管材壁厚动态过程能力指数的在线测量方法

On-Line Measurement Method for the Pipe Wall Thickness of Dynamic Process Capability Index

管材壁厚均匀性及外径尺寸精度是精密管材较为重要的控制指标。在分析了超声波壁厚测量原理和管材壁厚闭环控制方法的基础上,探讨了基于统计过程控制中的静态过程能力指数Cpk在精密管材挤出过程中的应用,以管材壁厚为控制指标,选定样本容量后随采样周期的更新样本递推更新的动态过程能力指数Cdpk的算法。通过合理地设置采样周期、样本容量、目标值、控制偏差,得到准确反映设备运行稳定性与产品质量的性能指标-管材壁厚实时动态过程能力指数。实验证明,当壁厚实时动态过程指数值在控制范围内时,管材壁厚偏差及均匀度均达到要求,当壁厚实时动态过程能力指数变化范围为1.37~1.58时,壁厚变化范围为-27.7~14.9μm。从而证明了壁厚实时动态过程能力指数作为衡量精密挤出稳定性及制品几何精度的可靠性。

The uniformity of thickness and the accuracy of external diameter were the most important control indexes for precision pipe. Based on the analysis of pipe wall thickness measurement by the ultrasonic principle and closed-loop feedback control method, the application of static process capability index （ Cpk） that was based on statistical process control in the process of precision pipe extrusion was discussed. An algorithm of dynamic process capability index （ Cdpk ） was also discussed in which the pipe wall thickness was the control index. And after the sample size was selected, the sample of the algorithm would he updated with the updating of the sampling period. By setting the sampling period, the sample size, the target value and the control deviation reasonably, the real-time Cdpk of pipe wall thickness could be obtained, which could accurately reflect the stability of the equipment and the quality of the product. It was shown in the experiment that when the real-time Cdpk was in the control range, the pipe wall thickness deviation and uniformity were at the required level. When the real-time Cdpk ranged from 1.37 to 1.58, the deviation of wall thickness ranged from - 27.7μm to 14.9μm. It was proved that the reliability of Cdpk would be as an indicator of precision extrusion stability and precision of product geometry.