What is a Seismic Transducer? Inertial Reference Sensor • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is a Seismic Transducer? Inertial Reference Sensor • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is a Seismic Transducer? Inertial Reference Sensor

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Understanding Seismic Transducers

Definition: What is a Seismic Transducer?

Seismic transducer (also called seismic sensor or inertial transducer) is a vibration sensor that uses an internal seismic mass (proof mass) suspended by springs or other compliant elements as an inertial reference to measure the absolute motion of the sensor base. When the sensor housing vibrates, the inertial mass tends to remain stationary in space (above the mass-spring system’s natural frequency), and the relative motion between the vibrating housing and the relatively stationary mass is measured and converted to an electrical signal representing vibration.

The term “seismic” comes from earthquake measurement (seismometers) where this principle was first used—a suspended mass remains relatively stationary while the ground moves beneath it. In machinery monitoring, both velocity transducers and accelerometers are seismic transducers, though the term is most commonly associated with velocity pickups.

Operating Principle

Mass-Spring-Damper System

  • Seismic Mass: Suspended inside sensor housing
  • Spring: Mechanical springs or flexures supporting mass
  • Damping: Air, magnetic, or fluid damping
  • Transduction: Relative motion converted to electrical signal

Frequency Response Regions

  • Below Natural Frequency: Mass and housing move together (poor response)
  • At Natural Frequency: Resonance (amplified but distorted)
  • Above Natural Frequency: Mass stationary, housing vibrates (good measurement region)
  • Usable Range: Typically > 2× natural frequency

Types of Seismic Transducers

Velocity Transducers (Moving-Coil)

  • Magnet suspended by springs inside coil
  • Relative velocity generates voltage (electromagnetic induction)
  • Natural frequency typically 8-15 Hz
  • Usable above 16-30 Hz
  • Directly measures velocity

Accelerometers

  • Piezoelectric: Piezo crystal senses mass force
  • MEMS: Capacitive or piezoresistive sensing
  • Higher natural frequency (10-30 kHz)
  • Usable from ~1 Hz upward
  • Measures acceleration

Seismic vs. Non-Seismic Sensors

Seismic Sensors (Inertial Reference)

  • Accelerometers, velocity transducers
  • Measure absolute motion
  • Mounted on vibrating structure
  • Internal mass provides reference
  • Most common for machinery monitoring

Non-Seismic Sensors (External Reference)

  • Proximity probes (eddy current)
  • Measure relative motion between two surfaces
  • Require stationary mounting point
  • Measure shaft motion relative to bearing
  • Used for shaft vibration measurement

Advantages of Seismic Design

Self-Contained Reference

  • No external reference needed
  • Can mount anywhere on vibrating structure
  • Measures absolute motion in inertial space

Versatility

  • Single sensor type for many applications
  • Temporary or permanent installation
  • Portable between machines

Limitations

Frequency Response Limitations

  • Cannot measure below ~2× natural frequency reliably
  • Velocity transducers poor below 15-20 Hz
  • Trade-off: low natural frequency (good low-frequency response) vs. large size

Measures Housing Motion

  • Measures bearing housing, not shaft directly
  • Housing vibration ≠ shaft vibration (affected by bearing stiffness, structure)
  • For direct shaft motion, need proximity probes

Applications

Machinery Condition Monitoring

  • Bearing housing measurements
  • Overall vibration trending
  • Bearing defect detection
  • General machinery diagnostics

Structural Vibration

  • Building and foundation vibration
  • Seismic monitoring (earthquakes)
  • Ground vibration from machinery

Modal Analysis

  • Measure structural response to impacts
  • Determine natural frequencies and mode shapes
  • Transfer function measurements

Seismic transducers, using internal masses as inertial references, form the foundation of vibration measurement in rotating machinery monitoring. Understanding the seismic principle—how suspended masses enable absolute motion measurement—explains both the capabilities and limitations of accelerometers and velocity transducers, the workhorses of industrial vibration analysis programs.

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