect to the object.
2. The method of claim 1 wherein, the light source is selected from a group comprising laser, incandescent light, LED and all such sources that would emit light in the range of 300nm-800nm.
3. The method according to claim 1, wherein the reflection is achieved by magnetically actuating a mirror.
4. The method according to claim 1, wherein the calibration is achieved by altering the state of at least one means for calibration.
5. The method according to claim 1, wherein the signal is an analog signal, further wherein the analog signal is converted to a digital signal capable of being displayed.
6. The method according to claim 1, wherein the motion detected is at least one of periodic motion and/or non-periodic motion of the object.
7. A system for a multiple motion measurement of an object, wherein the system comprises of :
An illumination unit;
a measurement unit; and
an analyser coupled to the illumination unit and the measurement unit.
8. The system of claim 7, wherein the illumination unit comprises of:
a pair of light sources;
a beam splitter for combining the light from the sources;
a reflector mounted coaxial to the beam splitter; and
a calibration arrangement mounted coaxial to the reflector;
9. The system according to claim 7, wherein the reflector is a mirror coupled to a magnetic actuator.
10. The system according to claim 7, wherein the calibration arrangement comprises of:
an optical flat mounted on a stage configured for two-axis rotational motion; and
a lens arrangement mounted on a stage that is configured for three-dimensional translational motion;
11. The system according to claim 7, wherein the calibration arrangement is configured for regulating the position and the angle of incidence of the light beam on the object.
12. The system according to claim 7, wherein the measurement unit comprises of:
a beam splitter;
a curved reflector mounted coaxial to the beam splitter; and
a pair of detectors wherein the first detector is coupled to the beam splitter and the second detector coupled to the curved reflector.
13. The detector according to claim 12, wherein the detector is a quadrant photo detector.
14. The analyzer according to claim 8, wherein the analyzer comprises of:
a calibrator configured for focusing and calibration of the illumination unit;
a convertor coupled to the calibrator, the convertor configured for converting the beam deflection signal to an electrical signal; and
a display coupled to the convertor and configured for viewing the displacement of the object.
,TagSPECI:MULTIPLE MOTION MEASUREMENT

FIELD OF INVENTION
The invention generally relates to the field of optical metrology and particularly to a method and a system for multiple motion measurement of objects along multiple axes.

BACKGROUND
Systems for ultra-precise motion measurement, each optimized for measurement of specific kind of motion, can be broadly classified as contact type systems and non-contact type systems. Contact type systems require the measuring unit to be mounted on the object whose motion is to be measured. Examples of contact type systems include strain gages, accelerometers and the linear variable differential transformers (LVDTs). Non-contact type systems, in contrast, do not require the measurement unit to be physically attached to the object. Examples of non-contact type systems include optical systems such as interferometers and laser Doppler vibrometers (LDV). For applications involving measurement of motion of MEMS stages, and of dynamic response of structures, non-contact type techniques possess significant advantages over contact type techniques, since they minimize spatial constraints, do not result in mass-loading and avoid challenges in integration of the measurement units. An important limitation of optical non-contact measurement techniques, however, is that they can typically measure motion only along one axis. For instance, the interferometer and the LDV measure displacement and velocity along the line-of-sight of the incident laser beams, respectively. Thus, measurement of motion along multiple axes necessitates the use of multiple units, thereby increasing the bulk and the complexities in alignment and calibration. A second limitation is that the optical measurement systems require relatively sophisticated signal processing in order to extract the measurements. This contributes to the increased cost of the systems.

BRIEF DESCRIPTION OF DRAWINGS
So that the manner in which the recited features of the invention can be understood in detail, some of the embodiments are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
Fig. 1 is a schematic representation showing the overall construction of the system, according to an embodiment of the invention.
Fig. 1(a) shows a schematic representation of illumination unit, according to an embodiment of the invention.
Fig. 1(b) shows a schematic representation of the construction of the measurement unit, according to an embodiment of the invention.
Fig. 2 shows a schematic representation of the measurement-optics subsystem employed to measure out-of-plane motion, according to an embodiment of the invention.
FIG.2 (a) shows the change in direction of the light spots on the detectors during an in-plane and an out-of-plane motion, according to an embodiment of the invention.
FIG.3 generally shows a schematic representation of the in-plane measurement arrangement, according to an embodiment of the invention.
FIG.3 (a) shows the light path from the source to detector for an in-plane motion measurement, according to an embodiment of the invention.
FIG. 3(b) shows the change in direction of reflected beam during an in-plane motion along X-axis, according to an embodiment of the invention.
FIG. 3(c) shows the change in direction of reflected beam during an in-plane motion along Y-axis, according to an embodiment of the invention.
Fig. 4 generally shows the measured voltage signals for in-plane motion of an object, according to an embodiment of the invention.
Fig. 5 generally shows the measured voltage signals for out-of-plane motion of an object, according to an embodiment of the invention.
SUMMARY OF THE INVENTION
One aspect of the invention provides a method for multiple motion measurement of an object. The method includes identifying a light source; obtaining a calibrated reflected beam from the identified light source, directing the calibrated reflected beam to at least one distinct surface on the object to obtain a deflection specific to the surface; detecting each of the deflected beam to obtain a signal; and analyzing the signal to obtain measurement of the motion of the object.
Another aspect of the invention provides a system for multiple motion measurement of an object.

DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a high precision multiple motion measurement system based on the principle of optical beam deflection. The system can measure the motion of both macro-scale and micro-scale objects along all six axes. The measurement bandwidth is over 1MHz and resolution can be less than a nanometre along the linear degree of freedom and less than a micro radian along the angular degree of freedom. The design of the system enables automatic calibration of its sensitivity matrix and automatic scanning of the measurement beam on the surface of an object. These features of the measurement system enable it to measure both periodic and non-periodic motion.
One embodiment of the invention provides a method for multiple motion measurement of an object. The method includes identifying a light source; obtaining a calibrated reflected beam from the identified light source, directing the calibrated reflected beam to at least one distinct surface on the object to obtain a deflection specific to the surface; detecting each of the deflected beam to obtain a signal; and analyzing the signal to obtain measurement of the motion of the object.
The method described in brief herein above shall be described in detail. For multiple motion measurement of an object, a pair of light beam is directed on a mirror to obtain a calibrated reflected beam. The reflection is achieved by magnetically actuating the mirror. Calibration of the beam is achieved by altering the state of at least one means provided for calibration. The means of calibration is either a mechanical means or an electrical means or a combination thereof. The calibrated reflected beam is then directed to at least one distinct surface on the object to obtain a deflection specific to the surface. Each of the deflected beam is detected to obtain an analog signal. The analog signal is converted to a signal capable of being displayed. The displayed signal is then analyzed to obtain measurement of motion of the object. The measurement includes but is not limited to a linear displacement, an angular displacement or a combination thereof. Further, the linear displacement and the angular displacement is about all available axes with respect to the object.
A system for multiple motion measurement is also provided. The system includes an illumination unit; a measurement unit; and
an analyzer coupled to the illumination unit and the measurement unit. The illumination unit includes a pair of light sources. A beam splitter is provided for combining the light from the sources. A reflector is mounted coaxial to the beam splitter. In one embodiment of the invention, the reflector is a mirror coupled to a magnetic actuator. A calibration arrangement is mounted coaxial to the reflector. The calibration arrangement includes an optical flat mounted on a stage, which is configured for two-axis rotational motion. The calibration arrangement also includes a lens arrangement mounted on a stage, which is configured for three-dimensional translational motion. The calibration arrangement is configured for regulating the position and the angle of incidence of the light beam on the object.
The measurement unit of the system includes a beam splitter. A curved reflector is mounted coaxial to the beam splitter. A first detector is coupled to the curved reflector and the second detector is coupled to the beam splitter. Examples of detector include but are not limited to a quadrant photo detector.The analyzer includes a calibrator configured for focusing and calibration of the illumination unit. A convertor is coupled to the calibrator and is configured for converting the beam deflection signal to an electrical signal. A display is coupled to the converter for viewing the displacements of the object. Each of the units and parts of the system described herein above briefly shall be described herein below as exemplary embodiments.
Fig. 1 is a schematic representation showing the overall construction of the system, according to an embodiment of the invention. The system comprises of an illumination unit 1 for focusing and calibration and a measurement unit 3. An object holder 5 is provided for placing the object 7 whose motion measurement is to be obtained. Additionally, a microscope objective 9 is provided for measuring motion of micro electromechanical, MEMS devices. The microscope objective 9 enables simultaneous viewing of the object 7 and to appropriately choose the location of measurement on the object 7. An analyzer 11 is connected to the illumination unit 1 and the measurement unit 3. The analyzer 11 acquires the data and generates suitable commands to perform focusing, scanning and calibration.
Fig. 1(a) shows a schematic representation of an illumination unit 1, according to an embodiment of the invention. The illumination unit 1 comprises of a pair of light sources L1 and L2. The examples of light source include but are not limited to laser, incandescent light, LED and all such sources that would emit light in the range of 300nm-800nm. In one example of the invention, the light sources L1 and L2 are laser sources positioned perpendicular to each other. A beam splitter 13 combines the light emitted from the light sources L1 and L2. The beam emerging from the beam splitter 13 is reflected by a mirror 15. The mirror 15 is mounted on a structure 17 which is magnetically actuated by means of two coils C1 and C2. In an alternate embodiment of the invention, the actuation of the mirror 15 can be achieved by an electrostatic means, piezo-electric means or any such means suitable for actuation of the mirror 15 about an axis.
The reflected beam from the mirror 15 is then passed through a focusing arrangement and a calibration arrangement. The focusing arrangement comprises of a plurality of lenses (NOT SHOWN) to expand the reflected beam initially and then direct the reflected beam onto the calibration arrangement. The calibration arrangement comprises of an optical flat 19 and a lens arrangement 21. In one example of the invention, the lens arrangement 21 is a convex lens. Alternatively, the lens arrangement 21 can include a combination of lenses. The convex lens 21 is mounted on a stage 23 that is configured for three-dimensional translational motion. The optical flat 19 is a transparent glass and is mounted on a stage 25 configured for two-axis rotational motion. The calibration arrangement can be altered by altering the state of either the optical flat 19 or the convex lens 21 or the combination thereof. Further, the calibration arrangement is configured for regulating the position and the angle of incidence of the light beam on the object 7. Subsequent to calibration, the reflected beam is now focused onto the object.
Fig. 1(b) shows a schematic representation of the construction of the measurement unit 3, according to an embodiment of the invention. The measurement unit 3 includes a beam splitter 14. The beam splitter 14 is configured for partial redirection of the light reflected off by the object 7. One part of the light redirected is received by a photo detector 27 placed along the direction of the redirected light. The remaining reflected light from the object 7 is then focused through a convex lens 20. A curved reflector 29 is positioned in front of the convex lens 20. The position of the curved reflector 29 is configured for forming on its surface, the image of the point-of-incidence of the laser beam on the flat-region of the object 7. The light reflected from the curved reflector 29 is captured by a photo detector 31. In one example of the invention, each of the photo detector 31 and the photo detector 27 is a quadrant photo detector.
Measurement of Motion:
In one embodiment of the invention, the object 7 is assumed to possess two distinct regions on its surface, namely, a flat, smooth, reflective region and a curved reflective region of a given radius rS. The calibration and focusing optics focuses the light from the light source L1 on the flat region and the light from the light source L2 on the curved region. The light reflected off of the two regions is collected by the measurement optics. In order to discriminate between the light sourced by the two sources, the intensity of the sources can be modulated either at different frequencies or time-division multiplexed. Alternately, the light from the two sources can be collected by two different sets of photodetectors. In one of the embodiments of the invention, the intensity of the sources is pulsed in a manner to allow only one of the light sources to be switched ON, at any given time.The reflected light from the flat region is employed to measure out-of-plane motion while the reflected light from the curved region is employed to measure in-plane motion.
Measurement of out-of-plane motion:
Fig. 2 shows the schematic of the measurement unit employed to measure out-of-plane motion. Let the angle of incidence of the laser beam from the illumination unit 1 on the object 7 be ?. The convex lens 20 having a focal length of f is positioned to conform to the equation xy=f2 wherein x is the distance of the object 7 from the focal point and y is the distance of the curved reflector 29 from the back-focal point. The magnification offered by the lens is M=f/x. The light reflected from the curved reflector 29 is captured by the photo detector 31, which is placed at a distance S from the point of incidence on the curved reflector 29. The photo detector 27 is located such that the optical path length from the point of incidence to the photo detector 27 is (L+ l).
The out-of-plane displacement ?z of the object 7 and the corresponding rotation ??x both displace the laser spot on the photo detector 31 along the horizontal direction, thereby resulting in difference in optical power of the light incident on the left and right pair of quadrants. Likewise, an out-of-plane angular change ??y about the Y-axis displaces the laser spot along the vertical direction, thereby resulting in difference in optical power between lights incident on the top and the bottom pair of quadrants.
FIG.2 (a) shows the change in direction of the light spots on the detectors during an in-plane and an out-of-plane motion, according to an embodiment of the invention. The change in the direction is measured as the deflection of the spot from an original position 33 to a deflected position 35, the deflection being measured initially along the X-axis and then along the Y-axis. Alternatively, the deflection can be measured initially along the Y-axis and then along the X-axis.
Measurement of in-plane motion:
FIG.3 generally shows a schematic representation of the in-plane measurement arrangement, according to an embodiment of the invention. FIG.3 (a) shows the light path from the source to detector for an in-plane motion measurement, according to an embodiment of the invention. The in-plane motion measurement system includes a laser beam focused on a curved reflector 30 integrated on to the surface of the object 7. The radii of curvature of the curved reflector 30 along X- axis and Y- axis are denoted as rsx and rsy respectively. The reflected beam falls at the centre of the photo detector 27. FIG. 3(b) and 3(c) show the direction of the reflected beam for in-plane motion of the object, according to an embodiment of the invention. When the object 7 undergoes an in-plane motion ?x or ?y along the X- axis or the Y- axis, the direction of the reflected beam changes owing to the spatial variation in the local slope of the curved reflector 30. The reflected spot on the photo detector 27 undergoes proportionate displacement. The resulting difference in the optical power incident on the top and bottom halves of the photo detector 27 is proportional to ?x, whereas the difference between the right and left halves is proportional to ?y.
The measurement system as described herein can also be applied to measure in-plane angular change ??z of the curved reflector 30 rigidly mounted on the object 7 since such an angular change results in rotation of the major and minor axis of the laser spot on the detector by the same angle.
In general, the displacement of the object [?x ?y ?z ??x ??y ??z]T is related to the displacement of the spot [?a1x,1 ?a2x,1 ?a2y,1?a2x,2?a2y,2?a2z,2]T on the respective photo detectors by the following equation.

In the above equation, the quantities ?aix,j(i,j=1,2) and ?aiy,j(i,j=1,2) are the displacements of the spot along the X and Y directions on the ith(i=1,2) photo detector due to jth (j=1,2) source and the quantity ?a2z,2 represents the angular displacement of the spot about Z-axis on the photo detector 27. The parameter ? is angle of incidence at the curved reflector 29 whose radius of curvature is denoted as rc.
In one exemplary embodiment, the photo detector is a quadrant photodiode. The displacement of the spot results in preferential illumination of quadrants of the detector. If the voltage outputs from each of the quadrants of the ith(i=1,2) detector are labelled ViA,j, ViB,j, ViC,j and ViD,j, the signal ?Vix,j = ViB,j + ViD,j –ViA,j – ViC,j is proportional to the difference in optical power of the jth(j=1,2) source between the right and left halves of the ith detector. The signal ?Viy,j = ViA,j + ViB,j –ViC,j – ViD,j is proportional to difference in optical power of the jth source between the top and bottom halves of the ith detector. Likewise, the signal ?V2z,2 = V2B,2 + V2C,2 –V2A,2– V2D,2 is proportional to difference in optical power of the 2nd source between the diagonally opposite quadrants of the 2nd detector.
If in one exemplary embodiment, the light source is a radially symmetric Gaussian, the voltage outputs [?V1x,1 ?V2x,1 ?V2y,1 ?V2x,2 ?V2y,2 ?V2z,2]T of the measurement system are related to the spot displacements [?a1x,1 ?a2x,1 ?a2y,1 ?a2x,2 ?a2y,2 ?a2z,2]T on the photo detectors by a general formula:

wherein, Pi,j , ai,j and bi,j represents the total power, major and minor axis of the incident spot on the ith photo detector due to the jth source. kph is the optical power-to-voltage conversion gain of the detection circuit, given by kph = ?RivG , where ? is the spectral responsivity of the detector, Riv is the current-to-voltage (I-V) converter gain and G is the gain of the amplifier in cascade with the I-V converter. Further Dd represents the active diameter of both the detectors. The parameter is given by .
Industrial application:
In one specific example of the invention, light from a laser diode source (633 nm, 3 mW) is expanded, collimated and focused on the surface of the curved reflector. The surface normal to the reflector formed an angle to the optic-axis of the incident beam. The reflected light is collected by a quadrant photo-detector having an active diameter of 11 mm and a responsivity ? of 0.4A/W. The photo detector is placed at a distance of 8cm = (L+l) = 15cm such that the reflected spot is within the active area of the detector. The photo-currents from the quadrants of the detector are fed to the signal conditioning circuitry, composed of I-V converters of gain Riv and analog summation amplifiers of gain G that added the output voltages to yield the measurement signals V2x,2 and V2y,2.
In order to position the curved reflector precisely with respect to the incident laser beam and the detector, a three-dimensional micrometer based coarse-positioning stage and a piezo-based XYZ fine-positioning stage are employed. The piezo flexure stage mounted on the micrometer-stage is also employed to provide in-plane displacements in order to calibrate the sensitivity of the measurement system. Integrated strain gage based position measurement enable operating the stage in closed-loop. Actuation of the piezo-stage and acquisition of data from the signal conditioning circuitry are both performed by means of a real-time controller operated at 10 kHz update rate.
Measurement of in-plane motion of a macro-scale stage such as the piezo flexure stage is achieved by rigidly mounting a reflective sphere on the stage, focusing the laser beam on the reflector, and measuring the deflection of the laser beam due to in-plane motion of the stage. Accordingly, a smooth reflective metal sphere of radius 250µm is mounted on the piezo-stage. The angle ? between the surface normal of the reflector and the incident beam is set to 17o. Also, the coordinate axis of motion is aligned with the axis of the reflector. The stage is subsequently actuated in closed-loop by providing slow triangular waveforms in the time domain ?x(t) ?y(t) and ?z(t) along the X-,Y- and Z-axis, respectively. FIG. 4 generally plots the resulting outputs of ?V2x,2 and ?V2y,2 of the measurement system as function of the inputs ?x, ?y and ?z. Fig 4(a), 4(b), 4(c) particularly plot the variation of signals ?V2x,2 and ?V2y,2 of the measurement system as function of the inputs ?x, ?y and ?z respectively.The outputs for in-plane motion demonstrate linear relationship between ?V2x,2 and ?V2y,2 and ?x(t) ?y(t) respectively, with measurement sensitivities of 7.86V/µm and 7.58V/µm.
In order to measure out-of-plane displacement and angle change about the Y-axis, a similar setup as that described in FIG. 3 is employed, but with the exception that the object surface is flat. In this experiment, the object is an MEMS probe mounted on a nano-positioning stage, the angle ? is set to around 55°. The radius of the curved reflector is chosen to be rc=500µm. The focal length of the convex lens is chosen to be f=5cm and the magnification is chosen to be M=1.43. Finally, the distance of the two photo detectors is set to be L+l=17cm and S=6cm. In order to calibrate sensitivity to motion along the Z-axis, the piezo-actuator is actuated in closed-loop by providing slow triangular waveform ?z(t) along the Z-axis, and the output of the first photo detector is recorded. Likewise, to measure angular motion about the Y-axis, the laser light is focused on a micro-cantilever beam with integrated magnetic moment. An external solenoid is employed to actuate the beam and twist it by ±0.6mrad, and the corresponding output of the second photo detector is recorded.
FIG.5 generally shows the sensitivities of the measurement system to motion along these axes. Fig. 5(a) particularly plots the variation of ?V1x,1 as a function of ?z and Fig. 5(b) particularly plots the variation of ?V2y,1 as a function of ??y. It is seen that the two signals are proportional to ?z and ??y respectively, thereby indicating that sensitivities are independent of the magnitude of the linear and angular displacements. This simplifies measurement of multiple motion, of both periodic and non-periodic types.
Typical applications of the proposed system include measurement and control of the motion of nano-positioning stages, and characterization of the dynamic response, including mode shape and eigen-frequencies, of micro-electro mechanical systems (MEMS). Measurements of linear motion in three dimensions and two-axis angular motion have been experimentally demonstrated. The limitations/problems of optical measurement systems present in the art are overcome in two ways. Firstly, the optical measurement system is based on the optical beam deflection which is significantly different from the existing measurement systems that are predominantly based on interferometry and the Doppler Effect, specifically to the illumination and measurement arrangement. Secondly, the resulting measurement technique, and associated signal processing, is simpler than that of existing systems. It also enables achieving ultra-high resolution and high bandwidth in motion measurement along all axes. The measurement system as described herein yields the following result: The range of linear motion measurement is about 25 µm. The range of angular motion measurement is about 15 mrad. The measurement bandwidth is in a range of about 100 kHz to about 1MHz. The measurement resolution for linear motion measurement is <1nm within 1kHz bandwidth and the measurement resolution for angular motion measurement is ~1 µrad over a bandwidth of 1kHz.
The invention provides a method and a system for multiple motion measurement of an object. The method adopts a beam deflection to measure the deviation of the reflected beam from the object. The deviation can then be used for both in-plane and out-of-plane measurement. The method facilitates real time measurement of both periodic and non periodic motion. Further, the resolution is <1nm for linear displacements and <1µrad for angular displacements.
The foregoing description of the invention has been set to merely illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.