1. Introduction
In the CNC lathe industry, many domestic systems still rely on stepper motors as the primary driving components 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 failures. As the industry evolves, the SKY universal lathe system has emerged with a unique dual-position closed-loop control mechanism. It is fully compatible with standard high-speed 64-bit PC control cores, offers convenient automatic tool setting, and integrates powerful CAD/CAM/CNC functions that are not commonly found in foreign systems. This innovation provides significant economic benefits to users by enhancing performance and reducing downtime.
2. Control Principle
Traditional closed-loop machine tool position control systems typically use a speed loop combined with a position outer loop to form a feedback mechanism. However, these systems often struggle with nonlinear effects, leading to instability or self-sustained oscillations. To address this issue, the dual-closed-loop position control method based on angle-line displacement has advanced digital control technology. The dynamic structure of the position control system is illustrated in Figure 1.
The system consists of internal and external position loops. The inner loop is a rotational position loop, using an optoelectronic code wheel mounted on the motor shaft as the detection element, paired with an AC servo drive to create a tracking system with input θi and output θo. The external loop uses linear displacement sensors like gratings or inductive synchronizers to directly measure the machine table's movement, while the inner rotation servo system drives the table. A feedforward channel (Gc(S)) is also included, forming a composite control system that significantly improves tracking performance. Since the table's 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 is shown in Figure 2, divided into six main components: servo mechanism, position feedback, automatic tooling, switch control, spindle control, and spindle feedback.
(1) **Servo Section**: Based on the mechanical characteristics of the lathe, we use imported AC digital servo motors. For precise machining after transformation, a grating ruler is selected as the linear displacement sensor. Conventional 5V square-wave grating rulers have a resolution of 5μm, but can achieve precision up to 0.01mm. Their reading heads support a maximum fast-moving speed of 24m/min, ensuring the stability and accuracy of the CNC lathe. In practice, the system can reach a resolution of 0.5μm, with operating accuracy up to 1μm.
(2) **Spindle Section**: To enable real-time control of the spindle, we use inverters such as Mitsubishi, Panasonic, and Taian to manage the spindle motor’s speed and direction. Combined with the lathe’s mechanical transmission, this setup prevents vibration and torque drop at low frequencies. The system controls the inverter through PLC, allowing real-time adjustment of motor speed via a 0–10V analog signal. This ensures constant-speed cutting during machining. Additionally, a built-in optical encoder on the main shaft (C-axis) counts pulses and feeds back rotational speed to the computer, enabling software-based synchronous thread cutting. This method digitizes C-axis feedback, dynamically extracts synchronization data, predicts synchronization points, and performs flexible control, greatly improving system reliability and thread-cutting accuracy.
(3) **Tool Holder Section**: The tool holder system combines a computer and PLC to accurately control tool changes. The control flowchart is shown in Figure 3. When the computer issues a tool change command, the PLC receives the signal and activates the tool carrier motor to rotate forward. Position feedback signals from each tool position are compared with the commanded tool number. Once matched, the motor stops, and the PLC locks it in place. After locking, the PLC sends a completion signal to the computer, ending the tool change process.
(4) **Fast Automatic Tool Setting**: The system includes a quick and automatic tool setter that allows manual intervention when needed. Its working principle is illustrated in Figure 4. Two counterfaces, A and B, are used on the countersink module, with known positions. Under system control, the tool tip touches both surfaces, and X and Z coordinates are sampled to determine the tool nose position. If the tool tip is arc-shaped, the system calculates the center point based on geometric relationships, ensuring accurate tool positioning. With this 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 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 capabilities of a high-precision, reliable, and easy-to-operate CNC lathe. Integrated with powerful CAD/CAM/CNC software, it can process complex workpieces with high accuracy, delivering substantial economic benefits to users.
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