Use cases of FREQi card required to define frequencies for Frequency Response (FRF) Analysis.

Rajashri_Saha

OBJECTIVE – Use cases of FREQi card required to define frequencies for Frequency Response (FRF) Analysis.

Topic description: FREQi card defines the range of Frequencies in which response needs to be extracted in FRF Analysis. To create this, it is required to first create a Load collector with card image as FREQi and then select any of the 6 FREQ available cards in Optistuct to define the Frequency range.

Below is the detailed explanation of these cards and their use cases.

1. FREQ : This card will calculate the response for user defined frequency/frequencies.

By this FREQ crad response can be calculated at a single frequency or N no. of frequencies defined by user. Only for those frequency output will be generated.

 

For the above defined frequency, response will be calculated at 36.77Hz.

File used for this study is Freq.fem.

2. FREQ1: FREQ1 will give a response plot for a set of frequency in a linear increment with constant difference in the frequency.

F1 – Lower Frequency Bound                      DF – Frequency Increment                   

NDF–No. of Frequency Increment  

fi = F1 + DF * (i -1)

Where i = 1 to (NDF + 1)

File used for this study is Freq1.fem.

3. FREQ2: This will give a response plot for a set of frequency in a logarithmic increment.

F1 – Lower Frequency Bound                F2 – Upper Frequency Bound               

NF – No. of Logarithmic Increments

For the above mentioned FREQ2 values the response plot will start from 1Hz frequency and it will calculate response 99 logarithmic increment till 100 Hz. As it uses logarithmic increment lower frequency has more calculation points compared to the higher frequency.

File used for this study is Freq2.fem.

4. FREQ3: This will give a response plot for a set of frequency in a linear increment as well logarithmic increment.

F1 – Lower Frequency Bound                     F2 – Upper Frequency Bound    

NEF – No. of Increment in subset of frequency, CLUSTER – Position of calculation point

 

In FREQ3 frequency will form a subset (i.e) from F1  to 1^st Natural Frequency will form a set, similarly 1^st Natural frequency to 2^nd Natural frequency will form another subset, like this subset are formed till the F2 Frequency.

• NEF value is increments in the Subset (i.e) 1^st mode is 39.33 Hz so from 1Hz to 39.33Hz there will 99 increments.

File used for this study is Freq3.fem.

5. FREQ4: This will give a response plot for Natural frequency and nearby frequencies.  It is Used only in the Model Superposition method.

F1 – Lower Frequency Bound                F2 – Upper Frequency Bound   

            

• If NFM is 3, one response is calculated at the lesser frequency and other is on higher frequency of natural frequency. (i.e) if natural frequency is 39.33 Hz then one frequency is 35.4Hz  which is lesser frequency and other is 43.26Hz which is a higher frequency.
• FSPD is used to lesser frequency and higher frequency (i.e) (1-FSPD)*Natural Frequency for lesser frequency & (1+FSPD)*Natural Frequency for Higher frequency

File used for this study is Freq4.fem.

6. FREQ5:  This will give a response plot for Natural frequency or fraction of Natural frequency.  It is Used only in the Model method.

F1 – Lower Frequency Bound                F2 – Upper Frequency Bound               

FR – Fraction of frequency

If FR = 1 then calculation will be

 Fi = FR* Natural Frequency 

Here in table, we have given FR as 0.5, So 39.33*0.5=19.66

File used for this study is Freq5.fem.