## 2 Principle of Gear Clearance Elimination Using Double Servo Motor  
The heavy-duty vertical milling machine (Qiqihar Heavy CNC equipment Limited by Share Ltd., Qiqihar, China) studied in this paper has a maximum processing weight of 550 tons, a maximum rotary diameter of 25 m, and a very large rotational inertia. How to realize the positioning accuracy of high-precision C-axis in heavy-duty large inertia parts is a great challenge to the workbench, C-axis transmission chain, and control system. The C-axis of this specification machine tool is the world's largest CNC vertical milling lathe C-axis structure, its positioning accuracy requirements of \(\pm 4^{\prime\prime}\). If the traditional structure is adopted, it will be limited by the precision of C-axis drive components. In addition, due to the large inertia of the C-axis load, the large torque and inertia of the motor must be required when the single servo motor is driven, which is easy to exceed the capacity limit of the servo motor. The master-slave control technology with dual servo motor can solve the above problems well. The dual servo motors D1 and D2 drive their independent transmission chains, respectively (as shown in Figure 1), and engage with the large tooth ring of the work platform to form a complete closed-loopchain system. At the same time, the closed-loop feedback is carried out by the circular grating installed at the center of the main axis of the workbench, which can effectively improve the tracking characteristics and positioning accuracy of the C-axis system. The C-axis of the super-heavy CNC vertical milling lathe uses the same type of two motors in order ensure the C-axis forward and backward torque is equal. Through the master-slave function of the CNC system, the C-axis gear ring is always subjected to an inverse torque. The end gears of the two independent transmission chains of the servo motor D1 and D2 are always engaged in both positive and negative direction with the corresponding meshing teeth of the large gear ring. This control mode restricts the positive and negative direction of the large gear ring and eliminates the gear transmission backlash of the large gear ring at the end of the C-axis, so as to improve the positioning accuracy. In a dual servo motor system, D1 is the active drive motor and D2 is the driven drive motor. The servo motor D1 moves according to the motion parameters that are set by the numerical control system program. According to the instructions given by the numerical control system, the position ring and speed ring of the servo motor D1 are controlled. Servo motor D2, according to the numerical control system, has the function of master-slave control lag time \(\delta\)t. According to the feed torque ratio of the servo motor D1, the motor D2 follows the driven motion. The input speed and position instruction of the servo motor D1 are taken as the input instruction of the servo motor D2. The servo motor D1 of the active driving table is driven in the closed-loop working chain, and the driven servo motor D2 works in the open-loop state. The drive control that is received by D2 is the speed and position signal of the active servo motor D1, so as to improve the synchronization control accuracy and stability of the C-axis closed-loop system. From the principle of master-slave control, the clearance elimination process is realized in the feed phase. When the dual servo motor is in the acceleration and deceleration stages, the main drive motor D1 moves according to the specified signal, while the driven motor D2 always follows the active motor. When the main drive servo motor D1 acceleration process finished, driven motor D2 to follow the initiative and lag \(\delta\)t time to feed. Figure 2 shows the driving torque relation of the double motor.