JP2015184098A - optical encoder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical encoder with high utilization efficiency of light allowable mounting accuracy of an element and capable of obtaining stable output at high S/N ratio.SOLUTION: The optical encoder includes: a scale 5 with a signal pattern 6 formed linearly or circularly in which a reflective index periodically changes; a light emitting unit 1 that emits a beam of light toward the scale 5; a detection section 3 that detects one-dimensional distribution of the light reflected by the scale 5; and a diffraction grating 7 disposed between the light emitting unit 1 and the scale 5 for correcting the one-dimensional distribution of the light detected by the detection section 3.

Description

  The present invention relates to an optical encoder for optically detecting a pattern on a scale and measuring the position or displacement of the scale.

  An optical encoder is used in a servo system or the like to optically detect a rotational motion or translational motion of an object such as a motor to obtain a position or velocity. A general optical encoder detects a light emitting unit composed of an LED or the like, a scale on which a signal pattern for detecting a position or an angle is formed, and light on which the scale signal pattern is projected is converted into an electric signal. And a calculation unit for calculating a position or an angle from an electric signal detected by the detection unit.

  Regarding the configuration of the optical encoder, a scale is placed between the light emitting unit and the detecting unit, and the transmission type that detects the light quantity distribution of the light that has passed through the scale, and the light emitting unit and the detecting unit are arranged on the same side of the scale And a reflection type for detecting the light quantity distribution of the light reflected from the scale. In recent years, a reflection type configuration is often used because of a demand for a thinner encoder.

  FIG. 12 is a side view showing an example of a conventional optical encoder. The light emitting unit 51 and the detection unit 53 are installed on the substrate, and the scale 55 on which the signal pattern 56 is formed is installed in parallel to the substrate. Either the substrate or the scale 55 moves relatively along the X direction. In order to reduce the thickness of the encoder, the gap g between the scale 55, the light emitting unit 51, and the detecting unit 53 needs to be as narrow as possible. In such a narrow gap reflective optical system, since there is no space for placing optical components, a configuration that does not use a collimating lens is generally employed.

The light emitting unit 51 generally includes an LED (light emitting diode) or the like, and the radiation angle distribution D of light emitted from the light emitting region 52 usually has a Lambertian characteristic. The illuminance distribution by the light source having the Lambertian characteristic is a function of cos ⁴ θ using the light amount on the optical axis perpendicular to the light emitting surface as a reference and using the radiation angle θ formed by the irradiated light with the optical axis. Further, assuming that the optical axis on the irradiation surface of the detection unit 53 is the origin and the position normalized by the optical path length from the light emitting region 52 to the detection unit 53 is x, θ = tan ⁻¹ x, and the following equation (1) ), The light quantity distribution I is proportional to the fourth power of cosine.
I (x) = cos ⁴ (tan ⁻¹ x) (1)

  FIG. 13A is an explanatory diagram in which the optical system shown in FIG. 12 is developed on a plane, and shows how the light emitted from the light emitting region 52 irradiates the detection unit 53. FIG. 13B is a graph showing the light irradiation intensity distribution in the light receiving region of the detection unit 53. The horizontal axis normalizes the irradiation position by the optical path length from the light emitting region 52 to the detection unit 53, where x = 0 is the center through which the optical axis passes, X = 1 is the right end of the light receiving region, and X = −1 is the light received. Each corresponds to the left edge of the region. The vertical axis is also normalized by the maximum value of light irradiation intensity. It can be seen that the light irradiation intensity at the detection unit 53 exhibits a characteristic of rapidly decreasing from the center toward the left and right periphery.

  In the reflective encoder, the gap g is often designed to be about 1 mm because of the trade-off relationship between the blur due to the distance of the projected image and the margin for mounting the component. In order to secure a detection element having a length of about 6 mm at the position of the detection unit 53, a light receiving region of about x = ± 1 is required. In this case, the light irradiation intensity at the end of the light receiving region is about the maximum value. Decreases to 0.2.

  As a countermeasure against such light quantity fluctuation, Patent Document 1 below proposes that a plurality of detection elements 54 are installed in the detection unit 53 and the light receiving area of each detection element 54 is increased from the center toward the left and right periphery. As a result, the output of the detection element in the vicinity of the center where the light irradiation intensity is high and the output of the detection element in the vicinity of the end portion where the light irradiation intensity is low are substantially equal. As shown in the graph of FIG. The uniformity of the output from the detection element is improved.

JP 2009-168625 A

  In the optical encoder proposed in Patent Document 1, as shown in FIG. 13, so as to compensate for the light irradiation intensity that rapidly decreases from the center toward the left and right periphery, and to uniformize the output from each detection element, The light receiving area of each detection element is changed.

  However, when manufacturing an encoder, it is necessary to allow an element mounting error, so that the maximum value in the width direction perpendicular to the slit arrangement direction of the detection elements 54 is limited. For this reason, instead of increasing the width of the detection element 54 at the end of the light receiving region, the width of the detection element 4 near the center is decreased to reduce the amount of received light, thereby making the light quantity distribution uniform. As a result, the light utilization efficiency is reduced.

  In addition, since the light receiving sensitivity characteristic indicating the reciprocal of the light amount distribution curve as shown in FIG. 13B is given to the detection unit 53, due to mounting errors during assembly, temperature changes during operation, etc., When a deviation occurs in the positional relationship between the irradiation light amount distribution and the detection unit 53, the output uniformity is lost, and the position detection characteristics are deteriorated.

  An object of the present invention is to provide an optical encoder that has a high light utilization efficiency, can tolerate the mounting accuracy of an element, and can obtain a stable output at a high S / N ratio.

In order to achieve the above object, an optical encoder according to the present invention includes:
A scale in which a signal pattern in which the reflectance is periodically changed is formed in a linear or arc shape;
A light emitting unit that emits light toward the scale;
A detection unit for detecting a one-dimensional distribution of light reflected by the scale;
And a diffraction grating provided between the light emitting unit and the scale for correcting a one-dimensional distribution of light detected by the detection unit.

  According to the present invention, since the one-dimensional distribution of light is corrected using the diffraction grating between the light emitting portion and the scale, the irradiation intensity distribution can be made uniform without reducing the light utilization efficiency. . Moreover, the mounting accuracy of the element can be allowed, and a stable output can be obtained with a high S / N ratio.

It is a perspective view which shows Embodiment 1 of this invention. It is a side view which shows Embodiment 1 of this invention. FIG. 3A is an enlarged front view of the light emitting section and the diffraction grating, and FIG. 3B is a graph showing the irradiation intensity distribution on the detection element obtained by using the ray tracing simulation. It is a graph which shows the detail of irradiation intensity distribution on a detection element. It is a three-dimensional graph which shows the result of having calculated the fluctuation rate of irradiation intensity distribution. It is a contour map which shows the result of having calculated the fluctuation rate of irradiation intensity distribution. It is a perspective view which shows Embodiment 2 of this invention. FIG. 8A is an enlarged front view of the light emitting section and the blazed diffraction grating, and FIG. 8B is a graph showing the irradiation intensity distribution on the detection element obtained by using the ray tracing simulation. FIG. 9A is an enlarged front view of a light emitting unit having two separated light emitting regions and a blazed diffraction grating, and FIG. 9B is an irradiation intensity on a detection element obtained by using a ray tracing simulation. It is a graph which shows distribution. It is a perspective view which shows Embodiment 3 of this invention. 11A is a plan view of the tilted blazed diffraction grating as viewed from the Z direction, FIG. 11B is a front view as viewed in the Y direction, and FIG. 11C is a side view as viewed from the X direction. It is a side view which shows an example of the conventional optical encoder. FIG. 13A is an explanatory diagram in which the optical system shown in FIG. 12 is developed in a plane, and FIG. 13B is a graph showing the light irradiation intensity distribution in the light receiving region of the detection unit 53, and FIG. c) is a graph showing a distribution of detection outputs.

Embodiment 1 FIG.
FIG. 1 is a perspective view showing Embodiment 1 of the present invention, and FIG. 2 is a side view thereof. The optical encoder includes a scale 5 and a head 8 that can move relative to each other, a calculation unit 40, and the like. Here, a linear encoder in which the scale 5 and the head 8 are relatively moved along the linear direction is illustrated, but the scale 5 and the head 8 are relatively moved along the circumferential direction of the radius R, and the scale 5 is also a circle of the radius R. The present invention can be similarly applied to a rotary encoder formed in an arc shape. Here, for easy understanding, the moving direction of the scale 5 is the X direction, the normal direction of the surface of the head 8 is the Z direction, and the X direction and the direction perpendicular to the Z direction are the Y direction.

  The scale 5 is composed of a plate-like member, and a high-reflectance section and a low-reflectance section are alternately arranged on the lower surface thereof, and a signal pattern 6 in which the reflectance changes periodically is formed. In the case of a linear encoder, the signal pattern 6 is formed in a straight line shape along the X direction, and in the case of a rotary encoder, the signal pattern 6 is formed in an arc shape in contact with a straight line extending in the X direction.

  The head 8 includes a light emitting unit 1, a diffraction grating 7, a detection unit 3, and the like. The light emitting unit 1 is configured by an LED or the like and has a light emitting region 2 that generates light toward the scale 5. The detection unit 3 includes a plurality of detection elements 4 arranged in an array along the X direction, and detects a one-dimensional distribution of light reflected by the scale 5. The diffraction grating 7 is provided between the light emitting unit 1 and the scale 5 and has a function of correcting the one-dimensional distribution of light detected by the detection unit 3.

  The calculation unit 40 is configured by a microprocessor or the like, and has a function of calculating the position or displacement amount of the scale 5 based on the output signal from the detection unit 3.

  Regarding the optical system, as shown in FIG. 2, the light emitting region 2 of the light emitting unit 1, the signal pattern 6, the diffraction grating 7, and the detection surface of the detecting unit 3 are arranged in parallel to each other. The light emitted from the light emitting region 2 at an angle inclined with respect to the Z direction passes through the diffraction grating 7, undergoes intensity modulation by the signal pattern 6 when reflected obliquely by the scale 5, and then reaches the detection unit 3. To do. The signal pattern 6 and the detection unit 3 are arranged with a predetermined amount shifted from the light emitting unit 1 along the X direction.

  Further, the position and size of the diffraction grating 7 are set so that all of the light emitted from the light emitting region 2 and irradiating the signal pattern 6 can pass through the diffraction grating 7. The diffraction grating 7 is configured by arranging a large number of linear gratings extending in the Y direction along the X direction. When light passes through the diffraction grating 7, light diffraction occurs in a plane perpendicular to both the optical axis of the incident light and the X direction.

  FIG. 3A is an enlarged front view of the light emitting unit 1 and the diffraction grating 7, and FIG. 3B is a graph showing an irradiation intensity distribution on the detection element 4 obtained by using a ray tracing simulation. . In FIG. 3A, only light emitted in a specific direction from the center of the light emitting region 2 is illustrated, and illustration of the other light is omitted. The light emitted from the center of the light emitting region 2 generates 0th order diffracted light and ± nth order diffracted light (n is an integer of 1 or more). The exit angle of such n-order diffracted light is determined according to the wavelength of light, the angle of incidence on the diffraction grating 7 and the grating period. Hereinafter, for ease of understanding, the 0th order and ± 1st order diffracted light will be described as examples.

  As shown in FIG. 3 (b), if the one appearing by diffraction in the + X direction is + 1st order diffraction, the + 1st order diffracted light intensity distribution 9 appears at a position biased in the + X direction in the irradiation intensity distribution. Similarly, if the one that appears due to diffraction in the -X direction is -1st order diffraction, the -1st order diffracted light intensity distribution 10 appears at a position biased in the -X direction in the irradiation intensity distribution. There is also a certain amount of 0th-order diffracted light intensity distribution 11, and the total light intensity distribution 12 is obtained by adding all of them.

  The total light intensity distribution 12 spreads in the diffraction direction due to ± 1st-order diffraction, resulting in a mixture of 0th-order diffracted light and ± 1st-order diffracted light. Thus, the flatness near the center is improved. Such a total light intensity distribution 12 is expressed as follows: 1) The ratio of the intensity of the first-order diffracted light to the intensity of the 0th-order diffracted light is ξ, and the ratio of both is 0th-order peak intensity: 1st-order peak intensity = 1−ξ: ξ ) Depending on the ratio β = λ / Λ of the diffraction grating period Λ and the wavelength λ of light corresponding to the diffraction angle. For example, when ξ = 0.8 and β = 1.4, a flat distribution appears near the center as shown in the graph of FIG.

  In the optical encoder as described above, the distance between the scale 5 and the head 8 cannot be made very small due to limitations in assemblability and component tolerances. On the other hand, the image of the signal pattern 6 is detected with a high contrast within an allowable range. Since it is necessary to project to the part 3, it is difficult to make it too large. Because of this trade-off, the interval between the scale 5 and the head 8 is often set to about 1 mm.

  Further, the array length of the detection elements 4 of the detection unit 3 has a length of about 5 mm in order to detect a plurality of periods of the signal pattern 6, and the detection elements 4 pass through the signal pattern 6 from the light emitting region 2. An array length of detection elements corresponding to about two times the optical distance to reach, that is, an irradiation area is required. This irradiation area corresponds to an area of −1 ≦ x ≦ 1 when represented by a position x on the detector normalized by the optical path length as shown in FIG. 4, and the maximum value in that area is Max, and the minimum When the value is set to Min, the variation rate α in the region is calculated as α = 1−Min / Max.

This variation rate α depends on the ratio ξ of the first-order diffracted light and the ratio β of the diffraction grating period and the light wavelength, and the variation rate α was calculated in the range of 0 ≦ ξ ≦ 1 and 1 ≦ β ≦ 1.5. The results are shown in the three-dimensional graph shown in FIG. 5 or the contour map shown in FIG. Here, as described in FIG. 6, two inequalities L1 and L2 including a range in which the desired variation rate α is 0.3 or less can be defined.
ξ <a ₁ + b ₁ (β−1), a ₁ = 0.424−0.08α, b ₁ = 1.02−1.4α (L1)
ξ> a ₂ + b ₂ (β−1), a ₂ = 0.6 (0.3−α), b ₂ = 4/3 (L2)

  A region between these two inequalities L1 and L2 is a preferable region in which the desired variation rate α ≦ 0.3 can be maintained. It is preferable to design the diffraction grating 7 so as to fill this region.

  Although the diffraction grating according to the present invention can also be configured as an amplitude grating, by using it as a phase grating, the light utilization efficiency is approximately twice that of the amplitude grating, so that the SN ratio can be improved efficiently. Is possible.

  Further, in the present embodiment, the reflection type encoder having a higher necessity for lens reduction has been described. However, a similar configuration is possible in the transmission type encoder, and the collimating lens is arranged by bringing the light emitting unit and the detection unit close to each other. Any detection optical system that needs to be reduced can be applied to other than optical encoders.

Embodiment 2. FIG.
FIG. 7 is a perspective view showing Embodiment 2 of the present invention. FIG. 8A is an enlarged front view of the light emitting section 1 and the blazed diffraction grating 13, and FIG. 8B is a graph showing the irradiation intensity distribution on the detection element 4 obtained by using the ray tracing simulation. is there. This embodiment has the same configuration as that of the first embodiment, but differs in that a blazed diffraction grating 13 is used instead of the diffraction grating 7.

  The blazed diffraction grating 13 is blazed in a sawtooth shape inclined in one direction so that either + 1st order diffraction or −1st order diffraction has a high diffraction efficiency. Symmetrical lattice patterns with different blazed directions are formed in the A region 13a on the + X direction side and the B region 13b on the -X direction side with reference to the plane including the normal line of the scale 5 passing through the center of the light emitting unit 1. Is formed. That is, as shown in FIG. 8A, the A region 13a has a blaze angle inclined in the −X direction with respect to the normal direction of the diffraction grating, while the B region 13b has a normal line of the diffraction grating. It has a blaze angle inclined in the + X direction with respect to the direction. As a result, the diffracted light from the A region 13a is separated from the reference surface in the + X direction, and the diffracted light from the B region 13b is separated from the reference surface in the −X direction.

  As shown in FIG. 8B, since the efficiency of the diffracted light in the direction tilted in the + X direction is increased in the right A region 13a, the + 1st order diffracted light intensity distribution 9 is shifted to the position deviated in the + X direction. Appears. Similarly, since the efficiency of the diffracted light in the direction inclined in the −X direction is increased in the left B region 13b, the −1st order diffracted light intensity distribution 10 appears at a position biased in the −X direction in the irradiation intensity distribution. There is also a certain amount of 0th-order diffracted light intensity distribution 11, and the total light intensity distribution 12 is obtained by adding all of them.

  As described above, various diffraction efficiencies can be designed according to the relief shape of the diffraction grating, and the irradiation intensity distribution can be flattened by appropriately setting the diffraction grating constant as described in the first embodiment.

  Next, regarding the optical principle, a light beam emitted from the blazed diffraction grating 13 can be regarded as equivalent to a light beam that passes along the blazed diffraction grating 13 in the reverse direction and extends along the direction of the broken line in the figure. . For this reason, the light emission position behaves as if it moved closer to the center, the right light emitting area 2a appears to move leftward, and the left light emitting area 2b appears to move rightward. As a result, the X dimension of the light emitting region 2 is apparently reduced in the diffraction direction. The degree of reduction of the light emitting region 2 corresponds to the apparent amount of movement of the light emitting position, the width of the light emitting region is D, the diffraction angle of the blazed diffraction grating 13 is θ, and the grating surface of the blazed diffraction grating 13 and the light emitting region The optical movement distance is Go, and the apparent movement amount is Go × tan θ.

  When the amount of movement is D / 4, the centers of the right side light emitting region 2a and the left side light emitting region 2b overlap, and the apparent light emitting area becomes the smallest. Further, the amount of movement increases, and when the amount of movement exceeds D / 2, the apparent light emitting region width becomes larger than the base width. Therefore, in order not to enlarge the apparent light emitting region, it is preferable to arrange the blazed diffraction grating 13 so as to satisfy Go <D / 2 × cot θ (cot θ represents the cotangent of θ).

  Note that Gp shown in FIG. 8A indicates a physical distance. When a distance having a refractive index n is Ln and a distance having a refractive index 1 is La, the optical distance Go = Ln / n + La. The However, Gp = Ln + La.

  In general, in an optical encoder, in order to maintain the contrast of a projected signal pattern as far as possible, a point light source LED having a light emitting area diameter of 200 μm or less is often used. Since the amount of light emitted from the LED decreases as the light source becomes smaller, reducing the apparent light emitting area size while having as large a light emitting area as possible is effective in improving the SN ratio of the detection signal.

  FIG. 9A is an enlarged front view of the light emitting unit 1 having the two separated light emitting regions 2a and 2b and the blazed diffraction grating 13, and FIG. 9B is a detection element obtained using a ray tracing simulation. 4 is a graph showing an irradiation intensity distribution on 4. When the blazed diffraction grating 13 as described above is used, the apparent light emitting area becomes the smallest when the right light emitting area 2a and the left light emitting area 2b that are apparently moved overlap, and the light emitting areas 2a, 2a, Go = W / 2 × cot θ, where D is the width of 2b and W is the distance between the centers of the light emitting regions 2a and 2b. As for the deviation from the position, only the width of the light emitting region is allowed, and the blazed diffraction grating 13 is arranged in a range given by (WD) / 2 × cot θ <Go <(W + D) / 2 × cot θ. Is preferred.

Embodiment 3 FIG.
FIG. 10 is a perspective view showing Embodiment 3 of the present invention. This embodiment has the same configuration as that of the second embodiment, but is different in that an inclined blazed diffraction grating 14 is used instead of the blazed diffraction grating 13. The tilted blazed diffraction grating 14 has two blazed regions 14a and 14b as in the second embodiment, and the diffraction direction of each region is perpendicular to the dividing line of the region of the diffraction grating. It is configured to have a diffraction vector component not only in a direction but also in a direction in which the signal pattern 6 and the detection element 4 are viewed.

  11A is a plan view of the tilted blazed diffraction grating 14 seen from the Z direction, FIG. 11B is a front view seen in the Y direction, and FIG. 11C is a side view seen from the X direction. As shown by the oblique lines in the plan view of FIG. 11A, the regions 14a and 14b have a lattice inclined with respect to the central dividing line and are configured symmetrically with respect to the dividing line.

  The diffraction direction by the tilted blazed diffraction grating 14 is not only the vector component in the detection direction along the Y direction as indicated by the arrow in the plan view of FIG. 11A, but also the vector component in the + X direction in the right region 14a. The left region 14b has a vector component in the −X direction, and the diffracted light in the region 14a and the diffracted light in the region 14b are separated from each other. That is, as shown in the side view of FIG. 11C, the light 15 incident perpendicularly to the tilted blazed diffraction grating 14 is bent in the same vector direction 16. However, as shown in the front view of FIG. 11B, the light 15a incident perpendicularly to the right region 14a of the light 15 is bent in the vector direction 16a inclined in the + X direction, and the left side of the light 15 The light 15b incident perpendicularly to the region 14b is bent in the vector direction 16b inclined in the −X direction.

  In the configuration of the present embodiment, the diffraction grating period that satisfies the conditions for smoothing the irradiation intensity distribution is parallel to the direction perpendicular to the dividing lines of the regions 14 a and 14 b, that is, the arrangement direction of the signal pattern 6. It is defined by a component in the Y direction. Since the intensity of the emitted light from the light emitting region 2 is strongest in the vertical direction, the amount of light incident on the detection element 4 can be improved by tilting the vertical component toward the signal pattern 6 and the detection element 4. The configuration of this embodiment can also be applied to the configuration using the light emitting section having the two light emitting areas shown in FIG. 9 described in the second embodiment, and the amount of illumination light can be reduced without enlarging the light emitting area 2. Can be increased.

1 light emitting section, 2 light emitting area, 3 detecting section, 4 detecting element, 5 scale,
6 signal pattern, 7 diffraction grating, 8 head, 9 + 1st order diffracted light intensity distribution,
10-1st order diffracted light intensity distribution, 11 0th order diffracted light intensity distribution, 12 Total light intensity distribution,
13 blazed diffraction grating, 14 tilted blazed diffraction grating, 40 computing unit.

Claims (9)

A scale in which a signal pattern in which the reflectance is periodically changed is formed in a linear or arc shape;
A light emitting unit that emits light toward the scale;
A detection unit for detecting a one-dimensional distribution of light reflected by the scale;
An optical encoder, comprising: a diffraction grating provided between the light emitting unit and the scale and for correcting a one-dimensional distribution of light detected by the detection unit.   The optical encoder according to claim 1, wherein the diffraction grating is a phase grating.   3. The optical encoder according to claim 1, wherein the diffraction grating has a grating period that reaches the scale and the detection unit in a state where at least 0th-order diffracted light and ± 1st-order diffracted light are mixed.   4. The optical system according to claim 3, wherein the grating period of the diffraction grating is set such that an irradiation intensity distribution in the detection unit is flatter than a distribution when the diffraction grating is not present. Encoder. Λ is the wavelength of light, λ is the grating period of the diffraction grating, and λ / Λ = β is defined. The intensity ratio of the first-order diffracted light to the 0th-order diffracted light is 1-ξ: ξ, and the light intensity distribution in the detection unit The maximum value is Max, the minimum value is Min, α = 1−Min / Max is defined as the fluctuation rate α of the light intensity distribution, and the following relational expression is satisfied: The optical encoder described in 1.
ξ <a ₁ + b ₁ (β−1), a ₁ = 0.424−0.08α, b ₁ = 1.02−1.4α
ξ> a ₂ + b ₂ (β−1), a ₂ = 0.6 (0.3−α), b ₂ = 4/3
0 ≦ ξ ≦ 1
1 ≦ β ≦ 1.5   The blazed diffraction grating that is symmetrical with respect to a plane that passes through the center of the light emitting portion and includes the normal line of the scale is formed in the diffraction grating. The optical encoder described.   7. The optical encoder according to claim 6, wherein the blazed diffraction grating is inclined with respect to the reference plane so that the diffracted light has a vector component parallel to the reference plane and a vector component perpendicular to the reference plane. The following relational expression is satisfied, where Go is an optical distance between the light emitting part and the diffraction grating, θ is a diffraction angle of the blazed diffraction grating, and D is a width of the light emitting region of the light emitting part. The optical encoder according to 6 or 7.
Go <D / 2 × cotθ The light emitting unit has two separated light emitting regions, the optical distance between the light emitting unit and the diffraction grating is Go, the diffraction angle of the blazed diffraction grating is θ, and the width of the light emitting region of the light emitting unit. The optical encoder according to claim 6, wherein D is D, and a distance between centers of the light emitting regions is W, and the following relational expression is satisfied.
(W−D) / 2 × cot θ <Go <(W + D) / 2 × cot θ

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