Industrial Vibration Sensors: IEPE Accelerometers, Velocity Sensors and Proximity Probes

1. Introduction: The Vibration Monitoring Imperative

The reliability of rotating equipment is essential for operational continuity and safety in the Aerospace and Energy sectors. Excessive vibration is an early indicator of mechanical degradation, which can lead to unplanned downtime, costly damage and hazards to personnel. Rigorous vibration monitoring makes it possible to anticipate these failures, optimize maintenance cycles and extend the lifespan of assets.

This technical document provides an in-depth reference on the main types of industrial vibration sensors: integrated electronics (IEPE) accelerometers, velocity sensors (speed transducers), and eddy current proximity probes. It details their operating principles, specifications, areas of application and standards governing their use, providing a solid foundation for maintenance and reliability engineers.

2. Fundamentals of Vibration and Measurement

2.1. Vibratory Quantities: Displacement, Velocity, Acceleration

Vibration is an oscillatory movement characterized by three main physical quantities, linked by derivation and temporal integration:

• Displacement (D): Measures the physical movement of the object relative to a reference position. Expressed in micrometers (µm) or millimeters (mm). It is relevant for low frequencies and mechanical games.
• Velocity (V): Measures the speed of vibrational movement. Expressed in millimeters per second (mm/s). RMS velocity is an excellent indicator of vibration severity over a wide frequency range (ISO 20816 standards).
• Acceleration (A): Measures the rate of change of velocity. Expressed in meters per second squared (m/s²) or units of gravity (g, where 1 g ≈ 9.81 m/s²). Acceleration is sensitive to high frequencies and impacts, revealing bearing or gear faults.

For a simple harmonic motion with angular frequency $ \\omega = 2 \\pi f $:

• $ V_{peak} = \\omega \\cdot D_{peak} $
• $ A_{pic} = \\omega \\cdot V_{pic} = \\omega^2 \\cdot D_{pic} $

These relationships confirm that displacement is predominant at low frequencies, velocity at intermediate frequencies, and acceleration at high frequencies.

2.2. Principles of Transduction

2.2.1. Piezoelectric Effect (IEPE Accelerometers)

Accelerometers use the piezoelectric effect. A piezoelectric material (usually quartz or PZT ceramics) generates an electrical charge proportional to the mechanical force applied to it. A seismic mass is attached to the crystal; under the effect of vibrations, this mass exerts a force on the crystal, producing an electric charge. The integrated electronics (IEPE for Integrated Electronics Piezo-Electric, often called ICP® by PCB Piezotronics or CCLD by Brüel & Kjær) converts this charge into a low impedance voltage signal, directly proportional to the acceleration.

2.2.2. Electrodynamic Principle (Velocity Sensors)

Passive velocity sensors work on the principle of electromagnetic induction. A coil is suspended in a magnetic field created by a permanent magnet. When the coil moves relative to the magnet (as a result of vibration), a voltage is induced in the coil, proportional to the relative speed of movement. Piezoelectric (Piezo-Velocity) velocity sensors integrate an IEPE accelerometer and an internal integration circuit to provide a velocity signal.

2.2.3. Eddy Currents (Proximity Probes)

Eddy current proximity probes measure the non-contact relative displacement between the probe tip and a conductive metal target (usually the shaft of a machine). The probe contains a radio frequency coil that generates an electromagnetic field. When the metal target enters this field, eddy currents are induced in the target. The energy of these eddy currents absorbs part of the energy of the probe field, modifying its impedance. The conditioning electronics measure this change in impedance, which is directly correlated to the distance between the probe and the target.

3. Technical Specifications and Applicable Standards

3.1. General Vibration Standards

• ISO 20816 (formerly ISO 10816 and ISO 7919): This series of standards defines the measurement methods and criteria for evaluating the vibration severity of rotating machines (excluding reciprocating machines). Thresholds are based on RMS velocity for bearing measurements. The ISO 20816-3 standard is the most common for industrial machines with a power greater than 15 kW.
• NF E 90-300: French standards for monitoring and diagnostics of machines.
• ISO 5348: Specifications for vibration accelerometers.
• ATEX (Directive 2014/34/EU): Essential for potentially explosive environments (zones 0, 1, 2 for gas; 20, 21, 22 for dust). Sensors must be Ex ia (intrinsically safe) or Ex d (explosion-proof) certified.
• IEC 60529: Defines the Protection Index (IP) against the intrusion of solids and liquids (e.g. IP67, IP68).

3.2. Specifications of IEPE Accelerometers

• Sensitivity: 10 mV/g to 500 mV/g. The standard value is 100 mV/g for general monitoring of rotating machines, offering a good compromise between output signal and measurement range.
• Frequency Range: Typically 0.5 Hz to 10,000 Hz (±3 dB), with a resonant frequency above 25 kHz. Some low frequency models reach 0.1 Hz.
• Measuring Range: Up to ±50 g (for 100 mV/g) or ±500 g (for 10 mV/g).
• Operating Temperature: From -50°C to +120°C as standard. High temperature versions can reach +165°C. Beyond that, load sensors are used.
• Protection Index: Typically IP67 or IP68, with 316L stainless steel housings.
• ATEX certifications: Commonly available in II 1G Ex ia IIC T4/T6 Ga for the most critical areas.
• Cost: From €200 to €800 for standard models, more for ATEX or high temperature versions.

3.3. Velocity Sensor Specifications

• Sensitivity: 4 mV/mm/s is a typical value. Models up to 20 mV/mm/s exist for low speeds.
• Frequency Range: From 2 Hz to 1000 Hz (electrodynamic) or 1.5 Hz to 5000 Hz (piezo-velocity). This range covers imbalances, misalignments and mechanical play.
• Measuring Range: Up to 50 or 100 mm/s RMS.
• Operating Temperature: From -50°C to +120°C as standard. Specific models can reach +200°C or even +450°C (passive).
• Protection index: Frequently IP66, IP67 or IP68, in 316L stainless steel housing.
• ATEX certifications: Often available in II 1G Ex ia IIC T4/T6 Ga.
• Cost: From €300 to €1000 for standard models.

3.4. Eddy Current Proximity Probe Specifications

• Measuring Range: From 0.5 mm to 25 mm (standard), up to 50 mm for large probes. The range depends on the diameter of the probe head.
• Linearity: Typically ≤ ±1% of the measuring range (EM). High precision models reach 0.2% of MS.
• Resolution: In the order of micrometers (µm) or even nanometers for specific applications.
• Frequency Range (Dynamic): Up to 10 kHz as standard, with high dynamic versions reaching 80 kHz.
• Operating Temperature:Probe: from -30°C to +180°C. Electronic: 0°C to 70°C.
• Protection Index: Generally IP67 or IP68 for the probe itself.
• ATEX certifications: Frequently available in II 1G Ex ia IIC T4/T6 Ga for probes and their cables.
• Cost: Probe-cable-conditioner assembly from €800 to €2500 for the systems.

4. Selection and Sizing Guide

The choice of vibration sensor is critical and depends on the application, machine type, frequency range of interest, and environment. The table below provides decision support.

5. Good Installation and Commissioning Practices

The quality of the vibration measurement directly depends on the installation of the sensor.

• Surface Preparation: The mounting surface must be flat, clean, free of paint and roughness. A surface finish Ra < 3.2 µm is recommended.
• Mounting:
  • Stud Mount: Preferred method for accelerometers and velocity sensors. Ensures optimal mechanical coupling and faithful transmission of high frequencies. The tightening torque must comply with the manufacturer's specifications (eg: 2.7 Nm for an M6).
  • Adhesive Mounting: For inaccessible or non-machinable surfaces. Use a two-component epoxy adhesive, ensuring a thin and rigid layer. Limits the transmission of high frequencies.
  • Magnetic Mounting: Practical for temporary or diagnostic measurements. The coupling is less rigid and can attenuate high frequencies (> 1 kHz).
  • Proximity Probes: Mounted in a threaded bore passing through the housing, opposite the machined surface of the shaft. The initial air gap must be precisely adjusted according to specifications (eg: 1.27 mm ± 0.05 mm).
• Cabling: Use high quality shielded cables to minimize electromagnetic interference. IEPE sensor cables are often coaxial. Route cables away from sources of heat or intense electromagnetic fields (e.g. motors, speed controllers). Secure the cables securely to avoid friction and unwanted movements.
• Grounding: Proper grounding is essential to avoid ground loops and electrical noise. Use a single ground point.
• Calibration: The sensors must be regularly calibrated (according to ISO 17025 or NF X07-010 standards) to guarantee the accuracy of the measurements. The calibration period depends on the manufacturer's specifications and the criticality of the application, typically 12 to 24 months.

6. Failure Modes and Root Cause Analysis

Interpretation of vibration data is key to fault identification. Each type of mechanical failure generates a specific vibration signature.

6.1. Sensor Failure Modes

• Cable cut or short circuit: No signal or noisy signal. Often due to cable fatigue or mechanical damage.
• Electrical Noise: Interference from spurious signals, generally at 50/60 Hz (electrical network) or at higher frequencies (speed variators). Indicates a shielding or grounding problem.
• Zero Point Drift (Probes): Change in the reference position of the signal without physical variation. May be due to temperature changes or electronics issues.
• Signal Saturation: The signal exceeds the measurement range of the sensor or acquisition electronics, resulting in signal clipping.

6.2. Mechanical Failure Modes and Their Vibration Indicators

• Unbalance: Strong vibration at rotation frequency (1x RPM). Amplitude is proportional to speed.
• Misalignment: Axial and radial vibrations at 1x, 2x and sometimes 3x RPM. The ratio of axial/radial amplitudes is a good indicator.
• Bearing Faults: Specific fault frequencies (BPFI, BPFO, FTF, BSF) that appear well before catastrophic failure. Optimal detection in high frequency acceleration.
• Gear Faults: Meshing frequencies (GMF) and their harmonics, often with sidebands. Acceleration detection.
• Mechanical Play (Looseness): Harmonics of the rotation frequency (2x, 3x, etc.), often non-synchronous. May result in chaotic behavior. Visible in displacement and velocity.
• Rotor/Stator friction (Rub): Sub-synchronous, harmonic vibrations, and excitation of natural frequencies.

7. Predictive Maintenance and Condition Monitoring

The integration of vibration sensors into a predictive maintenance system makes it possible to move from reactive maintenance to proactive and conditional maintenance.

• Data Acquisition: The sensors are connected to local or remote acquisition systems (DAQ), transmitting data via industrial protocols (Ethernet/IP, Modbus TCP/IP, OPC UA) or wirelessly (LoRaWAN, Wi-Fi).
• Signal Analysis: Frequency analysis (Fast Fourier Transform - FFT) is the main tool. It decomposes the complex vibration signal into its frequency components, making it possible to identify specific signatures of defects. The analysis of temporal waveforms and the cepstrum complete this diagnosis.
• Alarm Levels:Alarm thresholds are defined according to ISO 20816 standards and machine specifications, considering zones A, B, C, D. Customized thresholds based on history and criticality can be implemented (e.g.: Alert at 4.5 mm/s RMS, Danger at 7.1 mm/s RMS for a Group 1 machine on rigid support).
• Trend and Prediction: Continuous monitoring makes it possible to follow the evolution of vibration levels and characteristic frequencies of defects. This provides a clear indication of the time remaining before failure (Time to Failure), allowing interventions to be planned accurately and the necessary spare parts ordered.
• SCADA/CMMS integration: Vibration data is integrated with SCADA (Supervisory Control and Data Acquisition) and CMMS (Computerized Maintenance Management System) systems for a comprehensive view of plant status and automated maintenance planning.

8. Sensor Comparison Matrix

This table compares the key features of the three types of sensors to make their selection easier.

9. Conclusion

Proper selection and implementation of vibration sensors are pillars of modern predictive maintenance. By understanding the physical principles, technical specifications and normative requirements (ISO 20816, ATEX, IEC 60529), engineers can deploy precise and reliable vibration monitoring systems. Whether detecting high frequencies with IEPE accelerometers, monitoring overall severity with velocity sensors, or precisely tracking shaft movement with proximity probes, each technology has its place in an integrated reliability program.

UNITEC-D GmbH, as a trusted supplier to the Aerospace and Energy industry, offers a comprehensive range of vibration sensors and monitoring solutions complying with the most rigorous standards (NF, AFNOR, EN). We help our customers choose CE, NF, ATEX and Nadcap certified components, thus guaranteeing unrivaled performance and safety. Explore our online catalog to find the solution tailored to your specific needs: https://www.unitecd.com/e-catalog/

10. References

1. ISO 20816-1:2016, Mechanical vibrations — Measurement and evaluation of machine vibrations — Part 1: General guidelines.
2. ISO 5348:1998, Vibrations and mechanical shocks — Calibration of accelerometers by translational shocks and continuous vibrations.
3. NF EN 60529:1992, Degrees of protection provided by the enclosures (IP Code).
4. NF EN 13463-1:2009, Protective devices and components intended for use in potentially explosive atmospheres - Part 1: Fundamental requirements and test methods.
5. Bently Nevada, Proximity Probe Handbook, General Electric Company, 2017.