Analog Input Channels
Figure 1. Accelerometers are transducers for measuring the dynamic acceleration of a physical device.
Accelerometers are transducers for measuring the dynamic acceleration of a physical device. The most common accelerometer measures acceleration only along a single axis. This type is often used to measure mechanical vibration levels. The second type is the triaxial accelerometer. This accelerometer is used to determine the type of vibration or the direction of acceleration.
Accelerometers designed to measure vibration are based on the piezoelectric effect. In a piezoelectric accelerometer, a mass applies force to a crystal creating a high-impedance charge, which results in a voltage across the crystal. Piezoelectric or charge mode accelerometers require an external amplifier to amplify the charge and to provide an impedance buffer. Easier to use accelerometers recommended for typical measurements include an integrated amplifier. These sensors are referred to as integrated electronic piezoelectric (IEPE) sensors.
To make accelerometer measurements, you will need amplification, current excitation, AC coupling, and filtering. It is also important to consider the dynamic range and frequency needs of your measurements.
The charge produced by piezoelectric accelerometers is very small yielding output voltage levels in the millivolt range. The piezoelectric crystal is a very high-impedance source and thus requires a high-input impedance low-noise detector. Thus, the sensor must be connected to a charge-sensitive amplifier to decrease noise and reduce the output impedance.
IEPE sensors have a charge-sensitive amplifier built inside them as close as possible to the transducer. This amplifier accepts a constant current source and varies its impedance with respect to a varying charge on the piezoelectric crystal. You can see this change in impedance as a change in voltage across the inputs of the accelerometer. Thus, the accelerometer uses only two wires per axis for both sensor excitation (current) and signal output (voltage).
As mentioned in the section above, accelerometers require an external current to be supplied to power the amplifier. In choosing a data acquisition system it is important to be aware of the excitation voltage the accelerometer requires. Depending on the sensor some platforms may not provide an adequate amount of excitation to work with a particular sensor.
The signal acquired from the sensor consists of both DC and AC components, where the DC portion offsets the AC portion from zero. AC coupling removes the DC offset in the system by means of a capacitor in series with the signal. An AC-coupled sensor system eliminates the long-term DC drift that sensors have due to age and temperature effect, dramatically increasing the resolution and the usable dynamic range of the system.
Vibration measurements are highly susceptible to noise. You can reduce this effect, however, by properly grounding the system. You can avoid improper grounding resulting from ground loops or floating nodes by ensuring that either the signal conditioning input or the sensor is grounded but not both. If the sensor is grounded, you must connect it differentially. If the sensor is floating, you should connect the signal conditioning system’s inverting input to ground.
To be sure that you are sampling the correct range of frequencies, add a lowpass filter before the sampler and the ADC. This ensures that you attenuate higher-frequency noise and that these aliasing components above the sampling rate do not distort the measurement.
Dynamic range is a measure of how small you can measure a signal relative to the maximum input signal the device can measure. Expressed in decibels, the dynamic range is 20 log (Vmax/Vmin). For example, a device with an input range of ±10 V and a dynamic range greater than 110 dB may have a voltage ratio of 106. Thus, with a maximum signal of 10 V, the smallest signal that you can see on the device is 10 µV. Thus, the input range and the specified dynamic range are important for determining the needs of your system.
Figure 2. The PXI platform provides a complete range of functionality for vibration monitoring and analysis with high-performance signal conditioning and up 272 synchronized channels per chassis.
The PXI platform provides a complete range of functionality for vibration monitoring and analysis with high-performance signal conditioning and up 272 synchronized channels per chassis. National Instruments offers a variety of PXI modules that provide IEPE signal conditioning, 24-bit resolution, antialiasing filters, up to 118 dB dynamic range, and up to 204.8 kS/s simultaneously sampled channels. You can choose modules with two inputs and two outputs or input modules with four, eight, and 16 channels.
Figure 3. NI CompactDAQ is recommended as a rugged and portable low-cost solution for vibration measurements.
NI CompactDAQ is recommended as a rugged and portable low-cost solution for vibration measurements. C Series vibration modules offer 24-bit resolution, ±5 V input range, up to 102 dB dynamic range, and up to 51.2 kS/s simultaneously sampled channels. NI CompactDAQ incorporates IEPE signal conditioning for accelerometers, antialiasing filters, and software-selectable AC/DC coupling.
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Sound and Vibration Software
National Instruments sound and vibration software provides a complete software solution for all acoustic; electroacoustic; noise, vibration, and harshness (NVH); and machine condition monitoring applications. Based on an open analysis capability and a flexible measurement library, the NI Sound and Vibration Measurement Suite and NI Sound and Vibration Toolkit present a unique software-based measurement approach to creating customized applications.