Overview
Bridge operators need vibration data that is objective, repeatable, and easy to collect without closing lanes, stopping rail traffic, or installing sensors on every structure. The Ommatidia Q2 multichannel laser Doppler vibrometer enables mobile, non-contact bridge vibration measurements from safe observation points, capturing 65 simultaneous velocity signals across the structure.
This application note summarizes two field demonstrations on operational bridges: one road bridge measured under normal traffic and one railway bridge measured during a train crossing. The goal is to show how Q2 can support mobile bridge monitoring campaigns, baseline creation, and follow-up inspections. The measurements are not presented as a structural diagnosis or certified modal assessment.
The Challenge: Scalable Bridge Vibration Monitoring
Permanent structural health monitoring is valuable, but it is not practical as a first step for every bridge in a network. Between periodic visual inspection and fixed instrumentation, asset owners need a mobile measurement layer that is fast, safe, and repeatable.
A useful mobile method should work from accessible positions, avoid contact with the structure, and capture enough spatial information to compare different points of the asset during the same operational event.
The Q2 Approach: 65 Simultaneous Non-Contact Measurements
Q2 is positioned where there is a clear line of sight to the target area. The instrument projects a line of laser beams onto the structure and records line-of-sight surface velocity at each point.
This setup supports several outputs from the same acquisition:
- Time-domain response during traffic or train passages
- Displacement estimates obtained by integrating velocity signals
- Frequency spectra and dominant vibration components
- Comparison between measurement points
- Quality checks using the internal accelerometer of the instrument
The internal accelerometer does not measure the bridge directly. It records motion of the instrument and its immediate environment, making it a useful field quality-control reference.
Field Demonstrations on Road and Railway Bridges
Both demonstrations were performed remotely, without contact sensors, asset modification, or service interruption.
Several 60 s acquisitions were recorded under live traffic from a distance of roughly 10–17 m. The beams were mainly positioned along a central girder, with an additional acquisition used to compare different visible zones.
A valid 80 s acquisition captured the response before, during, and after a train crossing. The train entered the bridge approximately 30 s into the record and took about 10 s to cross.
The selected figures show the kind of evidence Q2 can provide in a compact mobile campaign: beam locations, time-domain response, frequency content, and setup quality checks.
Figure 3. Road bridge example: surface velocity from one laser channel and displacement estimated by integrating the velocity signal.
Figure 4. Road bridge example: velocity spectrum from laser vibrometry and comparison with the internal accelerometer expressed as equivalent velocity.
Railway Bridge Train-Crossing Example
The railway bridge case shows the response to a single operational train crossing. The vibration level rises as the train enters the bridge, remains elevated during the crossing, and decays afterward.
The RGB capture documents the approximate positions of the 65 laser beams on the structure. This is important for repeatable campaigns, because future measurements can be compared only when the instrument position, line of sight, and illuminated zone are well documented.
Figure 6. Railway bridge example: velocity waveform from one laser channel during the train-crossing event and displacement estimated from the same record.
Figure 7. Railway bridge example: train-crossing spectrum and comparison between laser vibrometry and the internal accelerometer expressed as equivalent velocity.
What the Measurements Showed
The demonstrations show the value of combining non-contact access with simultaneous multi-point acquisition.
Q2 recorded vibration increases associated with normal road traffic and with the train crossing.
Different channels did not always show identical behavior. This spatial information gives more context than a single-point measurement and helps guide follow-up measurements.
The road bridge showed recurring low-frequency components across repeated measurements. The railway bridge showed clear spectral content before, during, and after the crossing — useful candidates for future tracking.
In both demonstrations, comparing vibrometry spectra with the instrument accelerometer helped assess whether specific components were dominated by bridge response or could be influenced by motion of the setup.
How Bridge Owners Can Use the Data
Q2 does not replace engineering assessment or regulatory inspection. Its value is to add an objective, repeatable measurement layer that can be deployed quickly across selected assets.
For bridge monitoring programs, this supports:
Because Q2 measures many points at once, each campaign can capture both temporal and spatial information in a single setup. This is especially useful when direct access to the structure is limited.
From Pilot Campaign to Monitoring Workflow
A practical Q2 workflow can start with simple, repeatable measurements and expand as baselines are built.
Choose bridges with safe access, clear line of sight, and operational relevance.
Record instrument location, distance, orientation, target zone, optical configuration, and operating conditions.
Capture several traffic or train passages to evaluate repeatability.
Review the Q2 accelerometer to identify intervals affected by motion of the setup.
Track dominant frequencies, relative amplitudes, event duration, and spatial response patterns over time.
Combine Q2 data with inspection history, maintenance records, models, or complementary instrumentation when needed.
A practical layer for mobile bridge monitoring
The field demonstrations show that the Ommatidia Q2 multichannel laser Doppler vibrometer can support mobile bridge vibration monitoring from safe, non-contact measurement positions. The system captured operational vibration events, measured multiple points simultaneously, and provided time- and frequency-domain information suitable for baseline creation and follow-up campaigns.
Q2 gives bridge owners a practical way to add dynamic measurements to inspection programs before committing to permanent instrumentation. Ommatidia can support bridge monitoring demonstrations, mobile inspection campaigns, operational modal analysis pilots, and asset baseline programs.