CN0588

The circuit shown in Figure 1 demonstrates a MEMS vibration sensing solution with included IEPE compatible interface circuitry at 8.75 V to 14.75 V bias and 4 mA current consumption.

Vibration Sensor Types Piezo Vs. MEMS

Piezoelectric (piezo) and microelectromechanical systems or MEMS are two different types of sensor technologies commonly used for various applications, including measuring physical parameters such as acceleration, pressure, and temperature. The choice between these technologies can impact the way the sensor responds to different input signals, including AC and DC signals, and how it affects the input bandwidth.

Traditionally, piezoelectric sensors have been the benchmark for vibration monitoring and are still the sensor of choice in the highest sensitivity applications, or where very high accelerations are involved. MEMS sensors were previously deemed unusable due to their limited bandwidth, g-range, and noise performance across frequency. However, recent advancements in MEMS sensor technology (especially with frequency response and noise performance) make MEMS sensors a viable alternative in many CbM applications.

Piezoelectric sensors may offer better noise performance than MEMS at higher frequencies, but at low frequencies MEMS sensors offer lower noise all the way to DC. Being able to measure these low frequencies is very useful for wind turbines, and other types of slow rotating machinery used in metal processing, pulp/paper processing, and food/beverage industries where slow rotating speeds of assets below 60 rpm (1 Hz) are common.

Overall, MEMS sensors are typically smaller, have a lower cost, lower power consumption, and now have a greater frequency response. Table 1 shows the key differences between these two sensor types.

Sensor

The core of the CN0588 is the ADXL1002 high frequency, single- axis MEMS accelerometer. The noise spectral density of the ADXL1002 is 25 μg/√Hz and its 3 dB bandwidth is 11 kHz with sensor resonance frequency of 21 kHz. The noise and bandwidth is comparable with a piezoelectric sensor while the ADXL1002 provides superior performance in temperature sensitivity, DC to low frequency response, phase response (and thus, group delay), shock tolerance, and shock recovery. The sensor has a linear (within ±0.1% FSR) measurement range of ±50 g, which is large enough to support the majority of CbM applications. The ADXL1002 also has an integrated full electrostatic self-test (ST) function and an overrange (OR) indicator that allow advanced system level features and are useful for embedded applications.

IEPE Interface

The IEPE interface is a 2-wire protocol, signal and ground. An instrument, such as data logger, supplies power via the signal line to the vibration sensor as current with an arbitrary voltage, typically between 10 V to 30 V. Because the signal line is supplied by a current source, the sensor device can modulate the acceleration data on the voltage rail. Therefore, the single wire can be used for both the power supply and the modulated output voltage of the sensor.

Figure 2 shows an example of the IEPE output with a bias voltage of approximately 11.75 V and a full-scale range swing of ±3 V.

Signal Conditioning

The circuit shown in Figure 1 must be supplied with a 4 mA constant current, which can be provided by an IEPE vibration measurement setup or a current source. Figure 3 depicts the signal conditioning circuitry of the CN0588. In this design, the ADR5045 precision voltage reference acts as the voltage source to the ADXL1002 as it generates a stable VDD to power the sensor. The LT6015 op amp is configured to amplify the filtered output of the ADXL1002 (2.5 V ± 2 V) by a factor of 1.5x. Then, the ADR5045 and ADR5043 are connected in series to introduce a DC offset of approximately 8 V. This configuration complies to the IEPE standard and operates within the voltage range of 8.75 V to 14.75 V. The VIEPE also supplies the LT6015 op amp in this signal conditioning circuit. A 4.7 kΩ resistor is used to regulate the voltage from the IEPE signal to at least the minimum current to supply the ADXL1002.