Impact Test with Software Trigger (DAQmx)

This VI performs a triggered acquisiton from two channels on the same device. The first channel should use a sensor mounted on an impact hammer. The second channel should use an accelerometer mounted on the structure under test.

The data from each channel is windowed and the frequency response is computed. The results of the frequency response measurement are displayed as magnitude and phase. The coherence is also graphed to evaluate the validity of the measured frequency response.

Requirements

Filename: impacttest_wswtrg.zip

Software Requirements

Application Software: LabVIEW Base Development System 8.0
Toolkits and Add-Ons: LabVIEW Sound and Vibration Toolset 4.0
Language(s): LabVIEW

Hardware Requirements

Hardware Group: Dynamic Signal Analyzers (DSA)
Hardware Model: USB-9233
Driver: NI-DAQmx 7.5

Reader Comments | Submit a comment »

Is there a flaw in this VI?
I've worked through the algorithms in the example, at it appears to me that there is a flaw. The intent is to detect a hammer impact and then save a record of a desired block length included a desired number of pre-trigger samples. In the subVI "Analog Triggering_DAQmx.vi", samples will be read from the waveform starting at index i-Np, where i is the index of the detected threshhold crossing and Np is the desired number of pre-trigger samples. Additional data are sampled and appended to ensure the desired total block length. This works fine as long as the impact event occurs at least Np samples after the start of the waveform. If i-Np<0 (not enought pre-trigger samples present), the pre-trigger samples simply start at the beginning of the waveform. This means that fewer than Np pre-trigger samples will be included in the final waveform output from the VI.
- John Anderson, Kenworth Truck Co.. john.anderson@paccar.com - Aug 27, 2009

Preferable for impact tests to Analog Level Trigger with Pre-Trigger Samples
I found this VI incredibly helpful, as I had started developing an impact test rig using ContAcq&Graph Voltage-Analog SW Trigger. The problem with using that was that the trigger position within the continuously acquired buffer was never fixed, meaning that occasionally you could miss an entire impact's data. Simply increasing the buffer size wouldn't have helped, as you could still receive a trigger at the end of the buffer. This VI takes a slightly different approach, meaning that it does not suffer from this problem and is able to capture the same number of datapoints each time.
- Tim Hoult, Imperial College. timothy.hoult@imperial.ac.uk - Aug 24, 2007