Bump Tests: Using a Hammer to Trigger Recordings

The information provided is for help with Commtest Instruments products:

vb instrument

QUESTION

How can I trigger a bump test recording when using a hammer?

ANSWER

For simple bump tests using peak hold averaging no triggering is required. See Bump Tests.

However, if you want to record, for example, a waveform using a modal hammer and a sensor on the machine structure you should trigger the recording to start just before the hammer strikes the surface. This can be achieved using the instrument in Manual Measure mode with a tach sensor detecting the motion of the hammer:

Attach some reflective tachometer tape on the side of the hammer and position the tach sensor so the hammer swings past it as you strike the machine.

Set up a measurement with the following parameters:
(Essential parameters are italicised. The others can be modified to suit your application.)

Domain	Time
Quantity	Acceleration
No. of Samples	2048
Duration	400 ms
N.o of Averages	1
Average Type	Linear
Tach Trigger	Single
Channel	CH1 & CH2

Press START to begin the settling process. When the instrument is settled it will display the message 'No tach signal'.
Swing the hammer at the machine; as it passes the tach beam a recording will start immediately.
Inspect the two waveforms on the screen pressing [8] to switch between channels 1 and 2.
If the signals look good press MENU then [2] to save both channels. Remember that the two channels must be saved to different measurement locations so that recordings don't overwrite each other.
To retry press START. The instrument will again display the message 'No tach signal'. Swing the hammer again and repeat the procedure from step 2.

FAQ ID: 13821 Last Reviewed: 01/11/2005