We connect a strain gage on one of the STG inputs of our SIRIUS.
A "tuning fork" is normally used for tuning the instruments of an orchestra. It is tuned to 440Hz, which is the standard pitch (note 'a'). In our demo tool a quarter bridge strain gauge with either 120 or 350 ohms resistance (is marked on the connector) is mounted on the steel, therefore we can measure the strain of the vibrations.
In the left upper section we find the amplifier settings. Set to "Bridge" and "Quarter bridge 3-wire", either 120 or 350 ohm (written on connector).
You directly see the according circuitry, how to connect the quarter bridge on the 9pin DSUB connector.
Select an appropriate range, if you use the highest, you don't have to care about overload (input voltage exceeding amplifier range).
We use a smaller range, e.g. 20mV/V. The higher ADC is now working in the 20mV/V input range, while the lower
ADC input range is 5% of it, 1 mV/V simultaneously.
So you get an amazing dynamic!
On the right side (Sensor settings), we select Physical quantity “Strain” or “Stress” and the unit. As the TEDS tab is shown, we see that this sensor is already equipped with TEDS chip, and all the settings are read from it automatically.
Before starting the measurement, we need to balance the strain gage. Click “Balance”, the output will go to 0 um/m, and the offset will be shown next to the button.
Because the natural frequency of the tuning fork is 440Hz, we have to think at which sample rate we want to digitize the signal. In theory, a factor of 2 (=880Hz, Nyquist criteria) would be sufficient, in praxis however it depends very much how the signal looks like. We suggest a factor of 10 or even 20 to get a good result.
So, the sample rate is still fine with 20 kHz.
Go again to "Storing", specify a filename, e.g. "tuning_fork_measurement". Then click "Store".
Now we switch to the second prepared screen called “Scope”. It is again a screen with only one instrument, the scope, maximized over the whole area.
- Switch to the Scope screen
- Hit the tuning fork, that we have an oscillating signal, then click the y-axis label for min/max scaling.
- Set the trigger to Auto, in the properties of the left.
- Move the trigger level up- or downwards with the mouse, until you get a triggered image.
- With the +/- buttons on the x-axis, you can adjust the time window shown.
Now we want to add an FFT instrument, to measure the resonance frequency of the tuning fork.
The third screen is called “Custom...”, but basically every screen can be adapted to your needs.
Go to the “Design mode”, either by clicking the hammer/ruler symbol on the left or the “Design” tab on the top.
The instrument toolbar will show up and display all available instruments. We pick the FFT.
Automatically the channel “AI 5” is assigned to the instrument. As you are in “Design mode” you can now freely adapt the size of the FFT and move it to your favourite location on the screen.
Following steps help to get your data displayed quickly with the FFT:
To be able to change the instrument, you need to exit Design mode first! Click again the Design mode tab.
- Set “y-axis” to “Log”
- Click on the y-axis lower limit, and change it to e.g. 0,001 um/m.
- Hit the tuning fork, let it vibrate, then click on the peak. A cursor will appear, showing the maximum of 439,5 Hz with the according amplitude.
Analysis folder view
After stopping the measurement, when you click two times on the Analysis button, you get to the Analysis folder view, it is like an Explorer. On the bottom, you get information about the channels and data header, and with the powerful search fields you easily find the data file you are looking for.
To learn more about strain gages and how to measure strain and stress, visit our Strain measurement course.