1 Introduction
In the CNC lathe industry, most of the current domestic use is based on a stepper motor as the driving element of the economic lathe CNC system. Most of these systems use single-chip microcomputers as their control cores, and they work in an open-loop manner with less functionality and higher failure rates. In today's transformation of the lathe, our SKY universal lathe system, with its unique dual-position closed-loop control, is fully compatible with the standard high-speed 64-bit PC control core, convenient automatic tool setting function, powerful CAD/CAM /CNC integrated function, play the advantages that foreign systems do not have, and bring good economic benefits to users.
2 control principle
The design of the conventional closed-loop machine tool position control system is based on the speed ring plus a position outer ring to form a closed-loop system. Because such systems are difficult to overcome the effects of nonlinear factors, they often do not work due to self-sustained oscillations. In order to overcome this kind of defect of the system, the double-closed-loop position control method of the angle-line displacement has brought the digital control technology to a new level. The dynamic structure of the position control system constituted by it is shown in FIG. 1 .
The system consists of internal and external double-position rings. The inner ring is a rotation position ring, the detection element is an optoelectronic code wheel mounted on the motor shaft, and the drive device is an AC servo system, thereby forming a follow-up system with input θi and output θo. The external position loop adopts linear displacement detection elements such as gratings and inductive synchronizers to directly obtain the displacement information of the machine table, and uses the inner corner rotation servo system as a driving device to drive the table movement, and introduces a feedforward channel composed of Gc(S). Forming a composite control system greatly improves its follow-up performance. Because the accuracy of the table is determined by the line displacement detection element, the influence of the machine clearance on the accuracy is theoretically eliminated.
3 system composition and application
System control part of the block diagram shown in Figure 2, is divided into six main parts: servo mechanism, position feedback, automatic tool, switch control, spindle control, spindle feedback.
(1) Servo section According to the mechanical characteristics of the lathe transmission part, we use the imported AC digital servo motor that matches. According to the precision requirement of the lathe after transformation, a matched grating ruler is selected as the line displacement detecting element. The resolution of the conventional 5V square-wave grating ruler is 5μm, its precision can reach 0.01mm, and the reading head has a maximum fast-moving speed of 24m/min, which can ensure the precision and stability of the CNC lathe. In fact, the system resolution can reach 0.5μm. The use of higher-precision detection elements has resulted in a system operating accuracy of 1 μm.
(2) Spindle part In order to achieve real-time control of the system to the spindle part, we use the matched inverters, such as Mitsubishi, Panasonic, Taian, etc., to control the spindle motor speed and steering. At the same time with the lathe's own mechanical transmission mechanism, it can avoid its vibration and torque drop phenomenon at low frequency. The system will realize the switching control of the inverter through PLC, and the motor speed can be controlled in real time through the 0~10V analog output. In this way, constant speed cutting in the machining process can be achieved.
In addition, a corresponding optical encoder is assembled on the main shaft (ie, C-axis), and its rotational speed is counted in pulses and fed back to the computer. The system can implement software synchronous thread cutting based on the counting results. This method digitizes the position feedback information of the C-axis, dynamically extracts synchronization information, predicts the pull-in synchronization point in real time, and performs flexible pull-in synchronous control accordingly. The software of multiple cutting processes is implemented in an uninterrupted manner. Synchronize. It greatly improves the reliability of the system and ensures reliable thread turning accuracy.
(3) The tool holder part is a combination of a computer and a PLC, which realizes the accurate control of the tool change of the tool holder. The control flow chart is shown in Figure 3. After the computer issues a tool change command, the PLC receives its signal and acts to control the positive rotation of the tool carrier motor. At the same time, the position feedback signal of each tool position of the tool holder is detected, and compared with the tool number command issued by the computer, until the same, the positive rotation of the tool carrier motor is stopped, and at the same time the PLC controls the tool holder motor to reverse lock. After the lock is in place, the PLC issues a tool change completion signal to the computer. At this point, the tool change action ends.
(4) Fast automatic knife countermeasures are assigned to the accessory knife counter, and a quick and automatic knife counter with manual participation is realized. Its working principle Figure 4 can be briefly explained. There are two counterfaces A, B on the countersink module, and their positional relationship is known. Under the control of the system, the tip of the tool touches the moments of the cutting surfaces A and B, and the X and Z coordinates are sampled to obtain the coordinates of the tool nose. This determines the position of the tool in the machine coordinate system. If the tool nose is arc-shaped and the tool tip touches the A and B surfaces, the sampled X and Z coordinates are the coordinates of the contact point of the arc and the tool face. At this time, the system will calculate the coordinates of the center point of the tool nose based on the geometric relationship between the tool nose radius and the counter tool surface to determine the tool position. With the fast knife setting, it takes only about 1 minute for a knife. And the high accuracy of the knife, greatly reducing the processing time, effectively improving the labor productivity and turning machining quality.
4 Conclusion
After the transformation of the lathe, such as the C6140 lathe of the Yunnan Machine Tool Plant, the C6150 lathe of the Taixing Machine Tool Plant, etc., can play the full functions of the universal CNC lathe, high precision, reliability, and easy operation. The machine tool is combined with the system's powerful CAD/CAM/CNC trinity software programming function, enabling it to process a variety of workpieces with high precision and complex shapes, thereby creating a good economic benefit for the user.
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