Typically, the signals that can be input to analysis devices such as FFT analyzers are voltage signals.
When measuring physical quantities such as acceleration, velocity, force, and sound pressure, detectors (sensors) that convert these physical quantities into voltages are used. For example, if there is an acceleration detector that outputs a voltage signal proportional to the magnitude of acceleration (m/s²), the measured voltage value can be converted to acceleration and displayed using its proportionality constant (sensitivity).
This series will introduce how to set up analysis equipment using various detectors (sensors) and amplifiers, and following on from the previous two installments, we will focus on acceleration detectors.
This time, we will introduce the procedure for calibrating acceleration detectors using a sensitivity calibrator and a calibration vibrator.
What are sensitivity calibrators and calibration vibrators for acceleration detectors?
Sensitivity calibrators and calibration exciters for accelerometers are exciters that vibrate at a predetermined magnitude (for example, 10 m/s²). By placing the accelerometer on top of this exciter and measuring the voltage value of the output signal (calibration signal), the sensitivity of the accelerometer (how many volts are output per 1 m/s²) can be determined.
Figure 1 shows our VX-1100 simple sensitivity calibrator for acceleration detectors and its specifications. This sensitivity calibrator vibrates with a vibration acceleration of 10 m/s² (rms).
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Figure 1 Simple Sensitivity Calibrator VX-1100
Analysis devices such as FFT analyzers are equipped with a function that allows them to input such calibration signals to calibrate their detectors.
RMS value (effective value), zero-peak value and peak-peak value
When expressing the magnitude of vibrations, in addition to the RMS value (effective value), the zero-peak value (0-p, single amplitude value) and the peak-peak value(pp, double amplitude value), which are the maximum values when viewed as a time waveform, are also used.
For example, a vibration acceleration of 10 m/s² in RMS value is approximately 14.1 m/s² when expressed as a zero-peak value, and approximately 28.2 m/s² when expressed as a peak-to-peak value. Figure 2 shows the relationship between RMS value and zero-peak value.
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Figure 2: RMS values and zero-peak values
This column describes the calibration procedure when the magnitude of the calibration signal is expressed in RMS values. If the vibration acceleration of a simple calibrator, calibration vibrator, etc., is given as zero-peak or peak-peak values, please convert it to RMSvalues using the following formula.
Calibration of an accelerometer using a calibration signal
Calibration using the calibration function of an analysis device such as an FFT analyzer can determine the sensitivity of an acceleration detector (how many volts are output per 1 m/s²). However, if the operation is incorrect or if the acceleration detector, cable, calibrator, or analysis device is not functioning properly, the correct sensitivity cannot be determined. When performing calibration, be sure to know the approximate sensitivity of the detector and verify that the sensitivity obtained from the calibration is correct.
During calibration, you may need to change some settings on the analysis device. For any settings that are changed from the measurement settings, please make a note of the original settings and revert them to the original settings after calibration.
- Regarding the CCLD or input source (sensor power supply) settings in the input settings, set it to ON (or 2mA/4mA, etc.) if you are directly connecting an accelerometer with a built-in preamplifier or using a charge converter. Set it to OFF if you are using a sensor amplifier or charge amplifier.
- As a general guideline, if the frequency range is approximately 3 to 100 times the calibration signal frequency, you can perform the calibration using that frequency range. If you need to change the frequency range, set it to approximately 3 to 10 times the calibration signal (for example, if the calibration signal is 159.2 Hz, set it to 500 Hz or 1 kHz).
- In the input settings, turn off any filters such as LPF/HPF, and any time-axis preprocessing such as absolute value processing, smoothing, or calculus. Set coupling to AC.
- The voltage range (input range) should be set to the smallest possible voltage range that does not cause an input overload when a calibration signal is input.
- For FFT analyzers and similar devices, the window function should be set to either a Hanning window or a flat-top window. The window function setting used during calibration may differ from the window function setting used during actual measurement.
- If your analysis device has an averaging function, averaging the calibration signal will result in a more accurate calibration. In the averaging settings, set the averaging mode to Power SP /Averaging and the averaging time to approximately 5 seconds or longer. If your device does not have an averaging function, perform calibration using instantaneous data.
Calibration procedures vary depending on the FFT analyzer or analysis device. Below, we introduce the calibration procedures for our FFT analyzers CF-7200/7200A, CF-4500, and multi-channel data stations DS-2000 (DS-0221 FFT analysis software, DS-0250 throughput disk software) and DS-3000 series.
Similar calibration should be possible with other analytical instruments, so please refer to the instruction manuals for each instrument.
How to set up the FFT analyzers CF-7200/7200A and CF-4500.
- [MAIN]→[INPUT]→[EU]→[CH1 (CH2)] And operate as follows: “Y EU ON” Bota
hmm 1 Press Major I'll do that.
- Press the "Y EU NAME" button 2 and set the EU NAME (unit name) to "m/s2".
- Press the "SP/EU" button 3 to display the SP/EU screen. Press the Search button 4 on the panel keys to display the search cursor 5 (red vertical bar), then move it to the right edge of the screen so that "X:OverAll" is displayed below the graph. Also, turn off the "Y LOG/LIN" button 6 on the panel keys to display the Y axis as LIN.
- Press the AVG button 7 on the panel keys to turn it on, then press the Start (➤) button 8.
- After the average stop, press the "SET VALUE" button 9 and enter the calibration signal value (10 m/s²) to perform the calibration.
- Press the "V/EU" button 10 under [MAIN]→[INPUT]→[EU]→[CH1 (CH2)] to confirm that the EU value has been set to the appropriate value.
* The CF-4500 is a 1-channel FFT analyzer, so there is no CH1/CH2 hierarchy.
How to configure the DS-0221 FFT analysis software.
- Navigate to [Input Menu] → [Units, Calibration], and in the Units and Calibration dialog box, under the Set tab, turn on Calibration 1, enter "m/s2" for Unit Name 2, and select "V/EU" for Calibration Value Setting 3. Leave the Physical Value (EU Value) as 1.
- Display the EU/SP tab in the Units and Calibration dialog. Move the handle 4 on the bar below the graph all the way to the right so that "X:OverAll" is displayed below the graph. Also, press the YUnit button 5 and set the spectral units to rms6 and Lin7 in the dialog that appears.
- Press the AVG button on the DS-0221 to perform the averaging calculation. After the averaging stops, enter the calibration signal value (10 m/s²) into box 8 above the setting button, and then press the setting button 9.
Next, press the START button on the DS-0221 to display the data and confirm that the Y value has become 10 m/s².
- We will perform the same calibration on other channels as well.
- Switch to the Set tab, verify that the physical value (EU value) is set to the appropriate value, and then click the OK button to close the dialog.
How to configure the DS-0250 throughput disk software
- Navigate to [Data Display Menu] → [Y-axis Scale], and in the scale settings dialog, set Y-axis LIN/LOG1 to "Lin" and rms/0-peak2 to "rms". With "Apply All" checked, press the OK button to close the dialog.
- Navigate to [Data Display Menu] → [Cursor Settings] and set Search Mode 3 to "Search". With "Apply to All" checked, press the OK button to close the dialog box.
- Move search cursor 4 (red vertical bar) to the right side of the graph so that "X:OverAll" is displayed below the graph. Press the FFT5 button on the DS-0250 to switch the graph display to power spectrum and confirm that the calibration signal is displayed.
- Navigate to [Input Menu] → [Units, Calibration], and in the Units and Calibration dialog box, under the Set tab, turn on Calibration 6, enter "m/s2" for Unit Name 7, and select "V/EU" for Calibration Value Setting 8.
- Display the EU/SP tab in the Units and Calibration dialog box. Select a channel, enter the calibration signal value (10 m/s²) in box 9 above the settings button, and press the settings button 10.
- We will perform the same calibration on other channels as well.
- Switch to the Set tab, verify that the physical value (EU value) is set to the appropriate value, and then click the OK button to close the dialog.
How to configure settings in real-time acoustic vibration analysis software
- Navigate to [Input/Output Settings Menu] → [Units, Calibration Settings], and in the Calibration Settings dialog, select Set
On the tab, turn on EU1 for the channel you want to perform unit calibration on. No other settings are required.
- Display the EU/SP tab in the calibration settings dialog. Set Unit Name 2 to "m/s²", 0dB Reference Value 3 to "1", and Lin/Log4 to "Lin". Leave the search cursor in its initial position, "Overall". Set the voltage range and frequency range appropriately. Note that the voltage range and frequency range changed here will revert to their original settings when the dialog is closed.
- Set the average processing time 5 to 5 seconds, turn on the Average (-X) button 6, and press the Start (➤) button 7. After averaging stops, enter the calibration signal value (10 m/s²) into Calibration Value 8 and press the Calibration Execution button 9. Press the Start button again to display the data and confirm that the Y value is 10 m/s².
- We will perform the same calibration on other channels as well.
- Switch to the Set tab, verify that the EU value is set to the appropriate value, and then click the OK button to close the dialog.
* The setup procedure varies depending on the software version. This procedure is for the latest version.
summary
When measuring the magnitude of vibration using acceleration values (m/s²), use the acceleration detector introduced in the previous two articles.
Methods for inputting the sensitivity value (or amplifier output sensitivity) numerically into an analysis device such as an FFT analyzer, or using the sensitivity calibrator or calibration vibrator for acceleration detectors introduced here, are available.
We will perform unit calibration using the calibration method described.
When measuring vibrations of floors or the ground, the magnitude of the vibration is measured using acceleration, as introduced in previous sections.
In some cases, the value is expressed in decibels, such as acceleration level or vibration level, rather than in m/s².
Next time, we'll discuss the units, calibration procedures, and analysis when expressing the magnitude of such vibrations in decibels.
Here's how to set up the device.
(Excerpt from the email newsletter issued on February 20, 2014)
