See also: Cord

The physical phenomenon of the vibrating cord connects three physical sizes:

• the tension of the cord
• the length of the cord
• the frequency of Resonance of the cord

The vibrating cord sensor makes it possible to determine the tension of the cord by measuring its frequency of Résonance (first mode of vibration). Its initial length is given during its manufacture. From the tension of the cord, one deduces his lengthening. This physical principle thus makes it possible to measure the structural deformation on which the sensor in its two ends is fixed. The measurement of the frequency of resonance is obtained via an electromagnetic reel. It is this same reel which will be used to excite the cord via an electric impulse and to thus allow measurement. Often the sensor included a second vibrating cord not-dependant on the structure in order to take account of the influence of the temperature to the measure.

Applications

Instrumentation of the Civil engineering

The principle of the vibrating cord is used in many sensors of Civil engineering or Hydrologie. The advantages of this type of sensors are indeed very numerous for these scopes of application:

• the electrical signal corresponding to the measurement of the frequency of Résonance of the vibrating cord can be transmitted without disturbance on a hundred meters of cable,
• the sensors with vibrating cord proved a very great reliability and robustness in time
• the frequencies of measurement can be with difficulty higher than the Hertz taking into account the energization of the cord, but that is not awkward in the field of civil engineering or hydrology, where the measured phenomena are slow.

The incremental position sensors (or constraint) to cord vibrating are commonly called: strain gauge extensometric or . It is also possible to measure efforts applied to a surface via flat Vérins. In Hydrology, the principle is used in the Piézomètre S, probes of intersticielles Pressions and Sondes of level.

Method of control of Civil engineering

The phenomenon of the vibrating cord is also used to check the tension of the Haubans and them Amortissement. It is then a question of measuring the vibratory Harmonique S of the stay under the influence of the traffic or on an excitation of the type impulse or maintained, using accelerometers.

The gauge of deformation: extensometric vibrating cord gauge with drowning in the concrete

The extensometric gauge with drowned in the concrete makes it possible to measure the deformations caused by the variations of constraints. It makes it possible to calculate the constraints where the moduli of deformation are known after corrections for the effects of temperature, creep and the autogenous reactions of the concrete. The principal concrete structures instrumented with these extensometric gauges are:

• the stoppings
• the nuclear plants
• bridges and viaducts
• the large buildings
• tunnel linings
• piles and boxes

They can be installed before the casting of the freshly-mixed concrete or placed beforehand in a small concrete block which, it, will be placed in the freshly-mixed concrete. It is also possible to inject the gauges or the small block in a préforé hole.

The extensometric gauge consists of a weld steel wire with two steel heads using a welding with the tin of weak modulus of deformation. The distance between the round flasks of the heads determines the base of measurement. A small side block with semi-length of the tube contains the electromagnets of maintenance and listening. An electrical resistance of one of the electromagnetic reels can also provide the temperature.

The reading of the vibrating cord gauges is carried out using a portable station or of a power station of data acquisition. It can be carried out in mode “deadened” or “maintained” according to the manufacturers.

The deformations between the heads of fixing at the ends of the sensor induce variations in the length and frequency of vibration of the cord. The relation frequency-elongation characterizes each type of extensometric gauge and makes it possible to calculate the unit strains starting from the extensometric coefficient K.

Music

See Cord (music) .

See too

Related articles

• Wave on a vibrating cord

• mechanical Wave

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