Executing the modal analysis of a structure allows users to analyze crucial modal parameters. The natural frequencies, damping and mode shapes of the unit under test help in adjusting the mechanical properties by optimizing the design and improving the structural behavior of the test unit.
In this case, the modal characteristics of a robot are acquired by performing experimental modal analysis. This robot is used in combats where the winner tries to destroy the opponent by causing damage or breaking crucial components. A quick modal survey reveals the natural frequencies, damping and mode shapes of the robot which helps users identify weak links in the structure. Hence, modal analysis plays a significant part in optimizing the design and performance of these robots.
A hammer impact test is carried out using a modal hammer and a uni-axial sensor to obtain the vibration characteristics of a robot. The short pulse induced with a modal hammer excites a wide range of frequencies. To avoid the mass loading effect that is induced with a roving response measurement, the modal test is carried out with a roving excitation method.
The efficient Spider-80X and the latest 10.0 release of the EDM Modal software is used to execute the hammer impact modal test.
Figure 1. Modal Hammer Testing of the robot
A mesh configuration of 30 measurement points is distributed through the blades to obtain good spatial resolution for the mode shapes. Using a flexible band and bungee cord, the robot is hung to imitate a free-free boundary condition (as shown in the experimental setup). The modal hammer with the metal tip is roved through the various measurement points. The responses to the impact excitations are captured using a uni-axial accelerometer that is placed accordingly. Measuring the excitation and response in the vertical direction facilitates in obtaining the out-of-plane mode shapes.
Figure 2. Robot Geometry
For this modal test, the modes up to 3.5 kHz frequency range are of interest and therefore a sampling rate of 8 kHz is set. A block size of 8192 is selected. A fine frequency resolution of 0.9765 Hz is produced with these configuration settings. Measurements of higher accuracy and reduced noise are obtained by linearly averaging 3 blocks of data at each measurement DOF.
The broadband spectrum from the metal tip on the modal hammer assists in exciting the modes up to a frequency range of 3.5 kHz. The large block size implemented helps ensure the natural decay of the structure response without introducing the conventional force-exponential window. Another added advantage of this block size is a finer frequency resolution. With this setup, there will be no leakage and a uniform window can be selected.
Figure 3. Hammer Impact Measurement of the robot
The coherence plot helps validate the measurement results; it looks good from the above screenshot. The valleys in the coherence plot occur at the anti-resonances which indicates that the response level is relatively lower at these corresponding frequencies. So overall, the inputs and outputs are well correlated in the desirable frequency range.
The DOF of the excitation and response for the measured FRF signals are switched automatically for this roving excitation test. This can be observed from the Modal Data Selection tab.
The FRF measurement shows good dominant peaks in the 0-3500 Hz frequency band. Overlapping the 30 measured FRFs, several modes can be identified. The good alignment of the peaks indicates the measurement results are good and there was no mass loading effect induced.
Figure 4. Modal Data Selection tab showing the magnitude and phase part of all overlapped FRFs
The Complex Mode Indicator Function (CMIF) is used to locate the modes in the desired frequency range. In addition, the summed FRF is also observed to identify the modes. The new Poly-X method is used to curve-fit the FRF’s to procure the following stability diagram. Six flexible modes are selected within the desired frequency range.
Figure 5. Mode Indicator Functions to locate and identify the modes in the desired frequency range
The stable poles (stable frequency and stable damping) are selected to obtain the natural frequencies and the damping ratios of the interested modes. The residue calculation facilitates in obtaining the mode shapes associated for each of the modes.
Figure 6. 1st Order Bending Mode of the Robot at 40 Hz
Figure 7. 1st Order Torsion Mode of the Robot at 345 Hz
Figure 8. Higher Order Bending Mode of the Robot at 509 Hz
Figure 9. A combination of Bending & Torsion Mode of the Robot at 2004 Hz
A quick sweep through the measured FRF dataset can also be carried out to visualize the deformation of the robot at each of the frequencies. With this spectrum data, the modes are uncoupled and hence the operational deflection shapes and the transition through these different frequencies can be analyzed and studied using the ODS function.
Figure 10. Frequency Domain ODS of the Robot
The results display the strength of the Spider-80X DAQ and the efficiency of the EDM Modal software to execute sophisticated modal test on intricate structures.
To learn more about EDM Modal software, visit: www.crystalinstruments.com/edm-modal-testing-and-analysis-software/