Method of detecting position of rectangular object and object detector

Abstract

A method of detecting a position of a rectangular object includes the steps of: capturing an image of the object by an image sensor having a rectangular image plane having four edges; detecting, for each edge of the four edges, a distance to a point of the image of the object closest to said each edge; and determining a position of a predefined point of the image of the object based on the detected distances.

1 . A method of detecting a position of a rectangular object comprising the steps of: 
 capturing an image of the object by an image sensor having a rectangular image plane having four edges;    detecting, for each edge of the four edges, a distance to a point of the image of the object closest to said each edge; and    determining a position of a predefined point of the image of the object based on the detected distances.    
   
   
       2 . A method as recited in  claim 1  wherein the step of determining includes the step of determining a position of the center point of the image of the object based on the distances.  
   
   
       3 . A method as recited in  claim 2  wherein a coordinate system having a first axis and a second axis is defined on the image plane, the first axis of the coordinate system being perpendicular to a first pair of opposite edges of the image plane, and the second axis of the coordinate system being perpendicular to a second pair of the edges of the image plane, and 
 the step of determining a position of the center point includes the step of:    determining first-axis coordinates of points closest to respective edges of the first pair of edges;    calculating a first-axis coordinate of the center point by averaging the first-axis coordinates determined in the step of determining first-axis coordinates;    determining second-axis coordinates of points closest to respective edges of the second pair of edges; and    calculating a second-axis coordinate of the center point by averaging the second-axis coordinates determined in the step of determining second-axis coordinates.    
   
   
       4 . A method as recited in  claim 3  further comprising the steps of calculating an angle θ which one of the edges of the image of the object forms with one of the four edges of the rectangular image plane using the first-axis coordinates determined in the step of determining first-axis coordinates, and the second-axis coordinates determined in the step of determining second-axis coordinates.  
   
   
       5 . A method as recited in  claim 4  wherein 
 the first-axis coordinates determined in the step of determining first-axis coordinates includes coordinate values R 1   x  and L 1   x  where R 1   x >L 1   x ; and    the second-axis coordinates determined in the step of determining second-axis coordinates includes coordinate values T 1   y  and B 1   y  where T 1   y >B 1   y ; and wherein the step of calculating an angle includes the step of calculating the angle θ by a following equation:            θ   =       tan     -   1       ⁢           T   ⁢           ⁢   1   ⁢   y     -     B   ⁢           ⁢   1   ⁢   y           R   ⁢           ⁢   1   ⁢   x     -     L   ⁢           ⁢   1   ⁢   x         .               
   
   
       6 . A method as recited in  claim 5  further comprising the step of rounding the angle θ to a nearest one of a predetermined set of angles.  
   
   
       7 . A method as recited in  claim 6  wherein the predetermined set of angles includes a series of angles increasing with a specific difference.  
   
   
       8 . A method as recited in  claim 1  wherein 
 the objects includes a rectangular retro-reflective strip, and    the step of capturing comprising the steps of:    turning on a lighting device;    capturing and storing a first image of the object by the image sensor while the lighting device is on;    turning off the lighting device;    capturing and storing a second image of the object by the image sensor while the lighting device is off; and    subtracting the second image from the first image.    
   
   
       9 . A method as recited in  claim 8  wherein the image plane of the image sensor has a plurality of pixels each producing a pixel signal having a plurality of signal levels; 
 the method further including a step of down-sampling the pixel signals of the plurality of pixels of the image sensor to 1-bit signals.    
   
   
       10 . A method as recited in  claim 9  wherein the step of detecting comprises the steps of, for each edge of the four edges of the image plane, scanning the down-sampled rectangular image plane starting from the each edge in a direction to an opposite edge until a point having a predetermined first value is found.  
   
   
       11 . An object detector for detecting a position of a rectangular object comprising: 
 an image sensor having a rectangular image plane having four edges;    a distance detector that detects, for each edge of the four edges, a distance to a point of the image of the object closest to said each edge; and    a position determiner that determines a position of a predefined point of the image of the object based on the detected distances.    
   
   
       12 . An object detector as recited in  claim 11  wherein the position determiner includes a center position determiner that determines a position of the center point of the image of the object based on the distances.  
   
   
       13 . An object detector as recited in  claim 12  wherein a coordinate system having a first axis and a second axis is defined on the image plane, the first axis of the coordinate system being perpendicular to a first pair of opposite edges of the image plane, and the second axis of the coordinate system being perpendicular to a second pair of the edges of the image plane, and 
 the center position determiner includes:    a first-axis coordinate determiner that determines first-axis coordinates of points closest to respective edges of the first pair of edges;    a first-axis coordinate calculator that calculates a first-axis coordinate of the center point by averaging the first-axis coordinates determined by the first-axis coordinate determiner;    a second-axis coordinate determiner that determines second-axis coordinates of points closest to respective edges of the second pair of edges; and    a second-axis coordinate calculator that calculates a second-axis coordinate of the center point by averaging the second-axis coordinates determined by the second-axis coordinate determiner.    
   
   
       14 . A n object detector as recited in  claim 13  further including an angle calculator that calculates an angle θ which one of the edges of the image of the object forms with one of the four edges of the rectangular image plane using the first-axis coordinates determined by the first-axis coordinate determiner, and the second-axis coordinates determined by the second-axis coordinate determiner.  
   
   
       15 . An object detector as recited in  claim 14  wherein the first-axis coordinates determined by the first-axis coordinate determiner includes coordinate values R 1   x  and L 1   x  where R 1   x >L 1   x;    the second-axis coordinates determined by the second-axis coordinate determiner includes coordinate values T 1   y  and B 1 Y where T 1   y >B 1   y , and where    the angle calculator includes a calculator of the angle θ by following equation:            θ   =       tan     -   1       ⁢           T   ⁢           ⁢   1   ⁢   y     -     B   ⁢           ⁢   1   ⁢   y           R   ⁢           ⁢   1   ⁢   x     -     L   ⁢           ⁢   1   ⁢   x         .               
   
   
       16 . An object detector as recited in  claim 14  further including a wireless transmitter that transmits the position of the predetermined point of the image of the object via wireless communication.  
   
   
       17 . An object detector as recited in  claim 11 , wherein the objects includes a rectangular retro-reflective strip, 
 the object detector further including:    a light source;    a light source controller that causes the light source to periodically emit a light;    an exposure controller that causes the image sensor to capture a first image while the light source is emitting a light and to capture a second image while the light source is not emitting a light;    and an image subtracting device that subtracts the second image from the first image.    
   
   
       18 . An object detector as recited in  claim 11  wherein the image plane of the image sensor has a plurality of pixels each producing a pixel signal having a plurality of signal levels; 
 the object detector further including a down sampling circuit that down-samples the pixel signals of the plurality of pixels of the image sensor to 1-bit signals.    
   
   
       19 . An object detector as recited in  claim 18  wherein the down sampling circuit includes a comparator having a first input connected to receive the pixel signals and a second input connected to a predetermined threshold level voltage.  
   
   
       20 . An object detector as recited in  claim 11  further including a wireless transmitter that transmits the position of the predetermined point of the image of the object via wireless communication.