The machining accuracy of the piston's outer surface significantly impacts the performance, fuel efficiency, and emissions of internal combustion engines. Pistons operate in harsh environments characterized by high temperature, pressure, and periodic impact, leading to substantial and uneven deformation. To match the shape of the piston under working conditions with a cylindrical body, the outer surface is typically designed as a contoured profile at normal temperature. Along its axis, it resembles a barrel shape, while its cross-section perpendicular to the axis appears elliptical.
To meet functional requirements, the outer circle of the piston is generally machined using turning. For profiled cross-sections, traditional mechanical simulation methods are used, but these come with challenges such as difficult mold manufacturing, rapid wear, low frequency response, and difficulty in adapting to different piston types. These limitations hinder improvements in both processing accuracy and efficiency.
In recent years, researchers have focused on applying numerical control (NC) technology to piston machining, achieving significant progress. Our PTC series of CNC turning systems for piston outer circles have been adopted by nearly 30 domestic companies, yielding excellent economic returns.
One of the key challenges in the CNC turning system is accurately inputting the piston's profile. Using a general NC program to describe the profile results in large and complex code. The maximum diameter (D), minimum diameter (d), and ellipticity (E = D - d) are defined based on the cross-section perpendicular to the piston's axis. Additionally, the orientation of the radial direction relative to the major diameter is described by an angle (f). These parameters are often provided as mathematical functions or point lists, requiring careful segmentation and validation.
Our system allows users to input or select these functions directly. The control system checks the validity of expressions, interval consistency, and function values. If data points are given, the system ensures their rationality, interval integrity, and fitting function range. It then uses cubic spline interpolation with freeform points and extends segments with linear slopes for smooth transitions.
For high-frequency response, conventional CNC systems have limited X-axis frequency (around 1 Hz), but pistons require higher frequencies (up to 120 Hz) and greater acceleration (over 9g). To address this, we introduced a second X-axis with a linear axis driven by voice coil motors and linear servos, offering over 135 Hz frequency and 13g acceleration.
High-resolution control is also critical. While standard CNC systems offer micron-level accuracy, the piston's large cross-section requires sub-micron precision. We use 12- or 16-bit DACs to control the linear axis displacement, ensuring the required accuracy.
For four-axis linkage, traditional systems are costly and unable to meet real-time demands. Instead, we use a dual-axis linkage method where the spindle encoder detects rotation angle, and the computer controls the linear axis in real time. This approach eliminates the need for a servo spindle, reducing costs and simplifying the system.
Linear axis control parameters are adjusted using a second-order dynamic model. By fine-tuning static stiffness, time constant, and damping ratio, we ensure stable performance. A speed sensor on the moving part helps maintain optimal damping, making adjustments easier and more reliable.
To address time delays from optical isolation at high control frequencies, we implement pulse stretching, high-speed optocouplers, time-slice multiplexing, and reentrant interrupt handlers, ensuring real-time operation.
The system hardware structure includes industrial controllers, digital I/O boards, D/A boards, and timing modules. Configurations such as pulse equivalent, acceleration/deceleration time, and signal polarity can be adjusted via a configuration file. Input signals are disabled during power loss or failure to prevent errors.
The software architecture runs in protected mode, using Pascal, C, and assembly languages. It supports multi-window interaction, Chinese character interfaces, and real-time control. Screen refresh is optimized to avoid conflicts with control cycles, and modules operate in parallel or exclusive modes depending on the task.
Overall, the system integrates advanced control methods, efficient hardware design, and user-friendly software to enhance the precision and efficiency of piston machining.
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