Jing Yongqing, Wang Chunlin, Le Zhong (Department of Mechanical Engineering, Dalian University of Technology, Dalian 116023, China), proposed a semi-closed-loop intelligent PID control method and the characteristics of the worm gear transmission mechanism in the turntable. A two-way pitch error compensation method for a precision CNC rotary table is proposed. The control algorithm and compensation algorithm have been applied to the processing control system of a space rocket nozzle.
When a CNC machine tool is required to have complex curve and surface machining capabilities, it must be equipped with a rotary axis, and the CNC rotary table is a direct component that performs the rotary feed function on the CNC machine. The numerical control rotary table with worm gear drive pair has large reduction ratio, can improve the torque of the motor, and has the function of “self-lockingâ€, which can keep the position of the workpiece after the motor loses power. Therefore, it has been widely used in practice. . At present, due to the high cost and difficulty in installing the feedback components, the full-closed control method is less used, and the semi-closed loop control is more used, as shown.
In the process of position control of the CNC rotary table, in order to achieve fast and stable operation, an intelligent PID algorithm must be developed; in order to ensure the accuracy requirements of the machine tool, the backlash error and pitch error of the worm gear drive pair must be performed. make up. In the process of debugging a spacecraft nozzle nozzle milling control system, the author found that for the CNC rotary table, if the single-axis pitch error compensation is used like the linear feed axis, it can not receive good results. For this reason, for the characteristics of the worm gear transmission pair, a two-way pitch error compensation method based on intelligent PID control is developed, which can well meet the position control requirements of the precision turntable.
1 The control system shown by the intelligent PID control of the CNC rotary table is a follow-up system controlled by following error. The system's speed loop and current loop control is done internally by the all digital AC servo unit. The position loop is implemented in the CNC system software. It adopts intelligent PID control, which has the advantages of strong robustness and simple algorithm. It is very suitable for real-time control of the computer in the second order. It is the control algorithm adopted by most CNC systems.
The motion process of the CNC turntable can be divided into three stages as shown. In the middle, it is the starting position of the turntable, and P2 is the target position of the turntable. At phase /1, the position control is in the speed tracking state and the turntable is rotated by programming the specified speed tracking command position. In phase B, the position control is in a transition state from dynamic to steady state, the turret decelerates and is about to run to the target position. In phase C, the position control is in the feed hold state and the turntable maintains the existing position.
According to different position control stages, different control algorithms and PID parameters should be used respectively, namely intelligent PID control.
In order to facilitate real-time control in milliseconds, the knowledge reasoning decision machine of intelligent PID control must be simple and fast, and directly identify phase/1, phase B and phase C as much as possible. In the system, the following error e is used as one of the inference information, and for the phase S, the delay counter counter is used to control the transition time. Therefore, it is possible to select the following error threshold agricultural and delay count constant G as the expert knowledge of the inference decision machine. The inference decision machine expression is: Develop an intelligent PID position control method in the actual system, as shown.
Knowledge Reasoning Decision Machine-Control SA Control B Intelligent P1D controller In stage 7, in order to eliminate static error, improve the ability of the actual position of the turntable to track the change of command position, improve the rapid response, and introduce the "proportional + feedforward" composite control. The output u(a) of the controller /1 is: u(a)=heart S+'eU) where the ruler/for the feedforward coefficient, S is the command coordinate increment, which is the proportional control coefficient, and e (the magical current sampling period) Follow error value.
The S phase is the transitional phase before entering the steady state. It is required to have as much overshoot as possible. Therefore, the feedforward action is cancelled, only the proportional control is used. The controller B output u(6) is: u(b)=Kf'e(k The C phase is the position retention phase. In order to ensure the system positioning accuracy and improve the system's anti-interference sensitivity to small disturbances, PID control is used. The controller e output u(c) is: where is the proportional control coefficient, and the inverse is the integral control coefficient. The heart is the differential control coefficient.
In the control system of a space rocket nozzle and slot milling machine that has been debugged, the PID control parameter value of the CNC rotary table is: /=0.2, '=0.1, '=0.2,=0.01, 2 two-way pitch error compensation backlash of the semi-closed loop system The compensation is simpler and will not be described here.
Pitch error compensation is one of the key functions of semi-closed loop CNC system and one of the main means to improve the positioning accuracy of the system. For the linear motion axis, the precision of the ball screw motion pair is high, and the single-direction pitch error compensation can meet the design and use requirements. For the number g turntable using the worm gear pair as the speed and indexing execution component, the difference in the error distance between the two directions in the same coordinate interval is large. If it is also like the linear motion axis, only the single-direction sinusoidal error compensation is used, and the compensation effect is not ideal. Therefore, when the CNC turret moves in both the forward and reverse directions, different sinusoidal compensation data must be used, that is, bidirectional must be performed. Pitch error compensation, the compensation process is as shown.
Among them, P; is the measured machine coordinate value, i is the error only interval index, i is given by the following formula: taking into account the positive and negative factors, the system correction algorithm is as follows: command coordinate / P; machine tool turntable back to the machine zero point, in the turntable A standard 36-face prism is mounted on the upper, and the pitch error compensation interval is 10°. The numerical control programming makes the turntable rotate in the positive direction. The absolute value of the actual coordinate value is rotated once every 101W. For example, after the medium-deflect line system is positively spaced from the medium, the absolute error of the forward rotation coordinate value of the turntable is measured as the “□†line. Show. It can be seen from the fold line data that the forward compensation data of the turntable has been valid, and the effect is clear. After the one-way pitch error of the turntable is oscillating, it is rotated by 36 rings and 0. The measured data is as shown by the middle line "A". . Due to the machining error of the worm gear pair, although the median error of the forward running of the turntable has been sent out, the negative media distance error is still very large, and the median error needs to be negatively sent. The measured coordinate error of the turntable after the two-way singer is as shown by the broken lines "□" and "O". It can be seen that the positioning accuracy of the system turntable has been greatly improved after the two-way suspected error compensation.
3 Conclusion In the position control of the precision CNC turntable, according to the different operation stages of the CNC turntable, the developed intelligent PID control algorithm works well, the turntable runs smoothly and the response speed is fast. Aiming at the operating characteristics of the worm gear pair in the precision CNC rotary table, the proposed two-way pitch error compensation method solves the problem of the offset error of such components. The intelligent PID algorithm and the two-way loss error compensation method can be used to realize precise position control of the precision CNC rotary table.
The PID control algorithm and error compensation method described here have been applied in the actual control system to verify the feasibility and correctness of the developed control scheme.
(Finish)
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