The Bently Nevada proximity amplifier is the core signal processing unit of the eddy current sensor system. It supplies power to the probe, converts tiny inductive reactance variations into standard voltage signals, and enables simultaneous output of static displacement (DC) and dynamic vibration (AC) signals.
1. System Composition (Three Core Components)
- Eddy Current Probe: A sensor fitted with a coil, installed close to the measured metal shaft.
- Extension Cable: Connects the probe to the proximity amplifier and matches system impedance.
- Proximity Amplifier: Performs power supply, oscillation, demodulation, amplification and signal output.
2. Core Working Principle
2.1 High-frequency Excitation (Power Supply for Probe)
The built-in high-frequency oscillation circuit inside the amplifier generates high-frequency alternating current at 1~2 MHz, which is transmitted to the probe coil via the extension cable to form a high-frequency alternating magnetic field at the probe end face.
2.2 Eddy Current Effect (Clearance Changes into Inductive Reactance Variation)
When the metal shaft approaches the probe, eddy currents are induced on the shaft surface, generating a reverse magnetic field that counteracts the original magnetic field, thus reducing the equivalent inductance and inductive reactance of the probe coil.
- Smaller clearance → stronger eddy current → lower inductive reactance
- Larger clearance → weaker eddy current → higher inductive reactance
2.3 Signal Demodulation (Convert Inductive Reactance to Voltage)
The detection and demodulation circuit inside the amplifier extracts clearance variation data from high-frequency oscillation signals, and converts it into DC voltage components (indicating static clearance) and AC voltage components (indicating dynamic vibration).
2.4 Amplification and Linearization (Convert Weak Signals to Standard Signals)
The demodulated weak signals are amplified by precision amplifier circuits and processed with linear compensation, ensuring a strict linear relationship between output voltage and probe-to-shaft clearance.
- Typical sensitivity: 7.87 V/mm (200 mV/mil)
- Output range: -2 V ~ -18 V (DC bias superimposed with AC signal)
2.5 Signal Output (Transmit to Monitoring System)
The amplifier outputs composite voltage signals:
- DC component: Corresponds to static shaft position and clearance, such as axial displacement and radial eccentricity.
- AC component: Represents dynamic vibration amplitude and frequency of the shaft.
Signals can be directly transmitted to Bently Nevada 3500 and other TSI monitoring systems for further analysis.
3. Key Features & Functions
- Dual-signal Output: Measures static displacement and dynamic vibration simultaneously for dual-purpose application.
- High Linearity: Strict linear correlation between voltage and clearance within specified measuring range (e.g. 0.25~2.5mm for 8mm probe).
- Strong Anti-interference Performance: Adopts high-frequency carrier and filtering design to suppress on-site electromagnetic interference.
- Impedance Matching: Forms a resonant system together with probes and extension cables to guarantee measuring accuracy.
- Power Supply Specification: -17.5V ~ -26V DC (typical -24V), working current ≤12mA.
4. Conclusion
By driving the probe to generate alternating magnetic fields via high-frequency excitation, the Bently Nevada proximity amplifier converts shaft clearance changes into coil inductive reactance variations based on the eddy current effect. Through subsequent demodulation, amplification and linearization processing, it outputs combined DC and AC voltage signals to realize non-contact measurement of shaft displacement and vibration.
As an essential core signal processing component of rotating machinery condition monitoring systems, it forms a complete measuring loop together with eddy current probes and extension cables, widely applied in vibration and displacement monitoring of large-scale units such as steam turbines, compressors and generators.
Its internal oscillation circuit produces 1~2 MHz excitation signals to power probe coils. Any clearance change between the measured metal shaft and the probe will lead to variation of coil equivalent inductance. The amplifier converts subtle inductive reactance changes into electrical signals via detection and demodulation circuits, and outputs composite voltage signals containing DC and AC components after precision amplification and linear adjustment.
The DC component reflects rotor static clearance and axial position, while the AC component characterizes dynamic vibration amplitude and frequency, supporting real-time monitoring and equipment protection after accessing TSI systems. Featuring high linearity, fast response and outstanding anti-interference capability, it operates stably even in harsh industrial environments with high temperature and intensive electromagnetic interference. It provides accurate and reliable raw data for unit safe operation, fault early warning and condition diagnosis, serving as an indispensable key part for on-line monitoring of rotating machinery.
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