Engineering Units (EU) and Unit Calibration - Part 2: The Case of Accelerometers -

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 when using various detectors (sensors) and amplifiers, and last time we covered an accelerometer with a built-in preamplifier.

This time, we will be looking at charge-output type accelerometers.

Charge output type accelerometer and preamplifier-integrated accelerometer

The signal output from the piezoelectric element used in an acceleration detector employing a piezoelectric element is a charge signal. Detectors that output such charge signals are called charge-output type acceleration detectors. Since this signal cannot be directly connected to a voltage-input type analysis device, an amplifier called a charge amplifier is used to convert the charge signal into a voltage signal. Our NP-2000 series acceleration detectors are charge-output type acceleration detectors.

Preamplifier-integrated accelerometer (voltage output type accelerometer) inside the detector Charge amplifier It incorporates a circuit equivalent to the detector, and from the detector Voltage signal This is the output. To use this type of accelerometer, you need to supply power to the built-in preamplifier. Our NP-3000 series accelerometers etc.“CCLD"This power supply method is called Accelerometer with built-in preamplifier is.

Accelerometers with built-in preamplifiers are easy to handle because they do not require a charge amplifier, but because they have a built-in amplifier, their maximum operating acceleration is low and their operating temperature range is narrow. Charge output type accelerometers are used when large accelerations (e.g., 2000 m/s² or more) are applied, such as in shock tests, or when used in high-temperature environments (e.g., 100°C or higher).

Figure 1 shows an example of connecting the preamplifier-integrated accelerometer introduced previously. Figures 2 and 3 show examples of connecting charge-output type accelerometers. Figure 2 shows an example of connecting using a small charge amplifier called a charge converter, such as our CH-6130/6140. A charge converter is a charge amplifier about the size of a slightly enlarged BNC plug that operates on a CCLD-type power supply and is used by attaching it to the input connector of a CCLD-compatible analysis device. Figure 3 shows an example of connecting using a charge amplifier such as our CH-1200A.

Setup procedure when using a charge output type accelerometer and charge converter

Charge output type accelerometers (such as our NP-2000 series) are used with a charge converter.
Using our CH-6130/6140, etc., our FFT analyzers CF-7200/7200A, CF-4500
Also, when connecting to the multi-channel data station DS-2000, DS-3000 series...
Here is the standard procedure.

The charge sensitivity is confirmed in the acceleration detector's shipping specifications sheet and test report. If the voltage sensitivity is listed as 0.307 pC/(m/s²), it means that this detector outputs a charge of 0.307 pC (picocoulombs) when it detects an acceleration of 1 m/s².

Furthermore, the gain is confirmed in the charge converter's specifications and test reports. According to Table 1, the CH-6140 is an amplifier that outputs 10 mV for a charge of 1 pC.

Table 1 Specifications of the charge converter (partial excerpt)

The combined voltage sensitivity of the accelerometer and charge converter is the product of the charge sensitivity of the accelerometer and the gain of the charge converter, which in the example below is 3.07 mV/(m/s²).

<Calculation example>:

Charge sensitivity of the accelerometer: 0.307 pC/(m/s²)
Charge converter gain: 10 mV/pC
The combined voltage sensitivity is 0.307 × 10 = 3.07 mV/(m/s²).

When an accelerometer and a charge converter are combined, the maximum usable acceleration is limited by the maximum output voltage of the charge converter. The maximum output voltage is 10 Vp-p at peak-to-peak and 5 V0-p at zero-peak. Dividing this by the combined voltage sensitivity gives the maximum usable acceleration, which in the example below is 1629 m/s².

<Calculation example>:
Combined voltage sensitivity: 3.07 mV/(m/s²)
Maximum output voltage: 10 Vp-p (5 V0-p)
Maximum operating acceleration 5 V ÷ 3.07 mV = 1629 m/s2

The method for setting the sensitivity of the analysis device is the same as for the preamplifier-integrated accelerometer. The charge converter requires a CCLD-type power supply, so turn on the CCLD function of the analysis device.

When the unit calibration function of the analysis device allows you to choose between "EU/V" and "V/EU" as the unit for setting the EU value, select "V/EU". If you interpret "EU" as m/s², this means you have chosen to set the voltage sensitivity in units of V/(m/s²). If the combined sensitivity is 3.07 mV/(m/s²), enter 0.00307, which is the unit converted to V (volts).

If the unit calibration function of the analyzer only allows you to set values in "EU/V" units, set it to 325.73(m/s2)/V, which is the reciprocal of 0.00307.

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 value of the voltage output when an acceleration with voltage sensitivity (1 m/s²) is detected.

How to set up the FFT analyzer CF-7200/7200A and CF-4500.

1. After connecting the detector to the FFT analyzer, 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 back on.
2. Set the unit name to be displayed on the Y axis to "m/s²". On this model, the superscript "2" is not included.
   I cannot use the power input, so I will simply enter "2".
3. Set the voltage sensitivity in V/EU. If the voltage sensitivity is specified in mV, set the value after converting it to V (volts).
4. To enable 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 configuration method for throughput disks

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 CCLD will turn off. In that case, reconnect the detector and turn it on again.
2. To configure the unit calibration function, go to [Input/Output Settings Menu] → [Unit Calibration Settings], and in the calibration settings dialog, turn EU ON, 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.

Setup procedure for connecting a charge output type accelerometer to a charge amplifier

Our CH-1200A charge amplifiers allow you to set the detector's charge sensitivity (input sensitivity) and output sensitivity (output range) separately.

When using this type of charge amplifier, set the detector's charge sensitivity to the charge amplifier's input sensitivity. Also, set the magnitude of the voltage signal output from the charge amplifier as the output sensitivity (output range). Set the value of the charge amplifier's output sensitivity (output range) in the unit calibration function of the analysis device.

With our CH-1200, when the detector's charge sensitivity is set to 0.307 pC/(m/s²), you can select the output range values shown in Table 2. Since the unit of the output range value is "mV/EU", select "V/EU" in the unit calibration function of the analyzer and set the output range value converted to "V".

Table 2 Output range values and EU values set by the unit calibration function

When using a charge amplifier whose output sensitivity (output range) is expressed in units of (m/s²)/V (EU/V), select "EU/V" in the unit calibration of the analysis device and set the output range value as the EU value.

summary

Last time, we introduced the procedure for setting up the analysis device based on the sensitivity values written in the shipping specifications for preamplifier-integrated accelerometers, and this time, we introduced the procedure for setting up the analysis device based on the sensitivity values written in the shipping specifications for charge-output type accelerometers.

Regarding the method of unit calibration for acceleration detectors, in addition to the methods introduced in the previous and current articles, there is also a method using a vibration calibrator. A vibration calibrator is a device that vibrates at a predetermined magnitude (10 m/s² in the case of our VX-1100). Using a vibration calibrator, you can perform unit calibration and operational checks on acceleration detectors and other devices.

Next time, we will introduce a method for unit calibration of acceleration detectors using a vibration calibrator.

(Excerpt from the email newsletter issued on December 19, 2013)