Suo Yao, Wang Yongqing, Wang Chunlin, and Lezhong (School of Mechanical Engineering, Dalian University of Technology, Dalian 116023, China) proposed a semi closed loop intelligent PID control method for the precise control of the position control of the rotary worktable. Based on the characteristics of the worm gear transmission mechanism in the turntable, a bidirectional pitch error compensation method for the precision numerical control rotary worktable was also proposed. The control algorithm and compensation algorithm have been applied in the milling groove machining control system of a certain aerospace rocket nozzle.
When complex curve and surface machining capabilities are required for CNC machine tools, a rotary shaft must be equipped, and the CNC rotary worktable is a direct component that completes the rotary feed function on the CNC machine tool. The CNC rotary worktable using worm gear transmission pair has a large reduction ratio, can increase the torque of the motor, and has a "self-locking" function, which can maintain the position of the workpiece after the motor loses power. Therefore, it has been widely used in practice. At present, due to factors such as high costs and difficulties in installing feedback components, fully closed-loop control is less commonly used, and semi closed-loop control is more commonly used, as shown.
In order to achieve fast and smooth operation during the position control process of the CNC turntable, an intelligent PID algorithm must be developed; To ensure the accuracy requirements of the machine tool, it is necessary to compensate for the reverse clearance error and pitch error of the worm gear transmission pair. During the debugging of the milling control system for a certain aerospace rocket nozzle, the author found that for a CNC rotary worktable, using unidirectional pitch error compensation like a linear feed axis does not achieve good results. Therefore, based on the characteristics of the worm gear transmission pair, a bidirectional pitch error compensation method based on intelligent PID control has been developed, which can well meet the position control requirements of precision turntables.
The control system shown in the intelligent PID control of the CNC rotary worktable is a follower system controlled by follower error. The speed loop and current loop control of the system are completed internally by the fully digital AC servo unit. The position loop is implemented within the system software of CNC, using intelligent PID control, which has the advantages of strong robustness and concise algorithm. It is very suitable for real-time computer control at the millisecond level and is currently the control algorithm adopted by the vast majority of CNC systems.
The motion process of a CNC turntable can be divided into three stages as shown. Zhong, Qiao is the starting position of the turntable, and P2 is the target position of the turntable. At stage/1, the position control is in speed tracking mode, and the turntable rotates according to the programmed speed tracking command position. At stage B, the position control is in a transitional state from dynamic to steady state, and the turntable decelerates and is about to run to the target position. At stage C, the position control is in the feed hold state, and the turntable maintains its current position.
According to different stages of position control, different control algorithms and PID parameters should be adopted, namely intelligent PID control.
In order to achieve millisecond level real-time control, the knowledge reasoning decision machine of intelligent PID control must be simple and fast, and try to directly identify stage/1, stage B, and stage C. In the control system, the following error e is used as one of the inference information, and the delay counter is used to control the transition time for stage S, in order to facilitate the implementation of millisecond level real-time control. So, the following error threshold value and the delay count constant G can be selected as expert knowledge for the inference decision machine. The expression for the inference decision machine is: Develop an intelligent PID position control method in an actual system, as shown in.
Knowledge reasoning decision machine - Control SA Control B Intelligent P1D Controller In stage seven, in order to eliminate static errors, improve the tracking ability of the turntable's actual position to command position changes, and enhance response speed, a "proportional+feedforward" composite control is introduced. The output u (a) of controller/1 is: u (a)=heart S+'eU), where ruler/is the feedforward coefficient, S is the instruction coordinate increment, is the proportional control coefficient, and e (phantom is the tracking error value of the current sampling period).
The S stage is a transitional stage before entering steady state, requiring as little overshoot as possible. Therefore, the feedforward effect is removed and only proportional control is used. The output u (6) of controller B is u (b)=Kf "e (k). The C stage is the position holding stage. To ensure the positioning accuracy of the system and improve its anti-interference sensitivity to small disturbances, PID control is used. The output u (c) of controller e is: where is the proportional control coefficient, inverse is the integral control coefficient, and heart is the differential control coefficient.
In the milling control system of a certain aerospace rocket nozzle that has been debugged, the PID control parameter values of the CNC turntable are:/=0.2, '=0.1,'=0.2, '=0.01. The reverse clearance compensation of the bidirectional pitch error compensation semi closed loop system is relatively simple and will not be repeated here.
Pitch error compensation is one of the key functions of a semi closed loop CNC system and one of the main means to improve the positioning accuracy of the system. For linear motion axes, the accuracy of ball screw motion pairs is relatively high, and the use of unidirectional pitch error compensation can meet the design and usage requirements. For a digital turntable that uses a worm gear and worm gear pair as the speed and indexing execution component, there is a significant difference in the distance error value when moving in both directions within the same coordinate range. If only unidirectional pitch error compensation is used like a linear motion axis, the compensation effect is not ideal. Therefore, when the CNC turntable moves in both directions, different pitch compensation data must be used, that is, bidirectional pitch error compensation must be performed. The compensation process is shown in the figure.
Among them, P; is the measured coordinate value of the machine tool, i is the error only interval index, and i is given by the following equation: considering the positive and negative sign factors, the system calibration algorithm is as follows: instruction coordinates/P; after the machine tool turntable returns to the zero point of the machine tool, a standard 36 sided prism is installed on the turntable, and the pitch error compensation interval is taken as 10 °. Numerical control programming causes the turntable to rotate in the forward direction. For every 101W rotation of the machine tool turntable, the actual coordinate value error is measured, as shown by the middle line "□" after the positive medium distance of the middle line system. From the line data, it can be seen that the positive compensation data of the turntable is effective, and the effect is clear. After realizing the one-way swim bladder distance error of the turntable, it turned from 36 to 0 in the negative direction, and the measured data is shown as the middle line "A". Due to the mechanical processing errors of the worm gear and worm gear pair, although the positive medium distance error of the turntable has been reported, the negative medium distance error is still significant, and it is necessary to report the negative medium distance error. The measured coordinate error of the turntable after the bi-directional nozzle is shown by the broken lines "□" and "O" in the middle. It can be seen that after bidirectional distance error compensation, the positioning accuracy of the system turntable has been greatly improved.
Conclusion: In the position control of precision CNC turntables, the intelligent PID control algorithm developed according to the different operating stages of the CNC turntables has achieved good results, with smooth operation and fast response speed. The bidirectional pitch error compensation method proposed and implemented for the operation characteristics of worm gear pairs in precision CNC rotary worktables has effectively solved the problem of pitch error compensation for such components. The comprehensive application of intelligent PID algorithm and bidirectional loss error compensation method can achieve position control of precision CNC rotary worktable.
The PID control algorithm and error compensation method described here have been applied in practical control systems, verifying the feasibility and correctness of the developed control scheme.
