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Simcenter Testlab Calibration

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Siemens Experimenter

(view in My Videos)

 

Simcenter Testlab Calibrationcalibration_icon.jpg 

 

This article details how to use Simcenter Testlab (formerly called LMS Test.Lab) to do calibration verification of transducers and their sensitivities.  It covers a few different calibration scenarios:

  • A single microphone
  • Multiple microphones with minimal software interaction
  • Triaxial accelerometer by adjusting signal to noise ratio to overcome crosstalk

In a calibration check, the voltage output per engineering unit of a transducer is verified.  Typically, for microphones this means checking the mV/Pascal output. For accelerometers the transducer output is usually in mV/g.

 

Equipment Needed for Calibration

 

The equipment needed to perform a calibration check for accelerometers and microphones is shown in Figure 1:

  • SCADAS data acquisition system
  • Computer with Simcenter Testlab software
  • Microphones or accelerometers
  • Microphone calibrator
  • Accelerometer calibrator
  • Appropriate cables

 

equipment_needed.pngFigure 1: Equipment needed to accelerometer and microphone calibration check.

A Single Microphone

 

First, a calibration check of a single microphone is discussed.

 

Channel Setup: Single Microphone

 

Select the ‘Channel Setup’ worksheet. An Excel-like table contains a list of all the channels in the connected SCADAS frontend. Each row corresponds to one channel on the frontend.

 

In the ‘Channel Setup’ worksheet (Figure 2):

  1. Verify the ‘Actual sensitivity’ field is in mV/Pa.
  2. Change the ‘Input mode’ to ICP.
  3. Set the channel group id to ‘Acoustic’.
  4. Turn on the input channel by checking the box next to the appropriate input.
  5. Attach the microphone to a dynamic channel on the SCADAS frontend.

 

channel_setup_single_mic.jpgFigure 2: In the ‘Channel Setup’ worksheet, turn on the input channel and use the proper settings.

Microphones are often ICP.  If non-ICP, polarized microphones are being used, a SCM-VM8 card can be used, and the polarization can be set to 200V and power supply set to 28V as needed. The input mode would be ‘Voltage AC’ rather than ICP.

 

Calibration: Single Microphone

 

Click on the ‘Calibration’ worksheet.  In this worksheet, the microphone calibrator information will be entered and a calibration performed.

 

A microphone calibrator produces a sound pressure at a specific frequency and amplitude level so the current sensitivity (in mV/Pa) can be calculated.  It is a good idea to recalculate the sensitivity value, as it can change over time, in order to ensure the accuracy of a measurement.

 

In the ‘Calibration’ worksheet:

 

1. Enter the specific information for the calibrator being used (Figure 3).

 

calibrator_settings_microphone.jpgFigure 3: Enter calibrator information at the top of the ‘Calibration’ worksheet.In this case, 1000 Hz and 94 dB RMS are settings from the calibrator:

  • Frequency -   Microphone calibrators can have different output frequencies, but 1000 Hz is common. This is because the A-weighting curve has 0 dB of attenuation or gain at that frequency, so it cannot affect the calibration value if it is applied or not applied.
  • Amplitude - Some calibrators have switches with different amplitude settings (for example: 94 and 114 dB).  Make sure the switch is in the correct position for the calibration.  An amplitude of 114 dB would be used if high amplitude level noise signals are to be measured.

 

2. Make sure the input box is checked ON as shown in Figure 4.

 

calibration_worksheet_single_mic.jpgFigure 4: In the ‘Calibration’ worksheet, turn on the channel to be calibrated.

3. Attach the calibrator to the microphone, as shown in Figure 5. Turn on the calibrator.

 

calibrator_and_microphone.jpgFigure 5: Position the microphone into the calibrator.

4. Press the ‘Check’ button in the Status Panel as shown in Figure 6. The message in the Calibration status area will change from ‘Not Active’ to ‘Checked’. If you have pre-gain or pre-weighting set on a channel, a warning will be issued listing the affected channels.

 

Check_and_Start.jpgFigure 6: Press the ‘Check’ and ‘Start’ buttons to start viewing calibration signal.

5. Press the ‘Start’ button. This initiates the calibration. The calibration time history and frequency spectrum show on the left hand side.

 

If the calibrator frequency measures as expected, the calibration starts.  An orange message will indicate that calibration is in progress as shown in Figure 7. This orange message area is designed to allow viewing from across a room so a single person can do the calibration.

 

calibrating_message.pngFigure 7: The orange message area indicates that calibration is in progress.

When the calibration is finished, the message area turns green as shown in Figure 8. The newly calculated calibration values are shown in a column called ‘New Sensitivity’.  The previous calibration values are shown in the column called ‘Actual Sensitivity’. 

 

Calibration_finished.pngFigure 8: The green message area indicates that calibration is finished.

6. Press the ‘Accept’ button to update the calibration values. The ‘New Sensitivity’ values will replace the ‘Actual Sensitivity’ values. Both fields will now have the same values, and will be active everywhere in Simcenter Testlab, including the channel setup. Be sure to save the project so the new calibration values are permanently stored.

 

Channel Status: What does ‘Sensitivity Difference’ mean? And why might it be OK?

 

The channel status could come back with several different messages (Figure 9):

  • ‘OK’ – Calibration successful.
  • ‘OK, Sensitivity Difference’ – The sensitivity difference message is due to comparison to the nominal sensitivity (see explanation below). If not using the nominal sensitivity field, and the value is close to expectations, then the calibration is successful.
  • ‘Failed, SNR NOK’ – SNR NOK is an abbreviation/acronym for ‘Signal to Noise Ratio Not OK’. This means there was a problem during calibration, and that it needs to be redone.

 

channel_status.jpgFigure 9: After calibration is finished, ‘Channel Status’ field can be different.

 The channel status ‘OK, Sensitivity Difference’ is acceptable in many cases.  If the calibration sensitivity is close to the expected value, it can be accepted:

  • The message often occurs when the nominal sensitivity field was not filled out in advance, and the difference between the newly calculated sensitivity value and the nominal sensitivity value is greater than 10%. This percent difference can be changed from the 10% default under the ‘Advanced…’ button in the Calibration worksheet.
  • The ‘Nominal Sensitivity’ is a representative value for a specific model of transducer. For example, a certain class of microphones may have a 45 mV/Pa sensitivity value, but individual microphones may have values like 45.34 mV/Pa, 44.56 mV/Pa, etc.
  • In this case, if the nominal value was set to 45 mV/Pa for this microphone, there would not be a significant difference between the newly calibrated value and the nomimal. The channel status would be ‘OK’.

In order to change the field called ‘Nominal Sensitivity’, scroll to the right in the ‘Calibration’ worksheet (Figure 10).

 

nominal_sensitivity.jpgFigure 10: The nominal sensitivity field can be changed by scrolling to the right in the Channels section of the Calibration worksheet.

With the appropriate nominal sensitivity value, the ‘Channel Status’ will be ‘OK’ after a calibration is performed correctly:

  • The nominal sensitivity can also be entered in the Channel Setup worksheet. The value of 45 mV/Pa is common for microphones, 10 mV/g or 100 mV/g are common for accelerometers.
  • Whether the nominal sensitivity was filled in appropriately, or not, the calculated calibration value is not affected.  The ‘Sensitivity difference’ message is only based on the comparison between the nominal value and the calculated value.

Rather than calibrating one microphone at a time, multiple microphones can be calibrated in a single calibration session.

 

Multiple Microphones in One Session

 

If there are several microphones to calibrate, they can all be done in one calibration session by a single person. This mode allows an operator to go into a test cell and calibrate multiple microphones without walking back and forth to the computer. Some microphone arrays consist of 30 or more microphones – that is a lot of walking back and forth!

 

This multi-calibration session will be illustrated using three microphones.

 

Channel Setup: Three Microphones

 

Attach three microphones to the SCADAS system input channels as shown in Figure 11.

 

In the ‘Channel Setup’ worksheet (Figure 12):

  1. Turn on the first three available dynamic channels by checking the box next to the input name.
  2. The channel group id of each input should be set to ‘acoustic’.
  3. The input mode should be set to ‘ICP’.

 

three_microphones_channel_setup.jpgFigure 12: Turn on the first three input channels in the ‘Channel Setup’ worksheet.

Now the microphones are ready for calibration. Go to the ‘Calibration’ worksheet.

 

Calibration Worksheet: Three Microphones

 

In the ‘Calibration’ worksheet, click on the ‘Advanced…’ button on the upper right side of the Calibration worksheet (Figure 13). Turn OFF the ‘Timeout’ option by unchecking the box. By default, this box is checked ON.

 

advanced.jpgFigure 13: Turn the ‘Timeout’ to OFF to calibrate multiple microphones under the ‘Advanced…’ button.

With the ‘Timeout’ OFF, an operator can simply move the calibrator from microphone to microphone during the calibration process.

 

In the ‘Calibration’ worksheet (Figure 14):

  1. Make sure the additional input boxes are turned on.
  2. Attach the calibrator to the first microphone.
  3. Press the ‘Check’ button and then the ‘Start’ button. Once the calibration of the first microphone is finished, the message area will turn white and give the message ‘Detecting…’ as shown in Figure 12.The software is looking automatically for a calibration signal on another channel.
  4. Remove the calibrator from the first microphone and move it to the next microphone. The microphones can be calibrated in any order. For example, the third microphone channel can be calibrated before or after the first one.
  5. Continue this process until all three microphones are calibrated.

 

Detecting.jpgFigure 14: With multiple channels selected in the ‘Calibration’ worksheet, the ‘Detecting…’ message area means the next microphone can be calibrated.

Once the three microphones are calibrated, the message area will turn green and give the message ‘Finished’. Press the ‘Accept’ button, and the new sensitivity values will be active everywhere in Testlab. Save the project to make sure the changes are permanent.

 

This process can also be used on accelerometers.

 

Triaxial Accelerometer

 

To calibrate a triaxial accelerometer (which can measure three different directions simultaneously), the process is very similar to multiple microphones.  However, adjusting the ‘Signal to Noise Ratio’ parameter may be required to do all three directions in a single session.

 

Channel Setup: Triaxial Accelerometer

 

Go to the ‘Channel Setup’ worksheet.

 

Attach the accelerometer to the SCADAS frontend through the dynamic input channel using the appropriate cables. Each of the X, Y, and Z directions need to be matched to the proper cables.

See the Knowledge Base article: ‘Cool triaxial accelerometer tips’ for some useful setup tips about ensuring the directions follow the ‘right hand rule’.

 

In the ‘Channel Setup’ worksheet (Figure 15):

  1. Ensure that the three input channels are turned on.
  2. Set channel group id for each input to ‘vibration’, and set input mode to ‘ICP’.

 

triaxial_accelerometer_channel_setup.jpgFigure 15: Turn on the first three input channels with the described settings.

Calibration: Triaxial Accelerometer

accel_calibrator.jpgFigure 16: Attach the accelerometer to the top of the calibrator.

 

In the ‘Calibration’ worksheet:

  1. Change the unit to g, the frequency to 159.2 Hz, and the level to 1 measured in Rms. These settings are specific to the calibrator being used.
  2. Click the ‘Advanced…’ button and ensure that the timeout feature is still turned off.
  3. It may also be necessary to increase the ‘Signal to Noise Ratio’ parameter under the ‘Advanced…’ button. See discussion below.
  4. Make sure that the first three input channels are turned on in the ‘Calibration’ worksheet.
  5. Attach the accelerometer itself to the top of the calibrator using bees wax. Ensure that the direction that is facing up is either the X, Y, or Z direction as shown in Figure 16.
  6. Turn on the calibrator. It is good practice not to attach the accelerometer while the calibrator is running.
  7. Select the ‘Check’ button and then the ‘Start’ button to begin the calibration of the first direction.
  8. When the message ‘Detecting…’ comes up, turn off the calibrator, rotate the accelerometer so that a different direction is facing the calibrator, and turn it back on.
  9. Do this until each direction of the accelerometer has been calibrated.
  10. Once the ‘Finished’ message comes up, select ‘Accept’, and the new sensitivities will be active throughout Testlab. Save the project to make the changes permanent.

 

Crosstalk and Signal to Noise Ratio (SNR)

 

With a triaxial accelerometer, it is possible that a calibration value will be calculated on an off-direction channel.  This should not occur.

 

If channels other than the intended direction of the accelerometer are being calibrated, crosstalk is occurring between channels within the accelerometer. Ideally, while one direction is being calibrated, the ‘Channel Status’ of the other two directions should say ‘Detecting, SNR NOK’ and not have a calibration value calculated.

 

Crosstalk is when the primary input is in one direction, and the other directions pick up minute vibrations from the calibration signal. The software sees the minute vibration as a calibration signal, and causes it to calculate an erroneous calibration on the off-directions (Figure 17).

 

crosstalk.pngFigure 17: Crosstalk between different directions on triaxial accelerometer causes calibration problems.

To solve this problem, the Signal to Noise Ratio (SNR) parameter can be adjusted. The SNR is criteria is used by the software to decide if a calibration signal is present on any given channel.  The dB difference between the background noise and the calibration tone is evaluated, and the SNR of the signal must be higher than the setting for calibration to occur.

 

On the primary channel being calibrated, there is a large difference between the signal and the noise floor of the instrumentation.  On the other channels not being calibrated, the crosstalk signal is barely above the noise floor. By setting the signal to noise ratio parameter higher, the off direction channels will not be calibrated.

 

Go to the ‘Advanced…’ button in the top right of the window, and, under ‘Limits’, change the SNR level to a higher value, for example 60 dB, as shown in Figure 18.

 

SNR.pngFigure 18: Change SNR level to a higher value than the default of 30 dB.

Simcenter Testlab will then calibrate each direction of the triaxial accelerometer separately, one at time.

 

In the case of the three single microphones, there was no crosstalk, so the default signal to noise ratio was acceptable.  Because the three inputs of the triaxial accelerometer are in one device, the crosstalk is much higher.

 

Questions? Email jacklyn.kinsler@siemens.com or contact Siemens PLM GTAC support.

 

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