Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P3/00—Measuring linear or angular speed; Measuring differences of linear or angular speeds
  • G01P3/36—Devices characterised by the use of optical means, e.g. using infrared, visible, or ultraviolet light
  • G01P3/38—Devices characterised by the use of optical means, e.g. using infrared, visible, or ultraviolet light using photographic means
• • 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
  • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
  • G01B11/04—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness specially adapted for measuring length or width of objects while moving
  • G01B11/043—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness specially adapted for measuring length or width of objects while moving for measuring length
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P3/00—Measuring linear or angular speed; Measuring differences of linear or angular speeds
  • G01P3/64—Devices characterised by the determination of the time taken to traverse a fixed distance
  • G01P3/68—Devices characterised by the determination of the time taken to traverse a fixed distance using optical means, i.e. using infrared, visible, or ultraviolet light

Definitions

• the present invention relates to a method of contactless measuring of the longitu- dinal movement and speed of a surface.
• the invention also relates to a device for performing this method.
• the invention can, for example, be used for determining the number of meters of material which pass past a certain position.
• the material can be paper, foil, sheet metal, packaging material etc. which move past the position on a moving web.
• DE 44 08 072 describes a device for measuring particles in a flowing medium. Two or more picture sensor units are used in order to take different pictures of the same object. Aligning these two pictures is difficult. This device is completely directed to solving problems of measuring in flowing media.
• the illumination geometry described in the text and shown in the figures cannot be used for measuring on solid surfaces.
• the illumination is furthermore adapted to give different types of light, e.g. different polarized light, adapted to the beam splitter which leads the pictures individually to the different picture sensor units.
• US 4 320 948 shows a movement-detecting device which performs measurements with the use of "center of gravity". That which is measured is actually the relative angular velocity and not the absolute movement.
• the problem which is solved according to this patent is to adjust a shutter time of a camera in accordance with the speed of an obj ect being photographed.
• FR 2 583 882 shows speed measuring where pairs of pictures from one and the same picture sensor unit taken at different occasions are compared with each other.
• the problem which is to be solved according to this patent is to measure the speed and position of a moving vehicle in relation to the ground.
• the conventional optics described and shown would give large measurement errors if this device would be used for accurate measuring of movement of e.g. surfaces of paper.
• One object of the invention is to permit accurate and cost-effective measuring of movement and/or speed of the movement of a surface at both low and high speeds. Another object of the invention is to permit a measurement of movement and/or speed on a relatively smooth surface on which it is often difficult to make an exact measurement, such as paper or plastic.
• the invention is based upon a method of measuring linear movement of a surface of an object, with imaging of a part of the surface of the object comprising at least partly the same surface parts, successive detecting and storing of imaged parts, correlation calculations based on the successive imaged parts, calculation of the movement of the surface of the object based upon the calculated correlation values.
• the invention is characterized by illumination of the surface of the object with a plane of light with essentially parallel light in order to generate a line of light on the surface of the object essentially parallel with the direction of movement of the surface of the object, and telecentric imaging of the line of light and the imaged part of the object.
• Illumination with a line of light which is relatively narrow means that: a) one can measure the movement of surfaces with a fine surface structure, such as plastic foils and fine paper, b) one can measure with high speed even with modest light power, c) the measuring position in the transverse direction can be defined by the position of the line of light and not the position of the sides on the picture sensor.
• Imaging of the line of light with telecentric illumination optics means that the points of the object are illuminated independent of the position of the movement, whereby a more accurate measuring can be obtained. Telecentric imaging technique improves the measuring performance considerably because:
• the laser light is pulsed especially in order to reduce the average effect of the light source. This also makes possible a constant exposure time for all speeds. It is instead the length of the pulse of light which can give rise to possible blurring caused by movement.
• the solution to the problem of accurate and cost-effective measuring of the movement and speed consequently lies in that the measuring object being investigated is illuminated from a, preferably laser-based, transmitter unit which generates a plane of light.
• the plane of light generates a narrow line of light on the object being measured.
• the line of light can be imaged at an angle or parallel to the plane of laser light by a matrix of photodetectors, where each detector is a pixel.
• the best illuminating geometry ought to be a plane of light parallel with both the optical axis and the direction of movement.
• the angle of incidence of the light should, however, be considerably different from the normal angle of instant, e.g. 30-40°.
• the line of light when it is formed by a laser can typically be approximately 10-20 pixels thick and 256 pixels long. (The length of the complete picture chip can thereby be longer, i.e. >256 pixels.)
• the angle of the laser light to the plane of the object being measured can, for example, be 25°.
• the light source suitably emits pulsed light and/or has a sampling time determined by the imaging device, whereby the pulse time and/or the sampling time are dependent on the earlier movement of the material being investigated.
• a special object of the invention can also be to provide compact sensor devices which do not require external electronic equipment.
• This can be provided through the use of a photodetector matrix of so-called intelligent type so that the calculating unit is a semiconductor surface common with the photodetector matrix and provided with software for performing at least a part of the correlation calculations.
• Fig. 1 illustrates a first embodiment of a device according to the invention.
• Fig. 2 illustrates a second embodiment of a device according to the invention and shows also an exemplifying, simplified block diagram for controlling, calculating and displaying which is suitable for both embodiments.
• a method of measuring the linear movement of a surface 5 of an object on an object being measured 1, which moves along a predetermined path.
• the surface 5 of the object is illuminated by a light source 3, preferably a laser, with a plane of light with essentially parallel light.
• a light source preferably a laser, even if theoretically it could be another type of light source.
• the laser 3 generates, e.g. by collimating and cylindrical lenses, a laser plane which gives a narrow laser line 4 on the surface 5.
• the laser line 4 is essentially parallel with the movement of the surface 5 of the object as shown by the arrow 2.
• the illuminated region imaged on the photomatrix can be a small number, e.g. less than 20 pixels, wide and relatively many, e.g. more than 250 pixels long, the narrower the laser line, the higher the intensity.
• the laser line 4 is imaged via a telecentric lens 8 by a matrix 7 of photodetectors.
• the telecentric lens 8 ensures that the same beam geometry is valid for all measuring points (pixels). Furthermore, this reduces the requirement that the measuring distance should be constant. It is, however, an advantage if the distance between the unit with camera and laser and the surface of the object is held as constant as possible so that the laser line and that which is imaged shall not move towards each other. If the material being investigated is being fed over rollers, it is therefore suitable to project the laser line on the material just before or after a roller.
• the scale of the imaging is determined first of all by the surface structure of the material, the signal variations required, the signal processing methods etc.
• the object being measured 1 is imaged at an angle ⁇ which is essentially normal to the surface 5 of the object.
• the laser light meets the surface 5 of the object at an inclined angle ⁇ . In this way a scanning light effect is obtained which accentuates the structure of the surface, i.e. its roughness.
• the angle of incidence of the laser light is consequently selected based on the surface structure of the object to be measured.
• the laser 3 is placed laterally in relation to the surface 5 of the object.
• Telecentric illumination from the laser together with telecentric imaging for the photodetector matrix 7 means that the geometry is the same for two consecutive registrations of the surface of the object. Telecentric imaging also means that the accuracy of the measuring of the movement and speed increases. Certain variations in the distance between the sensor and the surface of the object can be tolerated.
• the exposure time for each imaging must be held small in order to avoid blurring caused by movement. This can take place through the light source being pulsed and/or through regulating the exposure time of the camera.
• the photodetector matrix can be a one-dimensional detector array, but a two- dimensional matrix detector is preferable.
• the picture on the photodetector matrix 7 will in this case be some rows wide, which means that signals from corresponding points in several parallel rows can be added in each exposure, before the correlation calculations are performed. Correlation calculations are known in the prior art, for example, from US-4 708 021 (hereby incorporated by reference), and therefore such calculations are not described more closely here.
• the modulation in the signals obtained along the line becomes greater the fewer the rows which are added during columnwise adding. With better modulation in the signal, theoretically the possi- bility of accurate correlation should increase.
• the requirement that the material must move parallel to the laser light i.e. a structure on the surface of the material must follow the same pixel row during the whole movement increases. If a particle slides off the row of pixels, then valuable information will be lost. That gives a worse correlation result.
• time displacement ⁇ t For each measurement two short exposures with a certain time displacement ⁇ t are made.
• This time displacement ⁇ t can be freely chosen and adjusted dynamically, for example so that the time distance between two measurements approximately corresponds to half a row of pixels on the detector. In this way an extremely dynamic system is obtained which can handle large speed variations with the same relative accuracy. This also offers the possibility to measure a stationary object, in contrast to most of the other methods.
• the photodetector matrix 7 can be of so-called intelligent type, where part of the correlation calculations are performed in a calculation unit arranged on a semiconductor surface in common with the photodetector matrix.
• a suitable type of photodetector matrix denoted MAPP2200 (from F/P in Link ⁇ ping) is built into a camera specially constructed for the application.
• MAPP2200 is an intelligent picture chip comprising 256 parallel processors and an equal number of analogue/digital converters, which makes the camera extremely fast.
• the correlation between exposed pictures with the method according to the invention can be performed by means of software programmed into the camera for this purpose. A number of the calculations are performed directly on the picture chip.
• the laser 10 is placed behind or in front of the measuring line 11 seen in the direction of movement v and emits its line at an inclined angle ⁇ , selected to be more inclined the smoother the surface is.
• a lens 12 with a plurality of optical elements, e.g. lenses, ensures that the line is emitted tele- centrically. It is especially important here because the line 11 otherwise has a linearly varying intensity. Possibly the focusing optics is positioned inclined so that the laser line is held narrow (focused) over the whole of its length.
• a camera suitably with an MAPP-chip, images the line 11 telecentrically on its photodetector matrix (not shown). The camera is mounted so that the optical axis of the camera coincides with the normal to the surface of the object. The field of image of the camera is selected according to the maximum speed and the dimensions of the longitudinal surface structure of the object being measured.
• the light source is used as a laser.
• Laser light has, however, the disadvantage that there occurs a type of noise which is known as speckle noise.
• speckle noise This noise phenomenon occurs when coherent light if reflected against a surface which is structured, measured in wave lengths. The wave length for reflected wave fronts is the same but the wave fronts will not be in phase with each other. Random interference is formed through the light alternately being amplified or reduced at a point depending on how the wave fronts lie in phase with each other.
• a large shutter opening can be used in the camera lens. With a large shutter opening in the lens 8 (see Fig. 8), more speckles will be imaged on the picture field 7, and this gives a lower noise level in each pixel.
• the size of the receiving aperture is consequently selected depending on the size of the speckles and the required depth of field.
• the laser light is preferably pulsed.
• the time between the laser pulses does not have to be fixed but can be determined from earlier calculated movement values. Since the material to be investigated moves during the exposure time of the camera or the pulse time, the picture will have so-called blurring due to movement. Blurring due to movement means that the imaged structure will be evened out. This can lead to that the correlation function will not find a specific top when the different lines, which are to be correlated during the correlation procedure, have moved to lie alongside each other. In order to rectify this, the light pulses and/or the exposure time of the camera, i.e. the sampling time for each exposure, can be minimized.
• True movement can be calculated if the movements per minimum cycle time (exposure + reading/storage + time for calculating the correlation) is less than approximately 50 to 90% of the sensor's field of image.
• the reading can be performed continuously with correlation calculations between each picture exposure.
• an MAPP-based camera it is possible to take approximately 10 000 pictures per second.
• Each picture pixel value is analogue/ digital converted.
• the digital result is stored in a program-controlled register on the MAPP-chip.
• Each correlation calculation is performed between the last taken picture and the picture which is taken, for example, a half chip-length earlier.
• the analogue/digital-converted picture is correlated with the previous picture.
• the correlations are made with a picture taken several cycles earlier in order to achieve an adequate overlap between the two correlated pictures.
• the calculation of the speeds and thereby the choice of pictures which are to be correlated is controlled by a program with software adapted for the present invention.
• a calculating and control unit 16 such as a computer with software, performs the correlation calculations in order to investigate how far the material being investigated has moved between the exposures, between which the calculations are performed, calculates which of the stored exposures should be used for the correlations, especially if the material being investigated moves quickly, calculates if the laser 10 should be pulsed and with which frequency, and controls the laser in an adequate manner, calculates the exposure time of the camera 13 depending on the speed of the movement of the material 14.
• the measuring result is shown in a display unit 17, which can be a monitor and/or a printer or the like, and/or can be communicated to other units 18 for control and/or storage for later processing.
• Suitable materials to be investigated by means of the invention is e.g. moving paper, plastic, wood, wood fibre and metal webs.
• the maximal speed which can be measured is limited by the smallest exposure time + the reading time. At high speeds, it is not possible to measure the true movement. In that case, two pictures are exposed quickly after each other followed by calculation and determination of the movement. When the calculations are finished, two further pictures are taken etc. The time between the correlation calculations should be as short as possible. This is in order to increase the measuring accuracy through foraiing the average value. At low speeds, the above leads to that more intermediate pictures must be stored. This is because there should have occurred a sufficiently large movement between the two pictures which one intends to correlate between.

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Cited By (13)

Citations (5)

• 1997
  • 1997-05-22 SE SE9701911A patent/SE9701911D0/xx unknown
• 1998
  • 1998-05-19 AU AU76809/98A patent/AU7680998A/en not_active Abandoned
  • 1998-05-19 WO PCT/SE1998/000937 patent/WO1998053327A1/en active Application Filing

Patent Citations (5)

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