WO2014188894A1 - Method for self-calibrating a rotary encoder - Google Patents
Method for self-calibrating a rotary encoder Download PDFInfo
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- WO2014188894A1 WO2014188894A1 PCT/JP2014/062543 JP2014062543W WO2014188894A1 WO 2014188894 A1 WO2014188894 A1 WO 2014188894A1 JP 2014062543 W JP2014062543 W JP 2014062543W WO 2014188894 A1 WO2014188894 A1 WO 2014188894A1
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- scale
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- distortion parameters
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- 238000000034 method Methods 0.000 title claims abstract description 30
- 241000669069 Chrysomphalus aonidum Species 0.000 claims abstract description 14
- 230000006870 function Effects 0.000 claims description 10
- 230000008859 change Effects 0.000 claims description 5
- 238000012360 testing method Methods 0.000 claims description 3
- 238000012937 correction Methods 0.000 description 5
- 238000009434 installation Methods 0.000 description 4
- 238000004519 manufacturing process Methods 0.000 description 4
- 238000005259 measurement Methods 0.000 description 4
- 230000003287 optical effect Effects 0.000 description 3
- 230000000295 complement effect Effects 0.000 description 2
- 230000000737 periodic effect Effects 0.000 description 2
- 239000004065 semiconductor Substances 0.000 description 2
- 230000004075 alteration Effects 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 125000004122 cyclic group Chemical group 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 238000005286 illumination Methods 0.000 description 1
- 238000003384 imaging method Methods 0.000 description 1
- 238000003754 machining Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 238000005070 sampling Methods 0.000 description 1
- 230000001131 transforming effect Effects 0.000 description 1
Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING 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
- G01D18/00—Testing or calibrating apparatus or arrangements provided for in groups G01D1/00 - G01D15/00
- G01D18/001—Calibrating encoders
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING 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
- G01D5/00—Mechanical 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
- G01D5/26—Mechanical 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 infrared, visible, or ultraviolet light
- G01D5/32—Mechanical 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 infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical 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 infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/347—Mechanical 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 infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells using displacement encoding scales
- G01D5/34707—Scales; Discs, e.g. fixation, fabrication, compensation
- G01D5/34715—Scale reading or illumination devices
Definitions
- This invention relates to measuring devices and in particular to absolute rotary encoders for measuring absolute angle of rotation.
- CNC numerically controlled
- Typical encoders include a scale and a read-head.
- Optical encoders are typically used to measure absolute or relative linear positions or rotational angles.
- Relative encoders measure the relative position or angle within a period of the scale, and require counting the number of scale periods traversed to determine the absolute position or angle.
- Absolute encoders do not require memory or power to store the current position or angle, and can obtain these at any time, particularly at start-up.
- Optical encoders can be linear or rotary.
- a linear encoder measures a position, and a rotary encoder measuring an angle.
- Conventional absolute rotary encoders typically use multiple tracks and apply sine-cosine based interpolation method to achieve high resolution.
- a single track absolute linear encoder that uses a single scale and a single CCD/CMOS sensor is described in the parent Application. That encoder does not use the conventional sine-cosine based interpolation method. Instead, that encoder detects edges, or zero-crossings, in a scan-line, and fits a model to the edge positions to obtain high resolution absolute position information. That encoder acquires a ID image of the linear scale with a linear read-head.
- High accuracy rotary encoders are required in precise machining and manufacturing equipment. However, several errors can be introduced in the rotary encoder during manufacturing. These include errors in the scale pattern, installation of the scale on a rotary shaft, read-head alignment, and noise in electrical circuits.
- the spacing between the scale lines varies due to the circular nature of the scale.
- Another source of errors is eccentricity induced when the scale on a rotary disc.is arranged on a rotary shaft.
- out-of-plane motion (wobble), and misalignment in installation can also lead to variation of the distance between the read-head and the scale.
- the encoder can correct manufacturing variations, errors in the scale pattern, installation of the scale on the rotary shaft, read-head alignment, and noise in electrical circuits. During operation, temperature variations and mechanical vibrations can cause further distortions, further reducing accuracy.
- the center of the sensor receives more light compared to the sides. This results in vignetting, where the acquired ID image is brighter in the center and darker on the sides.
- Vignetting leads to errors in detected zero-crossings (edges), thereby reducing the overall accuracy.
- 6,598,196 drives the servo system on a predetermined trajectory such that encoder errors occur at frequencies outside the servo feedback loops. Such requirements increase calibration effort and time.
- U.S. 7,825,367 describes a self-calibrating rotary encoder where angular differences are determined as a Fourier series.
- Sine-cosine interpolation based rotary encoders can be calibrated as described in U.S. 8,250,901.
- the voltage data corresponding to sine and cosine of the rotation angle is fitted to an ellipse.
- Linear calibration parameters are obtained by transforming the ellipse into a circle.
- U.S. 7,825,367 describes a rotary encoder capable self-calibration.
- the rotary encoder includes a rotary disk with an angle code, a light source, and a linear sensor (CCD) that reads the angle code.
- a processing unit acquires reading values f(0) for predetermined angles.
- a difference between reading values ⁇ ( ⁇ + ⁇ ) and f(0) within a reading range on the linear sensor is g(0, ⁇ ).
- the difference is determined as a Fourier series.
- the rotation angle 0 at a location is obtained by analyzing the CCD image.
- the self- calibration is based on finding the rotation angle at two different locations, and analyzing the difference for use in the self-calibration.
- the embodiments of the invention provide a self-calibrating, single track, single read-head absolute rotary encoder.
- the encoder acquires measurements over one full rotation (360° ) or part of a full rotation.
- the encoder compensates for any errors or distortions introduced during manufacturing and later use as environmental or mechanical conditions change.
- the encoder can also compensate for vignetting due to illumination variations.
- the embodiments do not require multiple read-heads to cancel eccentricity errors. This significantly reduces the cost and complexity of the encoder.
- the embodiments do not require moving the motor at multiple speeds, or pre-determined trajectories for calibration.
- the invention also corrects for other installation errors, such as change of gap and shaft wobble.
- Fig. 1 A is a schematic of a rotary encoder according to embodiments of the invention.
- Fig. IB is a schematic of a circular scale of sectors according to embodiments of the invention.
- FIG. 1C is a schematic of the circular scale and a linear read-head according to embodiments of the invention.
- Fig. ID is a block diagram for calibrating the encoder of Fig. 1 A according to embodiments of the invention
- Fig. 2 is a graph of spatial frequency F ⁇ 6) with the rotation angle according to embodiments of the invention.
- Fig. 3 is a graph of spatial frequency variations due to noise according to embodiments of the invention.
- Fig. 4 is a graph of variations in spatial distortion parameter ⁇ ( ⁇ ) with the rotational angle according to embodiments of the invention.
- Fig. 5 is a graph of variations in spatial distortion parameter ⁇ ( ⁇ with the rotation angle according to embodiments of the invention.
- Fig. 6 is a graph of a fourth degree polynomial fitted to ⁇ ( ⁇ ) according to embodiments of the invention.
- Fig. 7 is a graph of scanline obtained by the ID sensor and depicts the vignetting
- Fig. 8 is a graph of a scaling factor according to embodiments of the invention.
- Fig. 9 is a graph of an offset factor according to embodiments of the invention.
- Fig. 10 is a graph of corrected sensor values after applying vignetting correction according to embodiments of the invention.
- a read-head can be a linear charge-coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS) to acquire ID images of a rotating circular scale.
- the ID image includes a linear array of pixels.
- the scale includes reflecting and non-reflecting regions arranged according to a de Bruijn sequence. The de Bruijn sequence is well suited, because the pattern itself is circular in nature.
- Fig. 1 shows a small portion of a circular scale 100 of an absolute encoder for one embodiment of our invention. Details.of.the scale.are described in U.S. Application 13/100092. The scale is used to determine a high-resolution phases P 120.
- the scale can include alternate light reflecting 101, and non-reflecting 102 marks or bits.
- the marks can also alternate between opaque and
- Each mark is B microns wide, which the scale resolution.
- the width B of each mark is a half-pitch. In one embodiment, B is 20 microns. Because of the relative small size of the marks, the example marks shown in the figures are not to scale.
- a read-head 1 10 is mounted at some distance and parallel to the scale.
- the read-head includes a sensor 1 1 1 , a (LED) light source 1 12, and an optional lens.
- the sensor can be a detector array of N sensors, e.g., N can be 512.
- the array can be complementary metal-oxide-semiconductor (CMOS) or charge coupled device (CCD).
- CMOS complementary metal-oxide-semiconductor
- CCD charge coupled device
- the read-head also be associated with a digital signal processor 1 15 connected to sensor and a memory. It is understood that other types of processors can be used.
- the marks or bits on the example scale 100 can be arranged on a rotatable disk 130 or shaft. The only requirement is that the marks are arranged sequentially for a particular code or non-periodic sequence.
- the marks are arranged as sectors of a circle on the scale 130.
- the read-head 111 includes a linear array of sensors 1 14.
- the read-head is centered tangentially at an offset 115 with respect to the center of rotation 1 16. Therefore, it is noted that sensor pixels near either end of the linear read-head observe a wider part of the sectors than the sensors near the center of the read-head. This leads to a distortion of the signal on the ID sensor.
- the DSP performs calibration of the encoder as shown in Fig. ID.
- the calibration can be performed offline, periodically or continuously during operation of the encoder.
- calibration samples 150 are acquired by the read- head 111 for rotational angles of the circular scale 101 for a full rotation of 360°, or some partial rotation.
- the partial rotation can be useful where the scale oscillates circularly instead of undergoing full rotations. Note that calibration samples can also be acquired for multiple rotations.
- Frequency F, and distortion parameters a and ⁇ 161 are estimated 160 from the calibration samples.
- the frequency F, and distortion parameters a and ⁇ can be directly stored in a memory, e.g., as a look-up .table, , and are sufficient to accurately determine phases of the encoder during operation.
- the table look-up may be advantageous if the look-up is fast, or takes less time and memory than evaluating the parametric function.
- a phase 195 of the encoder is determined 190 from test samples 151 and the modeled variations in the frequency F and distortion parameters a and ⁇ . It should be understood that the variations can be obtain from the raw parameters stored in the memory as a look-up table during operation. It is understood that the parameters can also be acquired during real-time operation of the encoder.
- Every subsequence has a. finite, length and. is unique, e.g.,. a. de Bruijn - sequences 103.
- Each unique sequence corresponds to a coarse phase angle. It is an object of the invention to self-calibrate the encoder so that a fine or precise angle is obtained.
- a Ar-ary de Bruijn sequence B(k, ri) of order n is a cyclic sequence of a given alphabet (number of angles) with size k, for which every possible subsequence of length n in the alphabet appears as a sequence of consecutive characters exactly once. If each B(k, ri) has a length k", then there are (k ⁇ h (n l ⁇ )/k n distinct de Bruijn sequences B(k, ri). When the sequence is truncated from front or back, the resulting sequence also has the uniqueness property with the same n. It should be noted that any non-periodic sequence with nonrepeating subsequences can be used.
- the detector array requires a field of view (FOV) of at least n bits for decoding to be possible.
- FOV field of view
- the field of view is designed to be 1-2 mm to have the desired accuracy.
- each bit of the sequence i.e., each half-pitch of the scale
- maps to at least two pixels in the linear detector array. This requires at least 16 X 2 32 pixels, which is well-below the number of pixels in conventional sensors.
- the number of pixels per half-pitch can be increased.
- the reflecting and non-reflecting regions are equi-angular, and not equi-distance when using a linear sensor, see Fig. 1C. Due to the circular scale, the width of reflecting/non-reflecting regions increases at the both ends of the sensor. Thus, the spatial frequency F is not constant along the sensor.
- z(i) be the detected zero-crossings (edge locations)
- P be the phase angle
- F be the frequency
- M be the number of bits between two successive zero-crossings z i) and z(i + 1). If we define
- parameters of the cubic model include the phase P, the spatial frequency F, and spatial distortion parameters a and ⁇ .
- This model accounts for the error due to non-uniform spacing of scale lines on the circular disk 130.
- N equations are obtained. For example, If there are N zero-crossings, z(l), z(20), then corresponding c ⁇ ), c(20) are known. These equations describe a linear system in unknowns P, F, a and ⁇ . We solve the linear system to obtain the values of P, F, a and ⁇ .
- Coarse_Position is the phase angle based only on the underlying code subsequence of the image.
- K can be 1024.
- the estimated parameters F, a and ⁇ 161 are expressed in terms of the actual rotary angle ⁇ as F 6), ⁇ ( ⁇ ), and ⁇ ).
- the embodiments considers the variations in the three parameters F(ff), ⁇ ), ⁇ ( ⁇ ). Due to imaging noise, there is a small variation (normal variation) in these parameters.
- the variation in these parameters should be normal variation over a one full rotation or part thereof, as ⁇ varies from 0 to 360 degrees.
- the spatial parameters F ⁇ 9), ⁇ ( ⁇ ), ⁇ ( ⁇ ) show a large variation than can be attributed to noise.
- Fig. 2 shows an example 200 of estimated F 6) with rotation angle over one full rotation.
- Fig. 3 shows frequency variations 300 in more details.
- the high frequency variations 301 are due to noise.
- the low frequency variations 302 are due to eccentricity wobble and gap change. It is an object of the invention to correct for these variations.
- these variations are modeled using a parametric function.
- the shaft with the scale is rotated a full (360° ) or partial rotation ( ⁇ 360°), and the encoder scale is sampled at several locations.
- the scale can be rotated every 2°, and sensor images corresponding to angles are stored in the memory. For all these angles, the estimated values of frequency and distortion parameters are stored along with estimated encoder phase P.
- a suitable parametric function or spline is used for modeling the variations in frequency and distortion parameters using least squares fitting.
- Fig. 4 shows the variation in a(ff) 400, which can be modeled using a fourth degree polynomial model with respect to the rotational angle ⁇
- ⁇ ( ⁇ ) t x + 1 2 0 + 1 3 0 2 + 1 4 0 3 + ⁇ 5 ⁇
- t , t 2 , t 3 , t 4 , and t 5 are model parameters.
- the model parameters are estimated using the least square fitting of estimated a ⁇ ff).
- Fig. 5 shows the variation 500 for ⁇ ).
- the model degree or form need not be same for all three parameters.
- the frequency F(0) can be modeled using a spline basis function, and a(ff) and ⁇ ) can be modeled using polynomial functions.
- Fig. 6 shows the estimated ⁇ ( ⁇ ) fitted to a fourth degree polynomial 600 for the full rotation. After curve fitting, the model parameters are stored in a memory of the DSP 1 15.
- the last encoder position can be used to determine the value of frequency and spatial distortion parameters for the current position. These values are used to determine the phase P.
- the frequency and distortion parameters can be iteratively determined along with the phase. This is useful at startup, where the last encoder phase is unknown or invalid.
- a first estimate of current rotation angle, frequency and distortion parameters can be obtained as described above.
- the parameters F, a and ⁇ for current position are re-determined using their respective models. Then the new value of these parameters is used to re-determine the phase P.
- the self-calibration is based on rotation angles at two different locations, and analyzing the difference for the self-calibration, and use it for calibration.
- Nakamura does not describe spatial frequency and distortion parameters.
- the encoder according to the invention is not based on the actual rotation angle as in Nakamura, but rather on the underlying frequency and distortion parameters that are used for modeling the zero-crossings at a particular rotation angle.
- vignetting correction can also be performed by acquiring measurements 700 during the rotation of the scale.
- a maximum pixel value m ⁇ (p) is a scaling factor 800
- a minimum pixel value m 2 (p) is an offset factor 900.
- the sensor values i(p) are modified 1000 as
- This modification ensures that the minimal intensity of each pixel is set to zero and the maximal intensity of each pixel is set to 255 as the encoder is rotated. This removes the vignetting effect.
- the steps of the method to perform self-calibration and vignette correction can be performed in the DSP, or similar microprocessor connected to a memory and input/output interfaces as known in the art.
Abstract
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Priority Applications (5)
Application Number | Priority Date | Filing Date | Title |
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DE112014002505.4T DE112014002505T5 (en) | 2013-05-21 | 2014-05-02 | Method for self-calibrating a rotary encoder |
KR1020157036050A KR101829521B1 (en) | 2013-05-21 | 2014-05-02 | Method for self-calibrating a rotary encoder |
JP2015552312A JP6143885B2 (en) | 2013-05-21 | 2014-05-02 | Method for self-calibrating a rotary encoder |
CN201480029584.3A CN105229424B (en) | 2013-05-21 | 2014-05-02 | Method for self-calibrating a rotary encoder |
TW103117466A TWI564548B (en) | 2013-05-21 | 2014-05-19 | Method for self-calibrating a rotary encoder |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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US13/899,025 US9423281B2 (en) | 2012-02-07 | 2013-05-21 | Self-calibrating single track absolute rotary encoder |
US13/899,025 | 2013-05-21 |
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WO2014188894A1 true WO2014188894A1 (en) | 2014-11-27 |
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PCT/JP2014/062543 WO2014188894A1 (en) | 2013-05-21 | 2014-05-02 | Method for self-calibrating a rotary encoder |
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JP (1) | JP6143885B2 (en) |
KR (1) | KR101829521B1 (en) |
CN (1) | CN105229424B (en) |
DE (1) | DE112014002505T5 (en) |
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WO (1) | WO2014188894A1 (en) |
Cited By (3)
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WO2017043249A1 (en) * | 2015-09-10 | 2017-03-16 | Mitsubishi Electric Corporation | Method and apparatus for determining position on scale |
JP2017531783A (en) * | 2015-09-22 | 2017-10-26 | 三菱電機株式会社 | Absolute encoder |
JP2018524554A (en) * | 2015-04-30 | 2018-08-30 | パーキンエルマー・ヘルス・サイエンシーズ・インコーポレイテッドPerkinelmer Health Sciences, Inc. | Autosampler, autoloader, and system and apparatus using them |
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TWI606228B (en) | 2015-10-23 | 2017-11-21 | 財團法人工業技術研究院 | Apparatus and method of automatic angle measurement |
DE102016101965A1 (en) * | 2016-02-04 | 2017-08-10 | Fraba B.V. | Method for calibrating a rotary encoder and rotary encoder for determining a corrected angular position |
DE102016115624A1 (en) * | 2016-08-23 | 2018-03-01 | Fraba B.V. | Method for calibrating a rotary encoder and rotary encoder |
US10551223B2 (en) * | 2017-03-20 | 2020-02-04 | Tt Electronics Plc | Method and apparatus for configurable photodetector array patterning for optical encoders |
JP2019158848A (en) * | 2018-03-16 | 2019-09-19 | 富士電機株式会社 | Absolute location information detection device, and absolute location information detection device control method |
US10886932B2 (en) * | 2018-09-11 | 2021-01-05 | Tt Electronics Plc | Method and apparatus for alignment adjustment of encoder systems |
TWI716246B (en) | 2019-12-31 | 2021-01-11 | 財團法人工業技術研究院 | Optical encoder |
TWI722886B (en) * | 2020-04-30 | 2021-03-21 | 國立陽明交通大學 | Rotary code disk and method for designing the same |
KR102388381B1 (en) * | 2020-06-18 | 2022-04-20 | 주식회사 져스텍 | Method and apparatus for linear position detection using De Bruijn sequence as scale ID |
TWI777686B (en) * | 2021-07-23 | 2022-09-11 | 禾一電子科技有限公司 | Electronic rotary encoder |
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- 2014-05-02 KR KR1020157036050A patent/KR101829521B1/en active IP Right Grant
- 2014-05-02 CN CN201480029584.3A patent/CN105229424B/en active Active
- 2014-05-02 WO PCT/JP2014/062543 patent/WO2014188894A1/en active Application Filing
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JP2018524554A (en) * | 2015-04-30 | 2018-08-30 | パーキンエルマー・ヘルス・サイエンシーズ・インコーポレイテッドPerkinelmer Health Sciences, Inc. | Autosampler, autoloader, and system and apparatus using them |
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DE112014002505T5 (en) | 2016-04-28 |
JP2016520794A (en) | 2016-07-14 |
CN105229424A (en) | 2016-01-06 |
TWI564548B (en) | 2017-01-01 |
KR20160011659A (en) | 2016-02-01 |
TW201504598A (en) | 2015-02-01 |
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JP6143885B2 (en) | 2017-06-07 |
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