This measurement column addresses frequently asked questions received by our customer support center and provides answers to those questions.
When using an accelerometer with a built-in preamplifier, it is necessary to supply power to the amplifier inside the accelerometer. Our accelerometers with built-in preamplifiers use a power supply mechanism called CCLD (Constant Current Line Drive). Some manufacturers may refer to this as IEPE or IPC, but it is the same mechanism.
Our FFT analyzers and data stations are equipped with CCLD-type power supply functions, allowing direct connection of preamplifier-integrated accelerometers. However, when connecting to oscilloscopes or A/D conversion boards, it is necessary to use a sensor amplifier with a power supply function.
Some sensor amplifiers have the ability to output the signal from a sensor either directly or simply amplified, as well as converting it into a signal called an RMS value. In such products, the output that is either directly or simply amplified is called the AC output, and the output that has been converted to an RMS value is called the DC output.
CCLD-compatible sensor amplifier
Our CCLD (Constant Current Drive) compatible sensor amplifiers include the 2-channel sensor amplifier SR-2210 and the 3-channel sensor amplifier PS-1300.
The SR-2210 2-channel sensor amplifier is a sensor amplifier equipped with a CCLD (Critical Collision Deposition) system for supplying power to sensors and amplifying signals. It also has a frequency correction function (A/C/FLAT) for use when measuring sound with a microphone connected, but when connecting an accelerometer, it should be used in FLAT mode. It does not have an RMS (Rough Value) output (DC output) function.
The 3-channel sensor amplifier PS-1300 is a sensor amplifier equipped with a CCLD method for supplying power to sensors and a signal amplification function. It also features integration, high-pass filter, and low-pass filter functions. By adding the PS-0131 RMS output function option to the PS-1300, it is possible to output a signal called the RMS value (DC output) instead of the normal signal output (AC output). When recording the time waveform of the acceleration signal itself or when performing frequency analysis on it, use the AC output. When recording only the magnitude of vibration without frequency analysis, use the RMS value output (DC output).
Vibration of a cell phone's vibrator
Figure 1 shows the time-domain waveform and power spectrum of a vibrator vibration acceleration measured by attaching an accelerometer to a mobile phone. The amplitude of the time-domain waveform was 13.37 m/ s², and the overall (RMS) power spectrum was 8.68 m/ s². Looking at the power spectrum, the fundamental frequency of the vibration was 200 Hz, and its amplitude (RMS) was 7.64 m/ s². Second-order and higher harmonic components were also observed.
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Figure 1. Time-domain waveform (top panel) and power spectrum (bottom panel) of a mobile phone vibrator's vibration acceleration.
The AC output of the sensor amplifier outputs the time-domain waveform shown in the upper part of Figure 1. The relationship between the acceleration value and the output voltage is determined by the sensitivity of the acceleration detector [mV/(m/ s²)] and the gain (amplification factor) of the amplifier.
AC output and DC output for mobile phone vibrator vibration.
Figure 2 shows the waveforms of the acceleration signal (AC output) from the mobile phone's vibrator and the RMS signal (DC output) obtained by processing that signal in the same way as the RMS output function of the PS-1300 sensor amplifier. The vibrator vibrated for 3 seconds and then stopped. The vibration approximately 1 second after stopping was caused by pressing a button on the mobile phone, but because the duration was short, it did not affect the RMS signal (DC output).
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Figure 2 shows the AC output (blue) and DC output (red) of the vibration acceleration of a mobile phone vibrator.
The PS-1300's RMS output function option uses an RMS circuit with a time constant of 1.18 seconds to determine the RMS value. Therefore, even when vibration begins, the output does not increase immediately, but takes several seconds to rise to 8.12 m/ s². After the vibration stops, it gradually decreases.
The maximum value of the acceleration signal (AC output) is 15.25 m/ s². The RMS value is the mean square of the time waveform, so the RMS value will always be equal to or less than the maximum value. For a sine wave, the RMS value is 1/√2 of the maximum value (0.707 times), for typical vibrations it will be a fraction of the maximum value, and for shock vibrations it will be even smaller.
If the vibration acceleration signal itself is not needed, and only the variation in the magnitude of the vibration needs to be measured, then the RMS output (DC output) of a sensor amplifier or similar device should be recorded. Recording the DC output value a few times per second is sufficient, and even less frequently is needed if the magnitude variation is small. However, short-duration shock vibrations will not be reflected in the RMS output (DC output). If the maximum value of the acceleration signal is also monitored to detect shock vibrations, the AC output must be recorded at a sufficiently fast sampling frequency.
summary
When recording the time waveform of the acceleration signal itself, or when performing frequency analysis on it, you need to use equipment that can acquire the signal at a sufficiently fast sampling frequency, such as an FFT analyzer, data station, or data logger. If you simply want to record the magnitude of the vibration, you can use relatively inexpensive equipment, such as a sensor amplifier with an RMS signal (DC output), to acquire data at a frequency of a few times per second or less.
Since our 3-channel sensor amplifier PS-1300 is often used for this purpose,
This time, we explained the difference between AC output and DC output of a sensor amplifier.
(Excerpt from the email newsletter issued on April 24, 2018)
