In this measurement column, we will address frequently asked questions received by our customer support center.
We are presenting the answers.
One of the most frequently asked questions about accelerometers is: what is the minimum acceleration, vibration velocity, and variable speed that can be measured?
There are some related to position. There is absolutely no background vibration or electrical noise, and the analysis device has no self-noise.
If it is sufficiently small, the smallest measurable acceleration value is the detector noise (charge output) of the accelerometer.
In the case of a type of accelerometer, it is determined by the input-referred noise of the charge amplifier.
Accelerometer detector noise
The detector noise of preamplifier-integrated accelerometers is listed in the product specifications. For our 3-axis accelerometers NP-3574 and NP-3578N20, it is 0.004 m/s² (rms) or less. This means that even in the absence of any vibration, a voltage signal equivalent to 4 mm/s² (RMS) will be output.
The error when measuring the time waveform of acceleration, or when measuring the maximum value of that time waveform, is approximately 4.
The unit is mm/s². If the measurement result is 40 mm/s², the error is ±4 mm/s² (±10%), and if the measurement result is 400 mm/s², the error is ±4 mm/s² (±1%). If a measurement accuracy of about 10% is acceptable, the minimum measurable acceleration will be 40 mm/s², which is 10 times the detector noise.
When measuring the effective value or the average of the power spectrum, the calculation is performed using the square of the acceleration.
Therefore, the influence of detector noise is reduced. The measurement result is 40 mm/s 2 In this case, the error is
1 − ｰ1 − (4/40) 2 = Approximately 0.005, which is 0.5%. This is an addition of about 10 times the magnitude of the detector noise.
For speed, sufficient accuracy can be achieved.
Regarding the smallest vibration velocity and displacement that can be measured, what is the relationship between acceleration values and vibration velocity/displacement values?
Since it depends on the frequency distribution of vibration, it cannot be estimated from the detector noise of the accelerometer.
No, it's not possible. It's necessary to measure and verify the vibration of the actual object being measured.
Noise in charge output type accelerometer and charge amplifier
When using a charge output type accelerometer, the noise is determined by the input-referred noise of the charge amplifier.
Yes, our CH-1200A charge amplifier has an input-referred noise level of 0.05 pC (rms) or less.
The sensitivity of our NP-2106 accelerometer is approximately 0.035 pC/(m/s²). 2 Therefore, in combination with NP-2106
In Waseda, 0.05 pC corresponds to 1.43 m/s. 2 This corresponds to the following: If the minimum measurable acceleration is 10 times the detector noise, then the minimum acceleration that can be measured with the NP-2106 is 14.3 m/s². 2 This is the case. NP-2106 is drop test.
This detector is for measuring large accelerations in impact and shock tests, so it is not suitable for measuring small accelerations.
I don't peel it.
Time waveform of detector noise
Figure 1 shows the time-domain waveform of noise measured with the NP-3574 accelerometer placed on a shelf.
This signal includes detector noise from the accelerometer itself, as well as background vibrations, electrical noise, etc.
Yes, it does. The sensitivity of the NP-3574 is 10 mV/(m/s 2), the detector noise is 0.004 m/s 2 (rms) Below.
Measurements were taken with a wavenumber range of 10 kHz and a voltage range of 31.6 mVrms.
The upper left of Figure 1 shows the time-domain waveform of the noise signal output from the accelerometer. Low-frequency background vibrations...
It contains electrical noise, with a peak value of 16.742 m/s². The lower left waveform is a double integral of the upper left waveform.
This is the displacement waveform obtained. Due to the influence of the low-frequency components of the original signal, the waveform slopes downward to the right.
The minimum value is -440 μm. This is the waveform of the displacement that is actually occurring.
No. This is an incorrect waveform obtained when numerically integrating a signal containing low-frequency noise.
is.
The upper right of Figure 1 shows the time waveform of acceleration with components below 25 Hz cut off using the inverse FFT function.
The peak value was 5.079 m/s². The bottom right shows the result obtained by double integration after cutting out components below 25 Hz.
The peak value in the displacement waveform was a very small 0.058 μm.
In actual measurements, in addition to the detector noise of the accelerometer, the data is also affected by background vibrations and electrical noise.
Therefore, it is necessary to measure the noise signal and confirm the measurable acceleration and displacement values.
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Figure 1 Time-domain waveform of detector noise
Top left: Acceleration waveform, Top right: Band-limited acceleration waveform.
Bottom left: Double integral waveform, Bottom right: Band-limited double integral waveform
Power spectrum of detector noise (1)
Figure 2 (green) shows the power spectrum of noise measured with the NP-3574 accelerometer placed on a shelf.
This shows that this signal includes detector noise from the accelerometer itself, as well as background vibrations, electrical noise, etc.
This includes the following: The NP-3574 has a sensitivity of 10 mV/(m/s²) and a detector noise of 0.004 m/s² (rms) or less. A 10-second averaging was performed with a frequency range of 10 kHz, 2048 sample points, and a voltage range of 31.6 mVrms.
The orange line in Figure 2 represents the voltage range of our DS-3000 data station when set to 1 Vrms.
This is the self-noise of the DS-3000 itself. It is louder than the noise of the NP-3574, so the voltage range
Small vibrations cannot be measured with a voltage of 1 Vrms. If the vibration of the object being measured is small, the voltage level
Lowering the voltage range should not cause an input overload.
Please set the range as small as possible.
The power spectrum is a breakdown of a signal into its frequency components, so the values of each frequency component are detected.
This value is considerably smaller than the instrument's own noise (4 mm/s²). Actual ambient vibration, electrical noise
The size of the vibrations, etc., must be measured and confirmed on-site. Then, the vibration of the object being measured must be
If it is sufficiently larger than the noise, it can be measured; otherwise, it will be affected by background vibrations and electrical noise.
We need to consider more sophisticated measurement methods.
By performing single and double integration on the power spectrum of noise, the vibration velocity and displacement can be determined.
You can calculate the converted noise spectrum. The vibration velocity and displacement values you want to measure,
Alternatively, by comparing the measured vibration velocity and displacement of the object being measured, it is possible to determine if measurement is possible.
We can consider this.
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Figure 2. Power spectrum of noise (10 kHz range)
Green: NP-3574, Blue: DS-3000 (31.6 mVrms range), Orange: DS-3000 (1 Vrms range)
Power spectrum of detector noise (2)
Figure 3 shows the power spectra of the noise of the NP-3574 and the self-noise of the DS-3000 (31.6 mVrms range, 1 Vrms range) measured at a frequency range of 1 kHz. The noise of the NP-3574 includes detector noise from the accelerometer itself, as well as background vibrations, electrical noise, etc.
Because the frequency range was lowered, components below 50 Hz and 20 Hz are more prominent. This measurement was performed
The location is a typical office, but even there, there are low-frequency components below 20 Hz and noise originating from the commercial power supply.
Frequency components of 50 Hz and 100 Hz have been observed. The vibration of the object being measured is sufficiently larger than these frequencies.
If possible, measurement is possible; otherwise, a measurement method that is less susceptible to ambient vibration and electrical noise can be used.
An attack is necessary.
When single and double integration are performed on the power spectrum of noise, the vibration velocity and displacement are obtained.
You can display the converted noise spectrum. You can also input the vibration velocity and displacement values you want to measure.
Alternatively, by comparing the measured vibration velocity and displacement of the object being measured, it is possible to determine if measurement is possible.
We can consider this.
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Figure 3. Power spectrum of noise (1 kHz range)
Green: NP-3574, Blue: DS-3000 (31.6 mVrms range), Orange: DS-3000 (1 Vrms range)
summary
This time, we'll introduce the minimum acceleration, vibration velocity, and displacement that can be measured by an accelerometer.
Ta.
The minimum acceleration value that can be measured by an accelerometer can be determined from its specifications (detector noise).
Yes, it is possible. However, the measurement site will be affected by ambient vibrations and electrical noise, so if the noise level at the measurement site is low...
We need to measure the speed and determine the maximum acceleration value that can be measured.
The relationship between acceleration values and vibration velocity/displacement values depends on the frequency distribution of the vibration, so the best possible value to measure is...
Small vibration velocity and displacement values cannot be determined from the specifications of the acceleration sensor.
After measuring the magnitude and frequency distribution of the vibrations, we will check what value can be measured.
It is necessary.
(Excerpt from the email newsletter issued on September 25, 2019)
