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Difference between revisions of "Advanced Swept Sine (A2S)"
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==Introduction== | ==Introduction== | ||
Traditionally transfer | Traditionally transfer functions of a system are measured using FFT based measurement with broadband excitation (as random noise or multi-sine). With only one average all frequency points of transfer function can be measured. Some limitations appear as: | ||
* Transfer function frequency resolution is limited to FFT resolution, | * Transfer function frequency resolution is limited to FFT resolution, | ||
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* Preventing overload, | * Preventing overload, | ||
* Measurement at any frequency | * Measurement at any frequency, | ||
* High frequency ratio, combined with high frequency resolution, | * High frequency ratio, combined with high frequency resolution, | ||
* Fine control of level, | * Fine control of level, | ||
* Measurement time optimized with frequency, | * Measurement time optimized with frequency, | ||
* Excitation level depending | * Excitation level depending on frequency, | ||
* No error between frequency lines due to non-linearity. | * No error between frequency lines due to non-linearity. | ||
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* First, sine generator sweeps smoothly to target frequency. | * First, sine generator sweeps smoothly to target frequency. | ||
* Then Advanced Swept Sine waits for a user-defined fixed or frequency | * Then Advanced Swept Sine waits for a user-defined fixed or frequency dependent delay in order to guarantee stabilization of system under measurement. | ||
* And finally using selected lines from an averaged cross-FFT measurement; transfer function for this frequency point is computed and saved. FFT average time can be either fixed or frequency | * And finally using selected lines from an averaged cross-FFT measurement; transfer function for this frequency point is computed and saved. FFT average time can be either fixed or frequency dependent. | ||
Advanced Swept Sine can adjust and limit generator output level in order to get either predefined or limited level at input and output of system under measurement. Level can be frequency dependent, for example to adjust displacement on a shaker at each frequency. | |||
=== | ===Splitting measurement into spans=== | ||
A complete measurement frequency range can be broken into up to 8 separate frequency spans; this multi-span capability is useful, for example, to define different frequency step sizes on the whole frequency range or to adjust excitation level on different frequency ranges. | A complete measurement frequency range can be broken into up to 8 separate frequency spans; this multi-span capability is useful, for example, to define different frequency step sizes on the whole frequency range or to adjust excitation level on different frequency ranges. | ||
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[[Image:sweptsine_advanced_02.png|700px]] | [[Image:sweptsine_advanced_02.png|700px]] | ||
In the last two ways, for each frequency point Advanced Swept Sine automatically adjusts generator output in order to get target | In the last two ways, for each frequency point Advanced Swept Sine automatically adjusts generator output in order to get target values with a user-defined tolerance. It stops adjustment after the user-defined maximum number of tries. | ||
The Advanced Swept Sine also includes user defined signal level limitations at system input or output. If measured value is greater than defined limit Advanced Swept Sine automatically lowers target level value and retries frequency point measurement. | The Advanced Swept Sine also includes user defined signal level limitations at system input or output. If the measured value is greater than defined limit Advanced Swept Sine automatically lowers target level value and retries frequency point measurement. | ||
Level adjustment function associated with signal limiting function can help | Level adjustment function associated with signal limiting function can help measurements, reducing input level at a resonance (or pole) to avoid non-linearity, or increasing level where there is anti-resonance (or zero) to get better measurement. | ||
===Multi excitations=== | ===Multi excitations=== | ||
For | For measurements on complex mechanical systems that need multi excitation with different amplitude or phase, the multi generator outputs of OROS 3-Series can be used. | ||
==Advanced Swept Sine implementation== | ==Advanced Swept Sine implementation== | ||
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[[Image:sweptsine_advanced_04_429.png|framed|none]] | [[Image:sweptsine_advanced_04_429.png|framed|none]] | ||
* The generator frequency (and/or the amplitude) sweeps to the new value | * The generator frequency (and/or the amplitude) sweeps to the new value. | ||
* The analyzer waits for the system under measurement to stabilize. This delay is specified as a minimum time and/or a minimum number of sine periods. This delay can also be set down to zero. | * The analyzer waits for the system under measurement to stabilize. This delay is specified as a minimum time and/or a minimum number of sine periods. This delay can also be set down to zero. | ||
* The analyzer does real measurement using averaging. The average time is specified as a minimum time (greater than 30 ms) and/or a minimum number of sine periods (greater than 10). | * The analyzer does real measurement using averaging. The average time is specified as a minimum time (greater than 30 ms) and/or a minimum number of sine periods (greater than 10). | ||
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[[Image:sweptsine_advanced_14.png|framed|none]] | [[Image:sweptsine_advanced_14.png|framed|none]] | ||
'''Note:''' for all these | '''Note:''' for all these types of graphs, all settings for the graphs available in NVGate are also available with Advanced Swept Sine by right clicking on the graph or graph window. | ||
===Edit menu=== | ===Edit menu=== | ||
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[[Image:sweptsine_advanced_15.png|framed|none]] | [[Image:sweptsine_advanced_15.png|framed|none]] | ||
It is possible to select the number of | It is possible to select the number of responses (depending of the hardware connected, that means this number of responses could be between 1 and 31). | ||
The next setting is the number of | The next setting is the number of inputs the user wants to control. Note that input 1 is not available because it is automatically the reference input. | ||
By clicking on <nowiki>'</nowiki>OK<nowiki>'</nowiki>, the inputs properties will be displayed. | By clicking on <nowiki>'</nowiki>OK<nowiki>'</nowiki>, the inputs properties will be displayed. | ||
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Editing settings is available through menu "Edit" <nowiki>>></nowiki> "Edit Settings" | Editing settings is available through menu "Edit" <nowiki>>></nowiki> "Edit Settings" | ||
This command opens a new window displaying <u>General Settings</u>, applicable for all spans, in the upper part and <u>Span Settings</u>, specific for each span, in the lower part. | This command opens a new window displaying <u>General Settings</u>, applicable for all spans, in the upper part and <u>Span Settings</u>, specific for each span, in the lower part. | ||
[[Image:sweptsine_advanced_16.png|framed|none]] | [[Image:sweptsine_advanced_16.png|framed|none]] | ||
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This setting defines the allowed tolerance between the target level and the measured level, which validates one frequency measurement point. | This setting defines the allowed tolerance between the target level and the measured level, which validates one frequency measurement point. | ||
'''Note''': the level limiting function (if enabled) has | '''Note''': the level limiting function (if enabled) always has priority on level tolerance. | ||
'''Max tries (from 1 to 100):''' | '''Max tries (from 1 to 100):''' | ||
This setting defines for a frequency measurement point the maximum number of tries in order to reach target level or to | This setting defines for a frequency measurement point the maximum number of tries in order to reach target level or to confrom with level limits. | ||
'''Tips:''' We recommend to set this value at least at 3 in order to process measurements with either reference or response level control or active level limiting. | '''Tips:''' We recommend to set this value at least at 3 in order to process measurements with either reference or response level control or active level limiting. | ||
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'''Note:''' this function cannot guarantee that a level greater than user-defined limit will never appear. It guarantees that a transfer function point measurement is validated only if the measured level is below this limit. | '''Note:''' this function cannot guarantee that a level greater than user-defined limit will never appear. It guarantees that a transfer function point measurement is validated only if the measured level is below this limit. | ||
'''Tips:''' this function is very useful when user wants to avoid | '''Tips:''' this function is very useful when user wants to avoid too large of a response level (with perhaps non linearity) due to a resonance when using constant excitation (and/or reference) level. | ||
'''RMS reference level limit:''' | '''RMS reference level limit:''' | ||
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This setting specifies if the frequency steps for the span will increase in a logarithmic (<u>ratio</u> between 2 consecutive frequencies is constant as in a geometric progression) or linear fashion (<u>difference</u> between 2 consecutive frequencies is constant as in an arithmetic progression). | This setting specifies if the frequency steps for the span will increase in a logarithmic (<u>ratio</u> between 2 consecutive frequencies is constant as in a geometric progression) or linear fashion (<u>difference</u> between 2 consecutive frequencies is constant as in an arithmetic progression). | ||
This setting defines the step between | This setting defines the step between two measurements. The Log sweep gives more points in low frequency, and less point in high frequency, compared to Lin sweep. It is useful for giving a plot in which attenuation or gain of n dB/octave appears as a line. | ||
'''Frequency point number (from 1 to 10 000):''' | '''Frequency point number (from 1 to 10 000):''' | ||
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'''Note:''' Large values produce higher frequency resolution at the cost of longer acquisition time. Boosted mode can be used to reduce measurement time. | '''Note:''' Large values produce higher frequency resolution at the cost of longer acquisition time. Boosted mode can be used to reduce measurement time. | ||
'''Tips: '''Number of frequency | '''Tips: '''Number of frequency points includes low and high frequency points. So, for example, to get frequency points from 200 Hz up to 800 Hz with 100 Hz linear spacing <nowiki><</nowiki>Frequency point number<nowiki>></nowiki> must be set to 7. | ||
'''Active:''' | '''Active:''' | ||
This setting | This setting lets you skip the span during measurement. If set to NO the Advanced Swept Sine smoothly stops the generator at processing end of previous span and goes to process next active span starting smoothly the generator. | ||
'''Tips:''' this capability can be used, for example, to redo transfer function measurements on a specific frequency range of interest. | '''Tips:''' this capability can be used, for example, to redo transfer function measurements on a specific frequency range of interest. | ||
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In the control Panel 3 tabs have been added with several settings of NVGate. | In the control Panel 3 tabs have been added with several settings of NVGate. | ||
* Reference and Response: Input 1 is called <nowiki>'</nowiki>Reference<nowiki>'</nowiki> and Input N (N is the number of the input selected by the user) <nowiki>'</nowiki>Response<nowiki>'</nowiki> and | * Reference and Response: Input 1 is called <nowiki>'</nowiki>Reference<nowiki>'</nowiki> and Input N (N is the number of the input selected by the user) <nowiki>'</nowiki>Response<nowiki>'</nowiki> and these two tabs displayed settings of these inputs. Between brackets is displayed the number of the input corresponding. | ||
[[Image:sweptsine_advanced_17.png|framed|none]] | [[Image:sweptsine_advanced_17.png|framed|none]] | ||
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===Saving settings=== | ===Saving settings=== | ||
Settings of Advanced Swept sine measurement are | Settings of Advanced Swept sine measurement are saved through Advanced Swept sine, using '''Save '''or '''Save''' '''as''' from the '''File''' menu. | ||
Settings are saved with the extension .sws. | Settings are saved with the extension .sws. | ||
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No visible difference in the coherence function | No visible difference in the coherence function. | ||
[[Image:sweptsine_advanced_20.png|framed|none]] | [[Image:sweptsine_advanced_20.png|framed|none]] | ||
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First measurement is an overview on the bandwidth 100 Hz to 10 kHz in order to determine resonances and anti-resonances. | First measurement is an overview on the bandwidth 100 Hz to 10 kHz in order to determine resonances and anti-resonances. | ||
The coherence and the transfer function have been displayed | The coherence and the transfer function have been displayed. | ||
[[Image:sweptsine_advanced_24.png|framed|none]] | [[Image:sweptsine_advanced_24.png|framed|none]] | ||
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[[Image:sweptsine_advanced_25.png|framed|none]] | [[Image:sweptsine_advanced_25.png|framed|none]] | ||
We | We notice a resonance around 1.5 kHz. | ||
In order to visualize properly these two phenomena, we use a multi-spans measurement with more points around interesting frequencies. | In order to visualize properly these two phenomena, we use a multi-spans measurement with more points around interesting frequencies. | ||
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</font>On this part we can see the anti-resonance on both graphs in order to determine the frequency absorbed by the muffler. | </font>On this part we can see the anti-resonance on both graphs in order to determine the frequency absorbed by the muffler. | ||
'''Note''': | '''Note''': These two examples are quite basic, but it is also possible to control the level on the response and/or on the reference with the level limiting of the Advanced Swept Sine solution. | ||
==Specifications== | ==Specifications== | ||