KR20160137819A - Displacement Sensor Using Astigmatism and Sensing Method thereof - Google Patents

Displacement Sensor Using Astigmatism and Sensing Method thereof Download PDF

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KR20160137819A
KR20160137819A KR1020150071651A KR20150071651A KR20160137819A KR 20160137819 A KR20160137819 A KR 20160137819A KR 1020150071651 A KR1020150071651 A KR 1020150071651A KR 20150071651 A KR20150071651 A KR 20150071651A KR 20160137819 A KR20160137819 A KR 20160137819A
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axis
displacement
translational motion
light emitted
lens
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KR101692152B1 (en
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이상헌
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안동대학교 산학협력단
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
    • G01C3/00—Measuring distances in line of sight; Optical rangefinders
    • G01C3/02—Details
    • G01C3/06—Use of electric means to obtain final indication

Abstract

Disclosed are a displacement sensor using astigmatism and a displacement measurement method using the same. The displacement sensor of the present invention includes a sensor part and a measurement part. The sensor part comprises: an object lens facing an object; a polarized beam splitter which reflects a light emitted from a laser diode to the object lens, and transmits a beam reflected from the object lens; a cylinder type lens where the beam transmitted from the splitter is incident; and a position sensing detector which is realized with four photo diodes, and where a light emitted from the cylinder type lens is formed. The measurement part can calculate and output a z-axial translational motion value (VT) of the object, an x-axial displacement (Vx) vertical to a z-axis by a rotary motion of the object, and a y-axial displacement (Vy) vertical to the z-axis and an x-axis by a rotary motion, using a voltage emitted from the four photo diodes by the beam formed on the position sensing detector. By the same measurement, the displacement sensor of the present invention can measure a pure translational motion only even though there is a rotation in an actual motion, by suggesting the method capable of measuring only the translational motion independent of the rotary motion.

Description

[0001] Displacement Sensor Using Astigmatism and Sensing Method [

The present invention relates to a displacement sensor for measuring multi-degrees-of-freedom motion, and more particularly, to a method for measuring a degree of freedom of movement of an object by using an astigmatism principle And a sensing method thereof

In general, a displacement sensor with high precision is required to measure the movement of a minute-sized structure. However, a commonly used laser interferometer or a capacitive displacement sensor can be applied only when the size of the object to be measured is not less than a certain size. Therefore, the optical lever system or the astigmatism type optical pickup apparatus, which is mainly applied to the atomic microscope, is utilized in a manner that the displacement of a minute-sized structure can be applied to a microscopic measurement point.

Astigmatism relates to image formation through a lens in three dimensions, which means that the imaging position of an object deviated from the optical axis is different between the tangential plane and the sagittal plane. The image of the object whose astigmatism result is deviated from the optical axis is formed to be long in an elliptical shape. A sensor for measuring the deformation or movement of an object using the astigmatism is a displacement sensor using astigmatism. For example, an objective lens applied to an optical pickup unit used in an optical disc drive focuses using an astigmatic-based sensor. The astigmatism-based sensor is smaller in volume and weight than the optical lever, providing designers with greater flexibility in building the system to which the sensor is applied. In the optical pick-up apparatus, it was possible to measure the displacement only in the focal length direction of the objective lens, but also to measure the inclination of the disk, and it was confirmed that the measurement of the multi-degree-of-freedom motion was possible

FIG. 1 is a diagram showing an optical internal configuration of a conventional optical pick-up unit. A beam emitted from a laser diode 101 is reflected by a polarized beam splitter 103 and passes through an objective lens 105 And is then reflected again on the optical disk m and passes through the splitter 103 and the cylindrical lens 107 to reach a four position detection detector (PSD) 110.

Here, the focal distance of the cylindrical lens 107 on the meridional plane differs from that on the sagittal plane, giving an astigmatic effect. Therefore, two focuses are generated by the astigmatism effect and the shape of the laser beam that is imaged on the position-detection detector 110 changes. When the position detection detector 110 is at a position corresponding to the focal length of the objective lens, the shape of the beam is circular, but at another point the beam is elliptical.

The beam shape information can be obtained by calculating an output value of a photodiode, which is each cell of the quadrant position detection detector 110, according to Equation (1).

Figure pat00001

Here, V a , V b , V c , and V d represent output voltages of respective photodiodes of the position detection detector 110, and V FE represents a focus error signal. If the focus error signal is 0, a circular beam is formed on the position detection detector 110 as shown in FIG. 1 (b), and the position detection detector 110 is focused. If the focus error signal is a positive value, a vertical oval beam is formed as shown in FIG. 1C. If the focus error signal is negative, an oval beam of a horizontal direction is formed as shown in FIG. The position detection detector 110 is out of focus.

Since the focus error signal appears in the form of an S-shaped curve with respect to the z-axis direction, it is possible to use an astigmatic displacement sensor using a linear interval between two peaks as a measurement interval.

On the other hand, since the S curve is a measurement result in a state in which the optical disk m is not twisted, only the translational motion in the z-axis direction can be measured. However, in the optical pick-up unit, even when the measurement object is tilted as shown in Fig. 2, the output value may change.

2 shows the position of the beam due to the tilting of the disc m in an in-focus state. In this case, as shown in FIGS. 2 (d) to 2 (f), there is a positional shift of the beam due to the twist, but since there is no change in distance on the optical axis (z axis) 1, there is no translational motion and the shape of the beam is constant. Since only the tilting with respect to the x and y axes is generated based on the coordinate axes given in Fig. 2, the beam moves to the vertical axis (x) and the horizontal axis (y) of the lens.

On the other hand, astigmatism occurs even when light inclined with respect to the optical axis of the lens is scanned. The tilting angle is very small and is expected to be less influential than the astigmatism caused by the cylindrical lens 107 and can be neglected. Therefore, distance information and torsion information on the z axis can be obtained from the shape of the beam and the position of the beam. That is, the distance information of the z axis is obtained from Equation (1), and the tilting value is obtained by using the output values of the photodiodes separated by x and y axes in the quadrant photodiodes of the position detection detector 110 by the following Equations 2 and 3 As well as the ability to measure it.

Figure pat00002

Figure pat00003

Vx 'is the x-axis direction displacement due to the rotational motion, and Vy' is the y-axis direction displacement due to the rotational motion. However, the measurement methods expressed by the equations (1) to (3) are related to each other, so that the amount of translational motion changes even if only the pure rotation (two degrees of freedom, tilting) is performed without translation (one degree of freedom) motion. This means that it is impossible to measure the motion of many degrees of freedom, and even if pure translation is desired, the value of translational motion may change if there is a fine rotation.

Therefore, the equations for translational motion measurements should be independent of the equation for rotational motion measurement and should not be dependent on each other. However, instead of Equation (1) for translational motion measurement, equations (2) and (3) for rotational motion measurement and an independent measurement method should be presented.

[Related Technical Literature]

1. Optical Focussing Device United States Patent No. 4,079,247 (March 14, 1978)

It is an object of the present invention to provide a displacement sensor and a sensing method thereof that can measure the measurement of a one-degree-of-freedom translational motion of an object using the principle of an astigmatism system, .

According to an aspect of the present invention, there is provided a displacement sensor including a sensor unit and a measurement unit. The sensor unit includes an objective lens opposed to the object, a polarization beam splitter for reflecting the light emitted from the laser diode to the objective lens and passing the beam reflected from the objective lens, And a position detection detector which is realized by four photodiodes and forms an image of light emitted from the cylindrical lens.

The measuring unit calculates the z-axis translational motion value (V T ') of the object using the voltage emitted from the four photodiodes by the beam image formed on the position detection detector, and outputs the calculated value as the following equation.

Figure pat00004

Figure pat00005

Figure pat00006

Here, V a , V b , V c , and V d are the output voltages of the four photodiodes, respectively.

According to an embodiment, the measuring unit may include an x-axis direction displacement (Vx ') perpendicular to the z axis due to the rotation of the object, a y-axis direction displacement (Vy') perpendicular to the x- ) Is calculated and outputted as the following equation.

Figure pat00007

Figure pat00008

The displacement sensor of the present invention can measure translational motion and rotational motion of an object based on four photodiode signals of a position detection detector.

However, according to the present invention, only a translational motion independent of rotational motion can be measured, so that only a pure translational motion can be measured even if there is a rotation in an actual motion.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view conceptually showing a translational motion measurement of a conventional optical pick-
FIG. 2 conceptually shows a tilting operation measurement of the optical pickup unit of FIG. 1,
3 is a block diagram of a displacement sensor of the present invention,
4 is a view showing a laser image picked up by the position detection detector,
5 is a graph simulating measured values in a conventional measuring method according to offset due to tilting when a circular laser beam is imaged without translational motion,
6 is a graph simulating measured values in a conventional measuring method according to an offset due to tilting when a elliptical laser beam is imaged without translational motion, and
FIG. 7 is a diagram showing a difference between a measurement method of the present invention and translational motion due to a conventional focus error signal.

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail with reference to the drawings.

Referring to FIG. 3, the displacement sensor 300 of the present invention includes a sensor unit 301 and a measurement unit 303. The displacement sensor of the present invention is not limited to this, and is a sensor for individually measuring the translational motion and the rotational motion of an object. The optical pickup unit of FIG. 1, an atomic microscope, and the like are examples.

The sensor unit 301 includes a laser diode 101, a polarized beam splitter 103, an objective lens 105, a cylindrical lens 107 and a quadrant position detection detector And a PSD (Position Sensing Detector) 110.

The beam emerging from the laser diode 101 is reflected by the polarizing beam splitter 103 and is then incident on the objective lens 105 and then reflected again on the surface of the object m to be incident on the splitter 103 and the cylindrical lens 107, And reaches the quadrant position detection detector 110. [

The measurement unit 303 receives the output voltages Va, Vb, Vc, and Vd output from the four photodiodes 110a, 110b, 110c, and 110d of the position detection detector 110 of the sensor unit 301, m) is measured.

Since the mathematical expression 1 and the mathematical expressions 2 and 3 are empirically obtained using the output values of the position detection detector 110, it is possible to mathematically model the shape of the laser beam B picked up by the position detection detector 110, Equation 1 and Equations 2 and 3 are not independent from each other as follows.

Equations (2) and (3) show the final beam position shift by tilting. In order to quantitatively calculate the offset amount of the beam center by the actual tilting, an optical pickup head output value model considering tilting is required. To this end, the relationship between the four photodiodes 110a, 110b, 110c, and 110d of the position detection detector 110 and the laser beam moved by tilting is derived using FIG.

4 conceptually shows a laser image captured by the position detection detector 110. It is assumed that the astigmatism occurs due to the difference in focal length between the meridional plane and the sagittal plane in the cylindrical lens 107. FIG. The four photodiodes 110a, 110b, 110c, and 110d of the position detection detector 110 are moved along the z axis direction (direction perpendicular to the drawing) according to the translational motion of the object m, And the beam B, which is an ellipse, is picked up by the translational motion of the object m. The lengths of the semi-major axis and the semi-minor axis of the elliptical beam B can be expressed by the following equations (4) and (5).

Figure pat00009

Figure pat00010

Here, t (z) is of the elliptical jangbangyeong lengths, s (z) is the length of the ellipse danbangyeong, r t, r s, f t, f s is finally astigmatic effect, taking into account the effect of various optical elements (R t , r s ) and focal length (f t , f s ) of the equivalent lens in the horizontal and vertical planes. Here, the constant Z is a variable indicating the distance in the z-axis direction (optical axis direction).

When there is a tilting or rotation of the object m, the center of the beam B deviates from the center of the position detection detector 110 as shown in Fig. In this case, the laser beam B can be expressed by an elliptic equation of the following equation (6).

Figure pat00011

Here, xm and ym move relative to the fixed coordinate system (X-Y) of the position detection detector 110 in the coordinate system of the laser beam (B). t and s are respectively calculated by equations (4) and (5) as the length of the axis.

The boundary lines c1 and c2 of the four photodiodes 110a, 110b, 110c and 110d can be expressed by the following Equation (7).

Figure pat00012

Equation 7 is derived on the basis of the coordinate system, and it is confirmed that the center of the laser beam B moves by -x 0 ', -y 0 ' when x 0 'and y 0 ' are offset on the position detection detector 110 have.

The output values Va, Vb, Vc, and Vd of the photodiodes 110a, 110b, 110c, and 110d of the position detection detector 110 are assumed to be proportional to the area of the beam imaged by the photodiode.

The area of the beam formed on the second diode 110b of the position detection detector 110 can not be obtained at once and the area of the laser beam B between the first to fourth diodes 110a, 110b, 110c, (B1, b2, b3, b4) using the intersections (P1, P2, P3, P4). The intersections P1, P2, P3, and P4 can be obtained by substituting Equation (7) into Equation (6). Accordingly, the area of each of the four portions (b1, b2, b3, b4) can be obtained by the following Equations (9) to (11).

Figure pat00013

Figure pat00014

Figure pat00015

Figure pat00016

The areas of the laser beams B in the remaining first, third, and fourth diodes 110a, 110c, and 110d can be obtained in a manner similar to the above. The area of the laser beam B picked up by the first to fourth diodes 110a, 110b, 110c and 110d is determined by the output voltages V a and V b of the first, third and fourth diodes 110a, 110c and 110d , V c , and V d .

By using this, simulating the case where the tilting occurs, the results shown in Figs. 5 and 6 can be obtained.

FIG. 5 shows output values of the first to fourth diodes 110a, 110b, 110c, and 110d according to offsets by tilting without translational motion. By limiting the angle of tilting, the laser beam B is always present in the position sensitive detector 110. Since there is no translational motion, the shape of the beam B is circular.

Referring to FIG. 5, (a) is a tilting value of Equation 2 according to an offset in the X-axis direction, (b) is a tilting value of Equation 3 according to an offset in the Y-axis direction, Signal value. (a) and (b), it can be seen that the output values of the tilting equations (2) and (3) change linearly by the offset. Therefore, when the output value is corrected, the offset values (xo ', yo') on the respective axes can be obtained by using the values of Vx 'and Vy' measured from (a) and (b) in FIG. However, it can be seen that the error signal value in FIG. 5 (c), that is, the translational motion of the z-axis wobble, is changed by the offset even though the shape of the beam remains constant in a circular form.

6 shows a case where the position of the object on the Z axis is fixed to a point different from the focal distance so that the shape of the beam is elliptical. FIG. 6A shows a tilting value of Equation 2 according to the offset in the X- Is the tilting value of Equation (3) according to the offset in the Y-axis direction, and (c) represents the error signal value. Here again, the offset values (xo ', yo') on the respective axes can be obtained using the values of Vx 'and Vy' measured from (a) and (b) of FIG. However, the error signal value in FIG. 6 (c), that is, the translational motion in the z-axis, can be confirmed by the offset even though the shape of the beam is constantly maintained as a constant ellipse. This indicates that independent degrees of freedom can not be measured.

Presentation of new methods

Therefore, the measuring unit 303 of the present invention is characterized in that (1) the area of the laser beam B is constant even when tilting occurs, and (2) the size of the translational motion corresponds to the area of the laser beam B The total area information of the laser beam B expressed by the following equation (12) is used to measure the displacement (i.e., translational motion) of the z axis without using the error signal of the equation (1) .

Figure pat00017

Equation 12 is obtained by adding the output values of the four photodiodes 110a, 110b, 110c and 110d of the position detection detector 110, and V T corresponds to the area of the laser beam B. [ However, since V T always has (+) value, the direction of translational motion can not be confirmed.

Therefore, since the focus error signal V FE indicates whether the laser beam B due to the translational motion is in-focus or out-focus, A measurement method can be derived.

Figure pat00018

Here, V T 'is the translational motion value of the object, which is the final output value of the translational motion measurement. FIG. 7 is a graph showing V T '(s 1) obtained by taking the sign of the error signal into consideration for the total area signal according to the distance in the z-axis direction, and it is possible to perform measurement according to the translational motion like the focus error signal (V FE ) see. It can be seen that the signal V T 'is less linear than the case of using the focus error signal, but a wide measurement area can be used.

Therefore, the measuring unit 303 of the present invention obtains the translational motion using Equation (13), and measures rotational motion using Equations (2) and (3).

In the present invention, a relational expression for the development of an astigmatism-based displacement sensor for multi-degree-of-freedom measurement is derived. Based on the derived relational expression, it was confirmed that the measurement method of uniaxial displacement and rotation about two axes is possible, and unlike the conventional method, the area of the whole beam is used together.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is clearly understood that the same is by way of illustration and example only and is not to be construed as limiting the scope of the invention as defined by the appended claims. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention.

Claims (4)

A polarizing beam splitter for reflecting the light emitted from the laser diode to the objective lens and passing the beam reflected by the objective lens, and a polarization beam splitter for reflecting the light emitted from the laser diode, A sensor unit having a lens and a position detection detector which is implemented with four photodiodes and in which light emitted from the cylindrical lens is imaged;
And a measuring unit for calculating and outputting a z-axis translational motion value (V T ') of the object using a voltage outputted from the four photodiodes by a beam formed on the position detection detector,
Figure pat00019

Figure pat00020

Figure pat00021
ego,
Wherein V a , V b , V c , and V d are output voltages of the four photodiodes, respectively.
The method according to claim 1,
The measurement unit calculates an x-axis direction displacement (Vx ') perpendicular to the z-axis by the rotational motion of the object, and a y-axis direction displacement (Vy') value perpendicular to the x-axis and the z- Output,
Figure pat00022

Figure pat00023

And the displacement sensor.
A polarizing beam splitter for reflecting the light emitted from the laser diode to the objective lens and passing the beam reflected by the objective lens, and a polarization beam splitter for reflecting the light emitted from the laser diode, 1. A displacement measuring method for a displacement sensor comprising a lens and a sensor unit having a position detection detector for forming light emitted from the cylindrical lens,
Calculating a z-axis translational motion value (V T ') of the object using voltages emitted from the four photodiodes of the position sensitive detector,
Figure pat00024

Figure pat00025

Figure pat00026
ego,
Wherein V a , V b , V c , and V d are the output voltages of the four photodiodes, respectively.
The method of claim 3,
Calculating and outputting a value of an x-axis displacement (Vx ') perpendicular to the z-axis due to the rotation of the object, and a value of a y-axis displacement (Vy') perpendicular to the z- Further comprising:
Figure pat00027

Figure pat00028

And the displacement is measured.
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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4079247A (en) * 1975-05-16 1978-03-14 Claude Bricot Optical focussing device
KR20040083012A (en) * 2003-03-20 2004-09-30 가부시키가이샤 키엔스 Displacement system and method for measuring displacement
KR20110078597A (en) * 2009-12-31 2011-07-07 신국선 System for providing position of beam's axis and method for measuring displacement thereof
JP2013022676A (en) * 2011-07-20 2013-02-04 Canon Inc Device for detecting deflection amount of laser light, device for measuring displacement, method of manufacturing optical element molding die and optical element

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4079247A (en) * 1975-05-16 1978-03-14 Claude Bricot Optical focussing device
KR20040083012A (en) * 2003-03-20 2004-09-30 가부시키가이샤 키엔스 Displacement system and method for measuring displacement
KR20110078597A (en) * 2009-12-31 2011-07-07 신국선 System for providing position of beam's axis and method for measuring displacement thereof
JP2013022676A (en) * 2011-07-20 2013-02-04 Canon Inc Device for detecting deflection amount of laser light, device for measuring displacement, method of manufacturing optical element molding die and optical element

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