The machining accuracy of the piston's outer surface plays a crucial role in determining the performance, fuel efficiency, and emissions of internal combustion engines. Pistons operate under extreme conditions—high temperature, high pressure, and periodic impact—which cause significant and uneven deformation and thermal expansion. To ensure that the piston’s shape aligns with the cylindrical body during operation, its outer surface is typically designed as a contoured profile at normal temperature. Along its axis, the shape resembles a barrel-like form, while its cross-section perpendicular to the axis is approximately elliptical.
To meet functional requirements, the outer surface of the piston is generally machined using turning. However, traditional methods rely on mechanical simulation, which involves complex molds that are difficult to manufacture, wear easily, and have limited frequency response. These limitations hinder improvements in both machining accuracy and efficiency. In recent years, researchers worldwide have explored the use of numerical control (NC) technology for piston machining, achieving notable progress. Our PTC series of CNC turning systems for piston outer circles have been adopted by nearly 30 domestic companies, delivering substantial economic benefits.
One of the key challenges in the CNC turning system for pistons is the input of the piston profile. If a general NC program is used to describe the contour, the program becomes large and complex. The maximum diameter (D), minimum diameter (d), and ellipticity (E = D - d) of the cross-section are usually provided in the form of mathematical functions or data points. These parameters are given in a piecewise manner, with smooth intervals and abrupt breakpoints between them. In our system, users can input these functions directly if available, or the system will validate the correctness of the expressions, segmentation intervals, and function values. If only discrete data points are provided, the system uses cubic spline interpolation and extends the curve smoothly at the ends.
To achieve high-frequency response, traditional CNC systems have an X-axis frequency response of around 1 Hz and acceleration of about 1g. However, for piston machining, the X-axis must operate at over 120 Hz with acceleration exceeding 9g. To meet this demand, we introduced a second linear axis with high-frequency response, using voice coil linear motors and servo components. This allows us to achieve a frequency response above 135 Hz and acceleration over 13g.
High-resolution control is also essential. While standard CNC systems offer micron-level precision, the piston’s large cross-sectional areas require sub-micron accuracy. We use 12- or 16-bit DACs to control the linear axis displacement relative to the X-axis, ensuring precise positioning.
For four-axis linkage control, using a conventional system would be costly and unable to meet real-time demands. Instead, we employ a dual-axis linkage method based on spindle rotation angle, allowing the system to control the linear axis in real time without requiring a servo spindle. This simplifies the design and reduces costs significantly.
To adjust the linear axis control parameters, we model the system as a second-order dynamic system and fine-tune it using speed sensors to provide optimal damping. This approach ensures stability and ease of adjustment.
Another challenge is the time delay caused by optical isolation in high-frequency control. We address this with pulse stretching, high-speed optocouplers, time-slice multiplexing, and reentrant interrupt handlers to maintain real-time performance.
The system hardware includes industrial controllers, digital I/O boards, D/A boards, and a timer board. It supports various motor types for the X, Z, and spindle axes, with configuration settings adjustable through a system file. The software architecture runs in protected mode, using Pascal, C, and assembly languages, with a user-friendly Chinese interface. Real-time operations are prioritized to avoid screen refresh delays, ensuring efficient system performance.
Overall, the system combines advanced control strategies, high-precision mechanics, and flexible software design to deliver reliable and accurate machining of piston outer surfaces.
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