Hammer Impact Test
Hammer Impact test is a method commonly used to determine natural frequencies of an object (machine, bridge, building etc). The following hardware setup is normally needed for this type of test:
- Multi-channel DAQ device
- Special impact hammer with built-in accelerometer sensor (connected to the first input of your DAQ device)
- Another accelerometer attached to the object being measured (connected to the second input of your DAQ device)
The idea of the test is to repeatedly generate energy impulses by impacting the object with the hammer, and at the same time, picking up the vibrations of the object with the second accelerometer. By analysing the relations between the impact (impulse) signal and the response of the object to that impulse, you can determine important natural frequencies of the object. Normally, many measurements would be done for one object on a different impact or response measurement positions.
How to perform a hammer test with SIGVIEW?
1. Attach your hammer and accelerometer to your DAQ device as explained above. In this example, we will use Data Translation DT9837, but any other multichannel DAQ would be used in a similar way.
2. Setup your Data Acquisition. We will use Time Synchronous Averaging technique to enhance measurement quality, as explained here
We will use 2 channels (one for hammer and one for other accelerometer) at the sampling rate of 10kHz. The recording will be started after a trigger (20% of full range) on channel 1 (impact channel). We will start recording 1000 samples (100ms) before the trigger. Only one block of data (10000 samples = 1s) will be recorded after each trigger. After that, the trigger will be re-armed, and the next recording will be done in the same window. We will do this 2 times, which will result in total of 3 blocks being recorded.
Usually, you would perform the calibration step so that the units of recorded signals match your hardware setup.
The resulting Workspace would look like this:
2. As a next step, before starting any acquisition, create any calculation windows needed for your analysis. Typically, you would use FRF magnitude and phase. Both functions can be found in the Signal Calculator. Here is an example for adding FRF Magnitude:
The Workspace should look like this:
You can set FRF magnitude window properties according to your analysis needs, for example switch from linear to dB Y-axis. Also, it is recommended to turn-off windowing because it may corrupt your impulse signal.
3. We will record 3 short signal blocks for each test, and for each block, new FRF magnitude and Phase will be calculated. We would like to average all 3 calculations. Therefore, Averager windows should be attached to both analysis windows. This will result in the following Workspace:
4. Now start Recording by pressing toolbar button while on DAQ window. You will see a message "Waiting for trigger...." inside DAQ window.
5. Use your hammer to impact the object under measurement 3 times (with at least 1s between impacts). On each impact, a new block will be recorded, FRF measurements and Averager windows will be updated.
6. At the end, both Averager windows will contain the results of the test. You can use Peak Detection function or manual inspection to find significant peaks (natural frequencies) in the averaged FRF magnitude result. All results can be stored in files, for example ASCII/CSV format for FRF calculations.
7. To start a new test, you can simply use the function "System control/Reset all windows". After that, you can proceed with the step 1 for the next test. Of course, the whole Workspace can be saved and reused so that you do not have to repeat all the steps each time you want to perform a test.