1.Shaanxi Key Laboratory of Ultrasonics, Shaanxi Normal University, Xi’an 710119, China 2.School of Information Engineering, Yulin University, Yulin 719000, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11674206, 11874253, 11964040)
Received Date:28 May 2020
Accepted Date:09 June 2020
Available Online:12 June 2020
Published Online:20 September 2020
Abstract:When the lateral dimension of the tool head is close to or greater than a quarter of the longitudinal wave length, the tool head will produce severe lateral vibration. The coupling of the lateral vibration and the longitudinal vibration makes the amplitude distribution of the tool head’s radiation surface uneven, which seriously affects the welding quality. To solve the problem of uneven amplitude distribution of the two-dimensional tool head’s radiating surface, in the paper we conduct an optimized design study on a two-dimensional ultrasonic plastic welding system. First, using the theory of phononic crystal dislocations, we construct a nearly periodic phononic crystal homogenous dislocation junction on a large-sized long strip tool head, and use the homogenous dislocation junction to change the regular lattice arrangement of the phononic crystal structure to adjust the position of the band gap and increase the width of the band gap, so that the operating frequency of the two-dimensional ultrasonic plastic welding system can be located in the band gap of the lateral vibration of the tool head, and the effective control of the lateral coupling vibration of the tool head can be achieved, thus optimizing the amplitude uniformity of the radiating surface of the tool head and increasing the amplitude gain. Although the homogenous dislocation junction structure improves the amplitude uniformity of the radiating surface of the tool head, the lateral dislocation effect of the homogenous dislocation junction causes the sound waves in the band gap frequency range to propagate along the dislocation channel, while the dislocation line channel is located in the middle of the tool head, which results in a larger displacement of the middle part of the tool head’s radiating surface, and a smaller displacement on both sides. Therefore, the further optimizing of the two-dimensional tool head is required. In this study, the nearly periodic phononic crystal inclined groove structure is used to better optimize the amplitude distribution uniformity of the radiating surface, and the influence of the inclined groove structure parameters on the longitudinal resonance frequency and amplitude distribution uniformity of the ultrasonic plastic welding system are analyzed, that is, the inclined groove can better improve the uniformity of the amplitude distribution than the straight groove, but the angle of inclination of neither the inner nor outer inclined grooves should be too large: the optimal range is 3°-6°. In addition, the difference in inclination angle between the inner inclined groove and the outer inclined groove should not be too large, and the angle difference from 0° to 2° is best. The simulation results show that the nearly periodic phononic crystal homogenous dislocation junction and inclined groove structure can optimize the two-dimensional ultrasonic plastic welding system, which provides a basis for further research on the theory of lateral vibration suppression. Keywords:near-period phononic crystal/ homogeneous dislocation junction/ inclined groove structure/ amplitude distribution uniformity
图 11 基于近周期声子晶体同质位错结系统的工具头辐射面位移分布图及与不开槽系统的位移分布对比图 Figure11. Displacement distribution diagram of tool head radiating surface of system based on near-period phononic crystal homogenous dislocation junction and its comparison with radiation surface displacement distribution with non-grooved system and system.
为了进一步提高二维超声塑料焊接系统焊接面纵向位移的均匀度, 沿工具头的X方向加工了4个斜槽(换能器、复合变幅杆、工具头的长度、宽度、高度以及槽子的高度和宽度尺寸均保持不变), 构造基于近周期声子晶体斜槽结构的大尺寸二维工具头, 优化后的工具头结构和各部分尺寸如图12所示, 工具头长、宽、高均不变, 斜槽的宽度和高度也不改变, 仅把中间两个槽沿槽的中心法线倾斜4°, 外侧两个槽沿槽的中心法线倾斜5°. 图 12 基于近周期声子晶体斜槽结构的工具头的结构尺寸图 Figure12. Dimensional diagram of tool head based on near-period phononic crystal inclined groove structure.
同样利用COMSOL Multiphysics获得图13所示的系统振型图和图14所示的位移分布图. 为了更清晰地看到近周期斜槽结构对工具头辐射面振幅分布均匀度的优化, 图14给出了两种结构的纵向位移分布对比图. 从图14可以看出, 相比近周期声子晶体同质位错结结构, 近周期声子晶体斜槽结构能更好地控制横向振动, 有效地提高辐射面振幅分布均匀度. 图 13 基于近周期声子晶体斜槽结构的二维超声塑料焊接振动系统的结构图和振型图 Figure13. Structural diagram and modal diagram of two-dimensional ultrasonic plastic welding vibration system based on a near-period phononic crystal inclined groove structure.
图 14 基于近周期声子晶体斜槽结构的工具头辐射面位移分布及其同近周期声子晶体同质位错工具头辐射面位移分布对比图 Figure14. Displacement distribution diagram of tool head radiating surface of system based on near-period phononic crystal inclined groove structure and its comparison with the displacement distribution of radiating surface of tool head with homogenous dislocation in near-period phononic crystal
由3.2节可知, 斜槽的引入使得系统工具头辐射面的振幅分布更加均匀, 即斜槽的结构参数是决定振幅均匀度的一个重要因素, 为了找到最佳的斜槽结构参数, 方便设计, 本研究利用COMSOL Multiphysics仿真分析了斜槽结构参数对基于近周期声子晶体斜槽结构的超声塑料焊接振动系统纵向谐振频率和振幅分布的影响规律, 这里的斜槽结构参数主要包括斜槽高度、斜槽宽度和斜槽倾角, 仿真结果如图15和图16所示. 图16中的dr表示振幅变化范围, 即辐射面纵向位移最大值减去辐射面纵向位移最小值, 即dr越大, 辐射面纵向位移的变化范围越大, 振幅分布越不均匀, dr越小, 辐射面纵向位移的变化范围越小, 振幅分布越均匀; 图中的θ1是外侧斜槽倾斜角度, θ2是内侧斜槽倾斜角. 图 15 斜槽高度对系统谐振频率的影响 Figure15. Influence of the height of the inclined grooves on the resonance frequency of the system.
图 16 斜槽高度对振幅变化范围的影响 Figure16. Influence of the height of the inclined grooves on the range of amplitude variation
图15和图16分别反映了系统谐振频率和振幅变化范围dr与斜槽高度的关系. 从图15可以看出, 当其他参数不变时(斜槽宽度、外侧斜槽倾角和内侧斜槽倾角固定, 斜槽宽度固定为10 mm, 斜槽外侧倾角为5°, 内侧倾角为3°), 随着斜槽高度的增大, 二维超声塑料焊接系统的谐振频率先减小, 然后再逐渐增大. 从图16可以看出, dr随着斜槽高度的增大先减小, 再增大, 即辐射面的振幅分布均匀度随斜槽高度的增加先越来越均匀, 而后越来越不均匀, 由此可以看出, 斜槽高度的取值要在合适的范围内, 高度太小或者太大, 都会导致系统的振幅分布不均匀, 在此系统中, 斜槽的高度在60—75 mm之间时, 振幅均匀度最佳. 从图17可以看出, 当其他参数不变时(斜槽高度、外侧斜槽倾角和内侧斜槽倾角固定, 斜槽高度固定为60 mm, 斜槽外侧倾角为5°, 内侧倾角为3°), 随着斜槽宽度的增大, 二维超声塑料焊接系统的谐振频率逐步减小. 从图18可知, 当其他参数不变时, dr随着斜槽宽度的增大先减小, 再增大, 即系统的振幅分布均匀度随斜槽宽度的增加先越来越均匀, 而后越来越不均匀, 由此可以推出, 斜槽宽度的取值也要在合适的范围内, 宽度太小或者太大, 都会导致系统的振幅分布不均匀, 在此系统中, 斜槽的宽度在10—12 mm之间时, 振幅均匀度最佳. 图 17 斜槽宽度对系统谐振频率的影响 Figure17. Influence of the width of the inclined grooves on the resonance frequency of the system.
图 18 斜槽宽度对振幅变化范围的影响 Figure18. Influence of the width of the inclined grooves on the range of amplitude variation.
图19是系统谐振频率随两个变量—θ1外侧斜槽倾斜角度以及θ2内侧斜槽倾斜角的变化关系. 当θ2 = 1°时, θ1从1°变化到20°的过程中, 系统谐振频率与两者的关系可以用图19中的黑色线条表示; 同理, θ2 = 2°时, θ1从1°变化到20°的过程中, 系统谐振频率与两者的关系可以用图19中的红色线条, 以此类推. 图20是辐射面的振幅变化范围随两个变量θ1, θ2的变化关系, 其中, 上图反映了dr和θ1随θ2的变化曲线, 由于点过多, 将上图分成了图20(a)和图20(b)两部分, 图(a)显示了θ2从1°变化到5°时, dr和θ1随θ2的变化曲线, 同理, 图(b)显示了θ2从6°变化到10°时, dr和θ1随θ2的变化曲线; 下图反映了dr和θ2随θ1的变化曲线, 图(c)显示了θ1从1°变化到5°时, dr和θ2随θ1的变化曲线, 图(d)显示了θ1从6°变化到10°时, dr和θ2随θ1的变化曲线. 图 19 斜槽角度对系统谐振频率的影响 Figure19. Influence of the angle of the inclined grooves on the resonance frequency of the system
图 20 斜槽角度对振幅变化范围的影响 Figure20. Influence of the angle of the inclined grooves on the range of amplitude variation
从图19可以看出, 当其他参数不变时, 随着斜槽倾斜角度的增大, 二维超声塑料焊接系统的谐振频率整体呈先减小后增大的趋势. 从图20可知, 当其他参数不变时, 二维超声塑料焊接系统的dr取值随着斜槽倾斜角度的增大先减小, 再增大, 即系统的振幅分布均匀度随斜槽倾角的增加先越来越均匀, 而后越来越不均匀, 且: 1)外侧和内侧斜槽倾角均不宜过大, 外侧和内侧斜槽倾角越大, dr值越大, 系统振幅分布均匀度越差. 从图20可以看出, 本系统内、外侧斜槽倾角最佳范围为3°—6°. 2)外侧和内侧斜槽倾角二者角度相差也不宜过大, 外侧和内侧斜槽倾角相差越大, 系统振幅分布越不均匀, 图21给出dr较小时对应的内外倾角. 从图20和图21可以看出, 本系统内、外侧斜槽倾角角度相差0°—2°为最佳. 图 21 dr和内外侧斜槽角度差值的关系 Figure21. Relationship between dr and the angle difference of outer and inner inclined grooves.