Vibration measurement with IEPE accelerometers

IEPE accelerometers are the standard sensor for vibration measurement on test benches, machines and structures. They are robust and easy to cable, but need the right excitation, coupling, gain and sample rate. This article explains how an IEPE accelerometer works, how to size gain and sample rate for your frequency range, what to check before the first measurement, and which ICP DAS signal conditioners, DAQ modules and sensors fit.

How an IEPE accelerometer works

IEPE stands for Integrated Electronics Piezo-Electric. The piezoelectric element produces a charge proportional to acceleration, and a small amplifier inside the sensor converts it to a low-impedance voltage. The amplifier is powered over the same two wires that carry the signal, usually a coaxial cable.

The measuring device supplies a constant current of a few milliamps from a supply with a compliance voltage of roughly 18 to 30 V. With the sensor connected, a DC bias voltage appears on the line, and the vibration signal rides on top of it. The iSN-701-F15 accelerometer, for example, specifies a bias of 10 to 14 VDC. The input must therefore be AC coupled: a capacitor blocks the bias, and only the dynamic signal goes on to the amplifier and A/D converter.

Two consequences follow. The coupling capacitor forms a high-pass filter, so very low frequencies are attenuated. And the gap between compliance voltage and bias voltage limits how far the signal can swing before it clips.

Sensitivity, range and gain

Sensitivity is given in mV/g: a 100 mV/g sensor outputs 100 mV for 1 g. Choose sensitivity and gain so that the largest expected vibration does not clip, while the smallest vibration of interest stays well above the noise floor.

Worked example: an iSN-701-F15 accelerometer (100 mV/g, ±80 g) feeds an SG-3227 conditioner with a ±10 V output. The gain setting decides the usable range:

Gain also affects bandwidth. On the SG-3227 the upper cut-off frequency is 80 kHz at ×1 and ×10, and 50 kHz at ×100.

Sample rate versus frequency range

The sampling theorem requires a sample rate of more than twice the highest frequency. For spectrum analysis, a factor of 2.56 is a common choice because it leaves room for the anti-alias filter. Shock pulses and other time-domain waveforms need faster sampling still.

• Up to 1 kHz: at least 2.56 kS/s per channel
• Up to 5 kHz: at least 12.8 kS/s per channel
• Up to 10 kHz: at least 25.6 kS/s per channel

At the low end, the AC coupling sets the limit: the SG-3227 has a lower cut-off of 1.6 Hz with 0.1 µF coupling or 0.2 Hz with 1 µF, and the PET-AR400 lists 0.16 Hz. Check the accelerometer as well; the iSN-701-F15 is specified from 0.5 Hz.

On multi-channel inputs the rate can depend on the number of active channels. The PET-AR400 samples at 128 kHz with one channel, 64 kHz with two and 32 kHz with three or four, with a bandwidth of 12.8 kHz (−0.1 dB) for one or two channels and 6.4 kHz for three or four.

Setup checklist

• Set the excitation current according to the sensor data sheet. The SG-3227 offers 2, 4, 6 or 10 mA per channel; higher currents help drive long cables at high frequencies.
• Check the bias voltage before measuring. A value near zero points to a short circuit, a value near the supply voltage to an open circuit. The SG-3227 shows this per channel with LEDs: green for normal, yellow for open, red for short.
• Mount the sensor rigidly. Stud mounting gives the widest frequency range; magnets and adhesive pads lower the usable upper frequency.
• Avoid ground loops between machine and DAQ system, for example with isolated inputs or an isolating mounting base.

ICP DAS products for IEPE vibration measurement

Signal conditioner for existing DAQ hardware

The SG-3227 is a 2-channel IEPE signal conditioner. Each channel is set with DIP switches for excitation current (2, 4, 6 or 10 mA) and gain (×1, ×10 or ×100), and has a ±10 V voltage output. The output can go to any DAQ input with a ±10 V range, such as the simultaneously sampling PCIe-8620 board or the PET-7H16M Ethernet DAQ module.

Ethernet DAQ with IEPE inputs

The PET-AR400 has four differential IEPE inputs on BNC connectors with 4 mA excitation, a 24-bit A/D converter, jumper-selectable AC or DC coupling, and software or analog threshold triggering. Data is read by TCP streaming or Modbus TCP, with SDKs for Windows (including LabVIEW) and Linux, and the module can be powered by PoE.

Stand-alone logging and permanent monitoring

The AR-200 and AR-400 data loggers acquire 2 or 4 IEPE channels simultaneously with 3 mA excitation and 16-bit resolution, at up to 200 kHz (AR-200) or 125 kHz (AR-400) per channel. Data is stored on a microSD card for offline analysis, and recording can be triggered by push-button, timer, threshold or digital input.

For permanent monitoring, the iSN-701X powers one IEPE accelerometer with 4 mA, converts the signal into vibration characteristic values and raises an alarm when a value exceeds its threshold. The values are read over Modbus TCP or Modbus RTU; the iSN-701X-mA adds a 0/4–20 mA output.

Accelerometers

The iSN-701-F15 is a single-axis IEPE accelerometer with 100 mV/g sensitivity, ±80 g range and a frequency response of 0.5 Hz to 15 kHz, IP68 rated (without cable), with a 3 m or 6 m cable. The iSN-711 and iSN-713 vibration sensor modules use MEMS sensors rather than IEPE and suit low-frequency monitoring over RS-485.

Send the sensor type, frequency range and number of channels to our technical sales team, and we will help you put the measurement chain together.