Manual
1. default settings
After confirming the license conditions, the application is ready for use (a). Default settings have been selected in the app that provide the most precise and high-resolution results possible for the most common devices in terms of sampling rate, without impairing smooth operation (b). Depending on the device and measurement situation, adjusting the settings can significantly improve the results. A sample rate of 50 Hz is preset for acceleration recording. However, an increase in the sampling rate is strongly recommended for the acquisition of higher-frequency signals. However, the performance limits are quickly reached with sensors with a lower resolution, which are usually found in older or inexpensive devices. By reducing the FFT resolution, which is preset to 256, the computing load can be reduced and the capacity freed up can be used for a higher sampling rate.
2. vibration measurement
When you click on the Start button, the measurement begins, the sensor data is recorded and displayed in the acceleration-time diagram. In addition, the corresponding frequency spectrum is displayed parallel to the time signal (c). Click again to stop the measurement. The data can now be evaluated and analyzed. By clicking the Table button, the results can be viewed in tabular form (d). The time steps of the signal sampling and the recorded accelerations in the direction of the three coordinate axes are listed (e).
3. evaluation of the measurement
During the measurement, the maximum recorded acceleration amplitudes and the natural frequencies are displayed directly in the diagram for the respective directions. After stopping the measurement, there are various evaluation options. By tapping two points in the time signal, a range can be selected in which the damping is to be determined. A third tap determines the attenuation using the decaying amplitude method (c). This requires clearly separated eigenmodes. By clicking the Analysis button, the maximum acceleration and velocity amplitude as well as the maximum weighted vibration severity KBFmax according to DIN 4150-2 can be displayed. An evaluation of vibrations/shocks in accordance with ISO 10137, VDI 2038 and DIN 4150-3 is also integrated.
4. Saving and loading the measurement
The measurements can be saved in csv format using the Save button and used for further processing. The name of the file can be freely selected; the date and time of the measurement are preset. It is also possible to call up saved measurements again using the load function in the “iDynamics” app.
5. post-processing:
To achieve clear and precise measurement results with (less sensitive) mobile devices, post-processing of the raw data can be advantageous.
There are several post-processing options under the Post-Processing tab, which are briefly explained here
5.1 Filtering
Several filter algorithms are implemented in the app for post-processing the measurement recordings. Special algorithms for high-pass, band-pass and low-pass filters with Butterworth characteristics are implemented in accordance with the German standard: DIN 45669-1[29].
The following figure shows the application of the filters to the vibration measurements of a frame with two degrees of freedom. With the filter option, the different eigenmodes can be isolated and evaluated separately.
5.2 Smoothing
With small vibration amplitudes, the acceleration recordings of smartphone sensors have a relatively low signal-to-noise ratio. To reduce the disruptive influence of noise, a 5-point smoothing algorithm was implemented, which calculates the mean value of the four nearest values. Up to three smoothing iterations can be performed. However, caution is advised, as at low sampling rates, high-frequency signals may be “smoothed out” in a falsifying manner. The following figure shows the difference between a measurement before and after smoothing. It can be seen that applying the smoothing algorithm to a 1.8 Hz oscillation leads to excellent results, while it has a detrimental effect with a 12.5 Hz oscillation. The measurement shown here was carried out with a sampling rate of 50 Hz.
6. Current user limits
Since Android version 6.0 (2015), Google requires the installation of a 12-bit acceleration sensor and recommends the use of a 16-bit sensor for smartphones . By complying with these compatibility requirements, most of the latest smartphone sensors have sufficient resolution. For most devices, noise is the limiting factor that affects the quality of the measurements. The effective value of the measured noise acceleration can be used to compare the suitability of the devices with one another. The value is determined by taking longer measurements in a very quiet environment (e.g. on the floor slab in the basement) without disturbing ambient conditions (e.g. slamming doors, people walking, devices). The effective value of the noise during the entire measurement time can be read off under the “Analysis” tab with aRMS(t=∞). The following table contains empirical values for effective noise values of some smartphones.
A detailed description of the app with sample applications and evaluation options can be found in the Bauingenieur Ausgabe 05/2017*.
*Sadegh-Azar, H.; Feldbusch, A.; Agne, P.; Kögel, C.: Schwingungsuntersuchungen mit dem Smartphone und Tablet, Bauingenieur, Mai 2017
Have fun with the iDynamics app.
If you have any problems or questions, please send an e-mail to sdt.sekretariat(at)bauing.uni-kl.de