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).
Starting with this article, we will be introducing how to configure analysis equipment when using various detectors (sensors) and amplifiers.
This time, we'll be looking at accelerometers with built-in preamplifiers.
Analysis devices such as FFT analyzers typically have a function to convert such voltage values into physical quantities, which is called a unit calibration function. The generalized units of these physical quantities are called engineering units (EU). The term engineering unit (EU) will appear when explaining the unit calibration function. When actually using it, if you are measuring the acceleration of vibration, it will be easier to understand if you replace "EU" with "m/s²" (meters per second squared). When measuring velocity, you replace it with "m/s" (meters per second), etc., and when measuring force, you replace it with "N" (Newton), etc.
What is a preamplifier-integrated accelerometer?
The signal output from the piezoelectric element used in an acceleration detector is a charge signal. This signal cannot be directly input into a voltage-input type analysis device, so an amplifier called a charge amplifier is used to convert the charge signal into a voltage signal.
Accelerometers with built-in preamplifiers have a circuit equivalent to a charge amplifier built into the detector, and a voltage signal is output from the detector. To use this type of accelerometer, power must be supplied to the built-in preamplifier. Our NP-3000 series accelerometers and others use a power supply method called "CCLD".
CCLD stands for Constant Current Line Drive, and it uses a standard two-wire cable, such as a coaxial cable, to connect the detector to the amplifier and analysis equipment. While detector and analysis equipment manufacturers may refer to it by names or registered trademarks such as IEPE or ICP, it is the same method.
CCLD-type acceleration detectors can be directly connected to and used with analysis equipment that supports the CCLD method (Figure 1). When connecting to analysis equipment, data loggers, oscilloscopes, etc. that do not support the CCLD method, a sensor amplifier or similar device that supports the CCLD method (and can supply power to the detector) will be required.
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Figure 1. Example of accelerometer connection 1
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Figure 2 Example of accelerometer connection 2
When connecting a preamplifier-integrated accelerometer to a CCLD-compatible analysis device
This document describes the setup procedure for connecting preamplifier-integrated accelerometers (such as our NP-3000 series) to our FFT analyzers CF-7200/7200A, CF-4500, and multi-channel data stations DS-2000 and DS-3000 series.
Check the voltage sensitivity in the acceleration detector's shipping specifications sheet or test report. If the voltage sensitivity is listed as 9.75 mV/(m/s²), it means that this detector outputs a voltage of 9.75 mV (0.00975 V) when it detects an acceleration of 1 m/s².
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Figure 3 Accelerometer Shipment Characteristics Table (Partial Excerpt)
When the unit calibration function of the analyzer allows you to choose between "EU/V" and "V/EU" as the unit for setting the EU value, select "V/EU". If you read "EU" as m/s², this means you have chosen to set the voltage sensitivity in units of V/(m/s²). If the detector sensitivity is 9.75 mV/(m/s²), convert the unit to V (volts) and enter 0.00975.
If the unit calibration function of the analyzer only allows you to set values in "EU/V" units, set it to 102.56 (m/s²)/V, which is the reciprocal of 0.00975.
Additionally, we will turn on the CCLD function of the analysis device.
The following describes the setup procedure for our product. This section explains how to operate when a detector is connected to CH1. If you connect a detector to another channel, please set it up in the same way.
I believe similar settings are possible with other analysis devices as well. If there is no unit calibration function, the acceleration value can be calculated from the measured voltage value and the voltage sensitivity value (how many volts are output when an acceleration of 1 m/s² is detected).
- How to set up the FFT analyzer CF-7200/7200A and CF-4500.
-1. Connect the detector to the FFT analyzer, then turn on the CCLD function (the function that supplies power to the detector). If the detector is removed or the cable is disconnected, the "disconnection detection function" will activate and the CCLD will turn off. In that case, reconnect the detector and turn it on again.
-2. Set "m/s²" as the unit name to be displayed on the Y axis. Since this model does not allow superscript "2", enter a regular "2".
-3. Set the voltage sensitivity in V/EU. If the voltage sensitivity is specified in mV, set the value converted to V (volts).
-4. To turn on the unit calibration function, turn on “Y EU ON”.
If the FFT analyzer is stopped, changes to these settings will not be reflected in the displayed data. These settings will take effect the next time you start a measurement.
* The CF-4500 is a 1-channel FFT analyzer, so there is no CH1/CH2 hierarchy.
- DS-0221 FFT analysis, DS-0250 throughput disk setup instructions
-1. To turn on the CCLD function (the function that supplies power to the detector), go to [Input Menu] → [Voltage Range Setting] and set the input source to Sensor (4.0 mA) in the Voltage Range Setting dialog. If the specification of the detector's CCLD (drive power supply) is up to 2 mA, set it to Sensor (2.0 mA).
-2. To set the unit calibration function, go to [Input Menu] → [Units, Calibration], and in the Units and Calibration dialog box, turn on calibration, enter "m/s2" as the unit name, enter voltage sensitivity as the physical value (EU value), and select "V/EU" for the calibration value setting.
- How to configure DS-3000 ESUFEEL
-1. After connecting the detector, go to [Input/Output Settings Menu] → [Input Settings] and turn on CCLD in the Input Condition Settings dialog. When this setting is turned on, the detector will be supplied with a 4 mA power supply. If the detector is removed or the cable is disconnected, the "disconnection detection function" will activate and the CCLD will turn off. In that case, reconnect the detector and turn it on again.
-2. To set the unit calibration function, go to [Input/Output Settings Menu] → [Unit Calibration Settings], and in the calibration settings dialog, turn on EU, enter "m/s2" as the unit name, enter the voltage sensitivity as the EU value, and select "V/EU" as the EU type. You can also select the unit name from the drop-down list.
Connecting a preamplifier-integrated accelerometer to a sensor amplifier (Part 1)
If your analysis equipment, such as an FFT analyzer, data logger, or oscilloscope, does not support CCLD, connect a sensor amplifier between the detector and the analysis equipment.
If the sensor amplifier is the type that allows you to set the gain (×1, ×10, +0 dB, +20 dB, etc.), such as our SR-2200/2210 or PS-1300, you set the value by multiplying the detector's voltage sensitivity (unit: V/(m/s²)) by the gain multiplier using the unit calibration function of the analysis device. If the gain is expressed in decibels, convert it as follows: +0 dB = 1x, +10 dB = 3.16x, +20 dB = 10x, and +10 dB = 31.6x.
The settings for the analysis device are the same as in the previous section, except that CCLD is not turned ON.
Table 1 Conversion of EU values when gain is specified as a multiplier
| Detector voltage sensitivity | Amplifier gain | Amplifier gain |
| 0.00975 V/(m/s²) | ×1 | 0.00975 V/(m/s²) |
| 0.00975 V/(m/s²) | ×2 | 0.0195 V/(m/s²) |
| 0.00975 V/(m/s²) | ×5 | 0.04875 V/(m/s²) |
| 0.00975 V/(m/s²) | ×10 | 0.0975 V/(m/s²) |
Table 2 Conversion of EU values when gain is specified in decibels
| Detector voltage sensitivity | Amplifier gain | Set EU value |
| 0.00975 V/(m/s²) | +0 dB | 0.00975 V/(m/s²) |
| 0.00975 V/(m/s²) | +10 dB | 0.03081 V/(m/s²) |
| 0.00975 V/(m/s²) | +20 dB | 0.0975 V/(m/s²) |
Connecting a preamplifier-integrated accelerometer to a sensor amplifier (Part 2)
If you connect a sensor amplifier like our AU-2100, which allows you to set the voltage sensitivity (input sensitivity) of the detector and set the output sensitivity (output range) separately from the input sensitivity, set the value set for the output sensitivity (output range) of the sensor amplifier and set it in the unit calibration function of the analysis device.
As shown in Figure 4, when using multiple detectors, each detector has a different voltage sensitivity. Using this type of sensor amplifier allows you to unify the output range to the same value. Note that our AU-2100 is for single-channel use, so if you use three detectors, you will need three AU-2100 units.
Figure 4. Input sensitivity and output range of the sensor amplifier.
When the voltage sensitivity of the detector is set to 10 mV/(m/s²) with our AU-2100, you can select the output range values shown in Table 3. The unit of the output range value is "EU/V", so select "EU/V" in the unit calibration function of the analyzer and set the output range value as is. If "V/EU" is selected, find the reciprocal of the output range value and set that value.
Table 3 Output range values and EU values set by the unit calibration function
| Output range value | Set the EU value (EU/V) | Set the EU value (V/EU) |
| 1（m/s2）/V 2（m/s2）/V 5（m/s2）/V 10（m/s2）/V 20（m/s2）/V 50（m/s2）/V 100（m/s2）/V |
1（m/s2）/V 2（m/s2）/V 5（m/s2）/V 10（m/s2）/V 20（m/s2）/V 50（m/s2）/V 100（m/s2）/V |
1 V/（m/s2） 0.5 V/（m/s2） 0.2 V/（m/s2） 0.1 V/（m/s2） 0.05 V/（m/s2） 0.02 V/（m/s2） 0.01 V/（m/s2） |
If the output sensitivity (output range) of the sensor amplifier is expressed in units of V/(m/s²) (V/EU), select "V/EU" in the unit calibration of the analysis device and set the output range value as the EU value.
If the sensor amplifier can output a calibration electrical signal, this signal is often used for unit calibration, and we will explain that procedure in a future article.
summary
This time, we introduced the procedure for setting the voltage sensitivity, as written in the shipping specifications, to the analysis device when using an acceleration detector with a built-in preamplifier.
In subsequent articles, we will sequentially introduce the unit calibration procedure for charge output type acceleration detectors, as well as the calibration signal for sensor amplifiers and the unit calibration procedure using vibration calibrators.
(Excerpt from the email newsletter issued on October 18, 2013)
