MECHANICAL MOVEMENT ASSEMBLY FOR CANVAS BACKGROUND OF INVENTION 5 1. Field of the Invention The present invention relates to a mechanically moveable assembly, particularly suited for use on trucks or truck trailers, which can control a canvas used to enclose the payload, so that the canvas may be either extended or to be retracted along the frame to which the canvas is 10 connected, so as to obtain better convenience and easiness in operation and implementation, and to increase its practical efficacy in its overall implementation. 2. Brief Description of Prior Art 15 Generally, in order that the commodity loaded on a truck can be shaded so as to prevent the commodity from damage due to exposure to wind, sun and rain, a canopy frame is usually provided on the overhead of the truck and a canvas is thereafter attached to the canopy frame. Thus, the canvas not only provides required shading to prevent the 20 commodity from exposure to wind, sun and rain, but also to avoid the commodity from stealing. With respect to the canvas attached to the canopy frame, the canvas 1 in early stage is tied directly on the canopy frame, and it is unable to disassemble or retract the canvas as desired, so this may cause extremely inconvenience in use. Hence, associated industry has proposed a manual canvas retracting device, whereby the canvas can be either 5 retracted or put down. When the commodity needs canvas for shading, the canvas is put down by the manual canvas retracting device; when the canvas is to be out of use, it is retracted upward by the same. Above manual canvas retracting device can achieve the 10 predetermined function of retracting or putting down the canvas, however it is found in practical implementation that manual effort is the only choice used to control either retraction or put-down of the canvas by this manual canvas retracting device, and this may cause great inconvenience is use. 15 In view of the above defect, the inventor of the present invention proposes a novel curtain type movable canvas box structure according to the research and improvement conducted on the conventional product and based on his abundant experiences in both R&D and manufacturing 20 activities in related field, so as to achieve the purpose of better implementation value. SUMMARY OF THE INVENTION 2 The main object of the present invention is to provide a mechanically moveable assembly that can be used on existing truck canopy frames for controlling lateral movements of the canvas canopy, 5 wherein the lateral movements of the canvas canopy is controlled by a movement control unit, and the movement control unit has a push-up mechanism to push a movable seat to move so as to allow a gear to mesh with a rack, and controls a rotation mechanism to drive the gear to roll on the rack so as to control the canvas either to be stretched or to be 10 collapsed on the rail. Therefore, the present invention is more convenient and easier in operation and implementation so that practical efficacy can be increased in its overall implementation. The main object and efficacy of the mechanically moveable 15 assembly for a canvas in the present invention can be achieved by the following specific technical means. The mechanically moveable assembly for a canvas in the present invention comprises a rail and a movement control unit. 20 3 The rail has a sliding channel formed therein for accommodating pulleys of a plurality of sliding units. Each of the sliding units has a combining portion for attachment to a canvas. The rail further has a rack formed thereon corresponding to the sliding channel. 5 The movement control unit is provided with a base plate for connection with the combining portion of the sliding unit fitted inside the sliding channel of the rail, a push-up mechanism securely fitted on the base plate, and a movable seat movably fitted on the base plate 10 opposite to the push-up mechanism. In such a case, the push-up mechanism can push the movable seat to move upward or downward on the base plate. Further, a rotation mechanism is provided on the movable seat, and a gear corresponding to the rack of the rail is provided on the movable seat. When the push-up mechanism pushes the 15 movable seat to move upward on the base plate, the gear meshes with the rack of the rail so as further to be driven by the rotation mechanism for rotation purposes. BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS 20 The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred 4 embodiments and the accompanying drawings, wherein Figure 1 is a perspective exploded schematic view of the assembly of the present invention; Figure 2 is a perspective schematic view of the assembly of the 5 present invention in an assembled state; Figure 3 is a front view of the assembly of the present invention in an assembled state; Figure 4 is a front sectional view of the assembled structure of the present invention; 10 Figure 5 is a side sectional view of the assembled structure of the present invention; Figure 6 is a front view of the assembly of the present invention in a use state; Figure 7 is a front sectional view of the assembly of the present 15 invention in a use state; Figure 8 is a side sectional view of the assembly of the present invention in a use state; and Figure 9 is a front view showing the assembly of the present invention in a collapsed state. 20 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS As referring to Figures 1 and 2, the present invention comprises a 5 rail (1) and a movement control unit (2). The rail (1) has a sliding channel (11) formed therein for accommodating pulleys (121) of a plurality of sliding units (12). Each of the sliding units (12) has a combining portion (122) for attachment to 5 a canvas (3). The rail (1) further has a rack (13) formed thereon corresponding to the sliding channel (11). The movement control unit (2) is provided with a base plate (21) for connection with the combining portion (122) of the sliding unit (12) 10 fitted inside the sliding channel (11) of the rail (1), a push-up mechanism (22) securely fitted on the base plate, and a movable seat (23) movably fitted on the base plate (21) opposite to the push-up mechanism (22). In such a case, the push-up mechanism (22) can push the movable seat (23) to move upward or downward on the base plate (21). Further, a 15 rotation mechanism (24) is provided on the movable seat (23), and a gear (25) corresponding to the rack (13) of the rail (1) is provided on the movable seat (23). When the push-up mechanism (22) pushes the movable seat (23) to move upward on the base plate (21), the gear (25) meshes with the rack (13) of the rail (1) so as further to be driven by the 20 rotation mechanism (24) for rotation. As further referring to Figures 3 to 5, the gear (25) is unable to mesh with the rack (13) of the rail (1) and not driven by the rotation 6 power unit (24) to move on the rack (13) when the push-up mechanism (22) of the movement control unit (2) is not actuated. As referring to Figures 6 to 8, the push-up mechanism (22) of the movement control unit (2) is actuated, so that the push-up mechanism (22) pushes the movable 5 seat (23) upward to the rack (13) of the rail (1) to have the gear (25) finally mesh with the rack (13) and then allow the rotation power unit (24) to drive the gear (25) for rotation. The gear (25) enables to roll on the rack (13) as meshing with the rack (13), meanwhile, the pulley (121) of the sliding unit (12) can be driven to move within the sliding channel 10 (11) of the rail (1) as well due to the base plate (21) of the movement control unit (2) fixed with the combining portions (122) of the sliding unit (12). In such a manner, the canvas (3) fastened on the combining portion (122) of the sliding unit (12) can be controlled for stretching or collapsing purposes when pulled or the sliding unit (12) being pushed by 15 aforesaid components, as shown in Figure 9. Based on foregoing, it is apparent from the description of the structural constitution and the implementation of the present invention, that the present invention mainly has a movement control unit, a push-up 20 mechanism of the movement control unit being actuated to drive the movable seat so that the gear meshes with the rack, and the rotation power unit being controlled to drive the gear to roll on the rack so as to control the canvas either to be stretched or to be collapsed on the rail. 7 Comparing with conventional art, the present invention is more convenient and easier in operation and implementation so that practical efficacy can be increased in its overall implementation. 5 Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive 10 concept as defined by the appended claims and their equivalents. 8