[Core Summary] The disagreement in model selection for vibration monitoring is mostly not a technical challenge, but rather inconsistency in terminology: the mismatch between sensitivity, frequency response range, measurement range, and output type renders the data read out by the backend incomparable. This article provides definitions and interpretation guidelines based on high-frequency terms used on-site, with all numerical values marked as [please verify] and subject to the specifications on the nameplate.
1. How to understand core terms
Sensitivity
The output electrical signal magnitude corresponding to the unit input vibration quantity. The common unit for IEPE type is mV/g; for 4-20mA type, it is expressed using a full-scale correspondence, for example, [please verify] mm/s corresponds to 20mA.
Key points of on-site interpretation: The nominal value differs from the measured value, and the measured value has a deviation range (e.g., ±[please verify]%). Switching to a different sensor may cause a step change in the trend.
Frequency Response
The frequency range for reliable measurement is typically bounded by ±3dB or ±10% amplitude error, for example, from [please verify] Hz to [please verify] Hz.
Key points of on-site interpretation: The fault characteristic frequency must fall within a specific range. When the characteristic frequencies of the bearing outer ring and gear meshing are high and exceed the upper limit of frequency response, the readings are low, appearing as if the "equipment is normal.".
Measuring Range
The measurable upper limit of vibration amplitude is denoted in g for acceleration-type devices and in mm/s for velocity-type devices.
Key points of on-site interpretation: Choosing a larger measurement range results in insufficient resolution, and early minor degradation is drowned in noise; choosing a smaller range leads to signal clipping during large impacts.
IEPE
Integrated Electronics Piezo-Electric (IEPE) is an interface standard where the piezoelectric element and the preamplifier circuit are integrated within the sensor. It is powered by a constant current source, with the signal and power supply sharing the same coaxial cable, and outputs an alternating current voltage. It is also known as the ICP interface.
Key points for on-site interpretation: Output the original waveform, suitable for spectrum analysis and diagnosis. A dedicated acquisition card is required to provide constant current excitation (typically [please verify] mA), and it cannot be directly connected to a common PLC analog input.
4-20mA output
Convert the vibration magnitude into a 4-20mA DC current loop signal, with 4mA corresponding to zero and 20mA corresponding to the upper limit of the range; the two-wire system means that both power supply and signal are transmitted over the same two wires.
Key points of on-site interpretation: The output is a scalar (effective value of speed or acceleration) without waveform, which is sufficient for trend alarming but inadequate for spectrum analysis. The loop needs to be checked to see if the supply voltage can cover the sum of the minimum operating voltage drop of the sensor, line resistance, and sampling resistor voltage drop.
Mounting Resonance
The mechanical system, consisting of the sensor and its mounting structure, has its own resonant frequency. When the output is amplified near this frequency, the readings may be elevated, but it does not necessarily mean that the equipment's vibration has intensified.
Key points of on-site interpretation: The resonant frequency of the rigidly mounted stud is higher than that of the magnetic base, which in turn is higher than that of the handheld probe.
II. Terminology Comparison Table
III. Several concepts that are easily confused
Acceleration sensors, velocity sensors, and displacement probes are often collectively referred to as vibration sensors, but they have different applicable frequency bands: displacement probes are suitable for low-frequency applications and track the trajectory of the shaft centerline, velocity sensors cover mid-frequency ranges and align with ISO 10816 intensity evaluation, while acceleration sensors are geared towards high-frequency applications and are ideal for early detection of characteristics in bearings and gears. Choosing the wrong type means that the fault frequency band is not within the observation range.
Another point of confusion is the blanket statement that "4-20mA cannot be used for diagnosis." It is suitable for trending and alarm classification, and when spectrum is required, parallel IEPE channels can be connected at the same measurement point.
IV. Correspondence with the PC420 Series
The Wilcoxon PC420 series is a two-wire 4-20mA vibration/acceleration sensor that outputs a scalar value and is wired according to the current loop. It can be connected to existing PLC or DCS analog channels for continuous trend monitoring of fans, pumps, and motors. Sensitivity, frequency response, and measurement range are differentiated by model, and are subject to factory parameters [please verify].
After the terminology is standardized, the selection discussion will shift from "which sensor is better" to "which frequency band's faults need to be detected." It is necessary to verify the frequency response and range matching relationship according to the unit. You can contact the Shanghai Qinuo Technology team for assistance.
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