Vibrating wire sensors have been around for well over a century. Their initial concept can be traced back to 1888, yet commercial development did not begin until around 1930, pioneered by companies Maihak in Germany and by Telemac in France. Most of these vibrating wire sensors were designed for use in dams.
In the 1960s further developments in the UK by the Building Research Establishment, now BRE, and in Norway by the Norwegian Geotechnical Institute (NGI), were made available to the public through Gage Technique in England, and through Geonor in Norway.
Designs during this period tended to be highly complicated, but had the added advantage of providing some excellent results, in combining long-term stability and survivability with a high level of accuracy.
The 1970s saw the rise of Geokon of Lebanon, New Hampshire, US, who redesigned the basic vibrating wire sensors along more elegant lines and used the latest electronic techniques in their readout equipment.
Geokon’s low-cost designs became so popular that they were, and still are today, copied by numerous companies worldwide and thus Geokon is recognized as the world leader in vibrating wire sensors.
How has the vibrating wire sensor remained the preferred type for use in harsh geotechnical environments for all these years?
The answer lies in the basic simplicity of what is really a mechanical transducer comprising of a length of tensioned steel wire, which vibrates at a frequency depending on the strain in the wire. The plucking of the wire is carried out electronically through a simple electrical coil, which, after the pluck, becomes a sensor outputting an alternating electrical current with the same frequency as that of the vibrating wire.
This basic relationship between vibrational frequency and strain is then used in a variety of sensor construction configurations (see wwww.geokon.com) to measure strains, stresses, ressures, temperature, tilts, displacements and loads, in tunnels, mines, dams, bridges, foundations, piles, landfills, unstable slopes and many other geotechnical, civil and structural applications.
The relationships between the vibrating wire characteristics and movement is given by:
Where:
f = frequency of vibration
L = wire length
T = tension
m = wire mass per unit length.
What are the advantages of vibrating wire sensors?:
1st: They are very resistant to water intrusion, the instance of which would usually spell instant death to more sophisticated electronic devices.
The frequency output of a vibrating wire sensor is not affected by changes in contact resistance, leakage to earth, or in changes of signal cable resistance. Cable lengths of up to several kilometers can be used successfully to reach remote sensor locations.
2nd: The long term stability of vibrating wire sensors is legendary. The all-steel construction of the basic sensor leaves nothing to deteriorate with time. Vibrating wire strain gages and piezometers typically show zero drifts of less than 0.1 per cent of full scale per year (FS/year) and sensor life can be counted in decades. There have actually been reports of sensors installed over 60 years ago in European dams that are still functioning.
3rd: Vibrating wire sensors, unlike 4-20- m A sensors, have no delicate electronics to ‘fry’, so they have a very high resistance to lightning damage. The vibrating wire plucking coil can be given still further protection through the incorporation of a gas discharge tube fitted inside the sensor that short-circuits to earth if the voltage across the coil ever exceeds 40V.
4th: Vibrating wire sensors can be easily multiplexed and connected to automatic data acquisition systems (ADAS). Several companies, such as Campbell Scientific and Datataker, have developed dataloggers to handle Geokon type vibrating wire sensors ( which can also be adapted to all SCADA [Supervisory Control and Data Acquisition] systems). Improved electrical noise suppression techniques using Fast Fourier Transform (FFT) techniques, (e.g. Campbell Scientifics’ Model AV200) allow the vibrating wire signals to be measured accurately in electrically noisy environments.
Specialty vibrating wire sensors have also been developed to measure temperatures of up to 230 degrees C. They are particularly useful, where other types of sensors have failed, in monitoring small changes of pressure and temperature at great depths in steam-assisted, gravity drainage (SAGD) oil recovery wells.
Vibrating wire sensors are also able to survive in very high radiation environments and have been used to monitor nuclear waste disposal processes.
The one area in which vibrating wire sensors are at a disadvantage happens to be for measurement of dynamic (rapid changing) parameters – the length of time required for the pluck and read sequence is just too long.
However, for accurate and reliable quasi-static measurements in harsh environments, there is no equal to the vibrating wire sensor.
Schematic representation of the operating principles of a vibrating wire transducer for a geotechnical instrument An array of Geokon vibrating wire geotechnical instruments and accessories math
