Understanding Displacement Probes
Definition: What is a Displacement Probe?
Displacement probe (also called proximity probe, displacement sensor, or non-contact position sensor) is a sensor that measures the distance (gap) between the probe tip and a target surface without physical contact, outputting a signal proportional to displacement typically in micrometers or mils. In vibration monitoring, displacement probes are mounted permanently in machine casings pointing at rotating shafts to measure shaft radial position, axial position, and dynamic vibration with response from DC (static position) to several kilohertz.
Displacement probes, most commonly eddy current probes, are the standard sensor type for critical turbomachinery permanent monitoring systems because they measure actual shaft motion directly, provide absolute position information for clearance monitoring, and operate reliably at high temperatures and in contaminated environments where contact sensors would be unsuitable.
Types by Technology
Eddy Current Probes (Most Common)
- Use electromagnetic induction in conductive targets
- Industry standard for turbomachinery
- Range: 0.5-5 mm typical
- Frequency: DC to 10+ kHz
- Temperature: to 350°C
Capacitive Probes
- Measure capacitance between probe and target
- Very high resolution (nanometer capability)
- Works on non-conductive targets
- Primarily precision applications and research
Laser Displacement Sensors
- Optical triangulation or interferometry
- Non-contact, long range possible
- High accuracy
- Expensive, less rugged
- Troubleshooting and research use
Ultrasonic Displacement Sensors
- Time-of-flight measurement
- Long range (to several meters)
- Lower resolution than other types
- Specialized applications
Key Advantages
Direct Shaft Measurement
- Measures rotor motion, not bearing housing
- True shaft vibration unaffected by bearing or structure
- Essential for rotor dynamics analysis
DC Response
- Measures static position (0 Hz)
- Detects slow drifts and thermal growth
- Monitors average shaft position
- Accelerometers cannot measure DC
Absolute Position
- Provides shaft position relative to bearing centerline
- Clearance monitoring capability
- Detects rotor shifts from bearing wear
- Protection trip on excessive displacement
Standard Installation
XY Probe Configuration
- Two probes 90° apart (horizontal and vertical typically)
- Measures shaft position in two perpendicular directions
- Enables orbit analysis and 2D shaft motion visualization
- Standard for API 670 turbomachinery monitoring
Axial Probe
- Mounted facing shaft end or thrust collar
- Measures axial position and thrust bearing performance
- Protects against excessive axial movement
- One or two probes (redundancy)
Applications
Permanent Monitoring (Primary Use)
- Steam and gas turbines
- Large compressors and generators
- Critical pumps (API 610 specification)
- Continuous monitoring with alarm and trip functions
- API 670 compliance for machinery protection
Rotor Dynamics Testing
- Critical speed identification
- Startup/coastdown analysis
- Mode shape determination
- Damping measurement
Clearance Monitoring
- Track shaft position vs. seals, labyrinths
- Detect bearing wear allowing rotor shift
- Thermal growth measurement
- Prevent rotor-stator contact
Displacement probes are the premium sensor choice for critical rotating machinery, providing comprehensive shaft position and vibration information that enables rotor dynamics analysis, clearance monitoring, and machine protection. While more expensive and complex than accelerometers, their unique capabilities—DC response, absolute position, direct shaft measurement—make them essential for turbomachinery and other high-value rotating equipment requiring the highest level of monitoring and protection.
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