1. Introduction
Creating accurate maintenance schedules based on condition (Condition-Based Maintenance, CBM) on Ukrainian industrial enterprises requires a perfect understanding and application of vibration control tools. Failure of heavy rotating equipment — feed water pumps at power plants, exhaust fans at coking plants, compressors in the chemical industry or main drives of rolling mills — leads to significant financial losses due to emergency stoppages.
The main engineering task is to properly select and integrate primary vibration transducers. Sensors serve as the basis for diagnosing rolling element bearings, rotor imbalances, shaft misalignment and resonance phenomena. The application of certified equipment meeting ISO and DSTU requirements guarantees data accuracy, reduces the level of false alarms and extends the mean time between failures (MTBF) of critical equipment.
This article thoroughly discusses three main types of vibration sensors: piezoelectric accelerometers with integrated electronics (IEPE), velocity transducers, and non-contact eddy current displacement meters. Clear engineering criteria for their selection, installation, and failure analysis are provided, taking into account the specifics of the Ukrainian industrial base.
2. Fundamental Principles of Vibration Measurement
Physical processes occurring in mechanical nodes, considered during vibration measurement, are described by three interrelated kinematic parameters: displacement ($s$), velocity ($v$), and acceleration ($a$). From a mathematical point of view, they are related by differentiation with respect to time:
v = \frac{ds}{dt}, \\quad a = \frac{dv}{dt} = \frac{d^2s}{dt^2}In harmonic oscillation with frequency $f$ and amplitude of displacement $X_0$, the peak values are determined by the formulas:
v_{peak} = 2 \\pi f X_0, \\quad a_{peak} = (2 \\pi f)^2 X_0It follows from these relationships an important conclusion for the reliability engineer: low frequencies (from 0.1 to 10 Hz), such as the rotational frequencies of heavy slow-speed mills in mining and beneficiation plants (MBC), have significant displacement amplitudes with small acceleration values. Measurement of displacement is here the most informative. On the contrary, high-frequency processes (from 1 kHz to 10 kHz and above), which accompany damage to rolling element bearing tracks or gear meshing in reducers, are characterized by microscopic displacements, but large accelerations. For their registration, accelerometers with a wide frequency range are required.
Piezoelectric effect forms the basis of operation of most industrial accelerometers. Crystal structures (quartz or artificial piezoceramics based on lead zirconate titanate — PZT) generate an electric charge $Q$, proportional to the applied acceleration $a$:
Q = d_{33} \\cdot F = d_{33} \\cdot m \\cdot awhere $d_{33}$ — piezoelectric modulus, $m$ — mass of the seismic inert mass inside the sensor housing.
3. Technical specifications and standards
Installation and operation of vibration control systems must comply with strict international and national regulatory documents:
- ISO 10816 / ISO 20816: Basic vibration evaluation standards for industrial equipment using measurements on non-rotating parts. Define machine classes and condition zones (A, B, C, D).
- ISO 13373: Guidelines on general condition monitoring and vibration diagnostics of machines.
- DSTU ISO 7919: Vibration measurement on rotating shafts and assessment of their technical condition.
- IEC 60079: Explosion-protected electrical equipment (critical for the Ukrainian oil and gas and chemical industries, where sensors require marking Exia or Exd).
- CE and UkrSEPRO: Mandatory conformity certification for industrial valves and electronics implemented on the market.
Main metrological characteristics of industrial sensors include sensitivity (for example, 100 mV/g for accelerometers), linear amplitude range, resonant frequency of the sensitive element (usually above 25-30 kHz) and degree of dust and water protection according to standards IEC 60529 (IP67 or IP68 for submerged environments).
4. Selection & Sizing Guide
The correct choice of sensor type depends on the kinematics of the unit, rotational speed and defects to be detected. The following table shows the comparison of three main technologies according to key engineering criteria.
| Selection Criterion | IEPE Accelerometer | Dynamic speed converter | Eddy current displacement sensor |
|---|---|---|---|
| Measured parameter | Acceleration ($g$ or m/s²) | Speed (mm/s) | Absolute/relative displacement (µm) |
| Frequency range | 0.5 Hz — 10 000 Hz | 2 Hz — 1 000 Hz | 0 Hz (static) — 10 000 Hz |
| Basic purpose | High-frequency defects (bearings, gearboxes) | General vibration (imbalance, misalignment) | Vibration of the shaft in sliding bearings, axial displacement |
| Typical sensitivity | 10, 100 or 500 mV/g | 3.9 to 25.4 mV/(mm/s) | 7.87 V/mm (200 mV/mil) |
| Working temperature | From -50°C to +125°C (high-temperature up to +165°C) | From -40°C to +105°C | From -35°C to +180°C (probes) |
| Mounting | M6/1/4"-28, adhesive support, magnetic | M8 Screw or Flange Mounting | Threaded body M10x1 / 3/8"-24 UNF |
Sensor Selection Algorithm for the Reliability Engineer:
- Determine the construction feature of the node: rolling bearings require IEPE accelerometers; sliding bearings of turbines or powerful compressors require eddy current probes.
- Evaluate operating temperatures and the presence of aggressive environments (lubricants, cooling fluids, acidic vapors). Choose the housing material — AISI 316L stainless steel or titanium.
- Check the cable length: for IEPE systems powered through the signal cable, the line length is limited by the cable capacitance and the power supply current (usually up to 300 m without additional amplifiers). For current-output systems, cable calibration together with the driver (pre-amplifier) is required.
5. Best Practices for Installation and Commissioning
Reliability of the signal path in the vibration system is 50% dependent on the correctness of mechanical and electrical mounting. Errors at this stage lead to distortion of high frequencies or appearance of false harmonics in the spectrum.
Mechanical fastening
The highest natural frequency and the best high-frequency transmission is ensured by a direct threaded connection. The surface of the sensor platform must be machined (surface roughness Ra ≤ 1.6 µm), free of paint and rust. During tightening of the bolt (thread M6), a torque wrench with a torque within the range of 2.5–3.0 N·m must be used. Under-tightening reduces contact stiffness (lowers the sensor's resonant frequency), while over-tightening deforms the housing or strips the thread.
Epoxy adhesive adapters are used on thin-walled housings or aluminum covers of reducers where drilling holes is impossible. Use a two-component epoxy adhesive with high shear strength. Magnetic clamps are used exclusively for manual diagnostics with portable instruments; for continuous monitoring, magnetic fixation is not allowed due to the reduction of the upper frequency range limit to 1–2 kHz.
Electrical installation and interference protection
Industrial facilities are equipped with variable frequency drives (VFD), welding machines, and powerful transformers, which generate electromagnetic interference (EMI). Connection rules:
- Use exclusively screened twisted pair cable with copper braided shield (coverage not less than 85%).
- Ground the screen of cable only on one side (usually on the side of the receiving equipment or distribution cabinet), to avoid the formation of grounding loops and balancing currents.
- Lay vibration sensor cables in metallic conduits or separate troughs at a distance of no less than 300 mm from the power cables supplying the motors.
6. Typical types of failures and analysis of their root causes (Root Cause Analysis)
Operational analysis of false indicators or sensor failures saves time for chief engineers. Below are the main failure scenarios of vibration sensors:
- Bias Voltage Drift at Output: For IEPE sensors, the normal bias voltage (DC component) is approximately +10...12 V DC. If the voltage drops below or rises to the supply level (for example, +24 V), this indicates an internal short circuit in the piezoelectric crystal or an open circuit in the internal operational amplifier, usually due to moisture in the connector.
- Tribo-electric noise effect: Appearance of low-frequency emissions and "noise" on the oscillogram. The primary cause is the use of poor quality or poorly secured cable subjected to mechanical vibrations. Deformation of the cable's dielectric generates parasitic charges.
- Thermal shock: A sudden change in process temperature (for example, feeding hot product into a cold pump) causes generation of pyroelectric charge in the sensor crystal, resulting in temporary saturation of the measurement channel (saturation).
- Mechanical resonance mounting: If the natural frequency of the sensor assembly on a thin bracket coincides with the rotor speed, a false increase in amplitude occurs on this harmonic.
7. Predictive Technical Maintenance and Condition Monitoring (PdM)
Integration of vibration sensors into modern condition monitoring systems (SCADA, DCS, or specialized CMS — Condition Monitoring Systems) shifts the enterprise from reactive maintenance ("after failure") to a predictive maintenance strategy.
Continuous data flow from IEPE accelerometers is subjected to spectral analysis (fast Fourier transform — FFT). This allows detecting defects at an early stage:
- BPFO (Ball Pass Frequency Outer Race): Characteristic frequency of the defect in the outer race of a rolling bearing.
- BPFI (Ball Pass Frequency Inner Race): Inner Race Defect Frequency.
- BSF (Ball Spin Frequency): Defect Frequency of Rolling Element.
- FTF (Fundamental Train Frequency): Separator Frequency.
Modern monitoring systems also use envelope vibration methods (Envelope Detection or Demodulation) to extract high-frequency impact pulses of combustion or micro-cracks against the general industrial noise of the machine. At the alert/alarm thresholds, the system automatically generates an order for component replacement through the company's corporate ERP system.
8. Comparative Matrix of Modern Vibration Monitoring Technologies
For quick orientation during the design of automation systems, a comparison matrix of parameters of industrial vibration control solutions is provided.
| Characteristic | General-purpose IEPE | Vibration sensor with 4-20 mA output | Eddy current movement system |
|---|---|---|---|
| Type of output signal | Variable voltage is proportional to acceleration (AC) | DC current 4-20 mA (average vibration value) | Voltage (DC) is proportional to the gap and displacement |
| Data Processing Complexity | High (requires FFT analyzers or data acquisition cards) | Low (connected directly to PLC / PLC) | Average (requires gap calibration before start-up) |
| Channel measurement cost | Moderate | Economic | High |
| Main industries in Ukraine | Metallurgy, energy, machinery industry | LCD, pump stations, conveyors, fans | Steam turbines, high pressure compressors |
9. Conclusion
Choosing the correct vibration sensors is the foundation of reliable operation of any industrial enterprise. IEPE accelerometers, velocity transducers and eddy current probes perform different, yet complementary tasks within the structure of modern monitoring. Compliance with installation standards, regular checking of signal circuits and deep analysis of spectrograms eliminate unexpected equipment stoppages.
Company UNITEC-D GmbH (Augsburg, Germany) supplies the full range of certified industrial components, measuring transducers and mounting accessories for Ukrainian manufacturing enterprises. Our engineers are ready to provide qualified assistance in selecting equipment according to the specifics of your technological process.
Familiarize yourself with the assortment in the electronic catalog and select the necessary components: UNITEC-D E-Catalog.
10. References
- ISO 20816-1:2016. Mechanical vibration — Measurement and evaluation of machine vibration — Part 1: General guidelines.
- DSТU ISO 10816-3:2015. Vibration. Evaluation of machine vibration by measurements on non-rotating parts. Part 3. Industrial machines with nominal power exceeding 15 kW and nominal rotational speed from 120 to 15 000 rpm.
- IEC 60079-0:2017. Explosive atmospheres — Part 0: Equipment — General requirements.
- Harris, C. M., & Piersol, A. G. (2009). Harris' Shock and Vibration Handbook (6th ed.). McGraw-Hill Education.
- Whitepaper: «Industrial Vibration Sensors: Selection, Installation, and Signal Conditioning Best Practices». UNITEC-D Engineering Division, 2023.
