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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an electric curtain, particular to an electric curtain via accurately controlling a stop position of its covering sheet. 
         [0003]    2. Description of the Related Art 
         [0004]    Referring to  FIG. 1 , a conventional electric curtain  1  comprises a rail  10 , a rolling shaft  11  pivoted on the rail  10 , a power  12  applied to motivate the rolling shaft  11 , and a controlling device  13  applied to control the power  12 . Wherein, the rolling shaft  11  includes a covering sheet  111  rolled thereon. Moreover, the controlling device  13  includes a turning member  131  synchronizing with the rolling shaft  11 , a sensor  132  applied to correspondingly sense the rotation of the turning member  131 , a processing unit  133  connected to the sensor  132  to control the power  12 , a counter  134  connected to the sensor  132 , and a memory unit  135  connected to the processing unit  133 . 
         [0005]    While in manipulation of the electric curtain  1 , users need to set a lower limit position of the covering sheet  111 . Namely, when the covering sheet  111  initially descends to the lowest position, the turning member  131  is concurrently triggered to rotate and generate a revolution, whose revolution number is counted and converted into a data by the counter  134 . Further, the memory unit  135  saves the data as a standard value, and the raising and descending of the covering sheet  111  would depend on comparing an actual amount counted by the counter  134  with respect to the standard value. Once the number of revolution counted by the counter  134  equals the standard value saved in the memory unit  135 , the processing unit  133  would keep the power  12  from running. 
         [0006]    After setting up, users draw down the covering sheet  111  again by the power  12  driving the rolling shaft  11 , and then the turning member  131  is synchronically rotated for permitting the sensor  132  capable of detecting the rotation thereof and rendering the counter  134  to count the rotation number. Hence, when the number counted by the counter  134  equals to the standard value saved in the memory unit  135 , the processing unit  133  prevents the power  12  from running to impede the dropping of the covering sheet  111 . Whereas, when the covering sheet  111  is retracted upward, the processing unit  133  would trigger the power  12  to rotate the rolling shaft  11  until the revolution number counted by the counter  134  equals to the standard value saved in the memory unit  135 . Therefore, the covering sheet  111  would be retracted to the upper limit position base on above preset standard data. 
         [0007]    Obviously, the conventional operating rationale mainly depends on the counting value executed by the counter  134  relative to the standard data saved in the memory unit  135  for lifting and dropping the covering sheet  111 , so that the processing unit  133  accordingly determines the timing for stopping the power  12 . However, an accumulative imprecision of the counter  134  may be probably generated since a mal-sensing may affect the function of the sensor  132 , or a facile transmission inertia between the power  12  and the rolling shaft  11  may incur an inaccurate motion and result errors produced therebetween. Thus, the actual rotating quantity of the turning member  131  would inevitably deviate from the number counted by the counter  134  in a long term of using to cause the electric curtain  1  unable to precisely furl and drop the covering sheet  111  to the desired upper and lower limit positions. 
         [0008]    If an electricity outage happens unpredictably, the counter  134  forcedly interrupts the counting and has to restart its counting while resuming the power supply. In such situation, the counter  134  usually confuses the revolution number of furling the covering sheet  111  with the revolution number of lowering the covering sheet  111  and makes an inaccurately counting and even a shutdown of the controlling device  13 . Thus, the conventional electric curtain  1  needs to reset the setting, which causes an inconvenient using and requires improvements. 
       SUMMARY OF THE INVENTION 
       [0009]    Therefore, the object of the present invention is to provide an electric curtain that assists in accurately suspending the covering sheet at a preferable position in each operation and increasing a convenience of using. 
         [0010]    The electric curtain in accordance with the present invention mainly provides with a rail, a rolling shaft pivoted on the rail, a power located relative to the rail for driving the rotation of the rolling shaft, a zero device disposed at one side of the power, and a controlling device connected to the zero device and the power. Wherein, the zero device has a turning member concurrently rotated with the rolling shaft, an actuating member disposed correspondingly to the turning member, a sensor correspondingly positioned to the actuating member, and a counter connected to the sensor. Additionally, the controlling device includes a receiver connected to the sensor, a processing unit respectively connected to the receiver and the power, and a memory member connected to the processing unit. When a covering sheet is rolled by the rolling shaft to stop at its upper limit position to trigger the actuating member every time, the sensor would generate a signal and send it to the processing unit, for which to control the counter to resume its counting to zero so as to recount the succeeding descending operation base on a standard value stored in the memory member. Whereby, an accumulative inaccuracy of the counting in a long term of using can be avoided for efficiently stopping the covering sheet at a desired position without further adjustments while using in a long period and increasing a using convenience. 
         [0011]    The advantages of the present invention would be apparent in following embodiments with drawings, and like elements are denoted by same numerals throughout the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view showing a conventional invention; 
           [0013]      FIG. 2  is a perspective view showing a first preferred embodiment of the present invention; 
           [0014]      FIG. 3  is an enlarged view showing partial components of the first embodiment; 
           [0015]      FIG. 4  is a schematic view showing the first preferred embodiment while setting and descending; 
           [0016]      FIG. 5  is a flow diagram showing the dropping of the first embodiment; 
           [0017]      FIG. 6  is another schematic view showing the first preferred embodiment while furling; 
           [0018]      FIG. 7  is a flow diagram showing the furling of the first embodiment; 
           [0019]      FIG. 8  is a perspective view showing a second preferred embodiment of the present invention; and 
           [0020]      FIG. 9  is an enlarged view showing partial components of a third preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Before in greater detail, it is noted that the like elements are denoted by the similar reference numerals throughout the disclosure. 
         [0022]    Referring to  FIGS. 2 and 3  showing a first preferred embodiment of an electric curtain  3  comprises a rail  30 , a rolling shaft  31  pivoted on the rail  30 , a power  32  located on the rail  30  for driving the rotation of the rolling shaft  31 , a zero device  33  disposed at one side of the power  32 , and a controlling device  34  connected to the zero device  33  and the power  32 . Wherein, the rolling shaft  31  includes a covering sheet  311  swiveled thereby, and the covering sheet  311  is restricted to stop extremely at an upper limit position a and a lower limit position b. Further, the zero device  33  includes a turning member  331  concurrently rotated with the rolling shaft  31 , an actuating member  333  preferably installed on the turning member  331 , a sensor  334  positioned correspondingly to the actuating member  333 , and a counter  335  connected to the sensor  334 . Additionally, the controlling device  34  includes a receiver  341  connected to the sensor  334 , a processing unit  342  respectively connected to the receiver  341  and the power  32 , and a memory unit  343  connected to the processing unit  342 . 
         [0023]    Wherein, the power  32  could be triggered by a starter  321  which is directed to a wireless remote control or directed to a wired switch used by pressing. Herein, the starter  321  in the illustration adopts a wireless remote control. Moreover, to promote a preferable matching position of the actuating member  333  with respect to the sensor  334 , the actuating member  333  and the sensor  334  are coaxially disposed on an axle as illustrated. 
         [0024]    Successively, when the covering sheet  311  is rolled to stop at its upper limit position a, the actuating member  333  on the turning member  331  would concurrently rotate relatively to the sensor  334 , from which a signal is emitted for the receiver  341  of the controlling device  34  to receive. Accordingly, when the processing unit  342  receives the signal transmitted by the sensor  334 , the processing unit  342  would properly stop the operation of the power  32  and control the counting of the counter  335  turning to zero. Thus, for initially dropping or descending the covering sheet  311 , the counter  335  would preset its counting. 
         [0025]    Still referring to  FIGS. 2 and 3 , before a substantial operation, users need to set a proper lower limit position b of the covering sheet  311 . Wherein, the lower limit position b is directed where the covering sheet  311  is dropped to its extreme lowest position. Thereby, when the covering sheet  311  is dropped from the upper limit position a toward its lower limit position b, the turning member  331  would be concurrently rotated with the rolling shaft  31  to motivate the actuating member  333  to deviate from the sensor  334  as shown in  FIG. 4 . Accordingly, the counter  335  would not only synchronically count the rotating quantity of the actuating member  333  on the turning member  331  detected via the sensor  334  but also concurrently convert the quantity into a data. Whereby, the data would be saved in the memory unit  343  as a standard data basis for the next dropping of the covering sheet  311 . 
         [0026]    Referring to  FIGS. 3 ,  4 , and  5 , when users desire to drop the electric curtain  3  again after setting up, they can directly press the starter  321  to trigger the power  32  so as to motivate the rolling shaft  31  for rotably synchronizing the covering sheet  311  with the turning member  331 . Thence, the actuating member  333  on the turning member  331  would rotate correspondingly to the sensor  334  as arrowed in  FIG. 4 . When an actual rotating quantity of the actuating member  333  counted by the counter  335  equals the standard data basis saved in the memory unit  343 , the processing unit  342  accordingly controls the power  32  to stop dropping the covering sheet  311 . 
         [0027]    Reciprocally, referring to  FIGS. 2 ,  3 , and  6 , in furling the covering sheet  311 , the power  32  drives the rolling shaft  31  to gradually move the actuating member  333  on the turning member  331  close to the sensor  334  until the actuating member  333  and the sensor  334  are aligned as shown in  FIG. 6 . Further referring to  FIG. 7 , the sensor  334  would detect the rotation of the actuating member  333  on the turning member  331  to transmit a signal to the receiver  341  to promote the processing unit  342  for stopping the power  32  from running and simultaneously zeroing the counting of the counter  335 . Therefore, the counter  335  would resume its counting every time when the covering sheet  311  is drawn downward, thereby avoiding the mechanical inertia and cumulative errors that causes inexact downward and upward motions of the covering sheet  311  after using for a long period. Whereby, the present invention preferably obtains a precise controlling of the stop position of the covering sheet without frequent adjustments or settings of the covering sheet  311  in a long term of using, thus increasing a using convenience. 
         [0028]    Even an electricity outage occurs during the operation of the electric curtain  3 , a furling of the covering sheet  311  would be firstly proceeded after the power supply is resumed so as to allow the actuating member  333  moving relative to the sensor  334 . Namely, the rolling shaft  31  would furl the covering sheet  311  back to the upper limit position a, and the processing unit  342  would retrieve the counter  335  by zeroing while aligning the actuating member  333  with the sensor  334 , so that the electric curtain  3  appropriately restarts operating and efficiently conduces to a using convenience. 
         [0029]    Referring to  FIG. 8 , a second preferred embodiment of the present invention comprises elements, operations and functions similar to the first preferred embodiment, in which the electric curtain  3  still comprises a rail  30 , a rolling shaft  31 , a power  32 , a zero device  33 , and a controlling device  34 . Differentially, an adjusting member  336  is disposed at one side of the zero device  33  for preferably adjusting the position of the sensor  334 . Further, a governor  337  can be alternatively installed between the adjusting member  336  and the zero device  33  for controlling the output power of the adjusting member  336 . Accordingly, by means of the additive adjustment member  336  cooperating with the governor  337 , the position of the sensor  334  can be more delicately amended to collaborate with the actuating member  333  for achieving the merits of precisely controlling the covering sheet at the desired position, obtaining an efficient speedy adjustment, and attaining a convenient operation. 
         [0030]    Referring to  FIG. 9 , a third preferred embodiment of the present invention comprises elements, operations and functions similar to the first preferred embodiment, in which the electric curtain  3  still comprises a rail  30 , a rolling shaft  31 , a power  32 , a zero device  33 , and a controlling device  34 . Specifically, the arrangement of the interrelated elements in the zero device  33  can be either performed in the configuration like the first and second preferred embodiments or performed in the configuration like this embodiment. Namely, the actuating member  333  of the zero device  33  in this embodiment is set as a discrete part to separate from the turning member  331  on which a plurality of apertures  332  are equidistantly defined, and the sensor  334  is still correspondingly situated with respect to the turning member  331 . Accordingly, when the turning member  331  and the rolling shaft  31  are concurrently rotated, the sensor  334  would be able to sense the rotation of the turning member  331 , and the actuating member  333  would be triggered by the furling of the covering sheet  311  stopping at the upper limit position a. 
         [0031]    Further in setting, the covering sheet  311  is firstly dropped to the lower limit position b for the sensor  334  to simultaneously sense the relative rotation of the turning member  331 , enhancing the counter  335  to convert the rotation into a data for the memory unit  343  to save as a standard data basis serving for the succeeding dropping basis of the covering sheet  311 . In the succeeding operation for dropping the covering sheet  311 , the processing unit  342  would thence impede the power  32  from operating and stop the covering sheet  311  at a proper position when the rotating number of the turning member  331  counted by the counter  335  equals to the standard data basis saved in the memory unit  343 . In contrast, when the covering sheet  311  is retracted or furled to the upper limit position a, the covering sheet  311  propels the actuating member  333  for the sensor  334  to directly emit a signal; whereby, the receiver  341  receives the signal from the sensor  334 , and the processing unit  342  accordingly impedes the driving of the power  32  and concomitantly resumes the counting of the counter  335  to zero. Therefore, the counter  335  recounts when every time the covering sheet  311  is dropped. 
         [0032]    To sum up, the present invention takes advantages of the actuating member of the zero device cooperating with the correlated sensor. Thereby, the covering sheet furled to the upper limit position would force the sensor to emit a signal to the controlling device for stopping the power and returning the counting of the counter to zero, which benefits a newly counting in pursuance of a next descent of the covering sheet. Thus, the present invention decreases an accumulative error resulting from inaccurate movements of the covering sheet after using for a long time for ensuring a precise suspension of the covering sheet and promoting an operative convenience. 
         [0033]    Various modifications may be made in further embodiments described without departing from the spirit and scope of the invention.

Summary:
An electric curtain mainly utilizes a covering sheet furled at an upper limit position, which simultaneously impels an actuating member of a zero device to relatively cooperate with a sensor of the zero device, so that the covering sheet would not continue operating and a counter in the zero device would recount once a processing unit of a controlling device receives a signal from the sensor. Thus, the processing unit would drive the counter to recount in every following drop of the covering sheet to accurately control the suspending position without incurring an accumulative counting error and increase an operative convenience.