Patent Publication Number: US-2012038765-A1

Title: Object sensing system and method for controlling the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an object sensing system and method for controlling the same and, more particularly, to an object sensing system and method capable of effectively enhance sensing accuracy. 
     2. Description of the Prior Art 
     As touch technology advances, an electronic device with large size and multi-touch function will be widely used in daily life. Compared with other touch design, such as a resistive touch design, a capacitive touch design, an ultrasonic touch design, or a projective touch design, an optical touch design has the advantage of lower cost and is easier to use. 
     Referring to  FIG. 1 ,  FIG. 1  is a schematic diagram illustrating an optical touch system  1  of the prior art. As shown in  FIG. 1 , the optical touch system  1  comprises an indication plane  10 , two image sensing units  12   a ,  12   b , three light emitting units  14   a ,  14   b ,  14   c , and a processing unit  16 . The image sensing units  12   a ,  12   b  are disposed at opposite corners of the indication plane  10  respectively. The light emitting units  14   a ,  14   b ,  14   c  are disposed around the indication plane  10 . The processing unit  16  is electrically connected to the image sensing units  12   a ,  12   b  and the light emitting units  14   a ,  14   b ,  14   c . Each of the light emitting units  14   a ,  14   b ,  14   c  may be an independent light source (e.g. light emitting diode) or may consist of a light guide plate and a light source. 
     When the optical touch system  1  is being used, the processing unit  16  controls the light emitting units  14   a ,  14   b ,  14   c  to emit light simultaneously. When a user uses an object (e.g. a finger or stylus) to indicate a position on the indication plane  10 , the object blocks part of light emitted by the light emitting units  14   a ,  14   b ,  14   c . Afterward, the processing unit  16  controls the two image sensing units  12   a ,  12   b  to sense images relative to the indication plane  10 . Then, the processing unit  16  determines a coordinate of the position indicated by the object or other information relative to the object according to the images sensed by the image sensing units  12   a ,  12   b.    
     If the light emitting units  14   a ,  14   b ,  14   c  emit light simultaneously when the image sensing units  12   a ,  12   b  sense the images relative to the indication plane  10 , light emitted by the light emitting units  14   a ,  14   b ,  14   c  will overlap and disturb each other. Consequently, the quality of the sensed images will be affected, the sensing accuracy will be reduced, and the electricity will be consumed much. Furthermore, if the light emitting units  14   a ,  14   b ,  14   c  emit light simultaneously and the light emitting times are the same, the illumination of some specific positions around the indication plane  10  will be so high or so low that the sensed image quality will be also affected and the sensing accuracy will be also reduced. 
     SUMMARY OF THE INVENTION 
     Therefore, an objective of the invention is to provide an object sensing system and method capable of effectively enhance sensing accuracy. 
     According to one embodiment, an object sensing system of the invention comprises an indication plane, a first image sensing unit, a plurality of light emitting units and a processing unit. The indication plane is used for an object to indicate a position. The first image sensing unit is disposed at a first corner of the indication plane. The light emitting units are disposed around the indication plane. Each of the light emitting units is corresponding to at least one of a plurality of operation times, at least one exposure time is set within each of the operation times, and each exposure time is corresponding to at least one of the light emitting units. The processing unit is electrically connected to the first image sensing unit and the light emitting units. The processing unit controls the light emitting units to emit light according to each exposure time correspondingly and controls the first image sensing unit to sense a first image relative to the indication plane within each operation time. 
     According to another embodiment, a method of the invention for controlling the aforesaid object sensing system comprises steps of: relating each of the light emitting units to be corresponding to at least one of a plurality of operation times; setting at least one exposure time within each of the operation times, wherein each exposure time is corresponding to at least one of the light emitting units; and controlling the light emitting units to emit light according to each exposure time correspondingly and controlling the first image sensing unit to sense a first image relative to the indication plane within each operation time. 
     As mentioned in the above, the object sensing system and controlling method of the invention control each of the light emitting units to emit light according to the exposure time within each operation time and control the image sensing unit to sense an image relative to the indication plane within each operation time. In other words, the invention can adjust the exposure time of each light emitting unit individually according to different positions on the indication plane and the distance between each light emitting unit and the image sensing unit, so as to provide sufficient and stable illumination for the image sensing unit and enhance the image quality. Accordingly, the sensing accuracy of the object sensing system can be effectively enhanced. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an optical touch system of the prior art. 
         FIG. 2  is a schematic diagram illustrating an object sensing system according to one embodiment of the invention. 
         FIG. 3  is a flowchart illustrating a method for controlling the object sensing system according to one embodiment of the invention. 
         FIG. 4  is sequence diagram illustrating the operation times and the exposure times according to one embodiment of the invention. 
         FIG. 5  is sequence diagram illustrating the operation times and the exposure times according to another embodiment of the invention. 
         FIG. 6  is sequence diagram illustrating the operation times and the exposure times according to another embodiment of the invention. 
         FIG. 7  is sequence diagram illustrating the operation times and the exposure times according to another embodiment of the invention. 
         FIG. 8  is sequence diagram illustrating the operation times and the exposure times according to another embodiment of the invention. 
         FIG. 9  is sequence diagram illustrating the operation times and the exposure times according to another embodiment of the invention. 
         FIG. 10  is a schematic diagram illustrating an object sensing system according to another embodiment of the invention. 
         FIG. 11  is a flowchart illustrating a method for controlling the object sensing system according to another embodiment of the invention. 
         FIG. 12  is a schematic diagram illustrating an object sensing system according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 2 ,  FIG. 2  is a schematic diagram illustrating an object sensing system  3  according to one embodiment of the invention. As shown in  FIG. 2 , the object sensing system  3  comprises an indication plane  30 , a first image sensing unit  32   a , four light emitting units  34   a ,  34   b ,  34   c ,  34   d , a processing unit  36  and a reflecting unit  38 . The indication plane  30  is used for an object to indicate a position. The first image sensing unit  32   a  is disposed at a first corner of the indication plane  30 . The light emitting units  34   a ,  34   b ,  34   c ,  34   d  are disposed around the indication plane  30 . The reflecting unit  38  is also disposed around the indication plane  30  and located at the same side with the light emitting unit  34   c .  FIG. 2  is a top view of the object sensing system  3 . In  FIG. 2 , the reflecting unit  38  and the light emitting unit  34   c  are substantially located at the same or very close position, meaning that the projection position of the reflecting unit  38  on the periphery of the indication plane  30  is substantially the same or very close to that of the light emitting unit  34   c  on the periphery of the indication plane  30 . It should be noted that if the object sensing system  3  is observed from a side view, the reflecting unit  38  can be disposed above or under the light emitting unit  34   c . The processing unit  36  is electrically connected to the first image sensing unit  32   a  and the light emitting units  34   a ,  34   b ,  34   c ,  34   d . The reflecting unit  38  can be a flat mirror, a prism mirror, or other structures capable of reflecting light. Each of the light emitting units  34   a ,  34   b ,  34   c ,  34   d  may be an independent light source (e.g. light emitting diode) or may consist of a light guide plate and a light source. It should be noted that the number and arrangement of the light emitting units are not limited to the embodiment shown in  FIG. 3  and those can be determined based on practical applications. The first image sensing units  32   a  can be a Charge-coupled Device (CCD) sensor or a Complementary Metal-Oxide Semiconductor (CMOS) sensor, or the like. The processing unit  36  can be a processor capable of calculating and processing data. 
     When the object sensing system  3  is being used, the processing unit  36  will control the light emitting units  34   a ,  34   b ,  34   c ,  34   d  to emit light individually during a predetermined polling time. When a user uses an object (e.g. a finger or stylus) to indicate a position on the indication plane  30 , the object blocks part of light emitted by the light emitting units  34   a ,  34   b ,  34   c ,  34   d . At the same time, the processing unit  36  controls the first image sensing unit  32   a  to sense a first image relative to the indication plane  30 . Then, the processing unit  36  determines a coordinate of the position indicated by the object or other information relative to the object according to the first image sensed by the first image sensing unit  32   a . In this embodiment, since there are four light emitting units  34   a ,  34   b ,  34   c ,  34   d  emitting light individually during the predetermined polling time, the first image sensing unit  32   a  will sense four first images relative to the indication plane  30  during the predetermined polling time. It should be noted that when the light emitting unit  34   a  or  34   d  emits light, the light emitted by the light emitting unit  34   a  or  34   d  can be reflected by the reflecting unit  38 , so that the first image sensing unit  32   a  can sense a reflective image relative to the indication plane  30 , wherein the aforesaid first image comprises this reflective image. Furthermore, the aforesaid predetermined polling time represents the needed time for polling the position coordinate indicated by the object every time by the processing unit  36 . For example, if the frequency for the processing unit  36  to poll the position coordinate indicated by the object is set as 125 times per second, the needed time for polling the position coordinate indicated by the object every time by the processing unit  36  is equal to eight micro-seconds (i.e. the predetermined polling time). 
     In this embodiment, the aforesaid predetermined polling time can be divided into four operation times according to the number of light emitting units, wherein each of the light emitting units  34   a ,  34   b ,  34   c ,  34   d  is corresponding to at least one of the four operation times. At least one exposure time is set within each of the operation times and each exposure time is corresponding to at least one of the light emitting units  34   a ,  34   b ,  34   c ,  34   d . The processing unit  36  controls the light emitting units  34   a ,  34   b ,  34   c ,  34   d  to emit light according to each exposure time correspondingly and controls the first image sensing unit  32   a  to sense a first image relative to the indication plane  30  within each operation time. It should be noted that when the object sensing system  3  is booting, the exposure time of each operation time can be adjusted automatically according to pixel noise, needed image quality and other factors of the first image sensing unit  32   a . The aforesaid adjustment can be implemented by software design and it will not be depicted herein. 
     Referring to  FIG. 3 ,  FIG. 3  is a flowchart illustrating a method for controlling the object sensing system  3  according to one embodiment of the invention. Please refer to  FIG. 3  along with  FIG. 2 . The controlling method of the invention comprises the following steps. First of all, step S 100  is performed to relate each of the light emitting units  34   a ,  34   b ,  34   c ,  34   d  to be corresponding to at least one of a plurality of operation times. Afterward, step S 102  is performed to set at least one exposure time within each of the operation times, wherein each exposure time is corresponding to at least one of the light emitting units  34   a ,  34   b ,  34   c ,  34   d . Finally, step S 104  is performed to control the light emitting units  34   a ,  34   b ,  34   c ,  34   d  to emit light according to each exposure time correspondingly and control the first image sensing unit  32   a  to sense a first image relative to the indication plane  30  within each operation time. 
     Referring to  FIG. 4 ,  FIG. 4  is sequence diagram illustrating the operation times and the exposure times according to one embodiment of the invention. As shown in  FIG. 4 , the predetermined polling time is set as t 0 -t 8 . In this embodiment, the predetermined polling time t 0 -t 8  is divided into four operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  averagely according to the number of the light emitting units  34   a ,  34   b ,  34   c ,  34   d , and four exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  are set within the four operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  respectively. The four exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  are shorter than the four operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  respectively. In this embodiment, all of the operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  are equal to each other and do not overlap each other, and all of the exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  are equal to each other. In this embodiment, the processing unit  36  controls each of the light emitting units  34   a ,  34   b ,  34   c ,  34   d  to emit light according to each of the exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  correspondingly and controls the first image sensing unit  32   a  to sense a first image relative to the indication plane  30  within each of the operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8 . 
     Referring to  FIG. 5 ,  FIG. 5  is sequence diagram illustrating the operation times and the exposure times according to another embodiment of the invention. As shown in  FIG. 5 , the predetermined polling time is set as t 0 -t 8 . In this embodiment, the predetermined polling time t 0 -t 8  is divided into four operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  averagely according to the number of the light emitting units  34   a ,  34   b ,  34   c ,  34   d , and four exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  are set within the four operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  respectively. The four exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  are shorter than the four operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  respectively. In this embodiment, all of the operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  are equal to each other and do not overlap each other, and at least one of the exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  is unequal to other exposure times. As shown in  FIG. 5 , the exposure time t 2 -t 3  is equal to the exposure time t 6 -t 7  and is unequal to other exposure times t 0 -t 1 , t 4 -t 5 . In this embodiment, the processing unit  36  controls each of the light emitting units  34   a ,  34   b ,  34   c ,  34   d  to emit light according to each of the exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  correspondingly and controls the first image sensing unit  32   a  to sense a first image relative to the indication plane  30  within each of the operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8 . 
     Referring to  FIG. 6 ,  FIG. 6  is sequence diagram illustrating the operation times and the exposure times according to another embodiment of the invention. As shown in  FIG. 6 , the predetermined polling time is set as t 0 -t 4 . In this embodiment, the predetermined polling time t 0 -t 4  is divided into four operation times t 0 -t 1 , t 1 -t 2 , t 2 -t 3 , t 3 -t 4  according to the number of the light emitting units  34   a ,  34   b ,  34   c ,  34   d , and four exposure times t 0 -t 1 , t 1 -t 2 , t 2 -t 3 , t 3 -t 4  are set within the four operation times t 0 -t 1 , t 1 -t 2 , t 2 -t 3 , t 3 -t 4  respectively. In other words, the four exposure times t 0 -t 1 , t 1 -t 2 , t 2 -t 3 , t 3 -t 4  are equal to the four operation times t 0 -t 1 , t 1 -t 2 , t 2 -t 3 , t 3 -t 4  respectively. In this embodiment, all of the operation times t 0 -t 1 , t 1 -t 2 , t 2 -t 3 , t 3 -t 4  do not overlap each other, at least one of the operation times t 0 -t 1 , t 1 -t 2 , t 2 -t 3 , t 3 -t 4  is unequal to other operation times, and at least one of the exposure times t 0 -t 1 , t 1 -t 2 , t 2 -t 3 , t 3 -t 4  is unequal to other exposure times. As shown in  FIG. 6 , the operation time t 0 -t 1  is equal to the operation time t 3 -t 4  and is unequal to other operation times t 1 -t 2 , t 2 -t 3 , and the exposure time t 0 -t 1  is equal to the exposure time t 3 -t 4  and is unequal to other exposure times t 1 -t 2 , t 2 -t 3 . In this embodiment, the processing unit  36  controls each of the light emitting units  34   a ,  34   b ,  34   c ,  34   d  to emit light according to each of the exposure times t 0 -t 1 , t 1 -t 2 , t 2 -t 3 , t 3 -t 4  correspondingly and controls the first image sensing unit  32   a  to sense a first image relative to the indication plane  30  within each of the operation times t 0 -t 1 , t 1 -t 2 , t 2 -t 3 , t 3 -t 4 . 
     Referring to  FIG. 7 ,  FIG. 7  is sequence diagram illustrating the operation times and the exposure times according to another embodiment of the invention. As shown in  FIG. 7 , the predetermined polling time is set as t 0 -t 8 . In this embodiment, the predetermined polling time t 0 -t 8  is divided into four operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  according to the number of the light emitting units  34   a ,  34   b ,  34   c ,  34   d , and four exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  are set within the four operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  respectively. The four exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  are shorter than the four operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  respectively. In this embodiment, all of the operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  do not overlap each other, at least one of the operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8  is unequal to other operation times, and at least one of the exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  is unequal to other exposure times. As shown in  FIG. 7 , the operation time t 0 -t 2  is equal to the operation time t 6 -t 8  and is unequal to other operation times t 2 -t 4 , t 4 -t 6 , and the exposure time t 0 -t 1  is equal to the exposure time t 6 -t 7  and is unequal to other exposure times t 2 -t 3 , t 4 -t 5 . In this embodiment, the processing unit  36  controls each of the light emitting units  34   a ,  34   b ,  34   c ,  34   d  to emit light according to each of the exposure times t 0 -t 1 , t 2 -t 3 , t 4 -t 5 , t 6 -t 7  correspondingly and controls the first image sensing unit  32   a  to sense a first image relative to the indication plane  30  within each of the operation times t 0 -t 2 , t 2 -t 4 , t 4 -t 6 , t 6 -t 8 . 
     Referring to  FIG. 8 ,  FIG. 8  is sequence diagram illustrating the operation times and the exposure times according to another embodiment of the invention. As shown in  FIG. 8 , the predetermined polling time is set as t 0 -t 7 . In this embodiment, the predetermined polling time t 0 -t 7  is divided into four operation times t 0 -t 2 , t 1 -t 3 , t 3 -t 5 , t 5 -t 7  according to the number of the light emitting units  34   a ,  34   b ,  34   c ,  34   d , and four exposure times t 0 -t 2 , t 1 -t 3 , t 3 -t 4 , t 5 -t 6  are set within the four operation times t 0 -t 2 , t 1 -t 3 , t 3 -t 5 , t 5 -t 7  respectively. The exposure times t 0 -t 2 , t 1 -t 3  are equal to the operation times t 0 -t 2 , t 1 -t 3  respectively, and the exposure times t 3 -t 4 , t 5 -t 6  are shorter than the operation times t 3 -t 5 , t 5 -t 7  respectively. In this embodiment, at least two of the operation times t 0 -t 2 , t 1 -t 3 , t 3 -t 5 , t 5 -t 7  at least partially overlap each other. As shown in  FIG. 8 , the operation times t 0 -t 2 , t 1 -t 3  partially overlap each other and the overlapping portion is t 1 -t 2 . In this embodiment, the processing unit  36  controls each of the light emitting units  34   a ,  34   b ,  34   c ,  34   d  to emit light according to each of the exposure times t 0 -t 2 , t 1 -t 3 , t 3 -t 4 , t 5 -t 6  correspondingly and controls the first image sensing unit  32   a  to sense a first image relative to the indication plane  30  within each of the operation times t 0 -t 2 , t 1 -t 3 , t 3 -t 5 , t 5 -t 7 . 
     In other words, according to pixel noise, needed image quality and other factors of the first image sensing unit  32   a , if the illumination generated by the light emitting units  34   a ,  34   b  must be maximum, the operation times of the light emitting units  34   a ,  34   b  can be set to at least partially overlap each other, as shown in  FIG. 8 . Accordingly, the exposure times of the light emitting units  34   a ,  34   b  can be extended within the predetermined polling time so as to satisfy the illumination requirement. 
     Referring to  FIG. 9 ,  FIG. 9  is sequence diagram illustrating the operation times and the exposure times according to another embodiment of the invention. As shown in  FIG. 9 , the predetermined polling time is set as t 0 -t 7 . In this embodiment, the predetermined polling time t 0 -t 7  is divided into three operation times t 0 -t 3 , t 3 -t 5 , t 5 -t 7 . Two exposure times t 0 -t 2 , t 0 -t 1  are set within the operation time t 0 -t 3 , and two exposure times t 3 -t 4 , t 5 -t 6  are set within the operation times t 3 -t 5 , t 5 -t 7  respectively. The exposure time t 0 -t 2 , t 0 -t 1 , t 3 -t 4 , t 5 -t 6  are shorter than the operation times t 0 -t 3 , t 3 -t 5 , t 5 -t 7  respectively. In this embodiment, the exposure times t 0 -t 2 , t 0 -t 1  within the operation time t 0 -t 3  at least partially overlap each other and are corresponding to different light emitting units  34   a ,  34   b  respectively, wherein the overlapping portion is t 0 -t 1 , as shown in  FIG. 9 . In this embodiment, the processing unit  36  controls each of the light emitting units  34   a ,  34   b ,  34   c ,  34   d  to emit light according to each exposure time t 0 -t 2 , t 0 -t 1 , t 3 -t 4 , t 5 -t 6  correspondingly and controls the first image sensing unit  32   a  to sense a first image relative to the indication plane  30  within each operation time t 0 -t 3 , t 3 -t 5 , t 5 -t 7 . 
     Referring to  FIG. 10 ,  FIG. 10  is a schematic diagram illustrating an object sensing system  3 ′ according to another embodiment of the invention. As shown in  FIG. 10 , the main difference between the object sensing system  3 ′ and the aforesaid object sensing system  3  is that the object sensing system  3 ′ further comprises a second image sensing unit  32   b  electrically connected to the processing unit  36 . The second image sensing unit  32   b  is disposed at a second corner of the indication plane  30 , wherein the second corner is adjacent to the aforesaid first corner. In other words, the first and second image sensing units  32   a ,  32   b  are disposed at opposite corners of the indication plane  30 . Furthermore, since the object sensing system  3 ′ is not equipped with the reflecting unit  38  shown in  FIG. 2 , the light emitting unit  34   a  shown in  FIG. 2  can be removed accordingly. That is to say, the invention can be implemented in any object sensing system no matter the reflecting unit  38  shown in  FIG. 2  is disposed therein or not. It should be noted that the components with identical labels in  FIGS. 10 and 2  work substantially in the same way, so they will not be depicted herein again. 
     When the object sensing system  3 ′ is being used, the processing unit  36  will control the light emitting units  34   b ,  34   c ,  34   d  to emit light individually during a predetermined polling time. When a user uses an object (e.g. a finger or stylus) to indicate a position on the indication plane  30 , the object blocks part of light emitted by the light emitting units  34   b ,  34   c ,  34   d . At the same time, the processing unit  36  controls the first image sensing unit  32   a  to sense a first image relative to the indication plane  30  and controls the second image sensing unit  32   b  to sense a second image relative to the indication plane  30 . Then, the processing unit  36  determines a coordinate of the position indicated by the object or other information relative to the object according to the first image sensed by the first image sensing unit  32   a  and/or the second image sensed by the second image sensing unit  32   b . In this embodiment, since there are three light emitting units  34   b ,  34   c ,  34   d  emitting light individually during the predetermined polling time, the first image sensing unit  32   a  and the second image sensing unit  32   b  will sense three first images and three second images relative to the indication plane  30  respectively during the predetermined polling time. 
     It should be noted that since the object sensing system  3 ′ comprises only three light emitting units  34   b ,  34   c ,  34   d , the aforesaid determined polling time in associated with  FIGS. 4 to 9  can be divided into three operation times according to the number of the light emitting units  34   b ,  34   c ,  34   d . Also, at least one exposure time can be set within each operation time appropriately in similar manner mentioned in the above. The division of the operation times and the setting of the exposure times are substantially the same as the description in associated with  FIGS. 4 to 9  and they will not be depicted herein again. 
     Referring to  FIG. 11 ,  FIG. 11  is a flowchart illustrating a method for controlling the object sensing system  3 ′ according to another embodiment of the invention. Please refer to  FIG. 11  along with  FIG. 10 . The controlling method of the invention comprises the following steps. First of all, step S 200  is performed to relate each of the light emitting units  34   b ,  34   c ,  34   d  to be corresponding to at least one of a plurality of operation times. Afterward, step S 202  is performed to set at least one exposure time within each of the operation times, wherein each exposure time is corresponding to at least one of the light emitting units  34   b ,  34   c ,  34   d . Finally, step S 204  is performed to control the light emitting units  34   b ,  34   c ,  34   d  to emit light according to each exposure time correspondingly, control the first image sensing unit  32   a  to sense a first image relative to the indication plane  30  within each operation time, and control the second image sensing unit  32   b  to sense a second image relative to the indication plane  30  within each operation time. 
     Referring to  FIG. 12 ,  FIG. 12  is a schematic diagram illustrating an object sensing system  3 ″ according to another embodiment of the invention. As shown in  FIG. 12 , the main difference between the object sensing system  3 ″ and the aforesaid object sensing system  3 ′ is that there are two light emitting units  34   a ,  34   b  disposed between the first image sensing unit  32   a  and the second image sensing unit  32   b  of the object sensing system  3 ″. Furthermore, the object sensing system  3 ″ further comprises a reflecting unit  38  disposed around the indication plane  30  and located at the same side with the light emitting unit  34   c . Similar to  FIG. 2 ,  FIG. 12  is a top view of the object sensing system  3 ″. In  FIG. 12 , the reflecting unit  38  and the light emitting unit  34   c  are substantially located at the same or very close position, meaning that the projection position of the reflecting unit  38  on the periphery of the indication plane  30  is substantially the same or very close to that of the light emitting unit  34   c  on the periphery of the indication plane  30 . It should be noted that if the object sensing system  3 ′ is observed from a side view, the reflecting unit  38  can be disposed above or under the light emitting unit  34   c . The reflecting unit  38  can be a flat mirror, a prism mirror, or other structures capable of reflecting light. When the light emitting unit  34   a ,  34   b  or  34   d  emits light, the light emitted by the light emitting unit  34   a ,  34   b  or  34   d  can be reflected by the reflecting unit  38 , so that the first image sensing unit  32   a  can sense a reflective image relative to the indication plane  30 . It should be noted that the components with identical labels in  FIGs. 12 and 10  work substantially in the same way, so they will not be depicted herein again. 
     According to pixel noise, needed image quality and other factors of the first image sensing unit  32   a  and the second image sensing unit  32   b , if the illumination generated by the light emitting units  34   a ,  34   b  must be maximum when the object sensing system  3 ″ is being used, the operation times of the light emitting units  34   a ,  34   b  can be set to at least partially overlap each other, as shown in  FIG. 8 . Accordingly, the exposure times of the light emitting units  34   a ,  34   b  can be extended within the predetermined polling time so as to satisfy the illumination requirement. 
     As mentioned in the above, the object sensing system and controlling method of the invention control each of the light emitting units to emit light according to the exposure time within each operation time and control the image sensing unit to sense an image relative to the indication plane within each operation time. In other words, the invention can adjust the exposure time of each light emitting unit individually according to different positions on the indication plane and the distance between each light emitting unit and the image sensing unit, so as to provide sufficient and stable illumination for the image sensing unit and enhance the image quality. Furthermore, according to pixel noise, needed image quality and other factors of the image sensing unit, the invention can selectively make the operation times and/or exposure times at least partially overlap or not overlap each other and selectively make the operation times and/or exposure times be equal or unequal to each other, so as to satisfy different requirements of illumination and polling time. Accordingly, the sensing accuracy of the object sensing system can be effectively enhanced. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.