No.25 Charge output type accelerometer and charge amplifier
There are 2 types of accelerometers, one is the built-in preamplifier, and another is the charge output.
The charge output type is not the built-in preamplifier, thus suitable for the measurement of vibration such as impact and drop tests and can be used in higher temperature environment. When analyzing the charge signal output with the analyzer such as an FFT analyzer or Data station, it is converted into a voltage signal by a charge amplifier or charge converter and input to the analyzer.
The charge amplifier converts the charge signal output to voltage signal. A general charge amplifier can set the sensitivity of the accelerometer, and can set the output sensitivity (range) independently. Some have also LPF, HPF, and built-in integration function.
· Setting about the charge amplifier and the analyzer
CH-1200A can set the sensitivity of accelerometer (argument/multiplier) and the range (output sensitivity). When pressing [SENS/RNG] key, the setting mode is switched to the followings:
- Sensitivity mode- pC/EU in the mode display is turned on.
- Range (output sensitivity) mode- mV/EU in the mode display is turned on.
In the sensitivity mode, set the sensitivity (pC/(m/s2)) of the charge output type accelerometer connected to CH-1200A. The values that can be set in the argument section are 1.00 to 9.99, and the values that can be set in the multiplier section are 0.01, 0.10, 1.00, 10.0 and 100. Therefore, set the argument section and multiplier section separately according to the sensitivity of the accelerometer.
In the range mode (output sensitivity), set the signal range output fromCH-1200A. When inputting the CH-1200A output to the analyzer, set a value corresponding to the range (output sensitivity) in the unit calibration (EU) functionality of the analyzer. The sensitivity of accelerometer itself does not affect.
The range (output sensitivity) can be set in CH-1200A as shown in Table 1. The selectable range (output sensitivity) is limited by the value of the sensitivity multiplier of the accelerometer.
Since the maximum output voltage of CH-1200A is ± 10V, the maximum acceleration is determined according to the range (output sensitivity). In the case of the 1.00 mV/ (m/s2) range, 1 mV is output at 1 m/s2, so it becomes 10 V at 10,000 m/s2, and 10,000 m/s2 is the maximum acceleration. As for the maximum output voltage of the previous product, CH-1200, CH-1100 is ± 5 V, the maximum acceleration is half value of Table 1.
In the unit calibration of analyzer, set the unit name and EU value. The unit name is m/s2, EU type is V/EU. If the range (output sensitivity) is 1.00 mV/ (m/s2), set the EU value to 0.00100 as V converted from mV.
Table 1. Setting for the range (output sensitivity) of the charge amplifier and the unit calibration of the analyzer
| The range (output sensitivity) of the charge amplifier | The unit calibration of the analyzer | ||||
| The display of range | Output sensitivity | The maximum acceleration | Unit name | EU value | EU type |
| 0.01 | 0.0100 mV/(m/s2) | 1,000,000 m/s2 | m/s2 | 0.00001 | V/EU |
| 0.03 | 0.0316 mV/(m/s2) | 316,000 m/s2 | m/s2 | 0.0000316 | V/EU |
| 0.1 | 0.100 mV/(m/s2) | 100,000 m/s2 | m/s2 | 0.0001 | V/EU |
| 0.31 | 0.316 mV/(m/s2) | 31,600 m/s2 | m/s2 | 0.000316 | V/EU |
| 1 | 1.00 mV/(m/s2) | 10,000 m/s2 | m/s2 | 0.001 | V/EU |
| 3.16 | 3.16 mV/(m/s2) | 3,160 m/s2 | m/s2 | 0.00316 | V/EU |
| 10 | 10.0 mV/(m/s2) | 1,000 m/s2 | m/s2 | 0.01 | V/EU |
| 31.6 | 31.6 mV/(m/s2) | 316 m/s2 | m/s2 | 0.0316 | V/EU |
| 100 | 100 mV/(m/s2) | 100 m/s2 | m/s2 | 0.1 | V/EU |
| 316 | 316 mV/(m/s2) | 31.6 m/s2 | m/s2 | 0.316 | V/EU |
| 999 | 1000 mV/(m/s2) | 10 m/s2 | m/s2 | 1 | V/EU |
When the vibration exceeding the maximum acceleration which depends on the range (output sensitivity) of charge amplifier is applied to the accelerometer, the OVER lamp of the charge amplifier lights up and the output of the charge amplifier is saturated. It takes a few seconds to recover once the output is saturated. Please set the range (output sensitivity) of the charge amplifier in advance so that the OVER lamp does not light before or during the measurement.
· Setting about the model of accelerometer and the sensitivity of the charge amplifier
The models of accelerometer and the setting of CH-1200A, charge amplifier are shown as in table 2. The charge sensitivity of the charge output accelerometer as in table 2 is reference value.
The charge sensitivity varies each accelerometer, thus please set the sensitivity value described in the test report on delivery attached in the accelerometer.
The input conversion noise of CH-1200A is less than 0.05 pC(rms). The input conversion noise is converted to acceleration value as in table 2.
Table 2. Setting for the model of accelerometer and the sensitivity of the charge amplifier
| Charge output accelerometer | Charge amplifier | |||||
| Model name | Features | Sensitivity (typ.) | Maximum allowable acceleration | Sensitivity (argument) | Sensitivity (multiplier) | Input conversion noise (rms) |
| NP-2106 | Ultra-compact and light weighted | 0.035 pC/(m/s2) | 100,000 m/s2 | 3.5 | 0.01 | 1.43 m/s2 |
| NP-2110 | Compact and light weighted | 0.16 pC/(m/s2) | 10,000 m/s2 | 1.6 | 0.1 | 0.31 m/s2 |
| NP-2710 | Compact, high-temperature | 0.306 pC/(m/s2) | 22,600 m/s2 | 3.06 | 0.1 | 0.16 m/s2 |
| NP-2910 | Compact, general purpose usage | 0.3 pC/(m/s2) | 50,000 m/s2 | 3 | 0.1 | 0.17 m/s2 |
| NP-2810 | Compact | 1.2 pC/(m/s2) | 20,000 m/s2 | 1.2 | 1 | 0.042 m/s2 |
| NP-2120 | General purpose usage | 5 pC/(m/s2) | 8000 m/s2 | 5 | 1 | 0.010 m/s2 |
| NP-2560 | Light weight tri-axial | 0.04 pC/(m/s2) | 25,000 m/s2 | 4 | 0.01 | 1.25 m/s2 |
· Summary
When connecting the accelerometer directly to the analyzer such as FFT analyzer or Data station, please set the sensitivity of the accelerometer directly in the unit calibration function of the analyzer.
When the charge output type accelerometer is connected to the charge amplifier and the output of the charge amplifier is input to the analyzer, the sensitivity of the accelerometer is set to the general charge amplifier but not to the analyzer.
The value set in the unit calibration function of the analyzer is the charge amplifier range (output sensitivity).
Check how much the charge amplifier range (output sensitivity) is set, check the specification how many volts (mV) the charge amplifier outputs when the acceleration of 1 m/s2 is detected in that range, and set the unit calibration function of the analyzer according to the above value.
When the magnitude of the generated acceleration is changed as the measurement object or measurement conditions are changed and the charge amplifier range (output sensitivity) is changed, the unit calibration setting on the analyzer must also be changed.
The vibration exceeding the maximum allowable acceleration of Accelerometer or the maximum measurable acceleration depends on the charge amplifier range (output sensitivity) cannot be measured. Select the accelerometer according to the maximum value of vibration, and set the charge amplifier range (output sensitivity). If the charge amplifier is OVER during installation / removal of the accelerometer or preparation for measurement, please measure once the output of the charge amplifier is stable, or avoid significant vibration during installation, removal, preparation.
(YK)