Convert SRS/PSD/Time functions

The Function Toolkit for Response Dynamics lets you perform conversions from one function type to another:

• Time to Spectrum (FFT)Note: The software uses a single-sided transform, resulting in half-peak values for the amplitudes in the frequency domain. Therefore, to use the frequency function as an excitation, you must multiply the function by 2 to get results that match the time-domain results. For more information about the conversion algorithm, see Fast Fourier Transformation (FFT).

• Time to PSD

• PSD to Time

• Time to SRS

• SRS to Time

For example, suppose that PSD excitations are given as the specifications for your analysis, but you want to perform a Transient analysis. You can use the PSD to Time conversion command in the Function Toolkit for Response Dynamics to generate time functions according to the given PSD specifications.

Convert PSD to Time

To convert from PSD to time:

1. On the Response Dynamics tab, → Function Toolkit, click PSD to Time.

2. Specify the AFU that contains the PSD function to convert from, and select the PSD function.

3. Set the time increment and number of time points.

4. Choose the interpolation method in the frequency domain: Log-Log, Linear-Linear, Linear-Log, Log-Linear.

5. Specify the file name and AFU for the time function to create.

6. Click OK or Apply to generate the function.

Note:

For more information about the conversion algorithm used, see PSD-Time conversion functions.

Convert Time to PSD

To convert from time to PSD:

1. On the Response Dynamics tab → Function Toolkit, click Time to PSD.

2. Specify the AFU that contains the time function to convert from, and select the time function.

3. Specify the file name and AFU for the PSD function to create.

4. Click OK or Apply to generate the function.

Note:

For more information about the conversion algorithm used, see PSD-Time conversion functions.

Convert SRS to Time

To convert from SRS to Time:

1. On the Response Dynamics tab, → Function Toolkit, click SRS to Time.

2. Specify the AFU that contains the SRS function to convert from, and select the SRS function.

3. Under Conversion Method, select Damped Sinusoid or Wavelet.Select Damped Sinusoid to perform the conversion by assuming that the response spectrum function is a summation of decaying sinusoids. This method is useful because many field shock environments look like sums of decaying sinusoids. If there is a shock event in the field, it will likely be damped sinusoids by the time it hits your structure.If you select Damped Sinusoid, the software continues to iterate until the function generated by the decaying sinusoids sufficiently matches the response spectrum function being converted. Use the Conversion Iterations option to specify the maximum number of iterations to perform. Select Wavelet to perform the conversion using the wavelet method. The wavelet method assumes you have a pulse of the following form: where:b = f/N**T = *N/(2f)*N = an odd integerThis type of pulse matches certain field conditions, though it does not match as many as the Damped Sinusoid method. The Wavelet approach is best for driving shakers and for designing to shaker environments, if your design goal is to pass or simulate the shaker test.If you select Wavelet, the software continues to iterate until the function generated by the wavelet satisfactorily matches the response spectrum function being converted. Use the Octave Spacing option to specify the resolution of the band for the frequency range of interest. Use the Number of Trials option to specify the maximum number of trials to perform with this method. Use the Conversion Strategy option to specify the method to use for the internal convergence process. You can use a Random method, a Reverse Sine Sweep method (where the highest-frequency wavelet has no delay and each lower-frequency wavelet has a progressively longer delay), or a Combination of the two.

4. Use the Number of Time Points menu to specify the time period (in seconds) for the generated time function.

5. Specify the file name and AFU for the time function to create.

6. Click OK or Apply to generate the function.The two generated SRS functions are the positive and negative SRSs that result from the generated time history. Comparing these to the original SRS indicates how good the synthesized time history is. Note that you might need several attempts at the conversion to get a reasonable transient function, by comparing the original SRS function to the synthesized SRS function.

Convert Time to SRS

To convert from Time to SRS:

1. On the Response Dynamics tab, → Function Toolkit, click Time to SRS.Note: For this conversion, the response is assumed to be from a single degree of freedom system with an excitation.

2. Specify the AFU that contains the time function to convert from and then select the time function.

3. Use the Frequency Axis menu to select how the frequency points should be spaced for the resulting function.

4. Specify values to define the Frequency Minimum and Frequency Maximum. Depending upon the Frequency Axis option you select, you may also need to define either a Frequency Increment or specify the number of Points per Decade.

5. Specify the Damping Ratio to use for the conversion.

6. Use the Response Type menu to specify which type of result you want to use from the conversion.Select Absolute Maximum to use the absolute maximum response.Select Positive Maximum to use the maximum positive response.Select Negative Maximum to use the maximum negative response.In most cases, you will probably want to select the Absolute Maximum option.

7. Use Pad end of function with zeros if needed so the software will modify the input function to set the function slope at the ends of the time range to zero. This is done by checking the last two function points and if they are not zero, additional zero points are added at the end.Note: Non-zero values and slope at the ends of the time range may result in undesired large low frequency responses. Using this setting adjusts your input data to eliminate these effects.

8. Specify the file name and AFU for the SRS function to create.

9. Click OK or Apply to generate the function.

How do I

Import test data into Response Dynamics

Create nodal and enforced motion excitations

Create distributed load excitations

Create static excitations

Create a drop impact or constant velocity impact simulation

Calculate random RMS functions from PSD input

Using Fast RMS Fitted PSD functions

Create rotating force excitations

Correlate two PSD excitations

Create DDAM excitations

Learn more

Excitation loads

Response Dynamics Function Toolkit

Using pulse functions for shock analysis

Velocity Impact excitations

Rotating forces and unbalanced masses

PSD correlation

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Function requirements by excitation type

Function parameters by event type

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Convert SRS/PSD/Time functions, Simcenter 3D 2021.1 Series

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