Introduction

Ground Micro-Vibrations, also known as Seismic Vibration or Floor Vibration, refers to a kind of vibration noise with a small amplitude caused by non-earthquakes on the earth's surface. These vibrations are usually generated by natural vibration sources (such as wind, waves, etc.) or artificial vibrations (such as machine vibration sources, vehicles, etc.), that are a stable non-repetitive random fluctuation on the ground. Seismic Vibration is characterized by being of low frequency and small amplitude, and its displacement is generally only a few microns to tens of microns, with a frequency variation range between 0.3-5.0 Hz.

The Seismic Vibration (micro-vibration) test system consists of three parts: signal acquisition instrument, micro-vibration sensor, and vibration test and analysis system. The sensor converts the measurement reading of the vibration into an analog signal, which is transmitted to the signal acquisition instrument via a cable. The signal acquisition instrument collects the analog signal and converts the analog signal into a digital signal through the A/D conversion module. The sampling parameters are collected and controlled by the signal acquisition instrument and the on-site monitoring computer. Dynamic data collected on site can then be analyzed by dynamic test analysis software.

Micro-Vibration Signal Acquisition Equipment

Spider System

The Spider device is a highly modular, truly advanced and scalable dynamic measuring device for structural applications. It is a highly accurate portable system. Using multiple modules, the measurement distance can reach thousands of kilometers. It supports PTP protocol (or IEEE 1588) technology. PTP protocol utilizes clock synchronization between hardware and has a high synchronization accuracy of +/-50 ns.

The Spider's DSP has strong processing power with dual 24-bit A/D chips. This allows for a high dynamic range of measurement that can detect voltages as small as 6 μV. The ± 20 V per measurement channel eliminates the need to set the input range/amplification factor for the Spider, as is the case with conventional data acquisition devices. Regarding high dynamic range technology: dynamic range is defined as the ratio of the maximum voltage value to the noise floor of an input channel specified at a determined range. In principle, the 24-bit A/D chip has a maximum resolution of only -144 dB. At present we use a dual A/D chip acquisition technology, which can reach up to 160 dB dynamic range. The basic principle is that each input channel is designed with two A/Ds that cover two separate ranges; for example, one is +/-0.1 V, and one is +/-20 V. When the signal amplitude is less than or equal to 0.1 V, it is read by the “small” measurement chip, and when the amplitude is greater than 0.1 V it is read by the “large” measurement chip. These signals are then "put together" to create a complete set of sampling waveforms.

The figure below shows the Spider's noise floor at the 20 V range:

As you can see, the passband noise floor of the Spider device is about -140 dB, and the 20 V range corresponds to 26 dB, so the effective dynamic range of the Spider is: 26 dB- (-140 dB) > 160 dB. This technology leads to much higher signal measurement accuracy and is very convenient to setup and use.

Spider systems are ideal for micro-vibration testing applications such as testing of bridges, high-rise buildings, sensitive medical equipment, etc. Any structure, equipment, or object affected by routine vibrations or the external environment can necessitate this type of testing.

This method of measurement is limited by the length of the sensor cable in terms of distance, and an increase of the cable length will lead to attenuation and delay in signal transmission, affecting the quality of the signal and the measurement accuracy. This method is not suitable when the distance is measured at more than 100 kilometers at the same time.

GRS Equipment

The GRS is a rugged, lightweight, battery-powered dynamic data logger and real-time dynamic signal analyzer with unmatched performance and accuracy. It is ideal for a wide range of industries that require high-quality acoustic and vibration measurements. In addition to on-site real-time processing, these industries require fast, easy, and accurate data recording. It has the following features:

Rugged, weatherproof housing. It is designed to withstand strong winds, dust, and heavy rain.
Two external battery power guarantees 12 hours of full operation.
Solar panels can be used to power the GRS and charge its external batteries to facilitate continuous remote operation.
Four input channels are available, and 24-bit dual AD chips provide services. DSP implements patented technology (United States Patent No. 7,302,354) to achieve better than 150 dBFS. Micro-vibration signals can be acquired and analyzed with high precision.
Large storage space allows recording for up to several months.
The GPS receiver allows time synchronization data sampling with an accuracy of up to 100 nanoseconds using patented (United States Patent No. 11,611,946) GPS time synchronization technology.
Equipped with cellular and satellite modules that allow for remote operation.
Scheduling function for fully autonomous operation

The GRS combines high accuracy and portability with Crystal Instrument’s timestamp synchronization technology to simultaneously monitor channels spread across hundreds of miles.

Timestamp Synchronization Technology for GRS Devices: Crystal Instruments has introduced accurate time-stamping technology with GPS as a time source. This advanced technology is deployed in the CoCo-80X portable analyzer and GRS units. It has been successfully used by NASA to test the sonic booms created from X-59 aircraft [Link] . The time stamps are accurate up to 100 nanoseconds. Using this time-stamping technique, we have developed theories and algorithms to calculate the self-spectrum and cross-spectrum between measurement channels on different data acquisition units thousands of miles apart. This technology allows the simultaneous acquisition of data from very distant measurement channels without using any connected wires.

The Global Navigation Satellite System (GNSS) installed in the GRS or CoCo provides high-quality, accurate timing for demanding applications around the world. In addition to GPS (Global Positioning System), Galileo satellite navigation is also supported, making them compliant with national requirements. Enhanced sensitivity and concurrent constellation reception extend the coverage and quality even in the most challenging of signal environments. Measurement and positioning navigation reduces time jitter even at low signal levels and keeps synchronization in the field of view with only a single satellite.

Multiple GRS or CoCo units can be used to acquire data simultaneously, even if they are physically located hundreds of miles away from each other. These units do not share any direct hardware connection, other than their respective GPS signals. Precise time-stamping technology is implemented using a GPS time base, so that the acquired signals can be arranged in the post-processing software.

Clock, timing, and position signals can be obtained from the GPS chip inside each GRS or CoCo unit. A real-time, zero-latency hardware logic is implemented to timestamp the A/D sampling clock with the measured GPS time base.

The following is a diagram of how the timestamps of GRS or CoCo work:

The following example shows how to plot a signal after applying sample rate correction with an additional timestamp signal.

The first diagram depicts two signals in the time domain where the trigger point is located. Each signal is captured with a different GRS unit.

The time difference shown above is caused using a nominal sampling rate of each GRS unit, and it may vary slightly. The second plot shows the same transient event at the end of the recording after a first-order correction for the sampling rates of the two signals has been applied.

These two signals line up in a beautiful line in the time domain!

Micro-Vibration Sensors

Crystal Instruments' Spider-80X, GRS, and CoCo-80X dynamic signal analyzer uses high dynamic range measurement technology and has a background noise of less than 10−7 V/√Hz. However, if the local noise of the micro-vibration sensor itself is high, the measurement accuracy will also be reduced. Vibration sensors can use acceleration-type and velocity-type micro-vibration sensors. They are usually very sensitive and can detect tiny vibrations. The frequency range is very low, only 0.1 to several hundred Hz, with strong anti-interference ability, low signal-to-noise ratio, relatively large size, heavy mass, and easy to install. The installation method is simple, usually placed on a smooth surface of the vibrating object or fixed to a mass block by bolts, and the mass block is then buried under the surface. Several commonly used models are listed in the following section.

Accelerometers

393B04 [Sensitivity: 1V/g, Frequency Range: 0.06~450Hz, Spectral Noise: 2.9 μg/√Hz(1Hz)]
393B12 [Sensitivity: 10V/g, Frequency Range: 0.15~1KHz, Spectral Noise: 1.30 μg/√Hz(1Hz)]
393B31 [Sensitivity: 10V/g, Frequency Range: 0.1~200Hz, Spectral Noise: 0.06 μg/√Hz(1Hz)]
731-207 [Sensitivity: 10V/g, Frequency Range: 0.6~450Hz, Spectral Noise: 0.28 μg/√Hz(2Hz)]

When the Crystal Instruments system is combined with a micro-vibration sensor, the effective acceleration value can reach 10 -8 g/√Hz.

Applications for Long-Range Micro-Vibration Monitoring

The GRS equipment can be installed at key points on structures such as bridges, railways, and high-rise buildings to ensure that the monitoring points cover an area representing the structure. The distance between GRS devices is evaluated based on the actual measurement requirements, e.g. 1 km. Users can set the sampling rate, recording duration, and trigger conditions of the GRS device based on the monitoring requirements. Use the GRS scheduling function to set a test plan and then repeat the operation at specified intervals. Through the communication module, the GRS device supports remote operation and data transmission and is suitable for a wide range of monitoring points.

The GRS equipment collects micro-vibration data generated from the vibration of a bridge or other structure in real time with time-stamps and then transmits the monitoring data to a cloud central database for storage and analysis through the wireless network. EDM Post Analyzer can then be used to analyze the collected data in detail and assess the health of the structure. The collected input is imported into EDM Modal software to analyze the modes of the structure, generate the mode shape animation, and calculate the modal parameters. To conclude, a monitoring report is generated according to the analysis results, and maintenance and repair suggestions are proposed.

GRS units are designed to operate unattended for long periods of time. The long battery life of the units combined with solar charging allows the system to operate for days or weeks at a time.

Engineering Data Management (EDM) software allows users to manage different monitoring points, real-time signal display, historical data viewing, reports, and alarms. The real-time signal display allows remote viewing of signal data collected by the GRS. Historical data can be viewed and exported according to days, weeks, months, or years. The Customized Reporting Tool in EDM allows users to select a report time range and the information to be included in the report. A report is then generated based on a selected custom template.

Case Study

Non-destructive techniques such as operational modal analysis are widely used to study building vibrations and to identify changes in stiffness properties due to deterioration, defects, or cracks by analyzing the modal parameters. However, experimental modal tests on large buildings often face challenges due to the fixed placement of data acquisition systems and the extensive cabling required for sensor deployment, especially when measuring ambient excitations like wind or foot traffic. Distributed data acquisition systems, combined with advanced GPS timestamp technology, provide a practical solution by enabling precise synchronization of data from multiple, wirelessly connected units.

An experimental investigation using this approach was executed to capture a building's frequency signature from ambient excitations, aiming to analyze its dynamic properties for structural health monitoring.

A geometric 3D model of the building was developed using EDM Modal software. The measurement points and node points, corresponding to the building's columns on each floor, were created and refined within the editor table, which allowed a precise input of points and lines.

Three CoCo-80X units were employed for this modal test. One CoCo was designated to measure the acceleration responses of the reference sensor, which remained fixed at a specific location. The other two CoCo units acquired the X and Y responses from 14 uni-axial accelerometers. These units were roved through all measurement points. A detailed test plan was created to document the workflow, which was then uploaded to the CoCo units to facilitate the execution and acquisition of measurements. The measurements from all distributed CoCos were precisely synchronized using the patented GPS timestamp technology.

The 10 V/g uni-axial accelerometers were positioned at specific columns to measure the building's acceleration in response to ambient excitation. 100-foot cables were used to connect the distributed CoCo systems to the sensors placed at various measurement columns.

The CoCo device logged these raw time-stream responses, along with the corresponding GPS timestamps at the time of recording.

The entire measurement dataset was transferred into EDM Modal software, where the time-domain data was post-processed to compute the spectrum data.

The computed spectra was further processed to extract the building's modal parameters, i.e., its natural frequencies, damping ratios, and mode shapes.

Conclusion

Crystal Instruments provides excellent performance and portability in an efficient and reliable solution for micro-vibration health monitoring of bridges, high-rise buildings, industrial production, and more. Real-time monitoring and data analysis of micro-vibrations will help ensure the long-term stability and safety of test structures and safeguard its operations.

Micro-vibration Testing Applications