FIXED PULLEY STRUCTURE FIELD OF THE INVENTION The present invention relates to a fixed pulley structure 5 which has a falling protection. BACKGROUND OF THE INVENTION With reference to FIGS. 1 and 2, a conventional fixed pulley structure contains a base 10, a fixed wheel 11, a braking 10 device 12, and a driving device 13, wherein a rope 14 is rolled onto the fixed wheel 11 from a stopping piece 100 of the base 10. A retaining block 120 of the braking device 12 is pushed by a spring 121 to match with the stopping piece 100 so as to clamp the rope 12. When the rope 14 is pulled downwardly, it drives the retaining 15 block 120 to overcome the spring 121 and to swing downwardly and to drive the fixed wheel 11 to rotate forward. A first swing projection 130 of the driving device 13 is pushed by the first spring 131 to swing an angle and to retain between a post 133 of the pulling piece 132 and a groove 110 of the fixed wheel 11, such that 20 when the fixed wheel 11 rotates forward, the first swing projection 130 pushes the post 133 and drives the pulling piece 132 to rotate. As shown in FIG. 3, the pulling piece 132 has a protrusion 134, when the protrusion 134 rotates downwardly, it pushes an actuation member 135 to overcome the second spring 136 to swing 25 downwardly. Thereafter, the actuation member 135 pushes the 1 retaining block 120 to swing downward so as to release the rope 14, thus moving the object downward. Thereafter, when the rope 14 is pulled downward to drive the protrusion 134 to move away from the actuation member 135, the actuation member 135 is pushed by 5 the second spring 136 to move back to an original position, and the retaining block 120 is pushed by the spring 121 to move back to an original position, such that the retaining block 120 matches with the stopping piece 100 to clamp the rope 12 (as illustrated in FIG. 2). But such an operation can only lift and position the object, so the 10 object cannot be moved downward. Referring further to FIG. 4, when the rope 14 is released, it drives the fixed wheel 11 to rotate adversely. When an adverse rotating speed of the fixed wheel 11 is high to generate a centrifugal force by which a second swing block 137 overcomes an elasticity of 15 a third spring 138, and the second swing block 137 swings outwardly so as to be retained between the post 133 of the pulling piece 132 and the groove 110 of the fixed wheel 11. Thereafter, the fixed wheel 11 pushes the post 132 and drives the pulling piece 132 to rotate adversely via the second swing block 137 until the 20 protrusion 134 of the pulling piece 132 disengages from the actuation member 135, and the actuation member 135 is pushed by the second spring 136 to move back to the original position, and the retaining block 120 is pushed by the spring 121 to move back to the original position so as to match with the stopping piece 100 to 25 clamp the rope 12 (as shown in FIG. 2), thus having a falling 2 protection. However, the conventional fixed pulley structure cannot move the object upwardly. In addition the user has to pull the rope 14 with large force, thus operating the fixed pulley structure 5 laboriously. The present invention has arisen to mitigate and/or obviate the afore-described disadvantages. SUMMARY OF THE INVENTION 10 The primary object of the present invention is to provide a fixed pulley structure which is capable of overcoming the shortcomings of the conventional fixed pulley structure. To obtain the above objective, a fixed pulley structure provided by the present invention contains: a base, a fixed wheel, 15 a braking device, and a driving device. The base includes a first hole defined on an upper end thereof so as to fix an object thereon and a stopping piece formed on a right side thereof so as to roll a rope. The fixed wheel is rotatably connected with base and used 20 to fix the rope. The braking device includes a retaining member and a controlling member, the retaining member is fixed on the base and swings to a locking position and an unlocking position, wherein when the retaining member is located at the locking position, the 25 arcuate engaging face matches with the arcuate stopping piece to 3 clamp the rope; when the retaining member is located at the unlocking position, the arcuate engaging face moves away from the rope, the controlling member is coupled with the base and pushes the retaining member to the locking position or swings the retaining 5 member back to the unlocking position. The driving device includes an action loop, a post, two swing paws and two resilient elements, the action loop also has a plurality of recesses defined on an inner surface thereof, and two ends of the post are in connection with the action loop and the 10 retaining member, the two swing paws is fixed on the fixed wheel, after the fixed wheel rotates adversely, the two swing paws are driven to swing into the plurality of recesses, the two swing paws disengage from the plurality of recesses, and when an adverse rotating speed of the fixed wheel is up to a predetermined value, a 15 centrifugal force generates so that the two swing paws overcome an elasticity of the two resilient elements and swing into the plurality of recesses, thereafter the action loop and the post drive the retaining member to move to the locking position. Thereby, when the rope is pulled, the two swing paws are 20 pushed by the two resilient elements to disengage from the plurality of recesses, so as the fixed wheel rotates, the driving device and the braking device are not driven by the fixed wheel, thus saving pulling force. In addition, the controlling member is pulled to push the retaining member to swing to the locking position so that the 25 retaining member cooperates with the arcuate stopping piece to 4 clamp the rope securely, thus hanging the object still, thereafter the rope is pulled downwardly so that the retaining member moves back to the unlocking position, thus moving the object upwardly and downwardly. Also, as releasing the rope, the fixed wheel is 5 driven by the rope to rotate aversely so that the two swing paws are driven to overcome the elasticity of the two resilient elements and swing outwardly to retain with the plurality of recesses, and the action loop and the post drive the retaining member to swing to the locking position, thus having a falling protection. In addition, the 10 two swing paws retains with the plurality of recesses so as to positioning the object securely. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing the assembly of a 15 conventional fixed pulley structure. FIG. 2 is a cross sectional view showing the operation of the conventional fixed pulley structure. FIG. 3 is another cross sectional view showing the operation of the conventional fixed pulley structure. 20 FIG. 4 is also another cross sectional view showing the operation of the conventional fixed pulley structure. FIG. 5 is a perspective view showing the exploded components of a fixed pulley structure according to a preferred embodiment of the present invention. 25 FIG. 6 is a perspective view showing the assembly of the 5 fixed pulley structure according to a preferred embodiment of the present invention. FIG. 7 is a side plan view showing the assembly of the fixed pulley structure according to the preferred embodiment of the 5 present invention. FIG. 8 is a cross sectional view showing the operation of the fixed pulley structure according to the preferred embodiment of the present invention. FIG. 9 is another cross sectional view showing the 10 operation of the fixed pulley structure according to the preferred embodiment of the present invention. FIG. 10 is also another cross sectional view showing the operation of the fixed pulley structure according to the preferred embodiment of the present invention. 15 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 5, a fixed pulley structure according to a preferred embodiment of the present invention 20 contains a base 20, a fixed wheel 30, a braking device 40, and a driving device 50. The base 20 includes a first hole 21 defined on an upper end thereof so as to fix an object thereon, a second hole 22 formed on a lower end thereof so as to connect with a distal end of a rope 6 60. The base 20 also includes an arcuate stopping piece 23 formed on a right side thereof so as to insert the rope 60. The base 20 further includes a shaft 24 vertically mounted thereon, a positioning piece 25 parallelly disposed thereon, and four orifices 26 arranged 5 around the positioning piece 25. The fixed wheel 30 is rotatably connected with the shaft 24 and located at an inner side of the positioning piece 25 and used to fix the rope 60 so that the rope 60 is further inserted to a movable pulley (not shown) of a lower side of the fixed pulley structure. The 10 fixed wheel 30 has a groove 31 defined on an outer side thereof. The braking device 40 includes a retaining member 41 and a controlling member 42. The retaining member 41 is fixed on the base 20 and swings to a locking position and an unlocking position. The locking member 41 has an arcuate engaging face 43 on which a 15 plurality of conical teeth 44 are defined. When the retaining member 41 is located at the locking position, the arcuate engaging face 43 matches with the arcuate stopping piece 23 to clamp the rope 60. When the retaining member 41 is located at the unlocking position, the arcuate engaging face 43 moves away from the rope 60. 20 The controlling member 42 is coupled with the base 20 and pushes the retaining member 41 to the locking position or swings the retaining member 41 back to the unlocking position. Also, the controlling member 42 has a pulling piece 45 disposed on one side 7 thereof so as to be pulled by a user. The driving device 50 includes an action loop 51, a post 52, two swing paws 53 and two resilient elements 54. The action loop 51 is defined between the fixed wheel 30 and the positioning piece 5 25, and the action loop 51 has four fixing bolts 510 inserted into the four orifices 26 so that the action loop 51 rotates in a range and connects with the positioning piece 25. The action loop 51 also has a plurality of recesses 511 defined on an inner surface thereof, and two ends of the post 52 are in connection with the action loop 51 10 and the retaining member 41. The two swing paws 53 are fixed in the groove 31 of the fixed wheel 30 and on the shaft 24. After the fixed wheel 30 rotate adversely, the two swing paws 53 are driven to swing into the plurality of recesses 511. Each resilient element 54 is a tension spring and has two segments abutting against a head 15 end and a distal end of each swing paw 53, such that the two swing paws 53 disengage from the plurality of recesses 511, and when an adverse rotating speed of the fixed wheel 30 is up to a predetermined value, a centrifugal force generates so that the two swing paws 53 overcome an elasticity of the two resilient elements 20 54 and swing into the plurality of recesses 511, thereafter the action loop 51 and the post 52 drive the retaining member 41 to move to the locking position. Referring further to FIGS. 6 and 7, the fixed pulley 8 structure of the present invention further comprises two auxiliary wheels 70 fixed on the base 20 and used to roll the rope 60 therearound, thus forming three fixed pulleys. If only one auxiliary wheel 70 is fixed on the base 20 and is served to roll the rope 60 5 therearound, then two fixed pulleys are formed. If plural auxiliary wheels 70 are mounted on the base 20 and are applied to roll the rope 60 therearound, then multi- fixed pulleys are formed. As shown in FIG. 8, the two swing paws 53 are pushed by the two resilient elements 54 to be retained on two sides of the shaft 10 24, so one end of a respective one of the two swing paws 53 disengages from the plurality of recesses 511. Thereby, when the rope 60 is pulled downwardly, it drives the fixed wheel 30 to rotate forward, and the action loop 51 is not driven by the fixed wheel 30, so the retaining member is located at the unlocking position in a 15 normal state so that the rope 60 is pulled upwardly and downwardly so as to move an object upwardly or downwardly. As illustrated in FG. 9, when the controlling member 42 is pulled upward to push the retaining member 41 to swing upwardly, the arcuate engaging face 43 of the retaining member 41 cooperates 20 with the arcuate stopping piece 23 to clamp the rope 60 so as to position the object at a height. After the rope 60 is pulled downwardly, the retaining member 41 is driven by the rope 60 to swing back to the unlocking position, and the rope 60 is pulled 9 upwardly and downwardly to move the object upward and downward (as shown in FIG. 8). Referring further to FIG. 10, when the rope 60 is released, it drives the fixed wheel 30 to rotate adversely, and when the fixed 5 wheel 30 rotates adversely to generate centrifugal force, the two swing paws 53 overcome the elasticity of the two resilient elements 54 to swing outwardly so that one end of a respective one of the two swing paws 53 retains in one of the plurality of recesses 511, and the fixed wheel 30 pushes the action loop 51 to rotate adversely via 10 the two swing paws 53 and drives the retaining member 41 to swing upwardly via the post 52, thereafter the arcuate engaging face 43 of the retaining member 41 cooperates with the arcuate stopping piece 23 to clamp the rope 60 securely. Thereby, when the rope 60 is pulled, the two swing paws 15 53 are pushed by the two resilient elements 54 to disengage from the plurality of recesses 511, so as the fixed wheel 30 rotates, the driving device 50 and the braking device 40 are not driven by the fixed wheel 30, thus saving pulling force. In addition, the controlling member 42 is pulled to push the retaining member 41 to 20 swing to the locking position so that the retaining member 41 cooperates with the arcuate stopping piece 23 to clamp the rope 60 securely, thus hanging the object still, thereafter the rope 60 is pulled downwardly so that the retaining member 41 moves back to 10 the unlocking position, thus moving the object upwardly and downwardly. Also, as releasing the rope 60, the fixed wheel 30 is driven by the rope 60 to rotate aversely so that the two swing paws 53 are driven to overcome the elasticity of the two resilient 5 elements 54 and swing outwardly to retain with the plurality of recesses 511, and the action loop 51 and the post 52 drive the retaining member 41 to swing to the locking position, thus having a falling protection. In addition, the two swing paws 53 retains with the plurality of recesses 511 so as to positioning the object securely. 10 While the preferred embodiments of the invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all 15 embodiments which do not depart from the spirit and scope of the invention. 11