Patent Publication Number: US-8971583-B2

Title: Distance measurement apparatus and method

Description:
This application claims the benefit of Taiwan application Serial No. 100144244, filed Dec. 1, 2011, the disclosure of which is incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     The disclosure relates in general to a measurement apparatus and method, and more particularly to a distance measurement apparatus and method. 
     BACKGROUND 
     Distance measurement modules have long been a crucial technique in industrial applications, and are prevalent in applications including mobile robots, automated-guided vehicles (AGVs), product line inspections and industrial safety gratings. According to measuring techniques, current non-contact distance sensors are segmented into two types—a time of flight estimation method and a triangulation location method. The time of flight estimation method usually provides preferred accuracy and viewable angle than the triangulation location method. However, for calculating a time that light needs for traveling to and fro, the time of flight estimation method requires highly precise and extremely costly mechanism designs. 
     For applications of AGVs and safety gratings, the precision and viewable angle for sensing distance information are usually not regulated by strict standards. It infers that, when products based on the time of flight are adopted for the applications of AGVs and safety gratings, these over-qualified products are rather utilized for less significant positions such that implementation costs are in equivalence wasted. 
     SUMMARY 
     The disclosure is directed to a distance measurement apparatus and method. 
     According to one embodiment, a distance measurement apparatus is provided. The apparatus includes a line-shaped laser transmitter, an image sensing device and a processing unit. The line-shaped laser transmitter transmits a line-shaped laser, and the image sensing device senses the line-shaped laser to output a line-shaped laser image. The processing unit receives the line-shaped laser image, and segments the line-shaped laser image into several sub-line-shaped laser images. The processing unit further calculates a vertical position for a laser line in an i th  i th  sub-line-shaped laser image, and outputs i th  distance information according to the i th  sub-line-shaped laser image and a transformation relation. Wherein, i is a positive integer. 
     According to another embodiment, a distance measurement method is provided. The method includes steps of: receiving a line-shaped laser image; segmenting the line-shaped laser image into several sub-line-shaped laser images; calculating a vertical position for a laser line in i th  an sub-line-shaped laser image; and outputting distance information according to the i th  sub-line-shaped laser image and a transformation relation. Wherein, i is a positive integer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a distance measurement apparatus. 
         FIG. 2  is a flowchart of a distance measurement method. 
         FIG. 3  is a schematic diagram of segmenting a line-shaped laser image into several sub-line-shaped laser images. 
         FIG. 4  is a schematic diagram of a relation curve of the vertical position for a laser line in a sub-line-shaped laser image and a corresponding distance. 
         FIG. 5A  is a schematic diagram of a sub-line-shaped laser image. 
         FIG. 5B  is a schematic diagram of an ideal sub-line-shaped laser image without noise. 
         FIG. 5C  is a schematic diagram of a practical sub-line-shaped laser image with noise. 
         FIG. 6  is a schematic diagram of calculating a vertical position for a laser line by utilizing a brightness center algorithm. 
         FIG. 7  is a lateral view of a distance measurement apparatus. 
         FIG. 8  is a schematic diagram of a distance measurement apparatus for measuring an object under test. 
     
    
    
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     DETAILED DESCRIPTION 
     First Embodiment 
       FIG. 1  shows a block diagram of a distance measurement apparatus.  FIG. 2  shows a flowchart of a distance measurement method.  FIG. 3  shows a schematic diagram of segmenting a line-shaped laser image into several sub-line-shaped laser images.  FIG. 4  shows a schematic diagram of a relation curve of the vertical position for a laser line in the sub-line-shaped laser image and a corresponding distance.  FIG. 8  shows a schematic diagram of a distance measurement apparatus for measuring an object under test. Referring to  FIGS. 1 ,  2 ,  3 ,  4 , and  8 , a distance measurement apparatus  1  is coupled to a computer  83 , which is capable of recording distance information generated by the distance measurement apparatus  1 . The distance measurement apparatus  1 , applicable to a mobile platform, includes a line-shaped laser transmitter  11 , an image sensing device  12 , a processing unit  13  and a housing  14 . The line-shaped laser transmitter  11  transmits a line-shaped laser  82  to an object  81  under test. The image sensing device  12 , coupled to the processing unit  13 , senses the line-shaped laser  82  to output a line-shaped laser image  3 . In one embodiment, the line-shaped laser  82  is parallel to a horizontal plane, e.g., the ground. The housing  14  accommodates the line-shaped laser transmitter  11 , the image sensing device  12  and the processing unit  13 . 
     A distance measurement method, applicable to the distance measurement apparatus  1 , includes the steps below. In Step  21 , the processing unit  13  receives the line-shaped laser image  3 . In Step  22 , the processing unit  13  segments the line-shaped laser image  3  into a plurality of sub-line-shaped laser images  3 ( 1 ) to  3 (n), where n is a positive integer. In Step  23 , the processing unit  13  calculates a vertical position for a laser line in an i th  sub-line-shaped image of the sub-line-shaped laser images  3 ( 1 ) to  3 (n), where i is a positive integer and 1≦i≦n. 
     In Step  24 , the processing unit  13  outputs i th  distance information according to the vertical position for a laser line in the i th  sub-line-shaped image and a transformation relation, such as relation curve  4  of the vertical position for a laser line in the sub-line-shaped laser image and a corresponding distance in  FIG. 4 . For example, the i th  distance information is a distance between the distance measurement apparatus  1  and an object under test. Alternatively, the processing unit  13  outputs other distance information according to the i th  distance information, the vertical position for a laser line in another sub-line-shaped image and a trigonometric function. For example, according to the i th  distance information, a trigonometric function and the vertical position for a laser line in a j th  sub-line-shaped laser image  3 (j) of the sub-line-shaped laser images  3 ( 1 ) to  3 (n), the processing unit  13  outputs j th  distance information, where j is a positive integer and not equal to i. 
     When the distance measurement apparatus  1  and the distance measurement method are applied to a mobile platform, measuring errors resulted from moving the mobile platform can be reduced. Further, since the line-shaped laser transmitter utilized in the distance measurement apparatus  1  and the distance measurement method adopts a line-shaped light source rather than a dot light source, multiple sets of distances information can be obtained through one distance measuring process to increase the amount of distance information per unit time. 
       FIG. 5A  shows a schematic diagram of a sub-line-shaped laser image.  FIG. 5B  shows a schematic diagram of an ideal sub-line-shaped laser image without noise.  FIG. 5C  shows a schematic diagram of a practical sub-line-shaped laser image with noise. Referring to  FIGS. 1 ,  3 ,  5 A,  5 B and  5 C, in principle, a line-shaped laser image without noise is as shown in FIG.  5 B, with light spots being successively located in a continuously manner on a same horizontal position. However, as the brightness changes or is affected by the environment, a line-shaped laser image actually sensed may appear as that shown in  FIG. 5C , with three discontinuous light spots on the right being noise. 
     The foregoing processing unit  13  adaptively, continuously segments the line-shaped laser image  3  according to the laser lines in the line-shaped image  3 . In other words, according to the laser lines in the line-shaped laser image  3 , the processing unit  13  adaptively segments the line-shaped laser image  3  into the sub-line-shaped laser images  3 ( 1 ) to  3 (n). The width of the sub-line-shaped laser images  3 ( 1 ) to  3 (n) may vary due to different obstacle of the application environment. For example, the processing unit  13  determines whether a change occurs in the height of the laser lines. The processing unit  13  segments successive regions having the same vertical position for a laser line into one sub-line-shaped laser image. When the vertical position for a laser line changes, the processing unit  13  starts counting from a disconnected position of the laser line, and segments following successive regions having the same vertical position for a laser line into another sub-line-shaped laser image. Further, the processing unit  13  may also equally segments the line-shaped laser image  3  into the sub-line-shaped images  3 ( 1 ) to  3 (n) having an equal width. For example, according to a width W of the line-shaped laser image  3  and a maximum tolerable noise width N D , the processing unit  13  determines the number n of the sub-line-shaped laser images  3 ( 1 ) to  3 (n). The number n of the sub-line-shaped laser images  3 ( 1 ) to  3 (n) equals 
               W     2   ⁢     N   D         .         
It should be noted that, pixels where the noise occurs in the line-shaped laser image  3  are unlikely to successively locate at a same horizontal position. Therefore, to prevent the noise from being misjudged as a line-shaped laser, the maximum tolerable noise width N D  may be appropriately defined in practical applications. When the number of a plurality of successive light spots in the sub-line laser image is greater than or equal to the maximum tolerable noise width N D , the processing unit  13  determines that these light spots are a part of the line-shaped laser. Conversely, when the number of successive light spots in the sub-line laser image is not greater than the maximum tolerable noise width N D , the processing unit  13  determines that these light spots are not a part of the line-shaped laser. For example, the maximum tolerable noise width N D  equals 3. When the number of successive light spots in the sub-line laser image is greater than or equal to 3, the processing unit  13  determines that the light spots are a part of the line-shaped laser. Conversely, when the number of successive light spots in the sub-line laser image is not greater than 3, the processing unit  13  determines that these light spots are not a part of the line-shaped laser. Accordingly, by segmenting the line-shaped laser image  3  into the sub-line-shaped laser images  3 ( 1 ) to  3 (n), noise interference may be further reduced.
 
     The processing  13  performs a histogram calculation along a vertical direction of the i th  sub-line-shaped laser image  3 (i) to obtain a vertical position y i  for a laser line in the i th  sub-line-shaped laser image  3 (i). For example, shaded areas in the i th  sub-line-shaped laser image  3 (i) in  FIG. 5A  represent pixels having a higher grayscale. Along the vertical direction of the i th  sub-line-shaped laser image  3 (i), the processing unit  13  performs a histogram calculation for a sum of grayscales of pixels of each row. When the sum of grayscales of a particular row is greater than the sum of grayscales of other rows, it means that the sum of grayscale of this particular row is the highest. That is, the laser line is located on the pixels of this row. 
       FIG. 6  shows a schematic diagram of calculating a vertical position for a laser line by utilizing a brightness center algorithm. In another embodiment, to optimize the position accuracy, the processing unit  13  calculates the vertical position for a laser line by further adopting a brightness center algorithm. The processing unit  13  regards the vertical position y i  for a laser line obtained from the foregoing histogram calculation as a center, and selects a region having (2m+1)×(W/n) pixels according to the center. According to coordinates and the brightness of the pixels in this region, the processing unit  13  obtains a coordinate of a laser light spot through an approach similar to calculating for a center of gravity. The first sub-line-shaped laser image  3 ( 1 ) is taken as an example for calculating the brightness center: 
     
       
         
           
             
               
                 
                   
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     In Equations (1) and (2), (X c ,Y c ) represents the coordinate of the calculated brightness center, W is the width of the laser image  3 , n is the number of the sub-line-shaped laser images, m is a positive integer, y 1  is the y-axis height of the laser line of the first sub-line-shaped laser image obtained from the histogram calculation, (x i ,y i ) is the coordinate in the region of (2m+1)×(W/n) pixels, and l(x i ,y i ) is the corresponding brightness value. The processing unit  13  further replaces the vertical position y i  for a laser line with the coordinate of the brightness center Yc, and determines the distance to the object under test according to the coordinate Yc of the brightness center. The coordinates of the brightness centers of the second sub-line-shaped laser image  3 ( 2 ) to the n th  sub-line-shaped laser image  3 (n) may be similarly calculated as above. 
     Please referring to  FIGS. 1 and 7 ,  FIG. 7  shows a lateral view of a distance measurement apparatus. Further, an optical axis L 1  of the foregoing line-shaped laser transmitter  11  and an optical axis L 2  of the image sensing device  12  are parallel to each other and are located on a same vertical plane perpendicular to the line-shaped laser. A distance BL between the optical axis L 1  of the line-shaped laser transmitter  11  and the optical axis L 2  of the image sensing device  12  is smaller than or equal to 10 cm. The foregoing housing  14  has a line-shaped laser opening  141  and an image sensing opening  142 . The line-shaped laser opening  141  and the image sensing opening  142  are located from each other by a horizontal distance d, which may be adjusted according to a viewable angle range of the image sensing device  12 . For example, when the viewable angle range is 60 degrees, the horizontal distance d needs to provide an angle formed between a line connecting the line-shaped laser opening  141  and the image sensing opening  142  and the optical axis of the image sensing device  12  to be greater than 30 degrees. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.