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Experiment AM2.1, "Free and Forced Vibrations," aimed to enhance the understanding of the relationship between natural frequency and added mass of a Rigid Beam and Spring and a Simply Supported Beam. Dunkerley's Theory was also employed to calculate the natural frequency of the Simply Supported Beam without added mass. The TM1016 and TecQuipment's Versatile Data Acquisition System (VDAS) were utilized to carry out this experiment. In the first part, loads ranging from 400 g to 2.2 kg were added onto an exciter attached to a Rigid Beam and Spring System, and time period readings were recorded using the VDAS.
The natural frequencies were then calculated and compared to theoretical values. In the second part, masses were added to a Simply Supported Beam System, and the same procedure was followed. Dunkerley's theory was applied to determine the natural frequency of the beam without any added mass. The results for natural frequencies were in line with expectations, with the Rigid Beam and Spring System producing relatively small errors, and the Simply Supported Beam yielding minor errors.
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Both sets of errors can be attributed to a combination of systematic and human errors. However, a significant error was observed when comparing the natural frequency of the Simply Supported Beam to the theoretical value, likely due to certain assumptions made during the experiment.
Natural frequency is the frequency at which a body or system vibrates when released from an initial displacement from its equilibrium position, without the influence of repeated external forces [1]. This natural frequency varies with changes in the mass of the system [2].
Understanding the natural frequencies of a system is crucial, especially when other moving or vibrating components are present within the system. If these components start vibrating at the same frequency as the system's natural frequency, resonance can occur [3]. Resonance leads to significant oscillations with larger amplitudes, which can have disastrous consequences in structures such as buildings and bridges [4].
This experiment consists of two parts, both aimed at determining the effect of added mass on natural frequency but on two different systems. The first system involves a Rigid Beam and Spring. In this part of the experiment, it is assumed that the beam remains "rigid" without bending, and the exciter is treated as a single point mass at a specific distance along the beam [5]. Natural frequencies are then analyzed based on these assumptions. The second system is a Simply Supported Beam, where the beam is supported at both ends, and the center of the beam oscillates. In this part, it is assumed that the mass of the beam is negligible, and the exciter is considered as a single point mass, allowing the application of Rayleigh's theory to calculate the natural frequency [5]. The results from this second part are used to validate Dunkerley's theory, which states that the addition of the reciprocals of individual elements' angular velocity of a whirling shaft equates to the reciprocal total angular velocity of the shaft. This theory can also be applied to calculate the natural frequencies of a beam, as both systems follow the laws of simple harmonic motion [5].
The aim of this laboratory experiment is to enhance the fundamental understanding of free vibrations and the impact of added mass on a system, along with the associated calculations. The objectives of this laboratory experiment are as follows:
The main method for both parts of the experiment was identical, with adjustments made for the Rigid Beam and Spring setup and the Simply Supported Beam setup.
The "Free and Forced Vibrations TM1016" apparatus was used to conduct the AM2.1 Free and Forced Vibrations experiment.
The apparatus was used in conjunction with a Versatile Data Acquisition System (VDAS) configured with Channel 1 Displacement (y-axis) set to 2 millimeters and Timebase (x-axis) set to 50 milliseconds. The SET ZERO controls were employed to ensure that the VDAS trace was centered on the displayed chart. Initially, there was no added mass, and the VDAS was started. The top of the exciter was struck with a flat hand, causing the beam to oscillate with an initial amplitude of a few millimeters. After a few seconds, the VDAS was stopped, and the resulting waveform, as shown in Figure 3, was displayed on the screen.
The difference in the x-coordinates on the graph within VDAS was then determined, and Equation 1 was used to calculate the time period of the oscillation.
(1)
T = 2π⁄Δx
Equation 2 was subsequently employed to calculate the natural frequency, and this value was recorded.
(2)
f = 1⁄T
The mass holder, weighing 200 g, was then added to the system. The "Added Mass" field in VDAS was updated, and the procedure was repeated to calculate the new natural frequency, which was recorded. Subsequently, five 400 g masses were individually added to the mass holder, updating the "Added Mass" field each time, and the natural frequencies were calculated and recorded after each addition.
For the Simply Supported Beam setup, the following values were relevant to the system:
The total mass moment of inertia of the system could be calculated using Equations 3 and 4, as shown below:
(3)
ΣI = Ib + Ie
(4)
Ie = me * Le2
Using Equation 5, the theoretical natural frequency was then calculated.
(5)
ftheory = 1⁄2π * √ΣI⁄k
A graph of natural frequency against added mass was plotted, and the theoretical and experimental values were compared and analyzed by calculating the percentage errors using Equation 6.
(6)
Percentage Error (%) = |(ftheory - fexp)| * 100⁄ftheory
For the Simply Supported Beam setup, the following values were relevant to the system:
Using Rayleigh's theory that provides a corrected beam mass, the effective mass was calculated using Equation 7 below.
(7)
meff = mc + me
Since the exciter effectively acted as a clamp, the beam could be treated as two cantilevers. This allowed the use of Equation 8 to calculate the theoretical natural frequencies.
(8)
ftheory = 1⁄2π * √meff⁄k
After calculating the theoretical natural frequencies, Equation 9 was applied.
(9)
fbeam = 2 * ftheory - fexciter
Once these values were computed, a graph of fbeam against total exciter mass was plotted. The trend line was extended until it crossed the vertical axis, and this value was used to calculate the theoretical natural frequency of the beam itself.
Equation 10 was then employed to calculate the natural frequency of the beam from the theory.
(10)
fbeam = 1⁄2π * √mbeam⁄k
The two values were subsequently compared and analyzed using Equation 6 above, and the values of fbeam were further analyzed through regression analysis using Equation 11.
(11)
[1/f2 = a·x + b]
The results obtained in the experiment with the Rigid Beam and Spring are presented in Table 1 below.
| Added Mass (kg) | Total Exciter Mass (kg) | Iexciter | Itotal | Natural Frequency f (Hz) | Experimental | Theoretical |
|---|---|---|---|---|---|---|
| 0.00 | 4.20 | 0.67 | 1.16 | 6.45 | 6.83 | |
| 0.20 | 4.40 | 0.70 | 1.19 | 6.25 | 6.74 | |
| 0.60 | 4.80 | 0.77 | 1.26 | 6.06 | 6.57 | |
| 1.00 | 5.20 | 0.83 | 1.32 | 5.97 | 6.40 | |
| 1.40 | 5.60 | 0.90 | 1.38 | 5.88 | 6.25 | |
| 1.80 | 6.00 | 0.96 | 1.45 | 5.80 | 6.11 | |
| 2.20 | 6.40 | 1.02 | 1.51 | 5.63 | 5.98 |
Table 1 illustrates the variations in the natural frequency of the beam as mass is added to the exciter. It is evident that as the total exciter mass increases, the natural frequency of the beam decreases. Additionally, the results in Table 1 align with the same trend observed in the theoretical values for the natural frequency.
Percentage errors were calculated using Equation 6, and the results are presented in Table 2 below.
| Natural Frequency f (Hz) | % Error | Experimental | Theoretical |
|---|---|---|---|
| 6.45 | -5.53% | 6.83 | |
| 6.25 | -7.27% | 6.74 | |
| 6.06 | -7.75% | 6.57 | |
| 5.97 | -6.72% | 6.40 | |
| 5.88 | -5.89% | 6.25 | |
| 5.80 | -5.12% | 6.11 | |
| 5.63 | -5.79% | 5.98 |
Table 2 displays the percentage errors for the Rigid Beam and Spring experiment. These errors range from -5.12% to -7.75%. While these values may seem relatively large, it is important to note that they fall within the range of less than 10%. As a result, these errors can be considered reliable for the given experiment.
Table 3 below presents the results obtained in the experiment using the simply supported beam.
| Added Mass (kg) | Total Exciter Mass (kg) | Effective Mass (kg) | Natural Frequency f (Hz) (x10-3) | Experimental | Theoretical |
|---|---|---|---|---|---|
| 0.00 | 4.20 | 5.00 | 15.15 | 15.50 | 4.36 |
| 0.20 | 4.40 | 5.20 | 14.70 | 15.19 | 4.63 |
| 0.60 | 4.80 | 5.60 | 14.50 | 14.64 | 4.76 |
| 1.00 | 5.20 | 6.00 | 13.90 | 14.15 | 5.18 |
| 1.40 | 5.60 | 6.40 | 13.50 | 13.70 | 5.49 |
| 1.80 | 6.00 | 6.80 | 13.20 | 13.29 | 5.74 |
| 2.20 | 6.40 | 7.20 | 12.80 | 12.91 | 6.10 |
Table 3 clearly demonstrates a change in natural frequency as mass is added to the simply supported beam system. It highlights the relationship between added mass and natural frequency, with the experimental values closely following the same trend as the theoretical values.
| x10-3 | fbeam (Hz) | % Error | Experimental (2 Cantilevers) | Calculated |
|---|---|---|---|---|
| 1.14 | 29.6 | -22.92% | 38.4 |
Table 4 provides the values calculated from Graph 3 and Equation 10. There is an 8.8 Hz difference between the two values. This discrepancy is attributed to the variations in the results presented in Table 3 due to experimental errors. Additionally, it should be noted that the experimental fbeam represents the frequency of two cantilevers rather than a single solid beam.
Table 5 below displays the percentage errors calculated using Equation 6.
| Natural Frequency f (Hz) | % Error | Experimental | Theoretical |
|---|---|---|---|
| 15.15 | -2.26% | 15.50 | |
| 14.70 | -3.23% | 15.19 | |
| 14.50 | -0.96% | 14.64 | |
| 13.90 | -1.77% | 14.15 | |
| 13.50 | -1.46% | 13.70 | |
| 13.20 | -0.68% | 13.29 | |
| 12.80 | -0.85% | 12.91 |
Table 5 demonstrates the percentage errors for the Simply Supported Beam experiment. These errors are considerably smaller than those observed in the first part of the experiment. Given that their magnitude is below 5%, these results can be considered highly reliable.
| Total Exciter Mass (kg) | Experimental values | ||||||
|---|---|---|---|---|---|---|---|
| 4.20 | 4.36 | -0.820 | 0.672 | 4.384 | -0.796 | 0.633 | |
| 4.40 | 4.63 | -0.550 | 0.303 | 4.539 | -0.641 | 0.411 | |
| 4.80 | 4.76 | -0.420 | 0.176 | 4.847 | -0.333 | 0.111 | |
| 5.20 | 5.18 | 0.000 | 0.000 | 5.156 | -0.024 | 0.001 | |
| 5.60 | 5.49 | 0.310 | 0.096 | 5.465 | 0.285 | 0.081 | |
| 6.00 | 5.74 | 0.560 | 0.314 | 5.773 | 0.593 | 0.352 | |
| 6.40 | 6.10 | 0.920 | 0.846 | 6.082 | 0.902 | 0.814 | |
| Mean: | 5.18 | Total: | 2.407 | Total: | 2.403 | R2 Value: | 0.998 |
Table 6 presents the results of the regression analysis conducted on Graph 3. The R2 value of 0.998 indicates a very strong positive linear correlation among the experimental values, highlighting the reliability and consistency of the data.
In the experiment with the Rigid Beam and Spring, the experimental results exhibited variations from the theoretical values, with the largest percentage error being 7.75%. This error might have arisen due to the unfamiliarity of the apparatus among the users, as they may have needed more time to understand and familiarize themselves with it. Human error could also have been introduced during the quick process of starting and stopping the VDAS. Additionally, reading the change in the x-axis coordinates for calculations could have introduced some measurement error. To reduce errors of this magnitude, it is advisable to repeat the experiment, taking multiple readings and calculating an average. In the Simply Supported Beam experiment, the percentage errors were much smaller, with the largest being 3.23%. Since these errors are below 5%, these results can be considered reliable. Nonetheless, errors may still have occurred due to the factors mentioned above. Another potential source of error in both cases is the assumption that damping due to air resistance is negligible. If the VDAS was not stopped quickly enough, it could have influenced the readings.
In both systems, there is a negative relationship between Natural Frequency and Added Mass, although the specific relationship is not evident in the tables. A clearer relationship might become visible on a graph with more readings taken with higher added masses. However, by examining the equations (Equations 3, 5, and 8), it is clear that the relationship is proportional to the square root of the total exciter mass (i.e., √total exciter mass). Upon closer examination of the graphs, a slight curve in the relationship can also be observed.
When the values of √total exciter mass were plotted against Total Exciter Mass, a clear positive linear correlation between the two became evident. Regression analysis yielded an R2 value of 0.998, indicating a very strong linear pattern in the experimental values, likely due to the increase in Total Exciter Mass. The beam's natural frequency was determined using Dunkerley's Method. Although this method is typically used for whirling shafts, it can also be applied to vibrating beams as they both obey the laws of simple harmonic motion [5]. The value of 1.14x10-3 was read from the graph, and the natural frequency of the entire beam was calculated as 29.6Hz. When compared to the value of 38.4Hz calculated using Equation 10, this resulted in a percentage error of -22.92%. This substantial error can be attributed to the frequency obtained from the graph representing the natural frequency of two cantilevers rather than a single entire beam. Additionally, errors in the experimental values from the experiment, as seen in Table 5, also contributed to this error.
In conclusion, the experimental laboratory has revealed a clear negative relationship between natural frequency and added mass, which follows the relationship √total exciter mass. The natural frequency of the Simply Supported Beam in the experiment was found to be 29.6 Hz.
Table 7: Experimental and Theoretical Natural Frequencies Against Added Mass for a Rigid Beam and Spring
| Added Mass (kg) | Natural Frequency (Hz) Experimental | Natural Frequency (Hz) Theoretical |
|---|---|---|
| 0 | 6.452 | 6.8319 |
| 0.2 | 6.25 | 6.7396 |
| 0.6 | 6.061 | 6.5657 |
| 1 | 5.97 | 6.4045 |
| 1.4 | 5.882 | 6.2547 |
| 1.8 | 5.797 | 6.1149 |
| 2.2 | 5.634 | 5.9841 |
Table 8: Experimental and Theoretical Natural Frequencies Against Added Mass for a Simply Supported Beam
| Added Mass (kg) | Natural Frequency (Hz) Experimental | Natural Frequency (Hz) Theoretical |
|---|---|---|
| 0 | 15.15 | 15.4952 |
| 0.2 | 14.7 | 15.1943 |
| 0.6 | 14.5 | 14.6418 |
| 1 | 13.9 | 14.1454 |
| 1.4 | 13.5 | 13.6963 |
| 1.8 | 13.2 | 13.2875 |
| 2.2 | 12.8 | 12.9132 |
Table 9: Dunkerley's Theory - 1/f^2 Against Total Exciter Mass
| Total Exciter Mass (kg) | 1/f^2 x10^-3 |
|---|---|
| 4.2 | 4.3569 |
| 4.4 | 4.6277 |
| 4.8 | 4.7562 |
| 5.2 | 5.1757 |
| 5.6 | 5.4870 |
| 6 | 5.7392 |
| 6.4 | 6.1035 |
Laboratory Report: Changes in Natural Frequency. (2024, Jan 04). Retrieved from https://studymoose.com/document/laboratory-report-changes-in-natural-frequency
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