Automated impact device with non-synchronous impacts: a practical solution for modal testing during operation

Abstract

Previous study has shown that synchronization of phases between impacts and the cyclic load component should be avoided to improve the effectiveness of operational modal testing, i.e. impact-synchronous modal analysis in obtaining a cleaner frequency response function (FRF) estimation with fewer number of averages. However, avoiding the phase synchronization effect is rarely achievable with the current manual impact hammer because of the lack of control of the impact timing. We investigate how to improve FRF estimation in the presence of harmonic disturbances, such as those present in operating rotating machines. An auto impact device is therefore introduced to replace the manual impact hammer. This device ensures that impact intervals can be applied at non-synchronous instances with respect to the harmonic disturbance. We demonstrate that this new device is a viable option for operational modal testing. It allows significant improvement in FRF estimation and shows good correlation of modal extraction data with benchmark experimental modal analysis results.

中文概要

目的

目前手动冲击锤设备缺乏对冲击时间的控制,容易引起冲击相位和周期荷载的周期性响应的相位同步问题。本文旨在通过使用异步自动冲击激励的自动冲击装置代替同步冲击模态分析中传统手动冲击锤的方法来解决上述问题。

创新点

1. 引入具有可调冲击参数的自动冲击装置;2. 该装置可通过控制施加冲击的时间步来确保冲击和来自循环载荷组件的响应异步;3. 当周期性响应的相位与装置所施加的冲击信号不一致时,加速响应中未知力源的影响会被降到最小。

方法

1. 分别使用数字方波信号的波峰和波谷来控制自动冲击装置的“开”和“关”状态;2. 通过调控样本大小(1024 个)、采样率(50 000 个/秒)、占空比(0.5%)和冲击频率(97.78 Hz)(或周期)等参数得到不同的冲击图形。

结论

1. 使用可实现异步冲击的自动冲击装置可以估算第3 阶自然模态;2. 前3 种自然模态可以被成功确定并与基准实验模态分析结果具有良好的相关性,表现为低于1.67%的自然频率差异,1.79%~12.54%的阻尼比差异以及介于0.893 和0.925 之间的模态置信度。3. 针对谐波干扰对相位的影响,相比于使用手动冲击锤来增强频率响应函数估计和模态提取数据,使用可实现异步冲击的自动冲击装置是一种更可行的选择。