Understanding Seismic Transducers
A seismic transducer — also called a seismic sensor or inertial transducer — is a vibration sensor that uses an internal seismic mass (a “proof mass”) suspended on springs or compliant flexures as an inertial reference, allowing it to measure the absolute motion of the sensor base. When the housing vibrates, the relative motion between the suspended mass and the housing is converted into an electrical signal that represents the vibration. Depending on where the measurement frequency lies relative to the mass-spring system’s natural frequency, the sensor works in one of two regimes: above resonance the mass tends to stay still in space and the relative motion follows the housing displacement (the classic seismometer and velocity-pickup regime), while below resonance the small residual deflection of the mass is proportional to the housing acceleration (the accelerometer regime). The defining feature is that the reference is carried inside the sensor, so no fixed external datum is required.
The name “seismic” comes from earthquake instrumentation: a seismometer’s suspended mass remains relatively still while the ground heaves beneath it. In machinery monitoring, both velocity transducers and accelerometers are seismic transducers in this sense, although the term is most often associated with the classic velocity pickup.
1. Operating Principle
Mass-Spring-Damper System
Every seismic transducer is, at heart, a small mechanical oscillator with four functional parts:
- Seismic mass: a calibrated proof mass suspended inside the sensor housing.
- Spring: mechanical springs or thin flexures that support the mass.
- Damping: air, magnetic (eddy-current) or fluid damping that tames the resonance.
- Transduction: the element that turns the mass-to-housing relative motion into a voltage.
Frequency Response Regions
How the sensor behaves depends entirely on where the excitation frequency falls relative to its own natural frequency — and the two main sensor families deliberately work on opposite sides of the resonance:
- Below natural frequency (accelerometer regime): mass and housing move essentially together, and the small residual deflection of the mass is proportional to the housing acceleration. Piezoelectric and MEMS accelerometers work here, below their high mounted resonance.
- At natural frequency: the system resonates — output is amplified but distorted and unreliable, so measurement near resonance is avoided.
- Above natural frequency (seismometer regime): the mass effectively stays put while the housing vibrates around it, and the relative motion follows the housing displacement (or velocity). Displacement seismometers and moving-coil velocity pickups work here, above their low natural frequency.
- Usable ranges: a velocity pickup is conventionally used above roughly 2× its natural frequency, where its response has settled and is flat; an accelerometer is used well below its mounted resonance — typically up to about one-third of it for good accuracy.
2. Types of Seismic Transducer
Velocity Transducers (Moving-Coil)
- A magnet is suspended on springs inside a fixed coil (or vice versa).
- Relative velocity between magnet and coil generates a voltage by electromagnetic induction.
- Natural frequency typically 8–15 Hz.
- Usable above roughly 16–30 Hz.
- Measures velocity directly, needing no signal integration.
Accelerometers
- Piezoelectric types use a piezo crystal to sense the inertial force of the mass.
- MEMS types use capacitive or piezoresistive sensing on a micro-machined element.
- Much higher (mounted) natural frequency, typically 10–30 kHz.
- Unlike velocity pickups, used below resonance: usable from about 1 Hz up to roughly one-third of the mounted resonance frequency.
- Measures acceleration, which can be integrated to velocity or displacement.
3. Seismic vs. Non-Seismic Sensors
The seismic family is best understood by contrast with sensors that rely on an external reference.
Seismic Sensors (Inertial Reference)
- Accelerometers and velocity transducers.
- Measure absolute motion in inertial space.
- Mount directly on the vibrating structure.
- Carry their own internal mass as the reference.
- The most common choice for machinery monitoring.
Non-Seismic Sensors (External Reference)
- Proximity probes (eddy-current sensors).
- Measure relative motion between two surfaces.
- Require a stationary mounting point to look from.
- Typically measure shaft motion relative to the bearing.
- Standard for shaft-vibration measurement on machines with journal bearings.
4. Advantages of the Seismic Design
Self-Contained Reference
- No external reference frame is needed.
- The sensor can be mounted almost anywhere on a vibrating structure.
- It reports true absolute motion in inertial space.
Versatility
- One sensor type covers a great many applications.
- Suits both temporary surveys and permanent installations.
- Easily portable from machine to machine.
This versatility is why portable instruments rely on them. The two-channel Balanset-1A, for example, takes its readings from accelerometers clamped to the bearing housings — self-referencing seismic sensors that need no fixed datum, so an engineer can move quickly between measurement points and machines while balancing on site.
5. Limitations
Frequency Response Limitations
- Velocity pickups cannot measure reliably below roughly 2× their natural frequency; moving-coil types in particular respond poorly below 15–20 Hz. There is an inherent trade-off: a lower natural frequency gives better low-frequency reach but demands a larger, heavier sensor.
- Accelerometers lose accuracy as the measurement frequency approaches their mounted resonance; the practical upper limit (typically about one-third of the mounted resonance) depends strongly on the mounting method (see ISO 5348).
- At the very low end, accelerometer response is limited by the sensing element and amplifier electronics rather than the seismic suspension — typically down to about 0.5–1 Hz for standard industrial units.
Measures Housing Motion
- The sensor reports the motion of the bearing housing, not the shaft directly.
- Housing vibration is not the same as shaft vibration — it is filtered by bearing stiffness and the surrounding structure.
- Where the true shaft orbit is what matters, proximity probes are required instead.
6. Applications
Machinery Condition Monitoring
- Bearing-housing vibration measurements.
- Overall vibration trending.
- Bearing-defect detection.
- General rotating-machinery diagnostics.
Structural Vibration
- Building and foundation vibration surveys.
- Seismic monitoring of earthquakes.
- Ground-borne vibration radiated from machinery.
Modal Analysis
- Measuring a structure’s response to a calibrated impact.
- Determining natural frequencies and mode shapes.
- Building the frequency response functions used in modal analysis.
Seismic transducers, using an internal suspended mass as an inertial reference, form the foundation of vibration measurement on rotating machinery. Grasping the seismic principle — how a suspended mass enables absolute-motion measurement, and why velocity pickups work above their natural frequency while accelerometers work below their mounted resonance — explains both the strengths and the limits of these twin workhorses of every industrial vibration-analysis programme.
