Method for measuring flatness of sheet material and method for manufacturing steel sheet using the same

A method for measuring a flatness of a sheet material, comprising:

• forming a staggered bright and dark pattern made up of bright parts and dark parts in which the bright parts are disposed at first and second pitches respectively in first and second directions by light emitted from an LED light source including a plurality of LEDs disposed in a staggered manner at third and fourth pitches respectively in the first and second directions, and projecting the staggered pattern onto a surface of the sheet material such that the first direction of the staggered pattern lies along a lengthwise direction of the sheet material, and the second direction of the staggered pattern lies along a width direction of the sheet material;
• acquiring a pattern image by picking up an image of the staggered pattern with an image pickup device disposed at a location where the image pickup device can receive light in which the staggered pattern is specularly reflected at the surface of the sheet material;
• setting a shape measurement line extending along the first direction of the staggered pattern at a predetermined location in the acquired pattern image;
• averaging pixel densities on a straight line which passes pixels on the shape measurement line and extends in the second direction of the staggered pattern, and which has a length not less than twice the second pitch of the bright parts and calculating an average pixel density;
• calculating a distribution of the average pixel density along the shape measurement line;
• and
• calculating a surface shape of the sheet material along the shape measurement line based on the calculated average pixel density distribution, and computing a flatness of the sheet material based on the calculated surface shape.

A method for measuring a flatness of a sheet material, comprising:

• forming a bright and dark pattern made up of bright parts and dark parts, the bright parts being disposed at first and second pitches respectively in first and second directions by light emitted from an LED light source including a plurality of LEDs disposed at third and fourth pitches respectively in the first and second directions;
• projecting the bright and dark pattern onto a surface of the sheet material travelling in a lengthwise direction thereof such that the first direction of the bright and dark pattern lies along the lengthwise direction of the sheet material, and the second direction of the bright and dark pattern lies along a width direction of the sheet material;
• picking up an image of the bright and dark pattern with an image pickup device to acquire a pattern image;
• and
• analyzing the acquired pattern image to measure the flatness of the sheet material, wherein
• a two-dimensional camera with an electronic shutter which can set an exposure timing and an exposure time is used as the image pickup device, and
• a lighting timing and a lighting time of the LED are respectively made synchronous with an exposure timing and an exposure time set in the two-dimensional camera with the electronic shutter.

A method for measuring a flatness of a sheet material, comprising:

• forming a bright and dark pattern made up of bright parts and dark parts, the bright parts being disposed at first and second pitches respectively in first and second directions by light emitted from an LED light source including a plurality of LEDs disposed at third and fourth pitches respectively in the first and second directions;
• projecting the bright and dark pattern onto a surface of the sheet material travelling in a lengthwise direction thereof such that the first direction of the bright and dark pattern lies along the lengthwise direction of the sheet material, and the second direction of the bright and dark pattern lies along a width direction of the sheet material;
• picking up an image of the bright and dark pattern with an image pickup device to acquire a pattern image;
• and
• analyzing the acquired pattern image to measure the flatness of the sheet material, wherein
• as the LED, an LED that emits light of a single wavelength different from a peak wavelength of radiant light emanating from the sheet material is used, and
• a bandpass filter that passes only light having a wavelength of the emission wavelength of the LED and a wavelength close to the emission wavelength of the LED is disposed in front of the image pickup device.