CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Chinese Patent Application No. 201010609010.7 filed Dec. 28, 2010, which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to the mechanical and electrical designs for truckles of medical devices, in particular for truckles of mobile large-scale or heavy medical image diagnostic devices. 
     When a large-scale or heavy medical image diagnostic device needs to be moved frequently, the operators usually have to push with a lot of strength, so it is very inconvenient. For example, the mobile C-arm X-ray system needs to be moved and positioned frequently during a surgical operation and the device is moved by the handle. Moreover, in some special cases, such as on a soft floor (e.g. a carpet, etc.), since the material of the moving medium has a large friction coefficient, it is harder to move the devices. In addition, such devices are often equipped with more driving motors or include such components as high-energy X-ray generators, flat-panel detectors and cooling systems to meet the demand of the market, increasing the weight and the volume of the whole device. On the other hand, in order to make sure that the users can operate the devices conveniently, design specifications attempt to reduce the operating physical force required as much as possible. For example, one mobile C-arm X-ray system requires that on a tile floor, the starting push force is no more than 12.5 pounds and the rolling push force is no more than 6.0 pounds, and on a nylon carpet floor, the starting push force for a system with a display screen of 9 inches is no more than 45.0 pounds, the starting push force for a system with a display screen of 12 inches is no more than 50.0 pounds, the rolling push force for a system with a display screen of 9 inches is no more than 32.5 pounds, and the rolling push force for a system with a display screen of 12 inches is no more than 35.0 pounds. 
     SUMMARY OF THE INVENTION 
     A truckle for mobile medical devices is provided, characterized in that an electromagnetism torque balancing motor is mounted on the truckle to balance out the friction generated by the truckle. 
     The switch of the electromagnetism torque balancing motor is connected to the handle of the mobile medical device, and the electromagnetism torque balancing motor is powered on by pushing and pulling the handle. 
     The electromagnetism torque balancing motor is mounted on the bearing of the truckle. 
     There are two electromagnetism torque balancing motors, which are integrated at the two sides inside the truckle, respectively. 
     The power of the electromagnetism torque balancing motor on the truckle is selected according to the friction of the floor on which the medical device moves. 
     If the medical device moves on a soft floor, a high-power electromagnetism torque balancing motor is mounted on the truckle, so that the stall torque of the electromagnetism torque balancing motor is 50-70% of the friction torque. 
     If the medical device moves on a hard floor, a low-power electromagnetism torque balancing motor is mounted on the truckle, so that the stall torque of the electromagnetism torque balancing motor is 50-70% of the friction torque. 
     In another aspect, a mobile C-arm X-ray system is provided, including the above described truckle for mobile medical devices, and including a battery connected to the electromagnetism torque balancing motor on the truckle for supplying power thereto. 
     In yet another aspect, a mobile medical device is provided, including the above described truckle for mobile medical devices, and including a battery connected to the electromagnetism torque balancing motor on the truckle for supplying power thereto. 
     The embodiments described herein have such advantages as reducing the starting push force and rolling push force during movement of mobile medical devices to realize fast positioning of such kind of devices, and at the same time meeting doctors&#39; requirements on high mobility, maneuverability and flexibility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of an electromagnetism torque balancing motor; 
         FIGS. 2A and 2B  show schematic diagrams of an electromagnetism torque balancing motor; 
         FIG. 3  shows an appearance of a plurality of electromagnetism torque balancing motors; 
         FIG. 4  shows an appearance of a plurality of electromagnetism torque balancing motors; 
         FIG. 5  shows a mechanical structural diagram of an electromagnetism-torque friction-balancing truckle; 
         FIG. 6  shows a sectional view of the electromagnetism-torque friction-balancing truckle shown in  FIG. 5 ; 
         FIG. 7  shows a mechanical structural diagram of an alternative electromagnetism-torque friction-balancing truckle; 
         FIG. 8  shows a sectional view of the electromagnetism-torque friction-balancing truckle shown in  FIG. 7 ; 
         FIGS. 9A and 9B  show diagrams of a push force applied to front and back truckles. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Exemplary embodiments will be described in detail, but the present invention is not limited thereto. 
     Exemplary embodiments will be described in detail in conjunction with the drawings, but these embodiments are not intended to limit the present invention. The same components are denoted by the same reference numbers in different drawings. 
       FIG. 1  shows a schematic diagram of an exemplary electromagnetism torque balancing motor.  FIG. 2A  shows an exemplary torque motor before powering on and  FIG. 2B  shows that once the electromagnetism torque balancing motor is powered on, it provides a stall torque, i.e. a balance torque M 2 . 
       FIGS. 3 and 4  shows an appearance of a plurality of electromagnetism torque balancing motors. The electromagnetism torque balancing motor can be bought according to the desired parameters. 
       FIG. 5  shows the mechanical structural diagram of an electromagnetism-torque friction-balancing truckle.  FIG. 6  shows a sectional view of the electromagnetism-torque friction-balancing truckle shown in  FIG. 5 . Such a truckle is thinner, and an electromagnetism-torque balancing motor  15  is mounted on the bearing  16  of the truckle, with its switch connected to the handle of the mobile medical device (e.g. mobile C-arm X-ray system).  17  and  18  respectively represent positions where the handle is pulled and pushed. When pushing and pulling the medical device, the electromagnetism-torque balancing motor is powered on to generate a stall torque M 2 . The needed push force torque before balancing is M 1 .
 
 M 1= F   starting force   =f×R×G× ¼,
 
     Wherein F starting force  is the starting push force, f is the conversion factor for unit, R is the radius of the truckle, G is the gravity, which is 9.8 Newtons/meter (N/m), and ¼ represents that the push force is equally distributed on the four truckles. 
     If, for example, F starting force =12.5 lbs, f=0.45, R=0.01 m, and G=9.8N/m, then M 1 =12.5×0.45×0.01×9.8×¼=0.141 N·m. 
     In the exemplary embodiment, M 2  is about 60% of M 1 , so M 2 =M 1 ×60%=0.084 N·m. 
     In some embodiments, a larger stall torque M 2  may be desired. For example, some medical devices need to be moved on very soft carpet. For such medical devices, an electromagnetism-torque balancing motor of higher power may be used. 
       FIG. 7  shows a mechanical structural diagram of an alternative electromagnetism-torque friction-balancing truckle.  FIG. 8  shows a sectional view of the electromagnetism-torque friction-balancing truckle shown in  FIG. 7 . Such truckle is thicker and two electromagnetism-torque motors  15  are respectively integrated at two sides inside the truckle. In addition, a battery may be added on the mobile medical device (e.g. mobile C-arm X-ray system) to supply power to the electromagnetism-torque motors. Once the system is powered down, the system can still be moved substantial distances easily and flexibly using the power generated by the battery. 
       FIGS. 9A and 9B  show diagrams of a push force applied to front and back truckles.  FIG. 9A  shows that the starting push force of the truckle is F 1 , and the rolling push force is F 2 . Suppose that the friction torque is M 1 . When M 2  reaches 60% of M 1 , then  FIG. 9B  shows that the starting push force and rolling push force needed to move the truckle drops to F 3  when utilizing the electromagnetism-torque balancing motor. In the exemplary embodiment, F 3  is about 50-70% of F 2 , for example 60%. 
     The method of reducing friction resistance described herein can also be applied to other mobile medical image diagnostic devices and other mobile medical devices in addition to the mobile C-arm X-ray system. 
     The exemplary embodiments are only illustration. It shall be noted that those of ordinary skill in the art will be able to make many improvements, modifications and variations without departing from the spirit of the present invention. Such improvements, modifications, and variations shall be considered as falling within the scope of the present application.
A truckle for a mobile medical device is provided. The truckle includes at least one electromagnetism torque balancing motor mounted on the truckle and configured to balance out friction generated by the truckle.