Simcenter Testlab Calibration
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:
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:
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):
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).
In this case, 1000 Hz and 94 dB RMS are settings from the calibrator:
2. Make sure the input box is checked ON as shown in Figure 4.
3. Attach the calibrator to the microphone, as shown in Figure 5. Turn on 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.
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.
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’.
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):
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:
In order to change the field called ‘Nominal Sensitivity’, scroll to the right in the ‘Calibration’ worksheet (Figure 10).
With the appropriate nominal sensitivity value, the ‘Channel Status’ will be ‘OK’ after a calibration is performed correctly:
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):
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.
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):
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.
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):
Calibration: Triaxial Accelerometer
In the ‘Calibration’ worksheet:
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).
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.
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.
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