1. Introduction
In the modern CNC lathe industry, many domestic systems still rely on stepper motors as their primary driving elements for economical CNC lathes. These systems typically use single-chip microcontrollers as their control units and operate in an open-loop configuration, which limits their functionality and increases the likelihood of failure. However, with the ongoing transformation of traditional lathes, the SKY universal lathe system has emerged as a powerful alternative. It features a unique dual-position closed-loop control system, compatibility with high-speed 64-bit PC control cores, and advanced automatic tool setting capabilities. Additionally, it integrates CAD/CAM/CNC functions seamlessly, offering advantages that are not found in foreign systems. This results in significant economic benefits for users while improving overall performance and reliability.
2. Control Principle
Traditional closed-loop position control systems are usually designed using a speed loop combined with a position outer loop. However, these systems often struggle with nonlinear factors, leading to instability or self-sustained oscillations. To address this issue, the double-closed-loop position control method based on angle-line displacement has been developed, marking a major advancement in digital control technology. The dynamic structure of the position control system is illustrated in Figure 1.
The system consists of both internal and external position loops. The inner loop is a rotational position loop, where an optoelectronic code wheel mounted on the motor shaft serves as the detection element, and an AC servo system acts as the drive unit. This forms a tracking system with input θi and output θo. The outer loop uses linear displacement sensors like gratings or inductive synchronizers to directly measure the machine table’s position, while the inner rotation servo system drives the table movement. A feedforward channel (Gc(S)) is introduced to form a composite control system, significantly enhancing its tracking performance. Since the table accuracy depends on the linear displacement sensor, the impact of mechanical clearance on precision is theoretically eliminated.
3. System Composition and Application
The block diagram of the system control part, shown in Figure 2, is divided into six main components: the servo mechanism, position feedback, automatic tool changer, switch control, spindle control, and spindle feedback.
(1) **Servo Section**: Based on the mechanical characteristics of the lathe transmission system, we have selected imported AC digital servo motors. For precision requirements after transformation, a matching grating ruler was chosen as the linear displacement detection element. The resolution of the conventional 5V square-wave grating ruler is 5μm, but the system can achieve a precision of 0.01mm. Its reading head can move at a maximum speed of 24m/min, ensuring stability and precision. In practice, the system can reach a resolution of 0.5μm, and with higher-precision detection elements, the operating accuracy can be as fine as 1μm.
(2) **Spindle Section**: To enable real-time control of the spindle, we use matched inverters such as those from Mitsubishi, Panasonic, and Taian to control the speed and direction of the spindle motor. Combined with the lathe’s mechanical transmission, this helps avoid vibration and torque drop at low frequencies. The system uses PLC to switch inverters and controls motor speed through 0–10V analog output, enabling constant-speed cutting during machining. A corresponding optical encoder is also installed on the main shaft (C-axis), counting pulses and feeding back rotational speed to the computer. This allows software-based synchronous thread cutting, dynamically extracting synchronization information, predicting the pull-in point in real time, and performing flexible synchronization control. This greatly improves system reliability and ensures accurate thread turning.
(3) **Tool Holder Section**: The tool holder section combines a computer and PLC to accurately control tool changes. The control flow chart is shown in Figure 3. When the computer issues a tool change command, the PLC receives the signal and activates the tool carrier motor. Position feedback signals from each tool position are compared with the commanded tool number until they match, at which point the motor stops and reverses to lock the tool. Once locked, the PLC sends a completion signal back to the computer, ending the tool change process.
(4) **Fast Automatic Tool Setting**: The system includes a quick and automatic tool setting function that allows manual intervention when needed. As shown in Figure 4, the countersink module has two counter faces, A and B, with known positions. Under system control, the tool tip touches the surfaces of A and B, and the X and Z coordinates are sampled to determine the tool nose position. If the tool nose is arc-shaped, the sampled coordinates represent the contact point between the arc and the surface. The system then calculates the center point of the tool nose based on geometric relationships to determine the tool position. With fast tool setting, a tool change takes only about one minute, significantly reducing processing time and improving productivity and machining quality.
4. Conclusion
After transforming traditional lathes such as the C6140 from Yunnan Machine Tool Plant and the C6150 from Taixing Machine Tool Plant, the SKY universal CNC system fully realizes the potential of a versatile CNC lathe. It offers high precision, reliability, and ease of operation. Combined with the system's powerful CAD/CAM/CNC integrated programming capabilities, it can process complex workpieces with high accuracy, creating significant economic benefits for users. This makes the SKY system a reliable and efficient solution for modern manufacturing needs.
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